In Situ Microwave‐Assisted Fabrication of Hierarchically Arranged Metal Sulfide Counter Electrodes to Boost Stability and Efficiency of Quantum Dot‐Sensitized Solar Cells

Abstract This study describes preparation of metal sulfide counter electrodes (CEs) through one‐pot microwave‐assisted route to improve power conversion efficiency (PCE) of quantum dot‐sensitized solar cells at a lower cost. The CuS nanorods, Ni_0.96S nanoparticles, and PbS nanocubes are synthesized and deposited in situ on fluorine‐doped tin oxide substrate to serve as CEs without further post‐treatment. Effects of several reaction parameters including sulfur precursor (Na₂S, C₂H₅NS, CH₄N₂S), Cu concentration, reaction time, and choice of cation (Cu, Ni, Pb) on the CEs morphology, electrochemical characteristics, and PCE are studied. Furthermore, nanostructure formation and thin film growth are studied and correlated with PCE, from which morphology‐ and composition‐performance relationships can be inferred. Hierarchically assembled nanorod CuS CEs exhibit higher electrochemical stability in the S^2–/S_n^2– redox reaction. Together with the efficient charge transfer and higher diffusion coefficient of polysulfide redox at the electrode/electrolyte interface, deduced from electrochemical impedance spectroscopy and Tafel analyses, a PCE of 8.32% is achieved for the CuS CE. The enhanced photovoltaic performance is ascribed to the 1D CuS nanorods forming a diffusive structure which decreases charge transfer impedance and facilitates regeneration of polysulfide redox leading to a higher short‐circuit current density and fill factor.