Title:  Organic thin-film transistors with low voltage operation and high operation stability

Committee:

Dr. Bernard Kippelen, ECE, Chair, Advisor

Dr. Azadeh Ansari, ECE

Dr. Azad Naeemi, ECE

Dr. Shimeng Yu, ECE

Dr. Natalie Stingelin, ChBE

Abstract: Organic thin-film transistors (OTFTs) are the most proper candidate for the flexible technoogies, because they can be directly fabricated on top of the flexible substrates and the organic semiconductors are intrinsically flexible. However, there exists two technological hurdels for OTFTs to be commercialized, which are ppor operational stability and high operating voltage. In this thesis dissertation, possible solutions for the two problems were suggested. The first solution is using a bottom bilayer gate dielectric, which consists of HfO2 and CYTOP layers. The thickness of the bilayer gate dielectric can be reduced down to 17 nm. The thin bottom bilayer gate dielectric provided operating voltage as low as -1 V for the gate and drain voltages to achieve 1 µA of the drain-to-source current. The subthreshold swing values were reduced to 95 mV/dec which is comparable to that of crystalline Si transistors. The bilayer gate dielectric, the fabricated OTFT showed only 0.1 V of threshold voltage shift after applying DC bias stress for 24 hours. The second solution was analyzing the intrinsic contact resistance of OTFTs. A quantitative model for the intrinsic contact resistance was designed using Y-function method and transmission line method The model was applied to OTFTs which have the bottom bilayer gate dielectrics with a wide range of gate capacitance density (36.6 nF/cm2 to 231.7 nF/cm2). Applying the method and analyzing the intrinsic contact resistance, compared to the thick bilayer gate dielectric with 36.6 nF/cm2 of the capacitance density, bilayers with a high gate dielectric capacitance as high as 231.7 nF/cm2 can reduce the contact resistance values from 3.5 kOhm Ωcm to the values smaller than 1 kOhm Ωcm. The third method which is the observation of the operational stability during the DC bias stress was conducted in order to understand the characteristics of the operational stability, especially the positive current shift during the DC bias stress. In order to understand the mechanism, the charge injection characteristics from the gate electrode were modified by applying work function-changing materials on top of the gate electrode. These materials include PEIE and MoO3. By observing the DC bias stability of the PEIE-coated devices and MoO3-coated devices, it can be shown that the charge injection from the gate electrode can be responsible for making the large drain current overshoot at the early stage of the DC bias stress.