Growth kinetics of single crystal C8-BTBT rods using solvent vapor annealing

Abstract

Small molecules as organic semiconductors have been extensively studied due to their unique properties and their applications such as organic photovoltaics, organic light-emitting diodes, and organic field effect transistors (OFETs). Especially in the field of OFETs, the best performance is usually observed in the single crystalline state (long-range order) of conjugated small molecules rather than in their polycrystalline or amorphous state. Thus, relevant research efforts have been concentrated in this regard. Recently, a few groups have devoted their efforts to grow single crystals using organic semiconducting small molecules via solvent vapor annealing (SVA). An example is the re-crystallization of dioctylbenzothienobenzothiophene (C8-BTBT) by SVA on poly(methyl methacrylate)(PMMA) as polymer base film (PBF). They insist that re-crystallizing the small molecules by SVA is correlated with the miscibility of solvents and PBFs, because soluble polymer can uptake condensed solvent on the surface, thereby small molecules on polymer obtain mobility and eventually self-assemble and grow in the form of a rod via Ostwald ripening. However, it has not been attempted to understand the growth kinetics of C8-BTBT single crystal micro-rods. Here, we customized the solvent flow system to observe in-situ, the growth of C8-BTBT single crystal rods on PMMA during SVA at different conditions (pressure and temperature), and quantified the total volume over time under each condition. First, we varied the solvent vapor pressure using chloroform as a solvent. We find that the C8-BTBT single crystal is phase-separated within ~ 5 minutes from the precursor and they show different growth trends. At low pressure, C8-BTBT generally grows into short rods and coarsening of C8-BTBT rods does not significantly occur because the extremely small amount of choloroform remains on the substrate induced by dynamic equilibrium of adsorption and desorption (constant volume at 200 Torr). On the other hand, at high pressure, the amount of chloroform adsorbed on the substrate dramatically increases. Thus, the recrystallized C8-BTBT rods are re-dissolved. At 220 Torr, although there is some loss of C8-BTBT rods in total volume, longer rods form which is the adequate growth condition for real device applications. Next, we also varied temperature and find that if the temperature is high, the C8-BTBT rods grow short. This is because the higher excited state of PMMA hinders adsorption of chloroform on the substrate, which lets C8-BTBT molecules not diffusive. As a result, we conclude that C8-BTBT rods grow in long rod forms under the specific condition at 220 Torr and 28 ℃. Finally, we observed an unusual phenomenon, that is C8-BTBT rods are encapsulated in liquid chloroform on the substrate, and they grow and coarsen one another within this capsule. The results shown above could provide key information to fabricate the devices using various small molecule single crystals.