Investigation of Al2O3 diffusion barrier layer fabricated by atomic layer deposition for flexible Cu(In,Ga)Se2 solar cells

The use of Al2O3 fabricated by atomic layer deposition (ALD) as a metal diffusion barrier between the stainless steel substrate and the back contact layer in flexible Cu(In,Ga)Se2 (CIGS) photovoltaic (PV) devices was found to reduce metal ion diffusion from the substrate and reduce the number of defects at the CIGS absorber layer, as determined from the secondary ion mass spectrometry (SIMS) depth profile and quantitative defect analysis using C–V measurements. Cells with Al2O3 barrier layers were found to show higher efficiency and uniformity compared to cells with ZnO barrier layers. XRD pattern analysis showed the Al2O3 barrier layer's amorphous characteristic which can form a complex diffusion path. In addition, quantum efficiency (QE) analysis of the cells showed that the main advantage of using an Al2O3 barrier layer is derived from the increase in the current density due to the decrease in the number of recombination sites resulting from the decrease in the number of defects due to the amorphous nature of the layer. Therefore, cells with an Al2O3 barrier layer fabricated by ALD showed better average conversion efficiency and uniformity (11.23 ± 1.86%) compared to cells with a ZnO barrier layer fabricated by sputtering. Ongoing advancements in ALD processes make the use of Al2O3 barrier layers promising for obtaining large-scale flexible solar cells.