Bifacial semi-transparent ultra-thin Cu(In,Ga)Se2 solar cells on ITO substrate: How ITO thickness and Na doping influence the performance

Ultra-thin Cu(In,Ga)Se2 (CIGSe) is a promising absorber for thin-film solar cells, as it combines the advantages of low raw material consumption and high conversion efficiency. In addition, ultra-thin absorbers on transparent back contacts bring the advantage of semitransparency, which is essential for e.g. tandem or bifacial solar cells. This work optimizes ultra-thin CIGSe on In2O3:Sn (ITO) for application in bifacial semi-transparent ultra-thin (BSTUT) CIGSe solar cells. Firstly, 100–400 nm ITO were coated onto glass substrates, and it was revealed that the thickness of ITO influences its optical bandgap Eg due to the Burstein-Moss (B-M) shift. The band gap of 400 nm ITO increased by 0.14 eV compared to the 100 nm thick ITO, and the Voc of the related BSTUT CIGSe solar cells raised by 0.043 V as a result of the diminished Schottky barrier Φb at the ITO/CIGSe interface. Secondly, 0–8 mg of NaF used for post deposition treatment (PDT) of the CIGSe were applied to the BSTUT solar cells. Compared to the reference without NaF, 8 mg NaF PDT enhanced the carrier density NA from 2 × 1015cm−3 to 1.2 × 1016cm−3 and diminished the ITO/CIGSe Schottky barrier Φb by 0.21 eV. In conclusion, we found that NaF PDT can tune the carrier density of ultra-thin CIGSe on ITO, and both thicker ITO and higher NaF PDT dose can reduce the ITO/CIGSe Schottky barrier. These discoveries enable future optimization of BSTUT CIGSe solar cells.