The crystallinity of conjugated organic molecules (pi-OM) is determinant to the electrical properties of organic electronics. In particular, rubrene, a representative pi-OM, is known to exhibit highly coherent charge transport when the conducting channel consists of single crystal phase. However, there has been a lack in the fabrication of highly crystalline, patternable, and large-area rubrene thin film. In this work, we proposed a selective crystallization of rubrene thin film by modifying chemical and skeletal structure of underlying polymer interlayer. An UV irradiation in air allowed the molecular chains in polymer interlayer to be transformed into oxidatively degraded short chain and/or crosslinked network. In particular, we verified that the latter limited the intrachain vibration that prevents the nucleation of rubrene. Freezing molecular vibrations of polymer interface corresponds to successive phase transition of rubrene thin film from amorphous to orthorhombic spherulite during thermal annealing. Furthermore, in this system, we found that the thickness of polymer interlayer significantly determines the grain size of rubrene: the higher thickness, the larger size of spherulitic grains. Our study indicates that molecular vibration of underlying interlayer is possible to affect the assembly of pi-OM.

Selective crystallization of conjugated organic molecules by freezing molecular vibrations of polymer interlayer

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