Satisfying both interfacial- and bulk requirements for organic photovoltaics: Bridged-triphenylamines with extended p-conjugated systems as efficient new molecules

Abstract

In the domain of organic photovoltaic (OPV) devices, the design of new materials faces the extraordinary challenge to satisfy two opposite requirements: the new materials should exhibit weak intermolecular interactions at the donor-acceptor interface, but strong intermolecular interactions and good charge transport properties in the bulk. In an effort to progress in this direction, here we present three new diphenylethenyl substituted derivatives of methylene-bridged triphenylamine, synthesized by condensation of dimethylmethylene-bridged triphenylamines with 2,2-diphenylacetaldehyde. The synthesized compounds were found to be capable of glass formation with glass transition temperatures in the range of 65–130¿°C. The ionization potentials of these derivatives were found to be in the range of 5.29–5.68¿eV. The time-of-flight hole drift mobilities measured at room temperature well exceeded 10-2¿cm [2]/Vs for all the synthesized compounds. By combining the compounds with fullerene C70 acceptor in bilayer organic solar cells, a power conversion efficiency of 1.9% was obtained. Density functional methods-based calculations indicate that this result can be explained by considering edge-on orientations at the donor-acceptor interface. In this case, the designed architecture of the compounds has an effect to sufficiently “hide” the donor HOMO from a direct and easy contact with C70 LUMO(s), thus maintaining low level of geminate donor-acceptor charge recombination, without losing hole-transport properties in the donor bulk. These two design strategies operate consequently in different parts of the molecules, which is central to the success of the new compounds. Our study shows consequently a possible strategy for a simultaneous improvement of both interfacial- and bulk properties of molecules for OPV applications.