Tamotsu Okamoto

Improved performance of Cu(InGa)Se2 thin-film solar cells using evaporated Cd-free buffer layers

Polycrystalline Cu(InGa)Se2 (CIGS) thin-film solar cells using evaporated InxSey and ZnInxSey buffer layers are prepared. The purpose of this work is to replace the chemical bath deposited CdS buffer layer with a continuously evaporated buffer layer. In this study, a major effort is made to improve the performance of CIGS thin-film solar cells with these buffer layers. The relationship between the cell performance and the substrate temperature for these buffer layers is demonstrated. Even at the high substrate temperature of about 550°C for the buffer layer, efficiencies of more than 11% were obtained. Furthermore, the I−V characteristics of the cells using these buffer layers are compared with cells using CdS buffer layers fabricated by chemical bath deposition method. We have achieved relatively high efficiencies of over 15% using both the ZnInxSey and the CdS buffer layers.

Characterization of Highly Efficient CdTe Thin Film Solar Cells by Low-Temperature Photoluminescence

Highly efficient CdTe thin film solar cells prepared by close-spaced sublimation (CSS) method with a glass/ITO/CdS/CdTe/Cu-doped carbon/Ag structure were characterized by low-temperature photoluminescence (PL) measurement. A broad 1.42 eV band probably due to VCd–Cl defect complexes appeared as a result of CdCl2 treatment. CdS/CdTe junction PL revealed that a CdSxTe1-x mixed crystal layer was formed at the CdS/CdTe interface region during the deposition of CdTe by CSS and that CdCl2 treatment promoted the formation of the mixed crystal layer. Furthermore, in the PL spectra of the heat-treated CdTe after screen printing of the Cu-doped carbon electrode, a neutral-acceptor bound exciton (ACu0, X) line at 1.590 eV was observed, suggesting that Cu atoms were incorporated into CdTe as effective acceptors after the heat treatment.

Prospects of Thickness Reduction of the CdTe Layer in Highly Efficient CdTe Solar Cells Towards 1 µm

This study focuses on the technique for the stable growth of CdTe (1.44 eV) with thickness near its absorption length, 1 µm, by close spaced sublimation (hereafter CSS) process, in order to achieve high conversion efficiency. X-ray diffraction (XRD) spectroscopy was carried out to examine the microstructure of the films. Current-voltage (I–V) characteristics, spectral response and other features of the solar cells using these CdTe films were investigated to elucidate the optimum conditions for achieving the best performance in such thin (1 µm) CdTe solar cells. Thickness was found to be reduced by controlling the temperature profile used during CSS growth. The temperature profile was found to be an important factor in growing high-quality thin films. By controlling the growth parameters and optimizing the annealing temperature at different fabrication steps, we have succeeded, to date, in achieving cell efficiencies of 14.3% (open-circuit voltage (Voc): 0.82 V, short-circuit current (Jsc): 25.2 mA/cm2, fill factor (F.F.): 0.695, area: 1 cm2) with 5 µm, 11.4% (Voc: 0.77 V, Jsc: 23.7 mA/cm2, F.F.: 0.63, area: 1 cm2) with 1.5 µm and 11.2% (Voc: 0.77 V, Jsc: 23.1 mA/cm2, F.F.: 0.63, area: 1 cm2) with only 1 µm of CdTe layer thickness at an air mass of 1.5 without antireflection coatings. This is important for establishing a strong foundation before developing a new structure (e.g., glass/ITO/CdS/CdTe/ZnTe/Ag configuration) with a back surface field of wide-bandgap material (e.g., ZnTe).

Improved performance of CdTe thin film solar cells through controlling the initial stage of the CdTe layer deposition by close-spaced sublimation

Abstract Control of the initial stage of the CdTe deposition was investigated by modifying the substrate and the source temperature profiles in close-spaced sublimation (CSS) process for improving the performance of CdTe thin film solar cells. In the modified temperature profile, the substrate temperature was increased with higher rate than that of the source temperature. By using the modified temperature profile, the conversion efficiency was improved as compared with the conventional temperature profile, and 15.3% efficiency (Voc: 0.811xa0V, Jsc: 26.3xa0mA/cm2, FF: 0.718, 1xa0cm2, AM 1.5) was achieved.

Growth and characterization of In2Se3 epitaxial films by molecular beam epitaxy

Structural control of In2Se3 films was attempted by molecular beam epitaxy (MBE). In2Se3 epitaxial films with layered structure based on zincblende structure were successfully grown on (0 0 1)GaAs substrates at low temperatures (Tg ⩽ 520°C at VIIII ratio of 10) and at low VIIII ratios (VIIII ⩽ 30 at 500°C), and polycrystalline In2Se3 films with defect wurtzite structure were grown at high temperatures (Tg ⩾ 540°C at VIIII ratio of 10) and high VIIII ratios (VIIII ⩾ 90 at 500°C). In the epitaxial In2Se3 films with layered structure, the direction of c-axis corresponded to [1 1 1] and [1 1 1] directions in the (0 0 1)GaAs substrates. Furthermore, exciton emission located at around 579 nm was observed in the photoluminescence spectra of the In2Se3 films at 4.2 K.

Development of Cu(InGa)Se{sub 2} thin film solar cells with Cd-free buffer layers

Cu(InGa)Se{sub 2}(CIGS) thin film absorbers were fabricated by a three-stage method using a coevaporation apparatus. As a Cd-free buffer layer, ZnSe, In{sub x}Se{sub y} and ZnIn{sub x}Se{sub y} and ZnIn{sub x}Se{sub y} buffer layers have been deposited on the CIGS absorber continuously in the same apparatus. Atomic layer deposition (ALD) was employed as a growth technique for ZnSe. This technique offers a good thickness control as well as a good surface coverage. By irradiating with a solar simulator, all the solar cell parameters increased drastically for the first 50 minutes of the irradiation and then saturated at longer irradiation times. This phenomenon did not appear for the cells with a CdS buffer layer. The best efficiency of ZnO/ZnSe/CIGS thin film solar cells with about 10 nm thick ZnSe buffer layer was 11.6%. On the other hand, ZnO/In{sub x}Se{sub y}/CIGS thin film solar cells showed very stable characteristics under the light illumination, and initial measurements show an efficiency of 13.0%.

Doping and intermixing in CdS/CdTe solar cells fabricated under different conditions

Thin film CdS/CdTe solar cell structures have been investigated by spatially resolved cathodoluminescence (CL) spectroscopy. A postgrowth CdCl2 treatment of the CdTe layer was found to homogenize the distribution of acceptor-like defects or impurities leading to optimized p-type conversion of the polycrystalline CdTe. For values of the growth temperature (TG) of about 600u200a°C, the intermixed region between the CdS layer and CdTe grains is surprisingly thin. However, for TG as large as 630–650u200a°C, a gradual decrease of the CdTe band gap due to sulfur intermixing appears to be present up to 0.6 μm from the CdS/CdTe interface. The CL spectra of the CdS window layer exhibit two broad bands centered at 1.72 (red) and 2.04 eV (yellow). The yellow one is quenched by the CdCl2 treatment, indicating passivation or promoted outdiffusion of Cd interstitials.

Raman Study of Epitaxial Ga2Se3 Films Grown by Molecular Beam Epitaxy

Epitaxial Ga2Se3 films grown by molecular-beam epitaxy were characterized by Raman spectroscopy. Raman spectra of the obtained Ga2Se3 films showed two broad peaks at 250 cm-1 and 300 cm-1, and one sharp peak at 155 cm-1. The intensity of the sharp peak strongly depended on the VI/III ratio (Se beam flux/Ga beam flux), and maximum intensity was obtained for the film grown with a VI/III ratio of 150. The experimental spectra were analyzed in terms of an axially symmetric lattice-dynamical model for the first time. It is determined from both calculation and experiment that the sharp peak at 155 cm-1 shows A1 symmetry, which originates in the vacancy-ordering of Ga2Se3.

Anomalous anisotropy in the absorption coefficient of vacancy-ordered Ga 2 Se 3

The structural and optical properties of Ga2Se3 on (100)GaP and (100)GaAs prepared by molecular beam epitaxy have been investigated. The electron diffraction studies revealed that the superstructure was formed in [011] direction by the spontaneous ordering of native gallium-vacancies in the defect zinc blende structure under a selenium-rich growth condition, and very large absorption anisotropy (Δα>104cm1) was observed in the vacancy-ordered Ga2Se3. Furthermore, polarization dependence of photoconductivity due to the absorption anisotropy was observed in the vacancy-ordered Ga2Se3 on (100)GaAs.

Molecular Beam Epitaxy and Characterization of Layered In2Se3 Films Grown on Slightly Misoriented (001)GaAs Substrates.

In2Se3 epitaxial films with a layered structure were successfully grown on (001)GaAs substrates by molecular beam epitaxy. X-ray diffraction and electron diffraction studies revealed that the crystallinity of the films was improved by using slightly misoriented (001)GaAs substrates. Furthermore, electrical and optical anisotropies were observed in layered In2Se3 films. It was found that the conductivity of layered In2Se3 epitaxial films along the a-axis was much larger than that along the c-axis, and that the absorption coefficient of layered In2Se3 epitaxial films for light polarized parallel to the a-axis was much higher than that for light polarized parallel to the c-axis.