Hideyuki Kunugita

Metal halide perovskite materials (MHPs) are promising for several applications due to their exceptional properties. Understanding excitonic properties is essential for exploiting these materials. For this purpose, we focus on CsPbBr3 single crystals, which have higher crystal quality, are more stable, and have no Rashba effect at low temperatures compared to other 3D MHPs. We have estimated exciton energy positions, longitudinal-transverse splitting energy, and damping energy using low-temperature reflection spectra. Under high excitation intensity, two biexciton emissions (M-emission) and exciton–exciton scattering emission (P-emission) were observed. We assign the two M-emissions to the emission to the states of longitudinal and transverse excitons, i.e., ML and MT emissions. From the energy position of the MT emission, the biexciton binding energy has been estimated to be ∼2 meV. By analyzing P-emission obtained from the back side of the sample, we have estimated the exciton binding energy to be 17.8–23.7 meV. This estimation minimizes the influence of the wavenumber distribution in the scattering process. In addition, time-resolved transmittance measurements using pulsed white light have revealed the group velocity dispersion. Comparing experimental results with theoretical calculations using the Lorentz model clarifies that exciton dynamics in CsPbBr3 can be described with a simple Lorentz model. These insights enhance the understanding of exciton behavior and support the development of exciton-based devices using MHPs.

It is well known that the surface trap states and electronic disorders in the solution-processed CH3NH3PbI3 perovskite film affect the solar cell performance significantly and moisture sensitivity of photoactive perovskite material limits its practical applications. Herein, we show the surface modification of a perovskite film with a solution-processable hydrophobic polymer (poly(4-vinylpyridine), PVP), which passivates the undercoordinated lead (Pb) atoms (on the surface of perovskite) by its pyridine Lewis base side chains and thereby eliminates surfacetrap states and non-radiative recombination. Moreover, it acts as an electron barrier between the perovskite and hole-transport layer (HTL) to reduce interfacial charge recombination, which led to improvement in open-circuit voltage (V-oc) by 120 to 160 mV whereas the standard cell fabricated in same conditions showed Voc as low as 0.9 V owing to dominating interfacial recombination processes. Consequently, the power conversion efficiency (PCE) increased by 3 to 5% in the polymer-modified devices (PCE=15%) with V-oc more than 1.05 V and hysteresis-less J-V curves. Advantageously, hydrophobicity of the polymer chain was found to protect the perovskite surface from moisture and improved stability of the non-encapsulated cells, which retained their device performance up to 30 days of exposure to open atmosphere (50% humidity).

We developed anew and simple solvent vapor-assisted thermal annealing (VA) procedure which can reduce grain boundaries in a perovskite film :for fabricating highly efficient perovskite solar cell (PSCs). By recycling of solvent molecules evaporated from an as-prepared perovskite film as a VA vapor source, named the pot-root VA (PR-VA) method, finely controlled and reproducible device fabrication was achieved for formamidiniurn (FA) and methylammonium (MA), mixed cation-halide perovskite (FAPbI(3))(0.85)(MAPbBr(3))(0.15). The mixed perovskite was crystallized on a low-temperature prepared brookite TiO2 mesoporous scaffold When exposed to very dilute solvent vapor, small grains in the perovskite film gradually unified into large grains, resulting in grain boundaries which were highly reduced and improvement of photovoltaic performance in PSC. PR-VA-treated large grain perovskite absorbers exhibited stable photocurrent-Voltage performance with high fill factor and suppressed hysteresis, achieving the, best conversion efficiency of 18.5% for a 5 X 5 mm(2) device and 15.2% for a 1.0 X 1.0 cm(2) device.

Formamidine (FA) and cesium (Cs) cations were introduced into quasi-two-dimensional (2D) perovskites as B site cations. The unique crystalline growth of the resulting (n-C6H13NH3)(2)FAPb(2)I(7), which promotes charge transport in photovoltaic solar cells, was confirmed, as was the stability of this material. The photovoltaic properties of (n-C6H13NH3)(2)FAPb(2)I(7) were found to be superior to those of other homologous quasi-2D perovskite compounds.

The nature of metal oxide scaffold played a pivotal role for the growth of high quality perovskites and subsequently facilitates efficient photovoltaics devices. We demonstrate an effective way to fabricate a low-temperature TiO2 brookite scaffold layer with a uniform and pinhole-free layer for enhancing photovoltaic properties of perovskite solar cells. Various concentrations of TiCl4 were used to modify brookite TiO2 for efficient charge generation and fast charge extraction. We observed that the brookite layer with an appropriate TiCl4 treatment possesses a smooth surface with full coverage of the substrates, whereas TiCl4 treatment further improves the contact of the TiO2/perovskite interface which facilitates charge extraction and drastically influenced charge recombination. The surface treated brookite scaffolds perovskite devices showed an improved performance with an average power conversion efficiency more than 17%. The time resolved photoluminescence showed that the treated samples have obvious effect on the charge carrier dynamics. The striking observation of this study was very low appearance of hysteresis and high reproducibility in the treated samples, which opens up the possibilities for the fabrication of high efficient devices at relatively low temperatures with negligible hysteresis via facile surface modifications. (C) 2017 Author( s).

We have measured photoluminescence spectra in CH3NH3PbBr3 at low temperatures and found a nonlinear emission at the lower energy side of the exciton resonance. Considering that this signal shows a rapid decay, we have assigned it as originating from the exciton-exciton inelastic scattering process called P-emission. The energy difference between this P-emission and the free exciton resonance is 20-30 meV, which is consistent with recent studies of the exciton binding energy. (C) 2016 The Japan Society of Applied Physics

We have investigated coherent LO phonon properties in zinc-based II-VI widegap semiconductors, focusing on phonon-plasma coupled modes. By a careful treatment of the time evolution of the signals in ZnS, ZnSe, and ZnTe, we found a frequency upshift as the pump intensity increases. Using a classical coupled oscillator model, we have explained the pump intensity dependence of both the shift and the decay rates by a mixing of highly damped two-photon generated plasma. From the linear dependence between them we can estimate the photo-excited carrier mobilities, leading to a new powerful estimation method to measure the mobility. (C) 2015 Optical Society of America

We have measured the temperature dependence of the reflection and photoluminescence spectra of a lead bromide hybrid material (CH3NH3PbBr3) using high-quality macroscopic single crystals. Single crystals provide clear and sample-independent reflection spectra that permit discussion of the properties of the photoexcited carriers. We have found that excitons still exist in CH3NH3PbBr3, even at room temperature. We have also determined the temperature dependence of the exciton resonance energy and the line width.

An inclusion complex consisting of a boronic acid fluorophore (C1-APB) and beta-cyclodextrin (beta-CyD) acts as a supramolecular sugar sensor whose response mechanism is based on photoinduced electron transfer (PET) from the excited pyrene to the boronic acid. We have investigated the PET process in C1-APB/CyD complexes by using time-resolved photoluminescence (TRPL) measurements at room temperature, and have succeeded in estimating the electron-transfer time to be about 1 ns. We have also studied the effects of CyDs on the PET process by comparing two kinds of CyDs (alpha-CyD, beta-CyD) under different water-dimethylsulfoxide (DMSO) concentration conditions. We found that the CyDs interacting with the boronic acid moiety completely inhibits PET quenching and increases the monomer fluorescence intensity.

We have experimentally compared the excitonic properties of hybrid multiple quantum wells, (C6H5-C2H4NH3)(2)PbBr4 and (C4H9NH3)(2)PbBr4, using photoluminescence, reflection, and photoluminescence excitation measurements. We focused on the contribution of the image charge effect (ICE) to the excitonic energy structure in these materials which have different dielectric constants of the barrier layers. We have found that the binding energies of the 2s and 3s excitons are considerably enhanced by ICE, while the contribution of ICE to the 1s excitons is smaller because of the small Bohr radius, which is comparable to the well width. DOI: 10.1103/PhysRevB.87.125421

We have investigated experimentally excitonic properties in organic-inorganic hybrid multi quantum well crystals, (C4H9NH3)(2)PbBr4 and (C6H5-C2H4NH3)(2)PbBr4, by measuring photoluminescence, reflectance, photoluminescence excitation spectra. In these materials, the excitonic binding energies are enhanced not only by quantum confinement effect (QCE) but also by image charge effect (ICE), since the dielectric constant of the barrier layers is much smaller than that of the well layers. By comparing the 1s-exciton and 2s-exciton energies, we have investigated the influence of ICE with regard to the difference of the Bohr radius.

Optical properties of InN grown by the epitaxial lateral overgrowth (ELO) method have been studied using photoluminescence (PL) and excitation-correlation (EC) measurements. The PL spectrum is analyzed by free-electron recombination band (FERB) model, which shows that the ELO sample has a very low background carrier concentration (n=5.5*10(16)[cm(-3)]). EC measurements show that the dependences of the band gap renormalization and Auger effect on the carrier concentrations are similar in spite of the different physical origins.

The optical properties of InGaN/GaN multiple-quantum-well (MQW) nanocolumns whose emission wavelength is in the yellow-to-red region (2.1 similar to 2.3 eV) have been investigated using time-resolved photoluminescence (PL) measurements. The analysis of the PL decay curves using a stretched exponential function shows that the radiative and nonradiative rates in the red region are almost the same as those in the yellow region. Long-lived migrating carriers also contribute the strong PL intensity in the red region. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Optical properties in a single crystal of the organic-inorganic hybrid perovskite quantum-well material (C4H9NH3)(2)(CH3NH3)Pb2Br7 have been investigated. This material has double-layer sheets of corner-sharing [PbBr6](4-) octahedrons constituting the inorganic well. The estimated longitudinal-transverse (LT) splitting, exciton exchange energy, and biexciton binding energy are about 60 meV, 17 meV and 26 meV respectively. We can also study well-width dependence in angstroms by comparing with (C4H9NH3)(2)PbBr4 which has single-layer sheets comprising the inorganic well. This is the first time the well-width dependence of excitonic properties in hybrid organic-inorganic quantum materials is investigated. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Anatase TiO2 is well known to exhibit high photocatalytic activity. It is believed that photo-excited carriers create activated species by reacting with water and oxygen at the crystal surface, and such activated species decompose organic pollutants. However, the relaxation and diffuse process of the photo-excited carriers is not understood sufficiently. In this study, we investigated such dynamics of photo-excited carriers by transient absorption spectroscopy in anatase TiO2 epitaxial thin film. Moreover, the transient absorption signal and photoluminescence (PL) results are compared for the first time. © 2011 IEEE.

The relaxation dynamics of photo-excited carriers of indium nitride (InN) films and nanocolumns were examined using degenerate pump-probe measurements at room temperature. We measured two InN films and nanocolumns with different background carrier densities, and performed numerical calculations incorporating band-filling and bandgap-renormalization effects, as well as LO phonon scattering. We found that the intrinsic relaxation properties of InN can be understood by considering the density of states and electron occupation number of the conduction band. It was also revealed that the decay dynamics of InN are not affected by the carrier recombination time under the appropriate conditions. In addition, we examined the differences in carrier relaxation properties between films and nanocolumns.

Optical properties in a single crystal of the organic-inorganic hybrid perovskite quantum-well material (C4H9NH3)(2)(CH3NH3)Pb2Br7 have been investigated. This material has double-layer sheets of corner-sharing [PbBr6](4-) octahedrons constituting the inorganic well. The estimated longitudinal-transverse (LT) splitting, exciton exchange energy, and biexciton binding energy are about 60 meV, 17 meV and 26 meV respectively. We can also study well-width dependence in angstroms by comparing with (C4H9NH3)(2)PbBr4 which has single-layer sheets comprising the inorganic well. This is the first time the well-width dependence of excitonic properties in hybrid organic-inorganic quantum materials is investigated. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fast and slow photocarrier dynamics in indium nitride films have been investigated using femtosecond transient measurements at room temperature. The behavior of the decay dynamics is found to be strongly dependent on the excitation energy and the background carrier density. We have found that the slow decay component disappears under optimized conditions due to equilibrium between band filling and bandgap renormalization effects. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3272916]

We have measured the optical properties of wurtzite InN nanocolumns and film by photoluminescence (PL) Measurements at temperatures from 5 to 300 K and analyzed the PL spectra by fitting with the free-electron recombination bound (FERB) model. For the top-linked InN nanocolumns, we observed strong PL intensity compared to the InN film sample. The PL spectra were asymmetrical with low-energy tails and a red-shift of the PL peak energy position was observed with increasing temperature. However, For the separated InN nanocolumns, we observed weak PL intensity and symmetrical PL spectra. Analyzing the spectra shape of the top-linked InN nanocolumns at 5 K using the FERB model, we evaluated the intrinsic bandgap energy and carrier density of InN nanocolumns to be 0.69 eV and 2.5 x 10(17) cm(-3), respectively. (C) 2009 Published by Elsevier B.V.

We report the first observations of resonantly excited coherent optical phonons in bulk ZnTe using noncollinear optical parametric amplifiers As the pulse photon energy approaches the bandgap, the amplitudes of coherent phonons increase Since pulse fluence and polarization dependencies show that generation mechanism of resonantly excited coherent phonons IS impulsive stimulated Raman scattering, this enhancement can be concluded to be the result of the resonant Raman effect (C) 2009 Published by Elsevier B V.

We have studied the emission mechanism of InGaN/GaN nanocolumns. We extracted only the effect of localized states and investigated the difference between InGaN/GaN single-quantum-disc (SQD) and InGaN nanocolumns by means of photoluminescence and photoluminescence excitation measurements. The difference between the localized states was interpreted by the band tail model. We conclude that the InGaN nanocolumn has widely distributed localized states, while the InGaN/GaN SQD has a single localized state..

Ultrafast and slow photo-earner dynamics in InN films were investigeted using femtosecond transient measurements. We found the slow component disappears under the optimized condition due to the equilibrium of band filling and bandgap renormalization effects

We report the first observation of E(2) coherent phonon modes in CdS using a noncollinear optical parametric amplifier. We studied the intensity, temperature and excitation energy dependences of the high- and low-frequency E(2) modes. We obtain that generation mechanism under non- and near resonant regions is impulsive stimulated Raman scattering (ISRS) and E(2) modes are barely affected by FrOhlich interaction. As temperature increases, E(2) modes are affected by anharmonic effect. The observed phonon lifetime of the low- and high-E(2) modes in CdS is 243 +/- 37 and 11.4 +/- 0.4 ps, respectively, at 83 K. From fitting, we can estimate the lifetime of the high-E(2) mode at the low-temperature limit to be 49.3 ps.

We have studied the emission mechanism of InGaN/GaN nanocolumns. We extracted only the effect of localized states and investigated the difference between InGaN/GaN single-quantum-disc ( SQD) and InGaN nanocolumns by means of photoluminescence and photoluminescence excitation measurements. The difference between the localized states was interpreted by the band tail model. We conclude that the InGaN nanocolumn has widely distributed localized states, while the InGaN/GaN SQD has a single localized state.

Photo-excited carrier relaxation dynamics in InN films were investigated using femtosecond transient measurements. We measured two samples with different background carrier densities, and discussed the influence of the carrier densities on the relaxation dynamics. We also found that the slow decay component disappears under the optimized condition because of the effects of equilibrium band filling and bandgap renormalization.

We analyzed photoluminescence (PL) spectra of In-rich In(x)Al(1-x)N nanocolumns (NCs; x = 0.80) and film (x = 0.82) using the free-electron recombination to bound (FERB) model. The FERB model fits well to the PL spectra of the In-rich In(x)Al(1-x)N film and NCs, which implies that light-emitting properties of In-rich In(x)Al(1-x)N are similar to those of InN. We obtained bandgap energies and electron carrier densities from the results of fitting the PL spectra.

We observed large enhancement of reflected second harmonic generation (SHG) using the one-dimensional photonic effect in regularly arranged InGaN/GaN single-quantum-well nanowalls. Using the effect when both fundamental and SH resonate with the photonic mode, we obtained enhancement of about 40 times compared with conditions far from resonance.

We show pseudogap maps of Anderson localization of light adopting the parameters of self-organized nanocolumn samples, which consist of random arrays of parallel nanosized columnar semiconductor crystals. The maps indicate the parametric dependence of the localization effect. To obtain the maps, we simulated light propagation in open random media using the two-dimensional finite-difference time-domain method and analyzed the simulation results by Fourier transformation. We found that the shape of the pseudogaps is close to the one of bandgaps in photonic crystals. We conclude that strong localization of light occurs because of interference by average Bragg diffraction, not strong Mie resonant peaks.

We studied resonant generation of coherent phonon in ZnTe by a nondegenerate EO sampling measurement. The pump energy dependence of the initial amplitude can be explained by the intense absorption of the pump pulse

We have investigated how the morphology and diameter of GaN nanocolumn affect the optical properties of exciton and biexciton. We measured photoluminescence (PL) of excitons in GaN nanocolumns at low temperature and found that the PL spectra depend on the column diameter. Changing the distance between the sample and the collimating lens, it was established that this phenomenon is due to the z-polarized PL radiated from the side surface. This result proves that the nano-scale morphology produces a change in the optical properties. In addition, to elucidate the mechanism causing the increase of the biexciton binding energy observed in a previous experiment, we calculated the exciton binding energies by the variational method and also estimated their sizes. From the calculated results we can expect that the only biexcitons experience spatial confinement effect even when the nanocolumn width is much larger than the exciton Bohr radius. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We measured transient absorption spectra with pump-probe technique to investigate the dynamics of photo-excited carriers at 300 K and 12 K in anatase TiO2 epitaxial film. Although the photoluminescence (PL) originated from self-trapped excitons (STEs) vanished at 300 K, we observed the change in probe signal. On the other hand, the intensity of the transient absorption signal at 12 K was much weaker than that at 300K. Therefore the carriers at 300 K become trapped at the surface, leading to a pump-probe signal, while at 12 K, the remained carriers inside the bulk arise from STE recombination. These results suggest that the pump-probe signal is derived from the trap state(s) near the surface. © 2009 IOP Publishing Ltd.

We report the first observation of E2 coherent phonon modes in CdS using a noncollinear optical parametric amplifier. We studied the intensity, temperature and excitation energy dependences of the high- and low-frequency E2 modes. We obtain that generation mechanism under non- and near resonant regions is impulsive stimulated Raman scattering (ISRS) and E 2 modes are barely affected by Fröhlich interaction. As temperature increases, E2 modes are affected by anharmonic effect. The observed phonon lifetime of the low- and high-E2 modes in CdS is 243 37 and 11.4 0.4 ps, respectively, at 83 K. From fitting, we can estimate the lifetime of the high-E2 mode at the low-temperature limit to be 49.3 ps. © 2009 IOP Publishing Ltd.

Photo-excited carrier relaxation dynamics in InN films were investigated using femtosecond transient measurements. We measured two samples with different background carrier densities, and discussed the influence of the carrier densities on the relaxation dynamics. We also found that the slow decay component disappears under the optimized condition because of the effects of equilibrium band filling and bandgap renormalization. © 2009 IOP Publishing Ltd.

We analyzed photoluminescence (PL) spectra of In-rich InxAl 1-xN nanocolumns (NCs x 0.80) and film (x 0.82) using the free-electron recombination to bound (FERB) model. The FERB model fits well to the PL spectra of the In-rich InxAl1-xN film and NCs, which implies that light-emitting properties of In-rich InxAl 1-xN are similar to those of InN. We obtained bandgap energies and electron carrier densities from the results of fitting the PL spectra. © 2009 IOP Publishing Ltd.

We show pseudogap maps of Anderson localization of light adopting the parameters of self-organized nanocolumn samples, which consist of random arrays of parallel nanosized columnar semiconductor crystals. The maps indicate the parametric dependence of the localization effect. To obtain the maps, we simulated light propagation in open random media using the two-dimensional finite-difference time-domain method and analyzed the simulation results by Fourier transformation. We found that the shape of the pseudogaps is close to the one of bandgaps in photonic crystals. We conclude that strong localization of light occurs because of interference by average Bragg diffraction, not strong Mie resonant peaks. © 2009 IOP Publishing Ltd.

We observed large enhancement of reflected second harmonic generation (SHG) using the one-dimensional photonic effect in regularly arranged InGaN/GaN single-quantum-well nanowalls. Using the effect when both fundamental and SH resonate with the photonic mode, we obtained enhancement of about 40 times compared with conditions far from resonance. © 2009 IOP Publishing Ltd.

We report the observation of extremely efficient energy transfer (greater than 99%) in an organic-inorganic hybrid quantum-well structure consisting of perovskite-type lead bromide well layers and naphthalene-linked ammonium barrier layers. Time-resolved photoluminescence measurements confirm that the transfer is triplet-triplet Dexter-type energy transfer from Wannier excitons in the inorganic well to the triplet state of naphthalene molecules in the organic barrier. Using measurements in the 10-300 K temperature range, we also investigated the temperature dependence of the energy transfer. © 2008 The American Physical Society.

We report the observation of extremely efficient energy transfer (greater than 99%) in an organic-inorganic hybrid quantum-well structure consisting of perovskite-type lead bromide well layers and naphthalene-linked ammonium barrier layers. Time-resolved photoluminescence measurements confirm that the transfer is triplet-triplet Dexter-type energy transfer from Wannier excitons in the inorganic well to the triplet state of naphthalene molecules in the organic barrier. Using measurements in the 10-300 K temperature range, we also investigated the temperature dependence of the energy transfer.

Exciton polariton in an organic-inorganic multiple-quantum-well (MQW) single crystal (C4H9NH3)(2)PbBr4 at low temperature has been investigated by photoluminescence excitation (PLE), reflection, and time-resolved photoluminescence measurements. Since (C4H9NH3)(2)PbBr4 has ideal two-dimensional excitons with an extremely large oscillator strength and forms self-organized MQW with a very short well-period (d similar to 10 angstrom), polaritonic coupling among excitons is strong and extends over a large number of wells. Therefore, observed MQW polariton features were the same as those of bulk polaritons. We have also investigated relaxation dynamics of the MQW polariton in the same framework as discussions on bulk polaritons. (C) 2007 Elsevier B.V. All rights reserved.

This paper presents an investigation into the relaxation process of photo-excited carriers in anatase TiO2 thin films, in which the concentration of oxygen vacancy is controlled by annealing at various temperatures in an oxygen atmosphere. The influence of oxygen vacancies on absorption spectra, photoluminescence (PL) spectra, and PL decay dynamics are discussed. (c) 2007 Elsevier B.V. All rights reserved.

We measured photoluminescence (PL) from excitons and biexcitons in GaN nanocolumns at low temperature and found that the PL spectra of excitons depend on the nanocolumn diameter. Taking into account the polaritonic effect of the excitons, calculation of the PL spectra revealed that the dependence on diameters causes a difference of PL intensity from side surfaces of the nanocolumns. At high excitation intensities, we also observed biexciton emissions and found that the biexciton binding energies are higher than those in bulk samples. Although the mechanism for the increase in the binding energy is not clear at present, we suppose that it arises from a spatial confinement effect due to the nanocolumn morphology. (C) 2007 Elsevier B.V. All rights reserved.