Tailoring the Band Gap in 3D Hybrid Perovskites by Substitution of the Organic Cations: (CH3 NH3 )1-2y (NH3 (CH2 )2 NH3 )2y Pb1-y I3 (0≤y≤0.25).

Tuning the optical properties of MAPbI3 (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 MAPbI3 by H3 NCH2 CH2 NH3 (H2 en). Surprisingly, there is 1:1 exchange of the monovalent cation MA by the dication H2 en. The charge compensation results from a deficit of Pb2+ , leading to a series MA1-2y (H2 en)2y Pb1-y I3 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 H2 en content stabilizes the cubic high-temperature (HT) form of MAPbI3 . 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:H2 en ratio in solution.