A fluorine-induced high-performance narrow bandgap polymer based on thiadiazolo[3,4-c]pyridine for photovoltaic applications

Thiadiazolo[3,4-c]pyridine (PT) has great potential in the construction of high-performance narrow bandgap (NBG) photovoltaic polymers. But to date the best power conversion efficiencies (PCEs) for PT-containing polymers are only around 6%. Herein, we report two PT-containing NBG polymers PDTPT-2T and PDTPT-2TF based on 2,2′-bithiophene (2T) and 3,3′-difluoro-2,2′-bithiophene (2TF), respectively. The effects of a fluorine substituent on optoelectronic properties are thoroughly investigated. The film absorption onset of PDTPT-2TF is 855 nm, bathochromic-shifted by 24 nm in comparison with that (831 nm) of PDTPT-2T. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels of PDTPT-2TF are down-shifted by 0.15 and 0.11 eV relative to those of PDTPT-2T, respectively. X-ray diffraction (XRD) patterns indicate that a more ordered structure is formed in the solid film of PDTPT-2TF. Furthermore, the miscibility between the polymer and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is significantly improved through the introduction of fluorine. Consequently, PDTPT-2TF exhibits a high PCE of 8.01%, while PDTPT-2T only shows a maximum PCE of 2.65%. The efficiency of 8.01% is the highest one for PT-containing polymers, and more importantly, it is achieved without any processing additives or post-treatments. This work indicates that PT would have great potential as a building block to construct high-performance photovoltaic polymers.