Effects of mixed solvent on morphology of CH3NH3PbI3 absorption layers and photovoltaic performance of perovskite solar cells

In this paper, a two-step method is used to prepare a high-quality CH3NH3PbI3 layer for carbon-based hole conductor-free perovskite solar cells (PSCs). In the first step, the solvent for the PbI2/DMF solution is changed to a mixed solvent of N,N-dimethylformamide (DMF) with dimethyl sulfoxide (DMSO) from pure DMF solvent. This process requires 3.5h for PbI2 films deposited from the PbI2/DMF solution to completely convert to perovskite in a methyl amine iodide (MAI)/iso-propyl alcohol (IPA) solution, and perovskite films have a rough surface with a few large crystals. This mixed solvent accelerates the conversion of PbI2 to CH3NH3PbI3, resulting in smooth CH3NH3PbI3 films without residual PbI2. Only 15min are required for PbI2 films to completely convert to perovskite when the volume ratio of DMSO to DMF is 1:4. As a result, the photovoltaic performance, especially on the fill factor (FF) of PSCs fabricated from DMF/DMSO solvent, improved mainly due to the better contact of perovskite/carbon interface and faster charge transport of the perovskite layer.