Sodium-doped molybdenum back contacts for flexible Cu(In,Ga)Se2 solar cells

Abstract

The presence of Na in Cu(In,Ga)Se2 (CIGS) is well known to improve the solar cell performance. To incorporate Na into the CIGS absorber, Na-doped Mo (MoNa) back contact layers were grown on stainless steel foils. Three different back contact designs deposited from MoNa sputtering targets with Na concentrations of 3at.%, 5at.% and 10at.% were investigated. A multistage CIGS evaporation process at low (~450°C) substrate temperature was used to deposit the absorbers.Measurements from time-of-flight secondary ion mass spectroscopy depth profiles indicate that Na is preferentially collected at the internal interfaces and out-diffuses from the grain boundaries of the multilayer MoNa back contact into the CIGS absorber. From the [Ga]/([In]+[Ga]) grading profiles, a more pronounced Ga dip was found with increasing Na concentration. Moreover, at high Na concentrations (10at.% MoNa target), a change in the CIGS texture was observed by X-ray diffraction. Best solar cell efficiency of 14.4% was achieved for the 5at.% MoNa sample without antireflective coating, which is a significant improvement compared to the 9.8% efficiency measured for the Na-free reference.