J. Phys. Chem.
1983,87,3239-3244
during the process. It can therefore be easily inferred that dimerlike molecules should be able to display triplet CD, provided that some amount of electron delocalization causes the pure exciton model to break down. That such a breakdown of the EM occurs in binaphthyls and bianthryls is apparent from the optical activity of their singlet p band, which is short axis polarized in the monomers and should be optically inactive according to the exciton model (see eq 6), contrary to observation. The CD of this transition has been accounted for in several ways, which all amount to assuming some conjugation through the 1-1’ bond%,@and hence electron delocalization between monomers in addition to the exciton energy delocalization. But if electron delocalization occurs in binaphthyls, excitation will lead to real charge displacements over the whole dimer molecule, and there must be no more re(40)I. Hanazaki and H. Akimoto, J. Am. Chem. SOC.,94,4102(1972).
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strictions on displaying CD in the triplet spectrum. The amount of electron delocalization depends on the orbital overlap between the moieties. Increasing the interplanar angle towards 90° causes a reduction of delocalization and decreases the intensity of TT-CD (see Figure 2). Finally, it is apparent that TT-CD of binaphthyls happens to follow the same general trends as ground-state optical activity with respect to changes of the interplanar angle, a fact illustrated by the empirical correlation shown in Figure 3.
Acknowledgment. We are greatly indebted to Professor V. Prelog for a generous gift of optically pure spirobifluorenes. This work was supported, in part, by the Centre National de la Recherche Scientifique under A.T.P. 2661. Registry No. I, 18531-94-7;11,35294-28-1;111,39648-67-4;IV, 86334-02-3;V, 86289-51-2;VI, 86334-03-4;VII, 86289-52-3;VIII, 86289-53-4;IX, 86289-54-5;X, 86289-55-6;XI, 86289-56-7;XII, 86289-57-8.
Chemical Passivation of Carrler Recombination at Acid Interfaces and Grain Boundaries of p-InP Adam Heller,’ Harry J. Leamy, Barry Miller, and W. Dexter Johnston, Jr. Bell Laboratories, Murray Hill, New Jersey 07974 (Received: January 28, 1983)
Incorporation of submonolayers of silver into surfaces and grain boundaries of p-InP increases the current collection efficiency at the p-InP/Ti Schottkyjunction and at the InP/VCl3-VCl2-HC1, pInP/EuC13-EuC12-HC1, and p-InP(Rh)/H2-HC1semiconductor-liquid junctions. Charge collection scanning electron microscopy (EBIC) shows a 1000-fold increase in current collection for junctions between chemically vapor-deposited p-InP and titanium when the InP is treated with silver prior to junction formation. A 600-fold increase in output in some samples, and an equivalent solar-to-electric power conversion efficiency of 7%, have been observed in photoelectrochemical cells made with small-grained, chemically vapor-deposited p-InP films on graphite.
Introduction The term “surface modification of semiconductor photoelectrodes” encompasses two areas of research. One of these is aimed at reducing photocorrosion by mediation of charge transfer through an organic or metal organic layer between photoelectrodes and reactants in solution.’-15 The (1) Wrighton, M. S.; Austin, R. G.; Bocarsly, A. B.; Bolts, J. M.; Hass, 0.; Legg, K. D.; Nadjo, L.; Palazzotto, M. C. J. Am. Chem. SOC. 1978,100, 1602. (2)Bocarsly, A. B.; Walton, E. G.; Bradley, M. G.; Wrighton, M. S. J. Electroanal. Chem. 1979,100,283. (3)Bolts, J. M.; Bocarsly, A. B.; Palazzotto, M. C.; Walton, E. G.; Lewis, N. S.; Wrighton, M. S. J . Am. Chem. SOC.1979,101,1378. 1978,100,5257. (4)Bolts, J. M.; Wrighton, M. S. J. Am. Chem. SOC. (5)Bolts, J. M.; Wrighton, M. S. J . Am. Chem. SOC.1979,101,6179. (6)Bocarsly, A. B.; Walter, E. G; Wrighton, M. S. J. Am. Chem. SOC. 1980,102,3390. (7)Bruce, J. A.; Wrighton, M. S. J. Electroanal. Chem. 1981,122,93. (8)Lewis, N. S.;Wrighton, M. S. ACS Symp. Ser. 1981,No. 146, 37. (9)Dominey, R. N.; Lewis, N. S.; Bruce, J. A.; Bookbinder, D. C.; Wrighton, M. S. J . Am. Chem. SOC.1982,104,467. (IO) Fan, F. R. F.; Whealer, B. L.; Bard, A. J.; Noufi, R. N. J. Electrochem. SOC.1981,128, 2042. (11)Leempoel, P.; Castrc-Acuna, M.; Fan, F. R. F.; Bard, A. J. J. Phys. Chem. 1982,86,1396. (12)Yoneyama, H.;Muraw, Y.; Tamura, H. J. Electroanal. Chem. 1980,108, 87. (13)Noufi, R.; Frank, A. J.;Nozik, A. J. J. Am. Chem. SOC.1981,103, 1849.
objective in the other is reorganization of surface and grain boundary states by chemical means so as to neutralize their undesired effects, such as loss in barrier height, recombination of carriers, and reduced mobility.16-19 Representative of such studies are those of Seager, Ginley,2G22 and others23on passivation of silicon grain boundaries by atomic hydrogen, those of ourselves on passivation of nGaAs surfaces and grain boundaries by chemisorbed submonolayers of ruthenium(II1) and lead(II)24-30and on (14) Skotheim, T.; Lundstrom, I.; Prejza, J. J. Electrochem. SOC.1981, 129,1625. (15)Nakato, Y.; Shioji, M.; Tsubomura, H. J. Phys. Chem. 1981,85, 1670. (16) Heller, A. ACS Symp. Ser. 1981,No. 146,57. (17)Heller, A. Acc. Chem. Res. 1981,14,154. (18)Heller, A. In “Frontiers of Chemistry”; Laidler, K. J., Ed., Pergamon Press: Oxford, 1981;pp 27-40. (19)Heller, A. J . Vac. SOC.Technol. 1982,20,2. (20)Seager, C. H.; Ginley, D. S. Appl. Phys. Lett. 1979,34,337. (21)Seager, C.H.;Ginley, D. S.; Zook, J. S. Appl. Phys. Lett. 1980, 36,831. (22)Seager, C. H.; Ginley, D. S. J. Appl. Phys. 1981,52, 1050. (23)Johnson, N.M.; Biegelsen, D. K.; Moyer, M. D.; Deline, V. R.; Evans, C. A. Appl. Phys. Lett. 1981,38,995. (24)Heller, A.; Parkinson, B. A.; Miller, B. “Proceedings of the 13th IEEE Photovoltaic Specialists Conference”; IEEE: New York, 1978;p 1253. (25)Parkinson, B. A.; Heller, A.; Miller, B. Appl. Phys. Lett. 1977,33, 521. (26)Heller, A,; Miller, B. Adu. Chem. Ser. 1980,184,215.
0022-3654/83/2087-3239$01.50/00 1983 American Chemical Society
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passivation of p-InP crystal faces by 6-10-A indium oxide,31-34those of Ginley et ai. on passivation of grain boundaries of layered chalcogenides by silver,35and those of Abruna and Bard% and Razzini et al.37on passivation of surfaces of layered chalcogenides by organic species. Here we discuss the passivation of surfaces and grain boundaries of p-InP by submonolayers of chemisorbed silver, which we reported, in part, in an earlier communicati~n.,~Orders of magnitude improvements are attained. For example, in small-grained chemically deposited p-InP films on graphite as much as a 1000-fold improvement in current collection at dry Schottky junctions and a 600-fold improvement in the efficiency of a semiconductor-liquid junction solar cell are observed.
Experimental Section Photocathodes. Single-Crystalline Photocathodes. Zn-doped single crystals of p-InP, grown by the liquidencapsulated Czochralski technique, were sliced to produce (100) or (111) wafers and polished on a rotating cloth wetted with 1% br~mine-methanol.~~ Ohmic contacts were made by evaporating 2000 A of Au-2% Zn on one of the faces and heating for 30 s. The wafers were cleaved to 6-10 mm long, 3-6 mm wide pieces. Back copper wire contact leads were attached with silver epoxy and the samples were mounted in epoxy. Polycrystalline Photocathodes. Type A . A block of Carbone-Lorraine 5890-PT grahite was sliced into 15 X 15 x 2 mm pieces by using a diamond wheel to reduce surface damage. These were cleaned for 15 s in methanol in an ultrasonic bath to loosen the free graphite, rinsed with methanol, blown dry in Nz, baked for 5 h at 800 "C in a hydrogen atmosphere, and stored in an evacuated desiccator. First InGaAs and then InP films were deposited by reacting the corresponding metal halides with arsine or phosphine on the graphite substrate, heated to 700 OC. The metal halides were formed at 830 O C in a stream of hydrogen plus hydrogen chloride passing over molten Ga and/or In. The metal halide stream was mixed with hydrogen-diluted arsine or phosphine containing dimethyl zinc for doping to 10'' ~ m - ~The . total gas flow was 5 layer was L/min. Initially, a 10 pm thick In0.52Gh.48As deposited in a 20-min process, and then InP was grown for 35 min to form a 20-pm layer on top of the InGaAs film. The resulting grain sizes were of 0.5-5.0 pm. Type B. These films, also Zn doped, had 3-4-pm grains. They were made by a process in which Zn-doped GaAs was first deposited on graphite, followed by InP.40
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(27) Johnston, W. D., Jr.; Leamy, H. J.; Parkinson, B. A.; Heller, A.; Miller, B. J. Electrochem. SOC.1979, 126, 954. (28) Heller, A.; Miller, B.; Chu, S. S.;Lee, T. Y. J. Am. Chem. SOC. 1979, 101,7633. (29) Nelson, R. J.; Williams, J. S.;Leamy, H. J.; Miller, B; Casey, H. C., Jr.; Parkinson, B. A.; Heller, A. Appl. Phys. Lett. 1981, 36, 76. (30) Heller, A.; Lewerenz, H. J.; Miller, B. Ber. &uw?Rges. Phys. Chem. 1980,84, 592. (31) Heller, A.: Miller, B. Lewerenz, H. F.: Bachmann, K. J. J . Am. Chem. Soc. 1980, 102, 6555. (32) Heller, A.; Miller, B.; Thiel, F. A. Appl. Phys. Lett. 1981,38, 282. (33) (a) Heller, A.; Vadimsky, R. G. Phys. Reu. Lett. 1981, 46, 1153. (b) HeUer, A.; Aharon-Shalom, E.; Bonner, W. A.; Miller, B. J.Am. Chem. SOC.1982,104,6942, ( c ) Aharon-Shalom. E.: Heller, A. J. Electrochem. SOC.1982, 129, 2865. (34) Lewerenz, H. J. Aspnes, D. E.; Miller, B.; Malm, D. L.; Heller, A. J. Am. Chem. SOC.1982,104, 3325. (35) Ginley, D. S.; Biefeld, R. M.; Parkinson, B. A.; Keung-Kam, K. J. Electrochem. SOC.1982, 129, 145. (36) Abruna, H. D.; Bard, A. J. J.Electrochem. SOC.1982, 129, 673. (37) Razzini, G.; Bicelli, L. P.; Pini, G.; Scrosati, B. J. Electrochem. SOC.1981, 128, 2134. (38) Heller, A.; Vadimsky, R. G.; Johnston, W. D., Jr.; Strege, K. E.; Leamy, H. J.; Miller, B. 'Proceedings of the 15th IEEE Photovoltaic Specialists Conference";IEEE: New York, 1981; p 1722. (39) Bonner, W. A. J. Cryst. Growth 1981,54, 21.
Heller et al.
Chemicals and Solutions. VC13-VC12-HC1 solutions were prepared from VzO5 (3 N, Cerac), Zn (5 N, Alfa), and concentrated HC1 (ultrapure, Alfa) as follows: 5.3 g of V205 was suspended by stirring in 55 mL of HzO, and then 65 mL of concentrated HC1 was added. The suspension was then purged with nitrogen and 5.5 g of Zn was added. The solution warmed up and changed color while the V205 dissolved. The reduction was stopped by decanting from the Zn when the redox potential dropped to -0.45 V vs. SCE. The EuC1,-EuC12-HC1 solution was made as follows: 0.25 mol of Eu203was dissolved in 0.25 L of HC1, diluted to 0.5 L, and then reduced in an electrochemical flow reactor (ECO Model 1000) with two graphite electrodes and a NAFION membrane until the potential reached -0.6 V vs. SCE. The total Eu2+plus Eu3+concentration was 1 M and the free HC1 concentration approximately 3 M. KAg(CN), (0.1 M)-KCN (0.1 M) solutions were made with AgNO, (3 N, Spectrum) and KCN (99.0%, Fisher). Electrochemical Cells. The cells were continuously purged with N2. Saturated calomel and Ag/AgCl/saturated KC1 were used as reference electrodes, the first in regenerative, the second in hydrogen-generating cells. The counterelectrodes were spectroscopic carbon rods in regenerative cells and IrOz-Ir(Ta03)4 coated titanium in hydrogen (and chlorine) generating cells. Radiosiluer (ll0Ag) Solutions. K"OAg(CN), (0.1 M)KCN (0.1 M) solutions were used to determine the amount of chemisorbed silver on the various p-InP faces. A 10-mC lloAg solution (1.8 mL) in 1 M HNO, was purchased from New England Nuclear Co. The acid was neutralized with 0.127 g of KOH, and then 0.175 g of AgNO, and 0.220 g of KCN were dissolved to make a 1 M KAg(CN),-l M KCN solution, which was 10-fold diluted. Chemisorption experiments were performed on polished crystals, lightly etched to minimize surface roughening, in HC1 solutions previously deoxygenated by bubbling nitrogen €or 30 min. (111)A (indium) and (100) faces were etched in 0.1 M HC1, and (111)B (phosphorus) faces in 0.5 M HC1, both for 1 min. Light Sources. A 100-W tungsten-halogen lamp rated at 8.3 A112 V was used at 7 A. This source produced a light limited current density of up to 75 mA/cm2, nearly 3 times the p-InP photocurrent observed under -AM1 sun. Electrochemical Instrumentation. Current-voltage behavior was monitored with potentiostat-galvanostats comprised of either a 175 PAR programmer and a PAR 173, or a PINE RDE-3. Charge Collection Microscopy. EBIC measurements were performed with a JEOL Model 35 scanning electron microscope with a Keithley Model 427 current amplifier. Polycrystalline p-InP/Ti Schottky junctions were made by evaporating -200 8, of Ti on the semiconductor film.
Results Figure 1A shows that, when silver ions are chemisorbed on a (100) surface of p-InP, previously freshly etched either in methanol-2% bromine-1:l ammonia cycles or in concentrated HC1, the i-V characteristics of the p-InPJVCl3-VClZ-HC1IC and the p-InP~EuC1,-EuClz-HCl~Ccells are improved. Figure 1B shows the change in the characteristics for a (111)B p-InP surface of the p-InPIVC1,VCl,-HClIC cell before and 1 h after dipping the photocathode into any of the silver sources, 0.1 M AgNO3,O.l M Ag(NH&N03-1 M NH40H, or 0.1 M KAg(CN)z-O.l (40) Bachmann, K. J.; Buehler, E.; Shay, J. L.; Wagner, S.Appl. Phys. Lett. 1976, 29, 121.
The Journal of Physical Chemistty, Vol. 87, No. 17, 1983 3241
Chemical Passivation of Carrier Recombination
V vs SCE
VOLTS
0
N
E
’0-
40
VOLTS
a
E
75.
20 -
Figure 2. Characteristics of the hydrogengenerating cell p-InP(Rh)l4 M HCil Ir02-Ir(Ta03), under tungsten-halogen irradiance equivalent to -3 AM1 suns. Upon photoelectrochemical plating of a
