Elucidating the Mechanism of an RbF Post Deposition Treatment in CIGS Thin Film Solar Cells

Abstract

The impact of a rubidium fluoride post deposition treatment (RbF-PDT) on the material and device properties of Cu(In,Ga)Se 2 (CIGS) thin films and corresponding solar cells is investigated. The structure and device properties of CIGS with different PDT duration are compared. With longer PDT duration, which equals a higher amount of RbF deposited on the CIGS absorber layer, a clear trade-off is observed between increasing open-circuitvoltage (V OC ) and decreasing fill factor (FF). An optimum of the PDT duration is found increasing the efficiency by about 0.8% (absolute) compared to the Rb-free reference device. The mechanisms behind the increased V OC are explored by various characterization methods and identified as a combination of increased carrier concentration and reduced recombination rates in the device. Possible origins for these mechanisms are discussed. Furthermore numerical simulations are used to analyze the detrimental effect of the PDT on the FF. It is found that thermally activated alkali migration into the transparent front contact could create acceptor states there, which could explain the observed FF-loss.

Limitation of Current Transport across the Heterojunction in Cu(In,Ga)Se2Solar Cells Prepared with Alkali Fluoride Postdeposition Treatment

“…This high performance was achieved by incorporating KF, RbF, or CsF into the absorber layer via PDT. Regarding the effects of alkali fluoride PDT on device parameters, an increase in the open‐circuit voltage ( V oc ) has been observed consistently, whereas short‐circuit current ( J sc ) and fill factor (FF) present an inconsistent trend in different contributions . For the context of this work, we note that some publications have reported the presence of a barrier for the bucking and/or photo current, resulting for instance in a rollover of the current–voltage ( J–V ) characteristics or a crossover between dark and light J–V curves after PDT …”

“…In addition, studies demonstrated that if Na is present at the heterointerface between ZnO and CIGS in superstrate solar cells, the high density of deep acceptor states at the heterointerface lowers the FF, and therefore the device performance . Notwithstanding the different preparation sequences in our substrate cells, a similar situation may nevertheless arise . Thus, introducing acceptor states at the CdS/ZnO interface induced by Rb migration could explain some of the nonidealities presented in RbF‐treated samples within the scope of our numerical simulations.…”

“…Finally, Ni/Al/Ni finger grids were evaporated onto the ZnO. Process details for the respective solar cell devices can be found in the study by Kodalle et al…”

“…There are indications that a barrier at the back contact at least contributes to the limitation of the current transport in actual devices in particular with PDT of the absorber . Such a barrier may be needed to completely reproduce measured J–V curves in numerical calculations.…”

Heavy Alkali Treatment of Cu(In,Ga)Se2Solar Cells: Surface versus Bulk Effects

“…Concerning absorber elements, in both cases, Cu depletion and In enrichment are observed at the grain boundary, the latter being slightly more pronounced when the RbF‐PDT is applied. Accumulation of Rb at grain boundaries and dislocation cores has also been observed by TEM and X‐ray fluorescence investigations of RbF‐treated Cu(In,Ga)Se 2 absorbers and has been proposed as the cause of reduced bulk recombination …”