Effects of NaF evaporation during low temperature Cu(In,Ga)Se2 growth

Co-evaporation of NaF during the 3rd stage of the low temperature Cu(In,Ga)Se2 multi-stage process is compared to post-deposition treatment (PDT) with NaF in view of their influence on the electronic and structural properties. In case of NaF co-evaporation, quantum efficiency losses in the near infrared region and thus lower short circuit current density cause a reduced efficiency compared to solar cells prepared with NaF PDT. The formation of a deep defect with activation energy of ~ 250 meV is measured by capacitance spectroscopy and can explain the deteriorated performance in such devices. In addition, NaF co-evaporation during the 3rd stage causes reduced grain size in the top part of Cu(In,Ga)Se2 and altered In, Ga, and Cu distribution.