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TECHNOLOGYSOLAR


Figure 1. The structure of a HIT solar cell


the 20th century, our consumption of fossil fuels, environmental destruction and global warming have become crucial problems. Under such circumstances, solar energy was noted as an effective measure against carbon dioxide emission and global warming. In order to introduce solar electricity, policies such as a feed-intariff are being promoted.


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As a result, clean but expensive solar cells are now widespread, and the solar cell industry is growing larger in scale. The total production capacity is dramatically increasing. The total production capacity of the PV industry was 10.7GW in 2009, which is more than a 51% increase over the 7.06GW of 2008. With the expanding market size, there is a growing need for improving the efficiency and suppressing the cost of solar cells.


SANYO has been providing HIT (Heterojunction with Intrinsic Thin layer) solar modules to the market since 1997. The HIT solar module consists of original SANYO high-efficiency solar cells known as HIT solar cells.


To meet the market demand for highefficiency low- cost solar cells, SANYO has made great efforts to not only to raise the conversion efficiency of HIT solar modules, but also to reduce costs. In order to reduce the cost of solar modules, the use of a thinner silicon wafer is an important approach because the silicon wafer accounts for a large part of the module cost. Therefore, without compromising efficiency, the company is also conducting research on cell thinning. They have recently raised the highest conversion efficiency in a practical sized solar cell to 23.0% [1]. In addition, the company has successfully applied high- efficiency processes to thin silicon solar cells of less than 100 µm in thickness and achieved a conversion efficiency of 22.8% [2]. In this article, the authors describe recent efforts to raise the conversion efficiency and reduce the production cost of the HIT solar cell. In addition, we briefly introduce other research efforts related to the HIT structure.


Structure of the HIT cell


Figure 1 shows the structure of a HIT solar cell. The HIT solar cell is an original SANYO structure in which an intrinsic (i-type) amorphous silicon (a-Si) layer and a p-type a-Si layer are deposited on a randomly textured ntype CZ crystalline silicon (c- Si) wafer, so that a p/n heterojunction is formed. On the opposite side of the c-Si wafer, i-type and n-type a-Si layers are deposited to obtain a Back Surface Field (BSF) structure. On both sides of the doped a-Si layers, Transparent Conductive Oxide (TCO) layers and metal grid electrodes are formed. All of the processes described above are done at low temperature (<200°C), so that any thermal damage to the components of the cell can be avoided.


By inserting the high-quality intrinsic a-Si layer between the c-Si wafer and the doped a-Si layer using a low-damage process, the surface dangling bonds of c-Si can be well passivated. This is based on our technologies for forming high-quality i-type a-Si. This effective passivation allows us to obtain a HIT solar cell with a high Voc compared to a general c-Si based solar cell fabricated by the thermal diffusion of ~900°C. Its high Voc leads to not only a high conversion efficiency but also an excellent temperature coefficient.


This good temperature coefficient of HIT solar cells can bring higher benefits to consumers because the annual amount of output power that the HIT solar cells produce is more than that of a


www.solar-pv-management.com Issue IX 2010


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