The relationship of structural properties of microcrystalline silicon to solar cell performance

A study on the microstructure of intrinsic, microcrystalline silicon (μc-Si:H) layers deposited with various silane concentrations is presented. The layers were fabricated in a large-area Plasma-Enhanced Chemical Vapor Deposition (PECVD) system on rough TCO-coated substrates. The microstructure was investigated by Raman spectroscopy and X-ray diffraction (XRD) in two different geometries. The structural properties are related to the performance of solar cells deposited together with the μc-Si:H layers. The crystalline volume fraction of the studied films ranged from highly crystalline to amorphous as derived from Raman spectroscopy and Grazing Incidence XRD. A hexagonal silicon phase, which is related to stacking faults and twins appeared in Bragg–Brentano and Grazing Incidence diffractograms, and revealed that these types of defects occur more pronounced on crystal planes that grow parallel to the sample surface. By evaluating the integrated intensity ratio of the {220} to {111} reflection of cubic silicon a small preferential orientation (texture) was observed for the high-crystalline material. Material close to the transition to amorphous phase (that leads to an optimum in solar cell performance) exhibits no preferential orientation. This shows that for the material under investigation — which is optimized for tandem solar cell with efficiencies above 12% — a preferential orientation is not a requirement to achieve device-quality material for good solar cell performance.