Facile Deposition of Mesoporous Pbl(2) through DMF:DMSO Solvent Engineering for Sequentially Deposited Metal Halide Perovskites

Sequential deposition is one of the most promising approaches toward achieving scalable fabrication of metal halide perovskite thin films. However, this fabrication approach conventionally lends itself to the incomplete conversion of the lead halide, which impacts the stability, performance, and reproducibility of functional devices featuring such thin films. In this work, we have overcome this limitation by using a simple solvent and process engineering approach. We show that through the use of an optimized dimethylformamide and dimethyl sulfoxide solvent mixture in the precursor solution, and through the judicious control of this solution, the substrate, and final annealing temperatures, highly uniform and mesoporous Pbl2 thin films can be deposited. The porous structure of these films is found to accelerate the interdiffusion of CH3NH3I (MAI) during the second-step process when carried out at room temperature, enabling their complete conversion into CH.NH.Pbl, perovskite. Detailed investigations using scanning electron microscopy, X-ray diffraction, grazing-incidence wide-angle X-ray scattering, thermogravimetric analysis, UV-vis absorption, photoluminescence, and time-of-flight secondary ion mass spectrometry have been used to provide mechanistic insights into the porous Pbl(2) film formation and the interdiffusion process. Solar cells based on planar fluorine-doped tin oxide (FTO)/TiO2/CH3NH3PbI3/spiro-OMeTAD/Au device architectures yield optimized device efficiencies of 19%, which is among the highest for this device structure and perovskite absorber material. The applicability of this enhanced sequential deposition method to other perovskite systems has been further validated through the fabrication of efficient FA(x)MA(1-x)PbI(x)Br(3-x) and CsPbIxBr3-x solar cells.