Synthesis and photovoltaic properties of a low bandgap donor-acceptor alternating copolymer with benzothiadiazole unit

A new donor-acceptor alternating copolymer as the donor material of the active layer in polymer solar cells has been synthesized. The alternating structure consisted of dithieno3,2-b:2′,3′-d]thiophene (DTT) donor unit and 5,6-bis(tetradecyloxy)benzo-2,1,3-thiadiazole (BT) acceptor unit. Both units were confirmed by ¹H NMR and elemental analysis. Since the BT unit has long alkyoxyl side chains, the polymer was soluble in common organic solvents. Optoelectronic properties of the copolymer (PDTTBT) were investigated and observed by UV-vis, photoluminescence (PL) spectra, and cyclic voltammogram (CV). UV-vis spectrum exhibited a broad absorption band in the range of 300-750 nm and a low bandgap of 1.83 eV. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of PDTTBT could be determined from the data of CV and UV-vis spectrum. Based on the ITO/PEDOT:PSS/PDTTBT:PCBM/Al device structure, the power conversion efficiency (PCE) under the illumination of AM 1.5 (100 mW/cm²) was 0.113%. It was found that PCE of 0.301% could be acquired under the annealing condition at 150 °C for 30 min. In addition, solar cells fabricated with the 1,8-octanedithiol (OT) additive in the mixture solvent or adding TiO_x optical spacer show efficiencies significantly improved over 15%.