An aperture-area conversion efficiency of 20.0% (intrinsic efficiency: 21 .O%) has been achieved for a 1 .Ocm2 CZ n-type single crystalline silicon (c-Si) solar cell, by using the "HIT (Heterojunction with Intrinsic Thinlayer)" structure on both sides of the cell. This is the world's highest value for a c-Si solar cell in which the junction is fabricated at a low temperature of below 200 'C.In this paper, the junction fabrication technologies and features of the HIT structure are reviewed. The stability under light and thermal exposure, and the temperature dependence on performance of a highefficiency HIT solar cell are also reported.
The chemical analysis of trace metallic contamination on a wafer can be achieved by using total reflection X-ray fluorescence (TRXRF) with HF condensation and with poly silicon encapsulation secondary ion mass spectroscopy (PC-SIMS). HF condensation can concentrate almost all atoms, such as Fe, Cr, and Ni, within 10 mm of the center of a wafer, which leads to lower detection limits. Poly silicon encapsulation eliminates the surface problems that tend to occur with normal SIMS, which results in good reproducibility. A combination of both methods is suitable for analyzing lower-level contaminations up to 10' atoms/cm2 level for many elements such as transient metals and lightly elemental metals. The application of the analyses to wet cleaning reveals the contamination removal and adhesion effects for various solutions and cleaning procedures, as well as variation between experimental cleaning batches.
A total-area conversion efficiency of 12.0% has been achieved for a 100 cm2 single-junction a-Si solar cell, by the use of a high-quality i-layer and other successful techniques such as a high-quality buffer layer treated by hydrogen plasma. For further improvement, the first exact numerical model has been proposed for submicron "textured" a-Si solar cells. Some important results have been obtained with the model. For example. the non-uniformity of the electric field in the textured structure was quantitatively analyzed. And, for a cell with a structure of glassflCO/a-Si/metal, the incident light is trapped inside the cell, similar to that in a fiber optics, mainly due to the small loss of light escaping from the a-Si to the glass.
The H2 dilution technique at a high deposition rate (RD) was investigated by depositing hydrogenated amorphous silicon (a-Si:H) under a high if power density of 750 mW/cm2, which is 20 times as large as that of conventional conditions. It was found that the H2 dilution ratio γ ( = [H2 gas flow rate] / [SiH4 gas flow rate]) tendency of the film properties, such as the H content (CH), optical gap (Eopt), SiH2/SiH and photoconductivity (σph) of a-Si:H is different for the high rf power (750 mW/cm2) and the medium rf power (75 mW/cm2) conditions. Under medium rf power, the CH, Eopt and SiH2/SiH decrease as γ increases. Under the high if power, on the contrary, the CH and Eopt, monotonously increase while maintaining a low SiH2/SiH and a high σph of 10-6 S/cm as γ increases. These results suggest that increasing the rf power enhances the H incorporation reactions due to H2 dilution. It is thought that a high rf power causes the depletion of SiH4 and hence the extinction of H radicals, expressed by SiH4 + H* → SiH3* + H2, is suppressed. A high H radical density enhances the incorporation of H into a-Si:H, resulting in very wide-gap a-Si:H with a high CH, Consequently, very wide-gap a-Si:H with device-quality (Eopt of 1.82 eV with an (αhv)1/3 plot, corresponding to > 2.1 eV with Tauc's plot, and σph of 10-6 S/cm) can be obtained at a high RD of 12 Å/s without carbon alloying.
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