Mapping grain orientation of high mobility thienothiophene copolymer thin films by transmission electron microscopy and image analysis

In this work, poly(2,5-Bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene (pBTTT), a state-of-the-art solution-processable semiconducting polymer with field-effect hole mobility as high as 0.1-1 cm²/V s, was used for grain mapping. The large, terraced crystals of pBTTT that form in spin-cast thin films after heating to a mesophase on a hydrophobic substrate extend laterally for microns and vertically throughout the film thickness, making it a suitable material for imaging compared with other semiconducting polymers. By using dark-field (DF) transmission electron microscopy (TEM), we found that the characteristic terraced structure of a pBTTT with tetradecyl side chains (pBTTT-C₁₄) is made up of micron- or submicron-sized crystalline grains which contain even smaller nanocrystals.