Analytical expressions for the illuminated current-voltage characteristics of coupled photodiode-electrocatalyst fuel forming devices are derived. The approach is based on combining solid-state diode behavior with metal electrochemistry via the diode equation and the Butler-Volmer equation (charge transfer coefficients: αA = αC = α) or the Tafel equation (|η|118mVne), respectively. The analytical expression for the current-voltage behavior of the coupled photodiode-electrocatalyst device (αA = αC = 0.5) and an isolated photodiode is plotted, compared, and augmented with band diagrams at equilibrium, open circuit, short circuit, and the maximum power point to illustrate the effect of coupling an electrocatalyst to a photodiode. The applicability of the derived equations is then demonstrated by comparing with a recently reported high efficiency, thin film InP/InOxPy/Rh photoelectrosynthetic hydrogen evolution device.

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