Hisashi Miyazaki

Cu(InGa)Se2 thin film absorber with high Ga contents and its application to the solar cells

Abstract Cu(InGa)Se2 (CIGS) thin film absorbers with a high Ga content were investigated for solar cell applications. The CIGS and CuGaSe2 (CGS) films were deposited by the three-stage growth process using a molecular beam epitaxy apparatus incorporating an in situ surface temperature monitoring system. Measurements of the growing surface temperature during the second stage showed the chemical reaction between Cu and Ga to be slower than that between Cu and In. Raman spectroscopy of as-grown samples with a high Ga content, showed the presence of Cu2−xSe compounds, which are thought to be responsible for the degradation of the open circuit voltage of solar cells. The use of rapid thermal annealing (RTA) was studied to remove the metallic Cu2−xSe compounds from the surface layer with typical annealing conditions of 400 °C for 1 s. After the RTA process, the photovoltaic performance was found to be greatly improved, yielding a high conversion efficiency of 11.2% in CIGS solar cells with a high Ga content of 60%.

Efficiency Improvement of Cu(InGa)Se2 Thin Film Solar Cells with a High Ga Composition Using Rapid Thermal Annealing

Cu(InGa)Se2 (CIGS) thin films were grown by three-stage growth process using a molecular beam deposition system on a Mo/soda lime glass (SLG) substrate. The raman scattering spectroscopy of as-grown samples with a high Ga composition showed the presence of Cu2-xSe compounds, which are thought to be responsible for the degradation of the open circuit voltage of solar cells. Rapid thermal annealing (RTA) using an infrared lamp in forming gas (N2 95% + H2 5%) ambient under atmospheric pressure was examined in order to remove the Cu2-xSe compounds, and it was found that hydrogen is necessary to remove the Cu2-xSe compounds. The influence of RTA conditions on device performance was investigated. A Cu(In0.4Ga0.6)Se2 (energy band gap: Eg=1.37 eV) solar cell with an efficiency of 11.7%, particularly open circuit voltage (Voc) increased from 640 to 750 mV, was obtained by RTA at 400°C.

Chemical-Bath-Deposited ZnO and Mg(OH)2 Buffer Layer for Cu(InGa)Se2 Solar Cells

ZnO and Mg(OH)2 thin films were grown by chemical bath deposition (CBD) and applied to the fabrication of Cu(InGa)(SSe)2 (CIGSSe) solar cells. From the X-ray diffraction (XRD) measurements, the CBD-ZnO film was preferentially oriented toward the (002) plane and the CBD-Mg(OH)2 film was oriented toward the (001) plane. The optical and electrical properties of CBD-ZnO and CBD-Mg(OH)2 thin films on a fused silica substrate were investigated and it was found that the films showed a high resistivity and a high transmittance in the visible light wavelength regions. A cell efficiency of 14.3% (Voc: 557 mV, Jsc: 35.5 mA/cm2, F.F.: 0.721) was achieved using a CBD-ZnO buffer layer.

Studies of Thermal Anisotropy in 4H-, 6H-SiC Bulk Single Crystal Wafers by Photopyroelectric (PPE) Method

Thermal anisotropy in 4H-, and 6H-SiC bulk single crystal wafers was studied by the PPE method. The thermal diffusivities of the [1-100] and [11-20] orientations (^c-axis) samples were higher than those of the [0001] orientation (//c-axis) samples. Moreover, the thermal anisotropies of the lattice component and the carrier component were analyzed by Raman measurement.

Photoacoustic Spectra and Thermoelectric Properties of Amorphous Si/Au/Ge/Au Superlattice

The amorphous Si–Ge–Au superlattice shows highly superior thermoelectric properties. In this paper, we describe the photoacoustic spectra and thermoelectric properties of these materials with various Au concentrations and discuss the mechanism responsible for high thermoelectric properties. Amorphous Si/Au/Ge/Au superlattice was prepared by alternate deposition of Si, Ge and Au. The Au concentration dependence of electric and optical properties was measured. We investigated the band structure of amorphous Si–Ge–Au over a wide energy range by combining the electrical and optical measurements. The energy range was from 0.01 (the impurity level of the Au dopant) to 2 eV (band edge). We found that the Au atoms in amorphous Si–Ge not only produce strong potential fluctuations but also act as acceptor impurities.

Novel In(OH)3:Zn2+ buffer layer for Cu(InGa)Se2 based solar cells

A novel In(OH)3:Zn2+ buffer layer was proposed as an alternative for fabricating high-efficiency Cu(InGa)Se2 (CIGS) solar cells. The buffer layers were deposited by chemical bath deposition using ZnCl2, InCl34H2O and thiourea. The structural, electrical and optical properties of In(OH)3:Zn2+ thin films on quartz substrates were determined. Films with resistivity as high as 108 Ωcm and a transmittance of more than 80% have been obtained by regulating the growth conditions. CIGS thin-film solar cells with a ZnO/In(OH)3:Zn2+/CIGS/Mo structure were fabricated. The CIGS absorbers used in this study were prepared using two different deposition processes: three-stage co-evaporation and selenization/sulfurization. The influence of growth conditions of the buffer layer on device performance and device metastability was investigated. The presence of Zn2+ ions in the deposition bath was found to be an important parameter for achieving both device efficiency and metastability. A cell efficiency of 14% without the light-soaking effect was achieved using the In(OH)3:Zn2+ buffer layer.

Defect Distribution in N-Doped and Semi-Insulating 6H-SiC Bulk Single Crystal Wafers Observed by Two- and Three-Dimensional Light Scattering Tomography

We applied light scattering tomography (LST), which is powerful for rapid and nondestructive observation of structural defects in semiconductor single crystals, to investigate the defect distribution of 6H-SiC single crystal wafers for the first time. In conventional LST observation, a difference in tomograms between N-doped and semi-insulating 6H-SiC wafers was found. It is caused by the difference in optical absorption and scattered light in wafers. We successfully constructed three-dimensional (3D) LST images of the defects by rendering layer-by-layer two-dimensional (2D) LST images on different planes perpendicular to the direction of crystal growth. The 3D-LST images showed clearly the various defect distributions in depth direction.

Characterization of Defects in Semi-Insulating 6H-SiC Substrates Using IR Thermal Imaging Camera

We have detected defects micro-pipes and a cluster of impurities in semi-insulating 6H-SiC substrates using long-wavelength infrared thermal imaging camera (IR-camera) with 8 ~ 14 µm in non-destructive and non-contact. Also we have evaluated the thermal influence of defects on the entire substrates from the observation results of scanning laser microscope (SLM) and light scattering tomography (LST). Through the process, it was certificated that the defects in the substrates could be detected with relatively macroscopic scale (8  6 mm2). Moreover, through a temperature profile processing by a 0.1 K thermal resolution, we estimated thermal behavior of the defect areas in the 6H-SiC precisely. The IR-camera is considered as effective technique for evaluating the defects in the intermediate range between micro and macro scale.

Nondestructive Testing Technique for Defect Detection in LiNbO3 Using Infrared Thermal Imaging Camera

The existence of defects in lithium niobate (LiNbO3) degrades the specific properties required of LiNbO3 as substrate material, which is a major problem inhibiting device constructions. For the purpose of providing a convenient and quick tool for use in a nondestructive testing technique for defect detection, the feasibility of using an infrared thermal imaging camera (IR camera) is examined by comparing the results of IR camera use with those of two methods, scanning laser microscopy (SLM) and light scattering tomography (LST). We have succeeded in the detection of defects in the LiNbO3 substrate by observing the thermal properties of defects in a relatively wide field of view (8×6 mm2) compared with the case of using conventional microscopes. The maximum temperature difference of the defects in the LiNbO3 substrate was 0.47 K.

Proposal of novel measurement method for thermal diffusivity from infrared thermal movie

A brand new thermal diffusivity measurement method was developed. In this new noncontact and absolute measurement method, thermal diffusivity was measured from infrared movie data. The model of one-dimensional thermal conduction was constructed by taking into account the thermal flow other than one-dimensional thermal conduction in the sample. On the basis of this thermal conduction model, the analytical equation for calculating thermal diffusivity was derived. A single-crystal sapphire plate was used as a test specimen for the new method. The test specimen was arranged to cause one-dimensional heat conduction. Infrared movies were taken by using an infrared camera at room temperature. Then, thermal diffusivity was numerically calculated from the acquired movie data using the analytical equation. It was experimentally demonstrated that thermal diffusivity was measured with an accuracy of around 10% error, from an infrared movie of a single-crystal sapphire sample.