Morphology and Charge Transport Properties of P(NDI2OD-T2)/Polystyrene Blends
Linjing Tang, Benjamin Watts, Lars Thomsen, Christopher R. McNeill
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.