Alkali Dispersion in (Ag,Cu)(In,Ga)Se₂ Thin Film Solar Cells—Insight from Theory and Experiment

Abstract

Silver alloying of Cu(In,Ga)Se 2 absorbers for thin film photovoltaics offers improvements in open-circuit voltage, especially when combined with optimal alkali-treatments and certain Ga concentrations. The relationship between alkali distribution in the absorber and Ag alloying is investigated here, combining experimental and theoretical studies. Atom probe tomography analysis is implemented to quantify the local composition in grain interiors and at grain boundaries. The Na concentration in the bulk increases up to ∼60 ppm for [Ag]/([Ag] + [Cu]) = 0.2 compared to ∼20 ppm for films without Ag and up to ∼200 ppm for [Ag]/([Ag] + [Cu]) = 1.0. First-principles calculations were employed to evaluate the formation energies of alkali-on-group-I defects (where group-I refers to Ag and Cu) in (Ag,Cu)(In,Ga)Se 2 as a function of the Ag and Ga contents. The computational results demonstrate strong agreement with the nanoscale analysis results, revealing a clear trend of increased alkali bulk solubility with the Ag concentration. The present study, therefore, provides a more nuanced understanding of the role of Ag in the enhanced performance of the respective photovoltaic devices.

Performance Limitations of Wide‐Gap (Ag,Cu)(In,Ga)Se2 Thin‐Film Solar Cells

“…It was further found that the presence of Na in the absorber bulk plays a key role in the doping mechanism, usually leading to increased majority carrier densities. [63][64][65][66][67] As the solubility limit of Na in the ACIGS lattice (on V I ) increases with the Ag content, [68] it may be speculated that Na saturation is not reached during absorber processing, resulting in lower doping. Further studies are needed to reveal the exact relationship between doping, stoichiometry and Na distribution.…”

DLTS investigations on CIGS solar cells from an inline co-evaporation system with RbF post-deposition treatment

“…This could be linked with the spatial origin of the defects. From atom probe tomography (APT) measurements it is known that PDT has the effect that Rb [15] and other alkali [16] segregate at grain boundaries and interfaces. In this study, only a reduction of the trap density of E1 and not H2 is observed.…”

“…In contrast, the H2 trap is located in grain interior regions or is distributed more homogeneously through the layer. Alkali elements have a limited solubility in CIGS grain interiors [16] and therefore a significant passivation of the point defect H2 is not possible. Segregated at grain boundaries, the Rb atoms can occupy the defects and passivate them.…”

Surface/Interface Effects by Alkali Postdeposition Treatments of (Ag,Cu)(In,Ga)Se2 Thin Film Solar Cells

“…A comparison with the HAXPES results presented above indicates that there may be a correlation between the observed compositional changes and device properties for the CsF and RbF treated samples. The higher V OC for RbF as compared to CsF treated samples could possibly be connected to higher Ag surface content (thus higher acceptance of alkali elements in the absorber 33 and lower VBM expected), together with lower Cu content (known to promote band gap widening in CIGS). Still, no significant difference in the VB spectra has been observed between the RbF and CsF treated samples as discussed above, which might be masked by the increased bulk sensitivity of the HAXPES measurements, and thus the shift cannot be resolved.…”