Synthesis and properties of monodisperse multi-triarylamine-substituted oligothiophenes and 4,7-bis(2′-oligothienyl)-2,1,3-benzothiadiazoles for organic solar cell applications

Two novel series of monodisperse multi-triarylamine-substituted oligothiophenes, G₂-OT(n)-G₂ with thiophene unit (n) varying from 6 to 8, and 4,7-bis(2′-oligothienyl)-2,1,3-benzothiadiazoles G₂-OT(n)BTD-G₂ (n = 2, 4, 6) have been synthesized by the Suzuki coupling reactions. With an elongation of alkyl-substituted oligothiophene core or an incorporation of benzothiadiazole into the central core, the absorption and emission spectra of G₂-OT(n)-G₂ and G₂-OT(n)BTD-G₂ series redshift substantially with the optical gap reducing to 1.95 eV for G₂-OT(6)BTD-G₂. Alkyl-substitution onto oligothiophene backbone not only improves the solubility of the highly extended dendrimers but also renders coplanarity of the dendritic oligothiophene backbone at the excited state, which results in the enhancement of fluorescence quantum efficiency. The bulk heterojunction solar cells using these newly synthesized dendritic oligothiophenes as a donor material and [6,6]-phenyl C₆₁-butyric acid methyl ester (PCBM) as an acceptor material were fabricated and investigated which showed an increase in device performance as compared with those of the lower homologues. On increasing the loading of PCBM from 1.5 to 3 times in the active layer, there was also an enhancement in device performance with power conversion efficiencies of as-fabricated solar cells increasing from 0.18% to 0.32%. In addition, proper annealing procedure could significantly improve the device performance of the dendrimer-based photovoltaic cell.