Thickness‐Engineered FAPbI 3 Absorber Layer with Average Visible Transparency Above 24%

Semitransparent perovskite solar cells based on FAPbI 3 offer a compelling alternative to wide‐bandgap counterparts, overcoming halide segregation, instability, and the limited efficiency of quasi‐2D systems. Optical modeling reveals that thinning the FAPbI 3 absorber balances visible transmittance and performance, with 140 and 160 nm layers delivering average visible transmittance of 24.2% and 18.2% alongside power conversion efficiency of 15.9% and 18.0%. Surface passivation with 4‐fluorophenylethylammonium iodide enhances V OC by over 100 mV and raises efficiency by more than 1%, as confirmed by photoluminescence and transient optoelectronic analyses showing reduced nonradiative losses and extended carrier lifetimes. Crucially, both passivated and unpassivated devices exhibit excellent stability despite reduced thickness. These results demonstrate thin FAPbI 3 as a robust platform for potential efficient and stable applications. A preliminary test employing 10 nm Ag layer yielded a device with a light utilization efficiency of 1.8%, establishing a promising baseline for semitransparent photovoltaic applications.