Transparent Back Contacts for Superstrate <sc>(Ag,Cu)(In,Ga)Se</sc><inline-formula><tex-math>$_{\bf 2}$ </tex-math></inline-formula> Thin Film Solar Cells

Abstract

Molybdenum oxide (MoO3) and tungsten oxide (WO3) are considered as transparent back contacts for (Ag,Cu)(In,Ga)Se-2 thin film solar cells. MoO3 and WO3 films were deposited by reactive RF sputtering at room temperature in an Ar/O-2 ambient on (Ag,Cu)(In,Ga) Se-2 absorber layers with various Ga/(Ga + In) and Ag/(Ag + Cu) ratios. Determination of the valence band offsets by XPS showed that Ag-alloying of absorber layer changes the energy band alignment at the absorber-back contact interface with MoO3 and WO3 contacts. This produces a primary contact with lower valence band offset compared with Cu(In,Ga) Se-2 counterparts. The effect is less significant in films with Ga > 0.5 and Ag > 0.5 (corresponding to E-g > 1.4 eV) probably due to the different nature of ordered vacancy compounds forming near the surface phases.