Enhanced Light Transmittance of Electron Transport Layer through Bilayer SnO 2 for High‐Performance Semitransparent Perovskite Solar Cells

Semitransparent perovskite solar cells (ST‐PSCs) for building‐integrated photovoltaics (BIPV) face the challenge of achieving high efficiency due to significant light loss. The SnO 2 electron transport layer (ETL), utilized in n‐i‐p PSCs and prepared via the sol‐gel method, is susceptible to aggregation on substrate, resulting in light scattering that diminishes absorption of the perovskite layer. In this study, we propose a strategy that combines atomic layer deposition (ALD) and sol‐gel solution to deposit a bilayer SnO 2 structure to address these issues. The compact ALD SnO 2 layer enhances subsequent deposition of the sol‐gel SnO 2 layer, mitigating aggregation of SnO 2 nanoparticles. Moreover, ALD SnO 2 exhibits a lower refractive index compared to the sol‐gel SnO 2 film due to its lower ratio of Sn 4+ /Sn 2+ , creating a refractive index gradient that improves light transmittance. Consequently, the bilayer SnO 2 increases the short‐circuit current of wide‐bandgap ST‐PSCs with an energy gap of 1.66 eV up to 21.27 mA/cm 2 and boosts efficiency up to a certified value of 20.22 %. Furthermore, the device demonstrates an 81.4 % bifaciality and maintains 91.47 % of its initial efficiency after exposure to 1000 hours under 1‐sun white LED illumination.