**3.3.1 Tandem cell with multicrystalline Si**

In the tandem cell configuration in 1983, Hamakawa reported the structure that a-Si: H/µc-Si heterojunction cell has been investigated as a bottom cell (Hamakawa et al. 1982). Its advantage is that it does not require high temperature processing for junction formation, and the top a-Si:H cell can be fabricated continuously. A heterojunction cell that is reported in 1994 by Matsuyama consisting of a-Si:H as p-type and µc-Si as n-type, with a 10 pm thick µc-Si film fabricated by solid phase crystallization yielded an efficiency of 8.5%. By using a p-µc-Si: C/n-µc-Si/n-pc-Si heterojunction bottom solar cell a conversion efficiency of 20.3% and good stability can be gained. At least in this type of devices the highest efficiency reported up to now is 20.4% (Green et al., 2011).
