Understanding the Role of Fabry‐Perot Resonances in Maximizing Absorption for Ultrathin Sn Halide Perovskite Optoelectronic Devices

Abstract Tin (Sn) halide perovskites are promising substitutes for lead (Pb) halide perovskites, constrained by Pb toxicity concerns. However, Sn halide perovskites still face significant challenges, including limited phase stability and suboptimal film quality, which ultimately reduce device performance. Employing an ultrathin Sn halide perovskite may mitigate these disadvantages; however, inefficient light absorption limits their performance in optoelectronic devices. Here, a design principle aimed at maximizing absorption in ultrathin Sn halide perovskite devices is introduced by matching their inherent absorption wavelengths with Fabry‐Pérot (FP) resonances. In a device featuring a 46 nm‐thick formamidinium tin iodide (FASnI 3 ) layer, the intrinsic absorption wavelengths at 500 nm and 663 nm are aligned with FP resonances at 455, 490, 630, and 895 nm. This alignment enhances electric field intensities at the core absorption bands, significantly improving absorption within the FASnI 3 layer. Consequently, the ultrathin FASnI 3 optoelectronic device exhibits an increased open‐circuit voltage and reduces dark current, while maintaining a high photocurrent. This approach enables the fabrication of high‐performance devices—a photovoltaic cell with 7% power conversion efficiency and a photodetector with a specific detectivity of 1.40 × 10 12 Jones. The proposed strategy improves device performance and provides a versatile framework for various optoelectronic applications.