Roll-to-roll printed stable and thickness-independent ZnO:PEI composite electron transport layer for inverted organic solar cells.

• ZnO:PEI nanocomposite was developed as electron transport layer for use in OSCs. • Higher stability of ZnO:PEI ink stored under ambient conditions more than 6 months. • Improved mechanical properties of the ZnO layer replaced by ZnO:PEI film. • Less thickness dependence for ZnO:PEI as electron transport layer fabricated by roll-to-roll printing. Large-scale roll-to-roll (R2R) micro-gravure printing is one of the most effective fabrication techniques to achieve the commercialization of organic solar cells (OSCs). In the high-performance OSC devices, the electron transport layer (ETL) plays an important role. In this paper, a composite ink of ZnO and polyethylenimine (PEI) was developed to fabricate large-area ETL for the inverted OSCs through R2R micro-gravure printing. It was found that the modification of ZnO nanoparticles by PEI effectively improved ink stability and enhanced the mechanical property. Using R2R micro-gravure printing, the thickness and coverage of the ZnO: PEI films were well regulated through changing the printing roller speed and web speed. Consequently, a high-power conversion efficiency around 6% was obtained for the flexible PTB7-Th: PC 71 BM device with the thickness of R2R printed ZnO: PEI layer up to 120 nm, demonstrating the high printability of this ZnO: PEI composite material for the use in printed flexible OSCs as the interface layer. [ABSTRACT FROM AUTHOR]