Structural Distortion and Bandgap Increase of Two-Dimensional Perovskites Induced by Trifluoromethyl Substitution on Spacer Cations

In further advancing display technologies, especially for improved blue emitters, to engineer the bandgap of promising semiconductors such as hybrid perovskites is important. Present-day methods for tuning the bandgaps of perovskites, such as the incorporation of mixed halide anions, suffer drawbacks such as phase separation and difficulty in synthesis. Here we report a new 2D lead iodide perovskite that emits in the blue spectral region. We exploit an increased angular distortion of PbI42- octahedra to widen the bandgap of 2D metal halide perovskites. We synthesized 2D lead iodide perovskites based on (4-Y-C6H4CH2NH3)2PbI4 (Y = H, F, Cl, Br, I) and substituted the halogen atoms with a -CF3 group to create (4-CF3-C6H4CH2NH3)2PbI4 compounds. We observed that the CF3-substituted material exhibited a ∼0.16 eV larger bandgap than did the halogen-substituted materials. We used X-ray diffraction and density functional theory simulations and found that the blue shift can be assigned to the angular distortion of the PbI42- lattice, a distortion traceable to repulsive intermolecular interactions between the trifluoromethyl groups on oppositely-arranged spacers. These results add a degree of freedom in tuning 2D perovskites to selected bandgaps for optoelectronic applications.