Nanofiber growth and alignment in solution processed n-type naphthalene-diimide-based polymeric field-effect transistors

The nanofiber growth of an n-type conjugated polymer poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)}, P(NDI2OD-T2), was studied during solution processing. Highly reproducible fibrous nanostructures obtained at various dip coating speeds provide an insight into the growth mechanism of the polymer nanofibers during solution deposition and into the correlation between their morphology and charge-carrier mobility of organic field-effect transistors (OFETs). Remarkably, the nanofibers orient parallel to the dip coating direction on reaching 4 layers leading to a significantly improved charge transport between source and drain electrodes. We have observed that the morphology, thickness and orientation have great impacts on the charge carrier transport. We prove that the first fully covered fibrous layer is of minimum thickness for a charge carrier migration to happen in the transistor, whereas the mobility strongly rises in the subsequent 3-4 fibrous layers, which are considered to provide alternative pathways for charge carriers. The strong increase in the thickness of 3-4 layers is also attributed to the additional film alignment taking place during the dip coating process. Therefore, the type of fibrous nanostructures does not change with film thickness.