A photocathodic effect at the CdS-electrolyte interface

J. Electroanal. Chem., 130 (1981) 391--394

391

Elsevier Sequoia S.A., Lausanne - - P r i n t e d in The Netherlands

Preliminary note A PHOTOCATHODIC E F F E C T AT THE CdS--ELECTROLYTE INTERFACE

B. VAINAS and G. HODES

Department of Plastics Research, The Weizmann Institute of Science, Rehovot 76100 (Israel) J. DUBOW

Polytechnic Institute of New York, Dept of EE and Computer Science, 333 Jay St., Brooklyn, N Y 11201 (U.S.A.) (Received 9th October 1981)

There have been several reports in the literature on the p h o t o e n h a n c e m e n t of the forward current in semiconductor--electrolyte diodes [ 1--3 ]. A very small photocathodic effect was found for TiO2 in 1 M NaOH solution [1] while a larger effect was observed for CdS in polysulfide electrolyte [2] but only on the S (0001) face of the crystal. In both cases the p h o t o e n h a n c e m e n t effect was attributed to mediation by surface states. A similar effect on p - t y p e Se (in this case, a photoanodic effect) has been explained by a thinning of the space charge layer upon illumination [3]. In the present note we report more than an order of magnitude photoenhancement of the forward (cathodic) current at a CdS electrode on both the S and Cd faces. This effect is induced by mechanically damaging the surface, and disappears after thorough chemical etching. Single crystal CdS, approximately 1 ~ cm, cut perpendicular to the c-axis was used. Ohmic contact was made by rubbing with a G a i n alloy with subsequent soldering of In metal. The electrolyte was a deaerated aqueous solution of 0.1 M K3 Fe(CN)6 and 0.1 M K4 Fe(CN)~ kept under an argon atmosphere all the time. The electrochemical cell was of a conventional three electrode configuration with saturated calomel reference (SCE) and platinum foil counter electrodes. Magnetic stirring was used during the potentiostatic measurements. Surface damage of the CdS crystal was induced by polishing with dry fine crocus cloth leading to a network of scratches ca. 1 ~m in width. Since we found no appreciable difference in photocathodic behaviour between the Cd- and S-face, the results described here refer equally to both of them. Figure 1 shows the photoenhancement of the cathodic current (the photocathodic effect) obtained with the mechanically damaged electrode. The dark forward current at the damaged electrode appears at a relatively high forward bias. Thus, at a potential of --1000 mV vs. SCE which corresponds to a forward bias of 1220 mV (+220 mV vs. SCE is the redox potential of the

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electrolyte, i.e.zero bias),the cathodic current in the dark is only 12.1 # A / c m 2 . Also, current rise with potential is extremely slow in comparison to the ideal 60 mV/decade behaviour expected in the case of semiconductor electrodes [4]. These findings suggest an extremely non-ideal diode behaviour or, in the Schottky diode terminology, the mechanically damaged electrode exhibits a large n factor [5] (from Fig. 1, the average n is approximately 14 in the potential range between --0.6 and --1.0 V vs, SCE) which can be explained by the presence of a high density of interface states induced by the mechanical damaging. W e assume that in the dark the surface states are filledwith electrons up to the majority carrier Fermi level in the semiconductor (i.e.,assuming a fast charge transfer kinetics between the surface states and the conduction band of n-type semiconductor). An application of a forward bias in dark will then increase the negative charge on the surface states. The resulting charging of the Helmholtz layer capacitor causes an upward shift of the band edges thus making the decrease in the band bending less than the applied forward bias. Thus, part of the external voltage bias falls within the Helmholtz layer resulting in n> 1. Illumination will then deplete the negative charge on the surface states to the extent that they will be filled with electrons up to the quasi-Fermi level of the photogenerated holes at the interface [6] which under illumination is situated at a more positive electrochemical potential than the majority carrier Fermi level. This reduction of the negative charge on the surface will shift the band edges to a more positive energy (on the electrochemical scale). This results in a net reduction of the band bending at a constant forward bias, and an increase in the forward current. In other words, illumination

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"lowers" the fiat band potential. In Fig. 2 the differences between the total forward currents at an electrode potential of --1000 mV vs. SCE (curves b--e in Fig. 1) and the corresponding current in the dark (curve a in Fig. 1) are plotted as functions of the illumination intensity (both axes on a logarithmic scale). The linear dependence between log I~' (I~'is the increment of the forward current If by illumination) and log I ill (log illumination intensity) shown in Fig. 2 can be explained by the interdependence of the flat band potential, the surface charge and the illumination intensity. Thus, A Qss (the decrease of the electron charge in the surface states) is proportional to log ~fll if it is assumed that the population of a uniform density distribution of surface states in the bandgap energy range is determined by the position of the quasi-Fermi level of the photogenerated minority carriers at the interface, which is in turn logarithmically dependent on the concentration of the photogenerated holes and the illumination intensity. The flat band potential is determined by the electrical potential drop in the Helmholtz layer and is linearly modulated by the term AQssd/eeo where d is the Helmholtz layer width and e is its dielectric constant. Since the forward current is exponentially dependent on the extent of the band bending and thus on the flat band potential at constant bias or on AQss as was argued above, it is evident that log I~ is proportional to log/ill. Etching the electrode for 15 s in conc. HC1 produces a much improved diode characteristic whereby the dark cathodic current appears at much less negative potentials but the photocathodic effect is still apparent. Only after repeating the etching procedure four times and making cyclic voltammetry runs between them an almost ideal diode I--V behaviour is obtained (n~ 1.4) in the dark and a very small residual photocathodic effect appears upon illumination, as is shown in Fig. 3. These results can be explained by the gradual

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Fig. 3. I - - V characteristics of the system in Fig. 1 after etching with conc. HC1 as described in the text. elimination of the surface states with each subsequent etching. As suface state densities become very small the band edges are not expected to shift with the bias (n=l) and the photocathodic effect is thus eliminated. Several etching treatments as described above are required to remove the surface damage completely and thus the high density of surface states associated with this damage. In summary, we have demonstrated a strong enhancement of the forward current, obtained by illumination. This effect occurs only with a mechanically damaged surface of a CdS electrode and is almost absent after the damaged electrode has been etched with concentrated HC1. In general, similar effects can be expected if the mechanical surface damage has not been removed completely from semiconductor electrodes by sufficient etching. In view of the relatively high sensitivity of the above effect at low illumination levels (see curves b and c in Fig. 1) it might be considered as a possible photodetection device. ACKNOWLEDGEMENTS The authors thank Dr. Norbert Miiller, Dr. Uri Lachish and Dr. Krishnan Rajeshwar for many valuable discussions. REFERENCES 1 2 3 4 5 6

H. Morisaki, M. Hariya and K. Yazawa, Appl. Phys. Lett., 30 (1977) 7. H. Minoura and M. Tsuiki, Chem. Lett., (1978) 205. W. Gissler, J. Electrochem. Soc., 127 (1980) 1713. H. Gerischer in H. Eyring, D. H e n d e r s o n and W. Jost (Eds.), Physical Chemistry, Vol. IX A: An Advanced Treatise, Academic Press, New York, 1970, p. 463. See for example E.H. R h o d e r i c k i n M e t a l - - S e m i c o n d u c t o r C o n t a c t s , C l a r e n d o n Press, Oxford, 1980. S. Kar, J. Appl. Phys., 49 (1978 ) 5278.