Size and grain-boundary effects on the performance of polycrystalline CIGS-based solar cells

This work reviews the effect of Geometrical, physical and electrical parameters of the polycristalline CIGS thin film used as absorber materials in substrate Culn_0.7Ga_0.3Se₂(CIGS) solar cells. Two-dimensional device simulator Atlas SILVACO-TCAD was employed to study the performances of ZnO:Al/CdS/CIGS/Metal solar cell structure. The impacts of the grain sizes and the grain boundaries (GBs) in the polycrystalline p-CIGS absorber layer have been investigated. The variation of grain sizes in the CIGS bulk was studied and the corresponding design optimization was provided. The best energy conversion efficiencies have been obtained with large grain sizes higher than 2 μm for 3 μm-CIGS thick. The simulation results predict a strong detrimental effect of GBs recombination, which is enhanced by the presence of small width in the direction that attracts minority carriers. An efficiency of 17.1 % (with V_oc ≈ 0.68 V, J_sc ≈ 34 mA/cm² and FF ≈ 0.77) has been achieved with small width at about 3 nm. The presence of the valence-band offset in the absorber layer is benign to solar cell performance by limit the carriers recombination. The valence-band offset is predicted to be 0.4 e V in magnitude and localized to a very thin layer at the grain surface in which the surface reconstruction takes place. All these simulation results give some important indication to lead a higher efficiency of polycrystalline CIGS solar cells for feasible fabrication.