11% efficiency on CIGS superstrate solar cells without sodium precursor

Cu(In,Ga)Se₂ superstrate solar cells were prepared by vacuum evaporation of the elements on glass substrates coated with a ZnO/ZnO:Al bi-layer. Large grain size in the Cu(In,Ga)Se₂ layer was obtained with a modified three stage process while no additional source of sodium was used. As deposited devices showed a low fill factor due to a voltage dependent carrier collection. The fill factor and the open circuit voltage improved considerably upon light soaking. Efficiencies of up to 11.2% have been obtained. The capacitance voltage characteristics reveal a low effective carrier density in Cu(In,Ga)Se₂ of about 1.10¹⁴ cm^-3 for as-deposited cells. Light soaked cells show an increase in the carrier density by one order of magnitude which gives rise to a higher open circuit voltage. Additionally, measurements of the spectral response show an enhanced carrier collection under forward bias conditions.