C ont act r esi st ance, R
cont( Ω )
Fig. 4-13 The variation of the linear regime resistance of NPB-passivated under different UV-light illumination time
0 20 40 60 80 100 120 0.0
0.1 0.2 0.3 0.4 0.5
VD = -2V
UV illumination time ( minutes ) Field effect mobility ( cm2 / V-sec ) NPB passivation
without NPB passivation
Fig. 4-14(a) The field effect mobility vs UV illumination period plotted and summarized
Fig. 4-14(b) The variation of the linear regime field-effect mobility under UV-light illumination.( the literatures with SnO2 film)
( Electrochem. Solid-State Lett. vol 9, pp.G251 2006 )
0 20 40 60 80 100 120
UV illumination time ( minutes )
Threshold Voltage (V)
Fig. 4-15 The variation of the linear regime threshold voltage under different UV-light illumination times
0 20 40 60 80 100 120 Contact resistance, R cont ( Ω )
UV illumination time ( minutes ) Rfilm, NPB passivation
Rcont, NPB passivation Rfilm, without NPB passivation Rcont, without NPB passivation
Table 2 The transfer characteristics of the OTFTs before and after passivation with TiO2 ,NPB, NPB/Mg/TiO2/PDMSlayers compared with literatures
Chapter 5 Conclusion
5-1 Conclusion
In conclusion, the highly transparent NPB film was used as the passivation layer for pentacene OTFTs. After the passivation process, the OTFTs threshold voltage was reduced, the subthreshold swing was increased, and the on/off ratio was reduced, but the field-effect mobility was almost unchanged. It was clear that the NPB passivation will not damage the organic pentacene film.
The device degradation under UV-light illumination was prevented after capping the thermally-deposited NPB film. The high energy UV-light (175~254nm wavelength) was constantly applied onto the passivated device. After several periods time of 10, 20, 40, 80, and 120 minutes, the field-effect mobility of NPB passivated OTFTs gradually degraded from 0.48 to 0.30cm2/Vs. However, the other device without passivation will rapidly become invalid, and showed a larger threshold-voltage shift than the passivated OTFTs. The NPB film is suitable to protect the pentacene OTFTs from UV-light illumination.
We also discussed the effects of UV-light illumination on OTFTs. Through the analysis of atomic force microscopy (AFM), x-ray diffraction spectrum (XRD) and
electron Spectroscopy for Chemical Analysis (ESCA), we found the changes in both chemical properties and surface morphologies were significantly. The oxidation signal and destroyed pentacene grains were significantly appeared on the surface of pentacene film and seemed to be a dominated factor that influenced the OTFTs performance. Based on these experiment results, it may be supposed that the performance degradation was strongly related to changes in chemical bonding and grain morphology, but rather than changes inside the thin-film bulk.
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