Studies on flexible thin-film transistors based on pentacene and ZnO semiconductors

Abstract

Thin-film transistors (TFTs) based on organic semiconductors (OSCs) have received considerable attention in recent years. Intensive studies have made it possible to achieve device performances comparable to those of amorphous silicon TFTs. However, organic TFTs are not meant to replace single crystalline conventional inorganic TFTs because of the upper limit of their switching speed. Consequently, organic TFTs are unsuitable for use in applications that require high switching speeds. Although organic TFTs cannot compete with the performance of TFTs based on single crystalline inorganic semiconductors, they have great potential for a wide variety of applications, especially for new products that rely on their unique characteristics, such as electronic newspapers, inexpensive smart tags for inventory control, and large-area flexible displays. In order to be implemented as viable solutions in these types of applications, they must offer a low operating voltage, low-temperature processing, compatibility with polymer substrates for lightweight and foldable products, large area coverage, and, especially, low-cost processing such as can be achieved using spin-coating, printing, and roll-to-roll steps. In addition, high capacitance is required to induce a high density of free carriers at the conducting channel, allowing TFT operation at low voltages. Previous generations of organic TFTs have required high operating voltages due to generally low mobility, making these devices inappropriate for applications that require high current output, such as driving unit devices for organic light emitting diodes. The smooth surface of the gate dielectric is also required because flexible TFTs with OSC films deposited on a low-cost metal sputtered or laminated-foil based gate dielectric have inherently large surface roughnesses, which significantly affect the charge transport of the organic TFTs. In particular, the reliable performance of flexible organic TFTs under bending conditions is required because applications of organic TFTs are commonly subjected to bending strains. From these demands, flexible low-voltage organic TFTs with a high capacitance and smooth gate dielectric based on a low-cost aluminum foil as a gate electrode fabricated by a roll-to-roll lamination process were demonstrated in Chapter 2 and 3. Additionally, the effects of bending stress on the structure and electrical characteristics of pentacene films as the OSC in flexible TFTs were also investigated in Chapter 4.In Chapter 2 and 3, flexible and low-voltage pentacene organic TFTs constructed with an aluminum foil gate electrode, which was fabricated by the simple and low-cost roll-to-roll lamination process was reported. Electropolishing the surface of laminated aluminum foil and spin-coating it with an additional thin polymer film resulted in a gate dielectric surface with a root-mean-square roughness of about 0.85 nm. These pentacene TFTs with a poly(？-methylstyrene) (PAMS)/anodized Al2O3 dual-layered gate dielectric exhibited a mobility of 0.52 cm2V？1s？1, an on？off ratio of 10^5, a subthreshold swing of 0.32 V/decade, and little hysteresis when operating at ？5 V. The 50-nm-thick pentacene films deposited on the PAMS/anodized Al2O3 gate dielectric？which still contained local surface dimples with microscale lateral sizes？showed a preferred ？thin-film phase？, as indicated by atomic force microscopy (AFM) and grazing-incidence X-ray diffraction (GIXRD) experiments. Conducting-probe AFM and GIXRD studies of the pentacene films on these dielectrics revealed that the growth and lateral percolation of the pentacene crystals are not significantly affected by the presence of microscale roughness of the molecule/dielectric interface. These results indicate that electropolishing surface-modification procedure, proposed for coarse-metal electrodes, can be applied to produce gate dielectrics for low-cost and flexible organic TFTs.In Chapter 4, the effects of bending strain on the structure and electrical characteristics of pentacene films in flexible devices were investigated. It was found that the volume fraction of ？bulk phase？ in the pentacene film increases from 10.7 to 27.7% under 1.1% of tensile strain but decreases to 3.5% under 1.0% of compressive strain. These bending-stress-driven phase transitions between the bulk phase and the thin-film phase in the pentacene film resulted in the changes in field-effect mobility, and were driven by the differences between the in-plane dimensions of the crystal unit cells of the two phases to reduce the external bending stress.TFTs based on oxide semiconductors have also been widely studied with the goal of providing feasible alternatives to conventional silicon technologies for applications that require transparent electronics. TFTs based on OSCs are generally used as unipolar devices, and exhibit mostly p-type semiconducting behavior. Although high-performance unipolar devices have been achieved with TFTs based on p-type OSCs, the use of TFTs based on n-type OSCs in complementary logic circuits has been limited by their instability in the presence of atmospheric water and oxygen. TFTs that use oxide semiconductors, such as zinc oxide (ZnO), indium zinc oxide, zinc tin oxide and indium gallium zinc oxide, exhibit mostly n-type semiconducting behavior, and have the advantages of high charge mobility, excellent environmental stability and high transparency in comparison to TFTs based on OSCs. However, TFTs based on solution-processed oxide semiconductors for low-cost electronics have difficulties of fabricating on flexible substrates because of the requirement for high temperature solution processing to achieve a high mobility. Here, the development of solution-processed ZnO transparent TFTs on a flexible substrate was demonstrated in Chapter 5. Furthermore, the development of high performance ambipolar TFTs and an inverter by combining the p-type OSC and the n-type oxide semiconductor was reported in Chapter 6.In Chapter 5, the development of solution-processed ZnO transparent TFTs with a poly(2-hydroxyethyl methacrylate) (PHEMA) gate dielectric on a plastic substrate was reported. The ZnO nanorod film active layer, prepared by microwave heating, showed a highly uniform and densely packed array of large crystal size (58 nm) in the [002] direction of ZnO nanorods on the plasma-treated PHEMA. The flexible ZnO TFTs with the plasma-treated PHEMA gate dielectric exhibited an electron mobility of 1.1 cm2V？1s？1, which was higher by a factor of ~8.5 than that of ZnO TFTs based on the bare PHEMA gate dielectric.In Chapter 6, high performance ambipolar TFTs and an inverter based on organic-inorganic bilayer structures composed of an upper pentacene layer and a lower atomic-layer-deposited ZnO layer were fabricated. The insertion of a dodecanoic acid (DA) self-assembled monolayer (SAM) into the interface between pentacene and ZnO resulted in an improvement in the morphology of the pentacene layer and in well-balanced ambipolarity with hole and electron mobilities of 0.34 and 0.38 cm2V-1s-1 respectively. The ambipolar TFTs with DA-treated ZnO exhibited a hole to electron mobility ratio of approximately 0.90, which was higher by a factor of ~2.8 than that of ambipolar TFTs with untreated ZnO. The introduction of a perfluorooctyltriethoxysilane SAM was also tested

the effects of the permanent dipole fields of the SAMs on the electrical and ambipolar characteristics of the hybrid TFTs were investigated.