Effect of Temperature on the Operation of a Photoelectrochemical Device: Studies on the n ‐ GaAs / Room Temperature Molten Salt Electrolyte Interface

Abstract

The effect of temperature on the photovoltaic performance of a photo‐electrochemical (PEC) system was evaluated by measurements on a model system comprising the pyridinium chloride (BPC)|carbon cell containing the ferrocene/ferricenium ion couple. Changes in the solar cell parameters for this system with temperature were monitored for the range from ambient to 85°C. The short‐circuit current densities showed a systematic increase over the temperature range while the open‐circuit voltage and fill‐factor (FF) showed a linear decrease at the rate of 3.6 mV/°C and 0.001/°C, respectively. The energy conversion efficiency (η) of the device showed a maximum at an intermediate temperature (∼50°C) beyond which the decline in and FFoutweighed the beneficial effect of temperature on , leading to a rapid fall‐off in η at temperatures above ∼60°C. The linear decrease of with increasing temperature was seen to be directly related to the increased dark current density at elevated temperatures. Analysis of the dark currentvs. voltage data by Schottky diode theory revealed the expected exponential dependence of the reverse saturation current density on temperature for the model PEC system. Measurement of the semiconductor flatband potential for the interface at varying temperatures revealed a further origin for the increase in dark currents with temperature. The shift in to positive potentials with increasing temperature reduced the effective activation energy (or barrier height) for electron injection from the electrolyte into the semiconductor in the dark. These results are discussed in terms of two mechanisms: (i) direct charge transfer between the semiconductor and the occupied redox levels in the electrolyte and (ii) electron injection into the semiconductor mediated by trap levels in the energy bandgap of the semiconductor. Finally, the effect of temperature on the Schottky diode parameters of the , cell is discussed in terms of enhanced surface and bulk carrier recombination effects at elevated temperatures. These lead to the observed deterioration in the fill‐factor because of the increased series resistance associated with recombination‐generation current flow paths.