Surface modification of CuS counter electrodes by hydrohalic acid treatment for improving interfacial charge transfer in quantum-dot-sensitized solar cells.

High charge transfer resistance and low electrocatalytic activity of counter electrodes (CEs) are mainly responsible for the poor photovoltaic performance of quantum-dot-sensitized solar cells (QDSSCs). Herein, a novel strategy has been successfully introduced for the first time to improve the electrocatalytic activity and charge transfer properties of a copper sulfide (CuS) CE by modifying it with the addition of hydrohalic acids (HHA). Through the suitable surface modification of HHA-incorporated CuS CE, the charge transfer from the external circuit to the CE surface was effectively facilitated. The electrochemical analyses suggest that charge transfer resistance is sufficiently reduced at the CE/electrolyte interface by using the HHA-treated CuS CEs. This improvement is mainly attributed to the high electrocatalytic activity of the modified CEs for the reduction of the polysulfide redox couple electrolyte in QDSSCs. The device constructed with TiO/CdS/CdSe/ZnS photoanodes and the hydrogen-fluoride-treated CuS (HFCuS) CE exhibits a power conversion efficiency of 4.25%, which is considerably higher than that of the device with the bare CuS CE (3.11%). These findings can facilitate the fabrication of highly efficient CEs for next-generation solar cells.