Approaching isotropic charge transport of n-type organic semiconductors with bulky substituents

Using the sterically demanding cyclohexyl substituents, the new Cy6–BQQDI n-type organic semiconductor demonstrates fine-tuned 2D brickwork assembly with isotropic charge transport, and high electron-carrier mobility in both single- and polycrystalline thin-film field-effect transistors.

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Organic semiconductors (OSCs) demonstrate great promises as candidates for next-generation flexible and lightweight electronics1. Electronic applications such as the logic-circuits and radio-frequency identifiers require high-performance hole-and electron-transporting OSCs (p- and n-type, respectively) for their design and constructions. While state-of-the-art p-type OSCs have achieved adequately high charge-carrier mobilities (µ), the n-type counterparts have not demonstrated matching µ with p-type OSCs to truly realize these high-end electronics. From a scientific point of view, the lack of high-performance n-type OSCs also impedes our understanding of their charge-transport properties.

What we can learn from various in-depth investigations of high-performance p-type OSCs is that there are several important molecular features that lead to excellent charge-transport capability, which we can also apply to the molecular design of high-performance n-type OSCs. The charge transport of small-molecule OSCs is essentially an intermolecular phenomenon facilitated by the molecular orbital overlaps between molecules in a crystal structure, which is quantified by the transfer integrals. Therefore, crystal packing structures that result in large transfer integrals are essential for high-performance n-type OSCs. However, molecules are undergoing perpetual thermal fluctuations in the solid state, which disrupt transfer integrals and the charge transport of OSCs2.

Figure 1. Common crystal packing structures that exhibit 2D charge transport.

In 2017, Troisi and coworkers reported that OSCs with 2D isotropic charge transport3, that is, a crystal packing structure with uniform transfer integrals in a 2D plane, show effective resilience towards molecular fluctuations compared to those with anisotropic charge transport (Figure 1). From this important study, we can learn that isotropic charge transport is critical in minimizing the detrimental effect of molecular fluctuations on charge transport, as well as achieving high µ. From a practical point of view, having an OSC with isotropic charge transport also allows engineers to construct electronic devices along any crystallographic direction on a crystalline thin film since every direction would render equally high electronic performances, which reduces the device fabrication time. The isotropic charge transport is also beneficial for polycrystalline thin-film devices since the molecules are oriented in a less orderly manner than they are in the single-crystalline thin films. 

While organic materials chemists are exceptional in fine-tuning intramolecular features such as HOMO/LUMO energy levels and their distributions, designing intermolecular features is not a walk in the park. We oftentimes have to go through a painful process of synthesis, purifications, growing single crystals, analyze them by X-ray diffractions to eventually see whether our initial design concept was valid or not. Therefore, the design of crystal packing structures that demonstrates n-type isotropic charge transport is, by all means, a challenging task for us chemists (but it does make a fantastic research project). 

Our research group has developed an air-stable high-performance PhC2–BQQDI4 n-type OSC in 2020 that exhibits an outstanding µ of 3.0 cm2 V-1 s-1 (Figure 2). The excellent OSC performance of PhC2–BQQDI is attributed to the large transfer integrals within a 2D brickwork-type packing structure and its intermolecular hydrogen-bonding interactions along other favorable intermolecular interactions provide resilience to molecular fluctuations. Despite these promising results, PhC2–BQQDI forms anisotropic transfer integrals caused by the misaligned molecules in the π-π stacking directions. We then started contemplating how to achieve isotropic charge transport using the BQQDI π-core, and we believed that we could achieve this goal if we were able to force the alignment of the molecules π-π stacking directions. 

Figure 2. Molecular structures, intermolecular orbital (LUMO) overlaps, intermolecular distances, transfer integrals, and crystal packing structures of PhC2– and Cy6–BQQDI.

In this work, we employed somewhat bulky substituents such as phenyl and cyclohexyl on the BQQDI π-core (Ph– and  Cy6–BQQDI) and investigated their charge transports. While Ph–BQQDI shows similar anisotropic charge transport as PhC2–BQQDI, the cyclohexyl substituents successfully forced the molecular alignment in the π-π stacking directions of Cy6–BQQDI in a near-ideal 2D brickwork packing structure (Figure 2). Cy6–BQQDI also demonstrates equal transfer integrals in both π-π stacking directions and this isotropic charge transport indeed contributes to its strong resilience to molecular fluctuations compared to derivatives like Ph– and PhC2–BQQDI. The solution-processed single-crystalline thin films of Cy6–BQQDI achieved a high µ of 2.3 cm2 V-1 s-1 and its vacuum-deposited polycrystalline devices also exhibit a high µ of 1.0 cm2 V-1 s-1. Cy6–BQQDI also demonstrated high µ ranged from 1.5–2.0 cm2 V-1 s-1 at various angles of the single-crystalline thin films, which confirmed its isotropic charge transport.

The results here provide us with a strategy to achieve isotropic charge transport using effective substituents and we are glad to further confirm the importance of isotropic charge transport in suppressing the detrimental effect of molecular fluctuations. Based on this work, our colleagues are now working hard to design new n-type (also p-type) OSCs that show isotropic charge transport, higher µ, and better solution-processability. 

Check out our paper for more details: 10.1038/s42004-021-00583-2

Here is an awesome photo of the Takeya Lab family.

References

  1. Klauk, H., Zschieschang, U., Pflaum, J. & Halik, M. Ultralow-power organic complementary circuits. Nature 445, 745–748 (2007).
  2. Eggeman, A. S., Illig, S., Troisi, A., Sirringhaus, H. & Midgley, P. A. Measurement of molecular motion in organic semiconductors by thermal diffuse electron scattering. Nat. Mater. 12, 1045–1049 (2013).
  3. Fratini, S, Ciuchi, S., Mayou, D., Trambly de Laissaediere, Troisi, A. A map of high-mobility molecular semiconductors. Nat. Mater. 16, 998–1002 (2017).
  4. Okamoto, T. et al. Robust, high-performance n-type organic semiconductors. Sci. Adv. 6, eaaz0632 (2020).

Craig P. Yu

PhD, The University of Tokyo

My research involves the design and synthesis of organic semiconductors and the investigation of their charge-transport properties.