Highly efficient photovoltaics and field-effect transistors based on copolymers of mono-fluorinated benzothiadiazole and quaterthiophene: Synthesis and effect of the molecular weight on device performance

A new D-A conjugated polymer, namely poly[(5-fluoro-2,1,3-benzothiadiazole-4,7-diyl)-alt-(3′,4′′di(2-octyldodecyl)-2,2′:5′,2′′:5′′,2′′′-quaterthiophen-5,5′′′-diyl)] (P4TFBT), was designed and synthesized by using mono-fluorinated benzothiadiazole (FBT) as the acceptor and quaterthiophene (4T) as the donor. Two samples of P4TFBT with the same polydispersity index but different molecular weights (low molecular weight: L-P4TFBT; high molecular weight: H-P4TFBT) were obtained. Both L-P4TFBT and H-P4TFBT showed similar absorption bands from 300 nm to around 800 nm in the visible region, and the close optical band gaps at ca. 1.60 eV. Bulk heterojunction solar cells based on the blend of L-P4TFBT or H-P4TFBT with [6,6]phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) exhibited high power conversion efficiencies (PCEs) of 5.80% and 7.45%, respectively. H-P4TFBT also revealed a high p-channel organic field-effect transistor mobility of 0.39 cm² V^-1 s^-1. These encouraging results indicate that P4TFBT is a promising low band gap material with high performance PSCs and OFETs simultaneously.