The Effects of Non-Uniform Electronic Properties on Thin Film Photovoltaics
Thin film polycrystalline photovoltaics, typically made with CIGS and CdTe absorber layers, are promising sources of renewable energy due to their high efficiencies (>10%) and low cost. However, a fundamental understanding of their device physics is lacking. One of the primary reasons for this is the difficulty in modeling and quantifying the influence of electronic non-uniformities on device operation. Contrary to conventional silicon cells, which are typically cut from a monocrystalline ingot with relatively homogenous electronic properties, CIGS and CdTe solar cells exhibit both intentional and unintentional spatial variations in electronic properties. Therefore, in order to develop the physics of these photovoltaics, it is necessary to develop new characterization techniques and methods to probe the spatial variation in electronic properties in these material systems.
CIGS solar cells grown at the National Renewable Energy Laboratory were studied using a variety of techniques to quantitatively determine the impact of both intentional and unintentional electronic non-uniformities on device performance. The method of electron beam induced current (EBIC) was extended to characterize the depth dependent electronic properties in compositionally graded absorbers. It was experimentally shown using this EBIC technique that gallium gradients along the depth of the CIGS solar cells improve carrier collection through the introduction of quasi-electric fields. Furthermore, the improvements in the EBIC technique allowed for a quantitative determination of the bulk diffusion length in compositionally graded samples. It was experimentally determined that the bulk diffusion length in CIGS solar cells has no correlation with the device open circuit voltage, in sharp contrast to silicon solar cells. This result could be due to either lateral non-uniformities in the CIGS electronic properties or different defects dominating recombination in the space charge region. Therefore, a theory of electroluminescence intensity was developed for thin film photovoltaics to quantitatively determine the influence of lateral electronic non-uniformities on device performance. Using a combination of electroluminescence and electron beam induced current, it was determined that lateral non-uniformities in the quasi-neutral bulk electronic properties do not dictate device performance. These results show the defect properties in the space charge region strongly influence device performance thereby providing a theoretical justification for the gallium depth profile observed in the highest efficiency CIGS solar cells. Furthermore, non-uniform secondary blocking barriers were experimentally detected on CIGS solar cells using electroluminescence and attributed to an unpinned Fermi-level at the CIGS/CdS interface combined with a conduction band offset. A discussion on how these results influence other characterization techniques and modeling efforts is provided.