Functionalized isothianaphthene containing polymers as n-type fullerene substitutes in organic photovoltaics

N-type donor-acceptor block copolymers were synthesized containing functionalized isothianaphthene (ITN) acceptors as possible fullerene substitutes for organic photovoltaics. The ITN monomer was functionalized with strong electron withdrawing substituents which resulted in a slight decrease in the polymers' HOMO and a significant decrease in the LUMO. In such manner, the energy levels of ITN polymers were tuned by varying the electron-withdrawing strength of the substituents. For instance, by incorporating two nitrile groups into the ITN unit, a polymer was synthesized (ITN-CN) with energy levels comparable to that of the fullerene n-types commonly used in bulk heterojunctions. The resulting polymers were employed in lieu of fullerene acceptors using poly(3-(4- n-octyl)-phenylthiophene) as the p-type polymer. Using ITN-CN, photoconversion efficiencies (PCE) of up to 0.30% were obtained.