Side-Chain Engineering of Conjugated Polymers: Distinguishing Its Impact on Film Morphology and Electronic Structure

Solution processing of conjugated polymers is key for low-cost processing of organic electronic devices. To improve solubility, introducing alkyl side chains is a commonly employed approach, known for its impact on film morphology. The morphology of polymer films is a key aspect of the structure–function relationship in organic electronic devices with a strong impact on their overall efficiency. Although planarity of conjugated backbones is advantageous for exciton and charge carrier mobilities and the overall degree of order, it leads to aggregation in solution. This reduces solubility but can result in interesting structures. Side-chain-mediated backbone torsion, in contrast, greatly enhances solubility, facilitating synthesis and control of molecular weight, but often impairs performance. Detailed insight into the impact of side chains on both morphology and electronic structure is therefore of high demand. We demonstrate time-resolved electron paramagnetic resonance spectroscopy to be perfectly suited to probe the orientation as well as the overall degree of order in conjugated polymer films, while simultaneously revealing details of the electronic structure. By systematically studying the impact of additional side chains, we distinguish their impact on the film morphology and electronic structure. Additional side chains decrease exciton delocalization but not the overall degree of order in the film. Delocalization is therefore only connected to backbone planarity. Using magnetophotoselection experiments, we additionally present clear evidence for the preferential face-on orientation of the amorphous polymer backbone on the substrate. This is crucial for the efficiency of both organic photovoltaic devices and organic light-emitting diodes.