Modulating surface morphology via potassium fluoride-assisted solution post-treatment enables VTD-SnS thin film solar cells to achieve over 4 % efficiency

초록

Tin sulfide (SnS) is an attractive absorber material for thin-film photovoltaics due to its earth abundance, nontoxicity, and higher light absorption coefficient. However, SnS films fabricated by vapor transport deposition (VTD) often suffer from poor surface quality, secondary phases, and unfavorable interfacial energetics, which lead to high recombination losses and limited device efficiency. Conventional surface treatments have shown limited success in simultaneously improving film quality and passivating surface defects. In this work, we demonstrate a simple post-deposition surface modification strategy using potassium fluoride (KF) solution to effectively engineer the surface of VTD-SnS films. The KF treatment enhances film uniformity, densification, and wettability, while X-ray photoelectron spectroscopy confirms the formation of a thin SnF2 layer accompanied by a reduction of surface SnO species. This surface modulation not only passivates defects but also increases the work function and improves band alignment with the CdS buffer, leading to more favorable interfacial energetics. Devices based on the optimized KF-treated SnS absorber achieve a power conversion efficiency of 4.10 %, compared to 3.42 % for untreated films, with DLTS analysis revealing reduced recombination sites. KF-induced surface modulation through SnF2-mediated defect passivation represents a scalable strategy to overcome key interfacial limitations and advance SnS thin-film photovoltaics.

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