Interfacial Width Measurements of Dielectric/P(NDI2OD-T2) Using Resonant Soft X-ray Reflectivity*

Interfaces between a conjugated polymer and a dielectric play a critical role in organic thin-film transistors, yet it's difficult to measure. Resonant Soft X-ray Reflectivity (R-SoXR) is a unique and relatively simple method to investigate such interfaces. By tuning the soft X-ray energies, we are able to selectively and quantitatively characterize the interfacial width and thicknesses of the films. In an effort to relate performance to interface structure, we have used R-SoXR to investigate polystyrene (PS) or poly(methyl methacrylate) (PMMA) as the top layer and poly{\{}[N,N9-bis(2-octyldodecyl)-naphtha-lene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5, 59-(2,29-bithiophene){\}} (P(NDI2OD-T2)) as bottom layer supported on a Si substrate. We found that the device with PS as dielectric has a higher threshold voltage, which correlates to the increased interfacial trapping due to increased interfacial roughness. The extension of R-SoXR to the energy of fluorine K absorption edge is also demonstrated.