A model for Schottky barrier-like heterojunction photoelectrodes is presented. This model allows the calculation of the current-voltage curves for such electrodes under different conditions of illumination and electrochemical charge transfer. SnO2-coated n-type Si electrodes in contact with the redox systems KJK4 Fe (CN)e and C12C1-show experimental photocurrent-voltage curves with the behavior predicted by the model. The effect of the charge transfer overvoltage and the expected current limitation due to photon control and/or redox ion diffusion are demonstrated. The feasibility of solar energy conversion through photoelectrolysis, by means of cells based on heterojunction photoelectrodes, is discussed.Photocorrosion is a general problem of photoelectrochemical cells where small bandgap semiconducting electrodes are used. To avoid this problem, a range of coatings has been applied to the electrode surfaces. Thin layers of metals (1-5) or of transparent and conducting oxides (6-8) have been deposited onto the semiconductor by different techniques. The resulting electrodes can mostly be described as Schottky barrier-like heterojunctions with an electrolyte front contact.SnO~ is a particularly attractive material for this purpose because of its high transmittance in the visibleinfrared range, its resistance against chemical attack, and because of the possibility of incorporating foreign atoms which modify both solid-state and interracial properties in a wide range (9-12). We shall consider the following electrodes composed of n-type Si-SnO2 heterojunctions. A schematic representation of the Si-SnO2-electrolyte system is given in Fig. 1. The photovoltage Vj is generated at the junction SiSnO2, which is a device of the Schottky barrier type (13). The electrochemical reaction, on the other hand, proceeds at the SnO~-electrolyte interface and is thus separated from the photoactive junction by the protective oxide film. This physical separation is advantageous in the definition of the current-voltage characteristics of the system since it permits first analyzing separately and then combining the concepts characterizing the photovoltaic and the electrochemical part of this photoelectrochemical cell.In this work, we derive a mathematical model for the treatment of heterojunction photoelectrodes. Furthermore, we shall use this model to discuss the performance of illuminated Si-SnO~ electrodes in contact with the redox systems I~/K4 Fe(CN)6 and C12/C1-. We disregard in our treatment the special case of such thin layers (< 50A) that can be tunneled by photogenerated minority carriers. Their protection against corrosion is not likely to last for long enough time since it is hardly possible to produce such thin layers without pinholes or larger defects. The SnO2 layers of about 800A thickness used in our experiments here were tight, and we could not detect any photocorrosion of the silicon after more than 30 hr of operation under solar-like illumination (14).
TheoryThe equivalent circuit corresponding to the consi~lered photoe...
ChemInform Abstract The experimental setup that allows to observe in situ the overall semiconductor surface and to perform electroreflectance (EER) and photoelectrochemical measurements is described. Lead electrodeposition on the CuInSe2 semiconductor surface is found to occur preferentially on localized regions. Systematic mapping of the polycrystalline CuInSe2 sample indicates that domains with a small donor concentration exist on the surface of the low-resistive n-type semiconductor. EER measurements on the low-resistive area show bands unpinning under polarization at potentials more cathodic than the flatband potential. This is attributed to an accumulation of negative charges on the surface during the electroreduction, which occurs at the less resistive area where the electroreduction of Pb occurs preferentially, but not in the low-doped domains of high resistivity where the bandedges remain pinned.
Photocurrent transients have been measured with better than 1 nanosecond time resolution in photoelectrochemical cells with insulator and semiconductor electrodes. The shape of the transients observed with semiconductor electrodes in the time window of several nanoseconds can be deduced from a simple equivalent circuit. The transient observed in the external circuit consists of two contributions, the first is the time integral and the second is directly proportional to the actual photoresponse. At low light intensity good semiconductor electrodes show only the first contribution. There is no direct influence of charge carrier discharge or of the Helmholtz‐layer as such on the fast transient in the nano‐second range.
Ein entwickeltes Modell für Photoelektroden mit Heteroübergängen vom Schottky‐Sperrschicht‐Typ erlaubt die Berechnung der Strom‐Spannungs‐Kurven für diese Elektroden unter verschiedenen Bedingungen von Belichtung und elektrochemischer Ladungsübertragung (Heteroübergangs‐Photozellen in Kontakt mit einem elektrochemischen System).
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