The bandgap of a-SiGe alloys can be continuously adjusted between 1.7 and 1.1 eV by varying the percentage of Ge. Calculations for idealized materials indicate that a bandgap near 1.2 eV would provide the maximum efficiency (see Figure 8.9c); unfortunately, 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, and these materials have not proven useful for PV application.
Figure 12.21 shows the J – V characteristics of a series of a-SiGe solar cells with different Ge concentrations in the г-layer (of constant thickness, and without a back reflector) ; the bandgaps are indicated in the legend. As the bandgap is reduced by incorporating more Ge in an г-layer, VOC goes down, as was previously illustrated in Figure 12.18. Since the narrower-bandgap materials catch more sunlight, JSC increases. The fill factors of the cells also decrease as the bandgap decreases; this could reflect either decline in the hole drift-mobility or an increase in defect density; of course, the greatest interest is in these effects for the “light-soaked” state.
Similar to the deposition of a-Si, a-SiGe devices made with high hydrogen dilution show improved quality and light stability . Optoelectronic properties (mobility, lifetime, collection length) of narrow-bandgap a-SiGe material are nonetheless inferior to those of a-Si.