High efficiency perovskite solar cells with synchronous optimization by formic acid vapor annealing process and composite electron transport layer

The thermal annealing process is significant for the crystallization of perovskite films ((FAPbI 3 ) 0.9 (MAPbBr 3 ) 0.1 ). However, conventional annealing methods suffer from non-uniform heating of film samples. Herein, we report a vapor annealing method that the perovskite film was annealed in formic acid vapor, generating a uniform thermal field to control crystallization. Simultaneously, HCOOH efficiently enhances sample crystallinity, passivates defects and suppresses deprotonation, establishing an optimal physicochemical environment for perovskite growth. In addition, the electron transport layer (ETL) composed of SnO 2 and ZnO nanoparticles (NPs) demonstrates enhanced electrical conductivity and light transmittance. Methylammonium Chloride (MACl) is introduced to suppress ZnO NPs-induced deprotonation. The experimental and theoretical results manifest the mechanism of inhibiting the deprotonation reaction by MACl. The stable power conversion efficiency (PCE) of the device prepared by this method achieves 23.41 % and maintains 83 % of initial PCE during the long-term stability test.