Alloys and Doping

As was discussed in Sect. 41.4.7, a-Si-based alloys can be deposited using a gas mixture of SiH4 with other gases such as GeH4, CH4, O2, or NO2, and NH3 for obtaining a-SiGex, a-SiCx, a-SiOx and a-SiNx, respectively. Among these alloys, only a-SiC, as a wide band-gap p-layer, and a-SiGe, as a low-band-gap absorber layer, have been used to produce solar cells. But a-SiGe with a band-gap below

1.3 eV is difficult to deposit with uniformity (Mackenzie et al. 1998; Paul et al. 1993). By taking advantage of the similar dissociation rate of GeH4 and disilane (Si2H6), a mixture of these gases permitted the fabrication of a-SiGe alloy with uniform films (Guha et al. 1987). As discussed in Sect. 41.4.6, a-Si can be doped и-type by mixing phosphine (PH3) or doped p-type by mixing it with diborane (B2H6), BF3, or trimethylboron [TMB, B(CH3)3] during deposition. Most cells have either ^c-Si or a-SiC as the uppermost p-layer. Amorphous SiC p-layers in p-i-и devices are usually deposited using a mixture of SiH4 and CH4 diluted in hydrogen (Tawada et al. 1982), leading to bandgaps of 1.85-1.95 eV. The ^c-Si p­layer for и-i-p devices is mostly made by PECVD, using high H dilution with high RF power at relatively low temperature. The optimum p-layer for a-Si и-i-p solar cells is a mixed phase of amorphous and nano-crystalline (Pearce et al. 2007; Rath and Schropp 1998) and is essential to improve Voc.

In solar cells made on stainless steel in the mp growth sequence, the final p-layer yielding the best open-circuit voltage is a boron-doped “nanocrystalline” silicon film (Guha et al. 1986). In superstrate cells, usually made on TCO-coated glass, the best open-circuit voltages have been achieved using boron-doped amorphous silicon-carbon alloys (a-SiC:H:B). High efficient solar cells with high open-circuit voltages use a-SiC:H:B p-layers and also include a thin (<10 nm) “buffer layer” of un-doped a-SiC:H between the p-layer and the intrinsic-layer of the cell (Arya et al. 1986; Hegedus et al. 1988; Sakai et al. 1990).

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