Thin film silicon alloys with enhanced stability made by PECVD and HWCVD for multibandgap solar cells

This paper presents the development of high deposition rate protocrystalline Si:H and μc-Si:H as obtained by hot wire chemical vapor deposition (HWCVD, or catalytic CVD) and very high frequency plasma enhanced CVD (VHF PECVD) and protocrystalline SiGe:H as obtained by conventional PECVD. Using the latter two low bandgap materials as absorber layer, it is feasible to obtain stable devices showing virtually no light-induced changes in fill factor. We have obtained stable proto-SiGe:H cells using high H₂ dilution. A disadvantage, however, is the very low deposition rate for proto-SiGe:H (∼0.6 Å/s). By HWCVD, much higher deposition rates have been achieved for silicon-based thin films (both proto-Si:H and pc-Si:H). During the last few years, improved control of parameters is also obtained in HWCVD, which presently makes this method interesting for reducing the production cost of thin film solar cells. Recently, we have demonstrated that protocrystalline Si:H can be obtained from undiluted silane at 250 °C in HWCVD at a high deposition rate of 1 nm/s. The light induced degradation in single junction n-i-p cells is less than 10%. Here, we also present for the first time p-i-n cells on commercial SnO₂:F with proto-Si intrinsic layers deposited at rates up to 3 nm/s, with again remarkable stability. Further, HWCVD microcrystalline materials with a crystalline fraction V_f of ∼50% possess excellent electronic quality: the dangling bond density extracted from computer simulations, is only 2 × 10₁₅ cm^-3 and therefore we could obtain fill factors of 0.77 and 0.75 in dual-junction stacked μc-Si:H/μc-Si:H cells, and proto-Si:H/μc-Si:H ´micromorph´ n-i-p type solar cells, respectively. Both VHF PECVD μc-Si:H made in the high pressure depletion regime and HWCVD μc-Si:H have excellent potential for multibandgap solar cells.