Charge Transport in Sub-Monolayer Networks of a Naphthalene-Diimide-Based Copolymer

Sub-monolayer networks of the electron-transporting semiconducting 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)) are realized through judicious choice of spin-coating solvent and control of solution concentration. These sub-monolayer networks provide a platform to study the effects of surface coverage, network connectivity, and orientational order on charge transport. It is found that charge transport through semiconducting polymer networks depends not only on the coverage/concentration of the material but also on the nature of the conduction paths themselves. Down to 40% surface coverage, current measured in a field-effect architecture decreases in line with the decrease in surface coverage, but drops precipitously below a surface coverage of 40%. Below 40% surface coverage, there is a marked decrease in network connectivity (quantified through the branching point density) and a decrease in orientational order. Furthermore, an increase in the density of dangling branches (dead ends) is also seen with decreasing surface coverage. Additional supporting data in the form of grazing incidence wide-angle X-ray scattering (GIWAXS), optical spectroscopy, and charge modulation spectroscopy are presented, which help to establish that long-range interconnectivity rather than local morphology dominates charge transport in P(NDI2OD-T2) sub-monolayers as well as thin films. Charge transport in sub-monolayer networks of the semiconducting polymer P(NDI2OD-T2) is found to be determined by network interconnectivity and orientational order and not solely by surface coverage. Below 40% surface coverage a strong decrease in current is observed which is connected to a decrease in branching point density, a decrease in orientational order, and an increase in the density of dead ends. image