Imaging nanostructures in organic semiconductor films with scanning transmission X-ray spectro-microscopy

Abstract

Thin films of organic semiconducting materials are of increasing technological importance in optoelectronic devices such as light emitting diodes (LEDs), lasers, field effect transistors (FETs) and solar cells. However, the morphology of such films is complex, often displaying three dimensional composition structure or molecular alignment effects. The structure of the polymer film incorporated into a device can strongly affect its performance characteristics, e.g. via the connectedness of polymer domains and to the device electrodes, or due to anisotropic material properties due to molecular alignment.

Scanning transmission X-ray spectro-microscopy (STXM) has been demonstrated to be an excellent tool for the study of organic materials due to its high spatial resolution (down to about 20 nm) and strong contrast based on a variety of spectroscopic mechanisms. In particular, tuning the probing X-ray beam to resonances in the near edge X-ray absorption fine structure (NEXAFS) spectra provides a mechanism for molecular-structure based contrast which is a very powerful tool for studying blends of organic components. A further advantage of STXM is that strategic use of spectroscopic information allows quantitative compositional analysis of imaged areas. Recent work at the PolLux STXM has demonstrated two new developments in the imaging of thin organic films: simultaneous surface and bulk imaging via an additional channeltron electron detector, and molecular orientation mapping via anisotropic near edge resonances.

Introduction

Thin films of organic semiconducting materials are of increasing technological importance in optoelectronic devices such as light emitting diodes (LEDs) [1], [2], lasers [3], field effect transistors (FETs) [4], [5], [6] and solar cells [7], [8]. However, the morphology of such films is complex, often displaying three dimensional composition structure or molecular alignment effects. The structure of the polymer film incorporated into a device can strongly affect its performance characteristics, e.g. via the connectedness of polymer domains [9] and to the device electrodes, or due to anisotropic material properties due to molecular alignment [10], [11]. For such reasons, knowledge of the structures within organic semiconductor thin films provides a clear path towards further advancement of organic optoelectronic device technologies. However, imaging the structure of thin organic films is challenging due to a combination of small length scales and the limited variation in elemental composition and density amongst organic materials. For example, ultra-violet, visible and infra-red microscopy techniques are limited in resolution by the wavelength of the probe beam, while electron microscopy and scanning probe microscopy struggle to achieve significant (and unambiguous) chemical contrast. Scanning transmission X-ray spectro-microscopy (STXM) provides an excellent opportunity for imaging nanostructures in organic thin films due to its combination of high resolution (down to about 20 nm [12], [13]) and strong, natural contrast via X-ray spectroscopy [13]. In particular, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy can be utilised to achieve strong contrast between organic materials based on differences in the types of chemical bonding present in the molecules [14].

In NEXAFS spectroscopy, X-ray absorption resonance peaks are observed at photon energies close to the binding energy of a core electron shell. These resonances correspond to an electronic transition from the core shell to an unoccupied (i.e. anti-bonding) state whose energy follows from the associated bonding orbital and thus the molecular structure of the sample material (as illustrated in Fig. 1) [14], [15]. For organic materials, the rich structure of the C K-edge NEXAFS spectrum provides a good fingerprint of the material and plentiful opportunities for strong contrast (by tuning the photon energy of the STXM probe beam) between organic materials without the need for labeling [13], [16].

The utility of STXM for imaging organic nanostructures is further extended via the ability to exploit linear X-ray dichroism in order to provide sensitivity to molecular order and orientation [14], [17], [18]. Since anti-bonding orbitals have a limited spatial extent, electronic transitions to such states also have a preferred direction and thus the intensity of the associated NEXAFS resonances depend on the alignment between the electric field vector of the X-ray beam and the transition dipole moment. Thus, by performing STXM measurements with a linearly polarized X-ray beam with a photon energy tuned to that of a dichroic NEXAFS resonance, contrast is observed between chemically identical molecules due to differing orientations with respect to the polarisation axis [18].

In this article, we present the advantages of STXM in imaging nanostructures in organic semiconductor films using the example polymers poly(9,9′-dioctylfluorene-co-bis(N,N′-(4,butylphenyl))bis(N,N′-phenyl-1,4-phenylene)diamine) (PFB) and poly(9,9′-dioctylfluorene-co-benzothiadiazole) (F8BT). The molecular structures and NEXAFS spectra of PFB and F8BT are presented in Fig. 2. Further details of the STXM studies showcased here can be found in the respective previous publications [19], [20], [21], [22], [23].