Correlations between mapping spectroscopic ellipsometry and solar cell performance for the Study of nonuniformities in thin (0.7 μm) Cu(In _1-xGa_x)Se₂ solar cells over large areas

In the scale up from small area solar cells to large area production modules, it is important to understand the effects of macroscopic nonuniformities in basic properties on the ultimate performance of the devices. In this study, we have spatially correlated non-uniformities in thin-absorber (∼ 0.7 μm) CIGS [Cu(In_1-xGa_x)Se₂] based solar cells with the corresponding performance parameters of small area devices. Non-contacting spectroscopic ellipsometry (SE) mapping of constituent layers over a 10 cm × 10 cm area has been performed step by step during batch processing, providing CIGS composition, as well as bulk and surface roughness thickness maps for each layer of the device structure. After film stack preparation and mapping, an 18 × 9 array of 0.5-cm²-area solar cells was fabricated in order to establish a spatial correlation between the composition/ thicknesses and cell performance maps over the same area. This approach serves to characterize the inherent non-uniformities that occur during large-area, thin-layer deposition, and is uniquely suited for industrial application. In addition, given sufficient non-uniformity, correlations between layer properties, deduced by SE at different spatial points of a large area cell structure, and the solar cell performance at those points enables expeditious optimization. As an example of the application of this capability, we have demonstrated the feasibility of depositing efficient, verythin CIGS in a three stage process, designed to reduce materials cost and increase throughput of solar modules.