Optimizing hydrothermal synthesis temperature for enhanced morphology and electrochemical performance of rGO@Cu₂S counter electrodes in quantum dot sensitized solar cells

Abstract This study investigates the impact of the hydrothermal synthesis temperature on the morphological, structural, and electrochemical properties of rGO@Cu 2 S composite counter electrodes (CEs) for quantum dot-sensitized solar cells (QDSSCs). Reduced graphene oxide–copper sulfide (rGO@Cu x S) composites were synthesized at different temperatures (140, 160, 180, and 200 °C) and characterized using x-ray diffraction (XRD), Raman spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, field-emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM). The electrochemical performance was evaluated using photocurrent density–voltage (J–V) measurements, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Tafel polarization analysis. These results demonstrate that the hydrothermal temperature plays a crucial role in determining the phase composition, surface morphology, and charge transfer efficiency of rGO@Cu x S CEs. At 180 °C, the phase transformation from CuS to Cu 2 S was confirmed by XRD, leading to enhanced electron mobility and reduced charge recombination. Raman and FTIR analyses indicated improved graphitization of rGO, facilitating superior electrical conductivity. FESEM and HRTEM revealed a well-distributed Cu 2 S nanoflake morphology that maximized the electrochemically active surface area. Electrochemical studies showed that the rGO@Cu 2 S-180 electrode exhibited the highest power conversion efficiency (PCE) of 5.322%, outperforming the other samples because of its superior short-circuit current density (J SC = 21.158 mA cm −2 ) and reduced charge-transfer resistance (Rct = 42.50 Ω). The optimized morphology and improved catalytic properties resulted in a 30.5% reduction in Rct compared to that of rGO@Cu x S-140, which correlated with a 61.5% increase in J SC .