Analysis of the mechanism for enhanced crystalline quality of wide-bandgap Cu(In,Ga)Se2 films by pre-deposited Ag

Abstract

Wide-bandgap Cu(In,Ga)Se₂ (CIGS) solar cells show prospective utility for both single-junction and tandem solar cell applications. However, poor absorber layer quality and severe interface recombination hindered its widespread application. Here, a straightforward technique centered on the utilization of pre-deposited Ag layers at the CIGS/Mo interface, by controlling the preferential growth and ion-exchange processes through a traditional three-stage co-evaporation process, is proposed. We found that Ag doping not only effectively preserves the preferential orientation of the pre-layer and suppresses the formation of intermediate phases but also contributes to reducing the lattice energy of the (In,Ga)₂Se₃ film. Moreover, through calculations with empirical findings, we have demonstrated that Cu-based compounds, characterized by lower formation enthalpies (ΔH) compared to their Ag-based counterparts, initiate displacement reactions upon attaining the chemical stoichiometry of the absorber layer. This orchestrated interplay expedites interdiffusion between distinct elements within the thin film, culminating in the achievement of a high-quality absorber. Ultimately, a wide-bandgap solar cell characterized by an E_g ≈ 1.4 eV and an efficiency of over 16% was successfully obtained under conventional preparation temperatures. This strategy requires minimal modification of existing processes and demonstrates the potential for enhancing the efficiency of wide-bandgap CIGS solar cells.