Synthesis and photovoltaic performance of novel thiophenyl-methylene-9H-fluorene-based low bandgap polymers

Three novel donor-acceptor polymers have been synthesized with Suzuki cross-coupling between thiophenyl-methylene-9H-fluorene donating unit with thiazolo[5,4-d]thiazole (PFTTTz), benzothiadiazole (PFTODTBT) or [1,2,3]triazolo[4,5-g]quinoxaline (PFTDTBTzQ) accepting unit. The polymer PFTTTz-THF prepared from tetrahydrofuran (THF) presented higher molecular weight and narrower polydiversity than that from toluene (PFTTTz-toluene), leading to higher photovoltaic performance in BHJ devices of these polymers blended with PC71BM ([6,6]-phenyl-C71-butyric acid methyl ester). These good solution processable polymers exhibit a bandgap of 1.66–1.93 eV and a low highest occupied molecular orbital (HOMO) energy level ∼ −5.32 eV. Polymers PFTTTz and PFTODTBT displayed strong absorption in the range of 300–650 nm, while PFTDTBTzQ showed a further 100 nm extended absorption band. Overall efficiencies over 1.5% are achieved for BHJ devices fabricated from blends of PFTTTz with PC71BM as active layer. A maximum power conversion efficiency of 2.21% is obtained by the use of interlayer poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3′-( N, N-dimethylamino)propyl)-2,7-fluorene)] (PFN) and Ca between active layer and Al anode.