Vertical type organic light emitting transistors (OLETs), which combined with the organic static induction transistors
(OSITs) as the vertical type organic transistors and conventional organic light emitting diodes (OLEDs), are promising
for flexible sheet displays. In our recent research, pentacene based OSITs fabricated on flexible plastic substrates
exhibited stable electrical characteristics under bending condition. However, several problems have remained in the
OSITs with low on/off ratio and low current value for use in the vertical type OLETs.
This paper describes our recent efforts to improve the performance of vertical type OLETs by optimizing the device
structure and organic materials used in the OSITs. Interface between organic source electrode and semiconductor is
important factor to control the device operation of vertical type organic transistors. CuPc layer was inserted between ITO
source electrode and pentacene film in order to improve the performance of OSITs. The results showed that both the high
on/off ratio more than 103 and the high current value of larger than 40 μA were achieved. In addition, vertical type OLET
and logic devices were fabricated using the OSITs for the realization of flexible sheet displays. These results exhibit that
the OSITs attract attention for application in organic flexible displays.
We propose a new type organic light emitting transistor (OLET) combining static induction transistor (SIT) with double hetero junction type organic light emitting diodes (OLED) using n-type zinc oxide (ZnO) films which works as a transparent and electron injection layer. The device characteristics of newly developed OLED and ZnO-SIT showed relatively high luminance of about 500 cd/m2 at 7.6 mA/cm2 and is able to control by gate voltage as low as a few volts, respectively. The crystal structures of the ZnO films as a function of Ar/O2 flow ratio and the basic characteristics of the thin film transistor (TFT) and SIT depending on the ZnO sputtering conditions are investigated. The results obtained here show that the OLET using ZnO film is a suitable element for flexible sheet displays.