A new polymer interface structure: where lying-down makes all the difference
13 June 2012 ONDL scientist Jing-Mei ZHUO reports the discovery of a polymer adlayer at the dielectric interface that is decisive for field-effect transport, at the International Conference on Simulation of Organic Electronics and Photovoltaics 2012 today. This resolves a long-standing mystery and one of the most puzzling features of interfacial charge transport in high-carrier mobility polymer organic semiconductor field-effect transistors in which the field-effect mobility varies tremendously across nominally identical polymer films prepared on different substrates. Zhuo described an entire gamut of evidence from near-edge X-ray absorption fine structure spectroscopy to device spectroscopy and modeling for the existence of an adsorbed monolayer that is unfavorably oriented at the dielectric surface. This is surprising because for a long time, it has been thought that the interface structure of bulk films of these stiff-rod pi-conjugated polymer materials is dominated by polymer-polymer interactions rather than polymer-substrate interactions. As it turns out, depending on interactions with the substrate, the adlayer can lead to a range of behaviours, including trapping of charge carriers that causes a weak to marked reduction in mobility across the channel. This “transition” layer depends on competition with solvent adsorption and hence the details of film preparation, where in particular the surface termination of the dielectric is critical. “Our results reveal the “missing link” to the structural model of pi-conjugated polymer interfaces that is needed to understand field-effect transport at the interface and how this interface limits transport, and hence ultimately the role of solvent and film processing conditions. This is key to making consistently high field-effect mobilities across different dielectric surfaces!“ says Zhuo.