Solar cells with thin Cu(In1-xGax)Se₂ absorbers: Optical analysis and quantum efficiency simulations

We present in-depth quantum efficiency analyses of of Cu(In,Ga)Se₂ (CIGS) solar cells. Ex-situ spectroscopic ellipsometry (SE) analysis is applied to partially and fully completed solar cells with standard thickness and thin CIGS absorbers. Optical properties and multilayer structural data are deduced and used to predict the maximum obtainable quantum efficiency spectra and short-circuit current densities (J_sc). We validate optical model development and the resulting quantum efficiency (QE) simulations with experimental results for CIGS solar cells incorporating standard 2.2 μm thick absorbers. We find that both the bulk CIGS layer and the CdS-CIGS interface layer serve as active layer components and together contribute 100% of the photo-generated current. Thus, essentially all photo-generated carriers are collected from these layers. Solar cells with thin absorbers were also fabricated and efficiencies of 13.2% at 0.73 μm CIGS thickness, 10.1% at 0.50 μm and 8.0% at 0.36 μm were obtained. Although J_sc is expected to decrease with decreasing absorber thickness due to reduced optical collection, modeling results suggest that electronic losses are also occurring upon thinning the absorber, ranging from ∼ 1.3 to 1.9 mA/cm² for cells with CIGS thicknesses from 0.73 to 0.36 μm, respectively.