Long-Lived Exciton Coherence in Mixed-Halide Perovskite Crystals

Abstract

Compositional engineering of the optical properties of hybrid organic− inorganic lead halide perovskites is crucial for the realization of efficient solar cells and light-emitting devices. We study the effect of band gap fluctuations on coherent exciton dynamics in a mixed FA 0.9 Cs 0.1 PbI 2.8 Br 0.2 perovskite crystal by using photon echo spectroscopy. We reveal a narrow homogeneous exciton line width of 16 μeV at a temperature of 1.5 K. The corresponding exciton coherence time T 2 = 83 ps is exceptionally long due to the localization of excitons at the scale of tens to hundreds of nanometers. From spectral and temperature dependences of the two-and three-pulse photon echo decay, we conclude that for low-energy excitons pure decoherence associated with elastic scattering on phonons is comparable with the exciton lifetime, while for excitons with higher energies, inelastic scattering to lower energy states via phonon emission dominates.

Spin-Dependent Exciton–Exciton Interactions in a Mixed Lead Halide Perovskite Crystal

“…FA 0.9 Cs 0.1 PbI 2.8 Br 0.2 exhibits a direct band gap of ε g ≈ 1.52 eV at T = 2 K with a pronounced exciton feature shifted toward lower energies by the binding energy of 14 meV . In ref , the exciton coherence time T 2 was found to be exceptionally long in the order of 100 ps, which was associated with the localization of excitons on spatial modulations of the bandgap on the nanometer scale. The localized excitons lose their coherence due to elastic scattering with phonons on time scales comparable to the exciton lifetime (≈90 ps), as follows from the spectral and temperature dependences of two- and three-pulse decay times.…”

“…We provide additional information on the studied crystal in Supporting Information section S1. The same sample was previously studied using transient four-wave mixing (FWM) spectroscopy in ref , where further details on the growth technique can be found. FA 0.9 Cs 0.1 PbI 2.8 Br 0.2 exhibits a direct band gap of ε g ≈ 1.52 eV at T = 2 K with a pronounced exciton feature shifted toward lower energies by the binding energy of 14 meV .…”

“…Polarization-resolved pump–probe studies on CsPbBr 3 claimed a pronounced excitation-induced circular dichroism due to spin-dependent Coulomb interactions with a magnitude of about 12 meV at moderate exciton densities of roughly 2 × 10 17 cm –3 , which is significantly larger than the excitation-induced shifts and dephasing reported in bulk , and even two-dimensional perovskites and nanoplatelets . It has been demonstrated that at low temperatures the inhomogeneous broadening of optical transitions dominates over Γ 2 in organic–inorganic perovskites. − In this case, the nonlinear optical response is given by photon echoes which can be used to extract rich information on the homogeneous line width of the exciton and the role of EID.…”

Stable and Efficient Mixed‐halide Perovskite LEDs

“…Efforts to enhance the stability of mixed-halide perovskites involve a multidisciplinary approach. [95] Strategies include engineering the halide compositions, encapsulation techniques to minimize environmental exposure, and surface passivation to (c, d) Reproduced with permission. [92] Copyright 2021, Springer Nature.…”

Robust Free Excitons in CH3NH3PbI3Halide Perovskite Revealed by Four‐Wave Mixing

“…This results in the observation of the photon echo in the FWM signal in single crystals of MAPbI 3 [ 28,29 ] and mixed perovskites. [ 30 ] The FWM study, with the help of femtosecond pulses, led to the necessity of taking into account not only the free exciton but also the biexcitonic states. [ 29 ] In the works [ 24–27 ] FWM spectral properties were also studied by spectrally broad femtosecond laser pulses, which introduce the possibility of many‐body effects, potentially complicating the interpretation of the observed phenomena.…”