Developing flexible CIGS solar cells on stainless steel substrates by using Ti/TiN composite structures as the diffusion barrier layer

This study investigates the improved conversion efficiency of a Cu(In,Ga)Se2 (CIGS) solar cell developed on the flexible stainless steel (SS) substrates by using a Ti/TiN composite structure as the Fe ion diffusion barrier. The room-temperature photoluminescence measurement of TiN films with modulated nitrogen process flow showed improved donor-bound exciton recombination, and suppressed deep defect level emission, indicating that the crystallization of the thin films had improved. A three-stage CIGS evaporation process was adopted at a high substrate temperature of 550 °C for the high-quality absorption layer deposition. Secondary ion mass spectrometer measurements revealed that the Ti/TiN composite structure blocked Fe ion diffusion efficiently. An appropriate composition of sodium in the CIGS absorption layer was observed improving the open-circuit voltage and fill factor in the CIGS solar cells. Consequently, an optimal conversion efficiency of 8.9% of CIGS/SS solar cells with Ti/TiN composite structure was achieved without employing antireflective coating, which was close to the 9.1% value of CIGS cells fabricated on soda-lime glass substrates. The improved device performances in this study, demonstrating a high potential for developing high-performance flexible CIGS solar cells on SS substrates by using Ti/TiN composite structures as the diffusion barrier layer.