Influence of the redox couple concentration and activity of a NaOH/Na2S/S electrolyte on the performance of CdS thin-film photoelectrochemical cells

This work focuses on the interplay between redox couple activity and electrolyte concentration in terms of quantum cell efficiency and photocurrent in CdS thin-film photoelectrochemical solar cells. Optimization of the CdS thin-film electrodes was achieved through electrodeposition and chemical bath deposition, followed by controlled annealing. UV–visible electronic spectroscopy and Tauc measurements were used to determine that the energy gap of the CdS electrode was 2.4 eV. XRD confirmed the cubic structure of CdS, while SEM images revealed the agglomeration of CdS nanoparticles. The PEC performance with respect to different concentrations of NaOH/Na₂S/S electrolyte, that is, 0.25, 0.5, 0.75, and 1 M, was studied; the results revealed that the activity of the redox couple improved the efficiency. In this context, the ionic strength and redox solution activity were calculated by the Debye–Hückel equation. Specifically, a clear correlation was clearly obtained in this study between the PEC efficiency and solution activity (R² = 0.95 for the quantum cell efficiency and R² = 0.93 for the photocurrent density), which is greater than that obtained for the concentration alone, for which R² = 0.88 for the quantum cell efficiency and 0.83 for the photocurrent density. Consequently, the variation in ionic activity is one of the major parameters controlling the performance of PECs and, accordingly, solar energy conversion.