DMF as an Additive in a Two-Step Spin-Coating Method for 20% Conversion Efficiency in Perovskite Solar Cells.

DMF as an additive has been employed in FAI/MAI/IPA (FA= CH2(NH2)2, MA = CH3NH3, IPA = isopropanol) solution for a two-step multicycle spin-coating method in order to prepare high-quality FAxMA1-xPbI2.55Br0.45 perovskite films. Further investigation reveals that the existence of DMF in the FAI/MAI/IPA solution can facilitate perovskite conversion, improve the film morphology, and reduce crystal defects, thus enhancing charge-transfer efficiency. By optimization of the DMF amount and spin-coating cycles, compact, pinhole-free perovskite films are obtained. The nucleation mechanisms of perovskite films in our multicycle spin-coating process are suggested; that is, the introduction of DMF in the spin-coating FAI/MAI/IPA solution can lead to the formation of an amorphous phase PbX2-AI-DMSO-DMF (X = I, Br; A = FA, MA) instead of intermediate phase (MA)2Pb3I8·2DMSO. This amorphous phase, similar to that in the one-step method, can help FAI/MAI penetrate into the PbI2 framework to completely convert into the perovskite. As high as 20.1% power conversion efficiency (PCE) has been achieved with a steady-state PCE of 19.1%. Our work offers a simple repeatable method to prepare high-quality perovskite films for high-performance PSCs and also help further understand the perovskite-crystallization process.