Correlation of the π-conjugation chain length and the property and photovoltaic performance of benzo[1,2-b:4,5-b′]dithiophene-cored A-π-D-π-A type molecules

A-π-D-π-A type conjugated small molecules (COOP-nHT-BDT (n=1–4)) with a benzo[1,2-b:4,5-b']dithiophene (BDT) as the electron-donating core, 2-cyano-3-octyloxy-3-oxo-1-propenyl (COOP) as the terminal electron-accepting unit, and two regioregular oligo(3-hexylthiophene) (nHT) as π-conjugated bridges were synthesized and characterized. TGA results confirmed that all these compounds are thermally stable with decomposition temperatures higher than 340 °C. Broad absorption bands over 350 to 600 nm with small optical band gaps (E_g^opt) of 1.9–1.8 eV were measured for these compounds. Cyclic voltammetry measurements indicated that HOMO energy level of these COOP-nHT-BDTs increases slightly with the increase of nHT chain length, whereas all these compounds display similar LUMO energy level of −3.60 eV. These were supported by density function calculation results, that electronic wave functions of the HOMO orbitals are delocalized over the BDT-core with the adjacent 2–3 thiophene units, whereas the electronic wave functions of the LUMO orbital are mostly localized on the terminal COOP unit with 1–2 thiophene units. Gradually increasing of the photovoltaic performance was found for these compounds with the increase of the π-bridge chain length, and the COOP-4HT-BDT/PC₇₁BM (2:1, w/w) based device displayed a best power conversion efficiency of 5.14% with an open circuit voltage of 0.90 V, a short circuit current of 8.27 mA cm⁻²and a fill factor of 0.69. A positive correlation between long-term stability of PCE and chain length was also obtained. COOP-4HT-BDT based devices showed the highest device stability with only 15% decay after 320 h continuous illumination.