Tokuhisa Kawawaki

Efficiency Enhancement of PbS Quantum Dot/ZnO Nanowire Bulk-Heterojunction Solar Cells by Plasmonic Silver Nanocubes

For improvement of solar cell performance, it is important to make efficient use of near-infrared light, which accounts for ∼40% of sunlight energy. Here we introduce plasmonic Ag nanocubes (NCs) to colloidal PbS quantum dot/ZnO nanowire (PbS QD/ZnO NW) bulk-heterojunction solar cells, which are characterized by high photocurrents, for further improvement in the photocurrent and power conversion efficiency (PCE) in the visible and near-infrared regions. The Ag NCs exhibit strong far field scattering and intense optical near field in the wavelength region where light absorption of PbS QDs is relatively weak. Photocurrents of the solar cells are enhanced by the Ag NCs particularly in the range 700-1200 nm because of plasmonic enhancement of light absorption and possible facilitation of exciton dissociation. As a result of the optimization of the position and amount of Ag NCs, the PCE of PbS QD/ZnO NW bulk-heterojunction solar cells is improved from 4.45% to 6.03% by 1.36 times.

Enhancement of dye-sensitized photocurrents by gold nanoparticles: effects of dye–particle spacing

Photocurrents of a ruthenium dye-TiO(2) system are enhanced by gold nanoparticles (100 or 40 nm diameter) embedded in TiO(2). As dye-particle spacing decreases to 10 nm, enhancement factor and intensity of localized electric fields at the TiO(2) surface increase. A further decrease in the spacing suppresses the enhancement.

Dynamics of Charged Excitons and Biexcitons in CsPbBr3 Perovskite Nanocrystals Revealed by Femtosecond Transient-Absorption and Single-Dot Luminescence Spectroscopy

Metal-halide perovskite nanocrystals (NCs) are promising photonic materials for use in solar cells, light-emitting diodes, and lasers. The optoelectronic properties of these devices are determined by the excitons and exciton complexes confined in their NCs. In this study, we determined the relaxation dynamics of charged excitons and biexcitons in CsPbBr3 NCs using femtosecond transient-absorption (TA), time-resolved photoluminescence (PL), and single-dot second-order photon correlation spectroscopy. Decay times of ∼40 and ∼200 ps were obtained from the TA and PL decay curves for biexcitons and charged excitons, respectively, in NCs with an average edge length of 7.7 nm. The existence of charged excitons even under weak photoexcitation was confirmed by the second-order photon correlation measurements. We found that charged excitons play a dominant role in luminescence processes of CsPbBr3 NCs. Combining different spectroscopic techniques enabled us to clarify the dynamical behaviors of excitons, charged excitons, and biexcitons.

Potential-Scanning Localized Plasmon Sensing with Single and Coupled Gold Nanorods

Single plasmonic nanoparticles can potentially serve as optical sensors for detecting local refractive index changes. However, simultaneous monitoring of the scattering spectra from multiple nanoparticles is not practical. Here we perform potential-scanning localized surface plasmon resonance (LSPR) sensing. Gold nanorods are immobilized on an ITO electrode. Instead of collecting the full spectrum, as is done in conventional LSPR sensing, the electrode potential is scanned while the rod spectra are monitored at a single wavelength. We demonstrate that refractive index changes can be determined from single wavelength experiments and we further find that gold nanorod (NR) dimers exhibit higher refractive index sensitivities than single NRs in both potential-scanning and conventional wavelength-scanning based LSPR sensing.

Backward-scattering-based Localized Surface Plasmon Resonance Sensors with Gold Nanospheres and Nanoshells

Localized surface plasmon resonance (LSPR) sensors based on plasmonic nanoparticles are sensitive to changes in local refractive index, so that they are used for affinity-based chemical sensing and biosensing. Conventional LSPR sensors are generally based on transmission of light through the sensor and a sample solution, which could be colored or turbid. In this study, we develop backward-scattering-based LSPR sensors that can be applied to colored or turbid sample solutions. Au nanospheres (100 nm diameter) and Au nanoshells (25 nm thick) with SiO2 cores (80 nm diameter) are used as plasmonic nanoparticles and immobilized on a glass substrate. The refractive index sensitivities of the Au nanospheres and nanoshells are 128 and 278 nm RIU(-1), respectively, which are in good agreement with simulated values and the values for conventional transmission-based LSPR sensors. The Au nanoshells require a lower amount of Au for the same scattering intensity in comparison with the Au nanospheres. The backward-scattering-based LSPR sensing is possible with the Au nanospheres and nanoshells even in coffee as a colored and turbid sample.

Impact of Postsynthetic Surface Modification on Photoluminescence Intermittency in Formamidinium Lead Bromide Perovskite Nanocrystals

We study the origin of photoluminescence (PL) intermittency in formamidinium lead bromide (FAPbBr3, FA = HC(NH2)2) nanocrystals and the impact of postsynthetic surface treatments on the PL intermittency. Single-dot spectroscopy revealed the existence of different individual nanocrystals exhibiting either a blinking (binary on-off switching) or flickering (gradual undulation) behavior of the PL intermittency. Although the PL lifetimes of blinking nanocrystals clearly correlate with the individual absorption cross sections, those of flickering nanocrystals show no correlation with the absorption cross sections. This indicates that flickering has an extrinsic origin, which is in contrast to blinking. We demonstrate that the postsynthetic surface treatment with sodium thiocyanate improves the PL quantum yields and completely suppresses the flickering, while it has no significant effect on the blinking behavior. We conclude that the blinking is caused by Auger recombination of charged excitons, and the flickering is due to a temporal drift of the exciton recombination rate induced by surface-trapped electrons.

Hot Biexciton Effect on Optical Gain in CsPbI3 Perovskite Nanocrystals

Combining the superior optical properties of their bulk counterparts with quantum confinement effects, lead halide perovskite nanocrystals are unique laser materials with low-threshold optical gain. In such nonlinear optical regimes, multiple excitons are generated in the nanocrystals and strongly affect the optical gain through many-body interactions. Here, we investigate the exciton-exciton interactions in CsPbI3 nanocrystals by femtosecond transient absorption spectroscopy. From the analysis of the induced absorption signal observed immediately after the pump excitation, we estimated the binding energy for the hot biexcitons that are composed of an exciton at the band edge and a hot exciton generated by the pump pulse. We found that the exciton-exciton interaction becomes stronger for hot excitons with greater excess energies and that the optical gain can be controlled by changing the excess energy of the hot excitons.

Multiexciton recombination dynamics in CsPbBr 3 perovskite nanocrystals revealed by femtosecond transient absorption and single dot spectroscopies

Recently, metal-halide perovskite semiconductors have been extensively studied because of their potential use in solar-cell, photodetector, light-emitting-diode, and laser applications. In particular, perovskite nanocrystals (NCs) are gaining more and more attention because their optical spectra can be manipulated by changing the NC size and the halogen composition [1]. In contrast to the conventionally structured NCs such as colloidal CdSe NCs, high-quality perovskite NCs with unique cubic shapes and large sizes can be fabricated. However, their intrinsic photocarrier recombination dynamics have not yet been understood. Especially, the recombination dynamics of exciton complexes including biexcitons and charged excitons (trions) are complicated due to the fast Auger recombination [2]. Thus, a combination of different optical techniques for ensemble and single NC samples should be performed for a deep understanding of multiexciton dynamics in perovskites.