The control of polymorphism and thin film morphology in small molecule organic semiconductor thin films deposited by solution-shearing techniques is reviewed. To gain insights into the thin film crystallization is crucial to achieve high performing devices with high reproducibility
Organic semiconductors (OSCs) have emerged as promising materials for cost-effective production of new flexible electronic devices since they can be processed from solution and at temperatures compatible with polymeric substrates. In particular, small molecule OSCs have been applied as active materials in organic field-effect transistors (OFETs) exhibiting impressive field-effect mobility values. To raise industrial interest though, it is crucial that such deposition techniques are simple, cheap and compatible with up-scaling and high throughput processes such as roll-to-roll. In this direction, solution shearing techniques are highly appealing (Figure, left). However, small molecule OSCs are prone to structural modifications due to the presence of weak van der Waals intermolecular interactions. Therefore, the control of the crystallization in these materials is pivotal to achieve a high device-to-device reproducibility.
In this review article we report on the influence of polymorphism and morphology on the electrical characteristics of OFETs fabricated by solution-shearing techniques. We show that by modifying the coating parameters such as the coating speed or the temperature, among others, a control of the thin film morphology and structure can be achieved leading to optimized device performances (Figure, right). Further, the main characterization techniques for thin film structure are reviewed, highlighting the in situ characterization tools which can provide crucial insights into the crystallization mechanisms.
Sergi Riera-Galindo, Adrián Tamayo, Marta Mas-Torrent
Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain
Role of Polymorphism and Thin-Film Morphology in Organic Semiconductors Processed by Solution Shearing
ACS Omega 3, 2, 2329 (2018)
Left. Schematic illustration of some of the most common solution shearing deposition techniques. Right. Conceptual illustration of how crystal growth process and polymorph formation can be programmed during solution shearing process by varying the substrate temperature or the coating speed