Abstract
Single crystals represent a benchmark for understanding the bulk properties of halide perovskites. We have indeed studied the dielectric function of lead bromide perovskite single crystals (MAPbBr3, CsPbBr3 and for the first time FAPbBr3) by spectroscopic ellipsometry in the range of 1–5 eV while varying the temperature from 183 to 440 K. An extremely low absorption coefficient in the sub-band gap region was found, indicating the high optical quality of all three crystals. We extracted the band gap values through critical point analysis showing that Tauc-based values are systematically underestimated. The two structural phase transitions, i.e., orthorhombic–tetragonal and tetragonal–cubic, show distinct optical behaviors, with the former having a discontinuous character. The cross-correlation of optical data with DFT calculations evidences the role of octahedral tilting in tailoring the value of the band gap at a given temperature, whereas differences in the thermal expansion affect the slope of the band gap trend as a function of temperature.
Highlights
Single crystals represent a benchmark for understanding the bulk properties of halide perovskites
Adding bromides increases the band gap compared to corresponding pure iodides, and this favors tandem coupling with silicon in combined photovoltaic technologies.[8,9]
Supporting Information for further details). We show that their temperature-dependent optical properties are shaped by the cation type, which indirectly impacts on structural features like the octahedral tilting or the thermal expansion that are responsible for their optical response
Summary
Figure SI-4 shows the impact of either octahedral tilting (cubic → orthorhombic) or isotropic volume variation (of the cubic phase) on the band edge energies of the CsPbBr3 perovskite, relative to the vacuum level. Octahedral tilting increases the band gap by shifting the VBM away from the vacuum level, toward more negative energies, and the CBM toward more positive energies. We compare the structural parameters of all three perovskites in their corresponding room-temperature phases (i.e., orthorhombic for CsPbBr3 and cubic for MAPbBr3 and FAPbBr3) by means of the DFT calculations (Figure 4b). Though its effect is canceled out because of the random orientations or positions of the X-cations within the inorganic cage The combined experimental−theoretical analysis indicates two important parameters for the band gap determination in XPbBr3: octahedral tilting, which is fundamental for the band gap value, and the thermal expansion, which likely impacts the slope of the band gap as a function of the temperature
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