41.3.1 Multijunction Solar Cells
The conversion efficiency of the a-Si:H pin photodiode can be significantly improved by depositing two or three such photodiodes of different band-gap, one on top of another, to create a “multijunction” device. The main advantage to the tandem design over the single-junction is due to “spectrum splitting” of the solar illumination (Dalal and Fagen 1980), e. g. by using the a-Si:H with a band gap of 1.9 eV as front layer, followed by an a-SiGe:H alloy with lower band gap. More recently multi-junction cells of three consecutive layers with stepwise decreasing band gap have been used.
