Mapping of crystalline grain orientation for solution-processed semiconducting polymer thin films is key to understanding charge transport in electronic devices based on them and yet challenging. In this work, a high mobility thienothiophene copolymer, poly(2,5-Bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene (pBTTT), was used for grain mapping by dark-field (DF) transmission electron microscopy (TEM) and a pixel-by-pixel image analysis. The large, terraced crystals of pBTTT that form in spin-cast thin films after heating to a mesophase on a hydrophobic substrate was revealed to be made up of micron- or submicron-sized crystal grains which contain even smaller nanocrystals. Although grain size depends significantly on the film thickness and the volatility of the solvent used for spin-casting, the field-effect mobility and charge carrier hopping activation energy (extracted from variable temperature mobility measurements) are not much affected. This is attributed to the feature that grain orientation changes smoothly across the film, forming percolated charge transport pathways regardless of grain size.
Citation: Advanced Materials
Pub Type: Journals
Transmission electron microscopy (tem), dark-field tem, crystal orientation mapping, organic electronics, semiconducting polymers, transistor performance, structure property relationships, polymer crystals