Conjugated versus Nonconjugated Polymerized Small-Molecule Acceptors. Photovoltaic Response and Mechanical Properties

Recently, the power conversion efficiencies (PCEs) of all-polymer solar cells (all-PSCs) have surpassed 18%, principally reflecting the introduction of polymerized small-molecule acceptors (PSMAs). However, the effect of PSMA backbone conjugation on both device photovoltaic and mechanical properties has been sparsely investigated. Here we report the synthesis of two PSMAs having fully conjugated or partially nonconjugated backbones by copolymerizing a 2,2′-((2Z,2′Z)-((12,13-bis(2ethylhexyl)-3,9-diundecyl-12,13-dihydro[1,2,5] thiadiazolo[3,4- e ]thieno[2″,3″:4′,5′]thieno[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2- b ]indole-2,10-diyl)bis(methanylylidene))bis(6-fluoro-7-bromo-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))-dimalononitrile) (YF-Br) core with thienylene-vinylene-thienylene (TVT) and thienylene-ethyl-thienylene (TET) units, respectively. The resulting PYFTVT and PYFTET polymers are then blended with the conjugated donor polymer PM6 to fabricate all-PSCs. The results reveal that the PCE and device stability upon mechanical deformation are dominated by the PSMA backbone conjugation and molecular mass. Thus, for comparable molecular masses, the PYFTET-based blend exhibits higher crack onset strain and toughness than the PYFTVT-based blend. Furthermore, increasing the PSMA molecular mass enhances the mechanical ductility. Overall, these results convey important implications for developing future ductile and stretchable electronics.