Alternative alkali fluoride post‐deposition treatment under elemental sulfur atmosphere for high‐efficiency Cu(In,Ga)Se2‐based solar cells

Up to now, what we know about the impact of alkali post‐deposition treatment (PDT) on Cu(In,Ga)Se2 (CIGSe) absorber thin films is largely based on treatments performed in selenium atmosphere and only few studies have addressed the critical role of the chalcogen atmosphere during the PDT. The present study deals with an innovative process of alkali fluoride PDT under elemental sulfur atmosphere on co‐evaporated Cu(In,Ga)Se2 absorbers. With the aim to understand the effects of different the incorporated alkali element incorporated during the PDT, we investigate four different PDTs: CsF, NaF/RbF, RbF, and In + RbF—all under sulfur atmosphere. The treated absorbers are characterized by scanning electron microscopy, Raman spectroscopy, and photoluminescence spectroscopy. Our results show that for CIGSe compositions close to stoichiometry, forming a slightly Cu‐poor CIGSe at the surface during the PDT is beneficial. Cu(In,Ga)Se2/RbF(S) and Cu(In,Ga)Se2/In + RbF(S) exhibit the higher photoluminescence response probably due to decreased surface recombination. The quasi‐Fermi‐level splitting is in good agreement with the observed Voc difference between the treated and reference samples. The electronic properties of the Cu(In,Ga)Se2/In + RbF(S)‐based solar cells show a significantly improved performance with high Voc and FF. RbF post‐deposition treatment (PDT) under sulfur atmosphere could be a key parameter to further boost the Cu(In,Ga)Se2 (CIGSe)‐based solar cell efficiency. CIGSe/RbF(S) and CIGSe/In + RbF(S) absorbers show higher photoluminescence response than the CIGSe/REF (without PDT), CIGSe/CsF(S), and CIGSe/NaF/RbF(S) absorbers, suggesting a reduced surface recombination activity and/or increased concentration of radiative transitions. The electronic properties of the CIGSe/In + RbF(S)‐based solar cells show a significantly improved device performance due to their high open‐circuit voltage and fill factor values.