Efficient Solution-Processed Bulk Heterojunction Solar Cells by Antiparallel Supramolecular Arrangement of Dipolar Donor-Acceptor Dyes

Research on small-molecule-based organic semiconductors has undoubtedly been strongly influenced by xerographic photoconductors like triarylamines, the first important organic electronic materials in market products. Their development was strongly influenced by the B ssler model, which provided a rationale for the design of amorphous organic photoand semiconductors. According to this model, only compounds that lack dipole moments are considered promising for charge-carrier transport because the increased energetic disorder associated with dipole moments is thought to impede charge hopping. Recently, we questioned this paradigm in the field of organic photovoltaics (OPV) and successfully implemented highly dipolar merocyanine dyes as active components for light harvesting as well as exciton and hole transport in solution-cast bulk heterojunction (BHJ) solar cells. The rationale behind our concept was that highly dipolar donor–acceptor (D–A) substituted p systems (also called push-pull dyes) self-assemble into centrosymmetric dimers, thus effectively eliminating molecular dipole moments on the supramolecular and material levels. Two drawbacks of our BHJ materials, however, limited the acceptance of our concept so far. Firstly, the best solar cells were obtained for merocyanine dyes whose molecular scaffolds were equipped with rather bulky substituents that interfere with close face-to-face antiparallel dimerization. Secondly, the power-conversion efficiencies (h) under standard AM1.5, 100 mW cm 2 simulated solar illumination conditions for solution-cast BHJ cells with fullerenes—although significantly advanced by more sophisticated vacuum processing—could not be improved beyond 2.6%, which is significantly lower than the best solutionprocessed small-molecule-based BHJ devices fabricated with A–D–A and D–A–D chromophores, for example, acceptorsubstituted oligothiophenes (up to 3.7%) and triarylamines (up to 4.3%), diketopyrrolopyrroles (up to 4.4%), and squaraines (up to 5.2 %). Herein, we introduce dipolar D– A dyes with flat structures that undoubtedly form centrosymmetric dimers with perfectly cancelled dipole moments in the solid state. Solution-processed BHJ solar cells derived thereof exhibit power-conversion efficiencies up to 4.5–5.1% (dependent on light intensity), clearly placing D–A dyes now among the top-performing small molecules in the field of organic photovoltaics. Scheme 1 outlines the synthetic route that follows our earlier work on merocyanine dyes for photorefractive materials and the simple access to 5-dialkylamino-thiophene-2carbaldehydes by Hartmann. Detailed synthetic procedures and characterization data are described in the Supporting Information.