Abstract

Lead-halide hybrid perovskite crystals have emerged as potentially materials for solar cells due to their power conversion efficiency over 20% [1]. In this work we conduct a comparative study between two hybrids (CH₃NH₃PbBr₃ and CH₃NH₃PbCl₃) and an all-inorganic lead-halide perovskite (CsPbBr₃). Both have the general ABX₃ perovskite formula, with similar band gap (2.2 eV) and similar structural phase sequence, orthorhombic at low temperature, changing to tetragonal and then to cubic as temperature is increased [2]. Theoretical studies suggested that dynamic disorder, imposed by the rotation of anisotropic organic molecules in the inorganic octahedral network, plays an important role in the high photovoltaic activity presented by this perovskites [3,4]. In many materials, the quasi-elastic scattering spectral continuum is a characteristic of relaxational dynamics which can be observed by many techniques as well as Raman scattering [5]. In order to investigate this dynamic, low frequency Raman spectra were analyzed using the imaginary part of the Debye relaxation model for the central peak and the imaginary part of the damped Lorentz oscillator model for fit the Raman peaks (Figure 1). The experimentally measured Raman scattering spectrum is expressed as:

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