Field-induced electron spin resonance spectroscopy of organic
semiconductors
Shin-ichi Kuroda
Purpose of Research
In recent years, much attention is paid to organic electronic devices based on organic
semiconductors due to their flexibility and large area coverage. Typical devices areorganic light-emitting diodes, organic transistors and organic solar cells using πconjugated polymers and small molecules. Among them, organic transistors are drawing
interests due to their high mobilities exceeding those of amorphous silicon. The charge
carriers of organic transistors accumulate at the interface between the semiconductor andthe insulator. Observation of those carriers is an important problem in understanding
electronic processes occurring in the device, while their detection in the actual devices is
considerably limited because the interface is not easily accessible from the outside of thedevice.
In this research project, we focus on the observation of charge carriers in organic
transistors, by using field-induced electron spin resonance (ESR), we have developed,
which can detect spins of the charges at the device interface with a high sensitivity ( ~1011spins). ESR signals provide information concerning electronic states and conduction
mechanism of the organic interface, from which we may develop design principles ofhighly efficient organic devices. In addition, we will search metal transition and spintransition phenomena by high-concentration charge injection using ionic liquids and aim
at the development and control of new physical phenomena in organic semiconductors.
Methods
(a)

(b)

(c)

Figure 1 (a) Schematic structure of an organic field-effect transistor with solid gateinsulator. (b) Field-induced ESR signals of an ionic-liquid gated polymeric transistor.
(c) Molecular orientation of π-conjugated polymers at the device interface.
We first construct organic transistor device structures that fit into ESR cavity. As aninsulating material used for the transistor, we mostly employ SiO 2 solid insulating layer

on low-doped Si substrates developed by us for ESR measurements that do not deteriorate
Q-factor of ESR cavity. We also use ionic liquid insulating films capable of injecting highconcentration charge carriers. As for the organic semiconductors, thiophene-based
polymers having high crystallinity or small molecules are used. The electricalcharacteristics of those devices are measured to obtain the device parameters such as
mobility. Then the ESR measurements are carried out on those devices as collaborative
research. The g value and the line width obtained from ESR signals are determined andanalyzed with the aid of ESR simulation, in collaboration with the theoretical group todetermine the electronic or spin states, dynamics of carriers as well as interfacial
molecular orientations, which are crucial in determining the performance of devices.
Perspectives
Field-induced ESR is capable of clarifying microscopic information of device interface
which is difficult to observe by other methods and contributes to the development of highperformance devices. In addition, the transistor structure that can reversibly controls thenumber of carriers gives a new method of elucidating the structure-function relationshipof organic molecular aggregates by carrier injection and contributes to the developmentof future electronic materials and devices.