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)(2)PbI3 (commonly known as FAPbI(3)) has two polymorphs, a high temperature-stable black FAPbI(3) perovskite-type pseudocubic polymorph (alpha-phase) and a low temperature-stable yellow non-perovskite hexagonal polymorph (delta-phase). In order to understand the crystallization kinetics of the FAPbI(3) 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 FAPbI(3) 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 FAPbI(3)-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.