Shigeo Yata

Correlation between structure and optoelectronic properties of undoped microcrystalline silicon

The correlation between crystallinity and optoelectronic properties of undoped microcrystalline silicon is investigated. The use of the constant photocurrent method for measurement of microcrystalline silicon is discussed. This method measures the true absorption coefficient only if the crystallinity is a major fraction of a sample. If the crystalline volume fraction is less, it underestimates the absorption coefficient at smaller photon energies. At these energies, carriers are mainly photogenerated in the crystalline phase. Carriers generated in isolated grains contribute less to the photocurrent than carriers generated in grains which are embedded in percolation paths. The shape of constant photocurrent method spectra of microcrystalline silicon with a poor crystallinity differs from spectra of material with better crystallinity.

Optical confinement in high-efficiency a-Si solar cells with textured surfaces

Abstract The experimental spectral response and reflectance of high-efficiency a-Si solar cells are systematically investigated by using an optical simulation based on realistic optical properties of the transparent conducting oxide (TCO), a-Si, and metal electrode, in order to improve the spectral response. It is shown that a practically important optical loss results from absorption by the TCO, which is enhanced by the optical confinement effect. This suggests that improvement in the spectral response is possible by suppressing the optical confinement in the TCO.

Quantitative measurement and design of texture morphology for high-efficiency thin-film silicon solar cells

To decouple the trade-off relationship between short-circuit current ISC and open-circuit voltage VOC of thin-film silicon solar cells, which vary with texture morphology, it is necessary to first clarify the relationship between a solar cell’s properties and its texture morphology. We have developed a method for quantitatively measuring the texture morphology, which has enabled us to identify novel indices on the basis of texture width and angle individually correlated to ISC and VOC. A texturing process based on these indices should allow ISC and VOC to be improved independently.

Correlation between Structure and Optoelectronic Properties of Undoped Microcrystalline Silicon

Undoped microcrystalline silicon with various crystallinities was deposited by rf-glow discharge. We investigate the correlation between crystallinity and optoelectronic properties. The use of a constant photocurrent method for characterization of undoped microcrystalline silicon is discussed. In the case of high crystallinity this method measures the true absorption coefficient but with decreasing crystallinity, the constant photocurrent method underestimates the absorption coefficient at low photon energies. At these energies, carriers are mainly photo generated in the crystalline phase. Carriers generated in isolated grains give a smaller contribution to the photocurrent than carriers generated in grains forming percolation paths. This results in a strongly decreased absorption coefficient measured by the constant photocurrent method at photon energies of about 1.7 eV and below. Therefore, stable microcrystalline silicon with a high crystallinity can be easily distinguished from an unstable material with poor crystallinity.