In Figure 12.18 we present a summary of the open-circuit voltages (VOC) for amorphous silicon-based solar cells as a function of the bandgap of the intrinsic absorber layer ; the measurements include results from silicon-germanium, silicon-carbon alloy cells, and (separately) nc-Si:H cells. The measurements were done under standard solar illumination conditions. For amorphous cells, they show that the voltage output can be related to the bandgap of the cells by VOC = (EG/e) — 0.80; we shall refer to the difference of 0.8 V as the “VOC-deficit”. The fitting (and deficit) are consistent with the VOC (predicted by Equation 12.3) .
In the figure, we assigned nc-Si:H a bandgap of 1.12 eV, which is the same as that of c-Si . The deficit of 0.80V between EG/e and VOC in a-SiGe:H alloys shrinks to about 0.55V for the highest efficiency nc-Si:H cells (about 10%). The reduced deficit in nc-Si:H compared with a-SiGe:H alloys reflects smaller values both for the effective band densities-of-states NC and NV and for the band tail widths .
The useful thickness of cells made with the Ge-alloys of a-Si:H is apparently not greatly different from that of a-Si:H. This result is consistent with the limited measurements, indicating that hole drift mobilities do not change greatly with alloying ; electron drift mobilities are substantially affected by alloying.
The thickness of optimized nc-Si:H cells is nearly ten times larger than for a-Si:H. This is fortunate, because nc-Si:H has a lower absorption coefficient than a-Si:H throughout most of the visible spectrum. The increased thickness is enabled by the large increase in hole drift mobility (see
1.2 1.4 1.6 1.8
Bandgap EG (eV)
Figure 12.18 Open-circuit voltages and thicknesses for nc-Si:H, a-SiGe:H, and a-SiC:H singlejunction solar cells as a function of absorber layer bandgap [170, 171, 173]
Figure 12.16) in nc-Si:H compared to a-Si:H , which allows photocarrier collection from a larger thickness of material. The “micromorph” cell design we describe shortly takes advantage of these complementary properties to give larger efficiency than could be obtained with either a-Si:H or nc-Si:H alone.