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A systematic investigation has been made on output measurement of the a-Si//al-SiGe//a-SiGe triple stacked solar cells. Prototype modules incorporating triple a-Si cells fabricated in high deposition rate plasma CVD process with micro-wave power source[l] have been exposed outdoor for about a half to one year at three different geographical locations to evaluate the outdoor performance under cold, temperate and tropical climates. The results showed that a-Si performed better under higher temperature climate. Energy efficiency over 8% was obtained in Kuala Lumpur. Power rating under 1 sun, 50deg.C light soaking condition seems to be reasonable in Kyoto.

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A systematic investigations on high efficiency a-Si //poly-Si four-texminal structure tandem solar cell has been made both on theoretical and experimental approaches. It has been shown from an optimum design theory made on a realistic attainable best efficiency that the best combinations of tandem solar cell is a-Si / poly-Si silicon materials in its high achievable efficiency. In the optical design rule, priority of the photon utilization put on the poly-Si bottom cell. Employing high conductivity with wide optical band gap p type pc-Sic as a window material, n type IC-Si as back ohmic contact with BSF treatment. the conversion efficiency of 17.2% has been obtained for the poly-Si cell. An optimum design of the a-Si top cell has been experimentally made on p pc-Sic/ p a-SiC/i a-Si/n pc-Si/ITO structure, and an efficiency of 7.25% has been obtained with IOonm thick i-layer top cell. While the best efficiency of p-i-n single-junction solar cell with this structure is 12.3% so far with 5 h i-layer thickness deviced by Ag back electrode. With the IOOnm thick ultra thin top cell, a total conversion efficiency of a-Si // poly-Si fourterminal tandem solar cell as high as 21 .O% has been achieved.

A series of experimental investigations has been made on the a-Si // poly-Si tandem solar cell which is one of the most promised candidate of high cost-performance photovoltaic cell, e.g., high efficiency, low cost with almost no light induced degradation. Employing high conductivity with wide optical band gap p type microcrystalline SiC (μ-SiC) as a window material together with a-SiC as an interface buffer layer and also n type μc-Si as a back ohmic contact layer in the poly-Si based bottom cell, the conversion efficiency of 17.2 % has been obtained. Combining an optically transparent a-Si p-i-n cell as a top cell with an optical coupler between the top and the poly-Si bottom cell, a total efficiency of 20.3 % has been obtained so far on the four-terminal stacked mode structure. A systematic technical data for the optimization of cell structure variation on the developed tandem solar cells are presented and further possibility to improving the performance are discussed.