Self-formed grain boundary healing layer for highly efficient CH₃ NH₃ PbI₃ perovskite solar cells

Perovskite solar cells have attracted significant research efforts due to their remarkable performance, with certified power conversion efficiency now reaching 22%. Solution-processed perovskite thin films are polycrystalline, and grain boundaries are thought to be responsible for causing recombination and trapping of charge carriers. Here we report an effective and reproducible way of treating grain boundaries in CH 3 NH 3 PbI 3 films deposited by means of a Lewis acid-base adduct approach. We show by high-resolution transmission electron microscopy lattice images that adding 6 mol% excess CH 3 NH 3 I to the precursor solution resulted in a CH 3 NH 3 I layer forming at the grain boundaries. This layer is responsible for suppressing non-radiative recombination and improving hole and electron extraction at the grain boundaries by forming highly ionic-conducting pathways. We report an average power conversion efficiency of 20.1% over 50 cells (best cell at 20.4%) together with significantly reduced current-voltage hysteresis achieved by this grain boundary healing process.