Photon Echo Polarimetry of Excitons and Biexcitons in a CH₃NH₃PbI₃ Perovskite Single Crystal

Abstract

Lead halide perovskites show remarkable performance when used in photovoltaic and optoelectronic devices. However, the peculiarities of light−matter interactions in these materials in general are far from being fully explored experimentally and theoretically. Herein, we specifically address the energy level order of optical transitions and demonstrate photon echoes in a methylammonium lead triiodide single crystal, thereby determining the optical coherence times (T 2 ) for excitons and biexcitons at cryogenic temperature to be 0.79 and 0.67 ps, respectively. Most importantly, we have developed an experimental photon-echo polarimetry method that not only identifies the contributions from exciton and biexciton complexes but also allows accurate determination of the biexciton binding energy of 2.4 meV, even though the period of quantum beats between excitons and biexcitons is much longer than the coherence times of the resonances. Our experimental and theoretical analysis methods contribute to the understanding of the complex mechanism of quasiparticle interactions at moderate pump density and show that even in high-quality perovskite crystals and at very low temperatures, inhomogeneous broadening of excitonic transitions due to local crystal potential fluctuations is a source of optical dephasing.

Long-Lived Exciton Coherence in Mixed-Halide Perovskite Crystals

“…A comparison with pure FAPbI 3 crystals is not possible due to their fast degradation. We also note that the value of 0.8 ps for the exciton coherence time in a MAPbI 3 crystal was obtained by us in ref under the same conditions as for the mixed crystal in this work. The long-lived coherence in mixed FA 0.9 Cs 0.1 PbI 2.8 Br 0.2 crystal rather resembles results obtained in low-dimensional systems such as CsPbBr 2 Cl nanocrystals ( T 2 ≈ 25 ps in ref ).…”

“…19 These reports generated excitement about possible applications of perovskite semiconductors as bright quantum light sources (e.g., in quantum information processing), expanding the range of potential applications beyond photovoltaics and optoelectronics. So far, the coherence times in bulk perovskites were reported to be short (few picoseconds in MAPbI 3 15,20 ) as expected for free and weakly localized excitons.…”

Spin Dynamics of Electrons and Holes Interacting with Nuclei in MAPbI3 Perovskite Single Crystals

“…The TRDR signal shows to a good approximation to be a monoexponential decay with time τ X = 0.3 ns (see Figure d), which can be related to the exciton lifetime. Note, the exciton decoherence time T 2 X was determined to be below 1 ps …”

“…Note, the exciton decoherence time T 2 X was determined to be below 1 ps. 14 It is important for our study that at temperatures below 160 K MAPbI 3 crystals exhibit an orthorhombic crystal structure, 30 which causes an anisotropy of the spin-dependent properties, e.g., the g-factor anisotropy for electrons and holes shown below, in contrast to the isotropic cubic case. The structural phase transition with decreasing temperature from the tetragonal to the orthorhombic phase is accompanied by a steplike increase of the band gap by 113 meV undercutting the temperature of about 160 K. 30 In optical experiments it is evidenced by a corresponding high energy shift of the photoluminescence (PL) line, which is shown in Figure 2a.…”

“…A photovoltaic efficiency exceeding 25% is reached; , also long-standing challenges, like the limited lifetime of perovskite solar cells, come closer to a solution . Further, these materials and their nanostructures look promising for application in optoelectronic devices, , X-ray detectors, , and spintronics devices. − These perspectives demand a solid understanding of the fundamental properties of this class of perovskites, for which optical and magneto-optical techniques developed for the investigation of spin-dependent phenomena are valuable tools, as they give access to key band structure parameters. − …”