Article pubs.acs.org/Langmuir
Electropolymerized Polythiophene Photoelectrodes with Density-Controlled Gold Nanoparticles Yukina Takahashi,† Sakiko Taura,‡ Tsuyoshi Akiyama,§ and Sunao Yamada*,† †
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan § Department of Materials Science, School of Engineering, The University of Shiga Prefecture, 2500, Hassaka-cho, Hikone-city, Shiga 522-8533, Japan ‡
S Supporting Information *
ABSTRACT: A polythiophene thin film was fabricated on gold nanoparticle (AuNP)deposited indium−tin-oxide (ITO) electrodes with electropolymerization, whereas AuNPs were predeposited on the ITO surface. A photocurrent via photoexcited polythiophene increased with AuNPs which was attributed to the localized surface plasmon resonance. Investigation of the AuNP-density dependence on the relative enhancement of photocurrent revealed the maximum effect at 14% of AuNP-density, while 68% of AuNP-density exhibited smaller photocurrent than the polythiophene electrode without AuNPs. We have revealed that the effects of AuNPs saturate in the fairly low density region, and that the excess AuNPs even in the range of submonolayer resulted in the decrement of photocurrents.
■
INTRODUCTION Organic thin film solar cells are one of the most promising candidates for the next generation solar cells because of their potential advantages, such as lightness of weight, processability, flexibility, and so on.1−4 Conductive polymers such as polythiophene and polypyrrole have been often employed as a photoactive layer of the cells due to their attractive features of that, for instance, the properties of conductive polymers can be easily controlled by the introduction or change of appropriate functional substituents, and thus various polymers have been synthesized. However, such a synthetic approach for improving the cell performance leads to sophisticated design and high cost of cell materials. Skillful design of cell construction must be an alternative approach for improving the cell performance. As to effective conversion of the incident photon to the current, there seems to be considerable room to improve the cell performance. For instance, one of the reasons is a trade-off problem between light absorption and resistance; that is, a thicker photoactive layer for enough light absorption gives higher resistance of the cell, while a thinner photoactive layer decreases in the efficiency of light absorption. Bulk-hetero junctionarchitecture that intermixes the polymer donor and acceptor materials to form a percolated conducting network is one of the promising approaches for improving the photoelectric conversion efficiency.4−7 Yet, other innovative approaches should be continuously challenged. In recent years, we8−12 and other groups13−18 have developed photoelectrodes and solar cells with metal nanostructures of gold and silver. It is widely known that localized surface plasmon © 2012 American Chemical Society
resonance (LSPR) of noble metal nanostructures can be applied to the enhancements of photocurrent and luminescence signals from organic dye molecules. However, few researchers have mentioned quantitative effects of LSPR using plasmonic metal nanoparticles on the photocurrent enhancement of photoelectrodes consisting of an organic layer. In fact, it is practically difficult and complicated to experimentally evaluate rigorous effects of LSPR on the photocurrent enhancement. For example, when one tries to introduce the plasmonic metal nanoparticles in the layer of organic dyes by the spin coating method, one has to apply hydrophobic treatment the nanoparticles. However, the introduction of metal nanoparticles in the organic layer induces the changes of various parameters, such as the surface morphology of photoactive layer and the changes of the thickness of photoactive layer caused by the changes of the viscosity of dye solution for spin-coating. Thus, these experimental problems make difficulty in quantitatively evaluating the effects of LSPR of embedded metal nanoparticles. Recently, we found that the method of electropolymerization of bithiophene, giving the polythiophene layer on the electrode, was useful for the fabrication of photoelectric conversion film on the electrode.19,20 This method offers some advantages of applicability to various surface morphologies of the electrodes in wide dimension and of giving insoluble films. Therefore, it must be Special Issue: Colloidal Nanoplasmonics Received: January 16, 2012 Revised: March 12, 2012 Published: March 12, 2012 9155
dx.doi.org/10.1021/la300227j | Langmuir 2012, 28, 9155−9160
Langmuir
Article
TEM image, the mean size of AuNPs was found to be 18.9 ± 1.5 nm (Figure 1a). In addition, a sharp peak in the extinction
suitable to verify the quantitative effects of LSPR on the photoelectric conversion efficiency of the organic layer. From these few points, we have first investigated the effects of gold nanoparticles on the photoelectrochemical properties of an electropolymerized polythiophene layer that was fabricated on an indium−tin-oxide (ITO) electrode.
■
EXPERIMENTAL SECTION
Preparation of Electrode. The colloidal solution of gold nanoparticles (AuNPs) was prepared according to the reported procedure.21 In brief, 950 mL of 2.5 × 10−4 M HAuCl4·4H2O aqueous solution was heated to a boil. After 50 mL of sodium citrate aqueous solution (1.0 wt %) was added, the mixed solution was refluxed for 60 min, giving the monodispersed colloidal solution of AuNPs protected with citrate ion. The average diameter of resulting AuNPs was evaluated to be 18.9 ± 1.5 nm on the basis of transmission electron microscopy (JEM-200CX, JEOL) images. An ITO-coated glass plate (10 Ω/sq, 2 × 2 cm2) was cleaned ultrasonically in acetone and then in ethanol for 3 min each, following the treatment with ozone atmosphere (UV 253E, Filgen) for 15 min. The substrate was reserved in water until just before use. The geometric area of the electrode was evaluated to be 0.2 cm2 with an insulating polymer film by thermal adhesion at 100 °C. In order to effectively deposit the negatively charged AuNPs, the positive charge was implanted to the surface of ITO substrate as follows. Namely, the ITO substrate was immersed in a 42 mg mL−1 polyethyleneimine (PEI) aqueous solution containing 0.2 M NaCl for 20 min followed by rinsing with distilled water, giving the deposition of PEI on the ITO surface. Then the PEI-modified ITO electrode was immersed in the aqueous colloidal solution of AuNPs for different times (t = 0−24 h) so as to change the density of electrostatically depositing AuNPs onto the PEI-modified ITO electrode.22 Independently, dense packing of AuNPs on the PEI-modified ITO electrode was performed by a method using a liquid/liquid interface.23 The polythiophene layer was fabricated by anodic electropolymerization onto the PEI-modified ITO electrodes, as described before, under the conditions with and without AuNPs.19,20 Namely, electropolymerization of 2,2′-bithiophene was carried out at 1.4 V from a dichloromethane solution containing 1.0 mM 2,2′-bithiophene and 0.1 M t-Bu4NPF6, using a Ag wire and a Pt wire as reference and counter electrodes, respectively. The electrode potential was applied using a digital potentiostat (ALS model 650C, ALS Co., Ltd.) until the charge amount reached to 15.0 mC cm−2. Measurements. Photoelectrochemical measurements were carried out with a three-electrode setup using a potentiostat (HECS 318, Huso). A Ag/AgCl and a Pt wire were used as reference and counter electrodes, respectively. The area of the working electrode was adjusted to be 0.2 cm2. The aqueous solution containing 0.1 M NaClO4 and 5.0 mM 1,1′-dimethyl-4,4′-bipyridinium dichloride hydrate (methylviologen) as an electron acceptor was employed as an electrolyte. All photocurrents were measured at E = 0 V vs Ag/ AgCl. The monochromatic light from a 150 W Xe lamp was irradiated to the working electrode (photoelectrode). Extinction spectra of the films were measured with a UV−vis-NIR spectrophotometer (UV-3150, Shimadzu or V-670, JASCO). Surface morphologies of the samples were observed via field emission scanning electron microscopy (FE-SEM; SU8000, Hitachi High-Technologies) and atomic force microscopy (AFM; JSPM-5400, JEOL).
Figure 1. (a) TEM image of AuNPs and (b) extinction spectrum of the prepared colloidal solution of AuNPs.
spectrum was observed at approximately 519 nm which was typically attributed to LSPR of monodispersed AuNPs (Figure 1b). These results indicated that the monodispersed colloidal solution of AuNPs with uniform size was successfully prepared. Then, a PEImodified ITO electrode was immersed in the colloidal solution so as to electrostatically adsorb AuNPs. The density of AuNPs on the electrode with different immersion times was investigated. As a result, it was verified that the density was in proportion to the immersion time up to 6 h and then was saturated (Figure 2). The peak wavelength of adsorbed AuNPs, which was obtained by subtracting the extinction of ITO electrode, demonstrated a redshift to ca. 527 nm (Figure 2h). This red-shift should be ascribed to the change of refractive index of the surrounding medium of AuNPs because it is known that the refractive index of ITO (n = ∼2.1)24 is higher than that of water (n = 1.33). The maximum density of AuNPs on the electrode was ca. 30% with 6 h immersion (Figure 2d) through this method. More than 6 h immersion did not contribute to an appreciable increase of AuNP density even with 24 h immersion in this system. Therefore, dense packing of AuNPs on the PEI-modified ITO electrode was independently obtained by the method of using a liquid/liquid interface;23 the AuNP density of the electrode was 68%. Characterization of Electropolymerized Polythiophene Film. In the next step, polythiophene was electrodeposited
■
RESULTS AND DISCUSSION Densities of AuNPs on ITO Electrodes. It is quite important to elucidate the effects of plasmonic metal nanostructures on the photoelectric conversion. In this study, we have investigated the effects of AuNPs on the performance of photoelectrodes consisting of electropolymerized polythiophene and AuNPs. First, we have investigated the size and aggregation status of AuNPs in solution. From the analysis of a 9156
dx.doi.org/10.1021/la300227j | Langmuir 2012, 28, 9155−9160
Langmuir
Article
Figure 2. (a−f) SEM images of AuNPs with various densities deposited on the PEI-modified ITO electrodes. The density of AuNPs with (a) 5, (b) 14, (c) 20, (d) 28, (e) 29% from the method on immersion, and (f) 68% from the method using liquid/liquid interface. (g) Immersion time courses of AuNP-densities, calculated from SEM images. (h) Differential extinction spectra of deposited AuNPs after subtracting the PEI-modified ITO electrode.
onto the electrodes with and without AuNPs. Typical extinction spectra of electropolymerization polythiophene photoelectrodes are shown in Figure 3. Accordingly, these spectra indicated that polythiopnene films were successfully obtained in both conditions with and without AuNPs. Any peaks due to LSPR of AuNPs were not clearly observed in the spectra of those electrodes with AuNPs. It might be because the extinction intensities of AuNPs were relatively smaller than that of the polythiphene film in the prepared electrodes and/or the red-shifted wavelength of the peak could not be simply estimated because the refractive index of polythiophene depends on wavelength in the visible region.25 However, the existence of AuNPs after the electropolymerization has been confirmed by X-ray fluorescence analysis (EDX-800, Shimadzu) (Supporting Information 1). AFM measurements of the electrodes with scratching-up were carried out to investigate the film thicknesses (Figure 4a,b). Crosssectional profiles of the electropolymerized polythiophene electrode without AuNPs indicated that the film thickness was
Figure 3. Typical extinction spectra of electropolymerized polythiophene photoelectrodes with and without AuNPs.
estimated to be approximately 14.4 nm, while that of the electrode with 14% AuNPs, prepared with 3 h immersion in the colloidal solution, was estimated to be approximately 26.3 nm. Taking into 9157
dx.doi.org/10.1021/la300227j | Langmuir 2012, 28, 9155−9160
Langmuir
Article
Figure 4. AFM images and their cross-sectional profiles of scratched electropolymerized polythiophene photoelectrodes (a) without and (b) with AuNPs. Schematic illustration of the proposed models of the fabricated electrodes (c) without and (d) with AuNPs.
account the surface morphologies of electrodes measured by AFM, cross-sectional structures of the electrodes with and without AuNPs should be as schematically shown in Figure 4c,d. Here, increment of the film thickness was almost in accordance with the effect of calculated volume of introduced AuNPs (Supporting Information 2). The results support that the model of cross-sectional structure of the electrode with AuNPs should be reasonable. Effects of AuNPs Density on Photocurrents of Polythiophene. Monochromatic light was irradiated to the polythiophene photoelectrodes with various AuNPs densities from the rear side of the photoelectrode so that the effects of AuNP density on the photocurrents of polythiophene were investigated. All of the prepared photoelectrodes exhibited cathodic photocurrents with visible light irradiation in the range of 400−700 nm. It suggested that the photocurrents should be based on electron-transfer from photoexcited polythiophene to methylviologen or oxygen in the electrolyte as illustrated in Figure 5. Typical incident photon-to-current conversion efficiency (IPCE, the number of electrons generated in the external circuit divided by the number of incident photons) spectra of the electropolymerized polythiophene electrodes are shown in Figure 6a. The shapes of IPCE spectra were similar to those of
extinction spectra shown in Figure 3. The results indicate that the photocurrents are due to photoexited polythiophene. We also investigated the dependence of the photocurrent on the AuNPs density at the wavelength of 500 nm that is around the peaks of the IPCE spectra (Figure 6b). As a result, it was revealed that less than 30% of AuNPs density was effective to relatively increase photocurrents of the electropolymerized polythiophene photoelectrodes as compared with that without AuNPs. In addition, cathodic photocurrents of AuNPs electrodes without polythiophene were ignorable. Therefore, it is suggested that the increment of photocurrents should be attributed to enhancement of interactions between photon and polythiophene by LSPR of AuNPs. On the other hand, the photoelectrode with 2 nm thickness of Au film, which was fabricated by vapor deposition, did not exhibit significant enhancement effect as the photoelectrodes with 14% AuNPs (Figure 6a). It suggests that the enhancement effect should not be substantially attributed to the light scattering of Au or enhanced structural change of electropolymerized polythiophene with physical interactions of Au. The highest enhancement effect was observed with a AuNP density of 14%. In addition, more than 20% of AuNP densities exhibited smaller 9158
dx.doi.org/10.1021/la300227j | Langmuir 2012, 28, 9155−9160
Langmuir
Article
low density region. We previously reported that both the photocurrent and luminescence of photoelectrode consisting of tetraphenylporphyrin and 40 nm silver nanoparticles (AgNPs) showed that the enhancement effect based on LSPR of AgNPs saturated with the AgNPs density of 30−50%.9 Although the quantitative comparison is difficult because the effects of LSPR strongly depend on metal species, size and shape, the enhancement effect of that was saturated with submonolayered nanoparticles in this study, is in accordance with our previous report. On the other hand, the photoelectrode with the AuNP density of 68% demonstrated much smaller photocurrents than that of the electrode without AuNPs. It means that the dense packing of AuNPs even in the range of submonolayer should result in the decrement of photocurrents. It shows that AuNPs perform not only photocurrent enhancement by LSPR but also photocurrent decrement by quenching, and that the latter effect should be significantly observed with the dense packing of AuNPs. Interparticle Distances of AuNPs. Here, interparticle distances were estimated from the size and the density of AuNPs. When AuNPs are assumed to be deposited with equal spacing on the ITO electrode, each density of deposited AuNPs is evaluated to be, 5, 14, 20, 28, 29, or 68%, which gives the interparticle distance d as 62, 29, 21, 15 or 15, 3 nm, respectively (Supporting Information 3). In this study, the effective distance d1/2 (d1/2: half of the interparticle distance), which may reflect the photocurrent enhancement effect based on the enhanced electric field generated by LSPR of AuNP, from the above evaluation of d values can be defined as 31 nm > d1/2 > 11 nm. In general, the enhancement effects in spherical AuNPs are known to reach almost their diameters.26 Therefore, it is reasonable that the estimated d1/2 value is near the diameter of the AuNPs (18.9 ± 1.5 nm). In the case of the dye-sensitized solar cell system, on the other hand, AuNPs with 40 and 100 nm diameters have been reported to exhibit the most photocurrent enhancement effect when the spacing between the dye and AuNPs was at approximately 10 nm, and that the enhancement effect would be smaller when the spacing would be less than 10 nm because of quenching effects.18 Similar tendencies have been reported in the cases of photocatalytic activity27 and luminescence.26 Judging from these reposts, it is reasonable that the most effective AuNP density was approximately 14% in the photocurrents of electropolymerized polythiophene in this study. In the future, more effective enhancement effect of AuNPs is expected to be realized by using AuNPs with surface treatments to avoid quenching and/ or by developing the incorporation method of AuNPs threedimensional into the photoelectrodes.
Figure 5. Energy diagram of the electropolymerized polythiophene electrode. MV2+ means 1,1′-dimethyl-4,4′-bipyridinium dichloride hydrate (methylviologen).
■
CONCLUSIONS Electropolymerized polythiophene photoelectrodes with various densities of AuNPs were fabricated. The photoelectrode with a AuNPs density of 14% exhibited the largest photocurrent. Even in the range of submonolayer, excess AuNPs decreased photocurrents of the polythiophene. We have verified that the method offers potential usefulness for the optimal incorporation of plasmonic nanoparticles in organic thin film solar systems.
Figure 6. (a) Typical IPCE spectra of electropolymerized polythiophene photoelectrodes with 14% AuNPs (○), gold film of 2 nm (△), and without gold (●). (b) AuNPs-density dependence of enhancement factor on the photocurrent from electropolymerized polythiophene electrode irradiated at the wavelength of 500 nm.
■
enhancement effect of photocurrents. The almost same tendency of photocurrent profiles was also observed with illumination from the front side of the photoelectrodes. These results indicate that the effects of AuNPs saturate in the fairly
ASSOCIATED CONTENT
S Supporting Information *
Additional experimental details. This material is available free of charge via the Internet at http://pubs.acs.org. 9159
dx.doi.org/10.1021/la300227j | Langmuir 2012, 28, 9155−9160
Langmuir
■
Article
Scattering from Surface Plasmon Polaritons in nearby Metallic Nanoparticles. Appl. Phys. Lett. 2006, 89, 093103(1−3). (16) Nakayama, K.; Tanabe, K.; Atwater, H. A. Plasmonic Nanoparticle Enhanced Light Absorption in GaAs Solar Cells. Appl. Phys. Lett. 2008, 93, 121904(1−3). (17) Standridge, S. D.; Schatz, G. C.; Hupp, J. T. Distance Dependence of Plasmon-Enhanced Photocurrent in Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2009, 131, 8407−8409. (18) Kawawaki, T.; Takahashi, Y.; Tatsuma, T. Enhancement of DyeSensitized Photocurrents by Gold Nanoparticles: Effects of DyeParticle Spacing. Nanoscale 2011, 3, 2865−2867. (19) Roncali, J. Conjugated Poly(thiophenes): Synthesis, Functionalization, and Applications. Chem. Rev. 1992, 92, 711−738. (20) Takechi, K.; Shiga, T.; Motohiro, T.; Akiyama, T.; Yamada, S.; Nakayama, H.; Kohama, K. Solar Cells Using Iodine-Doped Polythiophene-Porphyrin Polymer Films. Sol. Energy Mater. Sol. Cells 2006, 90, 1322−1330. (21) Turkevich, J.; Stevenson, P. C.; Hillier, J. A Study of the Nucleation and Growth Processes in the Synthesis of Colloidal Gold. Discuss. Faraday Soc. 1951, 11, 55−75. (22) Decher, G. Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science 1997, 277, 1232−1237. (23) Suzuki, M.; Niidome, Y.; Terasaki, N.; Inoue, K.; Kuwahara, Y.; Yamada, S. Surface-Enhanced Nonresonance Raman Scattering of Rhodamine 6G Molecules Adsorbed on Gold Nanorod Films. Jpn. J. Appl. Phys. 2004, 43, L554−L556. (24) Yan, X.; Mont, F. W.; Poxson, D. J.; Schubert, M. F.; Kim, J. K.; Cho, J.; Schubert, E. F. Refractive-Index-Matched Indium-Tin-Oxide Electrodes for Liquid Crystal Displays. Jpn. J. Appl. Phys. 2009, 48, 120203(1−3). (25) Kymakis, E.; Amaratunga, G. A. J. Optical properties of polymernanotube composites. Synth. Met. 2004, 142, 161−167. (26) Anger, P.; Bharadwaj, P.; Novotny, L. Enhancement and Quenching of Single-Molecule Fluorescence. Phys. Rev. Lett. 2006, 96, 113002−113004. (27) Torimoto, T.; Horibe, H.; Kameyama, T.; Okazaki, K.; Ikeda, S.; Matsumura, M; Ishikawa, A.; Ishihara, H. Plasmon-Enhanced Photocatalytic Activity of Cadmium Sulfide Nanoparticle Immobilized on Silica-Coated Gold Particles. J. Phys. Chem. Lett. 2011, 2, 2057−2062.
AUTHOR INFORMATION
Corresponding Author
*Telephone: +81 92 802 2812. Fax: +81 92 802 2815. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors are grateful to Dr. T. Arakawa for his help with TEM measurements, and to Prof. H. Yonemura and Mr. H. Tahara for valuable discussion. This work was supported in part by a Grant-in-Aid for Scientific Research (No. 2241028 for S.Y.) from JSPS, A-STEP (No. AS231Z01250B for Y.T.) from JST, The Research Grant of Faculty of Engineering for Young Researchers (for Y.T.) from Kyushu University, and P & P (for Y.T.) from Kyushu University.
■
REFERENCES
(1) Tang, C. W. Two-Layer Organic Photovoltaic Cell. Appl. Phys. Lett. 1986, 48, 183−185. (2) Hiramoto, M.; Fujiwara, H.; Yokoyama, M. Three-Layered Organic Solar Cell with a Photoactive Interlayer of Codeposited Pigments. Appl. Phys. Lett. 1991, 58, 1062−1064. (3) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789−1791. (4) Hoppe, H.; Sariciftci, N. S. Organic Solar Cells: An Overview. J. Mater. Res. 2004, 19, 1924−1925. (5) Wudl, F. The Chemical Properties of Buckminsterfullerene (C60) and the Birth and Infancy of Fulleroids. Acc. Chem. Res. 1992, 25, 157−161. (6) Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. Effect of Postproduction Treatment on Plastic Solar Cells. Adv. Funct. Mater. 2003, 13, 85−88. (7) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15, 1617−1622. (8) Akiyama, T.; Nakada, M.; Terasaki, N.; Yamada, S. Photocurrent Enhancement in a Porphyrin-gold Nanoparticle Nanostructure Assisted by Localized Plasmon Excitation. Chem. Commun. 2006, 395−397. (9) Arakawa, T.; Munaoka, T.; Akiyama, T.; Yamada, S. Effects of Silver Nanoparticles on Photoelectrochemical Responses of Organic Dyes. J. Phys. Chem. C 2009, 113, 11830−11835. (10) Akiyama, T.; Aiba, K.; Hoashi, K.; Wang, M.; Sugawa, K.; Yamada, S. Enormous Enhancement in Photocurrent Generation Using Electrochemically Fabricated Gold Nanostructures. Chem. Commun. 2010, 46, 306−308. (11) You, J.; Arakawa, T.; Munaoka, T.; Akiyama, T.; Takahashi, Y.; Yamada, S. Silver-Nanoparticle-Assisted Photocurrent Generation in Polythiophene-Fullerene Thin Films. Jpn. J. Appl. Phys. 2011, 50, 04DK22(1−4). (12) You, J.; Takahashi, Y.; Yonemura, H.; Akiyama, T.; Yamada, S. Effects of Film Thickness on the Photocurrent Generation from Polythiophene-Fullerene Thin Films Containing Silver Nanoparticles. Jpn. J. Appl. Phys. 2012, 51, 02BK04(1−4) . (13) Wen, C.; Ishikawa, K.; Kishima, M.; Yamada, K. Effects of Silver Particles on the Photovoltaic Properties of Dye-Sensitized TiO2 Thin Films. Sol. Energy Mater. Sol. Cells 2000, 61, 339−351. (14) Catchpole, K. R.; Pillai, S. Surface Plasmons for Enhanced Silicon Light-Emitting Diodes and Solar Cells. J. Lumin. 2006, 121, 315−318. (15) Derkacs, D.; Lim, S. H.; Matheu, P.; Mar, W.; Yu, E. T. Improved Performance of Amorphous Silicon Solar Cells via 9160
dx.doi.org/10.1021/la300227j | Langmuir 2012, 28, 9155−9160
