Surface and Bulk Structural Characterization of a High-Mobility Electron-Transporting Polymer

Abstract

This publication investigates the microstructure of the high electron mobility polymer poly([ N,N′ -bis(2-octyldodecyl)-11 naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt -5,5′-(2,2′-12 biothiopene)) (P(NDI2OD-T2)) using a combination of grazing incidence X-ray diffraction (GIXD), near-edge X-ray absorption fine structure (NEXAFS), and optical spectroscopies. GIXD measurements confirm extended in-plane lamellae ordering and out-of-plane π−π stacking. Angle-resolved surface-sensitive NEXAFS measurements, however, indicate a lack of preferred in-plane or out-of-plane orientation of either the conjugated backbone or side chains at the P(NDI2OD-T2) film surface. This lack of pronounced orientation effects is attributed to a significant proportion of amorphous content in the film, with NEXAFS measurements sensitive to both amorphous and crystalline content while GIXD is sensitive only to coherent ordering. Furthermore, it is found that increased crystallinity in annealed thin films is not beneficial to the performance of top-gate bottom-contact field-effect transistors. In fact, both highly crystalline and amorphous P(NDI2OD-T2) devices exhibit similar device performance with saturation mobilities around 0.04 and 0.02 cm 2 /(V s), respectively, almost 1 order of magnitude lower than the high-performance devices where the pristine morphology has not been subjected to significant heat treatment. Changes in the optical absorption and Raman spectra of P(NDI2OD-T2) thin films with annealing are also presented and discussed.