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 (PEA2PbI4) and methylammonium chloride (MACl) to the formamidinium lead iodide (FAPbI3) precursor solution. Ultralow dose selected area electron diffraction studies prove that the high density of intragrain planar defects in FAPbI3 can be strongly reduced by adding PEA2PbI4 and fully eliminated by adding further MACl. Although PEA+ is too large to incorporate into FAPbI3, PEA2PbI4 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.