The mechanism of toluene-assisted crystallization of organic–inorganic perovskites for highly efficient solar cells

Abstract

We investigate the influence of solvent drenching in hybrid organic–inorganic perovskite (CH₃NH₃PbX) crystallization process with a non-solvent, toluene, during film fabrication process. We use three different precursor compositions, CH₃NH₃I (MAI):PbI₂, 3MAI:PbI₂ and 3MAI:PbCl₂ to unravel the crystallization mechanism with toluene drenching. The mixed halide precursor (3MAI:PbCl₂) results in the highest quality films with the toluene treatment, including high surface coverage, large grains, long PL lifetimes and high photoluminescence quantum efficiency (PLQE). The neat halide-based precursors (MAI:PbI₂ and 3MAI:PbI₂) with the treatment have increased photo-physical properties (PL lifetime and PLQE) and a surface coverage, but slightly decrease grain size in the film, while the 3MAI:PbI₂ precursor has still formed numerous pinholes in the film. Mechanistically, we visually observe that the toluene drenching accelerates the nucleation at early stage of crystallization in 3MAI:PbCl₂ precursor. X-ray diffraction pattern in this stage confirms the formation of both MAPbI₃ and MAPbCl₃, nucleation. During the crystallization process MAPbCl₃ is transformed into MAPbI₃ phase by the anion exchange. Toluene treatment strongly affects the ratio of MAPbI₃ and MAPbCl₃, nucleation and hence plays a critical role in deciding the final film morphology, their optoelectronic properties and hence their device performances.