山田 泰裕

Highly efficient anti-Stokes (AS) photoluminescence (PL) is observed from halide perovskite quantum dots (QDs) due to their strong electron-phonon interactions. The AS PL is particularly intriguing as it suggests the potential for semiconductor optical cooling if the external quantum efficiency approaches 100%. However, the PL quantum efficiency in QDs is primarily dominated by multiparticle nonradiative Auger recombination processes under intense photoexcitation, which impose limits on the optical cooling gain. Here, we investigate the Auger recombination of dot-in-crystal perovskites. We quantitatively estimate the maximum optical cooling gain and the corresponding excitation intensity. We further conducted optical cooling experiments and demonstrate a maximum photo-cooling of approximately 9 K from room temperature. Additionally, we confirmed that increasing the excitation intensity leads to a transition from photo-cooling to photo-heating. These observations are consistent with our time-resolved measurements, offering insights into the potential and limitations of optical cooling in semiconductor QDs.

Abstract Strong electron-phonon interactions are frequently considered the origin of the unique electrical and optical properties of lead halide perovskites. Electron-phonon interactions induce the formation of a polaron, which is a charge carrier dressed with a phonon cloud. The details of polaron formation are crucial for carrier transport since polaron formation leads to a larger effective mass of a carrier. Several mechanisms have been proposed regarding the physics of polaron formation in halide perovskites, but the details are still under active debate. While the Fröhlich interaction plays an essential role in ionic crystals, we also need to consider the strong phonon anharmonicity of halide perovskites that may lead to the formation of an unconventional polaron. In this review article, we discuss the uniqueness of perovskite semiconductors from the viewpoint of electron-phonon interactions. We review the experimental results and the proposed models concerning the effective carrier mass and carrier mobility. Finally, we briefly explain two physical phenomena related to strong electron-phonon interactions: strong anti-Stokes photoluminescence and slow hot-carrier cooling.

Carrier mobility is one of the most fundamental material parameters of semiconductors and requisite for device applications and interpretation of physical phenomena. We determined the electron and hole mobilities of a CH3NH3PbBr3 single crystal in the high-carrier density regime by combining AC Hall measurements under photoexcitation and two-carrier analysis. Both electron and hole mobilities were significantly enhanced by photo-doping and exceeded 300 cm(2) V-1 s(-1), which are comparable to the electron Hall mobilities of conventional inorganic semiconductors. Our experimental results indicate that charged dislocation scattering dominates the carrier transport at room temperature in the low-carrier density regime.

We studied the optical responses of organic-inorganic halide perovskite CH3NH3PbBr3 single crystals doped with heterovalent Bi3+ ions (electron densities up to 2.3 X 10(12) cm(-3)). The Bi doping causes no significant changes in the band gap energy but leads to an enhanced Urbach tail and photoluminescence blue shift. On the basis of the time-resolved photoluminescence measurements, we attribute the PL response to a shorter carrier lifetime induced by Bi doping, which results in a reduced photon recycling effect (i.e., the repeated emission and reabsorption of photons inside the crystal). We discuss the physical relation between Bi concentration and the optical and electric properties of Bi-doped CH3NH3PbBr3 and reveal the unique nature of the Bi3+ ion as a dopant in halide perovskites.

Organic-inorganic hybrid lead halide perovskites are currently a most attractive class of materials since they have emerged as a solar cell material that realizes both high efficiency and simple low-cost fabrication. The power conversion efficiencies of perovskite solar cells now exceed 22%, which is comparable to that of commercially available CIGS and CdTe thin film solar cells. The key to further improvement is understanding the physical origin of the high efficiency of the perovskite solar cells, and a tremendous effort to come closer to this target has been made through numerous experiments. In this review article, we discuss the optoelectronic properties of perovskite CH3NH3PbX3 (X = I and Br) solar cell materials. Special attention is given to the free carrier recombination and photon recycling (the re-absorption of photons emitted by radiative recombination of photocarriers) processes in CH3NH3PbX3 single crystals, because a deep understanding of these processes is crucial for improving the solar cell performance. Lead halide perovskites show unique optical properties, e.g., extremely high quantum efficiency of luminescence, small Urbach tail in the absorption spectra, and long lifetime of photocarriers, which all suggest a low density of defects in the crystals. Because of these features, photon recycling efficiently occurs and dominates the optical processes of thick crystals.

The dynamical processes of radiative recombination of photocarriers and reabsorption of emitted photons in CH3NH3PbBr3 single crystals are studied using time-resolved two-photon-excitation photoluminescence (PL) microscopy. We find that the PL spectrum and its decay dynamics depend on the excitation-depth profile. As the excitation depth increases, the PL spectrum becomes asymmetric, the peak energy redshifts, and the PL decay time becomes longer. These observations can be well explained by a simple model including photon recycling (photon emission and reabsorption) in thick samples with strong band-to-band transitions and high radiative recombination efficiencies.

The photophysics of trions in single-walled carbon nanotubes (SWCNTs) is reviewed briefly. The trion state is observed energetically below the exciton state in hole-doped SWCNTs, and shows a simple single-exponential decay. The decay dynamics of trions depends on the photoexcitation condition. The optical responses of trions can be discussed by considering the energy relaxation of the optically accessible trion state. Detailed studies of excitons, trions, and biexcitons provide a deep understanding of the material properties of SWCNTs and are required for the design of photonic devices based on SWCNTs. (C) The Author(s) 2017. Published by ECS.

We have investigated the dynamic optical properties of CH3NH3PbI3 (MAPbI(3)) 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 MAPbI(3) 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 MAPbI(3) thin films are more accurately described by a free carrier model, rather than an excitonic model even at low temperatures.

The fast-decaying component of photoluminescence (PL) under very weak pulse photoexcitation is dominated by the rapid relaxation of the photoexcited carriers into a small number of carrier-trapping defect states. Here, we report the subnanosecond decay of the PL under excitation weaker than 1 nJ/cm(2) both in CH3NH3PbI3-based heterostructures and bare thin films. The trap-site density at the interface was evaluated on the basis of the fluence-dependent PL decay profiles. It was found that high-density defects determining the PL decay dynamics are formed near the interface between CH3NH3PbI3 and the hole-transporting Spiro-OMeTAD but not at the CH3NH3PbI3/TiO2 interface and the interior regions of CH3NH3PbI3 films. This finding can aid the fabrication of high-quality heterointerfaces, which are required improving the photoconversion efficiency of perovskite-based solar cells.

The dynamics of lowest-energy E-11 excitons in undoped and hole-doped single-walled carbon nanotubes (SWCNTs) are investigated using transient absorption spectroscopy and theoretical calculations. In undoped SWCNT samples, E-11 excitons under resonant E-11 photoexcitation show faster decay than those under high-energy nonresonant photoexcitation. Doping SWCNTs with holes accelerates the E-11 exciton decay through exciton-hole interactions; moreover, both resonant and nonresonant photoexcitation in hole-doped SWCNTs lead to nearly identical decay profiles. Theoretical calculations, which are-based on a simple Model that accounts for bright, dark, and charged exciton states, agree well with the experimental results. We ascribe the fast E-11 exciton decay under the resonant E-11 photoexcitation to the redistribution of exciton populations in the E-11 bright exciton band, which includes optically accessible and inaccessible states; the rapid exciton redistribution is caused by exciton-hole scattering. Our study provides a clear understanding of the global features of exciton dynamics in SWCNTs.

We investigated optical and transport properties of transparent conducting La-doped strontium stannate epitaxial thin films prepared by pulsed laser deposition. The prepared films are transparent with no apparent optical absorptions in the wavelength range between 390 and 1600nm. The room-temperature electrical resistivity decreases with increasing carrier concentration and reaches values as low as 1 m Omega cm. The mobility shows the maximum of 40 cm(2) (V s)(-1) at a carrier concentration of about 9 x 10(19) cm(-3). At that concentration the predominant transport mechanism of the films changes from scattering by lattice defects to scattering by ionized dopants.

Photoelectronic responses of organic-inorganic hybrid perovskite CH3NH3PbI3 on mesoporous TiO2 electrodes are investigated. On the basis of near-band-edge optical absorption and photoluminescence spectra, the bandgap energy and exciton binding energy as a function of temperature are obtained. The exciton binding energy is much smaller than thermal energy at room temperature, which means that most excitons are thermally dissociated, and optical processes are determined by the photoexcited electrons and holes. We determined the temperature dependence of exciton binding energy, which changes from similar to 30 meV at 13 K to 6 meV at 300 K. In addition, the bandgap energy and the exciton binding energy show abrupt changes at 150 K due to structural phase transition. Our fundamental optical studies provide essential information for improving the device performance of solar cells based on halide perovskite semiconductors.

We utilized time-resolved photoluminescence (PL) spectroscopy to investigate the photocarrier recombination dynamics in CH3NH3PbI3 thin films as a function of the time elapsed from the films fabrication. We found that the PL lifetime gradually increased and began to level out once the age of the film reached similar to 30 h. Even under weak excitation, the PL dynamics depended on the excitation intensity in the fresh sample, while the mature sample displayed no excitation-intensity dependence associated with the PL dynamics. We submit that this can be explained by the fact that a significant number of defects are initially formed in CH3NH3PbI3 thin films fabricated by the sequential method and are spontaneously reduced by room-temperature annealing. Our results provide important insights for reducing the nonradiative recombination centers, which improves the power conversion efficiency of perovskite solar cells.

Although inorganic crystalline phosphors can exhibit high quantum efficiency, their use in phosphor films has been limited by a reliance on organic binders that have poor durability when exposed to high-power and/or high excitation energy light sources. To address this problem, Sn2+ -doped transparent phosphate films measuring several micrometers in thickness have been successfully prepared through heat treatment and a subsequent single dip-coating process. The resulting monolithic inorganic amorphous film exhibited an internal quantum efficiency of over 60% and can potentially utilize transmitted light. Analysis of the film's emissivity revealed that its color can be tuned by changing the amount of Mn and Sn added to influence the energy transfer from Sn2+ to Mn2+. It is therefore concluded that amorphous films containing such emission centers can provide a novel and viable alternative to conventional amorphous films containing crystalline phosphors in light-emitting devices.

The carrier upconversion dynamics in InAs quantum nanostructures are studied for intermediate-band solar-cell applications via ultrafast photoluminescence and photocurrent (PC) spectroscopy based on femtosecond excitation correlation (FEC) techniques. Strong upconverted PC-FEC signals are observed under resonant excitation of quantum well islands (QWIs), which are a few monolayer-thick InAs quantum nanostructures. The PC-FEC signal typically decays within a few hundred picoseconds at room temperature, which corresponds to the carrier lifetime in QWIs. The photoexcited electron and hole lifetimes in InAs QWIs are evaluated as functions of temperature and laser fluence. Our results provide solid evidence for electron-hole-hole Auger process, dominating the carrier upconversion in InAs QWIs at room temperature. (C) 2015 AIP Publishing LLC.

Valence control of polyvalent cations is important for functionalization of various kinds of materials. Indium oxides have been used in various applications, such as indium tin oxide in transparent electrical conduction films. However, although metastable In+ (5 s(2) configuration) species exhibit photoluminescence (PL), they have attracted little attention. Valence control of In+ cations in these materials will be important for further functionalization. Here, we describe In+ species using PL and X-ray absorption fine structure (XAFS) analysis. Three absorption bands in the UV region are attributed to the In+ centre: two weak forbidden bands (S-1(0) -> P-3(1), S-1(0) -> P-3(2)) and a strong allowed band (S-1(0) -> P-1(1)). The strongest PL excitation band cannot be attributed to the conventional allowed transition to the singlet excited state. Emission decay of the order of microseconds suggests that radiative relaxation occurs from the triplet excitation state. The XAFS analysis suggests that these In+ species have shorter In-O distances with lower coordination numbers than in In2O3. These results clearly demonstrate that In+ exists in a metastable amorphous network, which is the origin of the observed luminescent properties.

The dynamic optical properties of perovskite CH(3)NH(3)Pbl(3) 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.

We have studied photocarrier recombination and relaxation dynamics in CU2ZnSnS4 (CZTS) single crystals using a combination of various optical time-resolved spectroscopy. The band tails and deep defects govern primarily the photocarrier dynamics in CZTS. We pointed out the physical origins behind low power conversion efficiencies of CZTS-based solar cells. In addition, we found that the photocarrier lifetime increases as Na is doped in CZTS; this result implies that Na doping suppresses a part of nonradiative recombination centers in CZTS. Therefore, Na dopant is important to improve the photovoltaic performance of CZTS-based solar cells.

Perovskite oxide semiconductors have attracted a great deal of attention as new device materials because they show multifunctional properties beyond those of conventional semiconductors. With its unique electrical and optical properties, SrTiO3 is a key material in oxide electronics. In this chapter, we discuss the optical properties and carrier recombination dynamics of SrTiO3 bulk crystals. Electrondoped and strongly photoexcited SrTiO3 crystals show photoluminescence (PL) consisting of three components: a 2.5-eV green PL, a 2.9-eV blue PL, and a 3.2-eV band-to-band PL. The blue PL dynamics in the presence of high-density carriers is determined by nonradiative three-carrier Auger recombination. The PL spectra and dynamics of undoped SrTiO3 under strong photoexcitation are very similar to those of electron-doped SrTiO3. From the PL decay dynamics, we evaluate the electron density in the near-surface oxygen-deficient region in SrTiO3 bulk crystals and nanoparticles. PL spectra of other d0-type perovskite semiconductors, KTaO3, BaTiO3, LiNbO3, and LiTaO3, are also discussed in comparison with SrTiO3.

We studied the near-band-edge optical responses of solution-processed CH3NH3PbI3 on mesoporous TiO2 electrodes, which is utilized in mesoscopic heterojunction solar cells. Photoluminescence (PL) and PL excitation spectra peaks appear at 1.60 and 1.64 eV, respectively. The transient absorption spectrum shows a negative peak at 1.61 eV owing to photobleaching at the band-gap energy, indicating a direct band-gap semiconductor. On the basis of the temperature-dependent PL and diffuse reflectance spectra, we clarified that the absorption tail at room temperature is explained in terms of an Urbach tail and consistently determined the band-gap energy to be similar to 1.61 eV at room temperature. (C) 2014 The Japan Society of Applied Physics

Time-resolved photoluminescence (PL) measurements have been used to study the temperature-dependent photocarrier recombination dynamics in Cu2ZnSnS4 (CZTS) single crystals. We found a significant change of nearly four orders of magnitude of the PL decay time, from microseconds at low temperatures to subnanoseconds at room temperature. The slow PL decay at low temperatures indicates localization of the photocarriers at the band tails. Due to the large band tail states, the PL decay time depends strongly on both the photon energy and excitation density. It is pointed out that the drastically enhanced nonradiative recombination at high temperatures is one of the main factors that determine the power conversion efficiency of CZTS-based solar cells. (C) 2014 AIP Publishing LLC.

We studied the quantized exciton Auger recombination in undoped and hole-doped single-walled carbon nanotubes (SWCNTs) by means of transient absorption spectroscopy and theoretical calculations. In undoped SWCNTs, a fast decay component appears under strong photoexcitation owing to two-exciton Auger recombination. The exciton decay dynamics is well explained by the quantized exciton Auger recombination model that takes into consideration the dark-exciton state. In hole-doped SWCNTs, the fast decay component is drastically reduced even under strong photoexcitation. We calculated the temporal evolution of the exciton population in hole-doped samples by considering exciton hole interactions and the hole-number distribution in SWCNTs, and found it to be in good agreement with the experimental results. (C) 2014 The Japan Society of Applied Physics

We have studied the optical response and dynamical behavior of photocarriers in BiFeO3 thin films by means of transient absorption (TA) and photocurrent (PC) measurements. PC and absorption spectroscopy indicate that BiFeO3 thin films have an indirect band gap energy of similar to 2.4 eV. The TA and PC decay dynamics have fast (similar to 1 ns) and slow (similar to 100 ns) components that are attributed to the localization of free carriers to shallow trap states and the recombination of trapped carriers, respectively. The long decay time of the PC is caused by the thermal activation of trapped carriers into the conduction band. Long-lived trapped photocarriers can be linked to the ferroelectricity and give rise to unique photoinduced phenomena in BiFeO3.

In this paper, we describe the use of band-to-band photoluminescence (PL) as a tool for evaluating the quality of Nb-doped STO (Nb: 0.1 at. %) epitaxial thin films. We found that the films with the bulk-equivalent lattice parameters show a large variation in their band-to-band PL properties. In combination with the transport property characterizations; we-ascribe the variation to the change in the carrier density owing to the carrier compensation by a small amount of point defects, which cannot be detected in structural characterizations. We also show that the band gap-Orth-e film is 10 meV smaller than that of the single crystal. Our results imply that even a small amount of defects has strong influences on the physical properties of the Nb-doped STO thin films and that the band-to-band PL is useful for elucidating these influences. (C) 2014 The Japan Society of Applied Physics

The depth profile of the electron density near the LaAlO3/SrTiO3 heterointerface has been studied by means of time-resolved photoluminescence (PL) spectroscopy. A broad blue PL band is observed at 2.9 eV, originating from the two-carrier radiative recombination of interface-induced electrons and photoexcited holes. The PL lifetime of LaAlO3/SrTiO3 heterointerface is dominated by the three-carrier Auger recombination of electrons and holes and is sensitive to electron density. We tuned the probing depth by changing the excitation photon energy and evaluated the carrier-density profile using the relation between the carrier density and the PL lifetime. Our non-contact probe method based on PL spectroscopy indicates that the carriers are confined within several nanometers in depth near the LaAlO3/SrTiO3 heterostructures. (C) 2014 AIP Publishing LLC.

Using time-resolved photoluminescence and transient absorption measurements at room temperature, we report excitation-intensity-dependent photocarrier recombination processes in thin films made from the organo-metal halide perovskite semiconductor CH3NH3PbI3 for solar-cell applications. The photocarrier dynamics are well described by a simple rate equation including single-carrier trapping and electron hole radiative recombination. This result provides clear evidence that the free-carrier model is better than the exciton model for interpreting the optical properties of CH3NH3PbI3. The observed large two-carrier recombination rate suggests the promising potential of perovskite semiconductors for optoelectronic device applications. Our findings provide the information about the dynamical behaviors of photoexcited carriers that is needed for developing high-efficiency perovskite solar cells.

We studied the dynamics of photogenerated carriers in Cu2ZnSnS4 (CZTS) single crystals using femtosecond transient reflectivity (TR) and optical pump-THz probe transient absorption (THz-TA) spectroscopy. The TR and THz-TA decay dynamics consistently showed that free carriers have long lifetimes of up to a few nanoseconds. The excitation-photon-energy-dependent TR measurements revealed a slow picosecond energy relaxation of free carriers to the band edge in CZTS. The relaxation and recombination dynamics of free carriers were affected by nonradiative recombinations at the surface. Our results revealed a global feature of energy relaxation and recombination processes of free carriers in CZTS single crystals.

Photoluminescent (PL) properties related to Te4+ species in zinc borate glasses are examined. Broad emission was observed by the excitation of the PL excitation peak of Te4+ present at the optical absorption edge. The emission intensity of Te4+ in 5TeO(2)-50ZnO-45B(2)O(3) glass was thermally quenched in a temperature region over 100 K, suggesting that concentration quenching preferentially occurred. The lifetime of the emission was approximately 2.5 mu s, which is characteristic of relaxation from the triplet excitation state of an ns(2)-type center. (C) 2013 Optical Society of America

The photocarrier relaxation dynamics of an n-type LaAlO3/ SrTiO3 heterointerface is investigated using femtosecond transient absorption (TA) spectroscopy at low temperatures. In both LaAlO 3/SrTiO3 heterostructures and electron-doped SrTiO 3 bulk crystals, the TA spectrum shows a Drude-like free carrier absorption immediately after excitation. In addition, a broad absorption band gradually appears within 40 ps, which corresponds to the energy relaxation of photoexcited free electrons into self-trapped polaron states. We reveal that the polaron formation time is enhanced considerably at the LaAlO 3/SrTiO3 heterointerface as compared to bulk crystals. Further, we discuss the interface effects on the electron relaxation dynamics in conjunction with the splitting of the t2g subbands due to the interface potential. © 2013 American Physical Society.