Mikio Murozono

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.

Fabrication of Spherical Silicon Solar Cells with Semi-Light-Concentration System

A spherical Si solar cell with a semi-light-concentration system was successfully fabricated using a spherical Si crystal produced by a dropping method. The dropping method has great potential for low-waste and low-cost fabrication because the Si spheres can be made directly from dropping melted Si without the cutting and polishing processes of Si ingots. The fabricated Si spheres were generally multicrystalline due to the crystal growth through homogeneous crystal nucleation in containerless states. Spherical Si solar cells were fabricated using a Si sphere with a diameter of 1 mm as a solar cell substrate and then mounted on a reflector cup with a hexagonal aperture to complete the semi-light-concentration system. The current–voltage (J–V) measurement of the cell demonstrated an energy conversion efficiency of 10.4%. The key parameters for achieving higher efficiency are discussed with J–V data analysis, and quantum efficiency and laser-beam-induced current measurements.

15.1% Highly efficient thin film CdSCdTe solar cell

High-efficiency CdSCdTe solar cells with thin CdS film have recently been developed. Semiconductive layers of CdS via the CVD method and of CdTe via the CSS method were deposited on an ITO/#7059 substrate. Cell performance depends primarily on the thickness of CdS film, and the conversion efficiency is highest for a CdS film thickness of around 60 nm. Since the CdS film thickness decreases by about 30% during deposition of the CdTe layer, a thickness of 95 nm is required to obtain a 60 nm-thick CdS film after deposition of a CdTe layer. By observing the CdS film during the CdTe deposition process, a decrease was detected before CdTe layer completely covers the surface of the CdS film. By optimizing the thickness of CdS film, an efficiency of 15.12% for the best cell under AM 1.5 verified at JQA was obtained. This fabrication process has good reproducibility; 92.5% of 1 cm2 solar cells fabricated under the same conditions have efficiencies above 14%.

Reduction in Dislocation Density of Spherical Silicon Solar Cells Fabricated by Decompression Dropping Method

Spherical Si solar cells were fabricated based on polycrystalline Si spheres with a diameter of 1 mm produced by a dropping method. To decrease the cooling rates of Si spheres by decreasing the convection heat transfer to ambient, the Si spheres were dropped in a free-fall tower at a pressure of 0.2 atm. The conversion efficiency of low-pressure spherical Si solar cells was higher than that of normal-pressure spherical Si solar cells. Both Si spheres were polycrystals that consisting of crystal grains of about 200 µm. The distribution between electric active defects and the crystal quality were characterized by electron beam induced current (EBIC) measurements and transmission electron microscopy (TEM). By EBIC measurements, the low-pressure spherical Si solar cells were clearly observed to have the lower recombination sinks than the normal-pressure spherical Si solar cells. By TEM, the dislocation density in the low-pressure spherical Si solar cells was observed to be more reduced than that in the normal-pressure spherical Si solar cells. The dislocation density in the low-pressure Si spheres decreased because the reduction in the stress generated in the crystal grain.

Defect Evaluation of Spherical Silicon Solar Cells Fabricated by Dropping Method

Spherical Si solar cells are fabricated using polycrystalline Si spheres with a diameter of 1 mm produced by a high-speed dropping method. The distribution and types of electrically active defects in spherical Si solar cells have been directly characterized using electron-beam-induced current (EBIC) and transmission electron microscopy (TEM). Many recombination sink areas in grains and grain boundaries can be directly observed with EBIC in low-efficiency cells. The electrically active defects in grains are stronger recombination sinks than grain boundaries. The electrically active defect areas confirmed using EBIC were selectively etched with a Dash etching solution. TEM images revealed that the area showed a high dislocation density. These results suggest that the dislocations in grains deteriorate the performance of spherical Si solar cells.

Electric and Crystallographic Characterizations on Hydrogen Passivated Spherical Silicon Solar Cells

Hydrogen passivation (H-passivation) was performed to inactivate the electrically active defects in spherical Si solar cells fabricated by the dropping method. The atomic hydrogen activated by RF plasma passivated the dangling bonds in the crystal grains, and solar cell performance was improved. The distribution of defects and the crystal quality before and after H-passivation were observed by electron-beam-induced current (EBIC) and transmission electron microscopy (TEM). The recombination sinks were observed by EBIC as dark lines of grain boundaries, wide dark areas and local dark spots in the grains. The effect of H-passivation was observed at parts of the grain boundaries and the wide dark areas, but not at the local dark spots in the grains. TEM observation showed that the crystal grains of the areas with dark spots included many dislocations and that the crystal quality at the wide dark areas is microcrystalline (<100 nm). These results indicate that the bulk region of microcrystal grains and portions of grain boundaries are inactivated effectively by H-passivation.

Junction structure and physical properties of heteroepitaxial CdS/CdTe

Abstract The screen printed CdS/CdTe hetero-junctions formed on the (0001), (1120), (1010) surfaces of CdS single crystals have been studied by an X-ray double crystal spectrometer, electron diffraction, transmission electron microscopy (TEM), and EBIC. For CdS/CdTe formed on the CdS (0001) structure, the epitaxiality of the CdTe layer formed on (0001) of CdS was better than that formed on the other surfaces. The diffusion length of minority carriers formed on the single crystal CdS was found to be larger than that of sintered CdS/CdTe junction prepared by the screen printing method especially for the junctions grown on the (0001) plane of CdS.

Crystal Growth Mechanism of Spherical Silicon Fabricated by Dropping Method

We have investigated the crystal growth mechanism of spherical Si fabricated by a dropping method. The Si spheres were classified into two categories by surface morphology. The grain size of a Si sphere with a smooth surface is larger than that with a rough surface. To investigate the crystal growth mechanism of the spheres, Si samples with various crystal sizes were observed. Si sample size is related to Si droplet size of, which influences the ease of solidification. Large Si droplets take longer to solidify than small Si droplets. Si samples 4, 2 and 1 mm in diameter correspond to the initial, intermediate and final stages of crystal growth, respectively. In the Si sample 4 mm in diameter, a disk of (111) plane crystals is observed. This result suggests that the initial crystal growth of the Si spheres consisting of large grains involves the formation of a disk of (111) plane Si crystals. The crystal growth mechanism for a Si sphere 1 mm in diameter is proposed.

Microstructures, optical and photoelectric conversion properties of spherical silicon solar cells with anti-reflection SnOx:F thin films

The microstructures and optical and photoelectric conversion properties of spherical silicon (Si) solar cells were investigated and discussed. The surface of the spherical Si with a pn junction provided high crystallinity, and the lattice constant of the center of Si spheres is larger than that of the surface, which would be due to the lattice distortion by defect structures at the center of Si. The conversion efficiencies of spherical Si solar cells coated with SnOx:F anti-reflection thin films were improved by annealing. The optical absorption and fluorescence of the solar cells increased, and the lattice constants of SnOx:F anti-reflection layers decreased after annealing. The mechanisms of chemical reactions at the Si/metal interface were also discussed.

Crystal Characterization of Spherical Silicon Solar Cell by X-ray Diffraction

We have investigated the crystal quality of spherical Si solar cells by X-ray diffraction. A Si sphere with a diameter of 1 mm was fabricated by a dropping method. X-ray pole figure measurements confirmed that the Si sphere includes twin crystals. The spherical Si solar cells were fabricated using Si spheres. The spherical Si solar cells consisting of almost the same number of crystal grains exhibited various performance characteristics, because no clear correlation between the X-ray pole figure measurements indicating the grain number and the solar cell performance characteristics was confirmed. An X-ray rocking curve and a Dash etching method for the spherical Si solar cells indicated that the short-circuit current density and open-circuit voltage of the solar cells depend on the crystallinity of intragrains.