Effect of fluorination and symmetry on the properties of polymeric photovoltaic materials based on an asymmetric building block

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Abstract
Conjugated polymers based on an asymmetric dithieno[3,2-b:2′,3′-d]pyran (DTPa) donor and benzothiadiazole (BT), mono-fluorinated benzothiadiazole (fBT) or di-fluorinated benzothiadiazole (ffBT) acceptors were designed and synthesized. The introduction of fluorine substituents in the BT unit could not only enhance the electronegativity of the acceptors, but also change the symmetry of the BT derivatives, and thus affect the optical, electrochemical, and optoelectronic performance of the final polymers. With the increase of fluorine atoms in the BT unit, the peaks of the absorption spectra for these three polymers hypsochromic shift gradually, combined with the decrease of the HOMO energy levels. The asymmetric structure of fBT results in more complex multichromophore systems and consequently shows a broader absorption FWHM (full-width-at-half-maximum) of 229 nm in chloroform, as well as low absorption intensity and charge carrier mobility for polymer PDTPa-fBT. Finally, polymer solar cells based on these polymers demonstrate power conversion efficiency varying from 4.01% for PDTPa-BT to 3.70% for PDTPa-fBT and to 5.26% for PDTPa-ffBT. These results indicate that the symmetry of both electron-donating and electron-accepting building blocks in conjugated polymers could evidently influence the optical and photovoltaic properties, which might pave the way for the further development of novel photovoltaic polymers based on asymmetric building blocks.

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