Ammonium-Free-Layered-Perovskites with Alternating Cations in the Interlayer for Wide-Bandgap Solar Cells

Layered metal halide perovskites in the alternating cation interlayer (ACI) family typically exhibit small interlayer spacing, lowering exciton binding energy and promoting free carrier generation, favorable for photovoltaic devices. However, most reported ACI phase perovskites incorporate volatile methylammonium (MA) cations, susceptible to instability. Here, for wide-bandgap (>2 eV) solar cells, we demonstrate ammonium-free-ACI perovskites with equimolar cesium and formamidinium in the A-site producing predominantly n = 2 ACI phase, which forms spontaneously after spin-coating. Further device optimization via Rb-substitution in the bulk and the insertion of piperazine-1,4-diium chloride (PDCl) and PCBM between the perovskite and electron-selective C 60 layer results in a power conversion efficiency (PCE) of 10.6%, which is to date the highest for n = 2 layered perovskite solar cells. The encapsulated cell retains 90% of initial PCE after 400 h of maximum power point tracking, outperforming reported perovskite solar cells of the same bandgap, highlighting the value of avoiding ammonium species and halide mixing.