Morphology and Charge Transport Properties of P(NDI2OD-T2)/Polystyrene Blends

Abstract

While the strategy of blending commodity insulating polymers with conjugated polymers has been previously applied to modify the properties of organic field-effect transistors (OFETs), comprehensive studies on the evolution of morphology and OFET performance with film composition are scarce. In this contribution, films comprising the n-type semiconducting polymer P(NDI2OD-T2) and the insulating polymer polystyrene (PS) are explored in detail. In particular, the electrical properties and film morphology of blends with a P(NDI2OD-T2) concentration ranging from 0.125 to 100 wt % are studied. Two regimes characterized by distinct dependencies of device mobility as a function of P(NDI2OD-T2) content are observed: Above 12.5 wt % P(NDI2OD-T2), the OFET mobility exhibits a weak dependence on P(NDI2OD-T2) concentration despite the presence of strong lateral phase separation, while below 12.5%, the device mobility is observed to decrease strongly with decreasing P(NDI2OD-T2) concentration attributed to disrupted surface connectivity of the P(NDI2OD-T2) phase. UV–vis absorption spectroscopy and grazing incidence wide-angle X-ray scattering (GIWAXS) measurements indicate that the aggregation and crystalline packing are not strongly affected by the presence of PS. Atomic force microscopy and scanning transmission X-ray spectro-microscopy reveal lateral phase separation for blends with compositions of 87.5 to 25 wt % P(NDI2OD-T2). Enrichment of P(NDI2OD-T2) at the film surface is confirmed by near-edge X-ray absorption fine structure spectroscopy, with a high P(NDI2OD-T2) surface composition (>80 wt %) maintained down to an overall blend composition of 12.5 wt % P(NDI2OD-T2). The strong decrease in OFET mobility for P(NDI2OD-T2) concentration less than 25 wt % is attributed to a loss of P(NDI2OD-T2) surface connectivity at low P(NDI2OD-T2) loading. We also observe a transition from face-on to edge-on dominated configuration at low P(NDI2OD-T2) content with GIWAXS, consistent with an edge-on surface layer that persists down to monolayer surface coverage.