The material analyses have shown that the pentacene film and the dielectric properties are almost alike in these two PMMA-OTFTs. It is probable the differences of OTFT transfer characteristic and its photo responsivity are originate from the interface traps, which are created by the UV-light treatment. These UV-created interface traps should be act as negative sites, which may withdraw the electrons but accumulate the holes in pentacen film, near the interface of pentacene and PMMA dielectrics (UV-treated). Hence, the hole-accumulation will result in an upward-bended HOMO-band and build up an internal electric field near the interface.
When the PMMA-OTFTs are illuminated, the electron-hole pairs will be generated in the pentacene film. In UV-treated PMMA-OTFTs with build-in electric field, holes will be swept toward the PMMA-dielectric and the electrons will be swept toward bulk of pentacene. The process assisted with an additional build-in electric field will
come about faster than the case in untreated PMMA-OTFTs. Interestingly, from the analysis of saturation time-constant, the UV-treated PMMA-OTFTs show a smaller total saturation time than the untreated PMMA-OTFTs. It reveals that the process of threshold-voltage shift is faster in the UV-treated PMMA OTFTs. The result is good agreed with the assumption. Consequently, if the light-irradiation is removed, the electrons and holes begin to recombine. In the UV-treated PMMA-OTFTs, with a build–in electric field, the spatial separation between electrons and holes will be longer than that in untreated PMMA-OTFTs. As a result, the recombination rate will be reduced and show a prolonged recovering time. Therefore, we deduced from the recovering time-constant. Significantly, the total recovery time-constant is longer in UV-treated PMMA OTFTs than that in untreated PMMA OTFTs. Again, the result is good agreed to the proposed model. The correspondent model can be explained by the band-diagram and plotted in Fig. 4-14 (a), (b), (c), and (d). Furthermore, to verify the proposed assumption, we try to extract the saturation time-constant and the recovering time-constant of photo-generated current, as a function of gate-voltage.
The extraction equation is identical to the time-dependent threshold voltage shift:
1 2
the “saturation time-constant” of threshold shift under light-illumination; τ1', and τ2' present the “recovering time-constant” of threshold voltage after illumination is turned off. In Fig. 4-15(a) is the Saturation time-constant as a function of gate voltage. When the gate voltage increase, τ1 is almost the same but τ2 will decrease in untreated and UV-treated PMMA-OTFTs. In Fig.4-15(b) is Recovering time-constant as a function of gate voltage. When the gate voltage increase, τ1' is almost the same but τ2' will increase in untreated and UV-treated PMMA-OTFTs.
Since the higher gate-voltage will produce larger band bending and increase the internal electric field near the interface of organic semiconductor and gate-dielectric.
Evidently, a larger upward bending will reduce the saturation time-constant and prolong the recovering time-constant. All the results are agreed with the assumption of build-in electric field. Base on these experimental results, we may conclude that the UV-treated PMMA, which may create additional interface traps, should be the origin of these distinct responses in PMMA-OTFTs. The proposed “build-in electric field” model should be a suitable explanation to the observed results.
Chapter 5 Conclusion
5-1 Conclusion
In this study, we try to fabricate the OTFTs with solution-based PMMA gate-dielectrics and investigate the effects of UV-light treatment on PMMA gate-dielectrics. With the UV-light treatment, the threshold voltage and the subthreshold swing will be significantly altered, but the field-effect mobility remains almost unchanged. When the light-irradiation on these OTFTs, the devices with UV-light treatment will show a larger threshold voltage shift and a larger photo responsivity than those devices without UV-light treatment. In order to investigate the effect of UV-light treatment on OTFTs characteristics, the material analysis and the dielectric leakage-current measurement are proceeded. It is found that the variation of the semiconductor layer and the trapping in the dielectric layer should not be the dominated factors. By the time-dependent analysis of threshold voltage shift and photo-generated current, the saturated time and the recovering time show a strong correlation to the gate-voltage. That implies the electric-field in the organic semiconductor film will influence the behavior drastically. It is most probably the UV-light treatment may create the charged traps at the interface of the
PMMA-dielectric and the organic semiconductor, which will result in a build-in electric-field in the organic semiconductor near the PMMA gate-dielectric. Hence, the OTFTs with UV-light treatment will show different characteristics, such as a faster saturation time and a longer recovering time.
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Table I The comparison of the different polymer OTFTs electrical properties
Gate Dielectric
μ ( cm2/V-sec )
VT H
(V)
S.S.
(V/decade)
Cross-linked PVP 3 -5 1.2
On/off Current ratio
105
Polyimide
SiO2/PMMA 1.4 -12 0.71 106
106
1 Not
mention
Not mention
Table II The comparison of untreated and UV-treated PMMA dielectrics properties