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

Abstract

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.

Synthesis of Alternative Electron Acceptor Compounds

“…The same reaction has been performed with the same reactants in quinoline as solvent, instead of NMP, and the obtained results are listed in Table 2 (entries 2, 4,6,8). The measured yields are not significantly different with both solvents denoting the importance of using a high boiling solvent.…”

“…A major synthetic effort is devoted to the design of electron acceptor materials to be used in organic solar cells instead of fullerene derivatives, which are considered not ideal because their electronic properties are not easily tunable and they do not absorb much solar radiation [5,6].…”

A Combined Experiment and Computation Study of the Fused Polycyclic Benzimidazole Derivatives

“…Because of their stability, high light absorptivity and good electron transportation process of these derivatives have been extensively studied for developing different derivatives in organic electronics. The benzo [4,5]imidazo [2,1-a]isoindolone derivatives have already shown to be the promising candidates in optoelectronic device [21], light-emitting diodes [22,23], organic thin-film transistors (OTFTs) [24,25], organic photovoltaic's (OPVs) [26,27], organic electronics [28] and Organic Field Effect Transistors (OFET) [29][30][31].…”

Solution processed naphthalene diimide derivative as electron transport layers for enhanced brightness and efficient polymer light emitting diodes

“…[20][21][22] In addition, polymers developed from NDIs have shown promise as novel acceptors in organic solar cells, providing in excess of 1.5% device efficiencies, one of the highest for non-fullerene cells, with potential for extending device lifetimes. [23][24][25] The high electron mobilities of naphthalene diimides, coupled with their excellent air stability, give them considerable potential as air-stable electron injection layers. It is facile to chemically tune the electrical characteristics of naphthalene diimides using simple organic chemistry to modify either the conjugated backbone or the nitrogen atoms to introduce solubilizing side chains for easy solution processing.…”