Facile synthesis of high-performance nonfullerene acceptor isomers via a one stone two birds strategy

Abstract

Two isomeric fused undecacyclic electron acceptors (FUIC and i-FUIC) are designed and synthesized via a one stone two birds strategy based on core unit 4TT (four thienothiophene units condensed with three cyclopentadienyl rings) and end groups 2FIC (2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile). The only difference is the two attachment positions between 4TT and 2FIC, α, α-positions for FUIC and α, β-positions for i-FUIC. FUIC exhibits stronger and red-shifted absorption with an ultra-narrow bandgap of 1.22 eV (1.28 eV for i-FUIC), a down-shifted lowest unoccupied molecular orbital energy level of −4.06 eV (−3.99 eV for i-FUIC), the same highest occupied molecular orbital energy level of −5.31 eV and a higher electron mobility of 6.8 × 10⁻⁴ cm² V⁻¹ s⁻¹ (3.0 × 10⁻⁴ cm² V⁻¹ s⁻¹ for i-FUIC). Organic solar cells based on a blend of PTB7-Th:FUIC exhibit a power conversion efficiency as high as 11.2% with a high fill factor of 70.6%, higher than that of PTB7-Th:i-FUIC (10.3%).