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...
The long-range, short-range model of the nitrogen isoelectronic impurity in Ga(As, P) is discussed in terms of the results of recent photoluminescence and lifetime measurements. The predictions of the theory are shown to depend sensitively upon the strength of the coupling among the states produced separately by the long-and short-range parts of the potential: The strength of the coupling reflects the specific model of potential used in all cases treated. Strong coupling yields theoretical energies whose general composition dependence mirrors the features of the data. The effect of consideration of the L-conduction-band minima is smaller. Determination of potential parameters from experimental energies indicates a range-20-25 A. Luminescence calculations require an accurate treatment of the continuum contribution. Predictions of the theory are in good agreement with available data. Inclusion of L indicates the existence of an excited nitrogen state in GaP. No internal inconsistencies in the theory are found.
']?he growth of CdSe films electrodeposited on various substrates for solar photoelectrochemical cells has been studied by means of in situ optical reflectance experiments. The film growth rate, the faradaic efficiency of the electrodeposition process, and the dispersion of the film refractive index have been evaluated with this technique. The diffuse reflectance has also been measured during the plating process, and its behavior has been discussed.
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