Amorphous In-Ga-Zn-O Thin-Film Transistors for Next Generation Ultra-High Definition Active-Matrix Liquid Crystal Displays.

Next generation ultra-high definition (UHD) active-matrix flat-panel displays have resolutions of 3840x2160 (4K) or 7680x4320 (8K) pixels shown at 120 Hz. The UHD display is expected to bring about immersive viewing experiences and perceived realness. The amorphous In-Ga-Zn-O (a-IGZO) thin-film transistor (TFT) is a prime candidate to be the backplane technology for UHD active-matrix liquid crystal displays (AM-LCDs) because it simultaneously fulfills two critical requirements: (i) sufficiently high field-effect mobility and (ii) uniform deposition in the amorphous phase over a large area. We have developed a robust a-IGZO density of states (DOS) model based on a combination of experimental results and information available in the literature. The impact of oxygen partial pressure during a-IGZO deposition on TFT electrical properties/instability is studied. Photoluminescence (PL) spectra are measured for a IGZO thin films of different processing conditions to identify the most likely electron-hole recombination. For the first time, we report the PL spectra measured within the a IGZO TFT channel region, and differences before/after bias-temperature stress (BTS) are compared. To evaluate the reliability of a-IGZO TFTs for UHD AM-LCD backplane, we have studied its ac BTS instability using a comprehensive set of conditions including unipolar/bipolar pulses, frequency, duty cycle, and drain biases. The TFT dynamic response, including charging characteristics and feedthrough voltage, are studied within the context of 4K and 8K UHD AM-LCD and are compared with hydrogenated amorphous silicon technology. We show that the a-IGZO TFT is fully capable of supporting 8K UHD at 480 Hz. In addition, it is feasible to reduce a-IGZO TFT feedthrough voltage by controlling for non-abrupt TFT switch-off.PhDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111526/1/ekyu_1.pd

Organic Molecular Crystal Engineering via Organic Vapor-Liquid-Solid Deposition

Control over the size, shape, topology, orientation, and crystallographic phase of organic molecular materials is critical for a wide array of applications ranging from optoelectronics to pharmaceutical development. Herein, we demonstrate a relatively low-cost approach for fabricating single crystals with controlled sizes, shapes, microscale periodic features, preferred orientations and specific molecular packing modes. These features allow for the fabrication of intricate arrangements of single crystals for incorporation into complex device architectures, and potentially the endowment of tailored optical, electronic, thermal, and mechanical properties onto these materials. Patterning is achieved by utilizing an organic-vapor-liquid-solid (OVLS) deposition scheme paired with traditional photolithography methods. The OVLS approach involves spin coating a layer of a low vapor pressure solvent onto a substrate in order to drive up the critical nucleus size required for crystal nucleation, resulting in large grain sizes. This substrate is placed above a hot plate with the organic material to be sublimed. Our results show that millimeter-scale, ultrathin, planar organic molecular crystals can be grown on patterned substrates with rudimentary equipment (hot plate, spin coater, photoresist, photomask, UV source). We show that this technique is not only compatible with organic semiconductors, but also other organic molecular crystals such as pharmaceuticals

Doctor of Philosophy

dissertationIn Part 1, we demonstrate the fabrication of organic light-emitting devices (OLEDs) with precisely patterned pixels by the spin-casting of Alq3 and rubrene thin films with dimensions as small as 10　μm. The solution-based patterning technique produces pixels via the segregation of organic molecules into microfabricated channels or wells. Segregation is controlled by a combination of weak adsorbing characteristics of aliphatic terminated self-assembled monolayers (SAMs) and by centrifugal force, which directs the organic solution into the channel or well. This novel patterning technique may resolve the limitations of pixel resolution in the method of thermal evaporation using shadow masks, and is applicable to the fabrication of large area displays. Furthermore, the patterning technique has the potential to produce pixel sizes down to the limitation of photolithography and micromachining techniques, thereby enabling the fabrication of high-resolution microdisplays. The patterned OLEDs, based upon a confined structure with low refractive index of SiO2, exhibited higher current density than an unpatterned OLED, which results in higher electroluminescence intensity and eventually more efficient device operation at low applied voltages. We discuss the patterning method and device fabrication, and characterize the morphological, optical, and electrical properties of the organic pixels. In part 2, we demonstrate a new growth technique for organic single crystals based on solvent vapor assisted recrystallization. We show that, by controlling the polarity of the solvent vapor and the exposure time in a closed system, we obtain rubrene in orthorhombic to monoclinic crystal structures. This novel technique for growing single crystals can induce phase shifting and alteration of crystal structure and lattice parameters. The organic molecules showed structural change from orthorhombic to monoclinic, which also provided additional optical transition of hypsochromic shift from that of the orthorhombic form. An intermediate form of the crystal exhibits an optical transition to the lowest vibrational energy level that is otherwise disallowed in the single-crystal orthorhombic form. The monoclinic form exhibits entirely new optical transitions and showed a possible structural rearrangement for increasing charge carrier mobility, making it promising for organic devices. These phenomena can be explained and proved by the chemical structure and molecular packing of the monoclinic form, transformed from orthorhombic crystalline structure

Vertical Alignment Liquid Crystal Displays with High Transmittance and Wide View Angle

Structures, devices, systems and methods of using multi-domain vertical alignment liquid crystal displays with high transmittance, high contrast ratio and wide view angle in which at least one of the electrode substrates has circular or ring-shaped openings, such as holes or slits. Circular or ring-shaped patterns for openings and electrodes have not been used in the construction of a liquid crystal display. The new multi-domain vertical alignment (MDVA) liquid crystal display is particularly suitable for liquid crystal display television and computer monitor applications

Work Function Extraction of Indium Tin Oxide Used As Transparent Gate Electrode For MOSFET.

Recent commercialization has peaked interest in transparent conducting oxides being implemented in display technology. Indium Tin Oxide (ITO) is a popular transparent conducting oxide which has been utilized as high work function electrode in liquid crystal displays, solar cells, gas sensors and heat reflecting films. Indium Tin Oxide films exhibit excellent transmission characteristics in the visible and infrared spectrum while maintaining high electrical conductivity. High work function electrodes are used to inject holes into organic materials. In majority applications the ITO work function has an impact on the device performance as it affects the energy barrier height at the hetero-junction interface. Hence, the work function of ITO is of critical importance. In this thesis, the work function of ITO is extracted successfully from a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) device for the first time. Two MOSFET devices are fabricated using a four level mask under exact same conditions. Aluminum metal is used as a drain and source contact for both MOSFETs. One of the MOSFET has aluminum gate contact and transparent conducting ITO is used as gate contact for the second MOSFET. From the threshold voltage equation of both the fabricated MOSFETs, work function of ITO is extracted. Further optical transmission studies of ITO performed in the visible spectra are also reported in this study

Developing non-invasive processing methodologies and understanding the materials properties of solution-processable organic semiconductors for organic electronics

textEssential to the success of organic electronics, and in particular organic thin-film transistors, is the realization of stable, high-mobility, electrically-active organic materials that can enable low-cost solution-based processing methods. The development of viable solution-processable organic semiconductors helps make this possible. Consequently, understanding the materials properties of solution-processable organic semiconductors and how the processing conditions associated with device fabrication affect device performance are key to realizing low-cost organic electronics. In this work, we focused on understanding the processing-structure-property relationships of a solutionprocessable organic semiconductor, triethylsilylethylnyl anthradithiophene (TES ADT). Specifically, we demonstrated how a solvent-vapor annealing process can induce the crystallization of TES ADT post device processing. Bottom-contact thin-film transistors with annealed TES ADT routinely exhibit an average charge-carrier mobility of 0.1 cm 2 /V-s, which is sufficient to drive backplane circuitry in flexible display applications. Additionally, we demonstrated that the manner in which source and drain electrodes are defined significantly affects the performance of the resulting TES ADT thin-film transistors. Specifically, the yield of functioning top-contact TES ADT thinfilm transistors with electrodes defined by evaporation through a shadow mask directly on the organic semiconductor is low, and of the functioning devices, the charge-carrier mobility varies significantly (0.01 – 0.1 cm2 /V-s). In comparison, top-contact TES ADT thin-film transistors with electrodes defined separately and then laminated against the organic semiconductors have high yield and high charge-carrier mobility (0.2 ± 0.06 cm 2 /V-s). This result emphasizes the importance of adapting existing or developing new thin-film transistor fabrication techniques to overcome the materials limitations of organic semiconductors. Along the same vein, we also demonstrated an elastomeric stamp-based, solventless printing process, nanotransfer printing (nTP), for the additive patterning of copper electrodes and interconnects of feature sizes 1 – 500 μm. These printed copper patterns differ from similarly printed gold patterns in that they are not electrically conductive. Leaching the elastomeric stamps in hot toluene prior to printing, however, allowed us to routinely print conductive copper features with an average resistivity of 31 μΩ-cm. Another aspect of thin-film transistor fabrication that is crucial for optimal device performance (i.e., low off currents and low leakage currents) is the patterning and isolation of the organic semiconductor between neighboring devices. We demonstrated two novel techniques for patterning TES ADT. The first technique utilizes UV light in the presence of dichloroethane vapors to simultaneously pattern and crystallize TES ADT. TES ADT thin-film transistors patterned with this technique exhibit high chargecarrier mobility (0.1 cm2 /V-s) and low off currents (10-11 A). The second patterning technique uses a PDMS stamp to selectively remove TES ADT from the non-channel regions of the thin-film transistor. This technique can be used to pattern both as-spun and crystalline TES ADT thin films. Crystalline TES ADT thin-film transistors patterned with this technique exhibit an average charge-carrier mobility of 0.2 cm2 /V-s and low off currents on the order of 10-11 A, while amorphous TES ADT thin films that are first patterned and then crystallized exhibit an average charge-carrier mobility of 0.1 cm2 /V-s and off currents on the order of 10-10 A.Chemical Engineerin

Thin‐Film Transistors for Large Area Opto/Electronics

The present work addresses several issues in the field of organic and transparent electronics. One of them is the prevailing high power consumption in state-of-the-art organic field-effect transistors (OFETs). A possible solution could be the implementation of complementary, rather than unipolar logic, but this development is currently inhibited by a distinct lack of high performance electron transporting (n-channel) OFETs. Here, the issue is addressed by investigating a series of solution processable n-channel fullerene molecules in combination with optimized transistor architectures. Furthermore, the trend towards complementary circuit design could be facilitated by employing ambipolar organic semiconductors, such as squaraine molecules or polymer/fullerene blends. These materials can fill the role of p- or n-channel semiconductors and enable the facile implementation of power saving complementary-like logic, eliminating the cost-intensive patterned deposition of discrete p-and n-channel transistors. Alternatively, a patterning method for organic materials adapted from standard photolithography is discussed. Furthermore, ambipolar FETs are found to be capable of light sensing at wavelength of 400-1000 nm. Hence their use in low-cost, organic based optical sensor arrays can be envisioned. Another strategy to reduce the power consumption and operating voltages of OFETs is the use of ultra-thin, self-assembled molecular gate dielectrics, such as alkyl-phosphonic acid molecules. Based on this approach solution processed n- and p-channel OFETs and a complementary organic inverter circuit are demonstrated, which operate at less than 2 Volts. Finally, transparent oxide semiconductors are investigated for use in thin-film transistors. Titanium dioxide (TiO2) and zinc oxide (ZnO) films are deposited by means of a low-cost large area compatible spray pyrolysis technique. ZnO transistors exhibit high electron mobility of the order of 10 cm2/Vs and stable operation in air at less than 2 Volts. These results are considered significant steps towards the development of organic and transparent large-area optoelectronics