Characterisation of Ultra-Thin μc-Si:H Films for Silicon Heterojunction Solar Cells

The present work demonstrates the suitability of spectral ellipsometry (SE) to properly characterize ultra-thin (< 15 nm) hydrogenated microcrystalline silicon (μc-Si:H) and its possible advantages over alternative methods, e.g. Raman spectroscopy. For very thin μc-Si the undesirable amorphous and void-rich initial incubation layer can make up a significant fraction of the actual film, thereby adversely affecting the effective electrical and optical film properties. Based on SE measurements and a suitable model to fit the experimental data, the strong material inhomogeneities induced by the incubation layer can be analyzed and accounted for. This allows for (i) the non-invasive determination of the μc-Si:H film thickness and crystalline volume fraction and (ii) the evaluation of the material density. Using this approach, the growth behavior of μc-Si:H on different surfaces, the influence of doping precursor gases and different deposition parameters has been determined. A strong dependence between the film thickness and its density has been observed as well as its correlation with the electrical and optical properties in silicon heterojunction (SHJ) structures. Additionally, a good monocrystalline silicon (c-Si) surface passivation and high selectivity for the extraction of charge carriers from the absorber into the Transparent Conductive Oxide (TCO) electrode using n-type μc-Si:H as front layer is being demonstrated by high open-circuit voltages of up to 730 mV and good contacts to the TCO (both ITO and AZO). The approach described and demonstrated in this work provides a suitable tool to engineer the partially crystallized doped layers of SHJ cells with respect to their electrical and optical properties at a device relevant level.