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Investigating the effects of RbF post-deposition treatments in Cu(InXGa1-X)Se2 solar cells
Wands, Jake W.
Wands, Jake W.
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2023
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In order to meet global energy demand while reducing carbon emissions, large-scale deployment of renewable energy technologies is needed. Solar photovoltaics (PV) are a key technology and have become one of the cheapest options for new energy installations. Cu(InxGa1-x)Se2 (CIGS) based PV is a thin-film technology with potential benefits over market-leading silicon. Alkali halide post-deposition treatments (PDTs) have become an essential tool in maximizing the efficiency of CIGS solar cells. While Na and K have received the most research as the alkali choice, Rb has been shown to further improve performance, largely through open-circuit voltage (VOC) increases.
This thesis studies the effects of RbF based PDTs on the performance of CIGS solar cells. In all samples studied the RbF PDT increased VOC over an untreated reference sample. Device modeling and temperature-dependent current-voltage (JVT) experiments found that reduced recombination and bandtail width may be key factors in VOC improvements. An aging study also found a smaller decrease in VOC after aging in the RbF PDT device compared to the reference.
Despite the benefits of RbF, there are still some drawbacks. After aging, the RbF PDT device experienced a larger decrease in short-circuit current and fill factor (FF). Additionally, the RbF PDT device suffered from larger voltage-dependent collection efficiency (nC) losses. Low nC led to large reductions in FF and maximum power. It is theorized that low depletion width and the Ga/(Ga+In) gradient near the heterojunction are contributors to low nC.
Preliminary research on defect signatures suggests that the dominant trap state changes after RbF PDT. While the identity of the dominant defects is not known, this research identities an important area of future research.
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