365891 Understanding Organic Semiconductor Polymorphism Using High Speed in-Situ Optical and X-Ray Diffraction Methods
Organic electronics have been considered a leading candidate to make transparent and flexible electronics at a low cost. We have previously shown that the solution shearing method is a process that improves electrical performance for a range of OSCs, and the method is compatible with roll to roll industrial processing. This method can also tune the polymorph formation in OSCs, enabling high performance transistors without changing the OSC chemical structure. However, it is difficult to study the morphological and polymorph formations that enable high OTFT performance in situ. Not only does the thin film crystallize at a fast time scale, the evaporation front, where the crystal grows from the solution, is very small. The entire evaporation front can be less than 200 microns. Thus, the solution evolves into a crystallized thin film within seconds, and within an area less than 0.2 mm wide.
We use an X-ray ‘microbeam’ at the Cornell High Energy Synchrotron Source, with a beam width of < 20 microns, in conjunction with a high speed detector to resolve and follow crystallization from solution of the OSC during solution shearing. We have collected up to 100 frames per second X-ray images, and are able to create grazing incidence x-ray diffraction movies to easily see how crystallization occurs in the solution shearing system in real time. We also use an optical microscope trained at the evaporation front, which we can use to collect optical videos of the evaporation front at up to 10,000 frames per second. Being able to simultaneously study kinetic crystallization using both optical and X-ray movies helps us understand how different processing conditions result in various polymorphs. We study the model OSC 6,13-bis(triisopropyl)-silylethynyl pentacene (TIPS-pentacene) and show that confinement of the growing thin film plays a key role in forming metastable polymorphs, and that the film formation proceeds downwards from the air-solution interface. We generate metastable crystal polymorphs through other solution processing conditions as well. This is the first time such a fast rate of data collection has been utilized for grazing incidence X-ray diffraction.
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