(461g) MA and Br Free Phase Pure Dion Jacobson 2D Lead Halide Perovskites for Efficient and Durable Perovskite/Perovskite Tandems.

Three-dimensional (3D) metal halide perovskite solar cells have displayed tremendous progress in photo conversion efficiencies in the last decade. The tunable bandgaps enable mixed halide perovskite compositions to be used as the wide bandgap (WB) sub-cell in perovskite-perovskite tandems. However, halide segregation and ion migration under photo-illumination create bandgap instability that limits the long-term durability of these wideband gap subcells. 2D halide perovskites, on the other hand, have demonstrated better durability against moisture and photo illumination due to their large organic cations but still lag in efficiency due to >2eV band gaps and charge extraction barriers. However, the methylammonium (MA⁺) cation is hygroscopic and can still limit the stability of the perovskite. Here, we synthesize formamidinium (FA⁺) based 2D Dion Jacobson (DJ) perovskite with a band gap of 1.8eV, which is ideally suited for building bottom cells for perovskite-perovskite and perovskite/Si tandems. Films fabricated using the phase-selective memory seeds process preserved the phase purity of the n=3 2D perovskite. More importantly, FA 2D DJ perovskite solar cells demonstrate commercially viable durability against temperature and moisture. These properties make FA 2D DJ perovskites promising candidates for wide bandgap subcells in perovskite-perovskite tandem.