Self-Trapped Excitons in Organic Lead Halide Perovskites

Introduction:

Broadband emissions related to self-trapped excitons in the sub-bandgap region in organic lead halide perovskite (OLHP) have drawn attention in recent times due to their potential in optoelectronic applications. In this study, we have provided a systematic multi-technique approach to the formation of broad-band emission in OLHPs. We have observed that broad-band emission is closely related to the lattice fluctuation in OLHPs.

Methods:

Single crystals of methylammonium lead chloride (MAPbCl₃), methylammonium lead bromide (MAPbBr₃), and formamidinium lead bromide (FAPbBr₃) were grown using solvent evaporation at room temperature method. Temperature-dependent photoluminescence measurements were performed in Edinburg FLS920 spectrometer, synchrotron X-ray powder diffraction measurements were carried out at the P-02.1 beamline at PETRA III, DESY, and temperature-dependent Raman measurements were carried out in a micro-Raman spectrometer (in Via, Renishaw, UK).

Results:

Temperature-dependent photoluminescence study shows broad-band emission in MAPbCl₃ and MAPbBr₃ in the sub-bandgap region, which is missing in FAPbBr₃. Synchrotron X-ray powder diffraction analysis suggests cubic phase till 172, 235, and 265 K for MAPbCl₃, MAPbBr₃, and FAPbBr₃, respectively. The tetragonal phase transforms to an orthogonal phase at 167, 148, and 150 K, respectively. Interestingly, below 165 K in MAPbCl₃, conventional single Pnma symmetry cannot be fitted with low values of reliability parameters, suggesting the presence of another phase. Temperature-dependent Raman study shows stronger N-H---X hydrogen bonding between MA⁺ cation and PbX₆ octahedra in the orthorhombic phase of MAPbCl₃ and MAPbBr₃ than FAPbBr₃. The study suggests more tilting of the octahedral framework in both the MA-based compounds.

Conclusions:

In summary, our study suggests the emergence of broad-band emission in the two MA-based compounds is due to the transient lattice distortion due to the stronger N-H---X bonding. However, at the lower temperature of MAPbCl₃, the presence of double unit cells stabilizes the transient lattice deformation.