3238 tributions (e.g., vibrational and rotational states of the product) were found to be characterized by temperatures lower than the crystal temperature. J. Phys. Chem. 1984,88, 3238-3243 Materials Sciences Division of the US. Department of Energy under Contract No. DE-AC03-76SF00098. We acknowledge the San Francisco Laser Center for providing the laser system under NSF Grant CHE79-16250. M. Asscher thanks the Chaim Weizmann Post Doctoral Fellowship for providing a Postdoctoral Fellowship. Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, The Energetics of pin Photoelectrolysis Cells L. Fornarini: A. J. Nozik,* and B. A. Parkinson* Photoconversion Research Branch, Solar Energy Research Institute, Golden, Colorado 80401 (Received: May 31, 1983) The energetics of p/n photoelectrochemical cells, containing simultaneously illuminated p-type photocathodes and n-type photoanodes, have been investigated by using appropriate combinations of n-WSez, n-MoSe2, n-WS2, n-Ti02, p-InP, p-Gap, and p-Si semiconductor electrodes. The open-circuit photovoltages (Eocv) of the p/n cells were measured as a function of the redox reactions in the cell and as function of light intensity. For many semiconductor electrode combinations, the sum of Eocv plus the standard cell voltage for the net cell reaction (Uo) was found to be constant for a given pair of n- and p-type electrodes at a given light intensity. This constancy was shown to be equivalent to the constancy of the sum of the band bending in the semiconductor depletion layers and the sum of the electrode overvoltages. These results are explained by movement of the semiconductor band edges with changes in the redox reactions that occur at the semiconductor electrode. If special care is taken to produce semiconductor electrodes that do not show surface charging effects, Eocv was found to be independent of the redox reaction. This is the expected behavior for pinned band edges. The dependence of Eocv on light intensity for p-InP/n-MoSez cells was either about 60 mV or about 130 mV per decade increase in light intensity. The latter value occurred when both the p-InP and n-MoSez band edges were pinned, while the former value occurred when only the p-InP band edge was pinned. These conditions for band-edge pinning depended upon light intensity and the nature of the electrolyte. Introduction A photoelectrolysis device has the advantage over a purely photovoltaic device of providing all or some fraction of its output as storable energy. A photoelectrolysis device which employs a single semiconductor photoelectrode (connected to a metal counterelectrode) to provide the necessary photopotential to drive the photoelectrolysis of high-energy reactions must utilize either a large band gap semiconductor or an external bias. A large band gap material reduces the amount of solar radiation absorbed by the photoelectrode, and an external bias requires the input of expensive electrical energy, both of which reduce the practical solar conversion efficiency achievable in the device. One method for augmenting the effective voltage of a pho- toelectrolysis cell is to simultaneously illuminate ohmically con- nected p-type and n-type photoelectrodes which are immersed in an electrolyte.'^* Upon illumination the minority carriers are available at the conduction band edge of the p-type electrode and at the valence band edge of the n-type electrode, while the majority carriers recombine at the ohmic contact. The usable photopotential of this type of p/n cell is increased such that smaller band gap semiconductors can be used to drive more energetic electrode reactions, but at the expense of halving the maximum quantum yield for current flow due to the annihilation of the majority carriers at the ohmic contact. Such a p/n cell requires two photons per separated electron-hole pair and is roughly analogous to photosynthesis in green plants, where both the oxidation of water and the reduction of C 0 2 are also photodriven at two physically distinct photosystem sites.3 The ultimate utility of such a device for solar energy conversion would be governed by the band gaps of the repsective p- and n-type semiconductors, as well as by the position of the energy bands in the semiconductor with respect to the redox levels in the electrolyte. Small band gap materials with the conduction band t Current address: Institute of Chemical Physics, University of Rome, Rome, Italy. 0022-3654/84/2088-3238$01.50/0 of the p-type material at a rather negative potential and coupled to an n-type material with a valence band at a rather positive potential would be the most advantageous. Such a system for the efficient photoelectrolysis of HBr and H I has been described previ~usly.~ The prediction of which materials would be suitable for a given photoelectrolysis reaction would be straightforward if the energy position of the bands of the semiconductor would remain fixed (with respect to the solution redox levels) and independent of the redox species in the solution, the existence of surface states at the semiconductor/electrolyte interface, or whether the semiconductor was illuminated or not. Recent work on one-photoelectrode systems has shown that the band positions at the interface can be changed significantly by any or all of these The work reported herein attempts to analyze the effect of the sem- iconductor, redox electrolyte, and level of illumination on the open-circuit voltage of a p/n photoelectrolysis cell. Appro a c h We chose to study semiconductors which were stable in aqueous redox electrolytes and had relatively small band gaps (with the exceptions of TiOz and Gap). Stability is important because any (1) A. J. Nozik, Appl. Phys. Lett., 29, 150 (1976). (2) H. Yoneyama, H. Sakamoto, and H. Tamura, Electrochim. Acta, 20, (3) A. J. Nozik, Phil. Trans. R. SOC. London, Ser. A, 295, 453 (1980). (4) C. Levy-Clement, A. Heller, W. A. Bonner, and B. A. Parkinson, J. Electrochem. SOC., 129, 1701 (1982). (5) A. J. Bard, A. B. Bocarsly, F. R. F. Fan, E. G. Walton, and M. S. Wrighton, J. Am. Chem. SOC., 102, 3671 (1980). (6) A. J. Bard, F. Fan, A. S. Gioda, G. Nagasubramanian, and H. S. White, Faraday Discuss. Chem. SOC., 70, 19 (1980); A. B. Bocarsly, D. C. Bookbinder, R. N. Dominey, N. S. Lewis, and M. S. Wrighton, J. Am. Chem. SOC., 102, 3683 (1980); J. Gobrecht, H. Gerischer, and H. Tributsch, Ber. Bunsenges. Phys. Chem., 82, 1331 (1978). (7) J. A. Turner, J. Manassen, and A. J. Nozik, Appl. Phys. Lett., 37, 488 (1980). 341 (1975). 0 1984 American Chemical Society