The band-gap of a-SiGe alloys can be continuously adjusted between 1.7 and 1.1 eV by varying the percentage of Ge. However, the optoelectronic quality (i. e. defects and carrier transport) of a-SiGe degrades rapidly when the a-SiGe bandgap is reduced below about 1.4 eV. Figure 41.8 shows the J-V characteristics of a series of a-SiGe solar cells with different Ge concentrations in the i-layer (of constant thickness, and without a back reflector) (Agarwal et al. 2002).
As the band-gap is reduced, Voc is reduced and Jsc increases, since the narrower – band-gap absorbs more sunlight. But also the fill factor decreases as the band-gap decreases, probably because of a reduced hole drift-mobility or an increase in the defect density, hence lower lifetime and collection.
