Deep surface Cu depletion induced by K in high‐efficiency Cu(In,Ga)Se₂ solar cell absorbers

Abstract

Abstract:In this work we used K-rich glass substrates to provide potassium during the co-evaporation of CIGS absorber layers. Subsequently, we applied a post-deposition treatment (PDT) using KF or RbF to some of the grown absorbers. It was found that the presence of K during the growth of the CIGS layer led to cell efficiencies beyond 17%, and the addition of a PDT pushed it beyond 18 %. The major finding of this work is the observation of discontinuous 100-200 nm-deep Cudepleted patches in the vicinity of the CdS buffer layer, correlated with the presence of K during the growth of the absorber layer. The PDT had no influence on the formation of these patches. A second finding concerns the composition of the Cu-depleted areas, where an anti-correlation between Cu and both In and K was measured using scanning transmission electron microscopy (STEM). Furthermore, a steeper Ga/(In+Ga) ratio gradient was measured for the absorbers grown with the presence of K, suggesting that K hinders the group III element inter-diffusion. Finally, no Cd in-diffusion to the CIGS layer could be detected. This indicates that if Cd Cu substitution occurs, either their concentration is below our instrumental detection limit, or its presence is contained within the first 6 nm from the CdS/CIGS interface.

Secondary phase formation and surface modification from a high dose KF‐post deposition treatment of (Ag,Cu)(In,Ga)Se2solar cell absorbers

“…The use of a K‐rich/Na‐poor glass substrate together with a Cu‐[poor‐rich‐poor] co‐evaporation sequence can lead to the formation of Cu‐depleted patches at the surface of the absorber. Such patches are visible in this work but will not be further investigated here as they have already been extensively studied in previous works . Group I surface depletion can easily be identified in Figure looking at the Cu and Ag signal fluctuations in the no‐KF sample.…”

“…Then, all samples received a 15‐nm NaF precursor layer, made by thermal evaporation under a quartz crystal monitor control, to ensure a sufficient Na supply to the ACIGS absorber layer. The absorber layers were co‐evaporated following a Cu‐poor, Cu‐rich, Cu‐poor profile as described in Donzel‐Gargand et al, with average Ag/(Ag + Cu) (Ag/I) and Ga/(Ga + In) (Ga/III) ratios maintained for all the samples at 0.18 ± 0.01 and 0.40 ± 0.04, respectively. We should emphasize that the Ga/III profile was graded in the vicinity of the back‐contact leading to an optical band gap as determined from EQE measurements of about 1.22 ± 0.02 eV as detailed in the previous work using similar ACIGS absorber composition and grading …”

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

“…If the main effect of the PDT with heavy alkalis is in the bulk and not at the surface, the question arises if the heavy alkalis could not be added before or during growth, instead of after the growth by a PDT. Several studies that compare the addition of heavy alkalis before or during growth with the PDT exist . They all find that PDT is favorable, because the addition of heavy alkalis before or during growth leads to smaller grains, a deep defect, or increased inhomogeneity .…”

“…Several studies that compare the addition of heavy alkalis before or during growth with the PDT exist . They all find that PDT is favorable, because the addition of heavy alkalis before or during growth leads to smaller grains, a deep defect, or increased inhomogeneity . Thus, it appears that heavy alkalis during growth hinder the formation of large homogenous ordered crystals, and so it is necessary, to grow a high‐quality film first and then modify the grain boundaries by an alkali‐PDT to reduce their recombination activity.…”

No Evidence for Passivation Effects of Na and K at Grain Boundaries in Polycrystalline Cu(In,Ga)Se2 Thin Films for Solar Cells

“…Recently, several reports highlighted the formation of noncontiguous (island‐like) K‐In‐Se layers at the CIGSe/buffer interface with thicknesses of few nanometers after KF treatments. Therefore, it is assumed that the very thin, island‐like K‐In‐Se layers passivate the corresponding interfaces, leading to reduced recombination, thus, to increased open‐circuit voltage, without deteriorating the short‐circuit current substantially.…”