(Invited) Luminescence Spectroscopy of Metal-Halide Perovskites: A Solution-Processed High-Efficiency Photonic Material
Authors: Kanemitsu Yoshihiko 
Published: February 2020

Lead halide perovskites MAPbX3 [MA = CH3NH3, X = Br and I] have recently received much attention as a promising class of photovoltaic, laser, and light-emitting diode (LED) materials that enable low cost and high efficiency. To clarify the physical origin of the highly efficient mutual conversion between light and electricity in MAPbX3, we have studied the optical properties and photocarrier recombination processes in MAPbX3 thin films and single crystals. In this talk, we summarize our recent spectroscopic data of MAPbX3 and discuss their radiative and nonradiative recombination processes with respect to laser, LED, and photovoltaic applications.
Highly luminescent materials are required for excellent solar cell (SC) and LED devices, and bright LEDs can also work as high-power SCs. The energy conversion efficiencies of SC and LED devices depend on the luminescence quantum efficiency of the used semiconductor material. Therefore, luminescence spectroscopy is one of the most powerful techniques for characterizing and developing new materials and structures for SC and LED devices. Since solution-processed MAPbX3 thin films show high-efficiency photoluminescence (PL) and electroluminescence (EL), MAPbX3-based diodes should enable excellent SC and LED devices.
MAPbX3 perovskites have a direct band-gap structure and large optical absorption coefficients in the visible spectral region [1,2]. Their PL lifetimes are long, resulting in long carrier diffusion length. These intrinsic optical and transport properties are the physical origin of the observed high conversion efficiencies in MAPbX3-based SCs. Time-resolved PL measurements clearly show that the PL decay dynamics are explained by a simple model including single carrier trapping and the radiative electron-hole recombination [3]. The excitation intensity dependence of the PL decay time constant indicates that the density of the carrier traps and recombination centers is extremely low [3,4].
In MAPbX3 single crystals, an initial fast-decay component appears in the PL decay curve, which is not observed in polycrystalline MAPbX3 thin films [5]. Fast carrier diffusion determines the spatial distribution of carriers in the near-surface region. To clarify the impact of the spatial photocarrier distribution on the PL decay, the PL decay dynamics are studied by two-photon excitation spectroscopy [5,6]. Under one-photon excitation, a redshift of the PL peak is observed for later times after photoexcitation. On the other hand, the PL peak exhibits no redshift with time in case of two-photon excitation. In addition, the PL decay time constant for two-photon excitation is much larger than that for one-photon excitation [5,6]. We clearly show that the carrier diffusion process in the near-surface region causes the PL peak shift and the rapid decay of the PL intensity at the initial stage for one-photon excitation. The photon recycling is responsible for slow PL decays and long-range carrier diffusion [5,6]. These unique properties are suitable for photonic devices. Our optical studies provide fundamental information for realization of high-performance devices based on metal halide perovskites.
The author would like to thank Y. Yamada, T. Yamada, and A. Wakamiya for discussions. Part of this work was supported by JST-CREST.
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