Yongtaek Hong

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.

High-resolution organic polymer light-emitting pixels fabricated by imprinting technique

We have developed an approach to fabricate pixelated organic polymer light-emitting devices (OPLED) using an imprinting technique. The pixel array pattern was first defined in an insulating polymer layer on indium tin oxide glass by direct imprinting, followed by the spin-coating of OPLED polymers and cathode metal deposition. We demonstrated successful fabrication and operation of OPLED pixels of sizes from 50 μm down to 2 μm. Optoelectronic characterization is performed on these devices, and measured results show comparable device performance with OPLED pixels patterned by other methods. This fabrication scheme holds many merits such as easy to process, low-cost, high yield, expandable to flexible substrate, capable of repeated imprinting for large area arrays, and the potential to pattern submicron and nanoscale organic polymer light emitters.

Inkjet-printed stretchable single-walled carbon nanotube electrodes with excellent mechanical properties

Inkjet-printed single-walled carbon nanotube (SWCNT) thin films on stretchable substrates were developed, and their electrical properties were investigated. The 1- and 5-layer printed SWCNT thin films had sheet resistances of 169.76 and 19.08 Ω/sq, respectively, and maintained conductive properties under 100% tensile strain. A nitric acid treatment significantly improved the conductivity of the SWCNT electrodes, and after an initial increase, the 5-layer printed SWCNT electrodes showed a normalized resistance of less than 1.2 after 1000 cycles under 100% tensile strain. The potential of the inkjet-printed SWCNT thin films for stretchable electrode applications was demonstrated via integration with a light-emitting diode.

100 dpi 4-a-Si:H TFTs active-matrix organic polymer light-emitting display

In this paper, we report on 100 dpi four hydrogenated amorphous silicon thin-film transistors (4-a-Si:H TFTs) active-matrix organic polymer light-emitting display (AM-PLED). For this display, we have established the operational limitation of our 4-a-Si:H TFTs pixel electrode circuit by performing a load line analysis. Combining this result with the extracted pixel organic polymer light-emitting device (PLED) characteristics, we have found that the change of the AM-PLED pixel operating point, especially of a driving TFT, limits the operational range of AM-PLED pixel. The predicted results are compared with the measured data of 100 dpi monochromatic red light-emitting 4-a-Si:H TFTs AM-PLED. For our AM-PLED, we obtained luminance up to /spl sim/20 cd/m/sup 2/ and Commission Internationale de lEclairage color coordinates of (0.67, 0.33), which are calculated from the measured AM-PLED electroluminescence spectrum.

P-103: Novel Poly-Si TFT Pixel Electrode Circuits and Current Programmed Active-Matrix Driving Methods for AM-OLEDs

In this paper, we present the systematic analysis of the current-programmed poly-Si pixel electrode circuits with current-sink-type charging functionality and duty ratio controllability. Two different driving schemes for top-cathode and top-anode organic light-emitting device structures are described and compared. Their limitations and advantages are also discussed.

Opto-electronic properties of poly (fluorene) co-polymer red light-emitting devices on flexible plastic substrate

In this paper, we report on the multilayer poly (fluorene) co-polymer red light-emitting devices (PLEDs) fabricated on flexible plastic substrates. An organic hole transport layer (HTL) is inserted between PEDOT:PSS hole injection (HIL) and light-emissive layers (LEL). Since the highest occupied molecular orbital (HOMO) of the HTL is located between those of HIL and LEL, the insertion of HTL reduces the effective HOMO level offset between HIL and LEL, reducing the device operation voltage and producing comparable or better device efficiencies in comparison with the conventional PEDOT:PSS-only devices. Maximum emission efficiency, /spl sim/0.8 cd/A, power efficiency, /spl sim/0.7 lm/W, and external quantum efficiency, /spl sim/1.5%, have been obtained for multilayer red PLEDs.

Optoelectrical properties of four amorphous silicon thin-film transistors 200 dpi active-matrix organic polymer light-emitting display

We report on opto-electrical properties of a current-driven 200 dpi active-matrix organic polymer red light-emitting display (AM–PLED) based on four hydrogenated amorphous silicon thin-film transistor pixel electrode circuits. The AM–PLED luminance and effective light-emission efficiency were 30u2002cd/m2 and 0.3 cd/A, respectively, at the data current equal to 25 mA. The display electroluminescent spectrum has a peak located at and the full width at half maximum value of 644 and 95 nm, respectively, and Commission Internationale de l’Eclairage color coordinates of (0.66,0.33).

Amorphous silicon TFT-based active-matrix organic polymer LEDs

We report active-matrix organic polymer light-emitting displays (LEDs) based on a three hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) pixel electrode circuit that supplies a continuous output current to organic polymer light-emitting devices. The output current level drift induced by either process variations or device aging can be reduced in this design by adjusting the driver TFT operating point with the active resistor. Our first green light-emitting engineering prototype had a brightness of 120 cd/m/sup 2/ and fill factor of about 45%.

Integrating sphere charge coupled device-based measurement method for organic light-emitting devices

An integrating sphere charge coupled device (CCD)-based measurement system has been developed to accurately characterize the optoelectronic performance of organic polymer light-emitting devices (PLEDs). By theoretically analyzing a previously developed lens-coupled method and comparing it with the integrating sphere CCD-based method, we have found that the integrating sphere-based measurement method provides more stable reliable optical data in comparison with the lens-coupled measurement method. In addition, we demonstrate that inappropriate calibration of the PLED measurement system can greatly exaggerate device performance.

Air-stable organic polymer red light-emitting devices on flexible plastic substrates

Organic polymer red light-emitting devices (OPLEDs) with the double layer structure have been fabricated on flexible plastic substrates. Dow red emissive polymer and poly(3,4- ethylenedioxythiophene)/poly(styrene)(PEDOT/PSS) have been used as an emissive and a hole injection polymer, respectively. The spin coating technique was used to deposit different polymers. The absorption and the cyclic voltammetry spectra have been used to construct the band diagram of our OPLEDs. The following electrical and optical properties have been obtained for our OPLEDs: turn-on voltage, defined at 1 cd/m2=~3.0V; voltage and current density defined at 100 cd/m2=~6.5V and ~34mA/cm2; maximum emission efficiency =~0.25 cd/A; and maximum luminous efficiency =~0.1m/W. The extrapolated lifetime of unpackaged OPLEDs on flexible plastic substrate of about 1160 min for initial brightness of 100 cd/m2 has been obtained.