Bandgap Grading of a-SiGe i – Layers and of a-SiC

To enhance the fill factor of cells with a-SiGe i-layers, band gap grading is used to enhance the collection of holes (Guha et al. 1989; Zimmer et al. 1998). An asym­metric V-shaped band-gap profile is created by adjusting the Ge content across the i – layer. Wider-band-gap material is deposited closest to the p-layer.

Such a grading allows more light to be absorbed near the p – layer so that “slower” holes do not have to travel far to get collected. Also, the tilting of the valence band creates an electric field that assists holes generated in the middle or near the и-side of the i – layer to move toward the p-layer. With appropriate hydrogen dilution during growth and band-gap grading, a-SiGe cells can generate up to 27 mA/cm2 under AM1.5 light. The advantage of these graded band-gap junction is the improved light stability (Yang et al. 1991)

The bandgap of a-SiC can be adjusted between 1.7 and 2.2 eV, depending on the C concentration (Bullot and Schmidt 1987) However, a-SiC with enough C to increase the band-gap appreciably suffers significant decrease in electron drift mobility (Gu et al. 1994) and became highly defective after light-soaking (Li 1993).

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