Mapping Grain Orientation of High-Performance polythiophene Thin Films by Transmission Electron Microscopy and Image Analysis
Xinran Zhang1, Steven D. Hudson1, Dean M. DeLongchamp1, David J. Gundlach2
1 Polymers Division, MSEL, NIST
2 Semiconductor Electronics Division, EEEL, NIST
Poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes (pBTTTs) are among the best solution-processable semiconducting polymers in terms of field-effect charge carrier mobility (0.1 – 1 cm2 /V s). Their high performance is attributed to the large, terraced crystals that form in spin-cast thin films after heating to a mesophase on a hydrophobic substrate. However, the correlation of lateral pBTTT grain size with charge carrier mobility has remained unknown, because accurate determination of lateral pBTTT grain size has not been possible by either scanning probe techniques such as atomic force microscopy (AFM) or whole film microstructure analysis techniques such as X-ray diffraction (XRD). In this work, we use dark-field transmission electron microscopy (DF-TEM) and image analysis to show that the characteristic terraced structure of a pBTTT with tetradecyl side chains (pBTTT-C14) is made up of micron- or submicron-sized crystal grains which contain even smaller nanocrystals. The grain size is found to be dependent on the film thickness and the volatility of the solvent used for spin-casting, as well as how we define a grain boundary. We further correlate changes in grain size to the charge carrier hopping activation energy, extracted from variable temperature charge carrier mobility measurements.