Orientation of Charge-Carrier-Injected Molecules in Ambipolar Organic Semiconductor Devices: An Operando Electron Spin Resonance Study

Ambipolar organic devices that enable both p-type and n-type operations in a single device component have various attractive properties. In spite of the close correlation between the orientations of such molecules and charge transport properties, the orientations of molecules at which injected holes and electrons are located in these devices remain unclarified from a microscopic viewpoint. This issue was therefore studied at a molecular level by applying an operando electron spin resonance (ESR) technique to ambipolar organic devices. Typical organic semiconductors of pentacene and regioregular poly(3-hexylthiophene) (RR-P3HT) were used in the devices. Using this technique, we directly observed holes and electrons injected by gate bias voltage. Then, details of the orientations of charge-carrier-injected molecules were investigated based on the angular dependence of g values in the ESR spectra using density functional calculations of theoretical g tensors. The molecular orientations found using operando ESR and X-ray diffraction measurements were compared. Insights from microscopic observations with respect to the orientations of charge-carrier-injected molecules are important for the fundamental understanding of organic semiconducting materials and for improving the performance of ambipolar organic devices.