Synthesis of N-[4-Octylphenyl]dithieno[3,2-b:2′,3′-d]pyrrole-based broad absorbing polymers and their photovoltaic applications

Electron rich, fused N-aryl pyrrole based monomer namely 2,6-di(4,4,5,5-tetramethyl-1,3,2-dioxaborolan)-N-[4-octylphenyl]dithieno[3,2-b:2′,3′-d]pyrrole (N-aryl DTP) was synthesized and copolymerized with electron deficient 4,7-bis(5-bromo-3-octyl-2-thienyl)-2,1,3-benzothiadiazole (TBT) to afford alternating polymer PDTPTBT. The absorption band of PDTPTBT was found to cover the entire visible part of the solar spectrum (300 nm–750 nm) and the optical band gap was estimated to be 1.62 eV. In order to extent the absorption of PDTPTBT, another strong electron acceptor unit such as 2,1,3−benzothiadiazole or dimethyl-2H-benzimidazole was incorporated in polymer main chain by copolymerizing three different comonomers (N-aryl DTP, TBT and 4,7-dibromo-2,1,3−benzothiadiazole (B) or 4,7-dibromo-2,2-dimethyl-2H-benzimidazole (BI)) at 2:1:1 ratio to afford polymers PDTPTBTB and PDTPTBTBI, respectively. The absorption band of PDTPTBTB and PDTPTBTBI was found to be extended up to 1000 nm and 1200 nm, respectively, and the optical band gap was calculated to be 1.33 eV and 1.07 eV, respectively. The HOMO-LUMO energy levels of PDTPTBT, PDTPTBTB and PDTPTBTBI were found to be suitable for polymer solar cell (PSC) applications. The single layer PSCs fabricated with the configuration of ITO/PEDOT:PSS/PDTPTBT, PDTPTBTB or PDTPTBTBI:PC60BM(1:3 wt/wt)/LiF/Al showed maximum power conversion efficiency (PCE) of 2.04%, 1.01% and 0.70%, respectively.