Thin mono-Si, grown by vapor phase epitaxy, offers numerous advantages making it a potentially high-efficiency, low-cost, manufacturing technology. In this method, a crystalline Si film is grown on a monocrystalline reusable temporary-substrate that has a porous surface of suitably tailored characteristics. The epitaxial film (of appropriate thickness) is separated to become a free-standing wafer for solar cell fabrication. Alternately, cell fabrication is completed while the Si film is attached to the temporary substrate, followed by separation of the cell from the substrate.
The advantages of this technology include:
High quality mono-Si is obtained directly from the gas phase
No kerf/cutting losses
Low oxygen (no light induced degradation in P type due to B-O defects and lower manufacturing cost.
However, with this approach there are certain crystal defects and impurities that could impact the final performance of the solar cell product, which are not yet completely understood. Although cell efficiencies comparable to mainstream silicon PV are possible, it is expected that understanding the mechanisms of the specific defects, impurities, and impurity-defect interactions can lead to their mitigation with a concomitant increase in the cell performance. Hence, we have begun a study to determine the mechanism(s) that limit the efficiency of current cells and establish approaches to overcome these limitations. In this paper, we will describe the nature of defects and impurities in the epitaxial Si layer and their influence on the cell efficiency.
Our study has shown that defect generation mechanisms are of two types:
Type A- interface defects that originate/nucleate from factors such as surface cleanliness, quality of the porous Si at the surface, and factors related to the nucleation kinetics at the initial growth.
Type B- propagation of the interface defects and generation of bulk defects through thermal stress.
Type A defects are predominantly stacking faults (SF), while type B are primarily dislocations. Analyses of the solar cells have revealed an interesting behavior — in spite of the fact that SF density is low, they can have a controlling effect on the solar cell performance. We will discuss mechanisms of defect generation/propagation, and discuss approaches of mitigation.