Thin-film (TF) solar cells have recently gained attention due to their high energy conversion efficiency and sustainability to meet rising energy demands. The absorber layer is an important part of solar cells and is responsible for light absorption. Popular absorber layers used in TF solar cells include Cu(In,Ga)Se2 (CIGS), cadmium telluride (CdTe), and Copper Zinc Tin Sulphide (CZTS). However, these materials have drawbacks due to the toxicity of the materials used, scarcity, and/or high fabrication costs. To address these issues, this study proposes Copper Tin Sulphide (CTS) as a safer and more abundant alternative for TF solar cell applications. Numerical models for the absorber layer (CTS) and window layer (CdS) were conducted using a Solar Cell Capacitance Simulator (SCAPS-1D) to determine the optimised design. The thickness and Carrier Concentration (CC) of both CTS and CdS layers were varied to achieve an optimised energy band gap (Eg ). Data from these optimisations were used to predict the photovoltaic (PV) device output. The device demonstrated a conversion efficiency (η) as high as 22.56%, with short-circuit current (Jsc) of 35.23 mA/cm2 , open-circuit current (Voc) of 0.827V and Fill Factor (FF) of 77.32%, achieved with a CTS layer of 1´1018 cm-3 carrier concentration, 1,500 nm thickness, and 1.2 eV energy band gap, combined with a CdS layer of 1´1018 cm-3 carrier concentration and 120 nm thickness. The highest reported experimental efficiencies for CTS-based solar cells are 5.1% (Na-doped CTS) and 6.0% (Ge-alloyed CTS). These findings indicate significant potential for improving current TF PV devices. Further experimental investigation is recommended, and successful outcomes may contribute to the long-term sustainability of solar cell technology.