Crystallization of HC(NH₂)₂PbI₃ Black Polymorph by Solvent Intercalation for Low Temperature Solution Processing of Perovskite Solar Cells

One of the critical problems in fabrication of flexible perovskite modules and resolving their reliability issue remains the necessity to utilize high temperature annealing for synthesis of perovskite and electron transport layers. Here, we provide a breakthrough in addressing these challenges by demonstrating low temperature synthesis of both of these layers. HC(NH)₂PbI₃ (commonly known as FAPbI₃) has two polymorphs, a high temperature-stable black FAPbI₃ perovskite-type pseudocubic polymorph (α-phase) and a low temperature-stable yellow non-perovskite hexagonal polymorph (δ-phase). In order to understand the crystallization kinetics of the FAPbI₃ black polymorph, a PbI₂-NMP complex is fabricated via solvent intercalation between the adjacent I-Pb-I layers. Utilizing structural, electrical, and thermal analyses, the connection between solvent intercalation and the crystallization of the FAPbI₃ black polymorph is established. It is found that the solvent intercalation in the PbI₂ crystal causes lattice strain and the induced strain energy could reduce the activation barrier of the intermediate state and favor the crystallization of the FAPbI₃ black polymorph. The TiO₂ compact layer with a smooth surface, high crystallinity, and superior electron transport is also fabricated at room temperature by using a TiO₂ slurry composed of volatile solvents and TiO₂ nanoparticles. Using low temperature solution processed TiO₂ as electron transport layer, the FAPbI₃-based perovskite solar cell exhibits a conversion efficiency of 13.2% with significantly reduced hysteresis effect, benefiting from the low electron and hole trap state density. The low temperature process developed in this study holds great promise for flexible perovskite solar cells and perovskite tandem solar cells. (Chemical Equation Presented).