ZnO nanorods decorated with metal sulfides as stable and efficient counter-electrode materials for high-efficiency quantum dot-sensitized solar cells

Abstract

As a promising type of new-generation solar cells, the electrocatalytic activity and stability of counter electrodes (CEs) play a key role in the performance of QDSSCs (quantum-dot-sensitized solar cells) at present. Here, a facile solution-processing method for fabricating metal sulfides (CoS, NiS, CuS and PbS) on vertically aligned ZnO nanorods (NRs) has been demonstrated and used to produce efficient CEs in polysulfide electrolyte-based QDSSCs. Compared with bare metal sulfide CEs (CoS, NiS, CuS and PbS), the ZnO NR framework presents a larger surface area for loading more metal sulfide catalysts and easy accessibility of the electrolyte. Additionally, the metal sulfide catalyst with high catalytic activity plays the main role in the reduction of the oxidized polysulfide, while the ZnO NRs offer an excellent electron pathway for shuttling electrons to highly catalytic metal sulfide sites and facilitate charge transport during catalysis. Cyclic voltammetry measurements indicate that the ZnO/PbS CEs still retain good cyclability after 50 cycles, demonstrating super-stability, while the ZnO/CoS, ZnO/NiS, ZnO/CuS, and Pt CEs show obvious fluctuations. Therefore, the ZnO/PbS CE exhibits much higher catalytic activity with the polysulfide electrolyte than ZnO/CoS, ZnO/NiS, ZnO/CuS and Pt CEs. As a result, a QDSSC based on the ZnO/PbS CE achieves a power conversion efficiency (η) of 4.76%, which is attributed to the high fill factor (FF) of 0.566, and the η is much higher than that based on ZnO/CoS (2.75%), ZnO/NiS (3.12%), ZnO/CuS (4.10%) and Pt (1.54%) CEs. The excellent catalytic performance along with the facile preparation of ZnO NRs decorated with metal sulfide CE materials make them a distinctive choice among the various CEs studied.