Structure and interface chemistry of MoO3 back contacts in Cu(In,Ga)Se2 thin film solar cells

Molybdenum oxide (MoO3) is considered as a possible primary back contact for Cu(InGa)Se2 thin film solar cells for its potential as a transparent back contact for superstrate and bifacial devices. MoO3 films were deposited on Mo or ITO-coated soda lime glass substrates by reactive rf sputtering in an ambient of Ar + O2 with O2/(O2 + Ar) = 35% on which Cu(In0.7Ga0.3)Se2 alloy absorber layers were deposited using multi-source elemental evaporation. Scanning Electron Microscopy studies showed uniform coverage of the as-deposited MoO3 layer and good adhesion was obtained in all cases. X-ray Photoelectron Spectroscopy depth profile analysis showed that MoSe2 was not formed at the Cu(InGa)Se2 interface with either the Mo-MoO3 or ITO-MoO3 back contacts. Determination of the valence band offsets showed that the MoO3 layer at the interface changes the energy band alignment with Cu(InGa)Se2, producing a primary contact with lower valence band offset than ITO. Cu(In,Ga)Se2 thin film solar cells prepared using an as-deposited Mo-MoO3 back contact yielded a best conversion efficiency of 14%, with VOC = 647 mV, JSC = 28.4 mA/cm2, and FF = 78.1%. Cells with ITO-MoO3 back contact showed a best efficiency of 12%, with VOC = 642 mV, JSC = 26.8 mA/cm2, and FF = 69.2%