The outline and recent progress of thin-film solar cells and heterojunction with intrinsic thin-layer (HIT) solar cells

A solar cell is a large-area diode with a p-n or a p-i-n junction. Since the invention of the first solar cell with a p-n junction by G.L. Pearson et al. in 1954, various types of solar cells have been investigated. Especially in the decade since 2000, a wide diversity of cell structures, conversion efficiencies and production amounts for solar cells (photovoltaics) were developed. Among them, in the field of thin-film silicon solar cells, the adoption of a p-i-n junction, which means employing an intrinsic amorphous silicon (i-a-Si) layer between the p and n layers, and the stacked type (multi-junction) structure are very important. The i-a-Si layer between doped layers made it possible to obtain photovoltage from the thin-film silicon. And with the multi-junction structure, they have achieved high voltage, high collection efficiency and a low light degradation ratio compared with the single junction.