A high-resolution near-edge x-ray absorption fine structure investigation of the molecular orientation in the pentacene/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) pentacene/system

We present x-ray photoemission spectroscopy and highly resolved near-edge x-ray absorption fine
structure spectroscopy measurements taken on pentacene thin films of different thicknesses
deposited on a spin coated poly3,4-ethylenedioxythiophene:polystyrenesulfonate PEDOT:PSS
substrate. Thin films of pentacene were prepared by using organic molecular beam deposition in situ
using strictly controlled evaporation conditions. Our investigations show that pentacene thin films
on PEDOT:PSS are characterized by upright standing molecules. Due to the strong dichroic
behavior, the calculated values of the molecular orientation give a clear indication not only of the
real molecular arrangement in the films but also of a high orientational order. This high degree of
molecular orientation order is a characteristic already of the first layer. The films show the tendency
to grow on the PEDOT:PSS substrate following an island-fashion mode, with a relatively narrow
intermixing zone at the interface between the pentacene and the polymer blend. The peculiarity of
the growth of pentacene on PEDOT:PSS is due to the fact that the substrate does not offer any
template for the nucleated films and thus exerts a lateral order toward the crystal structure
arrangement. Under these conditions, the upright orientation of the molecules in the films minimizes
the energy required for the system stability.