Structural and morphological changes of P3HT films in the planar geometry of an OFET device under an applied electric field

We show that both the morphology and crystal structure of regioregular poly(3-hexylthiophene) (RR-P3HT) films in organic field effect transistor (OFET) devices with top surface contacts are altered, when an electric field is applied. Grazing incidence X-ray diffraction (GIXD) analysis reveals a dramatic decrease in structural order in the non-linear current regime and, by contrast, much less pronounced structural changes of the (h 0 0) peaks in the linear current regime. Using for the first time resonance soft X-ray reflectivity (RSoXR) on RR-P3HT to avoid radiation damaged and to enhance the scope of the attained depth resolved information, we show that these changes are located in a several tens of nanometer thin sub-surface layer of the active polymer in the device. Furthermore, the RSoXR data reveal that the changes are strongly related to the C 1s → π∗ and C 1s → σ∗-resonances. In situ AFM measurements of the RR-P3HT device surface support the reciprocal space analysis results by showing a substantial softening and lateral expansion of the RR-P3HT films, when 20 V are applied between the contacts. These morphological and structural changes are partially reversible in nature.