Boosting Bifacial Output Power of Ag‐CIGS Solar Cells by Optimizing ITO Back‐Contact Thickness for Tandem Applications

ABSTRACT Bifacial Cu(In,Ga)Se ₂ (CIGS) solar cells offer the unique advantage of harvesting light from both sides, enabling enhanced energy yield for diverse applications. However, their power conversion efficiency (PCE) remains constrained by two critical factors: (i) gallium oxide (GaO _x ) accumulation at the transparent conducting oxide (TCO)/absorber interface, and (ii) interface‐related recombination, which becomes more pronounced under rear‐side illumination. In this work, we demonstrate an optimized indium tin oxide (ITO) back contact strategy, where a pre‐Ga supply growth combined with Ag incorporation minimizes GaO _x interfacial formation and suppresses recombination losses. Systematic variation of ITO thickness revealed an optimal configuration that maximized device performance. The improved bifacial CIGS solar cells achieved 17.25% efficiency under front illumination, ranking second globally in the bifacial CIGS category, and 5.77% under rear illumination on glass substrates. We achieved outstanding device performance, delivering 19.05 mW cm ⁻² under 1.3‐sun (albedo) illumination and 23.03 mW cm ⁻² under 2.0‐sun illumination. When extended to four‐terminal (4 T) perovskite/CIGS tandem devices, this strategy resulted in 28.42 mW cm ⁻² under 1.3 sun albedo conditions and 32.26 mW cm ⁻² under 2.0‐sun illumination—representing the highest bifacial power‐generation density (BPGD) reported to date for bifacial 4T tandem architectures.