Alternative sodium sources for Cu(In,Ga)Se2 thin-film solar cells on flexible substrates

Abstract Sodium (Na) is an important doping element for Cu(In,Ga)Se 2 (CIGS) solar cells. However, when using Na-free flexible substrates like steel foil or polyimide film, it is necessary to ensure an efficient supply of sodium to achieve high cell efficiencies. The common incorporation methods for Na on these Na-free substrates are either to deposit a Na-containing precursor layer (e.g. NaF) onto the molybdenum (Mo) back contact prior to CIGS growth or to coevaporate a Na compound during CIGS growth. Another way is to incorporate sodium after CIGS growth by a post-deposition treatment with NaF. In this work, we tested two alternative Na doping methods which are well suited for a production line due to their easy controllability. One approach is to dope the molybdenum target with Na. With Na-doped Mo layers (Mo:Na) as the back contact, we could achieve efficiencies of 13.1% both on titanium (Ti) and stainless Cr steel foil using a single-stage inline CIGS process. With a low-temperature single-stage CIGS process on polyimide (PI) we reached an efficiency of 11.2% using a Mo:Na back contact. Another doping method involves sol–gel-deposited silicon oxide layers which contain Na (SiO x :Na). We have successfully deposited these sol–gel layers onto stainless steel foil by a roll-to-roll (R2R) method with short annealing times as needed in production. With these SiO x :Na layers we could achieve efficiencies of 13.7% on stainless steel foil and 11.5% on mild steel sheet using a single-stage inline CIGS process.