Nano-scale structure of polymers and small molecules
in organic electronic applications

Abstract

The thesis explores novel electron microscopy techniques for probing the molecular ordering in organic thin-films at high spatial resolution. The research is relevant to the wider field of optoelectronics as organic electronic devices, such as organic photovoltaics (OPV) and organic light emitting diodes (OLED), have become desirable as a potentially low-cost, flexible, and lightweight alternative to conventional inorganic technologies. The crystallinity in organic materials determines important properties, such as charge carrier mobility, but is extremely complex and difficult to characterise. Unlike standard inorganic crystals that have long-range order (LRO), organic materials tend to be disordered, with some regions of the material amorphous, and other regions semi-crystalline or crystalline. The study uses several electron microscopy methods to examine the nanoscale ordering in organic thin-films, including a variation of the fluctuation electron microscopy (FEM) technique performed in a scanning transmission electron microscope (STEM) to probe the medium-range order (MRO), as well as electron energy loss spectroscopy (EELS) to determine the orientation of π-stacking from the carbon K-edge fine structure. Multislice simulation (Chapter 3) is also used to examine the role of a range of parameters such as particle crystalline size, depth in the sample, atomic number, and para-crystallinity on the expected contrast in dark-field STEM imaging. The effect of these parameters is difficult to isolate by experiment, and therefore simulations are required to make estimates. The sensitivity to parameters such as the particle size was found to be similar to that estimated from experiment.

Chapter 4 presents the FEM results. STEM annular dark field (DF) imaging is used to measure the diffracted intensity of crystalline domains. By varying the inner angle of the annular detector used for DF imaging, it is possible to control the ‘coherence volume’, i.e. the volume over which diffraction takes place. This provides information on the degree of crystallinity as well as domain size. The measurements were first performed on a polycrystalline NiO thin-film test sample. It was found that the STEM technique overall showed the expected behaviour for NiO. After testing NiO, we examined samples of drop cast and spin coated TIPS pentacene by a conventional TEM at 200 kV, and found limitations in the information that could be extracted due to specimen damage under the electron beam. However, we found that useful information could be obtained on a 200 kV Talos TEM with direct electron detector and 4D STEM measurements. The drop cast TIPS pentacene diffraction pattern resembles previous reports in the literature. In particular, diffuse streaking due to pentacene molecular movement is seen. The molecular disorder was not uniformly distributed across the sample, as seen by virtual dark-field images taken from several points along the streak. A virtual dark-field image of the spin coated TIPS pentacene sample showed grains with an average size of 58.7 nm, and the diffraction pattern also showed Bragg reflections. The spin coated sample appears to have more paracrystallinity inside the grains than the drop cast sample, according to variance plots obtained from the same 4D STEM datasets.

EELS spectroscopy (Chapter 5) of the carbon K-edge fine structure is used to map the local orientation of organic thin-films like TIPS pentacene. A natural graphite sample was used to calibrate the π∗/σ∗ ratio as a function of specimen orientation for both TEM and STEM modes. Application of EELS spectrum imaging to a TIPS pentacene thin-film however showed that the minimum dose required for an accurate π∗/σ∗ ratio was several orders of magnitude above the critical electron beam dose for damage under the electron beam. Therefore, successful application of EELS to study molecular disorder requires strategies to overcome the beam damage in organic thin-films. These include increasing the energy of the incident electron beam (higher kV), using a cryo-microscope to cool the specimen, and direct electron detectors with higher detector quantum efficiency, as well as dose fractionation.