| 研究実績の概要 |
We investigated the dynamic optical properties of excitons in CH3NH3PbI3 (MAPbI3) perovskite single crystals and thin films using a combination of various laser spectroscopy techniques. Spectroscopic data clearly indicate a stable existence of free excitons in the single crystals at low temperatures. Based on the temperature-dependent measurements, we evaluated the exciton-phonon coupling and the exciton binding energy in the orthorhombic-phase single crystals. In contrast, in the thin films, we observed the optical response of photocarriers that can be described by a free-carrier model rather than an excitonic model even at low temperatures. The presence of the high-temperature tetragonal phase even below the phase transition temperature, which is confirmed by optical, THz transient absorption, and X-ray diffraction data, leads to the conclusion that a fast charge transfer from the low-temperature orthorhombic phase to the tetragonal phase prevents the formation of excitons in the thin films.
We also examined the effects of alkali doping and thermal annealing on the carrier transport in earth-abundant kesterite Cu2ZnSn(S,Se)4 (CZTSSe) thin films using THz transient absorption spectroscopy. We pointed out that the post-annealing treatment probably contributes to enhance the charge transport in CZTSSe. For alkali doping, we found that Na doping results in a longer carrier diffusion length.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
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理由
Apart from CH3NH3PbI3 (MAPbI3), which has been utilized mostly for single-junction perovskite solar cells because of its suitable bandgap for sunlight absorption, MAPbCl3 and MAPbBr3, whose bandgap energies lie in the blue and green-yellow spectral region, are expected to have high potential in applications for high-efficient visible light-emitting diodes and lasers. Although it is widely accepted that the photocarrier recombination dynamics in MAPbI3 is due to free carriers, the exciton binding energies in MAPbBr3 and MAPbCl3 are predicted to be larger than that in MAPbI3. The predominant excitations and the recombination mechanisms in MAPbBr3 and MAPbCl3, which play important roles in determining the device efficiency, are not clear. Therefore, our current focus is on finding experimental evidences to address these issues.
In addition to pristine halide perovskites, mixed-halide perovskites such as MAPb(I,Br)3, whose bandgap energy can be tuned controllably through halide composition, provide a promising class of materials for the top layer of tandem solar cells. To optimize the absorption of the sunlight for current matching, the bandgap energy of the top layer should be in a range from 1.7 to 1.8 eV. However, the power conversion efficiency (PCE) for such higher bandgap perovskites is still far below the PCE of lower bandgap perovskites and their theoretical limit. Thus, along with optimizing the bandgap energies for current matching, understanding the photovoltaic properties of mixed-halide perovskite absorbers is critical to achieving higher PCE tandems.
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| 今後の研究の推進方策 |
In order to exclude effects of grain structures and obtain the intrinsic photoelectrical properties of MAPbBr3 and MAPbCl3, corresponding single crystals grown by different methods including inverse temperature crystallization and anti-solvent diffusion crystallization will be used for optical and electrical characterizations. The fundamental near-band-edge responses will be examined by means of reflection, photoluminescence excitation and photocurrent excitation spectroscopy. Meanwhile, time-resolved behaviors of photocarriers will be examined using picosecond time-resolved photoluminescence and femtosecond transient reflection spectroscopy. To understand the nature of excitations and recombination mechanisms in MAPbBr3 and MAPbCl3, temperature dependent and excitation power dependent measurements will be performed. We will clarify the single-carrier trapping, two-carrier radiative recombination and Auger recombination rates in perovskites from excitation-power-dependent time-resolved photoluminescence and optical transient absorption measurements.
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