Photocorrosion decomposition potentials

Photocorrosion can be prevented by adding a redox couple to the electrolyte whose potential is more favourable than the decomposition potential such that the redox reaction occurs preferentially. When n-CdS is used as photoanode in aqueous electrolytes, the electrode is photocorroded since the reaction, CdS -1- 2h - S -1- Cd, occurs readily. By adding NaOH and sodium polysuphide to the electrolyte (Ellis et al, 1976), photocorrosion is prevented. The /S redox couple preferentially scavenges the photoholes. At the anode, sulphide is oxidized to polysulphide (free sulphur) and free sulphur is reduced back at the dark cathode. Similarly n-Si anodes have been stabilized by using a nonaqueous electrolyte containing a ferricinium/ferrocene redox couple (Legg et al, 1977 Chao et al, 1983). Unfortunately, a similar stabilization technique cannot be applied to photoelectrolysis cells. Some examples of electrode... 

The importance of kinetic factors in the photocorrosion of semiconductors has become increasingly clear during the past year. Of course, the thermodynamic calculation of decomposition potentials serves as a useful guide to the equilibrium situation, but detailed kinetic and mechanistic considerations are more immediately relevant to the problem of long-term photoelectrode stabilization. Gerischer has reviewed thermodynamics and kinetics of photodecomposition, and Cardon et have developed a detailed kinetic treatment. Similar... 

The menace of photocorrosion compels to restrict efficiency of the regenerative PEC cells. In fact, to increase efficiency (through the increase in photopotential) one has to increase the initial (i.e., preexisting in the dark) band bending in the semiconductor. For this reason, the reversible potential (p° of the redox couple in the cell with, say, n-type photoanode should be as positive as possible. Yet, at the same time it should not exceed the decomposition potential for the semiconductor, (p°dec,p Thus, one is forced to deliberately diminish the cell photopotential. As a result, the cell efficiency and stability vary with the solution redox potential in an opposed way. In reality the cell characteristics always are a result of a compromise between the requirements of efficiency and stability. (Therefore, one should take cautiously higher values of efficiency and service life of PEC cells given in published papers, they might be measured not for the same cell.)... 

Let us consider in more detail, using the above concepts, how a photocorrosion process occurs under the illumination of a semiconductor. Suppose that electron transitions at the interface between the semiconductor and solution do not take place in darkness in a certain potential range (the semiconductor behaves like an ideally polarizable electrode). This range is confined to the potentials of decomposition of the semiconductor and/or solution. The steady state potential of a semiconductor is usually determined in this case by chemisorption processes (e.g., of oxygen) or, which is the same in the language of the physics of semiconductor surface, by charging of slow surface states. It is these processes that determine the steady state band bending. 

Consider now the processes caused by the formation of quasilevels. As was noted above, the shift of Fn relative to F is very small for majority carriers (electrons) and can usually be neglected precisely, this was done in constructing Fig. 16b. But for minority carriers (holes) the shift of Fp can be very large. The shifts of both Fnx F and Fp increase with the growing intensity of semiconductor illumination, so that for a certain illumination intensity Fp may reach the level of the electrochemical potential of anodic decomposition Fdec, p, and Fn—the level of a certain cathodic reaction (for example, reduction of water with hydrogen evolution FHljH20). These reactions start to proceed simultaneously, and their joint action constitutes the process of photocorrosion. 

Let us consider, as an example, the photocorrosion behavior of GaAs and GaP in aqueous solutions. The levels of the electrochemical potential of decomposition reactions for these materials are shown in Fig. 16 together with the levels and... 

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