Abstract
AbstractState‐of‐the‐art Cu(In,Ga)Se2 (CIGS) solar cells are grown with considerably substoichiometric Cu concentrations. The resulting defects, as well as potential improvements through increasing the Cu concentration, have been known in the field for many years. However, so far, cells with high Cu concentrations show decreased photovoltaic parameters. In this work, it is shown that RbF postdeposition treatment of CuInSe2 solar cells allows for capturing the benefits from the improved absorber quality with increasing Cu content. A reduced defect density and an increased doping level for cells with high Cu concentrations close to stoichiometry are demonstrated. Implementing a high mobility front transparent conductive oxide (TCO), the improved absorbers with 1.00 eV bandgap yield a solar cell efficiency of 19.2%, and combined with a perovskite top cell a 4‐terminal tandem efficiency of 25.0% are demonstrated, surpassing the record efficiency of both subcell technologies.
Highlights
Structural defects play an important role in semiconductor materials and devices
For highly efficient CIGS solar cells, semiempirical optimization of the elemental composition lead to an optimum Cu to group-III element ratio (CGI) in the range of 0.80 to 0.90.[3,4,5,6] As a result of this off-stoichiometric, Cu deficient composition, a high density of native defects exist within the absorber layer.[7,8]
In this work we show that heavy alkali (RbF) post deposition treatment (PDT) is effective to overcome the recombination issues in CIS based solar cells with Cu concentration close to stoichiometry
Summary
Structural defects play an important role in semiconductor materials and devices. Point defects especially, such as vacancies, impurities, antisites, and interstitials, as well as the defect pairs of those, are known to influence the electronic properties of the materials. The implementation of a Cu deficient surface layer has shown to recover the VOC loss in those high CGI cells, while a decrease in current density remains, attributed to tunneling recombination.[17,21] Similar improvements of the absorber surface have been achieved with alkali treatments after etching of the secondary phases in Cu rich samples, [22,23] most recent results indicate this treatment may passivates defects that were at least partly generated by the etching in the first place.[24] This suggests that the front absorber surface limits the efficiency of Cu stoichiometric devices. It is important to reduce the front interface recombination, besides any other, in such devices
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