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.