We investigate Si nanocrystals fabricated by the rapid thermal oxidation (RTO) of an ultrathin chemical vapour deposition (CVD) amorphous Si (a-Si:H) film. It is found from the transmission electron microscope (TEM) observation that the ultrathin RTO film contains Si nanocrystals of around or less than 5 nm in size. The dynamic electrical conduction measurement of the RTO diode structure including the Si nanocrystals reveals novel features such as the N-shaped tunnel current versus gate voltage characteristics and the hysteresis. It is also found that the gate voltages at the first and second current rise are fixed and the current reduction in the fixed time interval is observed at the constant gate voltage. These findings can be explained by the fixed-amount electron charging effect at the Si nanocrystals and the consequent screening effect on the tunnel current flowing through the diode structure.

The relationship between average grain size on the surface of SnO 2 transparent conductive film and conversion efficiency of the a-Si:H solar cell was investigated. a-Si:H solar cells were fabricated on SnO2/glass substrates with various grain sizes.The cell structure was glass/p(SiC)-i-n/Al and the effective cell area was 4 x 10 -2cm2. The reflectivity from the glass substrate was reduced to about 7 percent with increasing the grain size from 0.1 to 0.8fim. and the short-circuit current was inceased from 12 to 14mA/cm2. A 7.9 percent of conversion efficiency was achieved using milky S n 0 2 film of 0 . 4 -~m average grain size at AM-lOOrnWicm2.T

The effect of optical confinement in textured antireflection coating (AR coating) was investigated. Textured ZnO films prepared by metalorganic chemical vapor deposition were applied to solar cells as AR coating. The reflectance of the cells decreased as the grain size increased with film thickness, especially at long wavelength. This reduction in the reflectance caused an increase in short-circuit current and spectral response of the cells at near-infrared. The effect of optical confinement in the cell by this film was shown as far as long-wavelength range was concerned.

Low-temperature n-type silicon epitaxial growth has been studied for the application to n þ /p homojunction solar cells. Epitaxial Si films can be grown at the substrate temperatures from room temperature (RT) to 200 C by plasma-enhanced chemical vapor deposition. Two-step growth is effective for obtaining high-quality layers above 150 C. In this growth, a very thin Si layer has been deposited on Si substrates at a low hydrogen-to-silane gas-flow ratio, and subsequently an epitaxial Si layer has been grown at a higher ratio. Smooth nucleation with reduced local stress has been enhanced in the initial stage of the epitaxial growth. An open-circuit voltage, V oc , of 0.610 V and a conversion efficiency, , of 13.54% have been obtained for an n þ /p cell, where the n þ layer was deposited at 200 C. Film growth mode changes below 100 C. The stress in the film during the epitaxial growth is relieved by introducing a small fluctuation of crystal orientation and the voids in the film at these temperatures. We have succeeded in fabricating a solar cell with a V oc of 0.608 V and an of 13.52% having an n þ layer deposited at RT.

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