Composition engineering and physicochemical properties of ultrasonic sprayed Cu ₂ (Zn, Al)SnS ₄ thin films: a combined experimental and simulation study

Achieving high performance efficiency of copper zinc tin sulphide (CZTS) thin film solar cells is a challenging issue, mainly hindered by a large open circuit-voltage deficit ( V OC ) owing in large part to the presence of various Cu–Zn related defects and associated non-radiative recombination. Cation substitution at the Zn site offers a promising route to suppress these defects and engineer the optoelectronic properties of the absorber layer. In this work, we present the first systematic study of partial Zn-site substitution by Al across a wide compositional range (0 < x < 0.75) in Cu 2 (Zn, Al)SnS 4 (CZATS) thin films deposited by the low-cost and scalable ultrasonic spray pyrolysis. X-ray diffraction and Raman spectroscopy confirmed the formation of the kesterite phase across all compositions, with structural degradation emerging at x = 0.75. The optical band gap decreases progressively from 1.51 eV to 1.25 eV with increasing Al content, a trend mechanistically linked to growing lattice microstrain and enhanced band tailing. Electrically, hole mobility improves substantially as x increases from 0 to 0.5, consistent with grain growth and suppression of donor-type intrinsic defects, before deteriorating sharply at x = 0.75 due to increased dislocation density. Furthermore, the numerical analysis of the photovoltaic performance of CZATS solar cells using one-dimensional solar cell capacitance simulator with experimentally measured data revealed that the highest simulated power conversion efficiency is achieved at the composition x = 0.25, demonstrating the benefit of moderate Al substitution for photovoltaic performance. These results establish an optimal substitution window ( x = 0.25−0.5) and provide a physically coherent framework linking Al content, kesterite defect chemistry, and solar cell performance, offering a practical and scalable pathway toward higher-efficiency CZTS devices.