Synthesis and photovoltaic properties of two-dimension-conjugated D-A copolymers based on benzodithiophene or benzodifuran units

A series of two-dimension-conjugated D-pi-A copolymers based on benzodithiophene (BDT) or benzodifuran (BDF) as the donor unit, benzothiadiazole (BT) as the acceptor unit and thiophene as the pi-bridge, were designed and synthesized for application as donor materials in polymer solar cells (PSCs). The polymers include PBDTT-BT based on BDT with a thiophene side chain, PBDTF-BT based on BDT with a furan side chain, PBDFT-BT based on BDF with a thiophene side chain and PBDFF-BT based on BDF with a furan side chain. The replacement of some of the thiophene rings in the donor unit with furan rings influences the UV-vis absorption spectra, electronic energy levels and band gaps of the copolymers. The HOMO levels of the copolymers based on BDF are ca. 0.1 eV up-shifted in comparison with that of the polymers based on BDT. While the furan side chain makes the LUMO level shift downwards a little. Power conversion efficiency (PCE) of the PSCs based on the polymers as the donor and [6,6]-phenyl C-71-butyric acid methyl ester (PC71BM) as the acceptor reached 1.85% for PBDTT-BT, 2.88% for PBDTF-BT, 4.42% for PBDFT-BT and 2.60% for PBDFF-BT. The results indicate that the replacement of thiophene rings by furan rings in the 2-D-conjugated BDT unit significantly influences the photovoltaic properties of the copolymers.