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.