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Based upon our observations the lowering of the electronic symmetry is apparently “clean” in the case of C6F50Ht,yielding, as we have noted above, a simple (and comparable) vibrational structure in both the ground and excited electronic states. Because of the almost identical vibrational symmetry, the excitation spectra of C6F6+and C6FsOHf are nearly identical (save for a shift in origin) and the observed emission spectrum characterizing the ground state of C6F50H+yields vibrational frequencies almost identical with the deperturbed frequencies revealed for C6F6+by the Jahn-Teller a n a l y ~ i s . ~ For C6F5CH3+the lifting of the degeneracy of the 2E1, state is probably not so “clean”. Indeed, it would appear based on the very irregular and perturbed emission spectrum reportedI5for C6F5CH,+_that the separation between the vibrationless levels on the A and X states is comparable to the vibrational intervals in the ground state. One would then have to conclude that electronically CH3 more nearly mimics F than does OH. On the other hand, the Jahn-Teller effect is clearly only indirectly affected by a change in vibrational symmetry.
s
In our recent studies of S Y I ~ - C ~ Hwe ~ Fhave ~ + noted that partial deuteration even though it lowers substantially the “vibrational” symmetry and causes appreciable changes in the vibrational frequencies, seems to leave the JahnTeller splitting unaffected. Both the linear and the quadratic Jahn-Teller constants appear to be in the lower symmetry D2H and DH2 species of the same magnitude as in the symmetric D3,, isotopic varieties. Apparently, even though considerable mass asymmetry occurs in these species, the electronic wave functions retain the higher symmetry, maintaining the electronic degeneracy of the ground-state ions. In a similar way we note that C6F5H+ lowers both the electronic and vibrational symmetry of C,$B+. We have previously reportedlg simple regular vibrational structure in both the ground and excited electronic states. The former observation is clearly consistent only with the “clean” breaking of the A and 2 state degeneracy. The vibrational frequencies of C6F5H+given in Table I11 show small but significant shifts from the other ions due to the different “vibrational” symmetry in C6F5Ht.
Photovoltage and Stability of an n-Type Silicon Semiconductor Coated with Metal or Metal-Free Phthalocyanine Thin Films in Aqueous Redox Solutions Yoshlhlro Nakato, Mltsuakl Shioji, and Hiroshl Tsubomura Department of Chemistty, Faculty of Engineering Science, Osaka Universiw, Toyonaka, Osaka, 560 Japan (Received: November 14, 1980)
An n-type silicon (n-Si) semiconductor coated with an evaporated thin film of metal phthalocyanine (MPc) or metal-free phthalocyanine (H2Pc) worked as a fairly stable photoanode in aqueous redox solutions. The photovoltage observed for a photocell, (n-SilCuPclFe3+/Fe2+aqueous solution (pH 4.2) IPt), was 0.50 V, only slightly less than that for a p-n junction Si photocell (-0.6 V). The action spectrum was similar to that of a bare n-Si electrode, except for a depression caused by photoabsorption by the CuPc film in the red region. The above wet photocell had current-voltage characteristics better than those for a solid photocell, (n-SilCuPclPd).
Introduction The photovoltaic effect at a semiconductor-solution interface has been attracting considerable attention in view of solar energy conversion. The main difficulties in utilizing such systems arise from the fact that most available semiconductors having small bandgaps, 1.0-2.0 eV, are subject to corrosion when operated in electrolyte solutions. We reported previously1 that n-type silicon (n-Si) or gallium phosphide (n-Gap) can be stabilized by coating the surface with a thin noble metal film. Unfortunately, the photovoltage observed was relatively small (e.g., 0.1 f 0.1 V for n-Si), and the metal film attenuated the light in a wide wavelength region. Morisaki and Yazawa,2 Tomkiewicz and W ~ o d a l land , ~ Bard et al.4 reported the (1) Y. Nakato, T. Ohnishi, and H. Tsubomura, Chem. Lett., 883, (1975); Y. Nakato, K. Abe, and H. Tsubomura, Ber. Bunsenges. Phys. Chem., 80, 1002 (1976). (2) H. Morisaki, T. Watanabe, M. Iwase, and K. Yasawa, Appl. Phys. Lett., 29,338 (1976);H. Morisaki, M. Ono, H. Dahkoshi, and K. Yazawa, Jpn. J. Appl. Phys., 19, L148 (1980). (3) M. Tomkiewicz and J. M. Woodall, J. Electrochem. SOC.,124, 1436 (1977). (4) P. A. Kohl,S. N. Franck, A. J. Bard, J. Electrochem. SOC.,124,225 (1977). 0022-3654/81/2085-1670$01.25/0
effect of coating n-Si or n-GaAs with stable metal oxide films such as Ti02, Sn02, FezO3,etc. Such systems responded mostly to light of energy higher than the bandgaps of the oxides. Recently, Wrighton et al.5 reported the stabilization of n-Si by binding ferrocenyl compounds to the surface with a silane coupling reagent. In the present paper we will report on the effect of coating n-Si with evaporated phthalocyanine thin films. Experimental Section Single crystals of n-Si in the form of wafers were obtained from Shin-etsu Semiconductor Co., Ltd. They were doped with phosphor, having a resistivity of ca. 0.4 Cl cm, and cut perpendicular to the (111)axis. The ohmic contact was made by evaporating indium metal. The n-Si electrode was polished with 0.3-pm alumina powder and then etched with CPD-2 (an etching solution containing hydrofluoric acid, nitric acid, and a small amount of bromine) for several seconds. After washing and drying, the phthalocyanine was evaporated under 2 X (5) J. M. Bolts, A. B. Bocarsly, M. C. Palazzoto, E. G. Walton, N. S. Lewis, and M. S. Wrighton, J . Am. Chem. SOC.,101,1378 (1979); A. B. Bocarsly, E. G. Walton, and M. S. Wrighton, ibid., 102, 3390 (1980).
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 12, 1981
Phthalocyanine-Coated n-Type Silicon Semiconductor
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U / V vs SCE Flgure 1. Current-potentlal curves for n-Si coated wlth a ca. 100-nm thick CuPc film in a ueous solutions containing (a) 0.05 M Fe(CN),3and 0.05 M Fe(CN)j- (pH 7.1), (b) 0.001 M I2 and 0.1 M I- (pH 7.1), and (c) 0.05 M Fe3+ and 0.05 M Fez+ (pH 4.2) (M = mol/dm3): (---) in the dark: (-) under illumination. eordx is the standard redox potential for the redox couple used.
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Flgure 2. Photocurrent vs. time curve for n-Si coated with a ca. 100-nm thick CuPc film in a solution containing 0.05 M Fe3+and 0.05 M Fez+ (pH 4.2). The electrode potential is kept at 0.4 V vs. SCE.
torr to the thickness of 20-100 nm at a rate of ca. 0.2 nm/s. The film thickness was monitored by a quartz crystal oscillator. Current-potential curves were measured with a Hokutodenko HA 101 potentiostat. A 250-W high-pressure mercury lamp was mostly used as a light source. Monochromatic light was obtained by using a 0.25-m monochromator and a 500-W tungsten lamp. The spectral distribution was measured with an Eppley thermopile. Deionized water and reagent-grade chemicals were used for preparing electrolyte solutions. Reagent-grade metal or metal-free phthalocyanines were used without further purification.
Results and Discussion Figure 1 shows current-potential curves for an n-Si electrode coated with a ca. 100-nm thick copper phthalocyanine (CuPc) film in aqueous solutions. Stable photoanodic currents were observed in the presence of redox couples such as Fe(CN)2-/Fe(CN)6k, I3-/I-, or Fe3+/Fe2+. In the absence of the redox couple, the photocurrent was negligibly small. Similar results were also observed for n-Si coated with zinc or metal-free phthalocyanine films. Figure 2 shows a photocurrent vs. time curve for a CuPc-coated n-Si electrode in a Fe3+/Fe2+solution. The photocurrent is stationary, with the photocurrent density as given. This result is in sharp contrast to the case of a bare n-Si electrode, where the photocurrent decreases to nearly zero in a few minutes, owing to formation of a thin
Flgure 3. (a) Action spectrum of the photocurrent for n-Si coated with a 100-nm thick CuPc film. (b) Absorption spectrum of a thin CuPc film evaporated on a glass plate.
insulating oxide layer on the surface. Very recently, Bard et al.6 reported that n-type CdS, CdSe, and GaP could not be stabilized even by coating with thick metal-free phthalocyanine films (1-1.5 pm). They explained the results by assuming small holes or cracks in the film, through which the electrolyte solution penetrates to the electrode surface. Such holes or cracks may also be present in the present case. Success in the present case can then be attributed to the quick oxidation and passivation of the n-Si surface which may be in contact with the electrolyte solution. The onset potential of the photoanodic current is nearly the same for various redox couples of different standard redox potentials, as is seen from Figure 1. It is therefore expected that the open-circuit photovoltage (V,) for an electrochemical photocell, (n-SilCuPcIredox electrolytelPt), is the higher, the more positive the redox potential of the redox couple in solution. In the case of a Fe3+/Fe2+couple, the V , was observed to be 0.50 V. This value is much higher than that observed for a metal/n-Si Schottky-type photocell1 and is even comparable to that of a p n junction Si photocell (-0.6 V).' The fill factor of the photocell was ca. 0.25, as also estimated from Figure IC. One may notice in Figure 1 that the photocurrent around the onset potential in a &-/I- solution increases with the potential more steeply than that in other redox solutions. This can probably be attributed to the increase of hole conductivity of a CuPc film, caused by penetration of a small amount of iodine into the film, acting as an acceptor impurity. Figure 3 shows the action spectrum of the photocurrent for a CuPc-coated n-Si electrode in a Fe(CN)63-/Fe(CN)6"solution, as compared with an absorption spectrum of a CuPc film on a glass plate. It is seen that the photocurrent starts to flow a t -1100 nm, or 1.1 eV, very close to the bandgap of an n-Si semiconductor. This result indicates that the photocurrent arises mostly from interband excitation of n-Si. The action spectrum shows a depression in a wavelength region from 550 to 750 nm, where a CuPc film absorbs light strongly. It was reported by several workers6V8-l0that thin films of CuPc or analogous com(6) C. D. Jaeger, Fu-Ren F. Fan, and A. J. Bard, 102, 2592 (1980).
J. Am. Chem. SOC.,
(7) Y. Hamakawa, Surf. Sci., 86,444 (1979). (8)N. Minami, T. Watanabe, A. Fujishima, and K. Honda, Ber. Bunsenges. Phys. Chem., 83,476 (1979), and papers cited there. (9) Fu-Ren F. Fan and L. R. Faulkner, J. Chern.Phvs., 69,3334,4441 (1978), and papers cited there.
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I
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n-Si
Nakato et al.
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Figure 4. Schematic energy band model for an n-Si electrode coated with a thin CuPc film in a redox solution.
pounds show a photovoltaic effect a t the interfaces with metal, semiconductors, or electrolyte solutions. However, the quantum yield of the photocurrent in these cases was much smaller than that for the interband excitation of n-Si (-1.0). Therefore, a CuPc film on n-Si is though to act as a light attenuator, rather than a photoactive material. The anodic photocurrent observed for a CuPc-coated n-Si electrode can be explained by a schematic energy band model, shown in Figure 4. It was reported from photoemission experiments under vacuum that the valence band edge at the surface of n-Si is -5.1 eV below the vacuum level,” while that of a CuPc film is at -5.0 eV.12 It can therefore be thought that the valence band edge of n-Si is slightly lower than that of the CuPc film at an n-Si/CuPc interface, as is shown in Figure 4. On the basis of such an energy correlation, it is highly probable that holes photoproduced in the valence band of n-Si are transferred to the valence band of CuPc and then migrate to the CuPc/electrolyte interface and oxidize the reducing agents in solution. As the CuPc film is generally believed to be a p-type material, a blocking contact might be formed betweeen it and the electrolyte solution in the dark. If this is the case, the existence of a steady photocurrent can be explained by taking account of the accumulation of holes in the film, which diminishes the barrier in the contact. Thus, a stationary state is attained, as is shown in Figure 4,if the n-Si electrode is anodic enough. It is also noted that hole transfer at the n-Si/CuPc interface may occur via a surface-trapped hole (surface state) on n-Si, as pro~~
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(10) T. Kawai, K. Tanimura, and T. Sakata, Chem. Phys. Lett., 56,541 (1978); Chem. Lett., 137 (1979). (11) G. W. Gobeli and F. G. Allen, Phys. Reu. A , 137, 245 (1965). (12) M. Pope, J.Chem. Phys., 36,2810 (1962); F. I. Vilesov, Fiz. Tuerd. Tela, 5 , 2000 (1963).
Flgure 5. Photocurrent-voltage characteristics for a solid photocell, (n-SIICuPclPd), In the air (-) and for the cell dipped in a 0.05 M Fe3+ -I-0.05 M Fez+ aqueous solution at pH 4.2 (-.--). Dark current for each case is shown by lines, --- and ..-,respectively.
posed for an n-type gallium phosphide electrode in contact with redox solution^.^^ We also measured the current-voltage characteristics of a solid photocell, designated as (n-SilCuPclPd). The results are shown in Figure 5. The cell was made by evaporating CuPc on n-Si in the same way as described before and then evaporating palladium to the thickness of ca. 20 nm. The open-circuit photovoltage (V,) between n-Si and the Pd film was observed to be ca. 0.26 V, ca. one-half that of the aforementioned wet photocell, (nSilCuPclFe3+/Fe2+solutionlPt). The V , of the solid photocell between Si and Pd was increased to ca. 0.38 V when the Pd side of the solid photocell was in contact with a Fe3+/Fe2+solution. The V , was still smaller than that of the above-mentioned wet photocell. It was reported that the hole conductivity of a ZnPc film was largely increased by wet air.14 The increase of V, by the contact of the solid photocell with the solution might therefore be attributed to an increased hole conductivity of the CuPc film due to the penetration of the solution through the porous Pd film. This is supported by an increase in photocurrent shown in Figure 5. The photocell was stable for more than several hours, indicative of almost no corrosion of n-Si or Pd. The above results are interesting in that they indicate that wetted photocells have current-voltage characteristics better than dried ones. ~
(13) Y. Nakato, A. Tsumura, and H. Tsubomura, presented at the Symposium on “Photochemical and Electrochemical Surface Science: Photoeffecta at Semiconductor-Electrolyte Interfaces” during the 179th National Meeting of the American Chemical Society, Houston, TX, March 1980; Y. Nakato, A. Tsumura, and H. Tsubomura, Chem. Lett., 127, 383 (1981); J. Electrochem. SOC.,in press. (14) H. Tachikawa and L. R. Faulkner, J. Am. Chem. SOC.,100,4379 (1978).
