Transparent Back Contacts for Superstrate (Ag,Cu)(In,Ga)Se Thin Film Solar Cells

Molybdenum oxide (MoO₃) and tungsten oxide (WO₃) are considered as transparent back contacts for (Ag,Cu)(In,Ga)Se₂ thin film solar cells. MoO₃ and WO₃ films were deposited by reactive RF sputtering at room temperature in an Ar/O₂ ambient on (Ag,Cu)(In,Ga)Se₂ 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)Se₂ 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.