Side-Chain Engineering Controls Molecular Packing and Vertical Electron Transport in Discrete n-Type NDI-T2 Oligomers
High-efficiency vertical charge transport in multilayer organic optoelectronic devices highly depends on molecular structure and arrangement. Using two T2-(NDI-T2)2 conjugated oligomers with ethylhexyl (EH) and octyldodecyl (OD) side chains as a model system, their effects on electron transport are investigated using, among other techniques, atomic force microscopy (AFM), conductive atomic force microscopy (C-AFM), grazing-incidence wide-angle X-ray scattering (GIWAXS), steady-state optical spectroscopy, and time-dependent density functional theory (TDDFT) calculations. The oligomers of both materials exhibit relatively disordered structures in the as-cast state, whereas annealing enhances structural order and leads to side-chain-dependent changes in molecular stacking. EH-based films show a smaller annealing-induced photoluminescence shift than OD-based films, suggesting less pronounced stacking and conformational rearrangements, consistent with TDDFT-supported exciton analysis. The current-voltage characteristics are analyzed using the space-charge-limited current (SCLC) model. After annealing, both materials exhibit improved vertical electron mobility. The EH-based films reach a maximum mobility of 1.2 x 10^-4 cm^2 V^-1 s^-1. The combined results suggest that the shorter EH side chains modify the out-of-plane packing environment and facilitate vertical electron transport. These results establish a structure-property relationship between side-chain structure, molecular order, exciton coupling and vertical mobility, providing a molecular design basis for optimising organic semiconductors through side-chain engineering.