First-principles analysis of band offsets, phase stability, and alkali incorporation in (Ag,Cu)(In,Ga)Se₂ solar cells

Over the years, thin-film photovoltaics based on Cu(In,Ga)Se₂ (CIGS) have undergone continuous improvements, with recent efficiency records indicating that CIGS is yet to unleash its full potential. Ag-alloying (forming ACIGS) is emerging as an exciting focus area, since ACIGS demonstrates better crystallinity and lower V_OC losses than CIGS alone. In this work, we investigate the fundamental origin of these improvements by performing detailed first-principles analysis of ACIGS alloys. Based on the computed electronic properties, we establish a correlation between the composition and conduction band minimum, which allows rational tuning of band offsets at ACIGS/buffer (CdS or Cd-free) interface with a desired band gap of the absorber. From a thermodynamic analysis, we reveal the existence of a miscibility gap that can induce phase separation and Ag grading during thin film growth. Furthermore, from a defect analysis, we show that Ag promotes solubility of alkalis during post-deposition treatment. The obtained results are verified by measuring device and material characteristics for different ACIGS films, which showed a good agreement with the theoretical predictions.