Influence of Na on Cu(In,Ga)Se2 solar cells grown on polyimide substrates at low temperature: Impact on the Cu(In,Ga)Se2/Mo interface

There are still open questions regarding the nature of the positive effect of the presence of Na on the performance of Cu(In,Ga)Se-2 based, chalcopyrite thin film solar cells, especially at low processing temperatures. Studying Cu(In,Ga)Se-2 thin film devices fabricated from low-temperature coevaporated absorbers on polyimide substrates by admittance and J-V-T measurements, characteristic properties are identified for different amounts of Na present during the growth. A roll-over behavior can be directly correlated with the Na-content. X-ray photoelectron spectroscopy shows the development of a MoSe2 phase at the back contact of the device. Efficiencies of 15.1% with MgF2 antireflection coating are demonstrated.