a-SiGe:H alloy material limitations and device considerations

The electron mobility of intrinsic amorphous SiGe films is found to depend on the hydrogen content. Deposition conditions that influence hydrogen content are discussed. The best C_H a-SiGe:H alloys of 1.3 eV bandgap are used in solar cells. Solar cell analysis is used to determine both hole and electron transport. It is shown that the electron mobility is reduced by alloying; however, the hole transport of the alloy is not significantly different from that of device-quality a-Si:H. Cell designs that minimize performance loss due to poor electron mobility are considered. The results obtained indicate that an ideal a-SiGe:H-based solar cell would be graded from a-SiGe:H near the front to a-Si:H at the back of the cell. Careful consideration must also be given to hole interface recombination at the contact as the holes generated deep in the i-layer have a reasonable chance to be allocated. The hole transport of the alloy is relatively good compared to that of a-Si:H or to the electron transport in a-S:G:H