Computer analysis of thin-film amorphous silicon heterojunction solar cells

A two-dimensional numerical computer analysis for thin-film-based hydrogenated amorphous silicon (i.e. a-Si : H) solar cells is presented. A comparative performance assessment for various absorbing layers such as a-Si, a-SiGe, a-SiC, combined a-Si+a-SiGe, a-Si/a-SiGe-graded layers and tandem (tdm) design is shown. The device performance is evaluated by implementing special surface recombination model, thermionic field emission model for transport at the heterojunctions and interface traps model at top and bottom side of the heterojunction interfaces. Single absorber with a graded design gives an efficiency of 10.1% for 800 nm thick multiband absorption. Similarly, tandem design shows an efficiency of 10.4% with a total absorber of thickness of 800 nm at bandgap of 1.75 eV and 1.0 eV for the top a-Si and bottom a-SiGe component cells. Numerical simulations predict improved efficiency for tandem design over large range of band discontinuity and interface trap density. Although comparable, the efficiency performance for graded design (i.e. absorbing layer of a-Si and a-SiGe graded layers) approach is very sensitive to the limited range of band discontinuity and interface trap density available at the heterojunctions.