Free Excitons and Exciton–Phonon Coupling in CH₃NH₃PbI₃ Single Crystals Revealed by Photocurrent and Photoluminescence Measurements at Low Temperatures

We investigated the exciton–phonon couplings and exciton binding energy in CH₃NH₃PbI₃ (MAPbI₃) single crystals using temperature-dependent photocurrent (PC) and photoluminescence (PL) spectroscopy. The PC and PL data provide clear evidence of the existence of excitons in orthorhombic-phase MAPbI₃. The temperature-dependent PC data were found to be less influenced by the bound excitons than the PL data, and thus the PC data reflect the intrinsic scatterings of excitons. We observed that the exciton–phonon couplings were strong in MAPbI₃ and determined the longitudinal optical phonon energy to be 16.1 meV. Moreover, on the basis of the temperature dependences of the PC and PL data, we evaluated the exciton binding energy to be 12.4 meV for orthorhombic-phase MAPbI₃ single crystals. Our findings pave a way for using simultaneous PC and PL measurements to determine precisely fundamental properties of perovskites

Free Carriers versus Excitons in CH₃NH₃PbI₃ Perovskite Thin Films at Low Temperatures: Charge Transfer from the Orthorhombic Phase to the Tetragonal Phase

We have investigated the dynamic optical properties of CH₃NH₃PbI₃ (MAPbI₃) perovskite thin films at low temperatures using time-resolved photoluminescence, optical transient absorption (TA), and THz TA spectroscopy. Optical spectroscopic results indicate that the high-temperature tetragonal phase still remains in the MAPbI₃ thin films at low temperatures in addition to the major orthorhombic phase. The fast charge transfer from the orthorhombic phase to the tetragonal phase is likely to suppress the formation of excitons in the orthorhombic phase. Consequently, the near-band-edge optical responses of the photocarriers in both the tetragonal and orthorhombic phases of the MAPbI₃ thin films are more accurately described by a free-carrier model, rather than an excitonic model even at low temperatures

Dynamic Optical Properties of CH₃NH₃PbI₃ Single Crystals As Revealed by One- and Two-Photon Excited Photoluminescence Measurements

The dynamic optical properties of perovskite CH₃NH₃PbI₃ single crystals were studied by means of time-resolved photoluminescence (PL) spectroscopy at room temperature. The PL peak under one-photon excitation exhibits a red-shift with elapsing time, while two-photon PL is time-independent and appears at lower energy levels. The low-energy two-photon PL can be attributed to emissions from the localized states because of strong band-to-band absorption and photon re-absorption of the emitted light in the interior region. We revealed that the PL behaviors can be explained by the diffusion of photocarriers generated in the near-surface region to the interior region. The excitation fluence dependence of the one-photon PL dynamics is also discussed in terms of the electron–hole radiative recombination and carrier diffusion effects