Takeshi Tayagaki

Carrier multiplication in carbon nanotubes studied by femtosecond pump-probe spectroscopy

Carbon nanotubes are one of the excellent materials for studying the many-body effects of excitons because of their unique band structures and large exciton binding energies. We studied exciton population dynamics in single-walled carbon nanotubes using pump-probe transient absorption measurements. The temporal profiles of the transient absorption signals depend on the excitation intensity and excitation photon energy. We observe carrier multiplication in carbon nanotubes at room temperature, when the excitation photon energy exceeds the third subband exciton energy.

Investigation of the open-circuit voltage in solar cells doped with quantum dots

Quantum dots (QDs) have attracted much attention for use in photovoltaic applications because of their potential for overcoming the limits of conventional single-junction devices. One problem associated with solar cells using QDs is that the open-circuit voltage (Voc) always decreases with the addition of QDs with respect to the reference cell without QDs. Here, we report the investigation of current–voltage characteristics in Ge/Si QD solar cells in the temperature range from 100 to 300 K. We show that even though Voc decreases with increasing temperature, it depends on the nominal Ge thickness, indicating that Voc reduction is primarily caused by a decrease in the bandgap energy of the cell. From photoluminescence decay measurements, we found that rapid carrier extraction from QDs occurred in the solar cells; this process eliminates the quasi-Fermi energy splitting between the QDs and the host semiconductor and causes Voc reduction in QD solar cells.

Enhanced carrier extraction from Ge quantum dots in Si solar cells under strong photoexcitation

We report studies of the carrier extraction mechanism in Si solar cells with Ge quantum dots (QDs), which enable the optical absorption of photons with energies below the band gap of the host. Photocurrent measurements revealed that the photocurrent in the QD solar cells increased superlinearly with increasing excitation intensity under strong photoexcitation, which differed greatly from the behavior of Si solar cells without Ge QDs. This nonlinear photocurrent generation indicates that the carrier extraction efficiency from QDs is enhanced under strong photoexcitation by nonlinear carrier extraction processes, such as two-step photon absorption and hot carrier generation via Auger recombination.We report studies of the carrier extraction mechanism in Si solar cells with Ge quantum dots (QDs), which enable the optical absorption of photons with energies below the band gap of the host. Photocurrent measurements revealed that the photocurrent in the QD solar cells increased superlinearly with increasing excitation intensity under strong photoexcitation, which differed greatly from the behavior of Si solar cells without Ge QDs. This nonlinear photocurrent generation indicates that the carrier extraction efficiency from QDs is enhanced under strong photoexcitation by nonlinear carrier extraction processes, such as two-step photon absorption and hot carrier generation via Auger recombination.

Mn―Mn couplings in Mn-doped CdS nanocrystals studied by magnetic circular dichroism spectroscopy

We report on the optical and magnetic properties of Mn-doped CdS nanocrystals coated with a ZnS shell layer (CdS:Mn/ZnS core-shell nanocrystals) by magnetic circular dichroism (MCD) spectroscopy. The magnetic field and temperature dependences of the MCD spectrum show paramagnetic behavior of the CdS:Mn/ZnS core-shell nanocrystals. The MCD intensity increases with the Mn concentration up to a few mol %, and then starts to decrease rapidly. This Mn-concentration dependence of the MCD intensity can be explained by the formation of Mn–Mn pairs in heavily doped nanocrystals.

Simultaneous enhanced photon capture and carrier generation in Si solar cells using Ge quantum dot photonic nanocrystals

We propose a novel solar cell structure with photonic nanocrystals coupled to quantum dots (QDs) for advanced management of photons and carriers. The photonic nanocrystals at the surface create an extra interaction between the photons and the QDs, which promotes light trapping. Photo-generated carriers can be efficiently transported by preparing vertically aligned QDs with electronic coupling. Implementation of the proposed structure was realized in crystalline Si solar cells with Ge QDs by development of a simple and practical formation method based on a wet chemical process without any lithography techniques. The wet process utilizes a periodically modulated etching rate induced by self-organized Ge QDs. The effectiveness of the proposed solar cell was demonstrated by the marked increase of the absolute conversion efficiency when compared with the control crystalline Si solar cells. It is found that light trapping by the photonic nanocrystals has a larger contribution to the efficiency improvement than the contributions from the carrier transport of the vertically aligned QDs.

Photocarrier recombination dynamics in highly excited SrTiO3 studied by transient absorption and photoluminescence spectroscopy

We studied photocarrier recombination processes in highly excited SrTiO3 crystals using pump-probe transient absorption (TA) and photoluminescence (PL) spectroscopy at room temperature. TA signals of nondoped SrTiO3 crystals clearly appear in the visible and infrared spectral region under intense interband photoexcitation, and TA spectra show Drude-like photon-energy dependence. Both TA and PL decay curves are well explained by the same simple rate equation including three-body Auger recombination and single-carrier trapping.

Energy transfer from semiconductor nanocrystal monolayers to metal surfaces revealed by time-resolved photoluminescence spectroscopy

We studied photoluminescence (PL) and energy transfer dynamics in CdSe nanocrystal (NC) assembled monolayers on Au surfaces. The close-packed CdSe NC monolayers were fabricated using the Langmuir–Blodgett method on spin-coated inert polymer films on rough-surface Au substrates. The PL intensity and PL decay time were dependent on the distance between the CdSe NCs and Au surfaces. These observations indicate that the metal-semiconductor distance and the luminescence-energy dependences of the energy transfer rate can be understood in terms of the Coulomb interaction between excitons in NCs and plasmons on the Au surfaces.

Recombination Dynamics of High-Density Photocarriers in Type-II Ge/Si Quantum Dots

The optical properties and dynamics of charge carriers under high-density photoexcitation in type-II Ge/Si quantum dots (QDs) are studied by time-resolved photoluminescence (PL) measurements at low...

Dynamics of Quantized Auger Recombination in CdSe Nanocrystals Studied by Femtosecond Intraband Pump–Probe Spectroscopy

The dynamics of quantized Auger recombination in CdSe nanocrystals was studied by femtosecond intraband pump–probe spectroscopy at room temperature. The intraband transient absorption signals increase linearly with excitation laser intensity, while the interband transient absorption signals saturate at high excitation intensities. We clarified that the excitation intensity dependence of the Auger recombination rate in nanocrystals is quantitatively explained by three-body photocarrier collision processes.

Infrared study of spin crossover Fe-picolylamine complex

Infrared (IR) absorption spectroscopy has been used to probe the evolution of microscopic vibrational states upon the temperature- and photo-induced spin crossovers in [Fe(2-picolylamine) 3 ]Cl 2 EtOH (Fe–pic). To overcome the small sizes and the strong IR absorption of the single crystal samples used, an IR synchrotron radiation source and an IR microscope have been used. The obtained IR spectra of Fe–pic show large changes between high-spin and low-spin states, for both the temperature- and the photo-induced spin crossovers. In contrast, the spectra in the temperature- and photo-induced high-spin states are relatively similar to each other, however they show distinct differences below 750 cm -1 . This demonstrates that the photo-induced high-spin state involves microscopically different characters from those of the temperature-induced high-spin state. The results are discussed in terms of local pressure and structural deformations within the picolylamine ligands, and in terms of their possible relevance...