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

Abstract

A new conjugated copolymer, PBTF-OP, based on meta-alkoxy-phenyl-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 donor material in polymer solar cells (PSCs). PBTF-OP possesses a similar molecular structure to the well-known 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 × 10⁻³ cm² (V⁻¹ s⁻¹) versus a HOMO energy level of −5.30 eV and a hole mobility of 1.11 × 10⁻³ cm² (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.