Nanoscale COMMUNICATION
Cite this: Nanoscale, 2013, 5, 4144
Received 5th February 2013 Accepted 18th March 2013
Plasmonic organic photovoltaic devices with graphene based buffer layers for stability and efficiency enhancement Emmanuel Stratakis,*ab Minas M. Stylianakis,a Emmanuel Koudoumasa and Emmanuel Kymakis*a
DOI: 10.1039/c3nr00656e www.rsc.org/nanoscale
Enhancement of photoconversion efficiency (PCE) and stability in bulk heterojunction (BHJ) plasmonic organic photovoltaic devices (OPVs) incorporating graphene oxide (GO) thin films as the hole transport layer (HTL) and surfactant free Au nanoparticles (NPs) between the GO HTL and the photoactive layers is demonstrated. In particular the plasmonic GO-based devices exhibited a performance enhancement by 30% compared to the devices using the traditional PEDOT:PSS layer. Likewise, they preserved 50% of their initial PCE after 45 h of continuous illumination, contrary to the PEDOT:PSSbased ones that die after 20 h. The performance increase is attributed to the improved photocurrent and fill factor owing to the enhanced exciton generation rate due to NP-induced plasmon absorption enhancement. Besides this, the stability enhancement can be attributed to limited oxygen and/or indium diffusion from the indium tin oxide (ITO) electrode into the active layer. The industrial exploitation of composite GO/NPs as efficient buffer layers in OPVs is envisaged.
1
Introduction
Organic photovoltaics (OPVs) offer potential advantages over their traditional inorganic Si based counterparts, including fabrication on light weight, large area and exible substrates and use of low cost and low temperature roll-to-roll (R2R) solutionprocessing techniques.1,2 Recent advances in OPVs have utilized bulk heterojunction (BHJ) devices reaching power conversion efficiencies of up to 8%3 and most recently to a record of 9.2% using an inverted structure.4 In a typical BHJ, the active layer is formed by a donor polymer, and an acceptor material5 is a
Center of Materials Technology and Photonics & Electrical Engineering Department, School of Applied Technology, Technological Educational Institute (TEI) of Crete, Heraklion, 71004, Crete, Greece. E-mail: kymakis@staff.teicrete.gr; Tel: +30 2810379895
b
Institute of Electronic Structure and Laser (IESL), Foundation for Research and Technology-Hellas (FORTH) and Dept. of Materials Science and Technology, Univ. of Crete, Heraklion, 71110 Crete, Greece. E-mail:
[email protected]; Tel: +30 2810391274
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sandwiched between a transparent anode and a metal cathode. However it is found that a buffer layer between the transparent anode and the active material facilitates extraction of photogenerated holes and ensures that electrons ow into the opposite cathode. This buffer layer, called the hole transport layer (HTL), enhances the conversion efficiency dramatically; the poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been most commonly used as the HTL.6 PEDOT:PSS has a good solution processability, transparency and it can smooth the ITO electrode surface to enhance the device stability. Due to its high work function (5.0–5.2 eV), it forms an ohmic contact with the BHJ, improving the open-circuit voltage (VOC) and charge collecting ability of the OPV devices.7 However, there are several drawbacks leading to OPV failure, which are directly related to the PEDOT:PSS. The acidic nature of PEDOT:PSS (pH 1) corrodes both the ITO electrode8 and the processing equipment9 at elevated temperatures, and can introduce water into the active layer, degrading the performance and long-term stability of the OPV devices.10,11 In addition, spin coated PEDOT:PSS lm morphology and conductivity vary over different lm regions, leading to inhomogeneous charge extraction in some locations and dead spots in others.12,13 PEDOT:PSS is also hygroscopic and its conductivity signicantly changes in the presence of moisture.14,15 Furthermore, PEDOT:PSS degrades signicantly at temperatures greater than 250 C16 while PEDOT manufacturer additives have both unknown and oen deleterious effects.17 These issues clearly illustrate the need for a chemically stable and mechanically uniform material to be used as the HTL replacing the PEDOT:PSS in OPVs. As a result, several attempts at replacing the HTL have been made using metal oxides,18–20 which are generally more stable in an ambient environment. However, these materials are deposited using cost-intensive high vacuum techniques that are incompatible with low-cost solution-processable and R2R large area manufacturing of OPVs. Thus, it is obvious that the development of cost efficient and simply processable HTL materials compatible with OPV
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Communication materials and R2R techniques is demanded. For this purpose, thin lms of graphene oxide (GO)21 and reduced GO (rGO)22,23 produced by spin coating have been studied as promising alternatives to PEDOT:PSS, due to their optical transparency and mechanical exibility. Recently, plasmonic metallic NPs have been identied as a breakthrough route for efficiency enhancement of OPVs.24 Different strategies of incorporating plasmonic NPs for light trapping into either the active25,26 or the buffer layers27 or at various interfaces28 within the OPV cell architecture were employed. In particular, the incorporation of NPs within OPVs enhances incident light absorption through the excitation of localized surface plasmon resonances (LSPRs) in their vicinity as well as surface plasmon polaritons (SPPs) along the intervening surfaces.29 More recently, surfactant free Ag and Au NPs were placed via dispersion from solution between the PEDOT:PSS and the photoactive layer interface. An efficiency enhancement by 20% was obtained, mainly due to LSPR induced by the metallic NPs, which led to a noticeable enhancement of the photocurrent.30 The same approach was followed by our group, utilizing carbon nanotubes as the transparent electrode, instead of ITO.31 In this work, we combine for the rst time a GO based buffer layer with plasmonic OPVs and report on the facile incorporation of Au NPs onto the GO HTL of BHJ solar cells. It is shown that the synergy of plasmonic NPs with GO HTL leads to a signicant improvement of both the PCE and stability compared to commonly studied PEDOT:PSS based devices. The PCE increase is attributed to the enhancement of the incident light harvesting within the device due to the excitation of LSPR effects in the vicinity of NPs. Besides this, the stability improvement could be originated from the limited oxygen and/ or indium diffusion from the ITO electrode into the active layer.
2
Experimental
2.1
Films fabrication
Graphite oxide was synthesized by the modied Hummers method and exfoliated to give a brown dispersion of GO under ultrasonication.32 The resulting GO was negatively charged over a wide pH condition, as the GO sheet had chemical functional groups of carboxylic acids. GO solution was dropped in ethanol (0.5 mg ml1) at pH 3.3 aer an oxygen plasma treatment for 2 min in order to make the ITO surface hydrophilic. The GO solution was maintained for a waiting period of 2 min and was then spun at 3000 rpm for 30 s, followed by 1 h baking at 100 C inside a nitrogen-lled glove box. The thickness of the lms was analogous to the number of spinning repetitions. By controlling the number of spin-coatings, GO layers with thickness ranging from 2 nm (1 coating) to 5.4 nm (4 coatings) can be obtained. Several lms were fabricated with a thickness of 2.0, 2.6, 3.9 and 5.4 nm, as measured by surface prolometry. 2.2
Au NP generation
Plasmonic Au NPs were generated by ultrafast laser ablation of an Au metallic target (99.99%) immersed in ethanol. This
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Nanoscale physical synthetic method provides the possibility of generating a large variety of NPs that are free of both surface-active substances and counter-ions.33 Two steps were followed for the NP generation;34 in the rst step high laser energies were used for the production of colloids of wide size distribution. In the second step the initial colloidal solution was illuminated using a focused femtosecond laser beam. During this process, the fragmented species recoalesce to form less dispersed and much more stable NPs in the solution.34 The NP sizes ranged from 12 to 27 nm with 85% of the NPs exhibiting sizes between 15 nm and 18 nm. 2.3
Device fabrication
The OPV devices were prepared using blends of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl) propyl-1-phenyl(6,6)C61 (PCBM) as active layers, GO as HTLs and Au NPs between those layers. The performance was optimized by changing the GO lm thickness. OPVs with PEDOT:PSS (VP AI 4083) HTLs, deposited by spin coating at 4000 rpm for 30 s followed by a 3 h annealing at 100 C inside a nitrogen lled glove box, were used as the control devices. For the plasmonic devices, Au NP solution (30% volume ratio) in ethanol was spincoated on the freshly prepared GO layer. The photoactive layer, consisting of P3HT (Rieke Metals) and PCBM (Nano-C) dissolved in 1,2-o-dichlorobenzene (DCB), was spin coated on top of the HTL layer. The wet lm was then transferred to a Petri dish and subjected to solvent annealing.35 Aluminum cathodes were nally thermally evaporated through a shadow mask. A post fabrication annealing was performed at 150 C for 5 min in nitrogen. The uniformity and roughness of the lms were determined using a surface prolometer (Veeco) measuring the height prole of steps formed between covered and uncovered substrate areas. Surface resistivity was measured using the fourpoint probe technique (Ecopia HMS). 2.4
Device parameters
Current–voltage (I–V) measurements were performed at room temperature using an Agilent B1500A Semiconductor Device Analyzer. For photovoltaic characterization the devices were illuminated with 100 mW cm2 power intensity of white light using an Oriel solar simulator with an AM1.5 lter through the glass/ITO side. For the stability measurements, J–V data were collected at 30 min intervals under simulated AM1.5; the devices were continuously irradiated in the open circuit mode. All measurements were performed in air.
3
Results and discussion
3.1
PV properties
The structure of the ITO/GO/P3HT:PCBM/Al devices as well as the energy level diagrams of the different materials used in the fabrication are shown in Fig. 1. Negatively charged GO contains oxygen functional groups including carboxyl and epoxy ones which disrupt the sp2 conjugation of the hexagonal graphene lattice in the basal plane. Therefore, GO can be represented as an insulator with a
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Fig. 1 Device structure of an OPV incorporating a GO buffer layer and the chemical structure of the GO, and the schematic energy diagrams of the flat band conditions of OPVs with GO and PEDOT:PSS HTLs.
large band gap of 3.6 eV, which is also conrmed from optical measurements.21 The work function of GO determined from ultraviolet photoemission spectroscopy (UPS) measurements is 4.9 eV.36 The HOMO and the LUMO of the P3HT and the PCBM are electrochemically determined to be approximately 5.2 and 3.53 eV below the vacuum level respectively for the P3HT,37 and 6.1 and 3.75 eV for the PCBM38 respectively. Fig. 2 shows representative surface prolometry steps of GO lms prepared by spin coating on ITO substrates. It is observed that sequential repetitions of the spin-coating process led to different GO lm thicknesses. A series of four GO lms were prepared, the average thicknesses of which were measured to be 2.0, 2.6, 3.9 and 5.4 nm, while the corresponding rms roughness values were 0.9, 1.0, 0.8 and 1.2 nm, respectively. The values are very low compared to the 3.6 nm rms roughness of the bare glass/ITO substrate, indicating that the spin coating of the GO lms serves to planarize the anode surface. This may be due to the fact that the repetitive spin coating lls the voids in the GO lm and makes the surface smoother. The optical transmission of the different GO lms with various thicknesses spin coated on ITO/glass substrates together with their corresponding thickness and surface roughness are presented in Table 1. It can be seen that as the thickness of the GO lms increases, the transmittance slightly decreases. Notably, the 2.0 and 2.6 nm GO lms on ITO are highly transparent, and even more transparent than the PEDOT:PSS. GO lms were used as the HTL in P3HT:PCBM devices. The average short-circuit current density ( JSC), open-circuit voltage (VOC), ll factor (FF), and power conversion efficiency (PCE) values for each set of devices (over 4 devices were fabricated for each set) are summarized in Table 2. It can be clearly seen that
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Fig. 2 Surface profilometry steps of GO films of three different thicknesses spin coated on ITO substrates of approximately (a) 2.0 nm, (b) 3.9 nm, and (c) 5.4 nm.
Table 1 Physical characteristics of ITO, PEDOT:PSS and spin coated GO films with different thicknesses
Sample
Thickness (nm)
Transmittance at 500 nm (%)
Surface roughness (nm)
PEDOT:PSS GO GO GO GO
40.0 2.0 2.6 3.9 5.4
89.5 90.3 89.9 88.7 88.2
1.2 0.9 1.0 0.8 1.2
the performance of the GO based device depends strongly on the GO layer thickness. Fig. 3 shows typical illuminated J–V curves of devices with no HTLs or with either PEDOT:PSS or GO with the optimum thickness. The device without a HTL (ITO-only) shows a JSC of 6.5 mA cm2, a VOC of 0.42 V and a FF of 42.5%, resulting in a PCE of 1.16%. This poor performance is due to the large leakage current caused by the low work function of ITO and the direct contact between ITO and PCBM. For the optimization of the device, a traditional PEDOT:PSS lm is incorporated as the HTL; the device exhibits a JSC of 9.15 mA cm2, a VOC of 0.6 V, and a FF of 51.7%, resulting in a PCE of 2.84%. The optimal
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Table 2 Summary of photovoltaic parameters of OPV devices containing different HTLs
ITO only PEDOT:PSS GO (2.0 nm) GO (2.6 nm) GO (3.9 nm) GO (5.4 nm)
Jsc (mA cm2)
Voc(V)
FF (%)
h (%)
6.51 9.15 6.65 9.04 9.59 8.02
0.42 0.60 0.43 0.54 0.60 0.60
42.5 51.7 36.2 43.8 50.7 44.1
1.16 2.86 1.04 2.09 2.90 2.12
Fig. 3 J–V characteristics of photovoltaic devices with no hole transport layer, with a 40 nm PEDOT:PSS layer, and a 3.9 nm GO film.
thickness for the device using GO as the HTL was found to be 3.9 nm. This device shows a PCE of 2.90%, a JSC of 9.59 mA cm2, a VOC of 0.6 V, and a ll factor FF of 50.7%. These results show that GO can effectively substitute PEDOT:PSS as the HTL between ITO and the photoactive layer with comparable device performance. For the GO lm with 2.0 nm thickness, its performance is comparable to the device without a HTL with an even poorer FF, indicating that the ITO substrate is only partially covered with the GO forming a discontinuous lm. Therefore, the GO acts as only a partial HTL to the device, resulting in higher recombination of electrons with holes at the ITO, reducing the hole transport. This nding is in disparity with previous reports, where the GO thin lms with a thickness of 2 nm showed the highest efficiency,21 mainly due to the different lateral dimensions of GO akes employed. By increasing the thickness of GO, the lm becomes continuous and a complete coverage of the ITO is achieved for a thickness of 3.9 nm, which provides the optimized conditions for the highest performance devices. Further increase of the GO thickness to 5.4 nm creates opposing effects on the device performance due to the decrease in transparency, and increased series resistance. It should be noted that the optimum GO lm (3.9 nm) exhibits an average transmittance at 350–650 nm of 88.7%, compared with the 89.5% of the PEDOT:PSS, but still its photocurrent is larger by 10%. This can be explained by the fact that the drawback in
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transmittance is tackled by the two-dimensional (2D) nature of the GO lm, which favours better hole charge transport, as observed in the case of the devices with carbon nanotubes.39 The 2D GO lm provides sufficient exposure of the photoactive layer to the illumination to efficiently create excitons at the P3HT/PCBM interface and collect holes through the GO layer, in contrast to the planar conguration of the PEDOT:PSS. Also, this explains the low performance of the thinnest GO lm with respect to the 3.9 nm optimum lm, which is produced by multiple spinning. The GO based device slightly outperforms the PEDOT:PSS, primarily as a result of increased JSC from 9.09 to 9.54 mA cm2, despite a small reduction in FF from 51.7 to 50.7%. The increase in the JSC can be attributed to the hole transport enhancement due to the 2D nature of the HTL lm. The slight decrease in the FF can be attributed to the insulating nature of the GO, which increases the internal resistance of the device, unless its thickness is kept at the minimum, ideally down to one monolayer. However, a full coverage of the ITO area is not possible. In order to validate this presumption, hole-only devices with the structure ITO/PEDOT:PSS/P3HT:PCBM/Au were fabricated for all the devices. The hole mobility was estimated from the J–V characteristics in the low voltage region, where the current is described by the Mott–Gurney square law: JSCLC ¼ (9/8)3o3rmh(V2/L3); 3o3r is the permittivity of the active layer, mh is the hole mobility, and L is the lm thickness.40 It should be noted that the obtained hole mobilities are not for the active layer, but for the complete device including the active and the HTLs. The hole mobilities of the devices prepared with PEDOT:PSS and GO as the HTLs are calculated from the currents in the square law region to be 1.2 103 cm2 V1 s1 and 1.38 103 cm2 V1 s1 respectively. To further expand the potential of GO as the HTL, the lifetime stability of the device when exposed to continuous solar illumination in air in comparison with the device with the traditional PEDOT:PSS was investigated. OPV devices were fabricated without encapsulation, in order to allow water in the air to reach them. The normalized PCEs of a traditional PEDOT:PSS and a GO based OPV as a function of exposure time are shown in Fig. 4. In each point a complete J–V curve is obtained. Between successive measurements, devices were continuously irradiated in the open circuit mode. For the rst 20 h, an abrupt aging of the pristine cell was detected, caused essentially by a single-step exponential decay, while the cell dies aer 20 h. In contrast, on the same time scale, the degradation rate of the GO-based device is much slower, preserving more than 70% of its initial PCE for over 25 h, and more than 50% of its initial value aer 45 h. This result is due to the fact that PEDOT:PSS is spin coated from highly acidic suspension (pH 1), which erodes ITO and causes indium migration into the photoactive layers. Also, water molecules are easily penetrated into the hygroscopic PEDOT:PSS layer, resulting in degraded device performance.41 This argument is also supported by the fact that the device based on PEDOT:PSS, in which the HTL was exposed in air for 4 hours before the photoactive layer was spincast, exhibits an “S-shaped” J–V characteristic, where the GO-based device does not. This behaviour is suggested to be due to the water
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Fig. 4 PCE as a function of exposure solar irradiation time normalized to their initial values for PEDOT:PSS and GO based OPV devices.
adsorption onto the photoactive layer caused by the hygroscopic PEDOT:PSS resulting from the formation of a PSS-rich resistive layer at the lm surface,42 which is not present in the GO based device. Therefore, the GO functions as a more stable interfacial layer and also has a more efficient passivation property against oxygen and moisture compared with PEDOT:PSS.22 Furthermore, it is possible that the degradation effects are from the ITO electrode, due to its loss of conductive tin and workfunction reduction,43 to be compensated by an increase in the GO conductivity due to thermal annealing reduction, caused by the continuous solar irradiation of the device. The effect of plasmon enhanced absorption processes on the performance of polymer–fullerene photovoltaic devices was enhanced by the higher exciton dissociation rate due to improved absorption within the active layer. SEM images of the best performed GO HTL of 3.9 nm thickness, before and aer the deposition of Au NPs, are shown in Fig. 5a and b respectively. It can be clearly seen that the physical deposition of Au NPs on the top of the GO lm does not alter the GO morphology, and a smooth bilayer is formed. In this respect, there is no indication of possible doping of the GO by the NPs. Devices with the ITO/GO/AuNPs/P3HT:PCBM/Al structure were fabricated and characterized with respect to their performance under continuous illumination. Fig. 6 shows the J–V curves of the PEDOT:PSS, GO and GO/NP based devices. It can be clearly seen that the introduction of Au NPs as an interfacial layer induces a signicant improvement of both the device photocurrent and the ll factor. Indeed, Table 3 displays that the GO/NP based devices outperform the PEDOT:PSS and the GO based ones showing a JSC of 10.2 mA cm2, a VOC of 0.61 V and a FF of 54.2%, resulting in a PCE of 3.37%. In order to investigate the underlying mechanism responsible for the enhanced performance of the devices, we have measured the incident photon-to-electron conversion efficiency (IPCE) curves of the device with the GO/Au NP HTL and compared with that using GO as the HTL. As shown in Fig. 7,
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Fig. 5 SEM image of a GO HTL before (top) and after the deposition of Au NPs (bottom).
within the wavelength range from 400 to 600 nm the photocurrent increased remarkably upon the incorporation of Au NPs, which complies with the enhanced JSC observed.
Fig. 6 J–V characteristics of photovoltaic devices incorporating PEDOT:PSS (40 nm), GO (3.9 nm) and GO (3.9 nm)/Au NP HTLs. In the inset, the PCE as a function of exposure solar irradiation time normalized to their initial values is shown.
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Table 3 Summary of photovoltaic parameters of OPV devices containing different HTLs
PEDOT:PSS GO GO/NPs
Jsc (mA cm2)
Voc (V)
FF (%)
h (%)
9.15 9.59 10.2
0.60 0.60 0.61
51.7 50.7 54.2
2.86 2.90 3.37
The corresponding percentage enhancement in IPCE calculated from the ratio of the two IPCE curves is also presented, together with the extinction (scattering plus absorption) crosssection of Au spheres embedded in a P3HT:PCBM medium and calculated using the Mie theory,44 and the experimental data for the NP size distribution. The dielectric constant of P3HT:PCBM was obtained from ref. 45. It is observed that the IPCE becomes enhanced in a broad spectral range (400 to 600 nm), while it maximizes at 580 nm. This peak regime practically coincides with that for which the optical absorption of the Au NPs embedded in a P3HT:PCBM medium becomes resonant due to plasmon absorption effects. Plasmonic NPs are strong scatterers of light at wavelengths near the plasmon resonance,
which is due to a collective oscillation of the conduction electrons in the metal. It thus can be concluded that the observed improvement can be due to the enhancement in the number of the photogenerated excitons46 near the NP-active layer interface. Finally it is important to note that the stability enhancement observed upon using the GO HTL, shown in Fig. 4, was preserved aer the NP incorporation (data not shown). It is well recognized that conversion efficiency and operation stability are equally important factors that have to be addressed before OPV devices can be commercialized. The present work demonstrates that the combination of GO and plasmonic layers provides composite GO/NP HTLs that lead to substantial enhancement in both the efficiency and stability of OPVs. It is expected that this simple, low-cost, solution-processable process should be useful and contribute to the future commercialization of OPV devices.
4
Conclusions
In summary the efficiency and stability of OPVs can be signicantly improved by the employment of the GO/NP bilayer as the HTL. The efficiency improvement is attributed to the unique plasmon absorption properties induced by the NPs, which led to a clear enhancement of the photocurrent and the ll factor. Besides this, the stability enhancement is due to slower oxygen and/or indium diffusion from the ITO electrode to the active layer. Furthermore, the utilization of the most efficient donor polymer polythieno[3,4-b]-thiophene/benzodithiophene (PTB7),4 instead of the P3HT could lead to a signicant efficiency enhancement, since the OPV device will exhibit a broader light absorption enhancement leading to signicantly higher efficiency. Likewise, the placement of NPs with various sizes and LSPR bands in both the interface and the photoactive layer has the potential not only to surpass the 10% efficiency barrier for a single cell, but to simultaneously enhance the cell stability.47,48 The approach used in this work is quite simple and may be suitable for integration in roll to roll fabrication for organic photovoltaics, since both GO and NPs are solution processable. Nevertheless, many parameters need to be still optimized including the Au NP density and size distribution.
Acknowledgements This research has been co-nanced by the European Union (European Social Fund – ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) – Research Funding Program: ARCHIMEDES III: Investing in knowledge society through the European Social Fund.
Fig. 7 (Top) IPCE of the devices incorporating GO (blue spheres) and GO/NP (red spheres) HTLs as a function of the wavelength of monochromatic irradiation. (Bottom) Spectral dependence of the IPCE enhancement after incorporating Au NPs (red spheres). The absorption spectrum predicted by the Mie theory for Au NPs, of the same size distribution as those deposited onto GO, in P3HT:PCBM (solid line) is also presented.
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Notes and references 1 H. Y. Chen, J. H. Hou, S. Q. Zhang, Y. Y. Liang, G. W. Yang, Y. Yang, L. P. Yu, Y. Wu and G. Li, Nat. Photonics, 2009, 3, 649. 2 G. Zhao, Y. He and Y. Li, Adv. Mater., 2010, 22, 4355.
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Nanoscale 3 S. H. Park, A. Roy, S. Beaupre, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee and A. J. Heeger, Nat. Photonics, 2009, 3, 297. 4 Z. He, C. Zhong, S. Su, M. Xu, H. Wu and Y. Cao, Nat. Photonics, 2012, 6, 591. 5 C. J. Brabec, S. Gowrisanker, J. J. M. Halls, D. Laird, S. Jia and S. P. Williams, Adv. Mater., 2010, 22, 3839. 6 F. Zhang, M. Ceder and O. Inganas, Adv. Mater., 2007, 19, 1835. 7 R. Po, C. Carbonera, A. Bernardi and N. Camaioni, Energy Environ. Sci., 2011, 4, 285. 8 H. Yan, P. Lee, N. R. Armstrong, A. Graham, G. A. Evmenenko, P. Dutta and T. J. Marks, J. Am. Chem. Soc., 2005, 127, 3172. 9 X. H. Yang, F. Jaiser, B. Stiller, D. Neher, F. Galbrecht and U. Scherf, Adv. Funct. Mater., 2006, 16, 2156. 10 J. Van de Lagemaat, T. M. Barnes, G. Rumbles, S. E. Shaheen, T. J. Coutts, C. Weeks, I. Levitsky, J. Peltola and P. Glatkowski, Appl. Phys. Lett., 2006, 88, 2333503. 11 F. So and D. Kondakov, Adv. Mater., 2010, 22, 3762. 12 A. W. Hains and T. J. Marks, Appl. Phys. Lett., 2008, 92, 1. 13 M. Kemerink, S. Timpanaro, M. M. de Kok, E. A. Meulenkamp and F. J. Touwslager, J. Phys. Chem. B, 2004, 108, 18820. 14 Y. Xia and J. Ouyang, J. Mater. Chem., 2011, 21, 4927. 15 A. M. Nardes, M. Kemerink, M. M. de Kok, E. Vinken, K. Maturova and R. A. J. Janssen, Org. Electron., 2008, 9, 727. 16 B. Friedel, P. E. Keivanidis, T. J. K. Brenner, A. Abrusci, C. R. McNeill, R. H. Friend and N. C. Greenham, Macromolecules, 2009, 42, 6741. 17 K. Norrman, M. V. Madsen, S. A. Gevorgyan and F. C. Krebs, J. Am. Chem. Soc., 2010, 132, 16883. 18 M. S. White, D. C. Olson, S. E. Shaheen, N. Kopidakis and D. S. Ginley, Appl. Phys. Lett., 2006, 89, 143517. 19 M. D. Irwin, B. Buchholz, A. W. Hains, R. P. H. Chang and T. J. Marks, Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 2783. 20 I. Hancox, L. A. Rochford, D. Clare, P. Sullivan and T. S. Jones, Appl. Phys. Lett., 2011, 99, 013304. 21 S. S. Li, K. Tu, C. C. Lin, C. W. Chen and M. Chhowalla, ACS Nano, 2010, 4, 3169. 22 H. P. Kim, A. R. M. Yusoff and J. Jang, Sol. Energy Mater. Sol. Cells, 2013, 110, 87. 23 J. Yun, J. Yeo, J. Kim, H. Jeong, D.-Y. Kim, Y.-J. Noh, S.-S. Kim, B.-C. Ku and S.-I. Na, Adv. Mater., 2011, 23, 4923. 24 E. Stratakis and E. Kymakis, Mater. Today, 2013, in press. 25 D. H. Wang, K. H. Park, J. H. Seo, J. Seier, J. H. Jeon, J. K. Kim, J. H. Park, O. O. Park and A. J. Heeger, Adv. Energy Mater., 2011, 1, 766.
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Communication 26 G. D. Spyropoulos, M. M. Stylianakis, E. Stratakis and E. Kymakis, Appl. Phys. Lett., 2012, 100, 213904. 27 J.-L. Wu, F.-C. Chen, Y.-S. Hsiao, F.-C. Chien, P. Chen, C.-H. Kuo, M. H. Huang and C.-S. Hsu, ACS Nano, 2011, 5, 959. 28 B. Wu, X. Liu, T. Z. Oo, G. C. Xing, N. Mathews and T. C. Sum, Plasmonics, 2012, 7, 677. 29 H. A. Atwater and A. Polman, Nat. Mater., 2010, 9, 205. 30 G. D. Spyropoulos, M. M. Stylianakis, E. Stratakis and E. Kymakis, Photon. Nanostruct., 2011, 9, 184. 31 E. Kymakis, E. Stratakis, E. Koudoumas and C. Fotakis, IEEE Trans. Electron Devices, 2011, 58, 860. 32 D. Li, M. B. M¨ uller, S. Gilje, R. B. Kaner and G. G. Wallace, Nat. Nanotechnol., 2008, 3, 101. 33 E. Stratakis, M. Barberoglou, C. Fotakis, G. Viau, C. Garcia and G. A. Shafeev, Opt. Express, 2009, 17, 12650. 34 S. Besner, A. V. Kabashin and M. Meunier, Appl. Phys. Lett., 2006, 89, 233122. 35 G. Li, V. Shrotriya, J. S. Huang, Y. Yao, T. Moriarty, K. Emery and Y. Yang, Nat. Mater., 2005, 4, 864. 36 B. R. Lee, J. W. Kim, D. Kang, S. Ko, H. J. Lee, J. Y. Kim, H. S. Shin and M. H. Song, ACS Nano, 2012, 6, 2984. 37 M. Al-Ibrahim, H. K. Roth, U. Zhokhavets, G. Gobsch and S. Sensfuss, Sol. Energy Mater. Sol. Cells, 2005, 85, 13. 38 C. J. Brabec, N. S. Saricici and J. C. Hummelen, Adv. Funct. Mater., 2001, 11, 15. 39 M. M. Stylianakis and E. Kymakis, Appl. Phys. Lett., 2012, 100, 093301. 40 V. Shrotriya, E. H.-E. Wu, G. Li, Y. Yao and Y. Yang, Appl. Phys. Lett., 2006, 88, 064104. 41 M. Jørgensen, K. Norrman and F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2008, 92, 686. 42 K. Kawano, R. Pacios, D. Poplavskyy, J. Nelson, D. D. C. Bradley and J. R. Durrant, Sol. Energy Mater. Sol. Cells, 2006, 90, 3520. 43 Y. Zhou, J. W. Shim, C. F. Hernandez, A. Sharma, K. A. Knauer, A. J. Giordano, S. R. Marder and B. Kippelen, Phys. Chem. Chem. Phys., 2012, 14, 12014. 44 http://www.philiplaven.com/mieplot.htm. 45 H. Shen, P. Bienstman and B. Maes, J. Appl. Phys., 2009, 106, 073109. 46 C. H¨ agglund and B. Kasemo, Opt. Express, 2009, 17, 11944. 47 B. Paci, A. Generosi, V. R. Albertini, G. D. Spyropoulos, E. Stratakis and E. Kymakis, Nanoscale, 2012, 4, 7452. 48 B. Paci, G. D. Spyropoulos, A. Generosi, D. Bailo, V. R. Albertini, E. Stratakis and E. Kymakis, Adv. Funct. Mater., 2011, 21, 3573.
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