Correlation of Small-Angle X-Ray Scattering and Solar Cell Performance of a-SiGe:H Alloys

Small-angle x-ray scattering (SAXS) measurements were made on a-SiGe:H alloys to study microstructure on the nanometer scale as a function of Ge content, and the results were compared with representative single-junction solar cell properties. Samples consisting of only the i-layer were used for SAXS. Above a Ge content of 20 %, a significant increase in SAXS was seen. From measurements made with the samples tilted relative to the incident x-ray beam, the increase in scattering is attributed to the appearance of elongated low density regions in the film, modeled as ellipsoidal microvoids, which are preferentially oriented perpendicular to the film surface and may be related to columnar-like microstructure. Flotation density measurements support the presence of low density regions. Initial and light-degraded measurements on corresponding solar cell structures do not show a correlation between SAXS and initial cell properties; there is, however, some evidence that the light-induced degradation is higher for cells with larger amounts of SAXS-detected microstructure and this needs further investigation.