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 SnO₂ 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 SnO₂ structure to address these issues. The compact ALD SnO₂ layer enhances subsequent deposition of the sol-gel SnO₂ layer, mitigating aggregation of SnO₂ nanoparticles. Moreover, ALD SnO₂ exhibits a lower refractive index compared to the sol-gel SnO₂ film due to its lower ratio of Sn⁴⁺/Sn²⁺, creating a refractive index gradient that improves light transmittance. Consequently, the bilayer SnO₂ increases the short-circuit current of wide-bandgap ST-PSCs with an energy gap of 1.66 eV up to 21.27 mA/cm² 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.