In situ studies of conjugated polymers by X-ray diffraction techniques

In this thesis conjugated materials are investigated, with a focus on (semi)conductive polymers and oligomers. These materials are promising candidates for use in organic electronics, such as thin-film organic field-effect transistors (OFETs) and organic photovoltaic cells (OPVs). The aim is to elucidate structure-property relationships of active layers of conjugated organic thin films that influence device performance.
The structural investigation spans length scales from millimeters, corresponding to macroscopic device architecture, down to nanometers in the range of single crystalline domain sizes, that govern molecular stacking and crystalline order. Such complex structural information is essential for correlating multiscale ordering with device performance.
X-ray diffraction is a common technique for structural studies of thin films. However, conventional X-ray laboratory sources lack the brilliance to resolve these features at an advanced level. Therefore, this work is carried out at high-brilliance synchrotron facilities that enable high spatial and temporal resolution X-ray scattering and diffraction studies of active thin film layers. Even at synchrotron light facilities, small- and wide-angle X-ray scattering at conventional beamlines provide average device structures, while advanced nano-focused beamlines enable spatially resolved measurements and localized mappings across device cross-sections with nanometer precision.
Besides high-resolution measurements, in situ experimentation is a central part of the thesis. In situ annealing studies show how thermal treatment alters structural properties and its effect on electrical device performance. Understanding annealing dynamics and appropriate thermal budgets is beneficial for processing optimization and for developing robust fabrication protocols.
In addition to annealing, other external factors may also have an influence on the structural organization of the film. This work reveals how structural order is affected by an external electric field during film formation, highlighting possibilities for morphology control. The application of a permanent electric field to P3HT induces non-equilibrium aggregation, leading to reduced crystallinity, smaller crystallite size, and increased structural disorder. Such changes provide the advantages to tune morphology, including expanded interfacial area and enhanced nanoscale heterogeneity. In contrast, an alternating electric field allows partial relaxation of polymer chains, resulting in a more equilibrium-like structure with preserved crystallinity.
Finally, in operando studies provide direct insights into the structure and electrical response, tracking voltage-induced structural alterations and lattice responses under working conditions.

The outcomes of this research include:
• In situ studies of conjugated polymer film formation reveal structural changes under an external electrical field. The quality of structural order and the size of crystalline domains can be modified during P3HT drop-casting. This effect is greater with a permanent electrical field than with an oscillating field. While the disordered morphology induced by a permanent field is unfavorable for charge transport in transistors, it can be advantageous for applications such as organic solar cells with increased interfacial area and nanoscale heterogeneity.
• The thermal protocol during annealing has a strong effect on structural order. In situ annealing studies show that a low thermal budget preserves and improves the microstructure of polymer-fullerene blends. In contrast, a P3HT:PCBM blend undergoes permanent, irreversible changes during the high thermal budget annealing with PCBM crystallization and phase separation. The structural order of PDOPT decreases until melted but recovers when returning to room temperature at a significantly enhanced level.
• X-ray nanobeam diffraction on quasi-freestanding P3HT films demonstrates the feasibility of high spatial resolution diffraction studies on organic thin films revealing local orientation variations. This method is ideal for the investigation of the local crystal structure of organic semiconductors while minimizing substrate induced contributions. Using an extremely focused X-ray beam (spot size approximately 150 nm), local variations in orientation and symmetry within the polymer network are directly detectable.
• Nanobeam grazing incidence diffraction (nanoGIXD) experiments revealed a strong gold reorientation and modifications of Au-polymer interfaces during device operation which can be detrimental for applications.
• In-operando nanoGIXD measurements revealed a significant anisotropy of the oligomer thin films, under source-drain applied voltages with strong structural variations for both in-plane and out-of-plane directions, including a +1.3 % tensile expansion of the π-stacking distance (d_020). The outcomes underline that high resolution X-ray diffraction techniques are excellent tools for spatially and temporally resolved studies of conjugated organic thin films. The results can support the device processing optimization and development of robust fabrication protocols.