Design of New Electron Acceptor Materials for Organic Photovoltaics: Synthesis, Electron Transport, Photophysics, and Photovoltaic Properties of Oligothiophene-Functionalized Naphthalene Diimides

A homologous series of six novel oligothiophene–naphthalene diimide-based oligomer semiconductors with a donor–acceptor architecture, NDI-nTH (n = 1, 2, 3, 4) and NDI-nT (n = 2, 3), was synthesized and used to explore a set of criteria for the design of non-fullerene electron acceptor materials for organic solar cells. Thin films of the oligomer semiconductors had optical band gaps that varied from 2.1 eV in NDI-1TH and 1.6 eV in NDI-3TH to 1.4 eV in NDI-4TH, demonstrating good potential for light harvesting and exciton generation. The LUMO energy levels of the oligomer semiconductors were similar (ca. −4.0 eV), but the HOMO levels varied from −5.5 eV in NDI-3TH and NDI-4TH to −6.1 eV in NDI-1TH, showing that suitable energy band offsets necessary for efficient photoinduced charge transfer could be achieved with current donor polymers. Single-crystal X-ray structures of NDI-3TH and NDI-4TH showed a slipped face-to-face π-stacking with short intermolecular distances (0.321–0.326 nm), which enabled facile self-assembly of single-crystalline nanowires from solution. Spin coated thin films of NDI-nTH and NDI-nT were mostly crystalline and had field-effect electron mobilities of up to (2–9) × 10–4 cm2/(V s). Bulk heterojunction solar cells incorporating one of the n-type oligomer semiconductors as the electron acceptor and poly(3-hexylthiophene) as the electron donor showed a power conversion efficiency of 1.5% with an open circuit voltage of 0.82 V and a bicontinuous nanoscale morphology.