One-step synthesis of solution processed time-dependent highly efficient and stable PbS counter electrodes for quantum dot-sensitized solar cells

Abstract

The performance of quantum dot-sensitized solar cells (QDSSC) is restricted due to the insufficient electrocatalytic activity and stability of counter electrodes (CEs), which are critical but challenging issues. The exploration of CE catalysts to match the redox couples has become a feasible route in the pursuit of high power conversion efficiency, stability, and reduction of fabrication costs. Electrodes were made by the deposition of a PbS thin film on a fluorine-doped tin oxide substrate at various growth times by a facile one-step chemical bath deposition approach. They were used as a highly catalytic CE without any post-treatments for QDSSCs filled with aqueous sulfide/polysulfide (S²⁻/S_n²⁻) electrolyte. The morphology, thickness, crystal structure, composition, and optical properties are dependent on the growth time of PbS and were characterized by SEM, XRD, XPS, EDS, and UV-vis analysis. In addition to surface morphology, the increase in the atomic ratio of S in PbS plays a key role in increasing the electrocatalytic activity of the CE. These PbS nano-cubes were used as an effective CE material in QDSSCs to show high electrocatalytic activity for catalyzing the reduction of the polysulfide electrolyte, which contributes to significant improvement in the short current density and fill factor. The QDSSC with the optimized PbS CE exhibits a power conversion efficiency of 4.61%, which is much higher than that of Pt CE (1.34%). Furthermore, this PbS CE shows high and consistent electrocatalytic activity toward polysulfide reduction, which was confirmed by electrochemical measurements. This leads to improved photovoltaic performance and superior stability of the QDSSC.