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Article
Competition Between Hot-Electron Cooling and Large Polaron Screening in CsPbBr Perovskite Single Crystals 3
Tyler J. S. Evans, Kiyoshi Miyata, Prakriti Pradhan Joshi, Sebastian Maehrlein, Fang Liu, and Xiaoyang Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00476 • Publication Date (Web): 10 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018
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The Journal of Physical Chemistry
Competition Between Hot-electron Cooling and Large Polaron Screening in CsPbBr3 Perovskite Single Crystals
Tyler J. S. Evans, Kiyoshi Miyata, Prakriti P. Joshi, Sebastian Maehrlein, Fang Liu, X-Y. Zhu* Department of Chemistry, Columbia University, New York, NY 10027, USA
ABSTRACT: Lead halide perovskites (LHPs) are solution processable semiconductors characterized by long carrier lifetimes. Recent studies have suggested that electrons and holes in LHPs interact with phonons to form large polarons on sub-picosecond time-scales and polaron formation may also slow down hot carrier cooling. Using femtosecond time-resolved two-photon photoemission (TR-2PPE) and transient reflectance (TR) spectroscopies, we follow the initial electron cooling and polaron formation dynamics in single-crystal CsPbBr3 perovskite. We find that the hot electrons cool down initially (≤0.2 ps) with rates of -0.64±0.06 eV/ps and -0.82±0.08 eV/ps at 300 K and 80 K, respectively. This weakly temperature-dependent rate is attributed to the initial relaxation of un-screened hot electrons by the emission of longitudinal optical (LO) phonons. On longer time scales, we observe dynamic changes in the photoemission cross-section and in the red-shift of the optical bandgap. We attribute these dynamic changes to large polaron formation from electron-LO phonon interaction, with temperature-dependent polaron formation time constants of τp = 0.7±0.1 ps and 2.1±0.2 ps at 300 K and 80 K, respectively. The increase in polaron formation rate with temperature is correlated with the broadening in phonon resonances, suggesting that phonon disorder and dephasing facilitate large-polaron formation. The large polaron formation rate is not competitive with the cooling rate of un-screened hot electrons in CsPbBr3, in contrast to hybrid CH3NH3PbBr3 (or CH3NH3PbI3) where the two rates are similar. This contrast explains the observation of long-lived hot carriers in the latter but not the former.
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1. INTRODUCTION Lead halide perovskites (LHPs), APbX3 (A = molecular or alkali metal cation; X = Cl, Br, I), have made their marks in highly efficient solar cells and light-emitting devices,1–6 thanks to the long carrier diffusion lengths and low recombination rates in these materials.7 The all-inorganic LHPs, in which the A site cation is Cs+, are more robust than their hybrid organic-inorganic counterparts and have drawn attention as robust materials for efficient light-emission8–11 including continuous wave polariton lasing.12 Many properties essential to device performance – charge carrier lifetimes, mobilities, and recombination rates in LHPs – have been unified in the large polaron model which assumes that each electron/hole is dynamically screened by the polar and soft crystalline lattice.13–15 The efficient screening diminishes the Coulomb potential of the resulting large polaron in scattering with other electrons/holes, with trapped charges, and with additional LO phonons, thus accounting for their remarkable properties. We have recently probed polaron formation dynamics in LHPs using time-resolved optical Kerr effect spectroscopy (TR-OKE) to follow the phonon activities upon charge injection. This measurement yielded room-temperature polaron formation time constants of τp = 0.3 ps in MAPbBr3 and 0.7 ps in CsPbBr3.15 The faster polaron formation in hybrid organic-inorganic LHPs is correlated with the presence of long-lived hot carriers in the former, but not the latter,16,17 suggesting that large polaron screening may contribute to the slowed cooling of hot carriers. Since the interaction of a nascent electron with LO phonons is responsible for both initial hot electron cooling18,19 and large polaron formation,20–22 how the competition between these two processes ultimately determines the fate of energetic carriers16,17 remains ambiguous. Using TR2PPE spectroscopy, we directly probe the electron distribution in the conduction band of singlecrystal CsPbBr3 following above-bandgap photoexcitation at both room and liquid nitrogen temperatures. Note that, at both temperatures, CsPbBr3 is in the low-temperature orthorhombic phase.23 We determine the initial (≤0.2 ps) energy relaxation rates of nascent hot electrons of RE = -0.64±0.06 eV/ps and -0.82±0.08 eV/ps at 300 K and 80 K, respectively. This weakly temperature-dependent relaxation rate is expected from the cooling of un-screened hot electrons by LO phonon emission, with an electron-LO phonon scattering time constant of τe-LO = 3.8±0.5 fs. In contrast to the weak temperature-dependence in hot electrons cooling, we find that polaron formation occurring on longer times is strongly temperature dependent, with time constants
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The Journal of Physical Chemistry
increasing from τp = 0.7 ± 0.1 ps at 300 K to 2.2 ± 0.3 ps at 80 K. Here τp is theoretically related to the build-up of electron-phonon correlation.24 We can attribute faster polaron formation at higher temperatures to higher phonon-phonon scattering rates and, thus shorter phonon lifetimes and broader phonon resonances. Our findings are consistent with known low-frequency Raman spectra of LHPs,25,26 and corroborate the crystal-liquid duality of LHPs.14
2. EXPERIMENTAL Sample Preparation and characterization. We grew single-crystals of CsPbBr3 under ambient conditions by vapor diffusion whereby anti-solvent vapor diffuses into a precursor solution to induced crystallization.27 The precursor was an 0.4 M solution of PbBr2 and CsBr (1:1 molar ratio) in dimethyl sulfoxide with methanol as an anti-solvent. Details on crystal growth and characterization have been reported elsewhere.15 Single-crystal samples with lateral sizes of a few mm were mounted on a native-oxide terminated Si(111) wafer with Ag epoxy and loaded into an vacuum system for laser spectroscopic measurements. Each crystal was freshly cleaved in an UHV sample preparation chamber (
