Influence of alkyl side-chain type and length on the thin film microstructure and OFET performance of naphthalene diimide-based organic semiconductors

Here we study the effect of side chain length and type on the microstructure and organic field-effect transistor (OFET) performance of solution-processed naphthalene diimide (NDI) thin films. Linear side chains with four (C4), five (C5), six (C6), eight (C8) and twelve (C12) carbon atoms are studied along with a branched ethylhexyl (EH) side chain. Interestingly, mobilities of up to ~0.2 cm2/Vs are achieved for short (C4) and long (C12) side chains with linear chains of intermediate length and the branched side chain producing lower mobilities. The observed mobility trends are explained in terms of the competing influence of changes in crystal packing and changes in thin film morphology with changes in side chain length. Shorter side chains produce changes in the lateral stacking of NDI units which promote higher mobility while longer side chains produce solution-processed thin films with higher film quality evidenced by larger domain sizes and lower orientational disorder. Side chain length is also found to strongly modulate the molecular orientation of the NDI core, with high edge-on orientations observed for long chains, and tilted orientations for short chains. Thin film microstructure is investigated using a range of techniques including atomic force microscopy, grazing incidence wide-angle X-ray scattering and near-edge X-ray absorption fine-structure spectroscopy.