Electronic Structure and Optical Properties of α-CH₃NH₃PbBr₃ Perovskite Single Crystal

The electronic structure and related optical properties of an emerging thin-film photovoltaic material CH₃NH₃PbBr₃ are studied. A block-shaped α-phase CH₃NH₃PbBr₃ single crystal with the natural <100> surface is synthesized solvothermally. The room-temperature dielectric function ε = ε₁ + iε₂ spectrum of CH₃NH₃PbBr₃ is determined by spectroscopic ellipsometry from 0.73 to 6.45 eV. Data are modeled with a series of Tauc-Lorentz oscillators, which show the absorption edge with a strong excitonic transition at ∼2.3 eV and several above-bandgap optical structures associated with the electronic interband transitions. The energy band structure and ε data of CH₃NH₃PbBr₃ for the CH₃NH₃⁺ molecules oriented in the <111> and <100> directions are obtained from first-principles calculations. The overall shape of ε data shows a qualitatively good agreement with experimental results. Electronic origins of major optical structures are discussed.