Electron Spin Resonance of Charge Carriers in Organic Field-Effect Devices

Abstract

We have developed a field-induced electron spin resonance (ESR) technique to detect charge carriers with spins that are generated at the interface of organic field-effect devices. ESR signals have been successfully observed in the devices of high-mobility organic materials, such as regioregular poly(3-alkylthiophene) (RR-P3AT), in the case of polymer, and pentacene, in the case of small organic molecules. ESR signals of these devices provide valuable information concerning the spin–charge relationship, the spatial extent of charge carriers and molecular orientations at the device interfaces. Instead of Al2O3 gate insulators initially used in devices for ESR, more conventional SiO2 gate insulators on the relatively low-doped Si substrates have been introduced in obtaining ESR signals without lowering the Q-value of the ESR cavity. We present gate-bias dependence of ESR signals of RR-P3AT devices with different gate insulators, where the spins tend to saturate above the charge concentration of about 0.2%, regardless of gate insulating materials. This fact indicates that the charge carriers of the polymers change from polarons with spins to spinless bipolarons or polaron pairs at higher doping levels. Anisotropic ESR spectra of unpaired π-electrons in the devices with SiO2 gate insulators have revealed preferential orientation of the molecular plane, consistent with the formation of self-organized lamellar structures of RR-P3AT as in the case of Al2O3 gate insulators. Motional narrowing of ESR spectra has been observed, depending on the alkyl-chain lengths of RR-P3AT, which could be related to the reported alkyl-chain dependence of carrier mobility.