Efficient polymer solar cells based on a copolymer of meta-alkoxy-phenyl-substituted benzodithiophene and thieno[3,4-b]thiophene

A new conjugated copolymer, PBTF-OP, based on meta-alkoxyphenyl-substituted benzodithiophene (BDT-m-OP) and 2-ethylhexyl-3- fluorothieno[ 3,4-b] thiophene-2-carboxylate (TT) was designed and synthesized for application as the donormaterial in polymer solar cells (PSCs). PBTF-OP possesses a similar molecular structure to the wellknown polymer PTB7-Th but different conjugated side chains on the BDT unit: meta-alkoxy-phenyl side chains for PBTF-OP and alkylthienyl side chains for PTB7-Th. Compared with PTB7-Th, PBTF-OP exhibits absorption with some blue shifts, while it possesses a deeper HOMO energy level of -5.45 eV and a slightly enhanced hole mobility of 1.25 x 10(-3) cm(2) (V-1 s(-1)) versus a HOMOenergy level of -5.30 eV and a hole mobility of 1.11 x 10(-3) cm(2) (V-1 s(-1)) for PTB7-Th. The PSCs based on PBTF-OP: PC71BM showed a higher power conversion efficiency (PCE) of 9.0% with a higher V-oc of 0.86 V in comparison with a PCE of 8.3% and a V-oc of 0.78 V for PTB7-Th. The results indicate that side chain engineering of BDT-based copolymers is an effective way to improve photovoltaic performance of polymer donors.