D-A conjugated polymers based on thieno[3,2-: B] indole (TI) and 2,1,3-benzodiathiazole (BT) derivatives: Synthesis, characterization and side-chain influence on photovoltaic properties

A facile synthetic strategy toward thieno[3,2-b]indole (TI) derivatives was developed by the Cadogan annulation method. Three donor-acceptor (D-A) conjugated polymers P1, P2 and P3 containing N-alkyl-TI derivatives and 4,7-dithien-5-yl-2,1,3-benzodiathiazole (DTBT) derivatives were successfully synthesized and applied to bulk heterojunction (BHJ) polymer solar cells (PSCs). Different side chains were introduced to TI units (for P2) or DTBT units (for P3), the results indicate that the bandgaps, energy levels and photovoltaic performance were finely tuned by the side chains in the TI-DTBT copolymer. Power conversion efficiencies (PCEs) based on the device structure of ITO/PEDOT:PSS/polymer:PC₇₁BM/Ca/Al exhibit a large distinction (1.61% for P1, 0.53% for P2 and 2.73% for P3) under optimal device fabrication conditions. The optimized devices based on P3:PC₇₁BM blends with a relatively higher mobility (2.84 × 10^-5 cm² V^-1 s^-1) show the best PCE under air mass 1.5 global (AM 1.5 G) irradiation of 100 mW cm^-2, which is in good agreement with its high current density and light absorption property. Accordingly, the TI unit can be used as the efficient donor units for D-A conjugated donor materials for PSCs application. In addition, this work suggests that the side chains on low bandgap polymers significantly impact their molecular energy levels and the observed PCEs of the corresponding BHJ solar cells.