Improving the Photovoltaic Performance and Mechanical Stability of Flexible All-Polymer Solar Cells via Tailoring Intermolecular Interactions

Naphthalene diimide (NDI)-based copolymers are promising polymer acceptors in all-polymer solar cells (all-PSCs), but their large crystal domains cause large-scale phase separation in all-polymer blend films. This limits the photovoltaic performance and mechanical stability of all-PSCs. Herein, we control all-polymer blend films by introducing a fluorinated copolymer of NDI and (E)-1,2-bis(3-fluorothiophen-2-yl)ethene (FTVT) (PNDI-FTVT) as a polymer acceptor for flexible all-PSCs. The copolymer PNDI-FTVT has a less crystalline structure and higher electron mobility than its nonfluorinated copolymer counterpart (PNDI-TVT). A blended film incorporating PNDI-FTVT exhibits a well-mixed morphology and improves the chain interconnectivity with a polymer donor, providing better charge transport pathways and enhanced mechanical resilience. The PNDI-FTVT-based flexible all-PSC exhibits enhanced photovoltaic performance in comparison with a PNDI-TVT-based flexible all-PSC (5.11-7.14%) as well as excellent mechanical stability in a flexible all-PSC (7.14-5.78%), maintaining 81% of its initial performance at a bending radius of 8.0 mm after 1000 bending cycles.