An optimum design of a a-Si//poly-Si tandem solar cell

A systematic investigation of a high efficiency a-Si//poly-Si four-terminal structure tandem solar cell has been made using both theoretical and experimental approaches. It has been shown from optimum design theory, used on a realistically attainable best efficiency, that the best combination of a tandem solar cell is a-Si/poly-Si silicon materials with respect to high achievable efficiency. In the optical design rule, priority of the photon utilization is put on the poly-Si bottom cell. Employing high conductivity with wide optical band gap p type μc-SiC as a window material and n type μc-Si as back ohmic contact with BSF treatment, a 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 a p μc-SiC/p a-SiC/i a-Si/n μc-Si/ITO structure, and an efficiency of 7.25% has been obtained with a 100 nm thick i-layer top cell. The best efficiency of a p-i-n single-junction solar cell with this structure is 12.3% so far with a 500 nm i-layer thickness device using an Ag back electrode. With a 100 nm thick ultra thin top cell, a total conversion efficiency of an a-Si//poly-Si four-terminal tandem solar cell as high as 21.0% has been achieved