Synthesis of new n-type isoindigo copolymers

Three new n-type copolymers were synthesized using the isoindigo monomer. 5-Octylthieno[3,4-c]pyrrole-4,6-dione (TPD), 5,5′-dioctyl-1,1′-4H-bithieno[3,4-c]pyrrole-4,4′,6,6′(5H,5′H)-tetrone (BTPD) and 3,6-bis(thiophen-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) were utilized as electron-withdrawing comonomers to obtain reduced or low bandgap n-type copolymers with deep HOMO and LUMO energy levels. The TPD and BTPD copolymers were synthesized using direct arylation polymerization and show bandgaps of 1.72 and 1.75 eV, respectively. Their LUMO and HOMO energy levels are also low at −4.2 and −6.0 eV, respectively. We investigated their electron mobility using thin film transistors and achieved electron mobility as high as 3.0 × 10−4 and 3.5 × 10−3 cm2 s−1 V−1 for the TPD and BTPD copolymers. The DPP copolymer was synthesized using Suzuki conditions and shows a low bandgap of 1.35 eV and a low LUMO energy level of −4.0 eV. The DPP copolymer exhibits an electron mobility of 2.7 × 10−4 cm2 s−1 V−1. All these polymers show interesting properties as potential electron acceptors in all-polymer solar cells.