Synergistic effect of thermal and light-soaking treatments on CZTSSe/ALD-ZTO solar cells

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is an attractive thin-film absorber based on earth-abundant and low-toxicity elements. Nevertheless, high-performance CZTSSe solar cells still typically employ CdS buffer layers, which pose significant limitations due to the hazardous nature of Cd and introduce parasitic absorption at short wavelengths. The use of alternative buffer layers is therefore crucial for further advancing this technology. Among the possible candidates, ZnxSn1−xO (ZTO) has emerged as a promising option due to its wide, tunable bandgap and benign constituents. In this work, ZTO buffer layers were deposited by Atomic Layer Deposition (ALD) at three different temperatures (150, 120, 90 °C). However, the as-deposited ZTO films exhibited high resistivity, which was mainly attributed to the controversial role of defect-related intragap states. To overcome this limitation, complete photovoltaic (PV) devices were annealed in air at 260 °C and subsequently exposed to light soaking. These post-deposition treatments synergistically improved the electrical conductivity of ZTO and reduced trap-assisted recombination, thereby enhancing final PV performance. Optical characterisation revealed a widened bandgap compared to conventional CdS, effectively suppressing parasitic absorption in the blue spectral region and enhancing short-circuit current density (JSC) as expected. The best-performing devices were obtained with ZTO deposited at 90 °C, yielding efficiencies (η) exceeding 7% by optimising the trade-off between junction quality and ZTO conductivity. These experimental findings are corroborated by numerical SCAPS simulations and confirm the potential of ALD-grown ZTO as a Cd-free buffer layer for kesterite solar cells.