Yung-Hui Yeh

High-contrast top-emitting organic light-emitting devices for active-matrix displays

Unlike previous high-contrast devices that all involve inserting extra layer(s) with optical purposes (e.g., absorption and interference) into the active region of devices, in this-letter we report a high-contrast top-emitting organic light-emitting device (OLED) that utilizes only optical characteristics of electrodes and anti-reflection coatings deposited outside the active region, thus reducing the complexity of devices. Furthermore, the device has an inherent microcavity which is beneficial to electroluminescence efficiency. The devices are readily compatible with the processing of active-matrix backplanes, and active-matrix OLED displays incorporating such high-contrast top-emitting devices were demonstrated to have improved readability under a strong lighting environment.

High-Performance Flexible a-IGZO TFTs Adopting Stacked Electrodes and Transparent Polyimide-Based Nanocomposite Substrates

We demonstrated flexible amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) on fully transparent and high-temperature polyimide-based nanocomposite substrates. The flexible nanocomposite substrates were coated on the carrier glass substrates and were debonded after the TFT microfabrication. The adoption of the Ti/IZO stacked electrodes as source/drain/ gain electrodes significantly improved the etching compatibility with other material layers, enabling successful implementation of flexible a-IGZO TFTs onto the transparent nanocomposite substrates by conventional lithographic and etching processes. The flexible a-IGZO TFTs exhibited decent mobility and mechanical bending capability. Field-effect mobility of up to 15.9 cm2/V · s, a subthreshold swing of 0.4 V/dec, a threshold voltage of 0.8 V, and an on/off ratio of >; 108 were extracted from the TFT characteristics. The devices could be bent down to a radius of curvature of 3 mm and yet remained normally functional. Such successful demonstration of flexible oxide TFTs on transparent flexible substrates using fully lithographic and etching processes that are compatible with existing TFT fabrication technologies shall broaden their uses in flexible displays and electronics.

Amorphous InGaZnO Thin-Film Transistors Compatible With Roll-to-Roll Fabrication at Room Temperature

High-performance amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) are successfully fabricated on a colorless polyimide substrate using a top-gate self-aligned structure. All thin films are deposited by roll-to-roll-compatible sputtering processes at room temperature. The maximum field-effect mobility is 18 cm2/V·s, the threshold voltage is -1.35 V, the subthreshold slope is 0.1 V/decade, and the on/off current ratio is about 105. The results highlight that excellent device performance can be realized in a-IGZO TFTs without compromising manufacturability.

Influence of Passivation Layers on Characteristics of a-InGaZnO Thin-Film Transistors

We investigated the influence of passivation-layer deposition on the characteristics of a-InGaZnO thin-film transistors (TFTs). The threshold voltage (VT) of the TFTs shifted markedly as a result of the mechanical stress induced by the passivation layers above. By adjusting the deposition parameters during the passivation process, the performance of the TFTs can be modulated. The a-InGaZnO TFTs after dual passivation exhibited good performance with a field-effect mobility of 11.35 cm2/V·s, a threshold voltage of 2.86 V, and an on-off ratio of 108.

Efficient White OLEDs Employing Phosphorescent Sensitization

We have investigated white-emitting organic light-emitting devices (WOLEDs) making use of both blue-phosphor-sensitized orange-red fluorescence and the residual blue phosphorescence. By carefully adjusting the concentrations the phosphor and the fluorophore in the emitting layer and choosing the carrier-transport layers in the device structure, WOLEDs containing a single phosphor-sensitized emitting layer (type-I devices) can give colors close to the equal-energy white (0.33, 0.33), CRI up to 75, and efficiencies up to (10%, 23 cd/A, 13.4lm/W). Furthermore, by doping a green phosphor into the poorly emitting electron-transport layer (type-II devices) to recycle excitons formed there, the EL efficiencies can be further enhanced up to (12.1%, 35.3 cd/A, 23.9lm/W). In both types of devices, the phosphor sensitization reduces population of triplet excitons in the emitting region and substantially mitigates the efficiency roll-off with the driving current or brightness that is often observed in all-phosphor OLEDs. At the brightness of 1000 cd/m2, both types of devices retain quantum and cadmium per ampere (cd/A) efficiencies similar to their peak values

Effects of Mechanical Strains on the Characteristics of Top-Gate Staggered a-IGZO Thin-Film Transistors Fabricated on Polyimide-Based Nanocomposite Substrates

In this paper, we had successfully implemented flexible top-gate staggered amorphous In-Ga-Zn-O (a-IGZO) thin- film transistors (TFTs) on colorless and transparent polyimide (PI)-based nanocomposite substrates using fully lithographic and etching processes that are compatible with existing TFT mass fabrication technologies. The use of the selectively coated release layer between the nanocomposite PI film and the glass carrier ensured smooth debonding of the plastic substrate after TFT fabrication. The TFTs showed decent performances (with mobility >; 10 cm2/V · s) either as fabricated or as debonded from the carrier glass. By bending the devices to different radii of curvature (from a flat state to an outward bending radius of 5 mm), influences of mechanical strains on the characteristics of flexible a-IGZO TFTs were also investigated. In general, the mobility of the flexible a-IGZO TFT increased with the tensile strain, whereas the threshold voltage decreased with the tensile strain. The variation of the mobility in a-IGZO TFTs versus the strain appeared smaller than those observed for amorphous silicon TFTs.

Top-Gate Staggered a-IGZO TFTs Adopting the Bilayer Gate Insulator for Driving AMOLED

We report the successful implementation of top-gate staggered amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) with decent performance and environmental stability by adopting the SiOx/SiNx bilayer gate-insulator stack. The PECVD SiOx and SiNx were used as the first and second gate insulators, respectively, in the TFT to simultaneously ensure the channel/gate-insulator interface properties for device performances and the water impermeability of the gate insulator for effective passivation of the channel layer. It was also found that the cleanliness of the back-channel interface (and thus the effectiveness of the source/drain etching process) is critical for the successful implementation of the top-gate staggered a-IGZO TFTs. In this paper, a two-step wet-etching process for source/drain was used to ensure the quality of the back-channel interface. Finally, we successfully integrated the top-gate staggered a-IGZO TFTs into a working 2.2-in active matrix organic light-emitting display panel, demonstrating the real use of the developed TFTs.

Hydrogenated Amorphous Silicon Solar Cells on Textured Flexible Substrate Copied From a Textured Glass Substrate Template

It is difficult to prepare textured surface on flexible substrate reproducibly for solar cell application. A novel method is developed that uses polyimide film to coat on a textured glass substrate as a template substrate and then peeled off as the flexible substrate. The surface morphology of the flexible substrate could faithfully reproduce the template as measured by atomic force microscopy. The hydrogenated amorphous silicon (a-Si:H) thin film solar cell was fabricated on the flexible substrate successfully. The results of I-V characteristics and spectral response confirm that the efficiency of textured solar cells increases as compared to that on a flat substrate. This technology will find application in making complicated reusable substrate for flexible electronics.

P-11: Amorphous In2O3-Ga2O3-ZnO Thin Film Transistors and Integrated Circuits on Flexible and Colorless Polyimide Substrates

A process was developed for fine fabrication of amorphous IGZO TFTs and integrated circuits on flexible and colorless polyimide substrates. TFTs with field-effect mobilities of ∼10 cm2/Vs and ring oscillators with propagation delay of 0.35 μs per stage were achieved on the polyimide substrates.

38.3: LTPS Active Matrix OLED Displays Incorporating High‐Contrast Top‐Emitting OLEDs

A 3.8-inch high-contrast top-emitting AMOLED display using the LTPS TFT backplane is demonstrated. The display incorporates low-reflection top-emitting OLEDs with improved efficiency, and thus exhibits clear readability under a strong lighting environment without need of extra contrast-enhancement films.