Effect of ramp rate on the growth and photovoltaic performance of CuSbS₂ absorbers synthesized via a two-stage process

Precursor layer deposition followed by sulfurization is a widely adopted method for fabricating thin-film solar cells. Among the process parameters, the temperature ramp rate during sulfurization plays a critical role in determining the quality of CuSbS₂ (CAS) absorber layers, yet its influence has not been systematically investigated. This study aims to optimize the ramp rate to enable the growth of high-quality CAS absorbers and improve solar cell performance. Cu/Sb/Cu precursor stacks were sulfurized at 450 °C for 5 min using ramp rates ranging from 2.5 to 20 °C/min. The slowest ramp rate of 2.5 °C/min resulted in complete intermixing of the metal layers to form CAS, but also led to partial decomposition, the formation of an Sb₂S₃ secondary phase, and reduced film thickness. Intermediate ramp rates of 5, 7.5, and 10 °C/min facilitated the formation of phase-pure CAS with large, micron-sized grains, and improved electrical properties. In contrast, a rapid ramp rate of 20 °C/min caused surface deterioration, void formation, secondary phases. Solar cells fabricated with CAS absorbers sulfurized at 2.5–10 °C/min exhibited power conversion efficiencies (PCEs) of 2.04–2.50%, short-circuit current densities (J_sc) of 12.20–13.77 mA/cm², open-circuit voltages (V_oc) of 528.7–568.9 mV, and fill factors (FFs) of 30.6–31.6%. However, the fastest ramp rate (20 °C/min) significantly degraded device performance. These findings demonstrate the crucial role of ramp rate in achieving high-quality CAS absorbers and enhancing solar cell efficiency.