We have already reported on the pentacene photo FET having a photo-sensitive gate dielectric layer (poly(Nvinylcarbazole):
PVK). This photo FET showed excellent photo-switching properties upon illuminating the gate
dielectric layer, because the gate capacitor was rapidly charged up by providing a large amount of photo-generated
charges in the dielectric layer to the gate capacitor. When the gate bias and photo-illumination were turned off,
accumulated charges diffuse from the channel region to the source electrode and the channel immediately becomes OFF
state. Therefore, this photo FET can switch ON-OFF states rapidly, however, memory property is poor. In this study,
we have tried to give a memory function to the photo FET. For keeping the accumulated charges at the channel region
even after turning off the gate bias and photo-illumination, we intentionally formed Schottky barriers at the interface
between the semiconductor and the DS electrodes by using aluminum as a low work function metal instead of gold.
As a result, the photo FET did not show any gate voltage modulations of drain currents under dark condition. On the
other hand, the photo FET showed the drastic increase in drain current upon illuminating the gate dielectric layer.
Further, this high-drain current state was kept even after turning off the gate bias and photo-illumination (written state),
probably because the accumulated charges at the channel region could not escape from the source electrode owing to the
Schottky barrier at the semiconductor/DS electrodes interface. From these results, we have concluded that we could
develop the novel organic rewritable optical memory.
We fabricated a novel type photo-FET using poly(<i>N</i>-vinylcarbazole) (PVK) as a photosensitive gate dielectric. For the
photo-FET, photo-illumination to the PVK insulator layer makes the field-effect mobility μ<sub>FET</sub> two orders of magnitude
higher than dark condition. In particular, under blue-light illumination condition the on-off ratio was also a few ten times
higher than dark condition. Furthermore, by introducing blocking layer between semiconductor layer and insulator layer lead.
We concluded that the improvement of the transistor properties resulted from effective charge accumulation at the conductive
channel by photo illuminations.
We fabricated a novel type photo-FET using poly(<i>N</i>-vinylcarbazole) as a photosensitive gate dielectric. For the photo-FET, photo-illumination to the PVK insulator layer make the field-effect mobility μ<sub>FET</sub> two order of magnitude higher than dark condition. In particular, under blue-light illumination condition the on-off ratio was also a few ten times higher than dark condition. We concluded that the improvement of the transistor properties resulted from effective charge accumulation at the conductive channel by photo illuminations.
We studied on the effect of nanoparticle dispersion on electrical properties of polymer field effect transistor (FET). Various kinds of powder materials were prepared and dispersed into P3HT films. In order to determine the electronic structures of particle-dispersed P3HT films, photoelectron emission spectra were measured. Using these spectra ionization potentials (I<sub>P</sub>s) of these films were determined. The shifts of IP originated from the particle dispersions were very small (from 4.69 to 4.72eV). Similarly, any differences were not observed between the electron absorption spectra of the particle-dispersed P3HT films and that of the pristine P3HT films. According to these results, no chemical reactions would occur between P3HT matrices and dispersed particles. On the other hand, Fermi levels of particle-dispersed P3HT films were obviously shifted from that of the pristine P3HT film. The shift was well correlated to the difference between the workfunction of P3HT and that of the dispersed material. Namely, the dispersion of the particles with the lower workfunction contributed to the decrease of workfunction of the P3HT film. Further, the decrease of workfunction of P3HT film resulted in the decrease of the off-current values of the P3HT-FET. This would be because the electrons transferred from the dispersed particles neutralized the excess holes in the P3HT film. Actually, the Ag particle dispersion remarkably improved on-off ratio of the P3HT-FET.
In order to develop a printable organic thin film transistor with high performance, it is required to develop not only printable materials and a printing process but also a suitable device structure for it. We have newly designed an organic field effect transistor with a diagonal configuration of source and drain electrode, named as the Top and bottom contact (TBC) configuration. It has several advantages due to its unique structure. For example, it can be prepared by a simple stacking process without any micro-machining process or related photolithography procedures. This is thus suitable for applying the simple printing technique such as a screen-printing. In the proposed structure, source and drain electrodes are arranged diagonally across the active layer. Therefore, the channel length can be controlled by the deposited active layer thickness. In this study, we have prepared the pentacene transistor with the TBC configuration. By inserting the extra insulator layer, leakage current between the top and bottom electrode was remarkably reduced. The output current
density was about two orders larger than the conventional organic transistor with a top contact configuration. These high performances are mainly due to the improvement of the carrier injection efficiency owing to the short channel length (ca.0.5μm).