Semiconductor Electrodes XV. Photoelectrochemical Cells with Mixed Polycrystalline n-Type CdS-CdSe Electrodes

Solid solutions of CdS and CdSe of different compositions were prepared by s inter ing pressed pellets and vacuum evaporation. The bandgap of the mixtures varied monotonical ly with percent composition be tween that of CdS and CdSe. Studies of the photoassisted oxidation of sulfide with these electrodes wi th a 1M Na2S, 0.1M NaOH electrolyte showed that the flatband potentials (Vfb) of the mixtures were shifted toward negative potentials with respect to pure CdS and CdSe. Solar cells using these electrodes formulated as (x )n -CdS (1 -x)CdSe/1M Na2S, 0.2M S, 0.1M NaOH/Pt are described. When x = 0.9, for a sintered pellet electrode, a power efficiency of 9% at a cell voltage of 202 mV (irradiat ion wi th 577 nm light of 1 mW/cm~ intensi ty) was obtained. There has been much recent interest in the application of n type cadmium chalcogenide (CdX, where X = S, Se, or Te) electrodes to the construction of photoelectrochemical cells for the conversion of solar to electrical energy (1-14). These materials have bandgap energies (Eg) which are small enough (~2.4 eV) to capture a reasonable fraction of the solar spect r um and can be operated for long time periods with l i t t le or no decomposition of the semiconductor electrode in suitable electrolytes (e.g., S 2 / S x 2 ) . Ceils with solar energy power conversion efficiencies (~p.~) of 7-8% (9) and monochromatic light efficiencies (~]p,m) of 9-10% (4) using single crystal CdX electrodes have been reported. The max imum efficiency of a photoelectrochemical cell uti l izing a redox couple with an equi l ibr ium potential, Vr~dox, for conversion of light to electrical energy with no over-al l change in electrolyte composition depends upon the flatband potential of the semiconductor, Vfb. For a cell with an n type semiconductor photoanode, the more negative the value of Vfb, the greater the possible output voltage (which has a m a x i m u m value Vredox Vfb) and the larger the max imum efficiency. Thus optimization of the semiconductor-electrolyte characteristics involves variation of Eg for opt imum match with the solar energy spectrum (15) and Vfb. Other semiconductor characteristics of importance include the doping level, which * Electrochemical Society Student Member. * * Electrochemical Society Active Member.