Influence of Orientational Disorder on the Optical Absorption Properties of the Hybrid Metal-Halide Perovskite CH NH PbI.

An experimental and theoretical investigation is reported to analyze the relation between the structural and absorption properties of CH NH PbI in the tetragonal phase. More than 3000 geometry optimizations were performed to reveal the structural disorder and identify structures with the lowest energies. The electronic structure calculations provide an averaged band gap of 1.674 eV, which is in excellent agreement with the experimental value of about 1.6 eV. The simulations of the absorption spectrum for three representative structures with lowest energy reproduced the absorption shoulders observed in the experimental spectra. These shoulders are assigned to excitations having similar orbital characters and involving transitions between hybridized 6s(Pb)/5p(I) orbitals and 6p(Pb) orbitals. The geometries of the three structures were analyzed and the effects of the inorganic frame and the CH NH cations on the absorption properties were estimated. It was found that both changes in the inorganic frame and the CH NH cations orientations impact the absorption spectra, by modifying the transitions energies and intensities. This highlights the role of CH NH cation in influencing the absorption properties of CH NH PbI and demonstrates that CH NH cation is one of the key elements explaining the broad and nearly constant absorption spectrum in the visible range.