Formation of MoSe2 layer and Ga grading in flexible Cu(In, Ga)Se2 solar cell via Na diffusion

•The role of Na diffusion on CIGSe solar cells was studied.•Na diffusion promoted Ga grading and MoSe2 layer formation.•The overall effect of Ga grading and MoSe2 layer formation on CIGSe solar cells was studied.•PCE increased from 5.75% to 8.73% upon Na diffusion.•The collection efficiency, response in the NIR region and back-junction properties were improved. In the present work, Na diffused CIGSe thin-film solar cells have been fabricated on the flexible stainless-steel substrate. Compared with the controlled CIGSe sample, Na diffused CIGSe layer showed the growth of the MoSe2 layer at the CIGSe/Mo interface. A 600 nm thick Mo:Na layer behaved as a source for Na diffusion throughout the CIGSe layer. Na diffusion assisted MoSe2 layer grown at CIGSe/Mo junction visualized by HR-TEM analysis. Compared with the CIGSe/Mo layer, a thin ~3–5 nm MoSe2 layer has been observed at the CIGSe/Mo interface up on Na diffusion. Additionally, the Ga grading has been confirmed from the SIMS depth profile analysis. The CIGSe/Mo junction properties are improved in terms of better adhesion at the back junction, altered junction property from Schottky to ohmic contact, and formation of the back surface field due to the presence of the MoSe2 layer and Ga grading. The overall device performance has been improved and the photo-conversion efficiency (PCE) has been increased from 5.75% to 8.73% due to Na diffusion for CIGSe solar cells as compared to CIGSe solar cells without Na diffusion. The impact of Na diffusion-assisted MoSe2 layer formation and Ga grading on junction properties has been evaluated using electrical characteristics.