Influence of Orientational Disorder on the Optical Absorption Properties of the Hybrid Metal‐Halide Perovskite CH₃NH₃PbI₃

Abstract

An experimental and theoretical investigation is reported to analyze the relation between the structural and absorption properties of CH3NH3PbI3 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 CH3NH3+ cations on the absorption properties were estimated. It was found that both changes in the inorganic frame and the CH3NH3+ cations orientations impact the absorption spectra, by modifying the transitions energies and intensities. This highlights the role of CH3NH3+ cation in influencing the absorption properties of CH3NH3PbI3 and demonstrates that CH3NH3+ cation is one of the key elements explaining the broad and nearly constant absorption spectrum in the visible range.

Effects of Bromine Doping on the Structural Properties and Band Gap of CH3NH3Pb(I1–xBrx)3 Perovskite

“…Such an accuracy is expected at this level of theory for MAPb(I 1−x Br x ) 3 and stays in agreement with our previous results on MAPbI 3 . 11 Both experimental and theoretical results show that the band gap increases linearly for bromine doping ranging from 0 to 10%. It can be mentioned that previous experimental results 12 have demonstrated that the band gap displays a quadratic behavior as a function of bromine composition in the entire 0−100% doping range.…”

“…Figure 2 shows the UV−vis absorption spectrum for different bromine atom concentrations. In our last work, Martynow et al 11 assigned the maximum absorption peaks for MAPbI 3 without bromine atoms. It is observed that the overall shape of the spectrum is not changed for different doping compositions.…”

“…The angles D 1 and D 3 describe the rotations of the octahedrons along the Z crystal axis, whereas D 2 and D 4 describe the rotations related to the tilting of the octahedrons. 11 The mean dihedral angle values (Figure 5) are obtained by averaging over the nine octahedrons in the supercell plane defined by the (−1, 1, 0) Miller index. Following the optimization of the supercell with variable-cell relaxation, it is observed that the doped MAPb(I 1−x Br x ) 3 structures remain nearly tetragonal with a ≈ b ≠ c (Figure 3).…”

“…9 One of the most important differences between MAPbI 3 and MAPbBr 3 is their band gap width. Both materials show a direct band gap, 10,11 but MAPbI 3 has a band gap narrower than MAPbBr 3 , with values of about 1.6 eV 11,12 and 2.0−2.2 eV, 12,13 respectively. This difference makes mixed lead halide perovskite MAPb(I 1−x Br x ) 3 a highly tunable material, in which the band gap can be modulated by changing the percentage of bromine doping.…”

“…19 For example, in a previous study on MAPbI 3 , we investigated the effects of changes in the inorganic frame geometry and the MA cation orientations. 11 Considering an ensemble of more than 3000 structures, standard deviations of about 0.1−0.2 eV were estimated for the band gap. Moreover, time-dependent density functional theory (DFT) calculations of the absorption spectrum provided an assignment of three experimental bands, and the impact of structural modifications on the transition energies and intensities was described.…”