Nanoparticle‐Assisted Crystallization Engineering Toward Efficient, Low‐Temperature‐Processed Sb ₂ (S,Se) ₃ Solar Cells

Hydrothermally deposited Sb 2 (S,Se) 3 is a promising absorber for solution‐processed thin‐film solar cells, yet its post‐treatment typically requires high‐temperature annealing (≈370°C), which inevitably causes severe selenium loss. This loss generates a compositionally disordered near‐surface region and degrades device performance. Here, we develop a simple PbBr 2 ‐assisted approach in which an in‐situ formed Pb(S x Se 1−x ) nanoparticle film initiates Sb 2 (S,Se) 3 crystallization at a substantially lower temperature. The nanoparticle film is shown to direct the microstructural evolution by accelerating grain coalescence and inducing (hk1)‐preferred crystallinity, producing a denser and flatter absorber with markedly suppressed selenium loss and reduced surface oxidation. As a result, the champion PbBr 2 ‐treated device delivers a PCE of 10.51%, representing a 16.8% improvement over the 9.00% control, mainly arising from enhanced surface selenium retention and improved interfacial band alignment as supported by compositional and electronic analyses. This work offers a practical materials‐engineering strategy for Sb 2 (S,Se) 3 photovoltaics that resolves the long‐standing trade‐off between high‐temperature crystallization and selenium loss, providing a pathway to stabilize surface composition while maintaining a high‐quality junction for higher device efficiency.