Sodium-doped Mo back contacts for Cu(In,Ga)Se2 solar cells on Ti foils: Growth, morphology, and sodium diffusion

Abstract

Sodium addition is necessary to reach high efficiencies with Cu(In,Ga)Se2 (CIGS) solar cells on metallic substrates. This can be achieved using DC-sputtered multilayer Mo back contacts including a sodium-doped layer (Mo:Na). In this study, 450 nm-thick Mo:Na layers were sputter-deposited on Ti foils using a working pressure ranging from 3.4 mTorr to 30 mTorr, and capped with 100 nm-thick, dense, pure Mo layers. The deposition pressure of the Mo:Na layers strongly affected their electrical resistance, optical reflectance, morphology, and crystal quality. The top Mo layer allowed controlling the electrical and optical properties of the back contact, but its own morphology was affected by the structure of the underlying Mo:Na layer. The Na concentration in the CIGS layer varied as a function of the deposition pressure of the Mo:Na layer, inducing changes in the absorber grain size and Ga distribution, as well as in the carrier density. Absolute conversion efficiency improvements over 3% were obtained using the sodium-doped back contacts on Ti foils, leading to a performance which was equivalent to the soda-lime glass reference.