Structure-dependent Photoluminescence from MA1-xFAxPbI3 with Evidence of Defect-suppression on FA-substitution

We present temperature-dependent photoluminescence (PL) spectra from MA1-xFAxPbI3 for x = 0.02, 0.15, 0.2, and 0.6 over the temperature range of ~12-300 K. These results show remarkable changes across the structural phase transitions together with presence of multiple peaks, prominently in the low temperature regime, that can be associated with different crystallographic phases apparently coexisting at low temperatures. We propose that the persistent dominant presence of the PL signal associated with the high temperature tetragonal/cubic phase in each case even at the lowest temperature can only originate from a structural relaxation at the surface of the grains of the low temperature phase, making the surface electronic structure close to that of the high temperature phases. Since the bandgap is smaller for the surface related phase, the photoexcited energy is significantly transferred to the surface region before deexcitation, giving rise to the dominance of this PL feature. Our results show that with an increasing FA content, this transfer of energy becomes more efficient with a complete suppression of the PL features of the low temperature bulk phase, while the x-ray diffraction shows a phase pure low temperature phase alone. Additionally, we show that the lowest energy PL feature that appears only at a critical low temperature is not associated with any crystallographic transition, but correlates well with the plastic crystal-like transition arising from the three-fold rotations of the MA unit around its C-N axis as determined by prior quasi-elastic neutron scattering studies. This suggests that the MA rotations suppress the formation of self-trapped excitonic features via electron-phonon interactions, while the feature begins to appear with increasing intensity as the MA rotations are frozen with a decreasing temperature below a certain temperature. This interpretation allows us to estimate the polaronic contribution to the stability of the self-trapped exciton in these systems to be around ~35 ± 3 meV. Interestingly, the PL spectrum progressively shows a suppression of contributions from the low temperature phases and the self-trapped exciton with an increasing FA content, leading to a single peak PL feature at all temperatures for the highest FA content sample investigated in this work.