Synthesis of a low bandgap polymer based on thieno[3,2-b]thiophene and fluorinated quinoxaline derivatives and its application in bulk heterojunction solar cells

• A novel copolymer based on mono-fluorinated quinoxaline and thieno[3,2-b]thiophene was synthesized.
• The polymer shows a low-lying HOMO of −5.34 eV with an optical bandgap of 1.65 eV.
• A PCE of 2.33% was achieved for the polymer solar cells blending with PC71BM.

To decrease the bandgap and maintain low-lying highest occupied molecular orbit (HOMO) level of TQ1 type polymer, a novel copolymer (TTFQ) using mono-fluorinated quinoxaline as electron-withdrawing unit and thieno[3,2-b]thiophene as electron-rich unit was designed and synthesized. The polymer was characterized by UV–vis spectroscopy and electrochemical cyclic voltammetry (CV) to evaluate its bandgap and molecular energy levels. Thanks to the planar π-conjugated thieno[3,2-b]thiophene segment and fluorinated quinoxaline structure, the resulting polymer TTFQ showed broad absorption area with a small bandgap of 1.65 eV and owned relative deep HOMO level of −5.34 eV. Polymer solar cells (PSCs) were fabricated with TTFQ as donor blended with [6,6]-phenyl-C71-butyric acid methylester (PC71BM) as acceptor. The power conversion efficiency (PCE) of PSC was 2.33% with an open-circuit voltage (Voc) of 0.74 V for an optimized TTFQ:PC71BM weight ratio of 1:1.