No abstract

Up to now, what we know about the impact of alkali post-deposition treatment (PDT) on Cu(In,Ga)Se 2 (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 coevaporated Cu(In,Ga)Se 2 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)Se 2 /RbF(S) and Cu(In,Ga)Se 2 /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 V oc difference between the treated and reference samples. The electronic properties of the Cu(In,Ga)Se 2 /In + RbF(S)-based solar cells show a significantly improved performance with high V oc and FF.

The original goal of our study is to synthesize by co-evaporation the phase that could be formed at the interface between polycrystalline p-Cu(In,Ga)Se 2 treated with KF and n-CdS. Hence, a new buffer layer, CdIn 2 S 4 (C24), deposited by co-evaporation is presented for the use in thin film solar cells, exhibiting device efficiencies as high as 16.2%, comparable to that obtained on a reference standard CdS-buffered device. The physico-chemical and optical properties of close to stoichiometry 400 nm-thick films of C24 show similar properties to what has been reported in the literature for single crystals. The layer stack used for solar cells is investigated by transmission electron microscopy, showing the formation of an ultrathin Cd-deficient C24 layer at the CIGSe/C24 interface, while a clear lattice match is observed at the C24/ZnO interface. Advanced electrical characterizations of the devices suggest that the output voltage and fill factor of the solar cells based on Cu(In,Ga)Se 2 /(PVD)C24 are limited by tunneling-enhanced recombination through extended band tail states. These results open new routes to explain the superiority of wet processes used for the junction formation compared to vacuum-based approaches.