Symmetry-Induced Ordered Assembly of a Naphthobisthiadiazole- Based Nonfused-Ring Electron Acceptor Enables Efficient Organic Solar Cells

Abstract

Nonfused-ring electron acceptors (NFREAs) havereceived increasing attention for use in organic solar cells (OSCs)because of their synthetic simplicity and tunable optical spectra.However, their fundamental molecular interactions and themechanism by which they govern the property−function relationsof OSCs remain elusive. Here, to investigate the effects of thestructural symmetry of NFREAs, two acceptor−donor−acceptor′−donor−acceptor (A−D−A′−D−A)-type NFREAs, 2,2′-(((naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole-5,10-diylbis(4,4-bis-(2-butyloctyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-6,2-diyl))-bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-in-dene-2,1-diylidene))dimalononitrile (NTz-4F) and 2,2′-(((benzo[c][1,2,5]thiadiazole-4,7-diylbis(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-6,2-diyl))bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))-dimalononitrile (BT-4F), are designed and synthesized. They have different A′ cores: NTz-4F has a modified centrosymmetric NTzcore, whereas BT-4F has a modified axisymmetric BT core. In pristine films, the NTz-4F, which has a centrosymmetric core, showssubstantially enhanced intermolecular interaction and microstructural crystalline ordering compared with BT-4F, which has anaxisymmetric core. Even in blends with poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8,-dione))] (PBDB-T), NTz-4F retains its highlycrystalline structure, whereas BT-4F loses crystalline packing. These changes in NTz-4F result in increased electron transport andsuppressed nonradiative voltage loss, resulting in a power conversion efficiency of 9.14% for PBDB-T:NTz-4F vs 7.18% for PBDB-T:BT-4F. This work demonstrates that centrosymmetric-structured cores are promising building blocks for high-performanceNFREA-based OSCs.