A-D-A-Type Oligothiophenes for Small Molecule Organic Solar Cells: Extending the π-System by Introduction of Ring-Locked Double Bonds

A series of novel acceptor-donor-acceptor oligothiophenes terminally substituted with the 1-(1,1-dicyanomethylene)-cyclohex-2-ene (DCC) acceptor has been synthesized. Structural, thermal, optoelectronic, and photovoltaic properties of the -extended DCCnTs (n = 1-4) are characterized and contrasted to the trends found for the series of parent dicyanovinyl (DCV)-substituted oligothiophenes DCVnT. The optoelectronic properties reveal the influence of the additional exocyclic, sterically fixed double bonds in trans-configuration in the novel DCCnT derivatives. A close correspondence for derivatives with equal number of double bonds, that is, DCCnTs and DCV(n + 1)Ts, is identified. Despite having the same energy gap, the energy levels of the frontier orbitals, HOMO and LUMO, for the DCC-derivatives are raised and more destabilized due to the aromatization energy of a thiophene ring versus two exocyclic double bonds indicating improved donor and reduced acceptor strength. DCC-terthiophenes give good photovoltaic performance as donor materials in vacuum-processed solar cells (power conversion efficiencies 4.4%) clearly outperforming all comparable DCV4T derivatives.