Pure‐Iodide Wide‐Bandgap Perovskites for High‐Efficiency Solar Cells by Crystallization Control

Abstract Wide‐bandgap perovskite is a vital part of perovskite‐based tandem solar cells. Currently, wide‐bandgap perovskites are typically based on mixed‐halide (I/Br) materials, but suffer from photoinduced phase separation. The pure‐iodide formamidine/cesium (FA/Cs) based FA x Cs 1− x PbI 3 perovskites with high Cs content are good candidates, whereas the control of crystallization is challenging due to the complex crystallization kinetics. Here, pure‐iodide FA 0.5 Cs 0.5 PbI 3 wide‐bandgap perovskite solar cells is reported. As an acidic diammonium salt, methylenediaminium dichloride (MDACl 2 ) is applied as an additive to control the whole crystallization process of perovskite films, including both nucleation and crystal growth. Starting from the solution chemistry, the MDACl 2 additive with acidity and strong solvation properties can effectively regulate the chemical composition of perovskite precursor, thus inhibiting the growth of undesired 1D intermediates during the nucleation process. Besides, the incorporation of larger‐sized MDA 2+ into the lattice compensates for the tolerance factor and accelerates the ion exchange reaction between FA + and Cs + in the crystal growth process. As a result, the crystallinity of the perovskite films is significantly improved, benefitting from the dual function of MDACl 2 . Finally, the efficiency of hole transport layer‐free carbon electrode‐based wide‐bandgap perovskite solar cells reaches 18.52%, which is the highest reported so far.