Intergrain Connection of Organometal Halide Perovskites: Formation Mechanism and Its Effects on Optoelectrical Properties

Two-step processes are commonly used for the fabrication of organic-inorganic perovskite solar cells; they convert a PbI₂ film to a perovskite film by dipping it in CH₃NH₃I (MAI) solution or spin-coating the MAI solution onto it. Dipping yields perovskite films with discrete and rough morphologies, whereas spinning yields films with smooth and connected morphologies. The residual MAI solution that remains after spinning is the key factor that governs the smoothness of the resulting morphology; centrifugal force has no influence. A perovskite layer forms as soon as the MAI solution is loaded onto the PbI₂ film, then the MAI residues left after spinning dissolve this outermost perovskite layer. The subsequent recrystallization of the dissolved perovskites increases the connectivity and smoothness of the crystals. The final morphology is dependent on the degrees of dissolution and recrystallization, which can be controlled by varying the processing conditions. A post-thermal treatment can be applied to induce the additional dissolution of the perovskites, which results in an increase in the final grain size while maintaining good connectivity. Combining these results, we fabricated an optimal film morphology that gives rise to perovskite solar cells with improved efficiency. The optimal perovskite film has a smooth and connected morphology as well as better carrier transport than rough and discrete films. This article provides fundamental understanding of the mechanism of formation during two-step processes of connected perovskite morphologies that can guide the further development of two-step processes for the fabrication of optimal perovskite morphologies.