Due to the quantum confinement effects, the bandgap of two-dimensional (2D) perovskites increases when the thickness of the inorganic slabs decreases. This could result in insufficient light absorption that limits the overall performance of perovskite solar cells. Here we report a series of small bandgap 2D perovskites using mixed butylammonium (BA) and formamidinium (FA) cations, i.e. (BA)₂(FA)_n−1Pb_nI_3n+1 (n = 1–5). In particular, 2D perovskite (BA)₂(FA)₂Pb₃I₁₀ shows a small bandgap of 1.51 eV, which is comparable to state-of-the-art three-dimensional perovskites. A strongly preferential out-of-plane crystallographic alignment of the inorganic perovskite component in the (BA)₂(FA)₂Pb₃I₁₀ film is achieved by the addition of thiourea in the precursor. This significantly improves charge transport and thus leads to highly efficient inverted solar cells with a planar structure of ITO/PEDOT:PSS/(BA)₂(FA)₂Pb₃I₁₀/PC₆₁BM/BCP/Ag. With the best power conversion efficiency of 6.88%, we demonstrated the highest PCE reported for FA based low-n (n < 4) 2D perovskite solar cells. By virtue of the stable 2D perovskites, the unencapsulated device retains 80% efficiency after storing in air with a humidity of 25 ± 5% for 25 days, indicating excellent stability against moisture and oxygen.

中文翻译：

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高度定向的二维甲ami鎓碘化铅钙钛矿，带隙为1.51 eV

由于量子限制效应，当无机平板的厚度减小时，二维（2D）钙钛矿的带隙增大。这可能导致光吸收不足，从而限制了钙钛矿太阳能电池的整体性能。在这里，我们报告了一系列使用混合丁基铵（BA）和甲ami（FA）阳离子，即（BA）₂（FA）_{n -1} Pb _n I _{3 n +1}（n = 1-5）的小带隙二维钙钛矿。特别是2D钙钛矿（BA）₂（FA）₂ Pb ₃ I ₁₀显示出1.51 eV的小带隙，可与最新的三维钙钛矿相媲美。通过在前体中添加硫脲，可实现（BA）₂（FA）₂ Pb ₃ I ₁₀膜中无机钙钛矿成分的强烈优先的面外晶体取向。这显着改善了电荷传输，并因此导致了具有ITO / PEDOT：PSS /（BA）₂（FA）₂ Pb ₃ I ₁₀ / PC ₆₁ BM / BCP / Ag平面结构的高效倒置太阳能电池。凭借6.88％的最佳功率转换效率，我们证明了FA基低n（n <4）2D钙钛矿太阳能电池。由于具有稳定的2D钙钛矿，未封装的设备在空气中以25±5％的湿度存储25天后仍能保持80％的效率，这表明它具有出色的防潮和防氧稳定性。