@phdthesis{oai:toyama.repo.nii.ac.jp:00005175,
 author = {Alam, Mir Waqas},
 month = {Sep},
 note = {Organic thin-film transistors (OTFTs) have attracted increasing interest for their potential advantages, such as, low cost, lightweight and possible fabricating on flexible substrate, as well as large-area feasibility. For constructing a high performance top-contact OTFT, it is very important to understand the influence of interfaces, which play a crucial role in the overall performance of the device. Mainly there are two kinds of interfaces in the OTFT. First is in between the semiconductor and the gate dielectric, where the conducting channel forms. The second one is in between the source/drain (S/D) electrodes and the semiconductor layer where charge carriers are injected.
First, I particularly discussed the latter interface. Organic and metal interfaces mostly limit the performance of the device and field effect mobility is also decreased. In earlier reports it has been shown that the diffusion between the metal electrode and pentacene reduce the hole　injection carrier at the interface which causes the increase in barrier height and contact resistance which effect the performance of the OTFTs. It is shown that the contact between the S/D electrodes and the organic semiconductor can be improved by inserting transition metal oxide layer as carrier injection layers. Because of good electronic properties, transition metal oxides such as molybdenum oxide (MoO_3), tungsten oxide (WO_3) and vanadium oxide (V_2O_5), and other oxides such as germanium oxide (GeO), titanium oxide (TiO_2) offer a unique opportunity to control the work function, and hence increase the charge-injection properties. Therefore, by modifying the organic/electrode interface, the S/D electrodes do not directly contact with pentacene layer and hence significantly reduces the contact resistance, barrier height and provides protection from diffusion and other chemical reactions, which increase device performance.
Second, I investigated that the enhancement of the charge injection and field effect mobility by inserting a thin (5 nm) MoO_3, GeO, WO_3 and TiO_2 interlayer between the Au electrode and pentacene layer in a top contact pentacene based organic thin-film transistor (OTFTs).In comparison with the pentacene-based OTFT with only-Au electrode, the device performance with bilayer electrode has been considerably improved. The device performance including field effect mobility, threshold voltage, and On/Off ratio of all the device after modification was highly improved, and the highest mobility of 0.96 cm^2/V1s, threshold voltage of -4 V, and highest on/off ratio of 5.2×10^4 were achieved in the device with 5 nm GeO. I further investigated the temperature dependence of I_D–V_D characteristics which showed strong temperature dependence in all the devices.
Third, the obvious temperature dependence of I_D-V_D curves in all devices suggests that the charge injection characteristics can be fitted by the Schottky emission mechanism. By plotting the relationship between ln(I) vs V^<1/2> and extrapolating straight lines to the ordinal point, the current at zero voltage I0 is determined. By using the values of I0, the relationship between ln(I_0/T^2) vs 1/T is plotted and from the resulting slope of extrapolated lines. While in case of bilayer MoO_3/Au shows 0.03 eV, WO_3/Au shows 0.05 eV, TiO_2/Au showed 0.04 eV and with only Au electrodes barrier height of 0.12 eV is achieved. The lowest barrier heights of 0.01 eV could be achieved in case of bilayer GeO/Au electrodes. It is assumed that the barrier height was dramatically reduced by inserting thin oxide layer between the Au and pentacene layer. Similarly, from surface morphology of pentacene, the root mean square roughness is also decreased after inserting metal oxide layer. The main factor for the improvement in the performance of the OTFTs with bilayer electrodes was explained in terms of the reduction in barrier height and smoothed surface roughness of active layer. Therefore, the combination of a thin oxide layer with Au as a bi-layer electrode is an effective way to improve the characteristics of OTFTs, which makes the device suitable for commercial applications.
Finally, I further reported the enhanced carrier injection in pentacene OTFTs with a thin MoO_3- doped pentacene layer between pentacene semiconductor and the S/D electrodes. Device performance including drain current, field effect mobility, and threshed voltage are improved by employing a MoO_3-doped pentacene thin layer. The barrier height at the Au/pentacene interface is lowered from 0.12 to 0.05 eV after inserting a MoO_3-doped pentacene thin layer between them., Article, 富山大学・富理工博甲第64号・Alam Mir Waqas・2013/09/27},
 school = {富山大学},
 title = {High-Performance Organic Thin Film Transistors with Special Oxide Contact Structures and Evaluation of Device Physics},
 year = {2013}
}