Optical losses in multi junction a-Si:H based solar cells and modules

A comprehensive optical model is described which is applicable to glass/textured CTO/a-Si:H/a-SiGe:H-based multijunction cells and allows the calculation of optical absorption in each layer of the solar cell. The major optical losses which limit the output current density of tandem cells using 1.72 eV/1.50 eV bandgap a-Si:H/a-SiGe:H and an ITO/Ag rear contact to about 20.8 mA/cm² (sum of both junctions) are identified and discussed. It is shown that improvements in the reflectivity and scattering properties of the rear contact may be expected to result in current densities of 22.3 mA/cm² in this type of cell using intrinsic layers of limited thickness. The use of low-cost materials, such as soda-lime glass and the aluminum rear contacts typically employed in the manufacture of large-area modules, should reduce the total current density available to 18.5 mA/cm²