Noble metal-free counter electrodes utilizing Cu2ZnSnS4 loaded with MoS2 for efficient solar cells based on ZnO nanowires co-sensitized with CuInS2-CdSe quantum dots

Noble metal-free counter electrodes using a co-catalytic loading of MoS2 nanosheets onto Cu2ZnSnS4 microspheres are investigated by means of CuInS2-CdSe quantum dot (QD) co-sensitized solar cells fabricated with single crystalline ZnO nanowires. An ex situ electrophoretic deposition route is employed to deposit QDs onto ZnO nanowires that are epitaxially grown on ZnO seed layered FTO substrates. Hydrothermally grown ZnO nanowires have also established an excellent stability against the bias conditions applied to fabricate the photoanodes of the devices. The superior photovoltaic performance of CuInS2-CdSe QD co-sensitized cells is compared with that of bare CuInS2 and CdSe counterparts, using current-voltage and incident photon-to-current conversion efficiency measurements. The present study also demonstrates an enhanced performance of the devices fabricated with 1.0 wt% of MoS2 loaded Cu2ZnSnS4 based counter electrodes in contrast to bare Cu2ZnSnS4. Hydrothermal loading of MoS2 onto Cu2ZnSnS4 generates an electrically interconnected network of Cu2ZnSnS4 microspheres, leading to facile charge transport in the counter electrode of the devices. MoS2 in its nanosheet form acts as an electrical bridge that interlinks the Cu2ZnSnS4 microspheres. Additionally, a favorable band energy alignment of Cu2ZnSnS4 and MoS2 stimulates the charge transfer dynamics in the Cu2ZnSnS4-MoS2 composite. Electron transport and recombination kinetics of the devices are measured using electrochemical impedance spectroscopy.