Effect of film thickness, type of buffer layer, and substrate temperature on the morphology of dicyanovinyl-substituted sexithiophene films

The influence of film thickness, type of buffer underlayer, and deposition substrate temperature on the crystal structure, microstructure, and morphology of the films of dicyanovinyl-substituted sexithiophene with four butyl-chains (DCV6T-Bu4) is investigated by means of X-ray diffraction (XRD) and X-ray reflectivity methods. A neat Si wafer or a Si wafer covered by a 15 nm buffer underlayer of fullerene C60 or 9,9-Bis[4-(N,N-bis-biphenyl-4-yl-amino)phenyl]-9H-fluorene (BPAPF) is used as a substrate. The crystalline nature and ordered molecular arrangement of the films are recorded down to 6 nm film thickness. By using substrates heated up to 90 °C during the film deposition, the size of the DCV6T-Bu4 crystallites in direction perpendicular to the film surface increases up to value of the film thickness. With increasing deposition substrate temperature or film thickness, the DCV6T-Bu4 film relaxes, resulting in reducing the interplane distances closer to the bulk values. For the films of the same thickness deposited at the same substrate temperature, the DCV6T-Bu4 film relaxes for growth on Si to BPAPF to C60. Thicker films grown at heated substrates are characterized by smaller density, higher roughness and crystallinity and better molecular ordering. A thin (up to about 6 nm-thick) intermediate layer with linear density-gradient is formed at the C60/DCV6T-Bu4 interface for the films with buffer C60 layer. The XRD pattern of the DCV6T-Bu4 powder is indexed using triclinic unit cell parameters.