| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 103 === Solution-based gelatin thin film was utilized as the gate dielectric material for organic field-effect transistors (OFETs) fabricated on flexible poly(ethylene terephthalate) (PET) substrate. Gelatin is a natural protein with good film forming and insulating properties, which was coated on PET by a low cost solution process. The performance of p-type pentacene OFETs were found to depend on the molecular weight of gelatin dielectric. The pentacene OFETs with 300 bloom gelatin as the gate dielectric exhibited the best performance with a very high average field-effect mobility (FE) of 16 cm2V−1s−1 and a low threshold voltage (VTH) of -1 V. The pentacene OFETs fabricated in the rearch also showed good flexibility under bending test. Under 0.34% compressive strain, the FE was slightly reduced from 14 cm2V−1s−1 (flatten) to 13.5 cm2V−1s−1 (bending in 0.34% compressive strain). The device performance was found decent enough to open the possibility of fabricating pentacene OFETs in applications of low-power consumption, low-cost manufacturing and being flexible structurally.
It is well known that electron transporting is also essential in circuits of complementary metal-oxide semiconductor (CMOS), which requires both p- and n-type transistors on a single device substrate. The present research found that gelatin also works well as the gate dielectric for n-type N,N-dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8) OFETs with FE and VTH values were (0.22 cm2V−1s−1, 55V) in vacuum and (0.74 cm2V−1s−1, 2.6V) in ambient air. The maximum voltage shift in hysteresis was also reduced from 10 V to 2 V when the operation environment changed from vacuum to ambient air, which was consistent with the reduction of the NSS value from 2.3×1012 cm-2eV-1 in vacuum to 5.8 ×1011cm-2eV-1 in ambient air. The improvement in the device performance is attributed to the charged ions generation owing to the water absorption in gelatin in ambient air.
The fabrication of ambipolar OFETs was also completed based on the bilayer structure of pentacene/PTCDI-C8 with gelatin as the gate dielectric. The ambipolar characteristics were found to depend on relative hole and electron currents that were affected by the moisture absorption in gelatin, layer sequence and relative thickness of pentacene and PTCDI-C8. In the PTCDI-C8/pentacene layer sequence, the hole current in pentacene was much higher than the electron current in PTCDI-C8 regardless of the change of layer thicknesses in ambient air and no ambipolar characteristics seemed to appear. However, the relative contribution from hole and electron currents depended on their relative layer thickness in the pentacene/PTCDI-C8 layer sequence in ambient air. The present work found that the optimum ambipolar performance occurs at pentacene (40 nm)/PTCDI-C8 (40 nm) with electron FE of 0.95 cm2V−1s−1 and hole FE of 0.85 cm2V−1s−1 in ambient air. In contrast, electron current became higher than hole current in pentacene/PTCDI-C8 ambipolar OFETs when moisture was extracted out of gelatin in vacuum. The optimum ambipolar performance occurred at pentacene (65 nm)/PTCDI-C8 (40 nm) with electron FE of 0.008 cm2V−1s−1 and hole FE of 0.007 cm2V−1s−1.
The present research also fabricated air ambient operated organic CMOS inverters using pentacene and PTCDI-C8 as semiconductor layers with a bilayer dielectric of tetratetracontane (TTC) and gelatin. The performances of inverters were greatly improved when gelatin was replaced by TTC/gelatin bilayer dielectric. With the TTC/gelatin bilayer, both types of OFETs show better pinch-off and current saturation in output characteristics and negligible hysteresis in transfer characteristics. The organic CMOS inverters can be operated at a voltage as low as 10 V with symmetric voltage transfer characteristics and small hysteresis. A high static gain of 60 can be obtained at a voltage of 12 V. Ambient air operated organic CMOS inverters with low operation voltage; negligible hysteresis and high static gain were realized.
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