Octylthieno[3,2-b]Thiophene (oTT) pi-Bridged Bithiophene-3,3 '-Dicarboximide Based Conjugated Polymers for Organic Photovoltaic Cells

Abstract

Donor-acceptor (D-A)-type conjugated copolymers, PBTTI and PTBTTI, were synthesized via Pd-catalyzed Stille cross-coupling polymerization of benzodithiophene (BDT) derivatives with different two-dimensional (2D) side chains as electron-donating groups and alkylthieno[3,2-b]thiophene pi-bridge 2,2'-bithiophene-3,3'-dicarboximide (ttBTI) as the electron withdrawing unit. The synthesized polymers were systemically characterized to compare the optoelectronic and photovoltaic properties. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of octylthio thiophene (aTT)-substituted BDT-based polymer (PBTTI) were down-shifted by decreasing the electron-donating effect on the BDT units, which resulted in the enhanced power conversion efficiency of 2.69% via the better HOMO and LUMO energy level match-up with PC71BM.