a-Si Basis Heterojunction Stacked Solar Cells
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a-Si BASIS HETEROJUNCTION STACKED SOLAR CELLS Y. HAMAKAWA, Y. MATSUMOTO, XU ZHANG-YANG, M. OKUYAMA H. TAKAKURA and H. OKAMOTO Faculty of Engineering Science, Osaka University Toyonaka, Osaka 560, Japan ABSTRACT
A systematic study has been made on a-Si basis heterojunction tandem Design rule and optimizations of cell parameters on structure solar cells. two and three stacked heterojunction solar cells are discussed on various material bottom junctions stacked with a-Si. A series of experimental trials has been made on some candidate materials. Technical data on cell fabrication processes, device performance are presented. In the -present stage of experiments, 13.3% efficiency on a-Si/poly-Si two terminals tandem solar cell and also on a-Si//poly- CdS/CdTe four terminals cell have been obtained. Some key technologies for future low cost massproduction processes such as growth of polycrystalline thin film for bottom cell are also discussed. INTRODUCTION In a recent few years, a remarkable progress has been seen in both physics and technologies of a-Si (hydrogenated amorphous silicon) solar cells (1). Now a-Si solar cell has a considerable size of the market, particulary in the consumer electronics field such as pocketable calculaters, wrist watches and battery rechargers (2). On the other hand, the field of power application has been still in the experimental phase, because there are still problems to be solved in the device performance including the efficiency and stability for out door sunlight. Therefore, in the present phase, an improvement of the conversion efficiency is the prime importance. As the results of recent R&D efforts from film quality improvements to new device structures such as a-SiC/a-Si heterojunction (3), superlattice window (4), randam reflection interface treatments (5) etc. More than 11% efficiencies are obtained on the laboratory scale small area cells (6,7), and also more than 7% efficiencies are presently quite common elsewhere in the industrial productions. (8). Another promised structure for further improvement of amorphous silicon basis solar cell is a heterostructure tandem solar cell. With this new structure, recently more than 12% efficiency has been reported on a-Si/poly-Si combination by Osaka University group (9), a-Si/a-SiGe by ECD group (10) for two terminal cells, and also a-Si//poly-CuInSe2 four terminal cells by ARCO group (11). Theoretical study has also been initiated by several groups. However, there are still no practically available design theory in this kind of cells so far. In this paper, some design rules to get high efficiency tandem cell are firstly presented. Secondary, a series of concrete calculations has been made on a-Si/poly-Si tandem solar cell. Then, experimental results on two terminal and four terminal cells are demonstrated. Finally, a new technology on fabricating thin poly-crystalline film is briefly discussed. CONCEPT OF OPTIMUM DESIGN Figure 1 shows an energy band diagram (a) and configuration of photon flux density D(x) penetration (b) in the three s
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