Two-dimensional benzo[1,2-b: 4,5-b ']difurans as donor building blocks for the formation of novel donor-acceptor copolymers

Four donor-acceptor copolymers combining the novel electron-donating moiety, 2,3,6,7-tetra(thiophen-2-yl)benzo[1,2-b : 4,5-b ']difuran (BDF) with either 2,3,1-benzothiadiazole (BT) or 2-octyl-2H-benzo[d][1,2,3]triazole (BTz) as the electron-accepting unit were prepared via the Stille cross-coupling reaction. Two different alkyl chains were attached on the BDF monomers and the impact on the polymers' solubility and thin film morphology was evaluated. The resulting polymers had optical bandgaps between 1.7-1.9 eV and HOMO levels that ranged from -5.60 to -5.85 eV. In thin films, the BTz copolymer showed absorption bands in the range of 400-625 nm. The BT copolymer exhibited two absorption bands: one from 350-450 nm and a broad absorption in the range of 450-750 nm. When the polymers were used as donor materials with the electron-acceptor PC71BM in bulk-heterojunction photovoltaic cells, power conversion efficiencies of up to 2.93% were obtained. These results indicate that BDF is a promising building block for the synthesis of donor materials for use in organic solar cells, due to the favorable energy levels. Although, additional modifications are needed to improve device performance.