Hideaki Ohyama

16.0% Efficient Thin-Film CdS/CdTe Solar Cells

High efficiency cadmium sulfide (CdS)/cadmium telluride (CdTe) solar cells have been developed using ultrathin CdS films having a thickness of 50 nm. CdS films deposited on indium tin oxide (ITO)/#1737 glass substrates by the metal organic chemical vapor deposition technique, and CdTe films subsequently deposited by the close-spaced sublimation technique were used for the fabrication of CdS/CdTe solar cells. A photovoltaic conversion efficiency of 16.0% under Air Mass (AM) 1.5 conditions has been measured by the Japan Quality Assurance Organization.

Characterization of CdS Thin-Film in High Efficient CdS/CdTe Solar Cells

Cadmium sulfide (CdS) thin films are the most commonly used window materials for high efficient cadmium telluride (CdTe) and chalcopyrite polycrystalline thin-film photovoltaic devices. High efficient CdS/CdTe solar cells with thin CdS films have been developed using ultrathin CdS films with a thickness of less than 0.1 µm. CdS films were deposited on transparent conductive oxide (TCO)/glass substrates by the metal organic chemical vapor deposition (MOCVD) technique. CdTe films were subsequently deposited by the close-spaced sublimation (CSS) technique. The screen printing and sintering method fabricated carbon and silver electrodes. Cell performance depends primarily on the electrical and optical properties of CdS films. Therefore we started to develop higher-quality CdS films and found clear differences between high- and low-quality CdS films from the analyses of scanning electron microscope (SEM), atomic force microscope (AFM), secondary ion mass spectroscopy (SIMS), thermal desorption spectrometry (TDS) and Fourier transforms-infrared spectrometry (FT-IR) measurements. As a result of controlling the quality of CdS films, a photovoltaic conversion efficiency of 10.5% has been achieved for size of 1376 cm2 of the solar cells under the Air Mass (AM) 1.5 conditions of the Japan Quality Assurance Organization.

Observation of ultrafast all‐optical modulation based on intersubband transition in n‐doped quantum wells by using free electron laser

A picosecond‐range all‐optical modulation based on the nonlinear interaction between interband‐ and intersubband‐resonant light in GaAs/AlGaAs multiple quantum wells was demonstrated. A free electron laser (FEL) with a very short pulse width (<10 ps) was used for the intersubband excitation. The dependence of the modulation depth on the wavelength of the intersubband‐resonant light was investigated by utilizing the wide tunability of the FEL. The result is in good agreement with the intersubband absorption spectrum of the multiple quantum wells measured by FT‐IR spectroscopy, which indicates that the observed modulation is indeed based on the intersubband transition induced by the FEL.

Evaluation of the CdS/CdTe interface using free-electron laser internal photoemission technique

The CdS/CdTe interface was investigated by the free-electron laser (FEL) internal photoemission technique. This technique is based on photocurrent spectroscopy utilizing the tunability and intense peak power of the FEL operative in the infrared range. We found two thresholds in the photocurrent spectrum, which can be identified as steplike band discontinuities. It is demonstrated that there is a mixed crystal layer of CdS1−xTex at the CdS/CdTe interface.

DETERMINING THE BAND DISCONTINUITIES OF ZNSE/GAAS AND ZNMGSSE/GAAS HETEROJUNCTIONS USING FREE ELECTRON LASER

The conduction band discontinuities of ZnSe/GaAs and ZnMgSSe/GaAs heterojunctions were investigated using the free electron laser (FEL) internal photoemission technique. This technique is based on the photocurrent spectroscopy utilizing the tunability and intense peak power of the FEL operative in the infrared range. We found the conduction band discontinuities of 113 (ZnSe/GaAs) and 180 meV (ZnMgSSe/GaAs). It is suggested that the band gap discontinuity between ZnSe and ZnMgSSe is δEc (conduction band): δEv (valence band)=0.45:0.55 at 77 K.

ULTRAFAST INTERBAND-LIGHT MODULATION BY INTERSUBBAND LIGHT (FREE-ELECTRON LASER) IN QUANTUM WELLS

Abstract Ultrafast interband-resonant light modulation induced by an intersubband transition in GaAs/AlGaAs multiple quantum wells is observed by using a free-electron laser (FEL) for intersubband excitation. The FEL operates with a picosecond pulse width and a wide wavelength tunability. The dependence of the modulation depth on the wavelength of the intersubband-resonant light shows good agreement with the intersubband absorption spectrum of the multiple quantum wells measured by Fourier transform infrared (FTIR) spectroscopy. We also investigated the relaxation time of the intersubband transition by a pump–probe experiment and found that it was less than the FEL micropulse width (∼3 ps).

Free electron laser annealing of N-ion-implanted 3C-SiC films

Infrared laser annealing at a variety of wavelengths (10.0–13.0 μm) was performed under room temperature for cubic silicon carbide (3C-SiC) films after N-ion implantation by using a free electron laser with features of wide-range tunability, ultrashort pulse operation (∼10 ps), and intense peak power (∼MW). Infrared absorption spectroscopy indicated that the annealing at 12.6 μm, which corresponds to the absorption peak of Si–C stretch mode, was effective for removing the crystalline damage induced by the ion implantation. On the other hand, Hall effect measurements showed an increase of carrier density for samples annealed at around 10.4 μm, whereas the absorption was weak at this wavelength. This important result is most likely attributed to the activation of N donors caused by the direct excitation of a local vibration mode associated with N atoms in the SiC matrix.

Measurement of Band Discontinuity at ZnSe/GaAs Boundary Using Free Electron Laser

We investigated the band discontinuity at the ZnSe/GaAs boundary using the free electron laser internal photoemission (FEL-IPE) technique. This technique is based on photocurrent spectroscopy utilizing the tunability and intense peak power of the FEL operative in the infrared range. We have found the threshold of the photocurrent near the photon energy of 110 meV which can be identified as the band discontinuity in the ZnSe/GaAs heterojunction.

Measurement of Semiconductor Heterojunction Band Discontinuities Using Free Electron Laser.

Measurements of the band discontinuity of semiconductor heterojunctions was carried out using the free electron laser internal photoemission (FEL-IPE) technique. This technique is based on photocurrent spectroscopy utilizing the tunability and intense peak power of the FEL operative in the infrared range. We found that there is a photocurrent threshold in the IPE spectrum of the ZnSe/GaAs and GaAlAs/GaAs heterojunctions which can be identified as the band discontinuity.

Free Electron Laser Annealing of Amorphous Silicon Carbide

We have applied a free electron laser (FEL) to crystallize amorphous silicon carbide (a-SiC) at room temperature. The FEL has two main characteristics, wavelength tunability and ultrashort-pulse operation (~10 ps) with intense peak power ( ~MW). The wavelength was selected to be 12.6 µ m which corresponds to the energy of the Si–C stretch mode in order to excite the lattice vibration directly. FT-IR measurements of the a-SiC film showed that the broad peak in the absorption spectrum became sharp after irradiation with the FEL. X-ray diffraction measurements of the irradiated sample revealed a peak related to SiC crystals. These measurements suggest that crystallization of a-SiC occurs at room temperature when irradiation with a FEL.