Tailoring the Band Gap in 3D Hybrid Perovskites by Substitution of the Organic Cations: (CH 3 NH 3 ) 1−2 y (NH 3 (CH 2 ) 2 NH 3 ) 2 y Pb 1− y I 3 (0≤ y ≤0.25)

Tuning the optical properties of MAPbI 3 (MA=methylammonium) is a key requirement to increase the efficiency of perovskite solar cells (PSCs). Simple precipitation from solution allows the partial substitution of MA in MAPbI 3 by H 3 NCH 2 CH 2 NH 3 (H 2 en). Surprisingly, there is 1:1 exchange of the monovalent cation MA by the dication H 2 en. The charge compensation results from a deficit of Pb 2+ , leading to a series MA 1−2 y (H 2 en) 2 y Pb 1− y I 3 with 0≤ y ≤0.25. This model has been supported by single‐crystal measurements and NMR investigations. The substitution results in a continuous shift of the band gap from 1.51 to 2.1 eV and a color change from black to orange‐red. The H 2 en content stabilizes the cubic high‐temperature (HT) form of MAPbI 3 . There is a linear correlation between band gap and unit cell volume. The substitution enables controlled band gap tuning because the extent of substitution is closely related to the applied MA:H 2 en ratio in solution.