Influence of the Cu(In,Ga)Se2 thickness and Ga grading on solar cell performance

Thin-film solar cells with Cu(In,Ga)Se-2 (CIGS) absorber layers ranging from 1.8 to 0.15 mum in thickness were fabricated by co-evaporation, with both homogeneous and Ga/(Ga + In) graded composition. The absorption of the CIGS layers was determined and compared with corresponding QE measurements in order to obtain the optical related losses. The material characterization included XRD as well as cross-sectional SEM analysis. Devices with CIGS layers of all thicknesses were fabricated, and down to 0.8-1 mum they showed a maintained high performance (eta similar to 15%). When the CIGS layer was further reduced in thickness the loss in performance increased. The main loss was observed for the short-circuit current, although the loss was not only due to a reduced absorbance. The open-circuit voltage was essentially not affected by the reduction of the CIGS thickness, while the fill factor showed a slight decrease. The fill factor loss was eliminated by introducing a Ga/(Ga + In) graded CIGS, which also resulted in an increased open-circuit voltage of 20-30 mV for all CIGS thicknesses. Device results of 16.1% efficiency at 1.8 mum CIGS thickness, 15.0% at 1.0 mum and 12.1% at 0.6 mum (total area without anti-reflective coating) were achieved. Copyright (C) 2002 John Wiley Sons, Ltd.