Additive strategy toward PbI2 film modifications for efficient perovskite solar cells fabricated in an air environment

• PenAAc boosts PbI 2 crystallinity and grain size; 5-AVAI ensures uniform PbI 2 layer for enhanced quality. • Optimized PbI 2 films promote efficient organic salt reactions, forming high-quality perovskite layers. • Distinct mechanisms of 5-AVAI (uniformity) and PenAAc (crystallization) on PbI 2 /perovskite films revealed systematically. In the two-step method for perovskite film fabrication, insufficient reaction between organic ammonium salts and PbI 2 affects the active layer quality, resulting in poor power conversion efficiency (PCE). In this work, we introduce pentanamine acetic acid (PenAAc) and 5-aminovaleric acid iodine (5-AVAI) as two efficient additives to modify PbI 2 layer during perovskite films fabrication via the two-step method in an air environment. After additive modifications, the quality of both PbI 2 layers is improved: PenAAc enhances crystallinity and increases PbI 2 grain size, while 5-AVAI induces a uniform and smooth PbI 2 layer. The optimized PbI 2 films facilitate the reaction between PbI 2 and organic salts, yielding a high-quality perovskite layer with enhanced crystallinity and reduced defect density. In contrast, the macromolecule structure of 5-AVAI does not diffuse after annealing, remaining in the PbI 2 layer and exerting minimal influence on the perovskite surface. However, dissociative CH 3 COO − groups in PenAAc molecules diffuse from the PbI 2 layer to the perovskite surface during annealing, forming strong interactions between I - and Pb 2+ with CH 3 COO − groups. This further improves perovskite film quality and accelerates charge carrier extraction/transport in devices. Consequently, additive-modified perovskite solar cells achieve excellent PCEs of 24.50 % (5-AVAI) and 24.65 % (PenAAc), significantly surpassing the control device (22.84 %). Moreover, hysteresis index is reduced from 4.20 % (control) to 0.33 % (5-AVAI) and 0.08 % (PenAAc). Additionally, the modified devices also demonstrated enhanced environmental stability across humidity, light, and thermal stress conditions.