Researchers from China's Jiangsu University, Zhejiang Institute of Quality Sciences and Tianjin University have developed an additive-engineering strategy combined with polymer encapsulation to significantly improve the performance and stability of lead-free CsSnI3 perovskite photodetectors.
Tin-based perovskites such as CsSnI3 are considered strong candidates to replace lead-based materials due to their lower toxicity, cost advantages, and favorable optoelectronic properties, including tunable bandgaps, high absorption coefficients, and strong carrier mobility. These features make them particularly suitable for photodetection across the visible to near-infrared range. However, their practical deployment has been hindered by poor stability and performance degradation, primarily driven by the oxidation of Sn2+ to Sn4+.
This oxidation process plays a central role in device instability. It promotes the formation of tin vacancies (VSn), increases defect density, and accelerates the phase transition of CsSnI3 from its photoactive black phase (B-γ-CsSnI3) to the non-photoactive yellow δ-phase. This transition reduces both light absorption and carrier mobility, ultimately degrading photodetector performance. Suppressing Sn2+ oxidation is therefore critical to maintaining both structural and electronic integrity.
To address this, the researchers investigated additive engineering using three different additives: SnF2, metallic Sn, and H3PO2. CsSnI3 precursor solutions and thin films were fabricated via a solution process, followed by device fabrication and systematic comparison.
All three additives were found to effectively suppress Sn2+ oxidation, but SnF2 delivered the most pronounced improvement. Mechanistically, SnF2 creates Sn-rich conditions that reduce the formation energy of tin vacancies, thereby limiting defect formation and self-doping effects. In addition, SnF2 promotes more uniform crystal growth, resulting in higher-quality perovskite films with improved electronic properties.
The optimized SnF2–CsSnI3 photodetector exhibited a responsivity of 0.299 A/W and a specific detectivity of 4.62×1011 Jones under 532 nm laser irradiation, demonstrating strong photoresponse and sensitivity.
To further improve environmental stability, the team introduced a polyvinylidene fluoride (PVDF) encapsulation layer. PVDF, a chemically stable and optically transparent fluoropolymer, acts as a barrier that limits exposure of the perovskite film to air and moisture. This encapsulation significantly reduced oxidation-related degradation.
As a result, the PVDF-encapsulated SnF2–CsSnI3 photodetector retained 76% of its initial performance after one week in air, compared to 62% for the unencapsulated device. This demonstrates a clear improvement in operational stability without adding process complexity, as the PVDF coating can be applied without additional purification or separation steps.
Overall, this work shows that combining additive engineering - particularly with SnF2 - and polymer encapsulation provides an effective route to mitigate oxidation, stabilize the black phase, and enhance the efficiency of tin-based perovskite photodetectors. The approach offers a practical pathway toward high-performance, environmentally friendly photodetection technologies.
