Heat-treatment-induced development of the crystalline structure and chemical stoichiometry of a Cu x S counter electrode, and the influence on performance of quantum-dot-sensitized solar cells.

Recently, various phases of CuxS (1 ≤ x ≤ 2) were extensively explored as superb counter electrode (CE) materials for quantum dot-sensitized solar cells (QDSSCs). Herein, hexagonal covellite CuS (HC-CuS) with hierarchical nanostructure was grown on porous Ti substrates by chemical bath deposition, and then heat treated in the temperature range of 150–450 °C under N2 atmosphere. The reaction process and the evolution of morphology, composition and crystalline structure of CuxS with the variation of heat treatment temperature were studied by XRD, SEM, EDX, TEM and XPS. The photovoltaic properties of TiO2/CdS/CdSe QDSSCs based on CuxS CEs showed an obvious dependence on the element stoichiometry and crystalline structure of the CuxS. With HC-Cu1.28S heat-treated at 230 °C as CEs, QDSSCs achieved a power conversion efficiency of 3.88% under one sun illumination (100 mW cm−2, AM 1.5 G), which was higher than the counterparts with other compositions. Electrochemical impedance spectroscopy, Tafel polarization and cyclic voltammetry measurement showed that the electrocatalytic activity of HC-Cu1.28S CE was much higher than that of other CuxS CEs, which supported the results of the enhanced short-circuit current density, open circuit voltage and filling factor.