Study of n-Type Semiconducting Cadmium Chalcogenide-Based Photoelectrochemical Cells Employing Polychalcogenide Electrolytes

Studies of CdX-based photoelectrochemical cells in X2_/Xn2- electrolytes are reported for X = S, Se, and Te. For eight of the nine electrode/electrolyte combinations we have demonstrated that the n-type semiconducting single-crystal CdX photoelectrodes are stable to anodic dissolution. Only for CdTe in S2_/Sn2- do we find that oxidation of the added chalcogenide does not quench the decomposition of CdX typically found in aqueous electrolytes. For all eight remaining electrolyte/electrode combinations the added chalcogenide is oxidized at the photoelectrode at a rate which precludes anodic dissolution of the CdX. For the stable combinations each electrolyte is capable of being oxidized at the photoelectrode and subsequently reduced at the dark counter electrode to complete a cycle where no net chemical change obtains. For all nine electrolyte/electrode combinations and for the CdX in alkaline H2O, the redox level associated with the oxidation of X2- or with O2 evolution is between the valence band and conduction band positions at the semiconductor-electrolyte interface. Thus, energetic requirements for X2- oxidation or O2 evolution from H2O are met in all cases, but apparently kinetic factors control whether oxidation of X2- or of H2O will be fast compared to anodic dissolution, which is also energetically feasible. For the stable electrode/ electrolyte combinations, conversion of optical to electrical energy can be accomplished with efficiencies of > 10% for monochromatic visible light. For CdTe or CdSe in the Te2-/Te22- electrolyte input power densities of > 500 mW/cm2 can be converted with a few percent efficiency with no deterioration of properties. Output voltages at maximum power conversion efficiency are of the order of 0.4 V.