Review Article An Affordable Green Energy Source

Review Article An Affordable Green Energy Source - Evolving through Current Developments of Organic, Dye Sensitized and Perovskite Solar Cells DOI:10.1080/15435075.2016.1171227 * S Ahmad Publishing models and article dates explained Received: 30 Apr 2014; Accepted: 23 Mar 2016 Accepted author version posted online: 11 Apr 2016 Abstract Intensive R&D efforts made in solar photovoltaic area has enabled Si, GaAs, CdTe, and CIGS devices exhibiting nearly ideal efficiencies. However, cost incurred per unit energy does not allow them to qualify for alternate energy source besides environmental issues at production floors. Consequently, search is still continuing not only to reduce cost by improved efficiency, reduced materials cost and simplified processing resulting in a cleaner and greener technology. Current successes in organic, dye-sensitized and perovskite solar cells with efficiencies in 10–15% range raise concrete hope for evolving a low cost and green technology involving solution processable organic/inorganic compounds extendable to roll-to-roll production. The current status and future prospects of these three promising routes are reviewed here highlighting the possibility of an emerging alternate green energy source to supplement the conventional ones at commercial level. Keywords Dye sensitized solar cells, green cells, photovoltaic energy source Abstract

energy

source, organic

solar

cells, perovskite

solar

Intensive R&D efforts made in solar photovoltaic area has enabled Si, GaAs, CdTe, and CIGS devices exhibiting nearly ideal efficiencies. However, cost incurred per unit energy does not allow them to qualify for alternate energy source besides environmental issues at production floors. Consequently, search is still continuing not only to reduce cost by improved efficiency, reduced materials cost and simplified processing resulting in a cleaner and greener technology. Current successes in organic, dye-sensitized and perovskite solar cells with efficiencies in 10–15% range raise concrete hope for evolving a low cost and green technology involving solution processable organic/inorganic compounds extendable to roll-to-roll production. The current status and future prospects of these three promising routes are reviewed here highlighting the possibility of an emerging alternate green energy source to supplement the conventional ones at commercial level. INTRODUCTION Silicon solar cells are the commonest photovoltaic (PV) devices to convert solar radiation into electrical energy currently being produced globally. The optimized design of Si solar cells has produced power conversion efficiency (PCE) of 25% on laboratory scale and 22% in large-scale production (Green et al. 2012) that has, of late, been revised to 29.43% (Richter, Hermle, and Glunz 2013), inching fairly close to the theoretical maximum of 31% for an ideal single p-n junction cell. In compound semiconductor multi-junction solar cells, the record efficiency of 44% (NREL 2012) and 44.4% (Sharp 2013), reported in recent past, has of late been improved to 44.7% (FISES; Soitec; CEA-Leti; Helmholtz Center Berlin, FISES; Soitec; CEA-Leti; Helmholtz Center Berlin 2013). In spite of having achieved these impressive values of PV solar cell

efficiencies, there are still problems like costly/scarce raw materials and involved technologies besides associated environmental issues connected with the production processes. The environmental pollutions and occupational health hazards, especially posed by the toxic chemicals either used or produced in SC manufacturing at various stages need a very careful attention as in case not contained properly, it may finally curtail the benefits of greenhouse gas emission reduction expected from the deployment of PV solar cell systems for future energy generation in place of fossil fuel based systems (Mulvaney 2013). Consequently, because of these constraints, efforts are still continuing to improve the solar cell performance for making them commercially viable in near future. In this context, the areas like device design optimization, defect control for improved lifetime, surface modification for device degradation and nanomaterials based device configurations, where some possibilities of further improvements do exist, are currently being examined for introducing the most probable alternatives (Binetti et al. 2013). Alternatively, lower cost versions of solar PV devices involving organic, organic/inorganic hybrid and dye sensitized solar cells are currently getting developed fast to explore meeting the part of energy shortfalls by complementing the share of the well-established silicon, gallium arsenide and copper-indium-gallium-selenide (CIGS) thin film devices (Arici, Meissner, and Sariciftci2004; Grätzel 2004; Huynh, Peng, and Alivisatos 1999; Shaheen et al. 2001). It is also important to note that the changes made by innovative molecular designs in a large variety of organic/polymeric molecules provide enough control to modify the material characteristics that are not only better to meet the requirement of PV energy conversion but also offer green technology alternatives with minimum environmental pollution and occupational health hazards. This kind of development of newer materials and simplified fabrication processes are expected to change the current status of PVSC acceptability by the industry in due course of time in future. In an alternate form of organic photovoltaic solar cells (OPVSCs), there is an inter-penetrating network of electron-donating and electron-accepting polymeric materials forming a percolating structure appropriate for PV energy conversion (Yu et al.1995). The photo-induced charge carrier transfers from a conjugated polymer to a fullerene derivative generates electrical power in a “bulk hetero junction” organic solar cell (Sariciftci et al. 1992). In hybrid solar cells, organic and inorganic materials are combined together availing the film forming properties of conjugated polymers in presence of inorganic semiconductors (Arici, Sariciftci, and Meissner 2003). It is significantly important to note that numerous organic materials used in OPVSCs are generally inexpensive, easy to process in solution form and their functional properties can be tailored by molecular design and chemical synthesis. In contrast, the inorganic semiconductors are easy to convert into nano crystals (NCs) that offer the advantage of having high absorption coefficients based on tunable size that affects band gap and the absorption band (Weller1993). Thus, a combination of organic and inorganic compounds is destined to offer better solutions in the development of future PV devices in general. For example, a concept of this kind led to the development of a dye-sensitized solar cell (DSSC) containing an electrolyte sandwiched between a light harvesting dye-sensitized semiconducting photoanode and a Pt-catalyst coated counter electrode. The photoanode is prepared by depositing a mesoporous thin film out of a range of wide band-gap metal oxide semiconductors like TiO2, ZnO2 or SnO2 on a transparent conducting − 3-

oxide (TCO) glass substrate. The liquid electrolyte contains the iodide/triiodide (I /I )-redox

couple in an organic solvent or in a solid form as a hole transporting p-type semiconductor. The counter-electrode (CE) is generally a thin film coating of Pt-catalyst on TCO-glass-substrate that has indium tin oxide (ITO) or fluorine doped tin oxide (FTO). Such a DSSC device converts solar radiation into electrical energy involving processes as briefly described in the following. The moment dye molecules absorb energetic photons from the solar radiation; electrons from the highest occupied molecular orbital (HOMO) are excited to the lowest unoccupied molecular orbital (LUMO) of the dye molecule forming electron-hole pairs as excitons. The excited state electrons from the dye molecule are subsequently injected into the conduction band of the mesoporous photoanode. On the other hand, electrons entering from the counter electrode (CE) side via redox couple electrolyte regenerate the oxidized dye molecules due to photo absorption rendering them again ready for photo-excitation. The injected electrons in semiconductor conduction band diffuse through the mesoporous film before they arrive at the anode from where they are released to the external circuit ultimately to return to CE where they are readily available for regenerating the oxidized redox species as mentioned above. In this way, the device generates electrical power from the light with no permanent chemical changes, taking place during the entire process. Since the first successful realization of DSSC reported in 1991, a number of significant improvements have, so far been made, but for last few years, the PCEs of these devices have stuck around 12% (Bisquert 2011). Nevertheless, further development of these PVSCs holds considerable promise involving relatively low cost solar cell technology of tomorrow due to the basic requirement of cost effective raw materials and simpler processing techniques that especially seem appropriate for automated large-scale manufacturing in due course of time (Bisquert 2011). In any kind of solar cell development, it is well known that the smaller difference between the efficiency of smaller area cells and the larger size panels is the indicator of the maturity of the fabrication technology. In case of DSSCs, this difference is still large primarily due to additional losses that occur while trying to fabricate larger area devices. In order to compensate for such losses in further scale-up of smaller area cells to larger size modules, the development of devices with still higher efficiencies becomes imperative along with rugged device packaging to ensure a longer life cycle. However, despite facing some sort of stagnation in the efficiency of DSSCs, the recent investigations carried out in search of better alternatives, do indicate towards some better prospects. In this context, it is rather useful to have a quick look into the salient features of these developments that were primarily undertaken to improve the performance of every individual component of a DSSC/OPVSC including hole transport layer, light sensitizer, photoanode and counter electrode besides exploring other conceptually different device configurations as well. It is imperative to consider improving the processes involved in electron injection as well as regeneration of the dye and the redox couple for ultimately realizing higher efficiency DSSC devices. Examining the overall reaction kinetics that is involved in the functioning of a DSSC, it can be seen that there is a competition between two sets of forward and reverse processes. The forward processes include electron injection into the semiconductor photo anode; regeneration of oxidized dye and redox couple and the backward process is primarily recombination of the photogenerated electrons before reaching the photo anode. The electron recombination also includes the oxidized dye molecules or electron acceptors in the electrolyte present there in a DSSC while

the photo injected electrons traverse relatively longer paths involving TiO 2 layer surrounded by dye and electrolyte molecules. Though, recombination process is slower than the forward processes mentioned above, but eventually, the competition between the forward and backward processes does affect the overall device efficiency. It has been possible to have DSSCs crossing the efficiency over 11% as a result of careful considerations of all these forward and backward processes in detail while designing the liquid electrolyte. The situation in quasi solid-state DSSC devices is lagging behind as can be seen later. −

−

In most of the conventional DSSCs, developed so far, an electrolyte with I /I3 redox couple is used to have higher conversion efficiency and improved reliability (Wang et al. 2005). Besides, having numerous advantages like low cost raw materials, simpler manufacturing process, lightweight and eco-friendly technology, the liquid electrolyte-based devices could not yet gain the industrial acceptance due to electrolyte leakage and corrosion of metal films in the vicinity. The −

–

lower redox potential associated with I /I3 redox couple is another limitation because of which it is not possible to increase the open circuit voltage (V OC) above 800 mV causing inefficient dyeregeneration. In addition, as I2-containing liquid electrolytes corrode metal thin films like Au, Ag and Cu; it restricts their use in preparing interconnects for fabricating larger size modules. In addition, the iodide ions are sensitive to some part of solar spectrum, which is not helpful. In order to get rid of these limitations, the development of a more stable, non-corrosive redox couple with −

–

higher redox potential than I /I3 redox couple is, thus, highly desirable. Despite having achieved a reasonable success in realizing DSSCs with efficiencies in excess of 10% and considerable life cycle, there are still a number of issues, as discussed above, that should be resolved while translating newer device structures to arrive at the options, which are not only more efficient but even less costlier and more environmentally friendly for their wide spread applications in future. In this context, it is, therefore, necessary to review the progress made recently in case of every component of a DSSC and then assess the situation regarding their possible modifications as future prospects. It is also getting clearer that finally an all solidstate configuration of DSSC is expected to emerge where most of the drawbacks of the current structures of DSSC and OPVSC devices are well taken care of by choosing the right kind of material compositions and combinations resulting in cost effective technologies capable of producing the larger area devices and modules for their large scale applications as renewable sources of alternate energy. In this context, very recently explored PVSCs employing Pb halide perovskite materials deserve special mention as in a shorter span of about few years 15% efficiency PV solar cells are quite common with further expectation of reaching around 20% level soon (Snaith 2010). An attempt has, therefore, been made here to examine various aspects of the progress made in the direction of developing high efficiency all solid state dye sensitized polymeric solar cells that would be better suited for commercial exploitation by assessing the existing status from device development angles. Besides improving device characteristics offering very high efficiency and reasonably longer lifecycle, the assessment of environmental impact of the process technology to be developed is equally important for making these devices commercially viable. For example, it is necessary to estimate the green house gas (GHG) emission status of the manufacturing technology targeted for future for which the standard technique developed by NREL is found very useful to deploy (NREL Analysis). A brief description of the process and some representative estimates for the current status of various technologies

involved in PV energy generation are also discussed in brief later. This shows that for a viable and green PVSC technology, it is a must to examine every aspect involved in realizing a cost effective and reliable technology not only from material angle alone but also from a device processing technology angles as a whole. The current review is aimed at highlighting these aspects for their practical applications. DSSC COMPONENTS For examining the basic limitations that are holding the overall efficiency of DSSCs at 10% and cost not yet appropriate for large-scale production due to costly rare and noble metals involved in the preparation of dye as well as counter electrode, it is important to know about the outcome of the recent investigations carried before considering the influence of using them with newer designs in their experimental realizations. The current DSSC device designs involve a number of layers (Yang, Zhang, and Meng 2011) stacked in series, including a glass substrate, transparent conducting layer, TiO 2 mesoporous photo anode, dye sensitizer, electrolyte redox couples (alternately hole transport layer) and catalytic counter electrode covered with sealing gasket. The layers are appropriately modified in case of flexible substrates keeping in view the relatively lower temperature processes. Electrolyte Redox Couples Currently, there are three main types of electrolytes that are used in a DSSC with their merits and demerits as briefly listed here (Yang, Zhang, and Meng 2011). The most common one involves −

−

+

acetonitrile solvent mixed with I /I3 and Li -ions as additive to improve the electron transport through adequate ion diffusion by entering into the pores of TiO 2 film, resulting in the highest efficiency, so far, except the problems of volatility and corrosion. The second category is that of inorganic ionic liquids (ILs) made of salt mixture that looks like solid with liquid like properties that are well suited for better ionic conductivity. However, these devices deteriorate in efficiency, in general, after their continuous uses over a period of time (Yang, Zhang, and Meng 2011). The third category is of solid electrolytes like spiro-MeOTAD or CuI where it is not that easy to fill the pores of TiO2 film due to CuI crystallization. However, this pore-filling problem is possible to get away with, to a large extent, by adding suitable crystallization inhibitor compounds (Konno, Kumara, and Kaneko 2007; Yang, Zhang, and Meng2011). In this context, it is anticipated that the liquid electrolyte filling into the TiO2 pores will form an intimate contact with the dye-coated nano-particles during device fabrication. Viewing the basic function of an electrolyte to provide diffusive ionic conduction path within the semiconducting layer (Wang 2009), comparison of the IPEC peaks of the devices using different types of electrolytes confirmed that the largest values were obtained in case of volatile organic solvents followed by those of IL-electrolytes. Despite producing highest efficiency devices, so far, the inherent limitations of these volatile organic solvent electrolytes (Wachter et al. 2008) encouraged the researchers to explore newer types of electrolytes such as ionic liquids (ILs), p-type inorganic/organic semiconductors, polymer and polymer gel electrolytes as possible alternatives in the recent past. A combination of room temperature ILs and low molecular weight organic solvents are found suitable for their uses in DSSCs after introducing the polar ligands that impart features like nonvolatility, chemical stability, non-inflammability besides higher ionic conductivity at room temperature except one drawback of high viscosity, which causes efficiency reduction due to

increased series resistance affecting the photocurrent generation in contrast to the conventional volatile organic solvent electrolytes. In case of p-type inorganic semiconducting HTL, the basic limitation is especially posed by the requirement of high temperature deposition causing possible damage to the dye. Keeping in view the requirement of diverse features of electrolyte as briefly mentioned above, it is better to examine them collectively (Li et al. 2006a; Nogueira, Longo, and De Paoli 2004). First of all, it is necessary that the electrolyte redox potential must match with the dye redox potential to facilitate dye regeneration and charge carrier transport between photoanode and CE after the sensitizer injects electrons into the semiconductor and the oxidized dye is reduced to ground state. Additionally, it must possess higher conductivity to permit fast diffusion of charge carriers while establishing intimate contact with mesoporous semiconductor and CE. Under these circumstances, it must possess long-term chemical, thermal, optical, electro-chemical and interfacial stability; eliminating the possibility of dye desorption and degradation from the oxide surfaces especially in case of liquid electrolyte and should not exhibit a significant optical absorption in the visible region of the solar spectrum for minimizing the interference with dye. For —

–

—

example, I3 in I /I3 redox couple, shows color that reduces the dye absorption and reacts with the injected electrons increasing the dark current. The earliest DSSCs using liquid electrolyte comprising of LiI/I 2, exhibited PCE of 7.1% followed by exploration of a large variety of other liquid electrolytes containing iodide/triiodide redox couple and organic solvents like acetonitrile, ethylene carbonate, 3-methoxypropionitrile, propylene carbonate, -butyrolactone and N-methylpyrrolidone (Fukui et al. 2006, Hao et al. 2006, Kebede and Lindquist 1999; Wang et al. 2003a, 2003c, 2003d; Wu et al. 2007). Detailed investigations carried out in this context revealed that the electrolyte components including solvent, redox couple and additives—all are important to decide the overall performance of the DSSCs (Wu et al. 2008). The interaction between non-aqueous solvents and iodine (Kebede and Lindquist 1999) helped in predicting iodide to triiodide transformation, which is subsequently correlated with the increase inVOC and reduction in JSC (Fukui et al. 2006; Hara et al. 2001; Wu et al. 2007). −

−

It is interesting to note that despite the mismatch between redox potential of (I /I3 ) electrolyte and the Ru-based dye complex, DSSCs developed using this redox couple exhibited the un-matched performance. Thus, in search for other alternative redox mediators, it is necessary to look for a −

−

more positive redox potential than (I /I3 ) couple for minimizing the VOC losses, due to the Nernst potential of the iodine-based redox couple and stopping the JSC leakage due to light absorption by −

−

triiodide ions. In this context, a variety of redox couples such as—SCN /(SCN) 3, −

−

+

+

+

+

SeCN /(SeCN) 3, (Co2 /Co3 ) and (Co /Co2 ) complexes and organic mediators such as 2,2,6,6−

−

tetramethyl-1-piperidyloxy were investigated to replace (I /I3 ) couple without much success (Min et al. 2011). Ideally, the redox couple should possess the following characteristics for its use as a DSSC electrolyte (Wolfbauer et al. 2001). First of all, it’s redox potential should match with that of the sensitizer to maximize VOC and should have higher solubility in the solvent to provide higher concentration of charge carriers along with higher diffusion coefficient in the solvent for better mass transport. Redox couple should, preferably, avoid having visible region absorption. Stabilities of the reduced and oxidized forms of the redox couple are imperative for longer operating life of the device. Highly reversible couple is necessary for faster electron transfers.

Finally, any couple, chosen for DSSC applications, should be chemically inert to all other components used in the cell making. –

Keeping these consideration in view, a number of alternate redox couples like Br /Br2, SCN

–

–

/SCN2, SeCN /SeCN2 and bipyridine Co(III/II) complexes were explored for DSSC applications but owing to their energy level mismatch with those of the dyes and/or their intrinsically lower diffusion coefficients in the electrolyte, lower efficiency devices were invariably realized as −

−

compared to those with the standard (I /I3 ) redox couple (Bergeron et al. 2005; Nogueira, Longo, and De Paoli 2004; Nusbaumer, Mcser, and Zakeeruddin 2001; Oskam et al. 2001; Sapp et al. 2002; Wang et al. 2005). In an in-depth analysis of charge carrier transport in the electrolyte, strong pairing of electrons +

with Li -ions was observed in absence of any band bending or depletion layer at the electrolyte/TiO2 NCs

interface

(Grätzel 2001;

Grätzel

and

Frank 1982;

Hagfeldt

and

Graetzel 1995), which was responsible for strongly correlated ambipolar diffusion of the electrons in TiO2 (Kopidakis et al. 2000; Nister et al. 2002; Solbrand et al. 1997; van de Lagemaat, Park, and Frank 2000; Zaban, Meier, and Gregg 1997). This phenomena of pairing was confirmed by noting the influence of adding LiI salt into the electrolyte by increasing JSC simply due to formation +

of additional Li -electron ambipolar pairs (Frank, Kopidakis, and van de Lagemaat 2004; Liu et al. 1998; Olson 2006). However, with the increased concentration of Li-ions, it also created additional problem by enhancing the possibility of combining with triiodide species, which consequently reduced VOC (Frank, Kopidakis, and van de Lagemaat 2004; Watson and Meyer 2004). This problem was, although, taken care of by adding larger size imidazole cations in the electrolyte to form a Helmholtz layer on the surface of TiO 2 film that blocked the direct +

+

contact of triiodide with ambipolar Li -electron pairs suppressing the interaction between Li electron pairs and triiodide leading to an improved VOC (Olson2006; Watson and Meyer 2004). Consequently, adding a small amount of 4-tert-butylpyridine and pyridine to the electrolyte improved VOC (Kusama and Arakawa 2004a, 2004b, 2004c, 2004d, 2005; Nazeeruddin et al. 1993) though with reduced JSC.However, only smaller amount of additive was found adequate to improve the device performance as the excess additive deteriorated the device properties in general (Lan et al. 2008; Wu et al. 2007). While optimizing the combination of redox couple and the electrolyte, low mass transport of the redox couple was another limiting factor, which needed higher concentration of redox couples for sufficient conductivity in the electrolyte. However, in doing so, the dark current increased with higher concentration of redox couples close to the dye attached to TiO 2 and the photocurrent generation could thus be interrupted by the absorption of visible light into the electrolyte (Kang et al. 2008). Currently, 1,3-dialkilimidazolium salts have been studied using di-cationic bis-imidazolium based ionic liquid (IL) electrolytes with alkyl and polyether chains that showed improved charge carrier transport wherein polyether groups improve the self-organization of the molecules increasing the charge transfer and wetting of TiO2 surface. This consideration resulted in device efficiency of 5.6% in case of di-cationic bis-imidazolium iodide where the fill factors exceeded more than 75% even in full sunlight irradiation, whereas the dilution of the same ionic liquid with a low-viscosity organic solvent often reduced the fill factor (Zafer et al. 2009).

In a very recent development, the substitution of terpyridine ligand to Cobalt bis (2,2′,6′,2′′terpyridine) complex enabled systematic tuning of the redox potential for optimizing V OC and the II

III

overall performance of the device. A [Co (Cl-terpy)2](TFSI)2/[Co (Cl-terpy)2] (TFSI)3 based electrolyte in combination with the Y123 D–π –A dye exhibited PCE of 8.7% under standard irradiation (Aribia et al. 2013). DSSCs comprising of ferrocene/ferrocenium redox couple with a metal-free organic D-A sensitizer, namely Carbz-PAHTDTT, exhibited efficiency of 7.5% at standard irradiation, primarily due to better matching of the redox potential of the ferrocene couple with that of the new − 3-

sensitizer. These devices exceeded the efficiency achieved in case of devices using I /I

electrolytes under comparable conditions, revealing the inherent potential of ferrocene-based electrolytes in future DSSC applications. This was followed by a systematic study of the charge transfer reactions as a function of the dye regeneration rate, recombination losses and recombination pathways on the reaction driving force to prepare guide-lines for selecting appropriate redox couple and sensitizer for fabricating high efficiency devices. During this exercise, it was noted that an increase in redox potential led to a higher efficiency due to higher VOC until a threshold reached where the driving force for dye regeneration became too small leading to rapid recombination between the oxidized dye and electrons in the TiO 2conduction band (Daeneke et al. 2012). A

cyclo-penta-di-thiophene-bridged II

D-A

sensitizer

named

as

Y123

along

with

III

[Co (bpy)3](B(CN)4)2 and [Co (bpy)3](B(CN)4)3 as redox couple, gave efficiencies up to 9.6 % under standard irradiation confirming the legitimacy of Co complexes as worthy alternatives to −

−

I /I3 redox couple, when used with properly designed organic sensitizers (Tsao et al. 2011). A 3+/2+

Co-complex with tridendate ligands [Co (bpy-pz)2]

(PF6)3/2 as redox mediator in combination

with Y123, adsorbed on TiO2, yielded an efficiency of 10% at standard irradiation with VOC = 1,000mV in a DSSC in which the high efficiency performance was the result of negligible absorption in the visible region and matching redox properties by selecting suitable D-A substituents on the ligand (Yum et al. 2012). In a recent development, adding terpyridine ligand to Co bis (2,2′,6′,2′′-terpyridine) complexes enabled systematic tuning of the redox potential for optimizing V OC and the overall PV performance

of

the

device.

A

combination

of

II

III

[Co (Cl-terpy)2](TFSI)2/[Co (Cl-terpy)2]

(TFSI)3 electrolyte with the Y123 dye yielded an efficiency of 8.7% under standard irradiation (Aribia et al. 2013). A number of alternative involving transition metal complex redox couples were thus examined for minimizing the driving force needed for dye regeneration and to optimize VOC. While searching for a better alternative, there are certain requirements to look for in an alternative redox mediator in order to obtain high efficiency DSSCs as mentioned in the followings. First of all, the redox potential should be as positive as possible to optimize V OC while providing a sufficient driving force for regeneration of the oxidized dye. The features of slower electron recombination and higher diffusion coefficient to avoid mass transport limitations are necessary for faster electron transfers at CE. Non-corrosive behavior towards metal thin films and negligible light absorptions along with improved photo-electrochemical stability are additional requirements to be there. Organic redox couples studied, in this context, include halogens (Ning et al. 2009; Rani, Suri, and Mehra 2011; Teng et al.2009), pseudo-halogens (Bergeron et al. 2005; Oskam et al. 2001; Wang

et al. 2004b), inter-halogens (Gorlov et al. 2007), hydro-quinones (Pichot and Gregg 2000), nitroxide radicals (Kato et al. 2012; Zhang et al. 2008) and sulfur-based systems (Li et al. 2010; −

−

3-

Tian et al. 2010b; Wang et al. 2010). Many of theses redox couples included Br /Br , SCN / −

−

−

(SCN)3 , SeCN /(SeCN)3 and thiolate/disulfide as well. The use of fast one-electron transition-metal redox couple, until recently, resulted in low V OC and JSC, because of enhanced recombination of the electrons in TiO 2 conduction band to the oxidized redox

species.

The

transition-metal

redox

couples,

investigated

so

far,

included

ferrocene/ferrocenium (Daeneke et al. 2011; Feldt et al. 2010; Gregg et al. 2001), Cu (I/II) (Bai et al.2011; Hattori et al. 2005), Co (II/III) (Nusbaumer, Mcser, and Zakeeruddin 2001; Nusbaumer et al. 2003; Sapp et al. 2002) and Ni (III/IV) (Li et al. 2010) complexes. Very recently, a new approach was proposed to design redox mediators involving the application +

+

of [Co(PY5Me2)(MeCN)]2 /3 complexes, where PY5Me2 is the pentadentate ligand 2,6-bis(1,1bis(2-pyridyl)ethyl) pyridine, to fine-tune the redox potential to match with the dye through coordinative interactions with the Co(II/III) centers. Application of electrolytes based on the 2+ 3+

[Co(PY5Me2)(NMBI)] /

complex combined with a commercial organic sensitizer resulted in

efficiency of 8.4% and 9.2% at a simulated light intensity of 1 and at 0.1 sun, respectively, higher 2+ 3+

than those from similar devices applying the [Co(bpy)3] / redox couple (Kashif et al. 2012). In order to address the issues of improving the ionic conductivity and
 long-term stability of PV devices, KI was added as crystal growth inhibitor along with diphenylamine (DPA)
 in poly (ethylene oxide) (PEO) for improving the charge transport in the electrolyte along with TiO2 nanocrystals.

These

devices

showed

a

conversion

efficiency

of

5.8%

under

standard
 irradiation with additional observation that DPA enhanced the interaction of the −

−

TiO2 NCs
 film and the I /I3 electrolyte leading to higher ionic conductivity of 3.5x10

–3

S/cm

without compromising on the electrochemical and mechanical stability. It was further confirmed by measurements that electron transport and electron lifetime were enhanced due to reduced iodine sublimation, which resulted in increased stability confirmed by preservation of 89% of the overall efficiency even after 40 days of operation (Agarwala et al. 2011). An environmentally friendly electrolyte, comprising of IL namely LiI(C 2H5OH)4–I2, was used in DSSCs with conversion efficiency of 4.9% under standard irradiation (Xue et al. 2004). A new type of IL electrolyte using IIQAI—imidazolium and quaternary ammonium salt and APII(hydroxyethyl, propyl, hexyl) was synthesized in which APII-hexyl was solid and IIQAI, APII(hydroxyethyl, propyl) were viscous liquids, showing better thermal stability compared to the commercial ILs of DMII. DSSCs using this electrolyte demonstrated efficiency of 6.3%, which was higher than that of the referenced from DMII devices at 6.2%) (Seo et al. 2011). A novel printable polymer gel electrolyte (PGE) with higher ionic conductivity based on polyvinyl (acetate-co-methyl methacrylate) [P (VA-co-MMA)] was prepared by soaking co-polymers in an organic electrolyte solution in acetonitrile (ACN) or 3-Methoxypropionitrile (MPN) containing − −

I3 /I as redox couple. The optimized quasi-solid-state devices demonstrated PCE = 9.10% under standard illumination whereas adding TiO2 NCs into the PGEs further enhanced ionic conductivity and the conversion efficiency to 9.40%. Subsequent results revealed that such quasi-solid state DSSCs had a better stability as they maintained 96.7% of the initial efficiency even after 1000 hours exposure to simulated sunlight. Besides, for the first time, large-area devices were

screen-printed on a 5 cm × 7 cm size substrate exhibiting efficiency better than 4% (Wang et al. 2013). While trying to usher in the large-scale DSSC production technology by enhancing the long-term stability of these devices, an altogether different approach was adopted to understand the influence of dye-molecules added to the liquid electrolyte. The motivation behind this approach was desorption of the dye molecules from the TiO 2 surface observed during long-term cycling leading to JSC degradation. For the first time, dye molecules desorption was suppressed by controlling thermodynamic equilibrium; by adding more dye molecules in the electrolyte that resulted in higher-level stabilities due to the suppressed dye-desorption (Heo, Jun, and Park 2013). A hybrid polymer gel electrolyte (PGE) comprising of poly-acrylonitrile (PAN) with graphene incorporated in a mixture of iodide redox electrolyte was prepared, for the first time, for quasisolid DSSC devices wherein the exfoliated 2-D graphenes dispersed in PGE facilitated the −

−

diffusion of I /I3 ion pairs better. By optimizing the ratio of graphene in the PAN PGE to maximize the ionic conductivity and photo electronic properties of the DSSC resulted in the efficiency of 5.41%, which was 1.5 times that of the liquid-electrolyte cell efficiency at 3.72% (Chan, Wang, and Chen 2013). Measuring ac conductivity and steady-state voltammetry it was possible to estimate the −

−

conductivities of I , I3 and cations in electrolytes comprising of hexa [methoxyethoxyethoxy cyclotriphosphazene] (MEE trimer) mixed with LiI, NaI, NH4I, and 1-methyl-3-propylimidazolium (PMII) and I2. From such a study it was concluded that the anionic conductivities were highest in the PMII system and decreased in the following order as: PMII > NH 4I > NaI > LiI. PV measurements of DSSC devices containing these electrolytes confirmed a similar order of performance. High polymeric polyphosphazene-plasticizer blended with a dissolved PMII/I2 electrolyte gave better performing DSSCs than equivalent poly (ethylene oxide)-plasticizer electrolytes (Lee 2010). A redox couple of Co (II)-bis[2,6-bis(1ʹ′-butylbenzimidazol-2ʹ′-yl) pyridine] demonstrated a significant promise rivaling the performance of the tri-iodide/iodide couple that was exclusively preferred for DSSCs. Though, such polypyridine based Co-complexes turned out to be better redox system because of their low visible light absorption, higher redox potentials and the reduced corrosiveness towards metallic conductors, but, otherwise resulted in lower efficiencies possibly due to the slow mass transport and faster back reaction of photo-injected electrons with the oxidized redox species coupled with the slow regeneration of the Co (II) at the cathode (Nusbaumer, Mcser, and Zakeeruddin 2001). However, the experimental realization of an improved efficiency of 6.7% using a D-π-A organic sensitizer-D35, in conjunction with the cobalt (III/II) tris-bipyridyl complex, [Co(III)(bpy)3](PF6)3/[Co(II)(bpy)3](PF6)2 couple triggered a renewed interest in this context. The tunable redox properties of Co-complexes by appropriate choice of − −

donor/acceptor substituents offered attractive alternate to the traditional I3 /I redox used in DSSCs. VOC in excess of 1,000 mV was observed in cells with such Co complexes (Yum et 2

al.2012). Since, D35 harvested sunlight only below 620 nm limiting JSC to 11 mA/cm , it’s spectral response was subsequently extended into the red region by incorporating a cyclo penta-dithiophene (CPDT) bridging unit in the D-π-A structure. The dye called Y123, in combination with 2

[CoII(bpy)](B(CN))/[CoIII-(bpy)3](B(CN)4)3 redox couple, exhibited JSC > 15 mA/cm under full

sunlight with PCE of 9.6% (Tsao et al. 2011). Later on, a complex with tridendate ligands 3+ 2+

[Co(bpy-pz)2] / (PF6)3/2 as redox mediator along with Y123 dye, adsorbed on TiO 2 used in 2.

devices showed PCE > 10% at 100 mW/cm This result established that the molecularly − −

engineered Co redox couple was a legitimate alternative to the commonly used I3 /I redox shuttle (Yum et al. 2012). In a detailed study of hole-transport in an organic complex called spiro-OMeTAD, it was observed that the electron-transport in TiO2 in DSSCs with liquid and IL-electrolytes were almost similar. Impedance spectroscopic measurements further clarified that the recombination rate being two orders of magnitude higher in spiro-OMeTAD based devices was the main cause of relatively lower PCEs. The anticipated enhancement in VOC due to a lower hole-Fermi level was offset by the excessive recombination losses. Under low potentials, the charge carrier transport was mainly due to the electron transport in the TiO2, but at higher potentials spiro-OMeTAD transport resistance took over by reducing the fill factor and efficiency of the devices. Thus, the enhanced recombination was noted as the major cause of the lower efficiency of these cells (FabregatSantiago et al. 2009). In search for suitable HTL, another organic conductor P3HT was examined against spiroOMeTAD and two dyes—D35 and M3. IPCE close to unity was noted in case of spiro-OMeTAD based devices resulting in an efficiency of 4.7% and 4.9% with D35 and M3, respectively, whereas P3HT devices showed lower efficiencies of 3.2% and 0.5% for the corresponding combinations. Photo-induced absorption measurements clarified the reason for this poor performance as the presence of incomplete dye regeneration and the polymer infiltration in case of P3HT, while spiro-OMeTAD regenerated the dyes very efficiently. This confirmed the importance of hole-conduction in the dye to optimize the energy conversion in such hybrid TiO2/dye/polymer systems (Yang et al. 2012). In the context of good hole-conductor for solid state DSSCs conducting polymer (CP) may offer −

−

significant advantages over the commonly used I /I3 redox couple as it can regenerate dyecations produced by electron-injection from photo-excited dye molecules to TiO2 and transport the positive charge to the cathode. However, the phenomenon of dye regeneration in a 3% efficient device using photo-electro-chemically deposited poly (3,4-eth-ylenedioxythiophene −

−

(PEDOT) was found orders of magnitude slower than either the I /I2 redox couple or the spiroOMeOTAD. This kind of slow dye regeneration was considered limiting JSC in these devices, which was thought to originate from the low dye to CP ratio (Mozer et al. 2010). −

3-

OPVSCs using carbazole dyes—TC301 and TC306 with Br /Br redox couple in CH3CN demonstrated efficiency of 3.68 and 5.22%, respectively due to efficient regeneration initiated by more positive HOMO levels (Teng et al. 2009). −

−

A solvent free IL-electrolyte comprising of SeCN /(SeCN)3 was developed for high efficiency DSSCs outperforming the iodide/triiodide redox couple at full sunlight by showing efficiencies in the range of 7.5–8.3% (Wang et al. 2004a, 2004b). Photo-chemically stable poly-ILs electrolytes comprising of poly (1-butyl-3-vinylimidazolium bromide) ([PBVIm][Br]) and poly [1-butyl-3-vinylimidazolium bis (trifluoromethanesulfonyl) imide] ([PBVIm][TFSI]) were synthesized for quasi-solid-state DSSCs exhibiting 4.4% efficiency under standard illumination (Zhao et al. 2011). Similarly, Bis-imidazolium based poly[1-butyl-3-(1vinylimidazolium-3-hexyl)-imidazolium

bis

(trifluoromethanesulfonyl)

imide]

(Poly

[BVIm][HIm][TFSI])

and

mono-imidazolium

based

poly

(1-butyl-3-vinylimidazolium

bis

(trifluoromethanesulfonyl)imide) (Poly[BVIm][TFSI]) electrolytes were prepared without any volatile solvent. Compared to mono-imidazolium electrolytes, bis-imidazolium electrolytes were thermally more stable and better conducting due to the charge transport networks formed in the gel electrolytes via the π-π stacked rings. The bis-imidazolium based gel electrolyte showed PCE of 5.92% under standard illumination (Chen et al. 2012). The influence of adding SiO2 NPs in propyl-methyl-imidazolium iodide (PMII) IL was studied with the conclusion that the presence of SiO2 NPs in PMII improved the charge transport of iodide/triiodide redox couple in the electrolyte and consequently enhanced the efficiency by 20%, relatively (Berginc et al. 2008). Exploring the influence of nanostructured materials in IL electrolytes further, NPs were used, for the first time, for solidifying ILs for quasi-solid-state electrolytes that yielded 7% efficiency at standard illumination when used with Ru-polypyridyl photosensitizer enabling the fabrication of flexible, compact, laminated all solid-state devices free of leakage (Wang et al. 2003b). A series of novel imidazolium iodides based ILs named as—NMIPHI, NAIPHI, and NBIPHI were synthesized with different functional groups for studying the dependence of diffusion coefficients −

−

of redox ions I and I3 on the molecular weight. Out of the three ILs examined, NMIPHI gave highest efficiency of 4.18% when it was used in a liquid electrolyte of a DSSC. When NMIPHI was mixed with PMII with a molar ratio of 1:1 in a solvent free electrolyte, the efficiency of the DSSCs was enhanced compared to that based on pristine PMII (Lim et al. 2012). A new type of IL called 5-mercapto-1-methyltetrazole 1-methyl-3-propylimidazolium salt—PMIT with di-5-(1-methyltetrazole)disulfide—T2 as the organic redox couple was developed very recently. Along with a catalytic carbon CE and two sensitizers—N719 and D102, it resulted in device efficiency of 4.30 and 3.83% under standard light illumination, respectively. The ink carbon CE exhibited better catalytic activity than standard Pt CE for the redox couple showing an attractive prospect of carbon materials for applications in DSSCs (Wu et al. 2013). Out of numerous IL electrolytes reported as alternative to the solvent based electrolytes, HMII, EMII, DMII, BMII and PMII are the most frequently used ones. PMII has the lowest viscosity and highest conductivity value at room temperature. Mixing non-active ILs like imidazoluim salts with various anions generally reduces the viscosities of ILs. In one case, 1-ethyl-3-methylimidazolium triflate (EMITf), 1-ethyl-3-methylimidazolium thiocyanate (EMISCN), -dicyanamide (EMIDCN), tricyanomethanide (EMITCM), -tetracyanoborate (EMIB(CN)4) and -bis (trifluoromethanesulfonyl) imide (EMITFSI) were added to PMII to decrease viscosity and to increase the diffusion of ionic liquids whereas in other csae, PMII performance was improved by adding system regulator chemicals

like

3-phenylpropionic

acid,

tert-butylpyridine,

Nalkylbenzimidazoles,

N-

methylbenzimidazoles and guanidinium thiocyanate where some of them improve VOC and some increase JSC (Cosar 2013). Incorporation of C nanomaterials like graphene, SWCNTs and a mixture of graphene and SWCNTs in 1-methyl-3-propyl-imidazolium iodide (PMII) were explored for preparing DSSC electrolytes. Graphene containing electrolytes displayed an increase in conversion efficiencies from 0.16% (for pure PMII) to 2.10% whereas in case of SWCNTs, the conversion efficiency increased from 0.16% to 1.43% and for the mixture of graphene and SWCNTs, it improved from 0.16% to 2.50%. These improved performances were ascribed to the fact that C materials served

simultaneously both as charge transporter in the ionic liquids and as catalyst for the −

electrochemical reduction of I3 ions besides IL-mediated self-organization of graphene and SWNTs into structured networks, which provided an efficient electron transfer. In addition, these electrolytes were stable up to 300 °C (Ahmad, Khan, and Gun’ko 2011). Two formulations of thermally long-term stable IL-based gel electrolytes were developed where they remained in gel form over a temperature range from room temperature to 80°C. In one case, higher PCE was noted due to enhanced conductivity by adding MPN whereas the other worked satisfactorily over a wide temperature range of 20-80°C showing excellent long-term durability (Chen et al. 2007). Alumina NPs were covalently surface-modified with an IL to improve their miscibility in1-methyl-3propylimidazolium iodide (MPII) for their use in preparing quasi-solid-state DSSCs. Detailed measurements revealed that the viscosity of the electrolyte continuously increased with the content of IL-Al2O3, and the fluidity almost disappeared completely when the MPII:IL-Al2O3weight ratio was 95: 5 or 90: 10. DSSCs fabricated with IL-Al2O3 were always more efficient than those with pristine Al2O3primarily due to the favorable interactions and good miscibility between MPII and IL-Al2O3, which in turn results in the formation of an interconnected channel pathway for ion transport. Using the double-layer structures with mesoporous TiO2beads, the efficiency increased 2

to 7.6% at 100 mW/cm , one of the highest values reported for quasi-solid-state DSSCs (Chi et al. 2013). Quasi-solid-state electrolytes synthesized with mesoporous silica called SBA-15 as framework material exhibited enhanced conductivity and the diffusion coefficients of poly iodide ions such as + + − I3 and I5 , which were about twice of those of I ．The optimized conversion efficiency of these devices with the quasi-solid-state electrolyte was 4.3% under AM 1.5 irradiation at 2 75 mW/cm light intensity (Yang et al. 2005)． Electrolytes prepared out of spherical and rod shaped SiO 2 NPs in polyethylene glycol methyl ether (PEGDME) exhibited higher PCE due to improved ionic diffusion. For instance changing the 2

shape of SiO2 NPs from spherical to rod like structure enhanced J SC to 12 mA/cm resulting in 38% increase in PCE. However, simultaneous increase in recombination due to rod like NPs was controlled by employing generation-5, polyester-32-hydroxyl-1-carboxyl-2, 2-bis (hydroxymethyl) propionic acid dendrons terminated in carboxylic acid moieties as co-adsorbents that resulted in 2

JSC = 14.1 mA/cm and PCE exceeding 4.5%. Long-term storage at room temperature demonstrated the superior stability of the oligomer electrolyte based devices in contrast to that of a volatile solvent electrolyte (Yoon et al. 2014). An organic ionic plastic crystal called 1-ethyl-1-methylpyrrolidinium bis (trifluoro methane sulfonyl) imide (P12TFSI), was synthesized and doped with RTIL 1-propyl-3-methylimidazolium iodide (PMII) for preparing solid-state electrolytes for fabricating PV devices that showed efficiency of 2

5.12% under simulated air mass 1.5 solar spectrum illuminations of 50 mW/cm (Li et al. 2012). A polymer gel electrolyte (PGE) comprising of polyacrylonitrile (PAN) with graphene incorporated in a mixture of iodide redox electrolyte was prepared, for the first time, for quasi-solid DSSC −

−

applications facilitating improved diffusion of I /I3 ion pairs. The optimized ratio of graphene in the PAN PGE maximizing the ionic conductivity and photo-electronic properties of the DSSC gave PCE of 5.41%, which was 1.5 times the value of the liquid-state cell (3.72%) (Chan, Wang, and

Chen 2013). Extending the search further, a combination of PEO: PEG (40:60 w/w) along with the iodides of Li, Na, K, ammonium and 1-ethyl-3-methylimidazolium were investigated for studying the influence of ion sizes on electrolyte conductivity where a strong dependence on cation radii was observed (Bhattacharya et al. 2009). Transparent Conducting Electrodes In a standard DSSC, the whole assembly is confined between two transparent conducting oxide (TCO) coated glass substrates, which not only act as current collecting electrodes but also provide mechanical support to the cell structure including the sealing used for protecting the cell materials from the ambient (Halme 2002). Fluorine-doped tin oxide (FTO) and indium tin oxide (ITO) are the most popular TCOs used in thin film PV cells. Keeping in view the necessary requirement of TCO to remain stable during heat treatment of TiO2 thin film in a standard DSSC fabrication, SnO2: F (FTO) has been the material to meet the goal. The sheet resistance of TCO on the glass plate is a compromise between two conflicting properties. For instance, thicker films of higher conductance have lower transmittance and vice versa. A variety of FTO glass plates are available commercially. While considering large-scale production of DSSCs, the use of flexible transparent plastic substrates offer advantages over rigid glass substrates. Flexible substrates are easy to use in continuous roll-to-roll process, making it possible to produce PV cells in large quantities and at high throughput and thereby at substantially lower costs. In addition to this, plastic substrate based cells would also be lighter, thinner and easier to handle and transport than cells sandwiched between glass substrates (Halme 2002). Since the polymers cannot stand high temperatures, SnO2: F film as such cannot be put there but room temperature sputtered ITO thin film can be used as TCO on the plastic substrates (Sommeling et al. 2000). For similar reason, the normal high temperature (450–500 °C) sintering of the nanostructured TiO2 films needs a low temperature sintering. The purpose of the high temperature treatment of TiO 2 films is to sinter NPs together along with the removal of surfactants/additives especially added to render the TiO2 colloidal solution viscous and easy to deposit, and in some cases to control the porosity of the sintered TiO2 film. For low temperature sintering of the TiO2 colloids at 100°C, plastic cells were fabricated without any organic surfactants (Pichot, Pitts, and Gregg 2000) in which the surfactant less TiO2 films adsorbed more dye due to larger inner surface area of the films resulting in better adhesion to the FTO coated glass surface than those prepared with surfactants, which may cause problems in long term stability. Omitting surfactants for low temperature sintering, however, resulted in reduced cell performance. For example, a cell with a 1 μm thick TiO2 film prepared without surfactants and sintered at 100°C exhibited PCE of 1.22% under 1 sun illumination (Pichot, Pitts, and Gregg 2000). In case of counter electrode, similarly, the thermal deposition of the counter-electrode at 385°C (Papageorgiou, Maier, and Graetzel 1997) must be replaced with room temperature electroplating or sputtering (Sommeling et al. 2000). An alternative to this was Pressing carbon or platinized SnO 2 powder onto the conducting plastic substrate was found as another alternative solution to the low temperature fabrication of CEs (Lindström, et al, 2001a, 2001b). ITO-coated poly ethylene terephalate (PET) (Lindström et al. 2001a, 2001b; Sommeling et al. 2000) substrates are commonly used in recently developed DSSCs. Although, there are several problems in using polymers as substrate in DSSCs as listed here. Easy permeation of

oxygen and water vapor in the polymers affect the DSSC performances adversely (Sommeling et al.2000). While ITO is the only suitable TCO-material for polymer foils, its exact behavior in the DSSC is not properly understood (Sommeling et al. 2000). The polymer itself may be unstable in presence of the electrolyte and release undesirable chemical compounds into the electrolyte (Sommeling et al. 2000). For instance, Pt deposition by electroplating may degrade cell performance (Olsen, Hagen, and Eric Lindquist 2000; Sommeling et al. 2000). It is further noted (Sommeling et al. 2000) that the performance of plastic DSSCs degrades with time, which is attributed to the degrading ITO-coating on PET rather than to the other factors listed above. A new manufacturing method, based on static or dynamic roller mill compression of TiO 2 powder suspension for working electrodes and platinized SnO 2 or carbon powder suspensions for CE onto ITO-coated PET-based substrates for nanostructured electrodes on plastic substrates, was 2

patented (Lindström et al. 2001a, 2001b). A 4.9% efficiency at low illumination level (10 mW/cm ) 2

with a cell area of 0.19 cm and about 3% efficiency at 1 sun illumination was reported with this preparation procedure (Lindström et al. 2001a, 2001b), and the current baseline efficiency is claimed to be 5% under 0.1 sun illumination. Special feature of this method is that no heat treatment is required for the compressed films. Furthermore, the continuous roller mill compression method is easily transferred to a large-scale roll-to-roll production line. Counter Electrode For higher efficiency DSSC devices, it is necessary to have as large V OC as it is practically possible by appropriately selecting the energy difference between the Fermi-levels of the illuminated transparent conductor to the nanocrystalline TiO 2 film and the Pt CE. Pt-catalyst on CE aids in regenerating the redox couple (McConnell 2002) efficiently by reducing the redox species acting as mediator to regenerate the sensitizer molecules after electron injection or collection of the holes via the HTL (Argazzi et al. 2004). A number of methods were developed for depositing of catalyst film on CE of a DSSC out of which chemical reduction of H2PtCl6 or Pt (NH3)4 Cl2 by NaBH4 based deposition of Pt-catalyst, was found simplest and of low cost (Yu et al. 2005). DSSC CEs prepared by electrochemical deposition of mesoporous Pt on FTO glass substrate using non-ionic surfactant octaethylene glycol monohexadecyl ether (C16EO8) exhibited PCE of 7.6%, compared to 6.4% observed in case of the devices with sputter-coated or thermally deposited metal films (Yoon et al. 2008). This kind of performance improvement was attributed to the higher values of J SC arising from the increase in the active surface area and light reflection as well as the decrease in the sheet resistance of the conventionally prepared films (Yoon et al. 2008). Similarly, low-cost electro-less 2

deposition of Pt resulted in a charge-transfer resistance of 1.24 Ohm-cm (Lin et al. 2010). A ‘twostep dip coating process’ developed with poly-Nvinyl-2-pyrrolidone (PVP)-capped Pt nanocrystals using a conditioner on TCO glass CE followed by annealing gave a charge transfer resistance of 2

0.56 Ohm-cm and this improvement was attributed to the removal of conditioner compound by the heat treatment at 270°C, which also improved the electrical contact between the Pt nanocrystals and the TCO glass, and thereby accelerated the charge transfer leading to a conversion efficiency of 8% (Lan et al. 2011). A modified Pt CE was fabricated by spin coating NiO film on top of a sputter coated/e-beam evaporated Pt thin film on a FTO glass substrate that resulted in conversion efficiency of 4.28% that exceeded those of conventional DSSCs with e-

beam evaporated Pt-CE at 3.16%, confirming the improved catalytic activity of NiO film coated on the Pt CE (Chou et al. 2010a, 2010b). Coming to a novel technique of preparing CEs, it is noteworthy that Pt NCs embedded in flexible carbon fiber paper replaced sputtered-Pt or PVP-capped Pt nanoparticles on TCO glass as CE in DSSCs (Lana et al. 2010). Not only did this CE show good conductivity without TCO glass but also possessed improved flexibility and adhesion imparting acceptable charge-transfer resistance 2

of 5.25 Ohm-cm in the devices showing 4.13% of cell efficiency under standard illumination (Lana et al. 2010). Similarly, a layered composite of graphene and Pt NPs was prepared by pulse laser ablation technique with different Pt loadings for CEs showing improved performance in contrast to conventional Pt thin film CE or unsupported Pt nanocrystals (Bajpai et al. 2011). A composition with 27% loading of Pt showed higher efficiency of 2.9% as compared to the cells using conventional Pt electrodes (Bajpai et al. 2011). Better catalytic efficacies of these −

composites were also reflected in the stronger reduction of I3 peaks in cyclic voltammetry scans. Suitable modifications were incorporated in room temperature magnetron sputtering for depositing Pt NPs on FTO films in form of nano islands where the size of the islands was possible to control by sputtering duration (Mukherjee et al. 2012). An optimum cell efficiency of 4.9% was measured in case of a sputtering duration of 45 s resulting in 2–3 nm in diameter islands with a 12

2

surface density of ~ 5.9 × 10 per/cm and consuming ~ 400 times less Pt material than that in a 50 nm Pt thin film, where similar photo-conversion efficiencies were observed (Mukherjee et al. 2012). In an attempt to replace Pt catalyst on CEs for reducing device cost, few configurations of carbonbased CEs using graphene, SWCNT and graphene-SWCNT composites were attempted (Kim et al. 2012) by electrophoretic deposition. DSSC cells with graphene-deposited CEs demonstrated the best PCE of 5.87% under standard illumination (Kim et al. 2012). Functionalized SWCNTs were deposited along with/without a layer of Ag on FTO-glass substrates for CE of a DSSC device where a thin film of SWCNT/Ag composite markedly increased the IPCE from 3.9% to 15.3% at 380 nm but at 550 nm it remained unchanged resulting in a device efficiency of 1.3% in case of a composition having SWCNT/Ag/acetyl acetone which was, in no way, inferior to that of DSSCs with a thin film Pt CEs having 1.25% device efficiency (Chou et al. 2010a). Nitrogen doped diamond like carbon (DLC) thin films were explored for CEs in place of Pt that was commonly used as CE/catalyst in DSSC devices. These films were grown on ITO glass substrates using cathode arc physical vapor deposition including four combinations namely— ITO/Pt/DLC (n-type), ITO/Cr/DLC (n-type), ITO/Cr/DLC (n-type)/Pt, and Glass/Cr/DLC (n-type)/Pt apart from standard ITO/Pt. PV performance of these devices with these CEs were evaluated in which an efficiency of 3.3% was found in case of ITO/Cr/DLC (n-type)/Pt layers. Even with simple glass substrate, the efficiency went up to 3.2%, making it clear that ITO glass substrate could be replaced with simple glass plate in this design still retaining the conversion efficiency high enough (Wang et al. 2011a). Vertically aligned CuInS2 thin films were prepared using one-step solvo-thermal process for CEs of DSSC devices resulting in a PCE of 6.33%, in contrast to that of sputter coated Pt-CE at 6.07% (Yang et al. 2013).

In a recent report, the detailed analysis of the results gathered from the development of non-Pt catalyst based CEs reveals two distinctly possible routes. In one case, materials such as activated carbon, carbon nanotubes, graphite and carbon black have been used with high efficiency of 9% with carbon black as counter electrode (Cha et al. 2010; Imoto et al. 2003; Kay and Grätzel 1996; Lee et al. 2010; Murakami et al. 2006; Papageorgiou et al. 1999; Papageorgiou, Maier, and Graetzel 1997; Ramasamy and Lee 2010; Suzuki et al. 2003; ). Although, CE was much thicker in these devices as compared to that of the Pt-CE. In the second approach (Lee et al. 2010b), conducting polymers (CP) were explored to enhance conversion efficiency up to 7.8% (Lee et al. 2009a, 2009b) including others of 7.1% with micro porous polyaniline (Li et al. 2008) and 7.93% with poly (3,4-alkylenedioxythiophene) PEDOT nanoporous layers prepared from electro-oxidative polymerization (Ahmad et al. 2010). During examining PEDOT for replacing Pt due to its high conductivity, electrochemical stability, transparency and catalytic ability (Ahmad, Deepa, and Singh 2007; Biancardo, West, and Krebs 2007; Ha et al. 2004) possibility of removing the use of TCO was also explored, in case possible, as it accounted for 60% of the total cost of the CEs. Thus, it was considered necessary to go for the development of cost-effective CEs by eliminating the use of both Pt and TCO as well. Working on these lines, Pt and TCO-free counter electrodes were developed using a highly conductive −

polymer with high catalytic ability to simultaneously act as a catalyst for I 3 reduction and charge transport. In this context, by tuning the electrical conductivity of the conducting polymer, Pt and TCO free DSSCs exhibited comparable cell efficiencies with those using a Pt/FTO counter electrodes. High conductivity PEDOT thin films were synthesized by a modified simple pre-solution/in situ polymerization

method

in

which

the

monomer

was

prepared

by

dissolving

3,4-

ethylenedioxythiophene (EDOT), poly (vinylpyrrolidone) as a matrix polymer and pyridine as a retardant in 1-butanol or ethanol along with an oxidant solution prepared separately by dissolving ferric p-toluene sulfonate (FTS) in 1-butanol or ethanol. The monomer and oxidant solutions were mixed together, followed by polymerization at a temperature ranging from 5–30 °C for 3– 72 hours. The pre-polymerized PEDOT solution was then coated onto a substrate to form a prepolymerized PEDOT film by post-polymerization at 70°C for 20 minutes, followed by washing in methanol and drying, resulting in very high electrical conductivity and smooth surface. The electrical conductivities of PEDOT films used in Pt and TCO-free DSSCs were controlled by adjusting the molar ratio of the monomer to the oxidant or to the retardant. These results clearly showed that carbon based materials not only offered ease in creating good physical contact with TiO2 film but also functioned as efficient charge carrier collectors at the porous interface (Lee et al. 2010). Using steel and Ni-substrates coated with carbon or fluorine-doped SnO2 checked the corrosive −

−

behavior of I /I3 redox couples. Metal layers gave higher FF for larger area devices due to their low sheet resistance. Efficiencies around 5.2% were reported in case of Pt-covered stainless steel and Ni-counter-electrode (Murakami and Grätzel 2008). In early days of developing DSSCs, a mixture of graphite and carbon black was also used as CE wherein a conversion efficiency of 6.7% was reported (Kay and Grätzel 1996). Using graphite increased the lateral conductivity of − −

CEs where carbon acted like a catalyst for the redox (I 3 /I ) couple reactions. Recently, carbon nanotubes were introduced as a new material for CEs to improve the performance of DSSCs

(Gagliardi et al. 2009) opening yet another new possibility of using combinations of NPs and carbon nanotubes for CE fabrication giving improved efficiency. For instance, using MW-CNT and − −

graphene nanocomposites in place of Pt to reduce the (I3 /I ) redox couple was compared with that using dry spun carbon-MW-CNT and graphene flakes independently. This combination, when deposited onto FTO glass substrate, showed efficiency of 7.55% compared to 6.62 and 4.65% in case of the MW-CNTs and graphene alone, respectively, in triiodide reduction reaction in DSSC (Velten et al.2012). Pt-free counter electrodes were, thus, fabricated using freestanding flexible SWCNT films called ‘bucky papers’ irradiated with microwave plasma. These plasma-treated ‘bucky papers’ developed into a vertically oriented, micron sized, pillar-like structures, while its base was still a dense random mesh of SWCNTs and this flexible film possessed larger accessible surface area and better catalytic properties. The plasma treatment improved the efficiency of these paper based CEs from 2.44% to 4.02%, which was quite comparable to that of using Pt thin film (4.08%) (Roy et al. 2012). Recent developments in large-area synthesis and transfer of high-quality graphene films onto various substrates appeared promising for their uses in flexible DSSCs (Kim et al. 2009). A combination of graphene and PEDOT was thus successfully employed in CEs of flexible DSSCs without additional TCO wherein graphene decreased the surface resistance along with the improvement in its mechanical flexibility. While placing graphene between PEDOT and substrate, it was noted that highly conducting PEDOT not only behaved equivalent to the combination of Pt and TCO put together rather aided electron transport by decreasing the surface resistance as well and thus leading to device efficiency of 6.26%, while devices with Pt/ITO and PEDOT counter electrodes gave 6.68% and 5.62%, respectively. Combination of graphene and PEDOT films, thus undoubtedly, proved to be the right alternative of Pt/ITO for solid state DSSCs (Lee et al. 2013). Photoanode Electron transfer from the dye to the semiconductor followed by the transport across the redox couple to regenerate the dye molecules decides the overall performance of a DSSC (Bisquert, Emilio, and Quinones 2006). For meeting these conditions effectively, generally mesoporous wide band gap semiconductors like TiO2, ZnO, SnO2, or Nb2O5 are considered out of which TiO2 is the preferred choice due to its stability, natural abundance, nontoxicity, low cost, and better device efficiency. Moreover, TiO2 films produce the largest values of JSC, VOC and the IPCE (Bandaranayake 2004). Further, the desired features of the deposited film including adequate transparency and larger surface area to maximize light harvesting; high charge carrier mobility and longer recombination lifetime for efficient photocurrent collection (DiCarmine and Semenikhin 2008); matching energy level with the excited dye molecules for facilitating electron injection (Jose, Thavasi, and Ramakrishna 2009); and ease in fabrication are the factors that help in this context. Structurally, the photoanode films perform well in case right kind of internal structures of NPs, NWs, NRs, and NTs as well as the particle size and morphology, crystallinity, phase content, chemical composition/doping, and surface groups, are selected for which extensive investigations have already been carried out to provide the required information (Benehkohal 2013). For instance, out of several crystalline phases possessing higher reflective index and transparency over a wide spectral range, only rutile and anatase phases are useful for DSSC applications (Mechiakh et al. 2010). It has also been experimentally observed that the final

crystal structure is decided by the deposition method based on sol-gel, chemical vapor deposition, physical vapor deposition, chemical bath deposition, reactive sputtering or atomic layer deposition (Quiñones, Ayala, and Vallejo 2010). Moreover, nanocrystalline TiO2 films provide sufficient anchorage to the dye molecules for which parameters like porosity, pore size distribution, light scattering, electron percolation and conduction band edge of the semiconductor, are all important to chose according to the dye and the electrolyte by adapting right kind of precursor chemistry, hydrothermal growth temperature, binder addition, doping, and sintering conditions (Kalyanasundaram and Grätzel 1998). For example, mesoporous anatase TiO2 beads were prepared (Chen et al. 2009) having larger effective surface area and controllable pore sizes that enhanced the electron diffusion lengths and extended electron lifetimes arising from the favorably interconnected and densely packed TiO 2 NCs inside the spheres. Further investigations revealed that N2 doping of TiO2 enhances the incident photon-to-current conversion efficiency within 380–520 nm and 550–750 nm (Ma et al. 2005) leading to improved JSC wherein the doping forms O-Ti-N bonds (Tian et al.2010a) that shifts the position of the flat-band edge causing improvement in VOC. Thus, expecting the nitrogen doping in TiO2to be associated with the improvement in both JSC and VOC, synthesis of a crystalline N2-doped TiO2 spheres using hydrothermal route was attempted in which the cell parameters of DSSCs improved accordingly exhibiting an efficiency of 6.01% (Xiang et al. 2011). There are various methods of preparing TiO2 photoanodes but the most common one involves screen-printing from a paste of NPs dispersed in a viscous medium followed by sintering at elevated temperature (~450C), which is rather a time consuming process (Benehkohal 2013). Moreover, high temperature thermal treatment for removing organic binders is not suitable for flexible substrates in which the maximum temperature should not exceed 150°C. However, such low temperature processed films do possess weak interconnection among the TiO 2 particles and poor substrate adhesion leading to increased resistance to electron transport and increased electrical resistance at substrate/thin film interface, which adversely affect the overall cell performance (Benehkohal 2013). Therefore, many efforts have been made to develop some lowtemperature methods for nanocrystalline TiO2 films, which include screen-printing followed by physical or chemical treatments such as compression, UV irradiation, hydrothermal crystallization or lift-off/transfer. Electrophoretic deposition (EPD) is another alternate procedure quite appropriate for low-temperature processing of flexible photoanodes offering several advantages like faster depositions; simpler equipment needed; besides easy adaptability to mass production. EPD employs deposition of charged particles that are suspended in a solution onto a substrate under the influence of an externally applied electric field. The deposited film is subsequently subjected to low/high temperature treatment depending on the substrate to improve its physical properties (Benehkohal 2013). It is possible to avoid unacceptable requirement of high temperature anneal in case of flexible substrates like plastics by using commercially available TiO 2-P25 powder for preparing uniform thick film on ITO-PET (plastic) or F: SnO2 (FTO) conductive glass substrates resulting in efficiency of 5.3% for the cell on TCO glass and 3.2% for ITO-PET plastic substrate. However, all solid-state DSSCs employing the same solid gel electrolyte and nanostructured TiO 2 film prepared in high temperature sol-gel process did exhibit much higher efficiency of 6% (Stathatos and Dionysiou 2010).

Micro/nano-composite of TiO2 NCs films, synthesized by the electro-hydrodynamic method, were found better for IL-filling and quasi-solid-state electrolytes resulting in PCE of 6.4% for ionic liquid electrolyte and 5.3% for quasi-solid-state electrolyte (Zhao et al. 2006). The suitability of TiO2 nanotubes in place of NCs for photo anode was examined by considering parameters like NT-geometry including length, diameter and morphology; crystal structure including amorphous, anatase and rutile phases as well as the factors affecting dye loading or electron mobility in detail. The highest PCE of the devices using such NTs reached 4% while for some mixed systems it was around 7% (Roy et al. 2010). A double layer comprising of ZnO nanowire arrays and poly-disperse ZnO NCs aggregates enhanced the cell efficiency because of the faster electron transport, relatively high surface area and enhanced light scattering resulting in PCE of 3.02%, which compared well with ZnO nanowire and nanocrystal devices having efficiencies of 1.04% and 2.56%, respectively (Zhang and Guozhong 2011). Ionic conductivity and
 long-term stability of TiO2 photoanode was improved using KI crystal growth inhibitor along with diphenylamine (DPA) charge transport enhancer added to poly 2

(ethylene oxide). These cells exhibited very large J SC = 14
 mA/cm with an efficiency of 5.8% −

−

possibly due to enhanced interaction of TiO2
 with (I /I3 ) electrolyte leading to higher ionic conductivity as well as electrochemical and mechanical stability. Further measurements confirmed that electron transport and lifetime were improved in DPA added electrolyte due to reduced iodine sublimation resulting in increased stability by retaining 89% of the overall efficiency even after 40 days of continuous operation (Agarwala et al. 2011). In a novel configuration, Ti-mesh photo-anodes having high transparency and improved bendability were used along with CNT-layer to fabricate flexible DSSCs resulting in lower sheet resistance, elevated JSC and enhanced sunlight harvesting exhibiting an efficiency of 5.3% as compared to 3.15% in case of all Ti-mesh photoanode under standard illumination (He et al.2012a). In the context of using other metal oxides, sol-gel processed ZnO photoanodes along with conventional Ru-dye and spiro-OMeTAD HTL were investigated with different precursors for controlling the film morphology by using polymeric additives and sintering (Boucharef et al. 2010). For example, adding ethyl-cellulose improved the dye loading of ZnO electrodes, while sintering at 350 °C removed the organic components. Using 800 nm thick porous ZnO electrodes with 2

N719 dye, the devices exhibited JSC = 2.43 mA/cm with cell efficiency of 0.50%. In another approach, solid-state DSSCs were fabricated using 11–12 µm long, vertically oriented ZnO nanowires in the anode and CuSCN as the solid HTL that demonstrated an efficiency of 1.7% (Desai et al. 2012). The influence of ZnO NW lengths on the performance of DSSCs was further studied using three different types prepared by multi-batch; continuous flow injection and NH3assisted CFI process out of which only CFI produced NTs were found suitable due to their favorable optical and structural properties and a 25 μm ZnO NWs array was synthesized by NH 3assisted CFI process for photoanodes of DSSCs that resulted in an efficiency of 3.92% (Chen and Yin2012). In another work, improvement in light harvesting properties of the ZnO films having large density of hollow cavities were prepared by calcining polystyrene spheres mixed during electro-hydrodynamic synthesis. These films did show strong light scattering and the efficiency was enhanced up to 5.5% under standard illumination (Sheng et al. 2009). DSSCs having composite photoanodes comprising of ZnO NW arrays coated on TiO 2 NCs demonstrated

efficiency of 4.52% exceeding the performance of both 0.90% of the ZnO NWs and 3.56% of the TiO2 NCs based cells wherein the improved efficiency was possibly due to large surface area of NCs, as well as fast electron transport and light scattering effects associated with NWs (Wang et al. 2011a). Two types of coaxial nanostructures namely—ZnS/ZnO coaxial NWs and hierarchical NWs consisting of ZnS NPs on ZnO NWs were synthesized for DSSC applications wherein improved efficiency was observed compared to those devices having bare ZnO NWs. In this connection, it was anticipated that ZnS layer in the coaxial structure retarded the back transfer of electrons to the dye and the electrolyte, while the coarse surface of ZnS NCs in the hierarchical NWs significantly enhanced the adsorption of dye molecules (Yu et al. 2010). While exploring the utility of ZnO nano tetrapods in photo anodes, varying the dye loading and film thickness realized 3.27% efficient cells but JSC continued increasing with the film thickness even beyond 35 μm. Further, mixing of SnO2 nanoparticles with the ZnO nano tetrapods increased the surface roughness leading to better charge collection and tunable light scattering that improved the efficiency to 6.31%. It was, however, confirmed that an ultrathin ZnO layer was spontaneously formed on the SnO2 NCs during preparation, which enhanced VOC besides shell type SnO2/ZnO NCs based composite films affected the recombination of charge carriers. Finally, a photoanode was successfully prepared using SnO 2 NCs/ZnO nano tetrapods in gel form for flexible DSSC demonstrating an efficiency of 4.91% on ITO-coated polyethylene-naphthalate substrates. The formation of a thin ZnO shell on SnO2 NCs, after ammonia activation, was also found useful in enhancing VOC and establishing better inter-particles contacts (Chen and Yang 2011). Sol-gel processed and sintered mesoporous TiO2 film were found very useful photo anode material for DSSC applications. Most of the high efficiency DSSC devices realized have used TiO2 films. An optimized photoanode film usually consists of two functional layers: the one at the bottom is a 10–12 μm thick transparent layer made of 20–30 nm anatase-TiO2 NPs, providing high surface area for dye adsorption and the top layer is a 4 μm thick film made of much larger anatase/rutile TiO2 particles of around 400 nm in diameter to scatter light back into the bottom layer and enhance light harvesting. The scattering effect is also possible to realize by adding an over layer comprising of NTs, beads, hollow spheres, or composite materials like TiO 2-Al2O3, TiO2-Nb2O5, and TiO2-ZrO2 (Benehkohal 2013). Alternatively, a variety of composite photoanodes were also realized by incorporating sub-micron size TiO2 particles or voids into the nano-sized anatase particles, or a blend of TiO2 NTs and NPs, NPs/NRs, and NTs/nanocube arrays offering better PV properties (Benehkohal 2013). Despite excellent performance of TiO2 NCs films in DSSCs, this material does have certain limitations that curtail further improvement in devices meant for large-scale applications. For instance, relatively smaller diffusion coefficients of electrons in TiO 2 NCs film pose problem as they slows down the charge carrier transport limiting the choice of redox couple in the electrolyte, which in turn limits the VOC and constrains the choice of the sensitizer. Further, the charge carrier transport in NC film often involves trapping and thermal de-trapping of electrons from distributed sub-band edge states, which is dependent on semiconductor Fermi level. The electron diffusion coefficient of 10

–4

2

cm /s in TiO2 NCs film compared to 10

–1

2

cm /s in the TiO2 single crystal

indicates towards possible improvement in the electron transport in TiO 2 film by changing the morphology of the photoanode material (Kopidakis et al. 2003; Martinson et al. 2008; O’Regan et al. 2006). Other drawback of TiO2 NC film is relatively lower porosity and complex paste preparation procedures whereas, ideally a good photoanode should have fast electron transport, high transparency, tunable surface area and adequate porosity besides, after all, cost-effective and scalable fabrication process (Ito et al. 2008; Papageorgiou, Maier, and Graetzel 1997). In this context, other photoanode materials, having one-dimensional nanostructures, which can effectively facilitate electron collection, have been studied as well. For example, in one case, aligned ZnO nanorods (NRs) array was employed in preparing DSSC photo anodes (Law et al. 2005) by coating a ZnO NC layer on a conducting glass substrate followed by oriented crystalline growth of ZnO resulting in conversion efficiency around 1.5% due to the small roughness factor. To overcome this limitation on ZnO NRs array, an array of TiO2 NTs with tunable roughness factor over 1000 was prepared by electrochemical anodization of Ti film. Thus, the DSSCs with these photo anodes gave conversion efficiencies approaching 7% (Zhu et al. 2007). Another alternate method of atomic layer deposition (ALD) was also explored using anodic alumina oxide (AAO) or silica as template to achieve a roughness factor >1500, exceeding NP films. This scalable technique allowed the fabrication of a diversity of metal oxides with 1-D nanostructures and the devices made of ALD coated ZnO NT photo anodes displayed excellent light harvesting and an energy conversion efficiency over 5% under standard illumination (Hamann et al. 2008). Electro-spinning is another simple and cost-effective technique to synthesize 1-D nanostructured semiconductor materials. Typically, the preparation of electro-spun nanofibers involves a gel containing inorganic precursors and an organic matrix polymer mixed in a solvent, which is electro-spun onto a grounded metal collector to form composite nanofiber mats. These mats were then calcined to decompose the organic components, resulting in pure inorganic semiconductor nanofibers. Using this method, semiconductor nanofibers with varied diameters, components and morphologies including random, oriented, core-shell, mesoporous and hollow structures were prepared on industrial scale (Greiner and Wendorff 2007; Li et al. 2006b). For instance, several groups prepared TiO2 nanofibers for DSSCs applications with improved charge collection as compared to TiO2 NCs film and the fabricated devices demonstrating energy conversion efficiencies over 6% in liquid and quasi solid-state device configurations (Song et al. 2005). Other materials such as SnO2, Nb2O5 and SrTiO3 were employed to make photo anodes but the conversion efficiencies of the devices using such materials were found still lower as compared to that based on TiO2 (Lenzmann et al. 2001; Onwona-Agyeman et al. 2005; Sayama, Sugihara, and Arakawa 1998). In a collaborative program of developing large area DSSCs by involving several research teams, 2

the maximum values of PCE observed were around 11% for cell areas 6% during sustained heating @ 80°C for an extended duration of 1,000 hours indicating them to be adequately fit for outdoor applications (Wang et al. 2003c). A batch of 7.4% efficient devices employing a new binary IL-electrolyte consisting of 1-propyl-3methylimidazolium iodide and 1-ethyl-3- methyl imidazolium tricyanomethanide in conjunction with the Ru-complex coded as Z-907Na exhibited a relatively long lifetime of its oxidized state. Devices employing N719 dye coated on thick TiO 2 films showed an efficiency of 9.18% under standard illumination (Nazeeruddin et al. 2003). The success of fabricating DSSCs possessing elevated temperature stability confirmed the utility of polypyridyl Ru-sensitizers by introducing the ligands like [Ru(dcbpy)(L)(NCS) 2, where L is N,Ndi(2-pyridyl)-dodecylamine, or N,N-di(2- pyridyl)-tetradecylamine] wherein the oxidized state was

found more stable than other polypyridyl Ru-sensitizers with thiocyanate ligand. These amphiphilic Ru-complexes produced device efficiencies of 8.2% at standard and ~ 8.7% at lower light intensity irradiations, respectively (Wang et al. 2004b). A heteroleptic polypyridyl Ru-complex Z910 possessing high-molar extinction coefficient was found highly efficient and stable sensitizer for solar cells exhibiting an efficiency of 10.2%. A detailed study of Ru(II)-polypyridyl sensitizers derived from N3 and N621 dyes, was conducted in which N3-derived (Bu4 N)2 [Ru(dcbpyH)2(NCS)2]—N719 sensitizer adsorbed on TiO2 films exhibited a remarkable efficiency of 11.18% at standard illumination (Wang et al. 2004a, 2004b). While studying the influence of hydrophobic hydrocarbon chains in amphiphilic Ru-dyes on the device performance it was observed that relatively longer hydrocarbon chains might lead to higher PCEs besides the same hydrophobic chain attached to the dye could suppress the recombination (Schmidt-Mende et al. 2005) . An ion-coordinating sensitizer—K51, containing triethylene oxide methyl ether (TEOME) at the 4,4′-position of a 2,2-bipyridine ligand was found better than non ion coordinating analogue— Z907 as this “super-molecular complex” performed better when used with a nonvolatile electrolyte or hole-transporting layer, exhibiting conversion efficiency of 7.8% or 3.8%, respectively. Additionally, K51 dye being of “ion-trapping” nature, inhibited the ions from reaching the TiO2 surface, thus, in a liquid electrolyte, there was very little reduction in VOC and for the solid+

state versions there was a significant increase in VOC with addition of Li ions resulting in enhanced light-harvesting and PCEs (Kuang et al. 2006). K60 dye was developed further by extending the π -system of the peripheral ligand giving an efficiency of 8.4% under standard illumination and over 9% under low-light irradiance of 30 mWcm

−2

when used with nonvolatile organic-solvent electrolyte. These devices exhibited

excellent stability when subjected to continuous thermal stress at 80 °C or light soaking at 60 °C for 1000 hours. Another new Ru-polypyridyl sensitizer K68 was synthesized that led to power conversion efficiency of 6.6% (Kuang et al. 2007a, 2007b; Kuang et al. 2008). Transition-metal-based sensitizers possessing higher molar extinction coefficients and better stability under thermal stress and light soaking were explored leading to the development of several dyes out of which K19 and K73 offered attractive features in terms of their improved PV performance and stability. For instance, K73 based devices with a nonvolatile electrolyte gave an unprecedented overall conversion efficiency of 9% under standard illumination and devices with K19 in combination with a binary IL electrolyte gave over 7.0% efficiency and maintained excellent stability under light soaking at 60°C for 1000 hours (Kuang et al. 2006; Wang et al. 2005). Further efforts were made to improve the performance of N3 dye by developing newer compounds such as—D6 and D5 containing oligophenylenevinylene backbones, each with one N, N-dibutylamino moiety wherein the devices showed improvement of 12% and 17% in the conversion efficiency, respectively, compared to the efficiency obtainable with N3 dye particularly due to higher molar absorption coefficients (Jang et al. 2006). Addressing to the optimization of interfacial charge transfer between the dye and the electrolyte in an investigation resulted in an IL-crystal electrolyte and amphiphilic Ru- dyes based solar cells giving an efficiency of 3.51% (Kawano et al. 2007).

A newer family of dyes denoted as K77 was prepared that gave more than 10.5% efficient devices with a volatile electrolyte Z675 (Kuang et al. 2007a, 2007b). Two newer Ru-complexes, SJW-E1 and CYC-B3 were synthesized by incorporating α-octyl-ethylene-dioxythiophene (OEDOT) and octyl-thiophene-substituted bipyridine ligands to increase the conjugation length for enhancing their molar extinction coefficient. The CYC-B3 and SJW-E1-sensitized solar cells exhibited conversion efficiencies of 7.39 and 9.02%, respectively (Chen et al. 2007). The difference in the performance based on the ancillary ligands opened the way to explore performance improvements by a two-dimensional conjugation enhancement and functional group substitutions in place of one-dimensional ones. A series of heteroleptic Ru-complexes such as [Ru(4,4′-carboxylic acid-2,2′ -bipyridine)(L)(NCS)2] (L = 5,5′-bis(4-octyl-

thiophen-2-yl)-2,2′-bipyridine-(9),5,5′-bis(N,N-diphenyl-4-aminophenyl)-2,2′-

bipyridine-(10),5,5′-bis(5-(N,N-di-phenyl-4-aminophenyl)-thiophen-2-yl)-2,2′-bipyridine-(11) 5,5′-bis(4-octyl-5-(N,N-diphenyl

−4-aminophenyl)-thiophen-2-yl)-2,2′-bipyridine-(12)

and were

synthesized where 9, 10, 11, and 12-sensitized solar cells exhibited power conversion efficiencies of 3.00%, 2.51%, 2.00% and 2.03%, respectively (Dai et al. 2011). In spite of the fact that the contribution of the sensitizer is confined to only a monolayer, still, for efficient cells it will always require a reasonably stable and long-living ruthenium-based dye. In other words, it is rather necessary to look for ruthenium-free sensitizers for reducing the over all device costs (Vougioukalakis et al. 2010). Besides trying a number of alternatives, a series of modifications were also introduced into the formulations of the early Ru(II) complexes as well that led to sensitizer formulations with amphiphilic properties and/or extended conjugation. These dyes demonstrated the properties including higher ground state of the binding moiety to improve the electrostatic binding onto the TiO2 surface at lower pH values; reduced charges on the dye to attenuate the electrostatic repulsion among the adsorbed dye molecules resulting in increased dye loading; better stability of the solar cells towards water-induced dye desorption, and shifting the oxidation potentials of these complexes compared to that of the N3 increasing the reversibility of the Ru(III/II) couple, leading to enhanced stability. There came a report of a Ru-dye incorporating an electron-rich hexylthio-chain along with, which was synthesized and used in DSSCs with volatile electrolyte and solid-state hole conducting layer demonstrating an impressive efficiency of 11.5% and 4.7%, respectively (Chen et al. 2009). A

polypyridyl

Ru-complex

‘cis-Ru(4,4′-bis(3,5-bis(5-hexylthiophen-2-yl)phenyl)-2,2′-

bipyridine)(4,4′-dicarboxyl-2,2′-bipyridine) (NCS)2, MC102′, with a high molar extinction coefficient 2

was synthesized and when used in DSSCs of 0.23 cm active area photo-electrode in combination with an electrolyte containing 0.6 M dimethylpropyl-imidazolium iodide (DMPII), 0.05 M I2, and 0.1 M LiI in acetonitrile, it resulted in an efficiency of 4.42% under standard illumination (Suresh et al. 2013). Solar cells with unprecedented stability under thermal stress and light soaking were fabricated using Ru-sensitizer namely—cis-RuLL′(SCN)2 (L = 4,4′-dicarboxylic acid-2,2′-bipyridine, L′ = 4,4′dinonyl-2,2′-bipyridine) in conjunction with a quasi-solid-state polymer gel electrolyte, reaching an efficiency of >6% in full sunlight (Wang et al. 2003b). These devices sustained heating for 1,000 hours @ 80 °C retaining 94% of its initial performance besides being stable under light soaking at 55 °C for the same duration under solar simulator. Efficient operation of these DSSCs relied upon the minimization of interfacial recombination losses (Clifford et al. 2004; Haque et

al. 2005; Karthikeyan, Wietasch, and Thelakkat 2007) for which the coating of inorganic barrier layers (Clifford et al. 2002; Palomares et al. 2002, 2003) saccharides (Handa et al. 2007a, 2007b) and

metal-assembling

dendrimers

(Nakashima

et

al. 2008;

Satoh,

Nakashima,

and

Yamamoto 2005) and the introduction of long alkyl chains to the dyes (Chen et al. 2006; Gao et al. 2008; Kim et al. 2008; Koumura et al. 2006; Moon 2011) were explored. While modifying the molecular structure to have better sensitization characteristics, incorporating the electron-withdrawing characteristics of bithiazole with thiophene, furan, benzene, or cyanomoiety as π -spacer, a series of dyes were synthesized containing thiophene-moiety between triphenylamine and bithiazole, resulting in enhanced responses in the red region of the solar spectrum and put to use in DSSCs. It was noted that HOMO and LUMO levels were tuned easily by introducing different π-spacers between the bithiazole moiety and cyanoacrylic acid acceptor. The overall efficiencies of the devices using these bithiazole dyes were in the range of 3.58– 7.51 %. Most significantly, the long-term stability of these devices with ionic-liquid electrolytes was tested for more than 1000 hours of light soaking and BT-II with a furan moiety exhibited better PV performance of up to 5.75 % efficiency (He et al. 2012a). On similar lines, employing tetra-hydro-quinoline derivative and 1-butyl-5-carboxy-3, 3-dimethylindol-1-ium as electron donor and acceptor moieties, respectively, the solar cell performance was found markedly dependent on the n-hexyl chains and the methoxyl unit. It was noted that including n-hexyl chains retarded charge recombination causing increase in electron lifetime and improved VOC whereas adding methoxyl moiety led to a higher J SC. Putting these dyes into solar cells resulted into an overall efficiency of 5.6% under standard solar irradiation. In addition, a considerably improved efficiency of 8.2% was reported in case one of the dyes, which was used in co-sensitizing mode showing a panchromatic response with a high IPCE exceeding 85% in the range of 400–700 nm (Cheng et al. 2013; Moon 2011). The introduction of phenanthrenequinone based π- conjugated bridge in a D-π-A structured dye was made, for the first time, for synthesizing three dyes namely—JH201, JH202 and JH203; out of which JH202 showed higher extinction coefficient and wider spectral response than others. When used in preparing DSSCs, 6.0% efficiency was demonstrated by JH203 based devices (Zhao et al. 2013). Another configuration of D-D-π-A type molecular structure was synthesized involving (E)-2Cyano-3- (5”- (4- ((4-(3,6-di-tert-butylcarbazol-9-yl) phenyl) dodecylamino) phenyl)—[2,2ʹ: 5ʹ, 2”terthiophene]–5—yl) acrylic acid where the PV devices demonstrated efficiency of 5.12%. The presence of double donor moieties (D-D-) not only improved electron-donating capability but also inhibited dye aggregation and prevented recombination of injected electrons with iodide/triiodide couple in the electrolyte (Namuangruk et al. 2012). Based on a special design meant for increasing the electron lifetime, alkyl functionalized organic dyes namely—MK-1 and MK-2 were developed and used in DSSC devices resulting in an efficiency of 7.7% under standard illumination (Koumura et al.2006). Very recently, a computer-aided design exercises were carried out on TA-St-CA dye sensitizer involving a number of electron-donating (ED) and electron-withdrawing (EW) units substituted into a π-conjugated oligo-phenylenevinylene bridge. This study successfully clarified that chemical modifications using ED substitutions were better than EW in reducing the HOMO-LUMO energy gap of the new dyes (Mohammondi, Mohan, and Wang 2013).

With reference to taking care of the dye loading in the mesoporous TiO 2 film, isophorone sensitizers—S4 and D-3 were synthesized for their uses in solid-state DSSC using spiroOMeTAD as hole-transporting layer wherein the dye-loading capability of D-3 was almost 1.5 times as that of S4, which led to higher light harvesting efficiency. The larger dipole moment of D3 could generate more negative charges close to the TiO2 surface than those in case of S4 resulting in a larger conduction band shift for D-3 based devices which increased VOC. Devices using D-3 dye showed an efficiency of 1.92% compared to 2.55% demonstrated by N3 based devices under the same standard illumination condition (Liu et al. 2012). An altogether new configuration of D-A-π-A organic dye named WS-9 was derived from another dye WS-2 by adding an n-hexylthiophene unit into the π-conjugation during its synthesis. Due to the presence of a strong electron-withdrawing benzothiadiazole unit in the π-bridge, the specific D-A-π-A organic dyes showed superior response compared to D-π-A configurations. When used in device fabrication, WS-2 aggregated faster as compared to strong anti-aggregation ability of WS-9. The optimized device efficiency of WS-9 reached 9.04% (Wu et al. 2012b). ZnO aggregates were optimized for DSSC applications by employing two dyes namely—indoline D149 and Ru-complex N719 and the fabricated solar cells showed maximum efficiency of 5.2% 2

and 4.5%, respectively. The monolithic version of devices with an active area of 2.4 cm were explored for the first time, exhibiting efficiency of 2.6% and 2.4%, respectively, confirming the superiority of D149 for its applications in low-cost ZnO based DSSCs (Ren et al. 2012). Organic dyes without a vinyl group were synthesized by using iso-quinoline cation as an electron acceptor in its molecular structure wherein the devices sensitized by JH304 dye exhibited the efficiency of 7.3% in comparison to 7.9 observed in case of the devices sensitized by the N719 dye. These JH304 devices exhibited excellent soaking stability under sunlight for 1000 hours (Zhao et al. 2013). While experimenting with a large number of synthetic organic dyes, it was natural to pay attention towards the possible exploration of natural dyes for solar cell applications. In this context, very recently, anthocyanin—a natural dye extracted from the flowers of Rhododendron, with three different colors—pink, red and violet, was used as sensitizer in DSSC devices after nitric and acetic acid treatment where the acetic acid-treated anthocyanin performed better. The device efficiency of the acetic acid-treated pink, red and violet dye was 0.35%, 0.36%, 0.28%, respectively, which was better than those observed in the nitric acid-treated and bare dyes (Kim et al. 2013a). Compared to the metal polypyridine complexes, a lesser number of organic dyes were examined for sensitizing the wide band gap semiconductors. Apart from the coumarin derivative, organic dyes such as porphyrins, phthalocyanines, perylene bis-amides, xanthenes and polyenes show low photon to electron conversion efficiencies. Another major factor responsible for the low efficiency of an organic DSSC was the formation of dye aggregates on the semiconductor surface for which a number of strategies have been considered using helical-shaped polysaccharide (Arunkumar, Forbes, and Smith 2005), aerosol OT (sodium 1,2-bis(2-ethylhexoxycarbonyl) ethanesulfonate) micelles (Khazraji et al. 1999), dendritic side chains (Smith 1999) and encapsulation by cyclodextrins (CDs) (Cacialli et al. 2002; Haque et al. 2004; Stanier et al. 2001; Willner, Eichen, and Willner 1994). However, these problems could be taken care of by using supra-molecular encapsulation strategies that

accommodated the individual dye molecules and prevented self-aggregation. In this process, the dye molecules were encapsulated in CD hosts that not only prevented the formation of dye aggregates, but also retarded the interfacial charge recombination dynamics. Encapsulated JK-2 dye inside CD cavities when used in devices with a polymer gel electrolyte, gave an overall conversion efficiency of 7.40%, which was the highest value for DSSCs based on the organic sensitizers reported till that time (Choi et al. 2009). Development of zinc-porphyrin sensitizers co-adsorbed with chenodeoxycholic acid (CDCA) on TiO2/Al2O3 films were reported in which the porphyrin aggregation on TiO 2 surfaces not only accelerated the rate of intermolecular energy transfer but also increased the rate of interfacial electron injection resulting in the efficiency of 6.5 and 6.8% with 10-µm TiO2 films in two specific combinations (Lu et al. 2009). Novel porphyrin-based dyes were synthesized by introducing tert-butyl groups onto phenyl rings to suppress dye aggregations; extending the porphyrin conjugation to improve the light harvesting and adding electron-donating group to enhance the charge-separation properties. These devices using such modified porphyrin dyes exhibited efficiency of 6.0% (Lee et al. 2009a). A number of organic dyes employing amines as electron donors, 2-(6-substituted-anthracen-2-yl)thiophene as the π -conjugated bridge and cyanoacrylic acid group as electron acceptor and anchoring group, were synthesized and used in fabricating DSSCs resulted in efficiency ranging from 1.62% to 2.88% under standard illumination (Yen et al. 2012). Two sensitizers active in near-IR region arising due to anchoring of one and two groups were synthesized for p-type DSSC devices where the performance of those using two groups was found better than that having only one group (Chang et al. 2012a). A variety of asymmetric organic sensitizers comprising of donor, electron conducting and anchors were designed and synthesized for anchoring them to the TiO 2 thin films along with a volatile electrolyte. The overall efficiency of these solar cells reached up to 7.2% in volatile electrolyte and 3.25% in solid-state organic hole transporting layer under standard illumination (Hagberg et al. 2008). In contrast to the above discussed approaches, a series of symmetrical A-D-A organic sensitizers containing 3,6 and 2,7-functionalized carbazole cores, connected to two anchoring cyanoacrylic acid termini via thienyl linkers were synthesized (Chu et al. 2012) to study the influence of the molecular planarity arising from the 3,6 and 2,7-functionalized carbazole cores on the performance of corresponding PV devices. Among these new dyes, the highest efficiency of 4.82% was reported in case of a DSSC device under standard illumination (Chu et al. 2012). A combination of three organic dyes was synthesized for their use in improving the performance of TiO2 electrodes based on the principle of co-sensitization that resulted in wide band coverage from 400 to 700 nm with IPCE exceeding 70%. The devices using such a combination of dyes −2

exhibited conversion efficiency of 6.5% (AM1.5, 80 mW cm ), the highest efficiency to date for dye-sensitized solar cells based on the co-sensitization of plural organic dyes (Chen et al. 2005). Another combination of three organic sensitizers involving 4,4-dimethyl—4 H—indeno [1,2-b] thiophene or 4, 4—dimethyl—4 H-indeno[1,2-b] thienothiophene in the bridged group was designed and synthesized for gel-based DSSC devices wherein the PV performance was found sensitive to the bridge unit. The optimized cell using a gel electrolyte gave an overall efficiency of

8.31% under standard illumination showing excellent long-term stability by exhibiting unchanged performance during the 300 hours light soaking at 70 °C (Choi et al. 2013). A series of bis-triphenylamine-based organic sensitizers with higher molar extinction coefficients was synthesized for modulating the range of absorption and its corresponding photon-to-current conversion efficiency by varying π-conjugation bridge thiophene from zero to two between the donor and the acceptor part. Among the tested DSSC devices, the highest power conversion efficiency of 5.67% was obtained from RC-13 (Chang et al. 2012b). A highly efficient organic dye was synthesized that gave an overall solar-to-energy conversion efficiency of 9.1% under standard illumination (Hwang et al.2007). The organic sensitizers having identical π-spacers and electron acceptors but different arylamine electron donors, carbazole, phenothiazine and diphenylamine were synthesized and employed in fabricating DSSCs to study the influence of the different electron donors on the photo physical, electrochemical and PV properties of the device structures involved therein. The overall conversion efficiencies observed varied from 1.77 to 2.03% (Wan et al. 2012). ALL SOLID-STATE DYE SENSITIZED SOLAR CELLS A conventional DSSC uses a combination of materials, where each one performs a specific task in photovoltaic conversion of light into electricity. In spite of showing reasonable efficiencies around 12% the technological development has slowed down primarily due to the leakage/evaporation of the liquid electrolyte that makes the manufacturing process difficult (Nazeeruddin et al. 1993). Because of these reasons, in current research, more emphasis is given to replacing the electrolyte with a solid material to eliminate the problems of sealing during device packaging and as a result of that the concept of all solid-state DSSC started emerging with a structure similar to the DSSCs except for the replacement of liquid electrolyte with a p-type semiconductor or “quasi solid” counterpart. Here, some suitable polymers are used as electrolyte since they are inexpensive and easily modified chemically to fit a wide range of technological requirements. For example, solid-state DSSCs using P3HT on mesoporous TiO2 photo anodes have already been demonstrated (Gebeyehu et al. 2001; Sicot et al. 1999; Smestad et al.2003) experimentally, but the efficiencies of these solar cells are still lower than that of conventional DSSCs. The effect of film deposition techniques and the influence of a compact TiO2 under layer was thus studied in detail while fabricating and characterizing a number of TiO 2/P3HT based DSSCs. From radiation harvesting efficiency angle, the use of inorganic nanocrystals in place of dyes implies specific advantage of the band gap tunability, which in turn could have the absorption spectrum properly matched to the spectral distribution of sunlight (Vogel, Hoyer, and Weller 1994). Two new concepts based on combining solid-state NCs-sensitized solar cell and NCs/polymer-blended solar cell were promoted, where HgTe nanocrystals were used to increase the radiation harvesting efficiency over a broad spectrum from 350 to 1500 nm. Another device, based on nanoporous electrodes prepared from size-quantized CuInS2 NPs, was successfully developed offering newer possibilities of realizing solid-state hybrid solar cells. In another attempt, solution grown bilayer hetero-junction hybrid solar cells using PbS nanoparticles and P3HT were demonstrated (Günes et al. 2006). The natural extension of conventional DSSCs is currently being examined in form of all solid-state DSSCs, by employing a p-type semiconductor or an organic HTL in place of the liquid redox couple electrolyte (Bach et al. 1998; O’Regan and Schwartz1998; Tennakone et al. 1998). The

requirement of providing good photo-response and higher charge carrier mobility within the same material is not that critical as the separation and transport of the photo-generated charge carriers follow sequentially. The motivations for developing all solid-state devices include features like simpler production process, less expensive fabrication technology than that involved in conventional DSSCs with their associated problems like electrolyte leakage, packing and corrosion, which are practically nonexistent in these devices (Fujishima and Zhang 2005). Despite several attractive inherent features as listed above, the development of all solid-state devices has not yet picked up the desired pace. It is interesting to note that unlike conventional DSSCs, all solid state DSSCs function more like p-n hetero-junctions (Fujishima and Zhang 2005). An all solid-state DSSC device employs a nanoporous TiO2 photo anode along with a solid-state HTL. A monolayer of charge transferring dye sensitizer is coated on the surface of NCs photoanode to inject electrons while the regeneration of the oxidized dye via HTL intercepts the recapture of the conduction electrons by the oxidized dye. The HTL is finally regenerated at the counter electrode by completing the circuit via electron flow through the external load (Li et al. 2006a). The nature of charge carrier transport between two electrodes makes the all solidstate DSSCs different from the conventional ones. The HTL based all solid-state cell involves electronic transport whereas in a conventional DSSC with liquid or polymer electrolyte, it is ionic transport. In all solid-state DSSCs, the charge transfer reactions occurring at the dye sensitized TiO2 film-HTL interface play major role in controlling the overall efficiency (Nogueira, Longo, and de Paoli 2004). A p-type semiconductor to be considered for fabricating an all solid-state DSSC should essentially have some specific features as listed here (Gebenyehu et al. 2002; Li et al. 2006; Smestad et al. 2003). For instance, it must efficiently transfer holes from the dye after electron injection into the TiO2 film for which it must go deeper into the pores of TiO 2 layer without degrading its quality besides being transparent in the visible region, or, in case it absorbs light, it must inject electrons as efficiently as the dye. Many inorganic p-type semiconductors like CuI, CuBr and CuSCN are found to fulfill these conditions and were examined to replace the liquid electrolyte of the conventional DSSC accordingly (Kumara et al. 2001; O’Regan and Schwartz 1998; Tennakone et al. 1995, 2000). A solid-state hybrid solar cell, employing CuI as HTL and cyaniding—a pigment extracted from flowers as dye, demonstrated an efficiency of 1% 2

at 80 mW/cm simulated sunlight (Tennakone et al. 1995). Using CuI as HTL and Ru-bipyridyl dye as sensitizer, the efficiency got considerably pushed up to 6% but it got reduced to 4.5% for 2

incident light intensities higher than 100 mW/cm (Tennakone et al. 1998). Photo-degradation of these all solid-state DSSCs was investigated by using space resolved photocurrent imaging technique wherein it was concluded that the formation of different morphologies arising due to CuI solution in TiO2 pores was primarily responsible for inhomogeneous photocurrent generation leading to the observed performance degradation (Sirimanne et al. 2003). The excess I2 in CuI film also caused reduction in JSC and degradation was thus explained as the modification of the interface of TiO2/CuI due to the release of iodine and formation of trace amount of Cu 2O and/or CuO. Though, cell stability improved with covering of the TiO 2 electrode by a thin MgO layer, still, it was almost impossible to eliminate the degradation in total (Tennakone et al. 2001).

Inferring from the above mentioned observation that different morphologies were formed due to inefficient penetration of CuI into the TiO2 pores, crystal growth inhibitors were introduced during hole conductor deposition, which, in turn, improved filling of the porous matrix, resulting in the formation of more complete and secure contacts of the hole collector and the dyed surface. The crystal growth inhibitor should, however, not leave any solid residue at the grain boundaries or other interfaces upon the solvent evaporation. A detailed study of the process of crystal growth inhibition was carried out where it was further noted that the crystallite sizes got reduced in case another compound—triethyl amine hydrocyanate (THT) was added and it resulted in cell efficiency of 3.75% (Kumara et al. 2002a, 2002b). Though replacing CuI by CuSCN improved the device stability due to appropriate band gap and band energy, but material deposition was rather difficult (Kumara et al.2001). For instance, electro-chemical deposition and n-propyl sulphide based CuSCN solution showed poor response (Kalyanasundaram and Grätzel 1998; Kumara et al. 2001; Tennakone et al. 2001). Even making a simple pressure contact between gold coated ITO glass substrate and the CuSCN surface showed a poor response with I SC = 10 μA and VOC = 450 mV, which was primarily ascribed to incomplete coverage of CuSCN over the dye covered TiO2 surface increasing the internal resistance. It was observed that J SC increased to 2

2 mA/cm after graphite painting on 
 CuSCN surface. At standard illumination, the finished 2

devices
 delivered JSC = 3.52 mA/cm , Voc = 616 mV and PCE = 1.25% (Kumara et al. 2001). In a subsequent attempt, using propyl sulfide based CuSCN dilute solution not only improved the pore filling but also enhanced PCE to 2% with enhanced stability (O`Reagan, et al, 2002). Compared to inorganic p-type semiconductors, organic p-type semiconductors possessed the advantages of easy film formation at low cost (Li et al. 2006). In another study, a dye sensitized hetero-junction solar cell using TiO2 with organic holeconductor—spiro-OMETAD was explored where photo-induced charge carrier generation at the hetero-junction was found to demonstrate an efficiency of 0.74% (Bach et al. 1998). The performance of all solid-state DSSCs using spiro-OMETAD was further improved by using 4-tertbutyl pyridine (tBP) compound that reduced the surface recombination due to modified surface states. By blending HTL with 4-tert-butylpyridine (tBP) and 
 Li(CF3SO2)2 N, the devices showed 2

VOC = 900 mV and JSC = 5.1 mA/cm and PCE = 2.56% at AM 1.5 illumination. It was further noted that the VOC increased with the tBP concentration in the HTL while at constant tBP concentration, an increase in Li-ion concentration enhanced JSC. Subsequent improvement in the efficiency of all solid-state dye sensitized solar cell pushed the PCE to 3.2% by modifying the dye adsorption in the presence of Ag ions in the solution (Krüger et al. 2002). Employing further improved TiO2 pore filling demonstrated enhanced efficiency of 4% (Schmidt-Mende and Grätzel 2006). It was thus confirmed that wetting and pore filling by the HTL played a decisive role in improving the cell efficiency as the cell resistance increased in case of incomplete pore filling leading to lower JSC. In this pursuit, several conducting polymers were also considered as HTL by especially examining their wettability. Solution cast polymers were found to penetrate into the pores easily where polymer molar mass was crucial for an efficient pore filling property (Nogueira, Longo, and De Paoli 2004). A dye sensitized poly-thiophene solar cells using a poly (3-butylthiophene) with and without a dye sensitizer were fabricated demonstrating very low efficiency (Sicot et al. 1999). Solid-state devices

using poly (3-octylthiophene)

as

hole-transporting layer

produced

Voc = 650 mV,

2

JSC = 0.45 mA/cm under

2

80 mW/cm (Gebenyehu

et

al. 2002).

Later

on,

polythiophene based solid-state dye sensitized solar cells with a Voc = 0.8 V, Jsc = 80– 2

90 μA/cm and a FF of 0.4 were reported (Smestad et al. 2003). However, PCE of the most solid state DSSCs employing organic p-type semiconductors were relatively low particularly under high light irradiation most probably due to higher recombination rate at the TiO2-HTL interface between the electrons in the conduction band/trap states and the oxidized HTL, lower conductivity of the HTL due to broad distribution of trap states in these compounds and lower connectivity between the HTL and the hole-collecting electrode (Nogueira, Longo, and De Paoli2004). Rendering the liquid electrolyte into a quasi-solid form using gelation was explored where the polymeric gel combines the cohesive properties of solids with the diffusive transport properties of liquids. Because of this specific feature, gels appear to be worth trying in all solid-state DSSCs (Nogueira, Longo, and De Paoli 2004; Ross Murphy 1998; Megahed and Scosati 1995). Although, gel formulations possess a high ionic conductivity but their mechanical strength is, in general, rather poor. Gel electrolytes were, therefore, suggested to use a liquid plasticizer and/or solvent containing the desired ionic salts into a polymer matrix, resulting in a stable polymer structure. The process of gelation converted a dilute or more viscous polymer solution into a high viscosity gel where cross-linking or thermosetting could be used for better mechanical strength. It was also possible to employ a chemical or physical crosslinking wherein covalent crosslinking resulted into irreversible gel formation but those based on physical crosslinking produced “entangled network” There were a number of polymer matrices that were developed involving poly (ethylene oxide), poly (acrylonitrile), poly (vinyl pyrrolidinine), poly (vinyl chloride), poly (vinyl carbonate), poly (vinylidene fluoride) and poly (methyl methacrylate) (Nogueira, Longo, and De Paoli 2004). Among the various quasi solid-state DSSC studies, one of the very first one employed a mixture of NaI, ethylene carbonate, propylene carbonate and polyacrylonitrile (Cao, Oskam, and Searson 1995). However, PCE was poor compared to the liquid dye sensitized solar cells, which was attributed to the low penetration of the polymer network into the TiO 2 film. In another study, quasi solid-state DSSCs were fabricated using poly (vinylidenefluoride-co-hexafluoropropylene (PVDF-HFP) to solidify a 3- methoxypropionitrile (MPN) and when used with an amphiphilic polypyridyl Ru-dye, the devices gave efficiency of 6% under full sunlight with improved stabilities under both thermal stress at 80°C and prolonged soaking with light (Wang et al. 2004). Despite reasonable PCE of quasi solid-state DSSCs, they lagged behind the conventional liquid electrolyte cells. It is quite likely that unstable gel electrolytes are still prone to leakage reducing the ionic conductivity leading to poor efficiency after storing the devices over longer duration. High ambient temperature also affected the cell performance wherein a solvent with higher vapor pressure could damage the sealing (Nogueira, Longo, and De Paoli 2004). The polymer electrolytes consisting of alkaline salts dissolved in a high molecular mass polyether or polypropylene oxide host were explored for device fabrication. Ideally, in a good polymer electrolyte, the polymer matrix should be an efficient solvent for the salt, capable of dissociating it and minimizing the formation of ion pairs. The salt solubility relies on the ability of the electron donor atoms in the polymer chain to coordinate the cations through Lewis type acid-base interaction. This interaction depends on the lattice energy of the salt and the structure of the host

polymer. The mechanism for ionic motion in polymer electrolytes results from a solvation-desolvation process along the chains that occur in the amorphous polymer phase (Armand 1987; Nogueira, Longo, and De Paoli 2004). All solid state DSSC devices fabricated by using poly (epichlorohydrin-co-ethylene oxide) and epichlomer-16 without additional plasticizer exhibited lower efficiency of 0.22%, which was further improved by an order of magnitude to 2.6% (Nogueira and De Paoli 2000; Nogueira, Durrant, and De Paoli 2001). Another set of devices gave 5.3% efficiency on flexible substrate using Al2O3 coated TiO2 electrodes and an I2/NaI-doped solid-state epichlomer-16 electrolyte (Haque et al. 2003). Still higher efficiency of 7% was reported with all solid-state dye sensitized solar cell incorporating polysaccharide solid involving redox electrolytes and an organic medium (Kaneko and Hoshi 2003). A solution grown p-type direct band gap semiconductor—CsSnI3 was synthesized for hole conducting layer in a DSSC comprising of CsSnI2.95F0.05 doped with SnF2, nanoporous TiO2 and N719 dye that exhibited efficiency of 10.2% (8.51% with a mask) wherein 1.3 eV band gap of CsSnI3 caused enhanced absorption on the red side of the spectrum (Chung et al. 2012). In a recent study, a number of electrolytes were prepared by soaking polyvinyl butyral in different solvents and their parameters like surface morphology, diffusion coefficient, and the ionic conductivity as a function of the electrolyte content were analyzed. Having optimized various parameters that were helpful in enhancing the overall all efficiency of DSSC, the fabricated 2

devices exhibited efficiency of 5.46% at 100 mW/cm with long-term durability over 3000 hours (Chen et al. 2013). Employing a solvo-thermal method, 0.3–2.0 μ m mesoporous anatase TiO2 microspheres were synthesized for solid-state DSSCs where an efficiency of 4.2% was observed primarily due to enlarged surface area and pore size leading to increased dye uptake and easy transport across solid electrolyte through mesopores in addition to the greater electron lifetime and superior light scattering ability (Jung et al. 2013). A solid polymer electrolyte comprising of poly (N-methyl-4vinyl-pyridine iodide), N-methyl pyridine iodide and iodine was synthesized with very high conductivity of 6.41 mS/cm for using in preparing devices based on conducting graphite paper, KI blocking layer and vacuum assembly that resulted in device efficiency of 5.64% under standard illumination (Wu et al. 2008). Alternate layers of mesoporous TiO2 and colloidal SiO2 nanoparticles were deposited to prepare a highly reflective counter electrode for maximizing light harvesting properties of a DSSC employing a combination of atomic transfer radical polymerization and sol-gel process with polyvinyl chloride-g-poly (oxyethylene) methacrylate graft copolymer as structure-directing agent for TiO2 and spin coating for commercially available silica nanoparticles. When used in fabricating the DSSCs along with a polymerized IL as solid electrolyte, the solid-state devices exhibited efficiency of 6.6 %, which was one of the highest reported in case of N719 dye-based devices (Park et al. 2013). Supra-molecular dyes were used in controlling charge recombination between photo-injected electrons and oxidized hole-transporting material, resulting in an enhancement in the performance of dye sensitized solar cell devices based upon such dyes (Handa et al. 2007a, 2007b).

A significant increase of two orders of magnitude of the conductivity of polymer electrolyte based on poly (ethylene oxide), LiI and I2 was measured after adding poly (ether urethane) that −

enhanced diffusion of I3 ions considerably. This additive was beneficial in enhancing V OC by shifting the band edge of TiO2 to a negative value (Zhou et al. 2009). Very recently, a batch of 9.4% efficient solar cells were successfully fabricated using hydrothermally grown 0.6 μm rutile TiO2nanorods along with CH3NH3PbI3 perovskite nano dots and Spiro-MeOTAD hole transporting layer. It was further noted that JSC and VOC decreased with increasing rod lengths correlating with charge generation efficiency while recombination remained unaffected (Kim et al. 2013). Ionic crystals comprising of 4-cyano-4′-hydroxybiphenyl and imidazolium synthesized as electrolyte for DSSCs demonstrated an efficiency of 5.11% @ 55 °C under standard illumination. Addition of 1-propyl-3-methylimidazolium iodine (PMII) into the electrolyte acted as a crystal growth inhibitor that was reflected in an enhanced efficiency of 6.55% @ 45 °C under the simulated air mass 1.5 solar illuminations at 50 mW cm

−2

accompanied by a better long-term

stability (Cao-Cen et al. 2012). Solid state DSSCs with 3.1% PCE were fabricated by using poly (3-hexylthiophene) along with graphite/carbon black CE under standard illumination (Wang et al. 2011b). A two-step aqueous solution polymerization process was developed to synthesize microporous hydrophobic

polyaniline

(PANi)

integrated

poly(hexamethylene

diisocyanate

tripolymer/polyethylene glycol) [poly(HDT/PEG)] gel electrolyte with ionic conductivity of 12.11 mS cm

−1

at room temperature. Morphological examinations of this electrolyte confirmed the −

−

micro porous structure, providing space for holding I /I3 liquid electrolyte besides the integration of PANi with poly (HDT/PEG) caused reduced charge-transfer resistance and enhanced electro −

−

catalytic activity for the I /I3 redox reaction. A batch of 6.81% efficient solar cells was successfully fabricated by sandwiching PANi integrated poly (HDT/PEG) gel electrolyte between a TiO2 anode and a Pt counter electrode, under standard illumination (Li et al. 2013). A solid-state electrolyte consisting of poly (ethylene oxide) and LiI without a filler was evaluated during the process of optimizing the concentration of KI for highest conductivity wherein a conductivity of 3.0 mS/cm was measured for 14.5 wt.% KI that gave efficiency of 4.5% in using this electrolyte in devices. It was concluded therefrom that the addition of KI in PEO/I 2/LiI electrolyte eliminated the crystallization of the polymer matrix and enhanced the ionic conductivity. The energy conversion efficiency of the device was further enhanced to 5.8% by incorporating a light scattering layer (Agarwala et al. 2011). In another work, poly (acrylonitrile-co-methacrylonitrile)/silica based gel electrolytes were employed to fabricate solid state DSSCs with the overall energy conversion efficiency of 3.0%, wherein the addition of silica filler to the copolymer [poly (acrylonitrile-co-methacrylonitrile)] matrix reduced the copolymer crystallinity that increased the mobility of iodide/triiodide couple (Akhtar et al. 2006). A mesoscopic nitrogen-doped TiO2 was synthesized for a solid-state DSSCs wherein the comparison showed the superiority of doped spheres to undoped ones. The detailed characterizations indicated that both the quasi Fermi level and the charge transport of the photo electrode were improved after being doped with nitrogen resulting in an efficiency of 6.01% (Xiang et al. 2011).

QUANTUM DOT SENSITIZED ALL SOLID STATE DSSC Foreseeing a number of advantages associated with using inorganic NCs in place of organic dyes, it is quite logical to think about exploring inorganic quantum dots (QDs) derived either from colloidal solution or produced in situ in place of the dye sensitizer molecules of the conventional DSSCs (Hoyer and Könenkamp 1995; Liu and Kamat 1993; Vogel, Hoyer, and Weller1994; Zaban et al. 1998). In this context, the possibility of altering the band gap for modifying associated absorption bands favorably by changing the size is expected to result in better light harvesting due to band edge type absorption and improved photo-stability of the electrodes by surface functionalizations of the NCs (Vogel, Hoyer, and Weller 1994). In addition, larger extinction coefficients of the NCs arising due to quantum confinement and intrinsic dipole moments help in rapid charge separation besides properly designed semiconductor NCs are, in general, robust inorganic entities (Alivisatos 1996). These expectations led to one of the earliest attempts in this context, to use smaller size NCs in embedding them into the pores of TiO2 photoanode. CdS and CdSe QDs were synthesized onto TiO 2 mesoporous film for preparing CdS/CdSe sensitized photo anode having complementary light harvesting properties for QD-sensitized solar cell applications. In a cascade structure of TiO 2/CdS/CdSe photo anode, the re-organization of energy levels between CdS and CdSe improved the electron injection and hole-recovery of CdS and CdSe QDs leading to a conversion efficiency of 4.22% in a TiO 2/CdS/CdSe/ZnS electrode, under one sun illumination (Lee and Lo 2009). Employing successive IL adsorption and reaction (SILAR) technique for self assembled monolayers (SAM) of phosphonic acid head groups on the surface of TiO 2 nanocrystals, 2–6 nm size CdS QDs were grown on TiO2 film for fabricating spiro-OMeTAD based DSSCs, which demonstrated 3x improvement in efficiency compared to the devices without SAM layers (Ardalan et al.2011). Feasibility of controlling the size of CdS QDs in the range of 1–10 nm by using ALD on TiO2 was demonstrated using device architecture with spiro-OMeTAD as the solid-state hole-conductor. From this experiment, it was established that ALD introduced a strategy by which material and optical properties of QD sensitizers could be adjusted not only by the size of the particles but also by changing the compositions along with (Brennan et al. 2011). Despite controlling the opto electronic properties of CdS QDs well, the QD sensitized devices have, in general, exhibited poor efficiencies due to enhanced recombination of charge carriers, which was attempted to reduce by introducing a thin layer of ALD Al2O3 in two types of device configurations like—TiO2-Al2O3-QD and TiO2-QD-Al2O3 with solid-state spiro-OMeTAD hole conducting layer. It was observed that Al2O3 layer did suppress the dark current and improved electron lifetimes consequently. A monolithic version of solid-state DSSC module using high stability mesoscopic carbon CEs was developed that offered better prospect for commercial exploitations. A typical module, consisting of five interconnected cells, exhibited an efficiency of 2.57% for a maximum output power of 156.9 mW. Further, a special kind of sealing was developed for these modules, which remained stable @ 60 °C (85% RH) while undergoing a thermal stress of −10–60 °C (Rong et al. 2012).

PbS and CdS QDs sensitized electrodes with Co-complex redox couples were employed as regenerative electrolyte in solid-state DSSCs wherein the PbS devices demonstrated 2% efficiency at 0.1-Sun illumination (Lee et al. 2009b). The role of linker molecules in QD sensitized solid state DSSCs was studied theoretically as well experimentally to clarify its influence on charge carrier separation, recombination and the following charge carrier transport. In this study, two typical examples of mercapto propionic acid (MPA) and cysteine (Cys) were examined with the conclusion that Cys based devices outperformed MPA based devices by demonstrating an efficiency of 2.7% at AM 1.5 illumination (Margraf et al. 2013). Solid-state DSSCs with individual CdS and CdSe QDs as well as combining the two in sequences like TiO2/CdS/CdSe and TiO2/CdSe/CdS were fabricated to study the possibility of cosensitization, which revealed that the devices based on TiO2/CdS/CdSe sequence were superior to both of the CdS and CdSe sensitized devices, whereas TiO 2/CdSe/CdS based devices were found inferior to TiO2/CdSe and TiO2/CdS based devices. These results clarified that introducing a CdSe layer between TiO2 and CdS was detrimental to the transport of excitons and, therefore, the co-sensitization effect could not be realized successfully. In addition, using a thin film of ZnS as passivating layer also affected the performance as seen experimentally. For example, introduction of ZnS improved efficiency from 2.9 to 3.7%, which was further boosted to 4.22% when Pt was replaced with Au counter electrode (Lee and Lo 2009). Coating of CdS QDs on mesoporous TiO2 photoanode using chemical bath deposition method −

−

was noted to improve the photo stability of I /I3 electrolyte exceptionally. QDs present on the surface of TiO2 film not only passivated the surface by introducing surface states that acted as hole traps, which otherwise would have degraded the cell performance, but also reduced electron recombination from QDs and TiO2 into the electrolyte and this attempt resulted in 1.24% efficient devices from this study (Shalom et al. 2009). SILAR synthesized CdS QDs were coated onto ZnO NWs and NCs, which were used in preparing QD sensitized solar cells. The highest device efficiency was measured in CdS sensitized devices using ZnO NCs based photoanode, which was due to significant improvement in JSC being ~3 times higher than that of the cell with NWs. Moreover, co-sensitizer combination involving ZnO-NC/CdS/CdSe showed higher efficiency than the one obtained with only CdS QDs as the sensitizer. Finally, adding a passivation layer of ZnS in form of ZnO-NC/CdS/CdSe/ZnS configuration showed an efficiency of 0.55% (Chou et al.2013). Two types of ILs namely—1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) and 1butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF 6]), were used to prepare carbon dots/ILs blend using IL-assisted electrochemical exfoliation. These carbon dot/IL combinations were employed in fabricating solid-state DSSCs, where 1-butyl-3-methylimidazolium iodide and LiI/I2 were added as performance enhancer. The overall efficiency of 2.71% and 2.41% for carbon dots/[bmim][PF6] and carbon dots/[bmim][BF4] based blend electrolytes were observed, respectively. These cells showed good stability under continuous illumination at room temperature without any further sealing (Xiong et al. 2013). A combined hetero-phase of TiO2-nanobelt (NB)/anatase TiO2 NC was fabricated via layer-bylayer assembly, where the NB-to-NC ratio could be varied by controlling the experimental conditions. This hetero-phase offered the advantages of the rapid electron transport in NBs and

the high surface area of NCs resulting in a photoelectric conversion efficiency of 7.54% better than that either with the naked NB photo anode or the mechanical mixing NB–NC photo electrode. The devices with optimal NB/NC ratio displayed the larger photoelectron injection drive force and the fastest interfacial electron transfer (Pan et al.2013). A method combining electro deposition with redox replacement was developed recently for faster deposition of thin films combining Pt/Pb nanoparticles on FTO glass substrate. The deposition 2+

progressed by cycling between the two steps in a single electrolyte bath containing both Pb and 2+ Pt ions. In the first step, Pb was electrodeposited on FTO glass at a potential E1and in the second step, Pt replaced the sacrificial Pb directly on the surface at open circuit conditions until the open circuit potential reached a value E2. This method allowed a quick deposition of Pt/Pb nanoparticles on FTO glass, which could possibly be explored for manufacturing purposes later on (Yliniemi et al. 2013). EXTREMELY THIN ABSORBER (ETA) SOLAR CELLS For light harvesting that is sufficient for efficient conversion of light into electricity, it is necessary to have some minimum thickness of the film containing TiO 2 and the sensitizer. For realizing the best of the sensitizer+TiO2 combination, it is necessary to reduce the thickness by improving the process of light harvesting by modifying the way two constituents are mixed together. In this context, study of extremely thin absorber (ETA) solar cell (Kaiser et al. 2001) was conducted employing a thin layer of CuInS2 or CdTe deposited on the TiO2 film along with CuSCN as p-type semiconductor (Ernst, Belaidi, and Könenkamp 2004). This kind of ETA solar cell was explored due to the special advantage of enhanced light harvesting arising out of enlarged surface area and multiple scatterings. Since photo-induced charge separation occurred over a much smaller length scale, it was possible to tolerate even higher levels of defects and impurities in thin film devices, where the minority carriers were required to diffuse several micrometers. ETA solar cell operates as a hetero-junction device with an extremely extended interface (Nanu, Schoonman, and Goossens 2005) employing TiO2/CuInS2 configuration, where 2 nm Al2O3 tunnel barrier and 10 nm In2S3 buffer layer were provided by ALD coating between TiO 2 and CuInS2 serving to 2

reduce the interfacial recombination losses. This kind of ETA cells produced J SC = 18 mA/cm , VOC = 0.49 V, FF = 0.44 corresponding to PCE = 4%. For controlling light scattering in nanostructured solar cells, a new type of ZnO structure was realized by combining electrochemical deposition with lithography and ALD for fabricating high efficiency ETA PV devices. A considerable improvement in light absorption was noted when urchin like ZnO nanostructures were coated with a thin layer of CdSe to study its influence on improving the charge carrier separation and reducing the recombination. Finally, ETA solar cells were fabricated using the optimum thicknesses of CdSe light absorber layer with CuSCN and ZnO urchin-like nanostructures that led to uniform absorption up to 90% in the range of 400– 800 nm solar spectrum and these initial devices showed an efficiency of 1.33%, which seems promising with further investigations (Elias, Philippe, and Michler 2012). Plasmonic enhancement of optical absorption was found viable in recent studies to improve the performance of DSSCs, where the plasmonic light concentrator reduced the TiO 2 thickness, in turn, making the charge carriers travel shorter distances accompanied by reduced recombination to reach the external circuit. But, using metal NPs for this purpose posed two problems namely— the corrosion from the electrolyte and the enhanced recombination. Though, enveloping metal

NPs by SiO2 shell mitigated the problem of corrosion, but then the dye molecules could not inject charge carriers into SiO2, which was finally resolved by applying another coating of TiO2 before incorporating them into a DSSC structure. The largest improvement in performance was observed not only because of broadband absorption but even the larger dipole moments brought about by coupled surface plasmon resonances. Thus a core-shell-shell type NCs involving Au@SiO2@TiO2architecture were synthesized and incorporated into DSSCs performing better than those of a Au@SiO2 system due to closer proximity of the dye molecules to the metal surface. Broadband enhancement of dye absorption resulted in a coupled plasmonic system leading to an efficiency enhancement from 2.81% to 5.52% as compared to those without metal nanoparticles (Sheehan et al. 2013). ORGANIC-INORGANIC HYBRID SOLAR CELLS Organic-inorganic hybrid solar cells have a combination of electron donor and acceptor layers of photoactive materials placed between two electrodes having work functions different from each other for collecting the free charge carriers generated by PV effect. The combination of light absorbing materials may thus involve a conjugated polymer and semiconducting NCs thin film as organic and inorganic part, respectively. The electron donor and acceptor photoactive layers may be organized in one of the three possible types of configurations namely—bilayer, bulk hetero junction and inter-digitated structures (Zhou et al.2011). A transparent conducting layer of ITO on a glass or flexible plastic substrate on one side and a vacuum deposited metal film of Al/ LiF/Al or Ca/Al directly onto the photoactive layer on the other side, are used as anode and cathode, respectively. The photo excited charge carrier generation in an organic material starts with bound electron-hole pairs—excitons that do not spontaneously dissociate into free charge carriers as such. Unlike the charge carrier transport in a conventional crystalline semiconductor energy band; it involves hopping between localized states in an organic semiconductor that results in finally very low mobilities. Organic semiconductors, being of one-dimensional molecular entities, possess highly anisotropic electronic and optical properties and their optical response is also confined to a narrow part of solar radiation spectrum. Since the optical excitations result in a transition from π to π* state, absorption bandwidth depends on the degree of conjugation and a tunable spectral sensitivity is thus achievable in conjugated dye molecules (Nelson 2002). The simplest configuration of an organic PV device employs an organic material that is sandwiched between two different conducting contacts involving ITO and a low work function metal such as Al, Ca/Al, or Mg/Al (Winder 2001). For PV effect, the bound excitons must dissociate releasing electrons and holes for which the presence of either metal-semiconductor or donor-acceptor semiconductor interface is necessary. Working of these devices involves Schottky barrier formation between the metal with the lower work function and the p-type organic semiconductor layer (Hoppe and Sariciftci 2004; Sze 1981), which gives rise to a depletion region due to band bending that dissociates the excitons (Hoppe and Sariciftci 2004). Because of short diffusion length in most of the organic solar cell materials, only those excitons generated in the region within ~ 20 nm from the contact contribute to the photocurrent (Hoppe and Sariciftci 2004). From this angle of very limited utility of charge generation capability of this single layer configuration, a bilayer device comprising of donor and acceptor material sequentially stacked together with a geometrical interface was considered alternately for improving the charge carrier

generation and separation (Hoppe and Sariciftci 2004; Pettersson, Roman, and Inganäs 1999; Peumans, Uchida, and Forrest2003; Rostalski and Meissner 2000), which is aided by a potential drop between donor and acceptor. This bilayer is now sandwiched between two electrodes with different work functions matching donor HOMO and the acceptor LUMO for efficient extraction of the corresponding charge carriers. Now it is clear that the charge carrier transport in bilayer hetero junction formed between undoped D and A-materials is due to the differences in the ionization potential and electron affinity of the adjacent materials. Upon photon absorption in the donor region, the electron is excited from the HOMO to the LUMO. In case, now an acceptor molecule A is in close proximity, the electron would be transferred to the LUMO of A, which is energetically preferred. The release in electron energy may then be used to separate the electron and the hole from their coulomb potential. This photo-induced charge transfer (CT) only occurs under illumination, since it needs the excitation energy of the electron in the donor to reach the LUMO in the acceptor (Hoppe and Sariciftci 2004). There are experimental indications (Murgia et al. 2001; Ruani et al. 2002; Toccoli et al. 2003) supported by theoretical considerations (Arkhipov, Heremans, and Bassler 2003) of the formation of an interfacial dipole between the donor and acceptor phases, independent of illumination (Hoppe and Sariciftci 2004), which can stabilize the charge-separated state by a repulsive interaction between the interface and the free charges (Arkhipov, Heremans, and Bassler 2003). Therefore, the success of the donor/acceptor concept lies, to a great extent, in the relative stability of the charge separated states: the recombination rate between holes in donor and electrons in the acceptor is several orders of magnitude smaller than the forward charge transfer rate (Hoppe and Sariciftci 2004; Nogueira et al. 2003; Sariciftci and Heeger 1997; Zerza et al. 2001). After the excitons are dissociated at the interface, the electrons travel within the n-type acceptor and the holes travel within the p-type donor material. In this process, the holes and electrons are physically separated from each other and their recombination is greatly reduced as it depends more on trap densities (Hoppe and Sariciftci 2004). Consequently, the photocurrent is linearly dependent on illumination intensity (Meissner, Siebentritt, and Günster 1992; Pettersson, Roman, and Inganäs 1999; Peumans, Uchida, and Forrest 2003; Rostalski and Meissner 2000; Sariciftci et al. 1993). For preparing a quality bulk hetero junction device, it is thus necessary to blend the donor and acceptor components so that each donor-acceptor interface is within a distance less than the exciton diffusion length (~ 20 nm) of each absorbing site (Hoppe and Sariciftci 2004). While in a bilayer hetero-junction the donor and acceptor phase contact the anode and cathode selectively, the bulk hetero-junction requires percolated pathways for the hole and electron transporting phases to the contacts. In other words, the donor and acceptor phase have to form a bicontinuous and inter-penetrating network, which is more sensitive to the morphology of the blend (Hoppe and Sariciftci 2004). Another device architecture, which is conceptually in between that of the bilayer and the bulk hetero junction, is a diffuse bilayer hetero junction device combining the advantages of the an enlarged donor-acceptor interface and a spatially uninterrupted pathway for the opposite charge carriers to their corresponding electrode (Hoppe and Sariciftci 2004). The diffuse interface is achieved in different ways: in case processed from solution, two thin films can be pressed together in a lamination procedure applying moderate pressure and elevated temperatures

(Granström et al. 1998; Hoppe and Sariciftci2004). Another way to achieve a diffuse interface is to spin cast the second layer from a solvent that partially dissolves the underlying layer (Brabec et al. 2002; Chen et al. 2000; Hoppe and Sariciftci 2004). Finally, the controlled inter-diffusion between an acceptor fullerene and donor polymer by annealing of a bilayer device results in an intermixed interfacial region (Drees et al. 2002; Hoppe and Sariciftci 2004). The ideal sensitizer for a single junction solar cell converting sunlight into electricity must fulfill the essential requirements including: absorption of all light below a threshold wavelength; efficiently injecting electrons into the conduction band of the oxide with unity quantum yield upon photo excitation; proper matching of the excited state energy level with the lower band edge of the conduction band of the oxide to minimize energetic losses during the electron transfer; sufficiently positive redox potential for efficient regeneration via electron donation from redox electrolyte; having attached carboxylate or phosponate groups for stronger anchoring to the semiconductor oxide surface; and stability against the continuous exposure of natural light over an extended period of time(Grätzel 2004, 2005; Polo, Itokazu, and Murakami 2004). While keeping these salient features in view, various kinds of compounds were investigated as sensitizers including porphyrins

(Kalyanasundaram

et

al.1987;

Kay

and

Grätzel 1993;

Mao

et

al. 1998),

phthalocyanines (Fang et al. 1997; Nazeeruddin et al. 1998, 1999), transition metal complexes (Kalyanasundaram and Grätzel 1998; Murakami et al. 2006), coumarin (Rehm et al. 1996) and transition metal complexes. Metal complex photosensitizers usually have anchoring ligands along with chromophoric groups that promote the adsorption onto the semiconductor surface. The ancillary ligands that are not directly attached to the semiconductor surface are used for tuning the photo absorption properties of the complexes (Polo, Itokazu, and Murakami 2004). Semiconductor NCs possess very interesting features for their use as photosensitizers for enhancing the light harvesting efficiency of PV solar cell device. In case, these semiconducting NCs are smaller than the size of the exciton in the bulk semiconductor, which is typically ~ 10 nm, their electronic structure deviates from that in an extended solid to more like a molecule having electronic and optical properties dependent on the material, of which they are made of and their size (Alivisatos 1996; Empedocles and Bawendi 1999; Murphy and Coffer 2002; Steigerwald and Eisrus 1990; Weller 1993). In this context, polymer-NCs blends are quite promising for fabricating PV devices wherein a substantial interfacial area, required for efficient charge separation, is provided by the NCs (Huynh, Peng, and Alivisatos 1999). To enhance electron collection, it is necessary to form a defined pathway to the appropriate electrode, which is generated at NCs/polymer interface. Successful fabrication of such PV devices was reported using a composite of 8 × 13 nm CdSe nanorods and P3HT, where CdSe nanorods were noted to chain in which the NCs stack along their axis. Devices made from thin films of 8 × 13 nm CdSe/P3HT 2

composite displayed JSC = 0.031 mA/cm and VOC = 0.57 V (Huynh, Peng, and Alivisatos 1999). In a similar device, with 4 × 7 nm CdSe NRs, the external quantum efficiency was 4% and the fill factor was 0.45. It was further observed that the rectification characteristics and VOC did not change with size. The difference in quantum efficiency resulting from a change in NCs size was primarily attributed to the differences in aggregation of nanorods within the polymer. In a complementary work, an efficient organic/inorganic hybrid solar cell was prepared by blending CdSe and P3HT together and it demonstrated a PCE = 1.7% under simulated AM 1.5 illumination

with CdSe nanocrystals of 7 × 60 nm size that covered a broad spectrum from 300 to 720 nm (Huynh, Dittmer, and Alivisatos 2002). While considering the tunable optical and electro-optical properties of semiconducting sensitizing materials for PV devices for optimizing their performances, nanosize QDs do possess certainly better features as compared to their bulk counterpart as the quantum confinement of electrons in a nanosize QD introduces discrete energy states around the conduction and valence band edges that in turn modify the effective band gaps which can be maneuvered by changing the QD size. In general, there are two routes to prepare QDs for this kind of applications. In one case, physical methods like lithography, physical vapor deposition, atomic/molecular layer/beam epitaxy and dry etching are involved whereas in the other primarily chemical methods involving colloidal chemistry are commonly involved. For example, colloidal QDs are synthesized in organic solvents, which can be mixed together with conjugated polymers in case they are soluble in the same solvent resulting in QDs having appropriate band gap and energy levels incorporated into conjugated polymer blends, forming bulk-hetero junction hybrid solar cells for PV applications (Borchert 2010; Zhou, et al, 2010a, 2010b; Xu and Qiao 2011). In most of these studies of hybrid solar cells, a number of semiconducting species such as CdS, CdSe, CdTe, ZnO, SnO 2, TiO2, Si, PbS, and PbSe QDs were used as electron acceptors (Zhou et al. 2011). Bulk hetero junction type hybrid PV solar cells have bright future despite lagging behind in the performance of fullerene derivative based organic PV solar cells especially due to their thin/flexible configurations produced at low cost,. For example, changing the diameters of the QDs, their band gaps and energy levels can be adjusted to meet the optimum conditions for photo generation and transport besides having the enhanced absorption coefficient due to quantum confinement. As a result, in the QDs/polymer system, both components have the ability to absorb incident light, unlike the typical polymer/fullerene system, where the fullerene contributes very little to the photocurrent generation (Diener and Alford 1998; Kazaoui and Minami 1997). In addition, NCs have also been synthesized in stable elongated structures on the length scale of 2–100 nm with desirable exciton dissociation and charge transport properties (Huynh, Dittmer, and Alivisatos 2002). Incidentally, the fabrication of hybrid solar cells started with CdSe QDs and these devices still continue to offer the highest efficiencies when compared to the devices based on other NCs. CdSe NCs have some specific properties, for example, of absorbing optical radiation from 300– 650 nm of solar spectrum, being good electron acceptors along with conjugated polymers, and having well-established synthesis protocols. A combination of CdSe QDs and conjugated polymer was used in the very first batch of such solar cell, which was reported in 1996 (Greenham, Peng, and Alivisatos 1996). Although, an external quantum efficiencies (EQE) of 10% estimated in case of 90 wt% composite did indicate an efficient exciton dissociation at the polymer/QD interface, but the poor efficiency of about 0.1% was primarily attributed to an inefficient inter-QD electron transport. This situation improved when non-spherical QDs were employed in cell fabrication (Peng et al. 2000). For example, elongated CdSe QDs or nano rods (NRs) were found better electron acceptor material in hybrid solar cells as reported in 2002, when used with P3HT (Huynh, Dittmer, and Alivisatos 2002). The optimized devices using 90 wt% pyridine treated 7 nm diameter/60 nm length CdSe NRs and P3HT exhibited PCE of 1.7%. Replacing chlorobenzene by 1,2,4-trichlorobenzene (TCB) as solvent for P3HT resulted in further improved efficiencies of

2.6% (Sun and Greenham 2006), which was further enhanced up to 2.8% when CdSe tetrapods (TPs) were used in place of NRs (Hindson et al. 2011; Sun et al. 2005). Recently, this efficiency was boosted up to 3.19% by using a lower band gap polymer PCPDTBT, which can absorb wider spectrum of the solar emission (Dayal et al. 2010). These elongated/branched NCs not only offer extended and directed electrical conducting pathways, also reduced the number of inter-particle hopping events required during electron extraction by the electrode. However, device performance is also affected by their surface modified solubility, which influences the charge transfer and carrier transport. Despite relatively higher intrinsic conductivity within the individual NCs, the electron mobility through the NC-network in hybrid solar cells is quite low, which could be mainly attributed to the electrically insulating organic ligands present there on the surface. For clarifying various issues in this context, the investigation of charge carrier injection and transport in CdSe NCs thin films covered with TOPO ligand and sandwiched between two metal electrodes were carried out (Ginger and Greenham 2000) where very low electron mobility of −5

2

−1 −1

10 cm V s

2

2

−1 −1

was measured as compared to the electron mobility of 10 cm V s

in bulk CdSe

(Rode 1970). The surface passivating ligands, involving oleic acid (OA), trioctylphosphine oxide (TOPO) or hexadecylamine (HDA) formed, in general, an insulating layer that prevented an efficient charge transfer between NCs (Greenham, Peng, and Alivisatos 1996; Huynh et al. 2003). In order to overcome this problem, post-synthetic treatment on the NCs was attempted employing ligand exchange from original longer alkyl ligands to shorter molecules like pyridine and chemical surface treatment followed by washing for reducing the ligand shell. A combination of ligand shell reduction and ligand exchange afterwards helped in improving the solar cell performance by enhancing the electron transport in the interconnected NC-network. State-of-the-art efficiencies of hybrid solar cells were reported after thoroughly washing the surface passivated NCs by methanol followed by reflux in boiling pyridine in inert atmosphere overnight (Huynh, Dittmer, and Alivisatos 2002). This treatment could possibly replace the insulating ligands with shorter and more conductive pyridine molecules. Similar treatment with materials like chloride (Owen et al. 2008), amine (Olson, Gray, and Cater 2009), and thiols (Aldakov et al. 2007; Sih and Wolf 2007) were examined using characterization methods like nuclear magnetic resonance (NMR), optical spectroscopy and electrochemistry and during this exercise, using butylamine ligand capping on CdSe/P3HT, the devices exhibited PCEs up to 1.77% (Olson, Gray, and Cater 2009). Shortening of the insulating ligands by thermal decomposition was also attempted with relative improvement of the PCEs of the CdSe/P3HT-based solar cells (Seo et al. 2009). However, NCs with smaller molecular ligands have tendency of aggregation in organic solvents (Huynh, Dittmer, and Alivisatos 2002; Huynh et al. 2003), giving unstable mixtures of NCs and polymer. Recently, a new type of surface treatment of CdSe NCs was reported, where a simple hexanoic acid-assisted washing procedure was adopted which imparted improved solubility, giving reproducible performance. This also led to higher loading of the CdSe NCs in the blend, which was helpful in providing efficient percolation network formed during the annealing step of the photoactive film. Devices with optimized ratios of NCs and P3HT exhibited reproducible PCEs up to 2.1% (Zhou et al.2010a). Additionally, investigations carried out on TOP/OA capped CdSe NCs suggested that the hexanoic acid treatment was also applicable to other ligand systems as well. In case of even two kinds of NCs having different sizes of 5.5 nm for HDA capped NCs and 4.7 nm for TOP/OA

capped NCs based devices exhibited same efficiency of 2.1%. Furthermore, using low band gap polymer PCPDTBT, optimized devices based on acid treated TOP/OA CdSe NCs exhibited the highest efficiency of 2.7% for CdSe QD based devices so far (Zhou et al. 2011). The experience in case of exploring materials other than CdSe has not been that encouraging. For example, ZnO NCs based hybrid solar cells did show efficiencies of 1.4 and 2% with MDMOPPV (Beek et al. 2005) and P3HT (Oosterhout et al. 2009) but it had problems of very low absorption and poor solubility in the solvents involved (Beek, Wienk, and Janssen 2006), which limited further improvements. Possibly, other materials like CdTe, PbS, PbSe, CuInS 2 and CuInSe2 could offer better acceptor properties due to longer wavelength light absorption. However, attempts made in this direction showed that CdTe NCs could not be used in absence of a suitable polymer combination and CdTe/MEH-PPV gave efficiency of only 0.1% (Kumar and Nann2004). Hybrid solar cells using MEH-PPV/CdSexTe1-x tetrapods did show some promise as it demonstrated decrease in efficiency from 1.1% starting from CdSe to 0.003% with CdTe (Zhou et al. 2006) due to an insufficient free charge carrier generation (van Beek et al. 2006; Zhou et al. 2006). The devices using vertically aligned CdTe NRs with P3OT gave efficiency around 1% (Kang, Park, and Kim 2005). Still lower band gap material NCs such as PbS; PbSe; CuInS2 and CuInSe2 were attempted but only with poor performance (Arici, Sariciftci, and Meissner 2003). From these attempts it is concluded that low band gap material NC/polymer system do not result in efficient exciton dissociation. Hybrid solar cells using RF plasma synthesized Si NCs and P3HT reported efficiency over 1% by changing the NCs size from 2 to 20 nm. Devices employing 3–5 nm Si NCs showed an efficiency of 1.47% under standard illumination (Liu, Holman, and Kortshagen 2009). While attempting in situ synthesis of PbS NCs in MEH-PPV without optimizing size distribution and concentration, an efficiency of 1.1% was reported (Watt et al. 2005). Similarly, a direct synthesis of CdS NRs in P3HT, led to hybrid solar cells with efficiencies up to 2.9% (Liao, Chen, and Liu 2009). Successful fabrication of hybrid solar cells employing CuInS 2 NCs in fullerene derivatives as well as a blend of CuInS2 and p-type polymer—PEDOT: PSS was reported in which improved PV response with external quantum efficiencies up to 20% was measured (Arici, Sariciftci, and Meissner 2003). In addition, hybrid devices based on TiO 2 blend with MDMO-PPV were also realized while studying the photo-induced electron transfer and PV response of MDMO-PPV: TiO2 bulk hetero junctions, where blending MDMO-PPV with the precursor got converted subsequently into Ti (iv) isopropoxide via hydrolysis in air in the dark resulting in the formation of a TiO2 phase in the polymer film. Such a device exhibited an external quantum efficiency of 11% (Hal et al. 2003). A planar D-A type hetero junction, prepared by sequential
 spin coating of CdTe and CdSe NCs followed by sintering at 200°C to remove the solvent residues was reported for air stable solution grown all-inorganic solar cells giving PCE = 2.1% under standard illumination. By varying electrode material, higher efficiencies were achieved in sintered nanocrystal cells; for instance, a Ca top contact capped with Al gave efficiency of 2.9% (Gur et al. 2005). Attempting to realize MEH-PPV and PbS NCs blends based solar cells by studying the influence of the surfactants on the PV performance of the hybrid devices using PbS NPs turned out in extremely low efficiency devices (Zhang et al. 2005). In another study, hybrid solar cells were

fabricated using a blend of poly thiophene and ZnO NCs from solution. Thermal annealing of the spin coated films resulted in the efficiency falling in the range of 0.49–0.9% under standard illumination. Further investigations indicated that the charge carrier generation was not sufficient, as a part of P3HT was not in contact with ZnO besides the coarse morphology also limited the device performance (Beek, Wienk, and Janssen 2006). Very recently, silver NPs were coated onto silicon NWs using metal-assisted chemical etching and electro less deposition for their uses in tandem solar cells accompanied by enhanced performance. The optimized devices with best AgNP-decorated silicon NWs improved the conversion efficiency from 2.47 to 3.23% (Liu et al. 2013). Flexible hybrid solar cells were fabricated using ZnO NRs array in bulk hetero junction configuration involving a blend of P3HT and PCBM. The ZnO NRs array was grown on the ITOcoated PET substrate using solution process followed by spin coating of conjugated P3HT and fullerene PCBM at 400 rpm on top of the ZnO NRs. Silver top electrode was sputter deposited on the photoactive layer resulting in a flexible solar cell configuration of PET/ITO/ZnO/ZnONRs/P3HT: PCBM/Ag that exhibited a conversion efficiency of 1.78% at standard illumination. According to the authors, this was the first report of organic/inorganic hybrid solar cells based on bulk hetero junction solar cell using conjugated polymer/fullerene photoactive layer and ZnO NRs array on flexible transparent substrates (Tong et al. 2012). PERVOSKITE SOLAR CELLS Organo metal tri-halide lead compounds of perovskite structure are currently being explored for producing PV solar cells with efficiencies in excess of 15% and having processes adaptable for commercial production as the vapor-deposition involved in depositing the absorber material is quite compatible with the process of fabricating well known organic solar cells (Ball et al.2013; Boix et al. 2014; Kojima et al. 2009; Kim et al. 2012; Noh et al. 2013). These semiconducting materials, having the stoichiometry of (CH3NH3)PbX3, (where X represented iodine, bromine or chlorine species), were first used as light-absorbing material in DSSCs in 2009 by depositing them onto the surface of TiO2 NC thin films. Such a light absorbing layer generates photon induced bound electron-hole pairs, which subsequently release free electrons and holes that are transferred to different charge carrier transport layers (HTL/ETLs) namely—TiO2 for the electrons and spiro-OMeTAD for the holes, carrying the charge carriers to separate electrodes and producing photovoltage VOC. It is important to note that these perovskite devices with efficiencies in the range of 12–15% are fast emerging as good competitor to the mature thin film solar cells (Boix et al. 2014). In order to explore the full potentials of the perovskite solar cells, it is better to know how to use this wonder material in the optimal manner. For instance, perovskite can be considered either as light absorber or nanostructured photosensitizer. It is useful to examine these possibilities for PV applications as described below. In case of light absorber application, it is essential to ensure fast injection of charge carriers into the respective transport layers, except none within the absorber. For this, the light absorber material must be dispersed well within the mesoporous layer for having the largest interface area to generate sufficient JSC (Snaith 2010). Examining CH3NH3PbI3 from this angle, it is noted that its band gap is fairly close to the optimum one for PV conversion, ensuring a good light absorption even in thinner mesoporous layers. However, the energy required for separating the electrons

and holes from photo-induced excitons should be provided by the existing energy band offsets, which is finally reflected in the achievable value of VOC. For instance, the energy band offsets of ∼ 0.07 and ∼ 0.21 eV available at the perovskite—TiO2 and perovskite—spiro-OMeTAD interfaces, respectively, make the influence of structural defects and disorders in the absorber less significant because both the processes of charge carrier separation and transport occur at the interface or outside the absorber material (Nayak et al. 2012). In case of perovskite used as photosensitizer, the distribution of energetic states in TiO 2 and hole transport layers are likely to influence the charge carrier transport via splitting of the Fermi levels and the population of band-tails. For using these energetic conditions, nanostructured entities like TiO2 nanorods and nano sheets (Etgar et al. 2012; Kim et al. 2013; Qiu et al.2013) are useful for charge carrier separation as well as transport employing some appropriate HTL with proper band alignment and enhanced hole mobilities (Bi et al. 2013; Heo, et al, 2013). Based on these considerations, it is easy to estimate losses incurring in a solid-state system. For instance, in case

of

devices

having

structural

compositions

of

TiO 2/Sb2S3/CuSCN

and

TiO2/CH3NH3PbI3/spiro-OMeTAD, the energy level differences of 1.3 and 1.22 eV coming from the position of the absorber band gap and the offsets of electron and hole injections, should ideally be available as VOC but the reported values @ 1 Sun are only 0.6 and 0.9 V (Boix et al. 2012a; Kim et al. 2012), respectively, indicating losses of ∼0.7 and ∼0.3 V in the Sb2S3and perovskite systems, respectively. However, from this, it may be concluded that recombination in perovskite solar cells is lower than that in the Sb2S3 devices but this is not corroborated by carrier lifetime measurements (Bi et al. 2013; Boix et al.2012b), which needs additional experiments for settling this issue. It is also noted that though increasing the absorber thickness does increase light absorption but simultaneously reducing the EQE and JSC. These observations highlight the importance of the recombination losses. Therefore, even while considering enhancing V OC, the processes controlling the charge carrier recombination must be examined carefully to minimize them for increasing the overall efficiency of perovskite solar cells (Kim et al. 2012, Heo, Jun, and Park 2013). Considering the existing energetics in mesoporous Al2O3 in place of TiO2, the injection of electrons from the photo absorber is almost ruled out. Instead, the extracted electrons flow only within the absorber itself and this situation is quite similar to that of thin film solar cells. In case, the binding energy of the photo-induced electron-hole pairs is close to the thermal energy, charge carrier separation is possible within the absorber like that in silicon solar cells and this is helpful as it doe not require additional energy to drive the dissociated electrons and holes (Shah et al. 2004). It is also noted that the generated electron-hole pairs with 50 meV of binding energy make the charge separation easier within the absorber and this ease combined with long charge carrier diffusion lengths is possibly the basic cause of good performance of perovskite devices in thin film configuration (Shah et al. 2004; Tanaka et al. 2003). Electron and hole carrier diffusion lengths are estimated around 100 nm in the perovskite layers along with electron or hole transport layers justifying the promising performance achieved in relatively thicker (∼350 nm) layers of CH3NH3PbI3−xClx, in a planar device configuration where higher permittivity can possibly enhance the charge generation still further (Ball et al. 2013; Eperon et al. 2014a, 2014b, Xing et al. 2013).

The perovskite layers not only absorb photons but also transport both free charge carriers— electrons and holes, benefiting therefore from ambipolar conduction (Ball et al. 2013). Higher conductivities

have

been

reported

for

perovskite

materials

such

as

CsSnI 3 and

CH3NH3PbI3 (Chung et al. 2012; Stoumpos, Malliakas, and Kanatzidis 2013), however, improving the carrier diffusion lengths is a better way of improving device efficiency. A possible route could, therefore, be to replace the metal cations or the halide anions, as reported for CH3NH3PbI3 perovskite compounds, where replacing I or Pb modified their conductivities (OnodaYamamuro, Matsuo, and Suga 1992). Sn-based perovskites have particularly shown higher conductivities (Mitzi et al. 1995, 1994), although devices fabricated using these materials did not show PV conversion. While exploring newer classes of perovskite compounds, it is essential that the crystallizing natures of these compounds are not constrained as the crystallinity determines the distribution of energetic states and hence important for conductivity and charge separation. Besides solution based deposition processes, alternate method of co-evaporation may also be employed for absorber with advantages (Mitzi, Prikas, and Chondroudis 1999). Perovskites made from different deposition techniques must be compared against single crystals either grown from solution

or

physical

vapor

transport

(Baikie

et

al. 2013;

Stoumpos,

Malliakas,

and

Kanatzidis 2013) in order to ensure low defect densities and less energetic disorder. In a thin film absorber, the bulk recombination within the perovskite becomes more important in contrast to interfacial recombination in the sensitized configuration and therefore the reduction of the energetic defects that can act as recombination centers or traps in the material becomes necessary to minimize for considerable efficiency improvement in the thin film configuration. There are a number of measurement techniques available for determining the relevant parameters that can improve the device performance (Boix et al. 2014). While exploring the PV properties of these materials, classical DSSC structures were initially retained employing single step deposition of the absorber material into the mesoporous TiO2 base followed by spin coating of spiro-OMeTAD HTM layer producing the device efficiencies around 10% due to the existence of the sufficient charge carrier injections from the perovskite film into both TiO2 and HTM layers as confirmed by the independent optical measurements (Boix et al. 2014). While attempting to improve the cell performance further, modification of the single-step absorber deposition into sequential one, wherein PbI2 is instantaneously converted into CH3NH3PbI3 within the pores, succeeded in producing 15% devices (Bi et al. 2013; Boix et al. 2024). Using similar processing conditions, mesoporous ZrO2 and TiO2 based solar cells produced efficiency of 10.8% and 9.5%, respectively. The ZrO 2 devices exhibited higher VOC and longer electron lifetime than the TiO2cells. In another attempt, the processing temperature was reduced considerably by using an insulating Al2O3 scaffold in place of mesoporous TiO2, which resulted in relatively lesser efficient devices of 10.9%, partly because of the absence of electron injection across the perovskite-alumina interface due to large gap between Al2O3 conduction band and LUMO of the perovskite (Boix et al. 2014). However, the photo-induced absorption spectroscopic measurements did confirm the existence of sufficient electron transport within the absorber layer and the injection of holes into the spiro-OMeTAD film, which could account for the observed PV performance observed experimentally (Boix et al. 2014). Though, the absence of electron injection into alumina did reduce the efficiency, to certain extent, as already indicated, but it otherwise enhanced VOC in absence of any voltage drop due to the involvement of band tail

states (Nayak et al. 2012). Al2O3 devices had faster charge collection compared to TiO 2 devices as confirmed by photocurrent decay measurements (Lee et al. 2012). Similar observations were made in case of non-injecting ZrO2 scaffold based devices as well (Bi et al. 2013). It was, thus, possible to achieve the highest VOC = 1.3 V based on similar consideration, along with a higher band gap Br-analogue and N, N′-dialkyl perylene diimide as HTL (Boix et al. 2014; Edri et al. 2013) substantiating the underlying cause further. The role of the HTL material on the solar cell performance was studied by employing tri-iodide lead perovskite-sensitized solar cells, fabricated using three different types HTLs involving spiroOMeTAD, P3HT and DEH exhibiting 8.5, 4.5 and 1.6% efficiencies, respectively (Bi et al. 2013). In addition, photo-induced absorption spectroscopic measurements carried out on these devices confirmed the existence of efficient hole injection across the perovskite-HTL interface with electron lifetimes varying in the following order as spiro-OMeTAD > P3HT > DEH. The observed differences in device performance could thus be assigned to the material dependent charge carrier recombination characteristics involved therein accordingly (Bi et al. 2013; Boix et al. 2014; Heo, Jun, and Park 2013). It was thus concluded that out of several different types of HTLs examined, spiro-OMeTAD and poly-triarylamine PTAA based HTLs turned out to be better than the energetically equivalent thiophene derivatives confirming the formation of superior interfaces with the perovskite absorber (Boix et al. 2014). Having established the ambipolar free charge carrier transport properties of perovskite compounds, efforts were even made to have PV conversion without involving the HTl/ETLs in hybrid solar cells (Chung et al. 2012; Etgar et al. 2012; Lee et al. 2012) while availing the ease of their solution-processability in device fabrication (Ball et al. 2013; Chung et al. 2012; Etgar et al.2012; Im 2011; Kim et al. 2012; Kojima et al. 2009; Lee et al. 2012; Noh et al. 2013). In this context, CH3NH3PbI3-xClx films with bulk crystalline properties were formed over a mesoporous Al2O3 scaffold (Ball et al. 2013). While comparing the performance of planar perovskite p-in heterojunction cells without mesoporous layer with those employing a meso-super-structured configuration with the perovksite compound fully infiltrating Al 2O3 scaffold, the measured efficiencies improved from 4.9% to 12.3%. However, the planar configuration, having almost 100% internal quantum efficiencies, still offered a promise for realizing more efficient architectures (Ball et al. 2013; Boix et al. 2014) from production point of view. Chemical synthesis of precursors involved in preparing perovskite thin films for solar cell applications is briefly described here (Eperon et al. 2014a, 2014b; Boix et al. 2014). Methylamine iodide (MAI) is synthesized by dissolving 33 wt% methylamine in ethanol, with drop-by-drop addition of 57 wt% HI in H2O at normal room temperature resulting in a white powder, which is stored in vacuum after drying the solution at 100°C. Next, 3:1 molar ratio of MAI to PbCl 2 were dissolved in one of the solvents like anhydrous DMF, DMSO or NMP having final concentrations 0.88 M lead chloride and 2.64 M MAI and stored in a nitrogen atmosphere. Subsequently, cells were fabricated on FTO glass substrates appropriately patterned for contacts by etching in 2 M HCl and zinc powder followed by thorough cleaning in 2% hallmanex detergent, acetone, propan-2-ol and oxygen plasma. A mildly acidic solution of titanium isopropoxide in ethanol was spin coated (@ 2000 rpm, 60 s), and annealed at 500 °C for 30 min for depositing hole blocking layer. For depositing thin film of perovskite layer, the non-stoichiometric precursor, as mentioned above, was spin-coated on the substrate in a nitrogen-filled glove box, at 2000 rpm for 45 s

followed by drying at room temperature in the glove box for 30 min and subsequent anneal at appropriate temperature and duration. For HTL, 0.79 M solution of spiro-OMeTAD in chlorobenzene, along with additives of lithium bis (tri-fluoro-methane-sulfonyl) imide and 4-tertbutylpyridine was spin-coated @ 2000 rpm and 60 s duration. The finished devices were left overnight in air for the spiro-OMeTAD to dope via oxidation. Finally, 60 nm thick gold electrodes were thermally evaporated to complete the devices structures (Eperon et al. 2014a, 2014b; Boix et al. 2014). In single-step perovskite film deposition onto mesoporous metal oxide films using a mixture of PbX2 and CH3NH3X in a common solvent, an uncontrolled precipitation takes place resulting in large morphological variations causing substantial spread in PV performance. Instead, a sequential deposition method, reported recently, is found better, wherein PbI2 in solution form is first introduced into a mesoporous TiO2 film followed by its conversion into the perovskite by introducing the solution of CH3NH3I resulting in instantaneous conversion with very uniform phase control over the perovskite morphology. Enhanced reproducibility combined with the realization of 15% efficient devices using this sequential deposition opens up the viability of producing the solution-processed PV cells of highest efficiencies and high stability for their commercial exploitation in near future (Burschka et al. 2013). In another study carried out for improving the fabrication process of perovskite solar cells, the photosensitive layers were prepared by co-evaporating methyl ammonium iodide and lead iodide in a PVD system, producing pure and smooth films with large crystallite domains followed by the deposition of polyTPD on one side and PCBM film on the other for effective blocking of holes and electrons, respectively. In addition, a thin layer of poly (3,4-ethylenedioxythiophene): poly (styrene-sulfonic acid) was used to smooth out the ITO surface of the transparent bottom electrode. The finished devices with the above described configuration exhibited VOC = 1.05 V, 2

JSC = 18.8 mA/cm and FF = 0.64, which produced 12.4% efficient devices, whereas larger area 2

cells (~1 cm ) showed a reduced efficiency of 10.9% (Malinkiewicz et al. 2014). For preparing flexible perovskite solar cells, the bottom contact of F-ITO was replaced by a layer of silver sandwiched between two layers of Al-doped ZnO. Because of smaller substrate sizes involved in this process, uniformity of the organic layer thickness was not that good and it produced 7% 2

efficient devices with JSC = 14.3 mA/cm ; VOC = 1.04 V and FF = 0.47. However, it was significantly important to note that the repeated bending of the above flexible solar cells produced had very little influence on the device’s performance. For example, 50 times repeated bending of the foils over a roll of 6 cm diameter produced very little efficiency reduction, demonstrating that the crystalline perovskites were compatible with the roll-to-roll processing of these solar cells (Boix et al. 2014; Malinkiewicz et al. 2014). Driven by this very fact that perovskite materials are ambipolar, solar cells with the configuration of TiO2/CH3NH3PbI3perovskite/Au were fabricated but these devices exhibited relatively lower PCE of 5.5%, which may be partly due to incomplete surface coverage (Etgar et al. 2012). However, these investigations did make it amply clear that the electrical transport in organic/inorganic metal halides constitutes of the ambipolar diffusion of electrons and holes and this is further supported by the recent observations where both electron and holes were extracted from a ∼350 nm thick layer of CH3NH3PbI3-XClX light absorber sandwiched between a compact TiO2 film and hole transporting spiro-OMeTAD (Ball et al.2013). A photocurrent density J SC ~

2

15 mA/cm was achieved in case of scaffold less structure or even higher with a minimum scaffold holding the perovskite thin film. Planar configurations of CH 3NH3PbI3 with other contacts 2

as PEDOT: PSS and various fullerene derivatives also displayed J SC> 10 mA/cm (Jeng et al. 2013). A recent work a planar heterojunction perovskite solar cell fabricated by vapor deposition produced 15% efficiency similar to the record of the mesoporous cell (Liu and Kelly 2013). This result confirms the existence of relatively long-range electron, hole transport in these materials. In one of the development of perovskite solar cells, a hybrid thin film configuration involving mixed perovskite of the type—CH3NH3PbI2Cl was sandwiched between selective contacts and these devices showed unprecedented reproducibility along with moderate efficiencies of 10.8% (Conings et al. 2014). A high quality polycrystalline perovskite thin film was prepared with full surface coverage, smaller surface roughness and grain sizes in micrometer range by using vapor assisted solution process. The solar cells prepared using such a perovskite material demonstrated device efficiency of 12.1% in planar heterojunction device configuration (Qi et al. 2014). Having realized the importance of low-temperature processing for making the process viable for economical commercial production, attempts were made to reduce the temperature from 500°C to 23 mA/cm and power conversion efficiencies of up to 14.2% (Eperon et al. 2014a). A new family of perovskite solar cells comprising of CH 3NH3PbI2Br absorber was synthesized and used along with one-dimensional TiO2 nanowire arrays exhibiting power conversion efficiency of 4.87% and VOC = 0.82 V, both exceeding the values reported in case of CH 3NH3PbI3 devices (Qiu et al. 2013). An electrodeposited ZnO compact layer followed by chemical bath deposition of ZnO nanorods enabled

lowering

of

processing

temperature

of

solution

based

flexible

perovskite

CH3NH3PbI3 solar cells wherein the conversion efficiencies of 8.90% were achieved on rigid substrates while the flexible ones yielded an efficiency of only 2.62% (Kumar et al. 2013). Perovskite

solar

cell

employing

~

0.6

μm

rutile

TiO 2 nanorod

sensitized

with

CH3NH3PbI3 perovskite nano dots exhibited an efficiency of 9.4%. Rutile nanorods were grown hydrothermally where lengths could be varied through the control of the reaction time. Infiltration of spiro-MeOTAD HTL into the perovskite-sensitized nanorod films demonstrated photocurrent 2

density, open circuit voltage and fill factor of 15.6 mA/cm , 955 mV, and 0.63, respectively. Cell performance was found depending on the nanorod length, where both photocurrent and voltage decreased with increasing nanorod lengths. A continuous drop of voltage with increasing nanorod length was assigned to the charge generation efficiency rather than recombination kinetics supported by impedance spectroscopic measurements displaying recombination independent of the nanorod length (Kim et al. 2013). Employing the technique of time-resolved spectroscopy to study the nature of charge carrier transport across the perovskite surfaces, cell configurations employing TiO 2 and Al2O3 thin films impregnated with lead iodide perovskite (CH3NH3PbI3) and organic HTLs were used and from there it was concluded that the charge separation involved electron transfer at both the junctions of TiO2 and HTL material and the charge carrier recombination was significantly slower in case of TiO2 films as compared to Al2O3 ones. It was further noted that lead halide perovskites constitute unique semiconductor materials allowing ultrafast transfer of electrons and holes at the two junctions simultaneously and transporting both types of charge carriers quite efficiently besides clearly showing superiority of TiO2 films and HTL materials (Marchioro et al. 2014).

Femtosecond transient optical spectroscopic measurements were carried out across the interfaces involving perovskite with ETL/HTL materials to estimate the long-range diffusion length from where it was estimated around 100 nm in solution processed CH3NH3PbI3 (Xing et al. 2013). In another study transient absorption and photoluminescence quenching measurements were carried out for estimating the electron and hole diffusion lengths, diffusion constants and life times in mixed halides—chloride + iodide type of perovskite compounds (Stranks et al. 2013). The diffusion length was estimated to be more than 1 micron in contrast to the diffusion length of 100 nm in triiodide perovskite compounds. These long carrier lifetimes as well as diffusion lengths together with exceptionally high luminescence yield are practically unknown in inorganic semiconductors and these properties are ideally suited for excellent photovoltaic conversion of solar radiation into electrical power (Boix et al. 2014). A vertical cavity optically pumped laser diode, successfully realized using photoluminescence quantum efficiencies exceeding 70% in mixed type perovskites like CH3NH3PbI3-XClX, free charge carrier formation within 1 ps and recombination occurring over a time scale of 10’s to 100’s of ns, which exemplifies another novel application of perovskite materials (Descler et al.2014). Successful

realization

of

5.5%

efficient

devices

having

a

configuration

of

TiO2/CH3NH3PbI3 perovskite/Au with no additional HTL after ascertaining the proper capping of the TiO2 film by the CH3NH3PbI3 confirmed the ambipolar nature of charge carrier transport of the perovskite layers. This is further supported by the recent reports where both electron and holes are extracted from a ∼350 nm thick layer of CH3NH3PbI3−xClx absorber sandwiched between a compact TiO2 film and hole transporting spiro-OMeTAD (Boix et al. 2014). Although, spiro-OMeTAD based HTL is quite often used in perovskite solar cells but it is a fairly expensive chemical and hence a number of lower cost replacements are explored. In a recent study, CuI was examined as an inorganic hole-conducting material as a possible alternative to spiro-OMeTAD. Although, the perovskite solar cells containing CuI HTL were less efficient in contrast to those using spiro-OMeTAD due to the presence of higher carrier recombination resulting in lower VOC, but the stability of these devices was worth noting a feature to be incorporated in developing low cost high-efficiency perovskite solar cells in future. Impedance spectroscopic measurements of these devices confirmed almost two orders of magnitude higher conductivity of CuI HTL as compared to spiro-OMeTAD, which, in principle, should lead to a higher FF improving the device performance, but still CuI-perovskite solar cells exhibited a power conversion efficiency of 6.0%, compared to 7.9% of the spiro-OMeTAD cells. This discrepancy was assigned to the lower value of VOC but it is anticipated that the CuI based solar cells could also be improved, in particular, by reducing the charge carrier recombination (Christians, Fung, and Kamat 2014). In an attempt to translate the fast growing perovskite solar cell fabrication technology into a commercially viable production process, some experiments were conducted recently employing a single thin film of the low-temperature solution-processed organometal trihalide perovskite absorber along with organic contacts that exhibited devices with efficiency of up to 10% on glass substrates and over 6% on flexible polymer substrates. Flexible perovskite solar cells were also developed successfully showing 7% efficient devices on PET substrates (Docampo et al. 2013). The success of overcoming most of the problems coming in the way of adopting the perovskite

technology, demonstrated the viability of these useful PV solar cells even on flexible processing platforms (Cristina et al. 2014). In a recent investigation it was shown that nanostructuring is perhaps not essential while depositing organometal halide perovskites thin films for solar cell applications. Instead, devices with a simple planar heterojunction configuration incorporating vapor-deposited perovskite as the absorbing layer could be made better than 15% efficient demonstrating that perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need of complex nanostructuring (Liu et al. 2013). Based on the experiences of various research groups involved in the development of perovskite solar cells, these devices should, in principle, be economical to fabricate using processes that are quite compatible with the existing organic solar-cell manufacturing technologies. Furthermore, since perovskites absorb light in a different part of the electromagnetic spectrum than that of Si, it is better to put the two types of devices together in tandem cell configuration. Here, a perovskite device at the top would absorb the higher-energy photons and the Si device below would take care of the lower-energy photons offering a more efficient combination better than the ones made from either silicon or perovskite alone (Boix et al. 2014). Commercial viability of perovskite solar cells would ultimately rely on improved costs per watt peak compared to the existing ones. With the compatibility of light-absorbers and electrode materials to low temperature processes including spray, blade coating and especially roll-to-roll printing, perovskite solar cells promise high efficiency, lightweight, cost-effective alternative with one third of energy payback period compared to that of silicon solar cells (Grätzel et al. 2012), appears as an extra advantage. Towards this goal, completely low temperature ( 300 mm diameter and ensuring a very large throughput, very high reproducibility and reliability with minimum defect density could finally bring down the cost of integrated circuits (ICs) to an affordable level. Therefore, it is quite likely to revise our possible options of cost reduction in case of DSSCs in a somewhat similar way. For example, for precise control of multiple thin film coatings on NCs using self-limiting nature of monolayer growth, being considered more critical to push the material performance, should not depend upon a process with inadequate precision (e.g. solution grown process alone) just because of cost consideration. Of course, the cost of a production type ALD with the option of fluidized bed CVD could be justified well in producing high efficiency all solid state DSSCs on a real mass scale production (Putkonen 2011). Monolayer, controlled ultra thin film depositions using some kind of self-limiting process is essential for this area of application and since ALD is known to possess this feature, it is easy to integrate into the fabrication process line appropriately. However, in case, it is possible to grow one monolayer in a self-limited manner either in a solution or from solution based processes, it will be the ideal situation from the cost reduction point of view that one would prefer to use in future. In this context, the process of NCs-colloidal solution-drop-drying mechanism reported sometime back is worth examining for its use with advantage. It was noted during evaporation of a NC-colloidal solution-drop on a surface in presence of an attractive particle-interface interaction, that an earlystage evaporation produced a two-dimensional solution of NCs at the liquid–air interface giving rise to NCs-islands that nucleated and grew into a self-assembled long-range ordered monolayers that were macroscopically compact. This drop-drying regime was noted to be simple, robust and scalable besides being independent of the substrate material and topography with a strong preference for monolayer film formation. This process was found useful for fabricating ultra thin films for a variety of devices like sensors, optical devices and magnetic storage media (Bigioni et al. 2005) and this could possibly be explored for preparing core-multiple shell and coaxial NRs/NTs structures for DSSC applications in a low cost manner. Room temperature ILs based electrolytes are currently emerging as useful electrolytes for DSSCs due to their negligible vapor pressure, high thermal stability, higher ionic conductivity and usable electrochemical properties (Lee et al. 2013b). Three types of systems including IL-based crystals, polymers and conductors were investigated in this context. A family of 5.87% efficient PV devices was reported using DMPII/KI/PEO combination of IL-crystals (Lee et al. 2013b). In IL-polymers category, electrolytes comprising of P-HI, NMB, GuNCS, HMII and AMII exhibited a cell efficiency of 6.95% (Lee et al. 2013b). From IL-conductor category, electrolytes using SD2, I2, Li[(CF3SO2)2 N], tBP and EMIB(CN)4, gave a cell efficiency of 2.85% (Lee et al. 2013b). Moreover, it is important to note that these electrolytes had superior long-term durability over the traditional organic solvent-based electrolytes. In addition, a redox system based on 2,2,6,6tetramethyl-1-piperidinyloxy (TEMPO) was synthesized (Lee et al. 2013b) offering significant features for substituting the widely used iodide/triiodide couples. This new IL-electrolyte with

TEMPO-imidazole complex can be considered as iodine free mediator system, providing dual channels for charge transportation within the DSSCs. Though Pt metal has been the most preferred choice for CEs because of its higher electrochemical activity but the associated higher cost and stability have been the major drawbacks for the delay in the large-scale commercialization of these devices (Calandra et al. 2010). It has also been commonly observed that the efficiency drops rapidly with time in the initial few days just after manufacturing due to dye molecule degradation and the electrolyte reaction with sealing material and CE. In this context, Pt CEs were attempted to be replaced by carbon-based materials including SWCNT, MWCNT and graphene based structures with improved device stability and not affecting energy conversion (Calandra et al. 2010). The conventional Pt bulk material was, therefore, replaced by Pt NPs, which improved the associated porosity and catalytic activity with their NC structure. In a subsequent attempt to replace Pt with a less costly metal replacement in this context, Ni-plated stainless steel counter electrodes were not only found helpful in protecting from the corrosion but also exhibited better photovoltaic performance in terms of lower charge transfer resistance compared to stainless steel counter electrodes (Zhu, Huang, and Ruan 2012). Functionalized graphene sheets (FGSs) were also examined as another useful alternative for DSSCs but it demonstrated cell efficiencies lower than those of Pt-based cells, which was ascribed to an order of magnitude larger charge-transfer resistance compared to that of Pt at no applied bias and approaching to that of Pt under biased conditions (Roy-Maythew, et al, 2010). While exploring tunability by inserting catalytic functional groups, it was noted that increasing the number of oxygen-containing functional groups as well as improving the porosity enhanced the associated catalytic activity of FGSs. In this context, FGSbased ink cast on a plastic substrate was employed in preparing CEs that eliminated the need for FTO substrate demonstrating lower efficiency than cells using Pt as CE (Roy-Mayhew et al. 2010). Tailoring the functionalization or morphology of the FGS electrodes could thus reduce charge-transfer resistance and facilitate the low-cost production (Roy-Mayhew et al. 2010) of catalytic and flexible CEs for DSSCs. Very recently, a multi-layer assembly of oppositely charged alternate layer of graphene oxide and poly (di-allyl-dimethyl-ammonium chloride) followed by an electrochemical reduction used in conjunction with Ru-complex C106TBA sensitizer exhibited conversion efficiencies of 9.5% and 7.6% in conjunction with low volatility and solvent free ionic liquid electrolytes, respectively (Xu et al. 2013). The new CEs exhibited good durability during 1000 hours exposure at 60°C under standard illumination during the accelerated tests (Xu et al. 2013). In light of the recent development of lead perovskite solar cells with efficiencies crossing >15% researchers are pursuing this route of low cost high efficiency PV solar cells with an aim of reaching 20% efficiency in very near future. Various aspects of perovskite solar cell development have been briefly reviewed in the main text. It is worth examining the promises foreseen to be translated into practical devices from these wonder materials. Considering the typical case of CH3NH3PbI3 thin film solar cell, a maximum of short circuit current 2

density of 27 mA/cm can be assumed as the onset of light absorption occurs around 800 nm 2

(Snaith 2010), which can be modified to 24 mA/cm assuming the availability of 90% of IPCE between 400 and 800 nm. Though the devices having absorber thickness in the range of 200– 300 nm show very high efficiencies, but the EQE slightly drops at wavelengths higher than

550 nm (Burschka et al. 2013) indicating a lack of absorption in this range but it can be taken care of by using plasmonic absorbers for better light harvesting (Atwater 2010; Oo et al. 2012; Wu et al. 2013c) in case of low absorber film thicknesses. It is also possible to suppress recombination to have VOC around 1.1 V for devices with TiO2 and spiro-OMeTAD. Taking into account these parameters, 20% efficient solar cells are possible to realize with a FF of 75%, which will ultimately supersede the a-Si cells and bringing them at par with polycrystalline Si solar cells (Boix et al. 2014). It is also noted that so far much of the development efforts have been centered around CH3NH3PbI3, however, there are other possible candidates from this family of materials that are suitable for solar cells (Boix et al. 2014) employing many other elements from the periodic table 2+

2+

2+

2+

2+

including Co , Fe , Mn , Pd , and Ge for their applications in PV devices and in that context the assessment based on Goldschmidt tolerance factor (Cheng et al. 2010; Goldschmidt 1937) becomes handy to identify the most stable structure. For instance, the replacement of Pb less toxic Sn is currently being examined, in this context, but its easy oxidation into Sn

2+

4+

with

state

starts behaving like a metal lowering its PV performance (Stoumpos, Malliakas, and Kanatzidis 2013). Other alternative that has been explored includes formamidinium cations. Similarly, employing longer chain organic components at the ‘A’ site, perovskite-type layer compounds comparable to the Ruddlesden-Popper series (Beznosikov et al. 2000) can be created for which theoretical calculations (Mosconi et al. 2013) are needed for identifying the appropriate families for their photovoltaic applications. Before concluding the current review, it is equally important to know about the evolving market potentials of this important PV technology of energy generation as analyzed by various agencies continuously engaged in watching the emerging market trends and the changing interest of the industries with time. For this purpose, the projections made by NanoMarkets and IDTechEx are taken as the basis for assessing the situation accordingly (Market-1 2013; NanoMarkets 2014; Ribeiro et al.2009). It is interesting to note that around 2010 the PV industries practically lost hope of entering into DSSCs area compared to the organic photovoltaic solar cells (OPVSCs), while noting their PCEs to be practically locked around 10%. In comparison, the OPVSCs with similar efficiencies were considered relatively more viable for large-scale commercial production (NanoMarkets2014). As a result of this general trend, the DSSCs were expected to wait for some more time before they came out of their R&D phase. However, the consistent efforts put in by various research teams globally, during last 5 years, have started changing the situation in this context after offering a number of technological breakthroughs to push the efficiency numbers considerably beyond 10% and fast approaching to 20%. The industry has also been equally involved in sorting out the lifetime-related issues that were a major problem faced. In the above-mentioned context, it is worth mentioning that in 2013 the EPFL, Switzerland, team could replace iodine-based liquid electrolyte with a solid-state perovskite material to achieve 15% efficient devices under standard test conditions. Similarly, the University of Basel (Swiss) and Merck explored to replace the iodine based liquid electrolyte with a Co-based system to increase the

stability

of

next-generation

DSSCs

while

retaining

their

cost-effectiveness

(NanoMarkets 2014). In yet another modification for cost reduction of DSSCs, electro-spun TiO2 nanofibers were prepared using the polymeric sol of a titanium precursor and Poly (vinyl

acetate) in acetic acid-dimethyl formamide mixture followed by 500°C thermal anneal. The continuous TiO2 nanofibers were further crushed into nanorice grains using grinding. The cells fabricated using these nanorice grains showed superior performance (Hamadanian and Jabbari 2012). Despite the fact that PV industries have been facing financial difficulties, the successful realization of >15% efficient DSSCs is being considered significant from its emerging markets point of view in the near future (NanoMarkets 2014). The DSSC manufacturers are, consequently, targeting on the economically viable and fast growing off-grid applications including building-integrated photovoltaic (BIPV) and low-light driven solutions for consumer electronics. Keeping these observations in view, the IDTechEx examined the growth of DSSCs market in the emerging areas of applications including indoor/outdoor posters/advertising/awnings; smart labels, portable and disposable electronics; electronics in apparel and emergency, military mobile devices; wireless sensors/actuators and automotive electronics; before projecting an estimate of $130 million market, in this context, by 2023 (Market-1 2013). Besides, there are other areas like bus shelters, steel roofing, facades, semi-transparent windows and many more, where the development work is progressing fast. However, the automotive sector market is rather picking up very slow, as it requires more stringent control on lifetime and efficiency performances, leading to just a few million dollars in market value by 2020 (Market-1 2013). The success of DSSCs in BIPV market depends mainly upon the availability of large-volume production of DSSC-modules with lifetimes of around 20 years—a real industry challenge. The industry is, however, investing in solving the scale-up issues for enhancing the production capacity adequate for meeting large-volume requirement of the BIPV market within the next three-five years. By that time, the relevant technology will mature and the stability and lifetimerelated issues that are still posing problems today should be resolved at the R&D level (NanoMarkets 2014). Applications of DSSCs in energy-harvesting is another fast growing area including applications like power supplies for sensors involving temperature, humidity, CO 2 concentration, and many more; remote control units, and charging devices. However, power supplies for information management systems in smart homes and large warehouses are long-term requirements (NanoMarkets 2014). Without introducing considerable improvements in terms of long-term stability of DSSCs, on-grid and utility-scale type energy generations are currently not considered viable. But the changes are certainly anticipated in near future as the developments succeed in improving the long-term stability under outdoor conditions. Rapid growth of indoor applications of DSSCs is another potential market of energy harvesting (NanoMarkets 2014). There are still a number of important issues that need to be sorted out, such as, photodegradation upon UV exposure and poor absorption in IR part of the spectrum besides taking care of hazardous volatile organic solvents escaping from the liquid electrolyte. However, the current developments of replacing liquid electrolyte with conducting polymers or ionic liquids will not only resolve the liquid electrolyte leakage and corrosion but even scale-up issues favorably. CONCLUSIONS Starting with bulk/porous thin films and liquid electrolytes, today, multilayered QDs, NTS, NRs and nano multipods are being explored in all solid-state DSSCs that are getting ready for their

mass scale production with efficiencies in excess of 15%. The primary reason for an early success of realizing liquid electrolyte based DSSCs, exhibiting such high efficiencies within a short span of time since their introduction, lies in the higher values of ionic mobility. However, it took some time in replacing the liquid with solid-state electrolyte, as it involved a systemetic search for the right composition with desired charge carrier transport properties. With the existing experience of organic and inorganic semiconducting NCs, it is anticipated to have something better than iodine redox couple by combining ionic liquids and QDs in very near future. Similarly, replacing Pt with carbon nanostructured species like nanofibers, NTs, fullerenes, ‘Bucky Papers’ and functionalized graphene sheets, is emerging as a viable alternative with added feature of cost reduction. Using NCs in place of mesoporous TiO 2 is found superior in controlling the porosity of the thin film. Once high conductivity graphene based materials are available in plenty at affordable cost, the dependence on SnO2 and ITO will be over as proven in early experiments mentioned in the main text. It is clearly foreseen on the basis of the current status of technology briefly discussed in this review that all solid state DSSCs based PV technology is certainly getting poised for entering into the industrial production phase after overcoming the major barrier of 10% efficiency with the introduction of several breakthroughs as discussed earlier. Acknowledgments The author gratefully acknowledges the support of the Confederation of Indian Industry (CII) and the Center of Excellence in Nanotechnology (COENT), CII Western Region, Ahmedabad, Gujarat, India. The challenging environment, created and duly supported by Mr. Anjan Das, Mr. GK Moinudeen, and the colleagues at the COENT, was especially enjoyed by the author while studying the current development of low cost solar cells and the help received is sincerely acknowledged. In connection with preparing the present manuscript, uses of various concepts developed over past few decades for improving the fabrication technology of photovoltaic solar cells, which are elaborately discussed in various excellent review articles and especially the present status of the device-related developments reported in recent publications, are sincerely acknowledged. The references provided here may not be that exhaustive as in most review papers mentioned above, but the contributions made by numerous authors in this area are all duly acknowledged, directly or indirectly, through the references currently included in the text as well as those mentioned in the referred review articles. References 

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