Transparent Cu1.8S and CuS thin films on FTO as efficient counter electrode for quantum dot solar cells

•Hydrothermal synthesis of Cu1.8/CuS thin films for efficient counter electrodes.•Dependence of product stoichiometry on starting copper precursor.•A solar conversion efficiency of 1.66% was achieved with CdS–TiO2.•Cu1.8/CuS counter electrodes exhibited high % IPCE and low Rct values.

The use of metal sulfides as counter electrode (CE) improves the short circuit current, open circuit voltage, fill factor, and power conversion efficiency (η) of quantum dot solar cells (QDSCs) compared to the conventional Pt electrode. Here we demonstrate a simple hydrothermal method for the preparation of copper sulfide thin film on FTO, containing Cu1.8S and CuS (Cu1.8S/CuS) which can serve as efficient CE for QDSCs. Hydrothermal deposition of Cu1.8S/CuS is carried out using thiourea as a source of sulfur and different copper salts as a source for copper. The product stoichiometry between Cu and S is observed to be dependent on the copper precursor. Under the illumination of one sun (100 mW cm−2), the CdS–TiO2 QDSCs incorporated with Cu1.8S/CuS CE exhibits η values of 1.66%, which is much higher than the value 0.61% obtained with Pt CE. Electrochemical impedance spectroscopy reveals low charge transfer resistance between polysulfide electrolyte and Cu1.8S/CuS CE, indicating the high catalytic activity of Cu1.8S/CuS CE. The results of this work show the potential application of hydrothermal synthetic method in making Cu1.8S/CuS CE instead of Pt, for large-scale transparent QDSCs in the future.

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