The Role of Dimethylammonium in Bandgap Modulation for Stable Halide Perovskites

Halide perovskites with bandgaps of 1.70-1.85 eV are of interest for multijunction photovoltaics. Mixing halides on the X site of the ABX(3)-structured perovskite system is a common way to reach these bandgaps, but this method introduces phase segregation pathways, limiting photovoltage. Recently, a new strategy for increasing the bandgap has been introduced, where cations normally too large to fit into the lattice, but compensated by smaller cations, are substituted on the A site. The mechanism underlying the increase of the bandgap with this strategy remained an open question. Here, we show that by partial substitution of the large dimethylammonium (DMA) cation at the A site of FA(x)Cs(1-x)PbI(y)Br(3-y) perovskites, a bandgap increase is observed not only when DMA is compensated by smaller Cs cations but also when only DMA is added, which is accompanied by an expansion of the crystal lattice. Our experimental findings suggest that adding DMA is causing an unexpected tilt in the perovskite octahedra, increasing the bandgap. Efficient solar cells based on 1.73 eV DMA-incorporated materials are extremely stable, retaining 96% of their original efficiency over 2200 h at 85 degrees C in the dark and 92% of their original efficiency after operation at 60 degrees C for 500 h. This octahedral tilting strategy is a promising route for attaining efficient and stable wide bandgap perovskite solar cells.