Thermally induced urea-assisted porous precursor film for efficient planar inverted water-based perovskite solar cells

Planar inverted water-based perovskite solar cells (PIW-PSCs) can be fabricated without toxic organic solvents. However, in the aqueous deposition route, organic ammonium salts (OAS) must penetrate the precursor film while simultaneously driving phase conversion. This deposition method results in slow and nonuniform conversion. Here, urea is used as a water-compatible additive that improves precursor wetting and promotes further development of the internal porous structure by the subsequent thermal treatment. The resulting porous precursor provides more accessible transport pathways for OAS, thereby promoting more complete conversion of Pb(NO₃)₂ into perovskite. Consequently, the resulting perovskite film exhibits enlarged grains, reduced defect density, and reduced residual compressive stress. The corresponding PIW-PSC achieves a champion PCE of 20.39%, while the PUS-L device shows stable power output for 20 min at a fixed bias corresponding to its maximum-power-point voltage. Unencapsulated PUS-L devices retain 84% of their initial PCE after approximately 1,000 h of ambient storage (30%-35% RH, ~ 25 ℃) and 68% after 450 h of thermal aging at 80 ℃ under N₂. This strategy combines the water compatibility, Pb²⁺ coordination, and thermally induced transformation of urea to regulate precursor-film morphology and facilitate OAS transport during the nitrate-to-halide conversion of Pb(NO₃)₂ precursors.