Secondary growth of three-dimensional lead halide perovskite during the alkylammonium salts induced “in-situ healing” strategy

Abstract

Two-dimensional (2D) precursor molecules-based surface treatment on three-dimensional (3D) lead halide perovskite (PVSK) has achieved huge successes, the in-depth understanding of the modification mechanism remains an urgent need. Here the effect of alkyl-chain length on the reaction dynamics between alkylammonium salts (XI) and 3D PVSK matrix is studied, through examination of surface morphological and crystallographic properties of the 3D PVSK matrix. It is observed that the average crystallite size of 3D PVSK increases as XI is either spin-coated on 3D PVSK or penetrated through carbon-electrode (during the “in-situ healing” process). Secondary growth is observed for 3D PVSK, which is related to ion-exchanging reactions. Prolonging alkyl-chain length favors the secondary growth. Besides, the formation dynamics of 2D PVSK are studied. Adding alkyl-chain length increases the yields. The observations are thoroughly discussed with respect to the steric-hindrance effect held by alky-chains of XI molecule. The improved crystallization of 3D PVSK and increased yields of 2D PVSK help accelerate charge extraction and reduce recombination across the interface between PVSK and carbon-electrode (CE). Tuning alkyl-chain length of XI molecules, and the mass ratio between XI molecules and carbon black could mitigate the “in-situ healing” effect. Power conversion efficiency (PCE) of the carbon-electrode-based hole-conductor-free planar perovskite solar cells has been upgraded from 14% to 17%, and further upgraded to 20.4% by utilizing relatively thick CEs. Thanks to the hydrophobicity of long alkyl-chains owned by XI molecules, prolonged stability has been achieved on unsealed devices at the high-moisture environment (RH ≈ 85%), meanwhile, shelf-stability up to 6400 h has been achieved. This study deepens the understanding of the 2D precursor-basing modification strategies.