Stress Relaxation for Lead Iodide Nucleation in Efficient Perovskite Solar Cells

Porous lead iodide (PbI₂) film is crucial for the complete reaction between PbI₂ and ammonium salts in sequential-deposition technology so as to achieve high crystallinity perovskite film. Herein, it is found that the tensile stress in tin (IV) oxide (SnO₂) electron transport layer (ETL) is a key factor influencing the morphology and crystallization of PbI₂ films. Focusing on this, lithium trifluoromethanesulfonate (LiOTf) is used as an interfacial modifier in the SnO₂/PbI₂ interface to decrease the tensile stress to reduce the necessary critical Gibbs free energy for PbI₂ nuclei formation. The relaxed tensile stress facilitates the more porous PbI₂ generation with larger particles and higher roughness, resulting in superior-quality perovskite films. Besides, this strategy effectively passivates the inherent electron traps of SnO₂ and smooths the interfacial energy levels, boosting the charge extraction and transfer. As a result, a champion power conversion efficiency (PCE) of 25.33% (25.10% stabilized for 600 s) is achieved. Furthermore, the device demonstrates exceptional stability, retaining 90% of its initial PCE at its maximum power point tracking measurement (under 100 mW cm^-2 white light illumination at ≈55 °C temperature, in N₂ atmosphere) after 600 h.