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 approximate to 55 degrees C temperature, in N-2 atmosphere) after 600 h.