Thin film crystallization of oligoethylene glycol-benzothieno benzothiophene: Physical vapor deposition versus spin coating

Abstract

The benzothieno-benzothiophene derivatives are among the best-performing organic semiconductor materials. Their triumphant application in electronic devices is correlated to their capability to form well-defined thin film structures featuring crystalline order at the substrate–organic interface. Thin films of the molecule 2,7-bis(2-(2-methoxy ethoxy)ethoxy) benzo[b]-benzo[4,5]thieno[2,3-d]thiophene (OEG-BTBT) are prepared by spin coating and physical vapor deposition using different experimental conditions. Atomically flat silicon oxide surfaces with defined surface energies are used as substrates. The crystalline morphology is studied for the monolayer regime up to thick films by X-ray reflectivity, grazing incidence X-ray diffraction and atomic force microscopy. Depending on the preparation process, substantial variations are found in the structure of the first monolayers. The highly flexible oligoethylene glycol side chains allow different monolayer formation processes. Also, the subsequent crystal growth strongly depends on the nucleation in the monolayer regime. Here, we observe two distinct nucleation scenarios, one directly at the substrate surface and one at the edges of sub-monolayer terraces. As a consequence, the mosaicity of the crystallites in thick films is strongly affected by the growth type. This work illustrates that different thin film preparation processes can result in different nucleation mechanisms for subsequent crystallization. Consequently, variations in the crystallographic order in device relevant films are observed.