Charge injection property of organic thin film devices is a key issue to
understand the device operation. Displacement current measurement (DCM) is a
powerful technique to probe the charge injection behaviors in terms of a
change in the apparent capacitance of test devices. However, it requires to
suppress actual current flowing through the device for investigating the
details of interface phenomena. We propose here the use of ionic liquids
(ILs) as a top contact insulator in organic metal-insulator-semiconductor
(MIS) structures. Because of the high stability and dielectric constant of
the ILs, the external applied voltage was applied mainly to the organic
layer with suppressing the actual current. The DCM curves of Pt
wire/IL/α-NPD/ITO structure were measured, and they actually show the
signals due to the hole injection from theITO to α-NPD layer and
accumulation at the IL/α-NPD.