Quinoidal propylenedioxythiophene dimers for air-stable n-type semiconductors: achieving crystallinity and solution processability†

Rapid expansion of the Internet of Things has fueled a growing need for environment-friendly organic semiconductor materials, circumventing the use of rare metals. A key area of focus is the development of n-type semiconductors that, similar to their well-established p-type counterparts, exhibit air stability and solution processability. Synthesizing high-performance, air-stable n-type semiconductor thin films requires a delicate balance between a π-conjugated framework, which facilitates efficient intermolecular interactions for charge transport, and the incorporation of side chains to enhance solubility for energy-efficient solution processing. This study departs from traditional small molecule and polymer approaches by exploring oligomer backbones, which offer greater structural design flexibility, to create novel n-type semiconductor materials. Specifically, inspired by the doped poly(3,4-ethylenedioxythiophene) family, we introduced a dicyanomethylene end-capped quinoidal (q) structure into propylenedioxythiophene (P) oligomers to induce n-type semiconducting behavior. We synthesized the shortest dimers, q2P and q2P^Hex, by incorporating dimethyl and dihexyl groups as the side chains, respectively. The differing side chains influenced both solubility and crystal structure, leading to the formation of two types of crystalline thin films with effective intermolecular interactions. Field-effect transistor characterization of these thin films demonstrated stable operation in air. Notably, q2P^Hex, with its higher crystallinity, exhibited a mobility 1000 times greater than that of q2P. These results demonstrate the successful achievement of n-type semiconductor characteristics with an excellent balance of crystallinity, solution processability, and air stability.