Tender Resonant X-ray Scattering (T-ReXS)

The performance of organic semiconductor devices critically depends upon the thin film microstructure of the solution-processed active layers. This microstructure is often a combination of crystalline and amorphous regions, and the network of both regions strongly underpin charged transport properties. This microstructure has for a long time proved difficult to characterise even in the most aligned (lamellar) samples. Typical crystallographic methods to determine microstructure, such as Wide Angle X-ray Scattering (WAXS), suffer from a lack of diffraction peaks required to differentiate between molecular packing models.

Resonant scattering offers enhanced morphological determination through stronger cross sectional scattering and varying optical contrast. Whilst diffraction from atomic planes is not accessible at resonant carbon energies due to the long X-ray wavelength (C-K-Edge 280 eV ~ 5 nm), many organic semiconductors are sulfonated (S-K-edge 2480 eV ~ 5 Å), and hence can offer resonant diffraction in the tender energy regime. By utilising Tender Resonant X-ray Scattering (T-ReXS), dynamically varying diffraction intensities can be correlated to changing structure factor which is sensitive to the atomic positions.

In this work, I explore T-ReXS in the context of grazing incidence (GI-WAXS) measurements of the common semiconducting polymer poly(3-hexylthiophene), P3HT, where complexity arises from influence of waveguiding, enhanced optical absorption, and polarisation. I show how resonant experiments can be performed to optimise conditions for organic semiconductors, and I also suggest what morphological information can be extracted from resonant diffraction experiments.

A particular focus of mine has been developing (python) programmatic infrastructure to undertake such work, which requires the analysis and modelling of resonant optical constant spectra. Tools include the development of Dr. Watt’s KKCalc (a Kramer’s Kronig Transform calculator for optical spectra), the introduction of pyNexafs (a toolkit for parsing and processing NEXAFS data), and XEFI (an X-ray Electric Field Intensity calculator for thin films). I also discuss some of the modern standards required to produce reliable and trustworthy open-source toolkits.

This work underpins T-ReXS to investigate other sulfonated organic semiconductors (such as PBTTT, N2200, etc) or alternative elements such as chlorinated (Cl-K-Edge 2820 eV ~ 4 Å) organic semiconductors.

Dr. Matthew Gebert is a postdoctoral research fellow in the group of Prof. Christopher McNeill at the Department of Materials Science Engineering at Monash University. Matthew previously completed his PhD in 2024, in Cond. Mat. Physics at the ARC Centre of Excellence in Future Low Energy Electronics, under the supervision of Prof. Michael Fuhrer.