The silicon heterojunction (SHJ) is one of the most attractive device structures for fabrication of high-efficiency silicon solar cells at low temperatures (<250ºC) with simple processing. The excellent surface passivation provided by H and extra band bending due to the larger band gap of aSi:H compared to c-Si makes well-designed a-Si:H emitters superior to conventional emitters made by dopant diffusion. Furthermore, a thin layer of a-Si:H, doped the same as the base wafer provides a back collector with very effective back-surface field that reduces the recombination velocity.
The current conduction through the a-Si:H collector is adequate and no localized current conduction windows are needed, in contrast to dielectric back-surface passivation layers. These outstanding properties open the path to many a-Si/c-Si heterojunction silicon solar cell designs, the most successful being the Sanyo HIT cell employing plasma enhanced chemical vapor deposition (PECVD) to deposit thin p/i (emitter) and n/i (collector) a-Si:H layers on n-type wafers.