Pt/CuSe: a New Counter Electrode for Polyiodide Reduction in Quantum Dot-Sensitized Solar Cells

The most common structure of a quantum dot-sensitized solar cell (QDSC) employs a metal sulfide counter electrode (CE) combined with a polysulfide electrolyte (S^(2-)/S_n^(2-)). Polyiodide electrolytes (I^-/I_3^-) have an advantage over polysulfide for their high photovoltage output. However, most semiconductor QDs will dissolve in polyiodide. This work presents a new double-layer CE material Pt/CuSe that is compatible with polyiodide. The electrocatalytic activity of three types of CE: Pt, CuSe and Pt/CuSe, are compared using cyclic voltammetry, Tafel plots and electrochemical impedance spectroscopy. Among the three CEs, the Pt/CuSe CE exhibits the best catalytic activity of low charge transfer resistance and a high current density. Pt/CuSe CEs are then applied to ternary Pb5Sb8S17 QDSCs that are stable in polyiodide. The QDSCs with Pt/CuSe CE exhibits a power conversion efficiency (PCE) of 3.71% (with ZnS passivation), which is significantly higher than cells with Pt (2.53%) and CuSe (2.68%) CEs. At the reduced intensity of 0.1 sun, the PCE further increases to 5.54%, a respectable efficiency for a QDSC. To our best knowledge, Pt/CuSe is the first composite CE for QDSCs using polyiodide and has potential for improving performance in QDSCs.