Direct Observation of Intragrain Defect Elimination in FAPbI₃ Perovskite Solar Cells by Two-Dimensional PEA₂PbI₄

Two-dimensional perovskites have widely been used to improve the efficiency and stability of perovskite solar cells and are generally believed to passivate defects at the grain boundaries of three-dimensional perovskites. Herein, we studied introducing various combinations of two-dimensional phenyl ethylammonium lead iodide (PEA 2 PbI 4 ) and methylammonium chloride (MACl) to the formamidinium lead iodide (FAPbI 3 ) precursor solution. Ultralow dose selected area electron diffraction studies prove that the high density of intragrain planar defects in FAPbI 3 can be strongly reduced by adding PEA 2 PbI 4 and fully eliminated by adding further MACl. Although PEA + is too large to incorporate into FAPbI 3, PEA 2 PbI 4 not only improves crystallization but also suppresses intragrain defect formation. As a result, a longer charge carrier lifetime, higher photoluminescence quantum yield, lower Urbach energy, and current–voltage hysteresis are achieved, resulting in a champion PCE of 23.69%, with an improvement of humidity stability.