Halide perovskites with bandgaps of 1.70–1.85 eV are of interest for multijunction photovoltaics. Mixing halides on the X site of the ABX₃-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_xCs_1–xPbI_yBr_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 °C in the dark and 92% of their original efficiency after operation at 60 °C for 500 h. This octahedral tilting strategy is a promising route for attaining efficient and stable wide bandgap perovskite solar cells.

中文翻译：

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二甲基铵在稳定卤化物钙钛矿带隙调制中的作用

带隙为1.70-1.85 eV的卤化钙钛矿对多结光伏电池很感兴趣。在ABX ₃结构的钙钛矿体系的X位置混合卤化物是达到这些带隙的常用方法，但是这种方法引入了相分离途径，限制了光电压。近来，已经提出了一种增加带隙的新策略，其中通常太大而无法放入晶格中但被较小阳离子补偿的阳离子在A位点被取代。该策略带隙增加的潜在机制仍然是一个悬而未决的问题。在这里，我们表明，通过FA _X Cs _{1– x} PbI _y Br _{3 – y}的A位上的大二甲基铵（DMA）阳离子的部分取代钙钛矿，不仅在较小的Cs阳离子补偿DMA时，而且在仅添加DMA的情况下，观察到带隙增加，伴随着晶格的膨胀。我们的实验结果表明，添加DMA会引起钙钛矿八面体的意外倾斜，从而增加带隙。基于掺有1.73 eV DMA的材料的高效太阳能电池极其稳定，在黑暗中在85°C下2200 h下保持200％原始效率，在60°C下运行500 h后仍保持92％原始效率。这种八面体倾斜策略是获得有效且稳定的宽带隙钙钛矿太阳能电池的有前途的途径。