Fluorine substituted thienyl-quinoxaline copolymer to reduce the highest occupied molecular orbit level and increase open-circuit voltage for organic solar cells

An alternating polymer (FTTQ) based on fluorine substituted thienyl-quinoxaline as electron-withdrawing unit was synthesized and characterized. FTTQ showed similar absorption property in comparison with that of the counterpart polymer without fluorine atom (synthesized TTQ). However, the low-lying highest occupied molecular orbit (HOMO) energy level of FTTQ was −5.30 eV, about 0.24 eV smaller than that of synthesized TTQ (−5.06 eV). In order to study the photovoltaic properties of the materials, polymer solar cells (PSCs) were fabricated with FTTQ as donor blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM) as acceptor, as well as with an alcohol/water-soluble conjugated polymer, poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7- fluorene)-alt-2,7-(9,9-dioctylfluorene)](PFN) as a cathode interlayer. The PSCs incorporating the PFN interlayer showed significant and simultaneous enhancement in open-circuit voltage (V_oc), short-circuit current density (J_sc) and fill factor (FF), which leaded to a PCE of 2.59% for an optimized FTTQ:PC71BM weight ratio of 1:2 with THF vapor annealing for 30 s, in comparison with that of synthesized TTQ-based device (PCE of 1.41%). These findings will be of importance for achieving high-performance of polymer solar cells by functional group substitution in low band gap polymers.