Microscopic insight into the impact of the KF post‐deposition treatment on optoelectronic properties of (Ag,Cu)(In,Ga)Se2 solar cells

Abstract

AbstractIt is attractive to alloy Cu(In,Ga)Se2 solar‐cell absorbers with Ag (ACIGSe), since they lead to similar device performances as the Ag‐free absorber layers, while they can be synthesized at much lower deposition temperatures. However, a KF post‐deposition treatment (PDT) of the ACIGSe absorber surface is necessary to achieve higher open‐circuit voltages (Voc). The present work provides microscopic insights to the effects of this KF PDT, employing correlative scanning‐electron microscope techniques on identical positions of cross‐sectional specimens of the cell stacks. We found that the increase in Voc after the KF PDT can be explained by the removal of Cu‐poor, Ag‐poor, and Ga‐rich regions near the ACIGSe/CdS interface. The KF PDT leads, when optimally doped, to a very thin K‐Ag‐Cu‐Ga‐In‐Se layer between ACIGSe and CdS. If the KF dose is too large, we find that Cu‐poor and K‐rich regions form near the ACIGSe/CdS interface with enhanced nonradiative recombination which explains a decrease in the Voc. This effect occurs in addition to the presence of a (K,Ag,Cu)InSe2 intermediate layer, that might be responsible for limiting the short‐current density of the solar cells due to a current blocking behavior.