Role of the A-Site Cation in Low-Temperature Optical Behaviors of APbBr₃ (A = Cs, CH₃NH₃)

Abstract

APbBr3 (A = Cs, CH3NH3) are prototype halide perovskites having bandgaps of 2.30–2.35 eV at room temperature, rendering their apparent color nearly identical (bright orange but opaque). Upon optical excitation, they emit bright photoluminescence (PL) arising from carrier recombination whose spectral features are also similar. At 10 K, however, the apparent color of CsPbBr3 becomes transparent yellow, whereas that of CH3NH3PbBr3 does not change significantly due to the presence of an indirect Rashba gap. With increasing the excitation level, evolution of the PL spectra, which are excitonic at 10 K, reveals the emergence of P-band emission arising from inelastic exciton–exciton scattering. Based on the spectral location of the P-band, exciton binding energies are determined to be 21.6 ± 2.0 and 38.3 ± 3.0 meV for CsPbBr3 and CH3NH3PbBr3, respectively. Intriguingly, upon further increase in the exciton density, electron–hole plasma appears in CsPbBr3 as evidenced by both red-shift and broadening of the PL. This phase, however, does not occur in CH3NH3PbBr3 presumably due to polaronic effects. Although the A-site cation is believed not to directly impact optical properties of APbBr3, our results underscore its critical role, which destines different high-density phases and apparent color at low temperatures.

Drastic transitions of excited state and coupling regime in all-inorganic perovskite microcavities characterized by exciton/plasmon hybrid natures

“…Figure 3c shows a threshold behavior in the fluence-dependent PL intensity detected at the lasing wavelength. These observations are very similar to previous studies, e.g., the work of Zhu et al, in which they assign this low-energy emission to the plasmonic emission and claim a normal photon lasing due to population inversion 44 . For our case, the density of the excited particle is estimated to be 1.6 × 10 18 cm −3 at I p of ~10 μJ cm −2 (see Note 2 in Supplementary Information).…”

“…At the pump fluence I p of ~3.8 μJ cm −2 , a PL signal with asymmetric spectral profile is observed in an energy range of ~2.3-2.4 eV. This is a typical spectral response of CsPbBr 3 crystal at RT 31 , whereas the origin of asymmetric spectral shape is still under discussion 8,31,44,45 . Figure 3b exhibits a colormap of the I p -dependent normalized PL intensities.…”

Mixed Dimethylammonium/Methylammonium Lead Halide Perovskite Crystals for Improved Structural Stability and Enhanced Photodetection

“…For example, increasing the size of the A‐site cation from Cs + to MA to FA narrows the bandgap due to decreased octahedral tilting. [ 8,9 ] Furthermore, cryogenic studies have demonstrated that the A‐site cation also impacts the temperature‐dependent optical [ 10–17 ] and structural properties. [ 14,15,18–24 ] Temperature‐dependent neutron/X‐ray diffraction and photoluminescence (PL) measurements on MAPbBr 3 revealed that the MA cation transformation from an ordered to a disordered state is correlated with an increased integrated PL intensity, and is also suggested to induce the orthorhombic to tetragonal phase transition.…”

Observation of 3D Biexcitons in Pristine‐Quality CH3NH3PbBr3 Single Crystals

“…[ 7,8 ] Regarding the latter, for example, the basic issue such as “the identity of optical excitation at room temperature” is still controversial, because there are contradicting evidences for two candidates, free carriers and excitons. [ 9,10 ] The key question is the stability of the exciton against thermal ionization of the constituents. The exciton binding energy φ X in 3D HPs such as MAPbX 3 (MA = CH 3 NH 3 , X = I, Br, Cl) ranges from 15 meV to 70 meV, [ 9,11,12 ] which is indeed comparable to room temperature (≈26 meV), and therefore, an exciton may or may not be stable.…”