Optimum front contact and growth conditions for microcrystalline silicon solar cells from hot-wire CVD

Hot-wire chemical vapor deposition (HWCVD) is a promising method for growing doped as well as undoped microcrystalline (μc)-Si films at rates of up to 20 Å/s. SnO₂ front contacts for solar cells, however, cannot stand HWCVD growth conditions, namely atomic hydrogen impact and elevated substrate temperatures of 400°C and above. Since SnO₂ is drastically reduced under HWCVD conditions, the authors propose to use ZnO instead and therefore present a detailed characterization of the ZnO/p⁺-μc-Si interface here. Nucleation and growth of B doped HWCVD μc-Si has been monitored by the combined use of in-situ ellipsometry (EL), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Varying the H₂ dilution of the process gases (flow ratio x=[H₂]/[SiH₄+B₂H₆]) significantly changes μc-Si deposition rate, crystallinity and morphology. XPS depth profiling and EL reveal that no chemical reduction of ZnO occurs even for highest H₂ dilutions