Directional Crystallization of C8-BTBT-C8 Thin Films in a Temperature Gradient
A directional crystallization of compound 2,7-dioctylbenzothieno[3,2-b]benzothiophene (C8-BTBT-C8) based on a temperature gradient approach has been used as a post-deposition process to control the crystalline morphology of thin films. C8-BTBT-C8 films of arbitrary thicknesses have been fabricated by spin-coating and subjected to the directional crystallization process. Dewetting is prevented by the liquid crystal phase of C8-BTBT-C8, allowing the formation of flat and rather uniform thin films with large crystalline domains. Spin-coating concentration, gradient magnitude, and cooling rate have been varied to explore a large set of crystallization conditions and to correlate them with thin film morphology. The latter has been characterized by a combined use of optical profilometer, polarized optical microscopy, X-ray reflectometry, and X-ray diffraction measurements. The characterization results put forward the roughness and crystalline order of thin films are improved after the temperature gradient treatments: 1. high cooling rates treatments (≥ 9 ℃.min-1) significantly reduce the roughness of high thickness films, leading good continuity and uniform of the films; 2. dendritic growth dominates not only the films with low thickness but also films with high thickness treated under low cooling rates (< 9 ℃.min-1); 3. the out-of-plane order and the in-plane alignment of the crystallites are also remarkably enhanced by the temperature gradient treatments.