5766
J. Phys. Chem. 1988,92, 5766-5770
Studies of Polycrystalline n-GaAs Junctions: Effects of Metal Ion Chemisorption on the Photoelectrochemical Properties of n-GaAs/KOH-Se-/*-, n-GaAs/CH,CN-Ferrocene'", and n-GaAs/Au Interfaces Sharon R. Lunt, Louis G. Casagrande, Bruce J. Tufts, and Nathan S. Lewis* Department of Chemistry, Stanford University, Stanford, California 94305 (Received: December 7 , 1987)
Current-voltage and spectral response properties were determined for polycrystalline n-GaAs photoanodes in contact with aqueous KOH-Se-IZ- and nonaqueous CH3CN-ferrocene+/O-LiC1O4electrolytes. The n-GaAs/KOH-Se-l2- system initially exhibited poor junction behavior, but chemisorption of Ru"', Rh"', Ir"', Co"', or OS"'ions onto the GaAs photoanode was found to yield improved I-V properties. The trend in I-V improvement correlated with improved electrocatalysis of SeZoxidation at pGaAs, n+-GaAs,and In,O? electrode surfaces. The n-GaAs/CH2CN system displayed excellent junction behavior and did not ;espond.to metal chemisoiption treatments. These results are consistent with thd mktal-ion-indukd improvement being predominantly due to electrocatalytic effects.
One of the key advantages of semiconductor/liquid junctions is the ability to access the surface junction under operating cell conditions and to perform in situ chemical treatments which improve the junction behavior. Some chemical methods have been shown to be strikingly effective in improving the performance of certain semiconductor/liquid and semiconductor/metal junctions.'-" In particular, treatment of GaAs surfaces with aqueous Ru"' ions has been shown to yield improved I-V characteristics for single-crystal n-GaAs photoanodes in 1.0 M KOH-1.0 M Se2--0.01 M Se22- electrolytes' and also to yield a decreased surface recombination velocity a t the n-GaAs/air interface.2a The GaAs/Ru'" results have been interpreted by use of a model in which deleterious gap states at surfaces and at grain boundaries are chemically reactive with the Rul" ions, causing the remaining density of gap states to be less effective at promoting carrier recombination processes.'2 For semiconductor/liquid junctions, improvement in the I-V characteristics arises primarily from a reduction in the ratio of surface recombination to minority carrier inje~tion.'~Thus, either electrocatalysis of minority carriers into the electrolyte or passivation of surface recombination could be primarily responsible for the I-V improvement in any given photoelectrode/electrolyte system. For strongly adsorbed, inner-sphere redox couples such as the Se-12- system described above, effects of electrocatalysis might be expected to be particularly important. In a recent study of the mechanism of I-V improvement of n-GaAs (100) single-crystal photoanodes in contact with KOH-Se-12- media, we proposed that electrocatalysis of Se2oxidation by chemisorbed metal ions was the dominant effect in the n-GaAs/KOH-Se-/2- ~ y s t e m . * Evidence ~ ~ ~ ~ ' ~which supported this conclusion was obtained from studies of the I-Vbehavior of In203 anodes, p-GaAs cathodes, and n+-GaAs anodes in the KOH-Se-12- electrolyte. In addition to the effect of electrocatalysis by metal ions, the passivation of surface recombination processes at n-GaAs/air interfaces2aand the reduction in recombination losses for polycrystalline n-GaAs/metal Schottky barriersI4 and polycrystalline n-GaAs/KOH-Se-12- liquid junction^^^^'^ do seem to suggest an effect of grain boundary passivation at the n-GaAs(Ru"') surface. To investigate this possibility, we have performed a study on the mechanism of I-Vimprovement of polycrystalline GaAs in contact with aqueous KOH-Se-12- solutions and in contact with Au Schottky barriers. These studies have been performed with a series of metal ions, all of which are efficient electrocatalysts for SeZoxidation and all of which yield improved I-V behavior in the single-crystal n-GaAs/KOH-Se-/z- cell. Additionally, we have investigated the I-V behavior of polycrystalline n-GaAs samples in contact with an outer-sphere redox coupled, ferrocene+/O, in CH$N solvent. This interface is informative mechanistically
* Address correspondence to this author at California Institute of Technology, Department of Chemistry, 127-72, Pasadena, CA 91 125. 0022-3654/88/2092-5766$01.50/0
because metal-ion-induced grain boundary passivation effects should still be apparent, but the effects of electrocatalysis by chemisorbed metal ions should be greatly reduced in this weakly adsorbing, electrochemically reversible redox ~ y s t e m . ' ~ , 'We ~ also describe the results of quantum yield vs wavelength studies which were performed to identify any changes in the minority carrier collection length as a function of metal ion chemisorption. Experimental Section All polycrystalline n-GaAs photoelectrodes were fabricated from material supplied by Professor S. Chu of Southern Methodist University. The material was an epilayer of the structure p+/ n/n+-GaAs which was grown on a W/graphite substrate with the n+ layer contacting the W. The grain size, morphology, and (1) (a) Parkinson, B. A.; Heller, A,; Miller, B. Appl. Phys. Lett. 1978,33, 521. (b) Parkinson, B. A,; Heller, A,; Miller, B. J. Elecrrochem. SOC.1979, 126, 954. (c) Miller, B.; Heller, A.; Menezes, S.; Lewerenz, H. J. Faraday Discuss. Chem. SOC.1981, No. 70, 223. (2) (a) Nelson, R. J.; Williams, J. S.; Leamy, H. J.; Miller, B.; Parkinson, B. A.; Heller, A. Appl. Phys. Lett. 1980, 36, 76. (b) Heller, A.; Lewerenz, H. J.; Miller, B. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 592. (3) (a) Hodes, G.; Fonash, S. J.; Heller, A.; Miller, B. Adu. Electrochem. Elecrrochem. Eng. 1984, 13, 113. (b) Heller, A.; Miller, B.; Chu, S. S.; Lee, Y.T. J. Am. Chem. SOC.1979. 101. 7633. (4) Heller, A.; Leamy, H. J.; Miller, B.; Johnston, W. D., Jr. J. Phys. Chem. 1983,87, 3239. (5) Bose, D. N.; Basu, S.; Mandal, K. C.; Mazumdar, D. Appl. .. Phys. Lett. 1986, 48, 472. (6) Folmer. J. C. W.: Turner. J. A.: Parkinson. B. A. Inora. Chem. 1985. 24,'4028. (7) Abruna, H. D.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 6898. (8) (a) Tufts, B. J.; Abrahams, I. L.; Santangelo, P. G.; Ryba, G. N.; Casagrande, L. G.; Lewis, N. S. Nature (London) 1987, 326, 861. (b) Allongue, P.; Cachet, H.; Clechet, P.; Froment, M.; Martin, J. R.; Verney, E. J. Electrochem. Soc. 1987, 134, 620. (c) Butler, M. A.; Ginley, D. S. Appl. Phys. Lett. 1983, 42, 582. (9) Abrahams, I. L.; Casagrande, L. G.; Rosenblum, M. D.; Rosenbluth, M. L.; Santangelo, P. G.; Tufts, B. J.; Lewis, N. S. Noun J . Chim. 1987, 11, 157. (IO) (a) Caley, C. E.; Kurnar, A,; Lunt, S. R.; Miskelly, G. M.; Rosenbluth, M. L.; Santangelo, P. G.; Tufts, B. J.; Lewis, N. S. Proc.-Elecfrochem. Soc., in press. (b) Tufts, B. J.; Abrahams, I. L.; Casagrande, L. G.; Lewis, N. S., submitted for publication. (1 1) White, H. S.; Abruna, H. D.; Bard, A. J. J. Electrochem. SOC.1982, 129, 265. (12) (a) Heller, A. ACSSymp. Ser. 1981, No. 146, 57. (b) Heller, A. J. Vac. Sci. Technol. 1982, 21, 559. (13) (a) Peter, L. M. Electrochemistry 1984, 9, 66. (b) Reiss, H. J. Electrochem. SOC.1978, 125, 937. (c) Albery, W. J.; Bartlett, P. N.; Hamnett, A.; Dare-Edwards, M. P. J . Electrochem. SOC.1981, 128, 1492. (d) Haneman, D.; McCann, J. F. J. Electrochem. Soc. 1982,129, 1134. (e) Kelly, J. J.; Memming, R. J . Electrochem. SOC.1982, 129, 730. (14) Johnston, W. D., Jr.; Leamy, H. J.; Parkinson, B. A.; Heller, A.; Miller, B. J. Electrochem. SOC.1980, 127, 90. (15) Heller, A. Acc. Chem. Res. 1981, 14, 154. (16) (a) Gronet, C. M.; Lewis, N. S.; Cogan, G.; Gibbons, J. Proc. Natl. Acad. Sci. U.S.A.1983, 80, 1152. (b) Lewis, N . S. Annu. Rev. Mater. Sci. 1984, 14, 95. (c) Casagrande, L. G.; Lewis, N. S. J. Am. Chem. SOC.1985, 107, 5793. (17) Van Ryswyk, H.; Ellis, A. B. J. Am. Chem. SOC.1986, 108, 2454. 1
0 1988 American Chemical Society
Polycrystalline n-GaAs Junctions
The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 5767
solid-state solar cell charcteristics of similar material have been published previously.'* For this study, the p+ layer was etched off with a 10-s exposure to 1% Br2-CH30H. Further etching yielded no difference in I-V characteristics until enough material was etched off to affect the value of the short circuit photocurrent. Photoelectrodes were fabricated by attaching the Wfgraphite substrates of the GaAs samples to a Cu wire with silver print, and the electrode assembly was then sealed into glass tubing with white epoxy resin. The electrochemical cell procedures, spectral response apparatus, and photoelectrochemical measurement techniques used in our laboratory have been described in previous publicat i o n ~ . ' ~ *All ' ~ ,electrochemical ~~ measurements were performed under potentiostatic control with a Pt counter electrode and a Pt reference electrode. The potential of the Pt reference electrode was generally about -0.95 V vs SCE in the aqueous KOH-Se-12cell (of composition 1.OM K2Se and 0.01 M K2S%in 1.0 M KOH, unless otherwise specified) and was +0.17 V vs SCE in the CH3CN-ferrocene+/O-LiC1O4 cell (0.09 M ferrocene, 0.5 mM ferrocenium, and 0.70 M LiC104 in dry, N2 purged solvent). Illumination was provided by a 3200 K ELH type tungstenhalogen bulb with a dichroic rear reflector. The polycrystalline electrodes were etched (1% Br2-CH30H for 10 s), rinsed with methanol, and dried with N 2 prior to introduction into the photoelectrochemical cell. Electrodes that received metal ion treatments were etched as above, immersed in the metal ion solution, rinsed with H 2 0 , and dried in a stream of N2 before use. The metal ion solutions consisted of 0.010 M MC13/0.10 M HCI (M = Ru, Os, Ir, Rh), and electrodes were immersed for 60 s at room temperature (unless otherwise noted). Also tested were aqueous p H 1.0 solutions of 0.010 M Co(bpy)3C13, Co("3)6Cb9 Ni("3)6Br2, and [Cr("3)dOH2)J (N03)3-NH4N03,with electrode immersion times of 60 s at room temperature. A convenient shorthand for these interfaces is GaAs(MY), where MY indicates the metal ion oxidation state initially present in the aqueous solution. Due to severe effects of cross-contamination that were observed in earlier studies of single-crystal n-GaAs photoelectrodes,1° all studies with a particular metal ion were performed with a dedicated set of electrodes, redox-electrolyte solution, and glassware. Schottky barriers were fabricated by depositing gold by filament evaporation at a base pressure of
