Recent Advanced Materials for Mesoporous Sensitized Solar Cells

Materials and processes for energy: communicating current research and technological developments (A. Méndez-Vilas, Ed.) ____________________________________________________________________________________________________

Recent Advanced Materials for Mesoporous Sensitized Solar Cells Getachew Alemu, Kun Cao, Mingkui Wang*, Yan Shen Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Road, 430074 Wuhan, P. R. China Email: [email protected] Numerous materials systems for dye-sensitized nanocrystalline solar cells (DSC) have been actively investigating since the seminal demonstrations by Grätzel in 1991. In this review we introduce current advancement in DSC with respect to renewable energy production, including components such as various nanocrystalline structures, suitable sensitizers, and convenient mediators. In this review we describe current advancement of DSC and improvement of component with various nanocrystalline structures, nanoparticle, one dimensional, and three dimensional to optimize conversion efficiencies. Keywords: Dye-sensitized solar cells, Sensitizers, Nanocrystalline,

1. Introduction Most of our electricity comes from power stations that use fossil fuels like coal and oil. The development of carbon-free sources of energy with affordable cost is one of the major scientific research focus and critical challenges of this century. [1, 2] Recent several attentions have been paid on alternative energy opportunities aiming for the climate change management, specially lowering the carbon intensity to stabilize atmospheric concentrations of greenhouse gases. [3, 4] Sustainable energy is usually known as the sustainable provision of energy that meets the needs of the present without compromising the ability of future generations to meet their needs. [4-5] there are several technologies that promote sustainable energy, including renewable energy sources, such as hydroelectricity, solar energy, wind energy, and so on. [6-8] . Among various type energy alternatives, solar energy is promising to satisfy the energy need of the world. The supply of energy from the sun is extremely high, that is about 3×1024 J per h, or about 105 times more than that the global population currently consumes. Photovoltaics (PV) is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect [8, 9]. Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material. Materials presently used for photovoltaic’s include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide. The photovoltaic effect refers to photons of light exciting electrons into a higher state of energy, allowing them to act as charge carriers for an electric current. The photovoltaic effect was first observed by Alexander-Edmond Becquerel in 1839 [10, 11]. Solar cells and modules based on crystalline and polycrystalline silicon wafers, the representatives of the so-called first generation of solar cells with the typical power conversion efficiency of up to 20 %, dominate the photovoltaic today and demonstrate light growth rates in the entire energy sector. Using sunlight as a renewable energy source has great potential. However, presently photovoltaic cells contribute only 0.04% to the world energy supply, leaving room for improvement of existing technologies and the development of new systems. Although inorganic solar cells govern the solar cell market, the manufacturing processes often involve costly, high vacuum, and numerous lithographic steps, resulting in a high production cost and high energy consumption. The second generation of photovoltaic cells is based on the use of semiconductors thin-film deposits produced generally using chemical vapor deposition techniques, typically plasma enhanced (PE-CVD). Third generation contains a series of new devices based on innovative technologies. These new devices include quantum dot cells, tandem/multifunction cells, intermediate band solar cells, hot-carrier cells and nonsemiconductor cells (polymer solar cells, dye-sensitized nanocrystalline solar cells, organic photovoltaics). It has been estimated that third generation PV technologies will achieve higher efficiencies and lower costs than first or second generation. Third generation photovoltaic cells are solar cells that are potentially able to overcome the Shockley-Queisser limit of 31-41% power efficiency for single band-gap solar cells. Among different type of solar cells, dye-sensitized solar cells (DSCs) have attracted much attention due to low cost, easy production, flexibility and transparency. [8, 9, 12]

2. Working principle and key components of DSCs 2.1. DSC working principle DSC has attracted much attention due to environmental friendliness and low cost of production since 1991. [13] The basic operating principle of DSC consists of light absorption by adsorbed dyes, charge separation and collection among

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the dye-sensitized nanocrystalline semiconductor. Charge separation occurs at the interfaces between sensitizer, semiconductor and electrolyte. The key components of DSCs are semiconductor oxide (such as TiO2, ZnO), the sensitizer (such as natural organic, metallic, and porphyrin), the electrolyte and the counter electrode. These components are well configuration as follow in Figure 1. Under illumination, sensitizer (S) absorbs a photon which leads to excited sensitizer state S*, followed by electron injection into the conduction band of the semiconductor. [14-15] This takes the sensitizer to an oxidized state S+. The original state of the dye can be restored by the electron donation from the electrolyte containing a redox couple. For example, iodide/triiodide couple is the preferred and effective redox couple, which is widely used in DSCs. The sensitizer is regenerated by iodide, which itself gets regenerated by the reduction of triiodide at the counter electrode. 2.2 The photovoltaic parameters The overall photon to energy conversion efficiency η, is a product of short-circuit current density (JSC), open-circuit photo voltage (VOC) and the fill factor (FF), divided by Pin (the total solar power incident on the cell) at air mass (1.5AM) [16].

η=

J sc × Voc × FF Pin

(1)

The precise JSC value produced by solar light can be derived by integrating to incident Photon conversion efficiency (IPCE) spectra over the spectral distribution of the standard AM 1.5 solar photon flux (JSC) represented

J SC =

 eIPCE (λ ) Is (λ )d λ

(2)

where e is the elementary charge, λ being the wavelength. In practically, it is important to perform this calculation for the confirmation of the JSC values determined with a solar simulator.

Figure 1. Schematic diagram operation principle of dye-sensitized cell

A common function of solar cells is convert light energy into electrical potential energy that can be supplied to an external electric circuit connected to the cell. The photovoltaic parameters are numerical description of conversion efficiency solar energy to electrical energy. A simple IPCE measurement involves measurement of the steady state iSC and the photon flux incident on the cell while scanning the light wavelength with a monochromatic [18, 19].

η IPCE (λ ) =

i SC (λ

)

q e φ DC (λ

)

(3)

where iSC is the steady state short circuit current density, ΦDC the steady state photon flux, λ the light wavelength, and qe the elementary charge. The ηIPCE of the DSC can be expressed as a product of the partial efficiencies of the light harvesting ηLH, electron injection ηINJ, and electron collection ηCOL processes (see Figure 2). η IPCE = η LH × η INJ × η COL (4)

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Figure 2. The photocurrent of the dye solar cell depends on the quantum efficiencies of light harvesting (ηLH), electron injection (ηINJ), and electron collection (ηCOL)

2.3 Key components in DSCs 2.3.1 Sensitizers In DSCs, sensitizer plays a key role in absorbing sunlight and transforming solar energy into electric energy. Sensitizers are classified into organic dyes and inorganic dyes. Inorganic dyes are mainly metal complex dyes such as complexes of Ruthenium, Osmium, Iridium, etc. Organic dyes mainly consist of fruit dyes and natural extract dyes. [19, 20] Various metal complexes and organic dyes have been synthesized and utilized as sensitizers. The highest efficiency of DSC sensitized by porphyrin compounds absorbed on nanocrystalline TiO2 reached 12.3%. [21] As for an efficient sensitizer, its high absorption range is the first requirement (see Figure 3). The (LUMO) of the dye should be localized near the anchoring group, while the HOMO of the sensitizer should lie below the energy level of the redox mediator. [19, 22]

Figure 3. Design principle of an organic dye for TiO2 photo anodes in DSC (from ref 23)

2.3.2 Nanostructure materials Over the past decades, various structured materials such as nanoparticles, nanowires, nanotubes, nanobelts, and assembling oxide aggregates [24, 25] have got much attention and intensively studied their application on optoelectronic, photovoltaic, photo-catalytic, and sensing devices. [26, 27] Among the unique properties of nanomaterials, the movement of electrons or holes in semiconductors is primarily governed by materials’ size and geometry. [28, 29] There are commonly three major ways of application of nanostructures for the design of solar energy conversion devices, including photosynthesis with donor-acceptor molecular assemblies and clusters, semiconductor assisted photo-catalysis to produce fuels, nanostructure semiconductor based solar cells. [29] Nanoparticles: One of the advantages of nanoparticles is their large surface area; however there are existence limitations in interfacial charge recombination between the photos generated electrons and the positive species in electrolytes (see Figure 4). In 1991 Grätzel et al reported an impressive breakthrough in power conversion efficiency of DSCs (about 7.1-7.9%), by using semiconductor films of nanometer-sized TiO2 particles sensitized in combination with a trimetric ruthenium complex [25]. The superiority of nanoparticles in creating large surface was demonstrated by a comparison between a flat film and a 10 μm-thick film that consisted of nanoparticles with an average size of about 20 nm. The mesoporous film made with nanoparticles presents a high porosity of 50-65%, and thus, increases largely in the surface area (almost 2000-fold compared to the flat counterpart). In addition to the film constructed with nanoparticles offering a large internal surface, on the material side, the anatase TiO2 with exposed (101) planes is also a key factor by reason of a good connection between the TiO2 and ruthenium-based dye molecules, which enables (1) the

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dye molecules to form high density monolayer chemisorptions on the nanoparticle surface, and (2) the electrons in dye molecules to inject into semiconductor with highly efficient. [28, 30]Also ZnO nanoparticles for application in DSCs have been extensively studied, partially due to the direct availability of porous structures with assembled nanoparticles and the simplicity of synthesis of nanoparticles via chemically based solution methods. Due to the mechanism of charge transfer between the dye and semiconductor, the strategy for studying DSCs with nanoparticulate ZnO films is almost the same as that adopted for nanocrystalline TiO2. It can be somewhat regarded as a straight implantation of TiO2-based Grätzel-type solar-cell technology, in which the TiO2 nanoparticles are replaced by ZnO nanoparticles while the dye. [28, 31]

Figure 4. Strategies to employ nanostructure assemblies for light energy conversion (from ref 1)

One-dimensional (1D) nanostructures: Recent advanced 1D nanostructure for DSCs are nanowires, [35] nanotubes, [36] nanocones, [37] and nanofibers [38]. They can be prepared by a variety of bottom-up and top-down approaches, such as CVD, solution chemistry, photo- and electron-beam lithography, nanoimprinting, etc. TiO2 and ZnO nanowires: One-dimensional nanomaterials can provide direct pathways for electron transport which can reduce recombination reaction. It has been reported that the electron transport in photo electrode comprised of single crystal nanowires is even 100 times faster than that in nanoparticle film (Figure 5). [34] Hydrothermal growth has been reported as a novel method for synthesis of TiO2 nanowire array on FTO glass substrate。Structural characterization demonstrate that these nanowires were rutile phase, and the growth was along [001] orientation with exposed (110) crystal plane. The ZnO nanowires were also emphasized to be superior to nanoparticles in electron injection kinetics. At a full sun intensity of 100±3 mW cm-2, the highest-surface-area devices with ZnO nanowire arrays were characterized by Jsc of 5.3-5.85mA cm-2, Voc of 610-710mV, FF of 0.36-0.38, [28] and overall conversion efficiencies of 1.2-1.5%。A further measurement on the time constants of electron injection shows a fast process for nanowires (about 3ps) (shown fig 5a). The value is about 200ps for the sample of nanoparticles. [29] The faster response of ZnO nanowires was ascribed to the 95% exposed (100) crystal plane of the nanowires, which was more favorable to yielding chemical bonding with the dye molecules in comparison to the nanoparticles. For all these merits in geometry and electrical property, ZnO nanowires have been suggested to be a promising photo electrode material which provides direct pathways for a rapid electron transport. The dependence of photocurrent density on the measured potential was investigated; by TiO2 nanowires reveal a saturated photocurrent at the bias of approximately -0.25 V under AM 1.5 illuminations. [29,31] Which Compared to 0.5-1 V of nanoparticle electrode to completely separation of photo generated electron-hole pairs, this indicated that nanowires possessed a low series resistance and had more effective capability of facilely separating the photo generated charges (shown fig 5b).[29, 31]

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Figure 5. (a) Schematic illustration of the source material preparation and the horizontal furnace system for the synthesis of ultralong ZnO NW arrays. [Ref, 23] (b) DSCs with TiO2 nanowire. [Ref, 22]

TiO2 and ZnO Nanotubes: Nanotubes are another one-dimensional nanostructure which gives surface area larger than that of nanowires or nanorods (Figure 6). [31] The fabrication involves a simple anodic oxidation of titanium foil in a fluoride-based solution at a constant potential. [29] The TiO2 nanotubes formed on the supporting titanium foils typically possess inner diameter in the 20-30 nm range, wall thickness in the order of ten several nanometers, and length that ranges from several tens micrometers to even millimeter, predominantly depending on the composition of electrolyte, applied bias, and growth time. [32, 35] An efficiency of 2.96% was achieved on the bamboo-type nanotubes that were grown under AV condition with a sequence of 1 min at 120 V and 5 min at 40 V for12 h, whereas the efficiency was only 1.90% for the smooth walled nanotubes with a same length. This result well demonstrates that the bamboo-type nanotubes might receive additional surface area for catching more dye molecules. An overall conversion efficiency of 2.3% has been reported for DSCs with ZnO nanotube arrays possessing a nanotube diameter of 500 nm and a density of 5.4×106 per square centimeter. [31]. However, it yields a relatively low conversion efficiency of 1.6%, primarily due to the modest roughness factor of commercial membranes. [29]

Figure 6. Front side illumination, integration of transparent nanotube array architecture into dye solar cell structure; (b) Schematic illustration of the procedure for fabricating crystallized TNT/FTO glass films. [From Ref. 31]

TiO2 and ZnO nanofibers: In addition to nanotubes and nanowiers, simple, low cost 1D nanofiber consists of polymers, ceramics and composites. [36] The nanofibers have high specific surface areas (102-103 m2 g-1) and larger pore sizes, [37, 38] which might solve the limitations of nanoparticle film-based DSCs due to their disordered geometrical structures and interfacial interference in electron transport. The nanofiber provides direct pathways for electron transport, giving larger electron diffusion length, and, thus improves the energy conversion efficiencies in DSCs.Various electro spun TiO2 and ZnO nanofibers have been used in DSCs. Recent report highest efficiency for electro-spun-based DSCs has reached 10.3%. [31] Son et al reported electro-spun TiO2 single-crystalline nanorods which were composed of nanofibrils with an islands-in-a-sea morphology (provided efficient photocurrent generation in a quasi-solid-state DSC, overall conversion efficiency of 6.2% under AM 1.5G illumination. The DSCs based on these ZnO nanofiber mats exhibited a conversion efficiency of 1.34% under 100 mW cm-2 illuminations. [31]. For example, Fujihara et al. reported a solar cell with two TiO2 layers: a ground electro spun TiO2 nanoparticle layer on a glass plate and then a TiO2 nanorod layer. It was found that the devices with a combination of electro-spun nanorods and nanoparticles showed improved conversion efficiencies over the entirely nanorod devices. Chuang et al also suggested that nanofiber-modified nanoparticles are very promising materials for the electrode structure of DSCs. In their work, TiO2 nanofibers were fabricated directly onto thick nanoparticle electrodes by using electro spinning and sol–gel techniques. The DSCs comprised of a nanoparticle/nanofiber electrode showed an incident photon-to-current conversion efficiency (IPCE) of 85% at 540 nm with conversion efficiencies of 8.14% and 10.3% for areas of 0.25 and 0.052 cm2, respectively. Here it is noted that the conversion efficiency of 10.3% is the highest reported efficiency for electro spun

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DSCs. Furthermore, an aligned or crossed architecture was fabricated with electro-spun method in order to enhance the exaction dissociation area and the charge conduction on the hybrid solar cell. Therefore, the photo-induced current (1.28 mA cm-2) and power conversion efficiency of the cells could be significantly improved by at least 50% under 1 sun conditions depending on the degree of aligning fibers. Yan et al [37, 38] also reported hybrid solar cells based on ordered electro-spun TiO2 and ZnO nanofiber arrays with conjugated poly (3-hexylthiophene) (P3HT), and they found that the organic-inorganic hybrid cells with 3 cross-aligned layers showed enhanced photovoltaic performance. DSCs with vertical nanofibers of electros-pun TiO2 as a photo electrode were recently reported, showing that Fig. 7f. The aligned nanofibrous TiO2 ribbons were firstly produced by electron spinning, and then erected to vertical nanofibers after post-treatment of rolling up, stacking and cutting. The conversion efficiency, short circuit current, and open circuit voltage of the resultant DSC were measured as 2.87%, 5.71 mA cm-2, and 0.782 V, respectively. [31]

Figure 7. (a) Schematic illustration of a hybrid solar cell introduced with aligned TiO2 or ZnO nanofibers via electro spinning.).[From Ref 23]

Three-dimensional (3D) nanoparticles materials: Recently, besides nanoparticles and one dimensional nanostructure, three-dimensional hierarchical nanostructures have got much attention due to their large surface area, effective light harvesting, charge transport and charge collection. There are various of 3D nanostructures such as ZnO dendritic, ZnO nanowires, ZnO moonflowers, ZnO aggregates, nanotetrapods ,branched ZnO nanowires, ZnO or TiO2 nanoforests,TiO2 nanotubes on titanium mesh, nanoporous TiO2 spheres, and hollowTiO2 hemispheres. [25, 26, 29] The current report demonstrates that 3D nanostructure photo electrodes can indeed improve photovoltaic performance (see Figure 8). For example, an efficiency of 3.27% for a photo anode film consisting was reported for ZnO nanotetrapods based DSCs, [31] which were much higher than those obtained for ZnO nanowire/rod/tube arrays (1.2-2.0%). Particularly, DSCs based on ZnO or TiO2 nanoparticle aggregates exhibited very high conversion efficiency of 5.4% [31] and 10.5%, respectively, possibly due to vary the high surface area, strong light scattering and short diffusion distance.. Furthermore, recent investigations have demonstrated that hybrid structures of ZnO nanotetrapods/SnO2 nanoparticles, [31] hierarchical nanotubes or TiO2 nanodendrites/nanoparticles may be promising photo electrodes for high-efficiency DSCs. ZnO nanotetrapods are of benefit to DSCs owing to their large specific surface area in view of high roughness factor and good interconnection of the arms, offering multiple pathways for electron transport. A photo electrode film consisting of the ZnO nanotetrapods (approximately 31 µm in thickness) was reported with an overall conversion efficiency of 3.27%, which was generally higher than those obtained for ZnO nanowire array.

Figure 8. Design and principle of a three-dimensional DSC.(a) TiO2 NWs grow vertically on the fiber surface for 3D DSC; (b) detailed structure of the 3D DSC [Ref. 36]

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2.3.3 Electrolytes The electrolyte is one of critical components dye sensitized solar cells and its properties have direct impact on the conversion efficiency and stability of the solar cells. Subsequent electron injection depends on the reducing ability of the electrolyte. The recently used electrolytes in DSC can be divided into: liquid electrolyte, quasi solid state electrolyte and solid electrolyte [38]. Liquid electrolyte could be grouped organic solvent electrolyte and ionic liquid electrolyte according to the solvent used. Organic solvent electrolytes were widely used and studied in dye sensitized solar cells due to their low viscosity, fast ion diffusion, high efficiency, easy to be designed and high pervasion into nanocrystalline film electrode [20]. It is responsible for the inner charge carrier between electrodes. It regenerates continuously the dye at the photo electrode, with the charge collected at the CE. The best results have always been obtained with the triiodide/iodide (I-3/I-) redox couple in an organic matrix, generally acetonitrile. The most noteworthy of the non-traditional electrolytes are room temperature ionic liquids (RTILs), quasi-solid state and solid state. These electrolytes are progressively viscous enabling increased stability. They appear to solve problems such as dye desorption, solvent evaporation and sealing degradation, however, until now their performance has been consistently low. A more viscous electrolyte diminishes regular charge diffusion and, therefore, requires higher concentration of the redox couple to maintain conductivity.

3. Summary and outlook The DSC has unique characteristics such as high energy conversion efficiency. Recent reported efficiencies have obtained to be 12.30%. DSCs are the recent engineering technologies which give innovative solutions for this millennium in many areas such as energy production and scientific research of environmental protection and application. These technologies are open new opportunities to focus some of the world's most pressing environmental issues, which include global climate change, clean energy production, depletion of nonrenewable resources. The recent advancement on DSC solar Engineering technology promising hope to satisfy current and future need of sustainable environment and forward promising clean energy opportunities. Also it is promising solution to current energy problem because it is low production cost, environmental-friendly raw materials, stable performance at non-standard of temperature and good efficiency under standard conditions Acknowledgment Financial support from the Director Fund of the WNLO, the 973 Program of China (2013CB922104, 2011CBA00703), the NSFC (21103057, 21161160445, 20903030, 21173091), the Natural Science Foundation of Hubei Province (NO. 2011CDB04), the Fundamental Research Funds for the Central Universities (HUST: 2011TS021, 2011QN040, 2012YQ027), the CME with the Program of New Century Excellent Talents in University (NCET-10-0416) and Scientific Research Foundation for the Returned Overseas Chinese Scholars, is gratefully acknowledged. The authors thank the Analytical and Testing Centre at the HUST for performing characterization of various samples.

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