Optimizing a CIGS Thin-Film Solar Cell with SILVACO ATLAS: Effects of Optical Bandgap and Absorber Electron Affinity

Abstract

This study uses TCAD numerical simulation to evaluate how key absorber parameters in Cu(In, Ga)Se 2 (CIGS) thin-film solar cells influence device performance, with the objective of identifying low-material, cost-effective optimization strategies. While crystalline-silicon (c-Si) still dominates the market despite its indirect bandgap and the thick wafers it requires, the CIGS pathway, featuring a direct, composition-tunable bandgap and a high absorption coefficient, on glass or flexible polymer substrates, offers a compelling alternative. The device investigated adopts the stack Al/ZnO/CdS/CIGS/Mo/PET and is simulated in SILVACO ATLAS (drift-diffusion transport coupled to Poisson and carrier-continuity equations) under conditions close to STC (AM1.5G, 27˚C, 1000 W·m − 2 ). Parameter sweeps cover the absorber optical bandgap g ∈ [ 1.14 , 1.50 ] eV, electron affinity χ ∈ [ 4.0 , 4.8 ] eV, and CIGS thickness d ∈ [ 0.1 , 3.0 ] μm, with p-type doping fixed at 1 × 10 16 cm − 3 . The results show that the short-circuit current density J s c is nearly invariant with respect to E g and χ once the absorber is sufficiently thick ( d ≥ 0.3 μ m ); a deviation appears at d = 0.1 μ m , attributed to stronger optical losses (residual transmission) and less efficient carrier collection. In contrast, the open-circuit voltage V o c increases markedly with E g over the investigated range (consistent with a reduced effective saturation current), which in turn raises the fill factor F F and the power-conversion efficiency η . The electron affinity χ has little influence on J s c (except for d < 0.3 μ m ), but it systematically impacts V o c , F F , and η \eta η via band alignment at the buffer/absorber interface. Within our parameter window, a maximum efficiency of about 27.05% is achieved for E g ≈ 1.50 eV with d = 3.0 μ m , where gains in V o c and F F compensate the near-invariance of J s c . Moreover, electron-affinity windows of χ ≈ 4.0 - 4.2 eV and χ ≈ 4.6 - 4.8 eV are shown to be favorable across 0.1 - 3.0 μm. These findings suggest that joint engineering of composition (Ga content) to tailor E g and of band alignment (through χ and the CdS/CIGS/ZnO interfaces) is a robust route to boost CIGS efficiency while minimizing material usage.