Sungkyu Park

All-organic active matrix flexible display

We have fabricated pentacene organic thin-film transistor (OTFT) driven active matrix organic light-emitting diode (OLED) displays on flexible polyethylene terephthalete substrates. These displays have 48×48 bottom-emission OLED pixels with two pentacene OTFTs used per pixel. Parylene is used to isolate the OTFTs and OLEDs with good OTFT yield and uniformity.

Contact-induced crystallinity for high-performance soluble acene-based transistors and circuits

The use of organic materials presents a tremendous opportunity to significantly impact the functionality and pervasiveness of large-area electronics. Commercialization of this technology requires reduction in manufacturing costs by exploiting inexpensive low-temperature deposition and patterning techniques, which typically lead to lower device performance. We report a low-cost approach to control the microstructure of solution-cast acene-based organic thin films through modification of interfacial chemistry. Chemically and selectively tailoring the source/drain contact interface is a novel route to initiating the crystallization of soluble organic semiconductors, leading to the growth on opposing contacts of crystalline films that extend into the transistor channel. This selective crystallization enables us to fabricate high-performance organic thin-film transistors and circuits, and to deterministically study the influence of the microstructure on the device characteristics. By connecting device fabrication to molecular design, we demonstrate that rapid film processing under ambient room conditions and high performance are not mutually exclusive.

Pentacene TFT driven AM OLED displays

Pentacene organic thin-film transistors (TFTs)-driven active matrix organic light-emitting diode (OLED) displays has been investigated. This letter addresses several process issues unique to this type of display which are important in achieving bright and uniform displays. A bottom contact structure was used to fabricate the pentacene TFT backplane. Polyvinyl alcohol and parylene were used to isolate the pentacene active layer and passivate the backplane. The low processing temperature may allow the use of polymeric substrates and lower cost processing. Uniform TFT performance is achieved with reasonably good mobility and on/off ratio on the backplane. The initial OLED display performance is also presented.

Correlation between microstructure, electronic properties and flicker noise in organic thin film transistors

We report on observations of a correlation between the microstructure of organic thin films and their electronic properties when incorporated in field-effect transistors. We present a simple method to induce enhanced grain growth in solution-processed thin film transistors by chemical modification of the source-drain contacts. This leads to improved device performance and gives a unique thin film microstructure for fundamental studies concerning the effect of structural order on the charge transport. We demonstrate that the 1∕f flicker noise is sensitive to organic semiconductor thin film microstructure changes in the transistor channel.

All-organic Active Matrix OLED Flexible Display

We have fabricated pentacene organic thin-film transistor (OTFT) driven active matrix organic light-emitting diode (OLED) displays on both glass and flexible polyethylene terephthalete (PET) substrates. These displays have 48 × 48 bottom-emission OLED pixels with two pentacene OTFTs used per pixel. Polyvinyl alcohol (PVA) and parylene were used to photolithographically pattern the pentacene active layer and isolate the OTFT backplane from the OLEDs. Pentacene OTFTs are able to easily supply the current required for OLED operation, but improvements in device uniformity and stability are of interest.

Fast ZnO Thin-Film Transistor Circuits

We report here five-stage ring-oscillator-integrated circuits fabricated using ZnO TFTs with signal propagation delays as low as 100 ns (>1 MHz oscillation frequency) for a 45 V supply voltage. These circuits also operate at a supply voltage as low as 10 V. To our knowledge, these are the fastest ZnO integrated circuits reported to date. We also designed ring oscillators with different source/gate and drain/gate overlap to investigate aspects of the circuit speed.

High Frequency Piezoelectric MEMS Devices

High frequency ultrasound array transducers are being explored for high resolution imaging systems. This increase in resolution is made possible by enabling a simultaneous increase in operating frequency (50 MHz to about 1 GHz) and close-coupling of the electrical circuitry. Several different processing methods are being explored to fabricate array transducers. In one implementation, the piezoelectric transducer is prepared by mist deposition of PbZr0.52Ti0.48O3 (PZT) films over Ni posts. In addition, a xylophone bar transducer has also been prototyped, again using thin film PZT as the active piezoelectric layer. Because the drive voltages of these transducers are low, close coupling of the electrical circuitry is possible. A CMOS transceiver for a 9 element-array has been fabricated in 0.35 mum process technology. The first generation CMOS transceiver chip contains beamforming electronics, receiver circuitry, and analog to digital converters with 27 Kbyte on-chip buffer memory.

Influence of the film microstructure on the electronic properties and flicker noise in organic thin film transistors

In organic thin-film transistors (OTFTs) the intrinsic properties of the semiconductor are generally masked by structural defects which localize and trap the charge carriers and contact effects, which limit charge injection and cause deviation from ideal linear and square-law behavior. Thus, their electrical characteristics are strongly dependent on the fabrication conditions. We perform current-voltage (I-V) and low frequency noise (1/f) measurements to demonstrate a direct correlation between the thin film microstructure and the performance of OTFTs. Using fluorinated 5,ll-bis(triethylsilylethynyl) anthradithiophene as a model system, we show the relevance of the grain size and grain distribution in the changes in the electronic properties in an OTFT. We present a simple method to induce enhanced grain growth in solution-processed TFTs by room-temperature chemical modification of the source-drain contacts. This leads to improved device performance, and gives an unique thin film microstructure for fundamental studies concerning the effect of structural order on the charge transport. We present evidence that the 1/f flicker noise is sensitive to the microstructure of the organic semiconductor thin film in the transistor channel and can be a diagnostic tool for the quality of the devices.

All-organic active matrix oled display

This paper reports on 48 times 48 pixels pentacene TFT driven active-matrix OLED displays on glass substrate. To our best knowledge these are the largest pentacene TFT driven displays