Exploring ionic liquid assisted chemical bath deposition of a highly uniform and transparent cadmium sulfide thin film for photovoltaic applications

Cadmium sulfide (CdS) is one of the most important semiconductor materials in solar cells. In this study, different concentrations (0–0.118 M) of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄) ionic liquid (IL) are introduced as a novel complexing agent in dilute chemical bath deposition of CdS thin films. To comprehend the effectiveness of different ionic liquid concentrations as the complexing agent, the structural, morphological, electrical, and optoelectronic properties of the films were investigated. X-ray diffractogram of the CdS thin film exhibited peaks attributed to wurtzite structure, with peak intensity enhanced dramatically after IL addition. From morphological studies, a pinhole-free and uniformly deposited CdS film with large grain size was observed upon inclusion of 0.069 M IL. Optical characterization has shown good transparency up to 85% from the UV-vis spectroscopy analysis. With the variation of the ionic liquid concentration, there was no major difference observed in the energy bandgap. However, an increment in carrier concentration and reduction in resistivity of the deposited thin films were observed. The film with 0.069 M IL showed the maximum carrier concentration value of 7.51 × 10¹⁴ cm⁻³ with the lowest resistivity. Incorporating the optoelectronic properties of the deposited CdS films, numerical simulations were performed to validate those as electron transport layers for perovskite solar cells with the device structure of FTO/CdS (CdS-0 to CdS-3)/CsSnBr₃/P3HT/Ag. Simulation results demonstrated that the fabricated CdS thin film fabricated with 0.069 M BMIMBF₄ would be a promising candidate in perovskite solar cells with an efficiency of around 16.5%.