Advancement of Microscopic Evaluation Techniques for Flexible Devices

We develop microscopic evaluation techniques for flexible electronics devices to improve and stabilize the device characteristics that are necessary for industrial applications. Newly-developed methods allow us to investigate much more detailed and accurate microscopic carrier transport properties than is the case with inorganic semiconductor devices. On the other hand, it often becomes quite difficult to apply the conventional evaluation techniques used for inorganic semiconductor devices. 

Topics：

1. Electron Spin Resonance Observation of Field-Induced Carrier Dynamics

The dynamics of carriers in organic field-effect transistors were investigated by the electron spin resonance (ESR) method. We successfully observed the motional narrowing effect of the ESR spectrum of gate-induced carriers.

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Phys. Rev. Lett. 100, 126601 (2008), Kotai-Butsuri vol. 43, No.6, 351 (2008). 

2.ESR Analysis of the Distribution of Trap States

Trap states in organic field-effect transistors were investigated by a low-temperature ESR method. From fine analysis of the ESR line shape, we determined the density of the trap states as a function of the spatial extent of wave functions.

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Phys. Rev. Lett. 104, 056602 (2010),Phys. Rev. B 85, 085211 (2012),Nihon Butsuri Gakkai-Shi Vol. 67, No. 10, 695 (2012).

3.ESR Analysis on the Domain Boundaries of Polycrystalline Devices

　 The potential barrier at domain boundaries of polycrystalline organic field-effect transistors were investigated by the ESR method. By using the parallel and perpendicular setups, we successfully determined the transport efficiencies for intra- and interdomain charge transport, respectively.

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Phys. Rev. B 85, 035308 (2012), Denshi Spin Science Vol. 10, No. 19, 82 (2012).

4.Visualization of Carrier Density in Organic TFT Channels

　 We have developed a technique of charge-induced elastic modulation interferometry for analyzing the local densities of accumulated charges in organic thin-film transistors. The method permits visualization of the temporal evolution of the charging and discharging of the channel layer.

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Appl. Phys. Lett. 95, 223301 (2009).

5.Modulation Spectroscopy of Organic Thin-Film Transistors

The effects on the excitonic absorbance of pentacene thin films were investigated using unipolar field-effect device structures that enable us to separate the effects of charge accumulation and field application.

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Phys. Rev. B 82, 161301(R) (2010). 

6.Investigation into the Origin of Bias Stress Effects in Organic Transistors

A scanning Kelvin probe microscope (SKPM) was used to investigate the origin of device instability in organic transistors. We found that bias-stress effects are caused by the hydrophilic/hydrophobic boundary on top of a gate dielectric layer in which the trapped charges are feasibly accumulated.

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Appl. Phys. Lett. 95, 223304 (2009).