Macroporous n-GaP in Nonaqueous Regenerative

In the highest-quality epitaxial GaP films, values of LD are only 1−10 μm,(20) while, ... Several questions regarding the viability of high-surface-ar...
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2009, 113, 11988–11994 Published on Web 06/22/2009

Macroporous n-GaP in Nonaqueous Regenerative Photoelectrochemical Cells Michelle J. Price and Stephen Maldonado* Department of Chemistry, Applied Physics, UniVersity of Michigan, 930 North UniVersity, Ann Arbor, Michigan 48109-1055 ReceiVed: May 12, 2009; ReVised Manuscript ReceiVed: June 12, 2009

Macroporous GaP photoelectrodes with wall thicknesses of approximately 500 nm and pore depths ranging from 0 to 45 µm have been prepared from nondegenerately doped single-crystalline n-GaP(100) with a minoritycarrier diffusion length of only 110 nm. The photoelectrochemical behaviors of planar and macroporous photoelectrodes have been assessed in nonaqueous regenerative photoelectrochemical cells operated under potentiostatic control and employing dry acetonitrile containing ferrocene/ferrocenium. Enhancements in the short-circuit photocurrents tracked increases in internal quantum yield measurements at long wavelengths for macroporous n-GaP. The observed photocurrent densities and spectral response measurements were consistent with values expected from GaP/liquid heterojunctions controlled by faradaic charge transfer to an outersphere, dissolved redox couple. Lower open-circuit photovoltages were observed with macroporous electrodes with increasing pore depth, consistent with distribution of the photocurrent across the entire macropore/ solution interface. Fill factors observed under these conditions did not systematically track pore depth in macroporous photoelectrodes. The noted changes in short-circuit photocurrents, open-circuit photovoltages, and fill factors with increasing porosity resulted in more than an order of magnitude improvement in the photoelectrode efficiency of macroporous n-GaP with 45 µm deep pores at 100 mW cm-2 illumination. The presented data show the level and type of enhancement in energy conversion efficiency that high-aspect-ratio electrode architectures can provide carrier-collection-limited materials such as GaP. Introduction Photoelectrochemical cells are attractive systems for the generation of energy-rich chemical fuels.1-4 Generally, larger operating cell voltages than those obtained with commercial silicon (Si) photovoltaics under solar illumination at 1.5 air mass units (AMU) are needed to drive kinetically slow, multielectron charge-transfer reactions involved in fuel production.3 For example, concentration overpotential and kinetic overpotential losses require a total cell voltage of at least +1.7 V to drive water electrolysis at rates equivalent to the incident solar photon flux.5 Due to fundamental limitations imposed by their band gap energies and bulk-recombination processes,6,7 even ideal semiconductor heterojunctions employing common small- to mid-bandgap semiconductors such as Si, gallium arsenide (GaAs), or germanium (Ge) cannot individually generate opencircuit photovoltages, Voc, larger than ∼0.8 V under typical solar insolation. In contrast, gallium phosphide (GaP) has a sufficiently large band gap energy (Eg ) 2.26 eV) and large charge carrier mobilities (µ ) 300 and 500 cm2 V-1 s-1 for electrons and holes, respectively)8 to support Voc g 1 V under solar illumination,9-11 but is still capable of absorbing a meaningful fraction of the solar spectrum. Unlike materials such as TiO2 with Eg g 3.0 eV, the Schockley-Quiesser limit for solar energy conversion efficiency at a single optimized GaP heterojunction under 1.5 AMU solar irradiation is 20%.12 Accordingly, GaP has long been recognized as a potentially useful semiconductor * To whom correspondence should be addressed. Phone: 734-647-4750. E-mail: [email protected]. Home Page: http://www.umich.edu/∼mgroup/.

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electrode material for photoelectrochemical energy conversion/ storage.13-17 From a practical perspective, GaP is also a technologically mature material commonly used in light emitting diodes where methods for doping, contacting, and depositing are well-known.18 Short carrier diffusion lengths, relative to the depth of visible light absorption in GaP, represent a key disadvantage to the utilization of GaP as a photoelectrode material in planar heterojunctions. The optical absorptivity, R, of light with energies near Eg is small, requiring a minimal bulk GaP material thickness of 28 µm (R-1) to capture light with λ ) 540 nm.19 The minority-carrier diffusion lengths, LD, in commercially available crystalline GaP are comparatively short. In the highestquality epitaxial GaP films, values of LD are only 1-10 µm,20 while, in single-crystalline wafers, minority-carrier diffusion lengths of ca. 10-100 nm are more typically observed.21,22 The lower minority-carrier diffusion lengths arise from sensitivity toward C impurities encountered during material preparation. For planar GaP heterojunctions, the substantial resultant mismatch between R-1 and LD severely limits the ability of GaP photoelectrodes to capture and convert visible light. Several groups have investigated nonplanar semiconductor heterojunction motifs for the purposes of decreasing the distance photogenerated charge carriers must travel before collection.21,23-29 Numerical models suggest that crystalline semiconductors with poor light- and minority-carrier-collection properties can still function effectively in solar energy conversion systems that decouple the path length of photogenerated minority carriers from the optical penetration depth (Figure 1).30 Comparatively  2009 American Chemical Society

Letters

Figure 1. Comparison of photogenerated charge carrier collection at (left) planar and (right) macroporous photoelectrodes. For visible light with λ g 460 nm and λ e 540 nm, values of R-1 range between 1 and 28 µm.

little work has been done to explore quantitatively this principle with GaP photoelectrodes, with few reports of the metrics for gauging the photovoltaic/photoelectrochemical performance enhancements. Vanmaekelbergh et al. previously demonstrated enhanced photocurrents at large applied biases using macroporous n-GaP in strongly acidic and alkaline solutions.21,31,32 Corrosion and interface instability are serious impediments to the analysis of GaP in aqueous photoelectrochemical systems under intense white light illumination33 and thus prevent direct quantitative assessment of the solar energy conversion properties of n-GaP photoanodes in water. Several questions regarding the viability of high-surface-area GaP photoelectrodes remain, including what sort of material length scales would be necessary to effect significant photoresponse changes and whether deleterious recombination at high-surface-area GaP and/or mass transport through deep macropores would too severely distort photoresponses to be useful. To ascertain the operational features of GaP photoelectrodes with high aspect ratios and known bulk optical, electrical, and electrochemical properties without complications from corrosion and other undefined faradaic processes, we report the photoelectrochemical responses of planar and macroporous nondegenerately doped n-GaP electrodes in dry acetonitrile solutions containing ferrocene/ferrocenium. Acetonitrile and similar nonaqueous electrolytes have proven useful in the study of chargetransfer processes at both planar10,34-36 and high-surface-area semiconductor electrodes25,37 and are used here to indicate the solar energy conversion properties of purposely structured GaP photoelectrodes with defined porosity in regenerative photoelectrochemical cells. The systems reported here do not suffer from limitations imposed by poor interfacial charge-transfer kinetics or issues related to the solid-state metallurgical processing of an intimate front contact. The conformal nature of these semiconductor/solution contacts instead reflects the upper limits of the attainable performance of low-grade GaP as the media for light absorption and charge carrier separation in lightsensitive heterojunctions. Experimental Section Single-side polished n-type GaP(100) 500 µm thick single crystals doped with sulfur at ND ) 5 × 1017 cm-3 were obtained from MTI, Inc. Sections were diced into 0.5 cm × 0.5 cm squares. Ohmic contacts were prepared by etching the back briefly (30 s) with concentrated NH4F(aq) (49% v/v, Transene), rinsing with distilled water (>18 MΩ cm, Barnstead Nanopure

J. Phys. Chem. C, Vol. 113, No. 28, 2009 11989 III purifier), soldering a thin, even film of pure indium on the back, and then annealing in forming gas (5% H2 in N2, Metro Welding) for 10 min at 400 °C. Electrodes were then prepared by attaching the GaP section onto a copper wire coil threaded through a glass tube and sealing with inert epoxy (Hysol C). Electrode areas were defined by the edge of the epoxy and were nominally 0.05 cm2. For samples suitable for microscopic analysis, a custom-built Teflon cell38 was used to define the electrode area rather than epoxy. A galvanostatic etching protocol was used in 1 M H2SO4 where each n-GaP electrode was anodically etched at a constant current of 100 mA cm-2 for a predetermined time.39 A custom-built constant-current circuit was used to direct the current between the n-GaP working electrode and a platinum gauze counter electrode. The etching solution was stirred vigorously, and the procedure was carried out in the dark; however, no efforts were made to deaerate the solution. After anodic etching, the n-GaP electrodes appeared various shades of yellow, depending on etching time (Supporting Information). Scanning electron microscopic analysis of these materials was conducted with a Philips XL-ESEM instrument operated at 15 kV with a secondary electron detector. Pore depths were assessed directly from electron micrographs. A quartz cell with an optically flat bottom serving as the window was used for photoelectrochemical measurements. Dry acetonitrile (Aldrich) was prepared with an MBraun solvent purification system. Battery grade lithium perchlorate (99.99%, Aldrich) was purchased in an ampule, opened in an inert atmosphere glovebox, and used as received. Ferrocene (Sigma) was sublimed and dried before use, and ferrocenium was generated electrolytically with a second compartment separated by a Vycor frit. The cell was assembled in an inert atmosphere glovebox and included a platinum gauze counter electrode and a luggin capillary reference electrode containing a platinum wire poised at the solution potential. After assembly, the cell was removed from the glovebox, connected to a Schlenk line, and kept under a positive pressure of argon gas (Metro Welding) during use. A platinum working electrode was used to measure concentration overpotentials and uncompensated solution resistances, typically less than 50 Ω. The data presented in the figures represent as-collected data without any correction for solution resistance. Tabulated data include correction for solution resistance losses which become more significant at higher current densities. Uncompensated solution “iR” drop represents purely an extrinsic limitation of the electrochemical cell geometry40 and not an inherent property of the GaP photoelectrodes, the focus of the present study. Prior to use, n-GaP electrodes were briefly etched according to a protocol proposed by Aspnes et al.,41 with slight modification. Electrodes were first immersed in 0.5% Br2 (Aldrich) in CH3OH (Aldrich) for 30 s, then rinsed with running water, and then dried under a stream of N2. Electrodes were then immersed in conc. NH4F(aq) for 30 s, rinsed with running water, dried under a stream of N2, and used immediately. Photoelectrochemical analyses under white light illumination were conducted using an ELH light source with a quartz diffuser and a custom-designed 1 cm thick quartz filter filled with water. Illumination power densities were calibrated with a thermopile (S302A, Thorlabs) and were set to 100 mW cm-2. Lower light intensities were achieved using combinations of appropriate quartz neutral density filters (Thorlabs). A CH Instruments 700 potentiostat was used to record steady-state current-potential data. All potentials are referenced to the solution potential defined by the ratio of the ferrocene and ferrocenium concentrations. Steady-state photocurrent-potential responses were re-

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Figure 2. (a-e) Scanning electron micrographs of cross sections of n-GaP (ND ) 5 × 1017 cm-3) electrodes after (top to bottom) 15, 30, 60, 90, and 120 min of anodic galvanostatic etching at 100 mA cm-2. The length scales in parts a-e are constant. (f) Top-down view of macroporous n-GaP after 15 min etch time. Scale bar ) 10 µm. (g) Top-down view of macroporous n-GaP after 120 min etch time. Scale bar ) 10 µm. (h) Measured pore depth as a function of anodic galvanostatic etching time at 100 mA cm-2.

corded at 0.01 V s-1. The shape of the response for all electrodes was invariant to scan rate at slow scan rates ( 1 were observed here; i.e., the responses are consistent with heterojunctions that are influenced by suboptimal interfacial processes. However, while surface recombination processes were not quantitatively assessed, the present data do show that highsurface-area GaP photoelectrodes do not innately possess too many surface trap states to preclude reasonably good photoresponses. Specifically, deleterious surface recombination at these macroporous n-GaP photoelectrodes is apparently not sufficient to distort77 the shapes of the photoresponses of the macroporous n-GaP photoelectrodes relative to the analogous planar photoelectrodes and the overall photocurrent densities and photoelectrode energy conversion efficiencies are improved by more than a factor of 10. Surface passivation strategies that minimize surface recombination and/or favorably shift the band edge energetics to push GaP into inversion conditions should undoubtedly improve the observed photoresponses but were not necessary to achieve the results reported herein. Galvanostatic etching of n-GaP substrates yields photoelectrodes with tunable physical dimensions. The ease and versatility of this process can be used to explore a wide range of n-type photoelectrode morphologies and porosities.42,78 These aspects should be useful to determine particular architectures that optimize optical (e.g., light trapping) and physical (e.g., doping) properties while minimizing the total material needed. Here, we only highlight one macroporous morphology-material combination. Whether this particular electrode form factor proves the most efficient design for GaP photoelectrodes remains to be seen. However, the salient features of this work are not just to highlight anodic etching as a method for fabricating macroporous GaP electrodes, per se. Rather, the data show that good photoelectrochemical performances ought to be attainable through other preparation schemes that yield GaP electrodes with form factors and doping comparable to the electrodes reported here. Bottom-up strategies for GaP electrode fabrication likely have more potential for impact than anodic etching of single-crystalline wafers for fabrication of solar energy technologies at scale. For example, methods are known for producing semiconductor nanowires with minority-carrier diffusion lengths as large as 10-6 m and may be used to generate analogous photoelectrode structures, as shown here.79,80 The

J. Phys. Chem. C, Vol. 113, No. 28, 2009 11993 present work establishes an encouraging benchmark for such materials in photoelectrochemical applications. Conclusion Photoelectrochemical energy conversion efficiencies of n-GaP in contact with dry acetonitrile solutions containing ferrocene and ferrocenium were improved by more than an order of magnitude by adopting a high-aspect-ratio photoelectrode architecture. N-type GaP with macropores possessing an approximate width and depth of 5 and 45 µm, respectively, converted light with λ e 510 nm into photocurrent with unity internal quantum yield despite a hole diffusion length of only 110 nm. Observed fill factors did not deteriorate with increased macroporosity, indicating mass transport was not limiting through the depth of the macroporous region. Open-circuit photovoltages decreased with increased interfacial area, consistent with the photocurrent being passed across the entire macroporous surface. The data thus indicate that the use of tailored electrode architectures is an effective strategy for overcoming the carrier-collection limitation of GaP in photoelectrochemical applications. Acknowledgment. We thank Dr. J. Mukherjee and Ms. B. Peng for their assistance with the preparation of reference electrodes and Dr. S. Parus for help with the design of the anodic etching setup. M.J.P. acknowledges the support of a Rackham Graduate Fellowship, Regents Fellowship, and the Applied Physics program at the University of Michigan. This work was supported by generous start-up funds provided by the University of Michigan. Supporting Information Available: Optical images of anodically etched GaP for various etch times, the solar irradiance spectrum for AM1.5 conditions and corresponding expected current density for unity quantum yield, the ELH lamp spectrum, transmittance spectra through 2 mm solution, and estimates of the current density from the ELH lamp corrected for solution transmittance are presented. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Parkinson, B. A. Acc. Chem. Res. 1984, 17, 431–437. (2) Gerischer, H. J. Electroanal. Chem. 1975, 58, 263–274. (3) Bard, A. J. J. Electroanal. Chem. 1984, 168, 5–20. (4) Archer, M. D. J. Appl. Electrochem. 1975, 5, 17–38. (5) Turner, J. A. Science 1999, 285, 687–689. (6) Rosenbluth, M. L.; Lieber, C. M.; Lewis, N. S. Appl. Phys. Lett. 1984, 45, 423–425. (7) Fonash, S. Solar Cell DeVice Physics; Academic Press: New York, 1982. (8) Berger, L. I. In CRC Handbook of Chemistry and Physics, 89th ed.; Lide, D. R., Ed.; CRC Press/Taylor and Francis: Boca Raton, FL, 2008; pp 12(89)-12(77). (9) Gronet, C. M.; Lewis, N. S. Nature 1982, 300, 733–735. (10) Kohl, P. A.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 7531–7539. (11) Sulima, O. V.; Sims, P. E.; Cox, J. A.; Mauk, M. G.; Mueller, R. L.; Reedy Jr., R. C.; Khammadov, A. M.; Paulson, P. D. In 3rd World Conference on PhotoVoltaic Energy ConVersion, Osaka, Japan, 2003; pp 737-740. (12) Werner, J. H.; Kolodinski, S.; Quiesser, H. J. Phys. ReV. Lett. 1994, 72, 3851–3854. (13) Dare-Edwards, M. P.; Hammett, A.; Goodenough, J. B. J. Electroanal. Chem. 1981, 119, 109–123. (14) Semiconductor Electrodes; Finklea, H. O., Ed.; Elsevier: Amsterdam, The Netherlands, 1984. (15) Halmann, M. Nature 1978, 275, 115–116. (16) McCann, J. F.; Handley, L. J. Nature 1980, 283, 843–845. (17) Petit, J. P.; Chartier, P.; Beley, M.; Deville, J. P. J. Electroanal. Chem. 1989, 269, 267–281. (18) White, T.; Carter, M. A.; Mottram, A.; Peaker, A. R.; Sudlow, P. D. Nature 1971, 232, 469–470.

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(19) Aspnes, D. E.; Studna, A. A. Phys. ReV. B 1983, 27, 985–1009. (20) Hamilton, B.; Wight, D. R.; Blenkinsop, I. D.; Harding, W. Phys. ReV. B 1981, 23, 5495–5510. (21) Vanmaekelbergh, D.; Erne, B. H.; Cheung, C. W.; Tjerkstra, R. W. Electrochim. Acta 1995, 40, 689–698. (22) Li, J.; Peat, R.; Peter, L. M. J. Electroanal. Chem. 1984, 165, 41– 59. (23) Maiolo, J. R.; Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Kelzenberg, M. D.; Atwater, H. A.; Lewis, N. S. J. Am. Chem. Soc. 2007, 129, 12346–12347. (24) Yang, F.; Forrest, S. R. ACS Nano 2008, 2, 1022–1032. (25) Maiolo, J. R.; Atwater, H. A.; Lewis, N. S. J. Phys. Chem. C 2008, 112, 6194–6201. (26) Kelly, J. J.; Vanmaekelbergh, D. Electrochim. Acta 1998, 43, 2773– 2780. (27) Shchetnin, A. A.; Drozhzhin, A. I.; Sedykh, N. K.; Novokreshchenova, E. P. Izmer. Tekh. 1978, 4, 35–36. (28) Yang, C. Y.; Heeger, A. J. Syn. Met. 1996, 83, 85–88. (29) Park, J. H.; Kim, S.; Bard, A. J. Nano Lett. 2006, 6, 24–28. (30) Kayes, B. M.; Atwater, H. A.; Lewis, N. S. J. Appl. Phys. 2005, 97, 114302(1)-114302(11). (31) Erne, B. H.; Vanmaekelbergh, D.; Kelly, J. J. J. Electrochem. Soc. 1996, 143, 305–314. (32) Erne, B. H.; Vanmaekelbergh, D.; Kelly, J. J. AdV. Mater. 1995, 7, 739–742. (33) Ellis, A. B.; Bolts, J. M.; Kaiser, S. W.; Wrighton, M. S. J. Am. Chem. Soc. 1977, 99, 2848–2854. (34) Segar, P. R.; Koval, C. A. J. Electrochem. Soc. 1988, 135, 2655– 2656. (35) Koval, C. A.; Austermann, R. L. J. Electrochem. Soc. 1985, 132, 2656–2662. (36) Casagrande, L. G.; Juang, A.; Lewis, N. S. J. Phys. Chem. B 2000, 104, 5436–5447. (37) Goodey, A. P.; Eichfeld, S. M.; Lew, K. K.; Redwing, J. M.; Mallouk, T. E. J. Am. Chem. Soc. 2007, 129, 12344–12345. (38) Maldonado, S.; Plass, K. E.; Knapp, D.; Lewis, N. S. J. Phys. Chem. C 2007, 111, 17690–17699. (39) Tiginyanu, I. M.; Langa, S.; Sirbu, L.; Monaico, E.; Stevens-Kalceff, M. A.; Foll, H. Eur. Phys. J.: Appl. Phys. 2003, 27, 81–84. (40) Gibbons, J. F.; Cogan, G. W.; Gronet, C. M.; Lewis, N. S. Appl. Phys. Lett. 1984, 45, 1095–1097. (41) Aspnes, D. E.; Studna, A. A. Appl. Phys. Lett. 1981, 39, 316–318. (42) Foll, H.; Langa, S.; Carstensen, J.; Christophersen, M.; Tiginyanu, I. M. AdV. Mater. 2003, 15, 183–198. (43) W ) [(2εsVbi)/(qND)]1/2, where εs is the dielectric of the semiconductor and Vbi is the built-in potential component of the equilibrium barrier height. The other terms have the same meanings as described in the text. (44) Kumar, A.; Lewis, N. S. J. Phys. Chem. 1990, 94, 6002–6009. (45) Green, M. A. J. Appl. Phys. 1976, 47, 547–554. (46) Srour, J. R.; Hopkins, M. A. IEEE Trans. Nucl. Sci. 1984, NS31, 5. (47) Emery, K.; Myers, D.; Rummel, S. In PhotoVoltaic Specialists Conference, Las Vegas, NV, 1988; pp 1087-1091. (48) Gronet, C. M.; Lewis, N. S. Appl. Phys. Lett. 1983, 43, 115–117. (49) Lieber, C. M.; Gronet, C. M.; Lewis, N. S. Nature 1984, 307, 533– 534. (50) Peng, K.; Xu, Y.; Wu, Y.; Yan, Y.; Lee, S. T.; Zhu, J. Small 2005, 1, 1062–1067. (51) Peng, K.; Wang, X.; Lee, S. T. Appl. Phys. Lett. 2008, 92, 163103(1)-163103(3).

Letters (52) Spurgeon, J. M.; Atwater, H. A.; Lewis, N. S. J. Phys. Chem. C 2008, 112, 6186–6193. (53) Voc ) (nkBT/q) ln(Jsc/γJ0), where γ is the ratio of the true electrode surface area to the projected geometric area. (54) Mel’nikov, V. A.; Golovan, L. A.; Timoshenko, V. Y.; Zheltikov, A. M.; Muzychenko, D. A.; Ukraintsev, E. V.; Laptinskaya, T. V.; Kashkarov, P. K. Laser Phys. 2004, 14, 660–663. (55) Rivas, J. G.; Lagendijk, A.; Tjerkstra, R. W.; Vanmaekelbergh, D.; Kelly, J. J. Appl. Phys. Lett. 2002, 80, 4498–4500. (56) Aroutiounian, V. M.; Arakelyan, V. M.; Shahnazaryan, G. E.; Hovhannisyan, H. R.; Wang, H. L.; Turner, J. A. Sol. Energy 2007, 81, 1369–1376. (57) Souza, F. L.; Lopes, K. P.; Nascente, P. A. P.; Leite, E. R. Sol. Energy Mater. Sol. Cells 2009, 93, 362–368. (58) Hu, Y. S.; Kleiman-Shwarsctein, A.; Forman, A. J.; Hazen, D.; Park, J. N.; McFarland, E. W. Chem. Mater. 2008, 20, 3803–3805. (59) Kennedy, J. H.; Frese, K. W. J. Electrochem. Soc. 1978, 125, 709– 714. (60) Jang, J. S.; Lee, J.; Ye, H.; Fan, F. F.; Bard, A. J. J. Phys. Chem. C 2009, 113, 6719–6724. (61) Prakasam, H. E.; Varghese, O. K.; Paulose, M.; Mor, G. K.; Grimes, C. A. Nanotechnology 2006, 17, 4285–4291. (62) Beermann, N.; Vayssieres, L.; Lindquist, S. E.; Hagfeldt, A. J. Electrochem. Soc. 2000, 147, 2456–2461. (63) Kay, A.; Cesar, I.; Gratzel, M. J. Am. Chem. Soc. 2006, 128, 15714– 15721. (64) Duret, A.; Gratzel, M. J. Phys. Chem. B 2005, 109, 17184–17191. (65) Kocha, S. S.; Turner, J. A.; Nozik, A. J. J. Electroanal. Chem. 1994, 367, 27–30. (66) Turner, J. A.; Deutsch, T. G.; Koval, C. A. J. Phys. Chem. B 2006, 110, 25297–25307. (67) Deutsch, T. G.; Head, J. L.; Turner, J. A. J. Electrochem. Soc. 2008, 155, B903-B907. (68) Ginley, D. S.; Chamberlain, M. B. J. Electrochem. Soc. 1982, 129, 2141–2146. (69) Yablonovitch, E.; Gmitter, T. J. Solid-State Electron. 1992, 35, 261– 267. (70) Stringfellow, G. B. J. Vac. Sci. Technol. 1976, 13, 908–913. (71) Gershenzon, M.; Mikulyak, R. M. Appl. Phys. Lett. 1966, 8, 245– 247. (72) Heller, A. ACS Symp. Ser. 1981, 146, 57–77. (73) Sturzenegger, M.; Prokopuk, N.; Kenyon, C. N.; Royea, W. J.; Lewis, N. S. J. Phys. Chem. B 1999, 103, 10838–10849. (74) Novotny, C. J.; Yu, E. T.; Yu, P. K. L. Nano Lett. 2008, 8, 775– 779. (75) Bard, A. J.; Bocarsly, A. B.; Fan, F. R. F.; Walton, E. G.; Wrighton, M. S. J. Am. Chem. Soc. 1980, 102, 3671–3677. (76) Lewis, N. S. J. Electrochem. Soc. 1984, 131, 2496–2503. (77) Anz, S. J.; Lewis, N. S. J. Phys. Chem. B 1999, 103, 3908–3915. (78) Yoriya, S.; Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 13770–13776. (79) Allen, J. E.; Hemesath, E. R.; Perea, D. E.; Lensch-Falk, J. L.; Li, Z. Y.; Yin, F.; Gass, M. H.; Wang, P.; Bleloch, A. L.; Palmer, R. E.; Lauhon, L. J. Nat. Nanotechnol. 2008, 3, 168–173. (80) Kelzenberg, M. D.; Turner-Evans, D. B.; Kayes, B. M.; Filler, M. A.; Putnam, M. C.; Lewis, N. S.; Atwater, H. A. Nano Lett. 2008, 8, 710–714.

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