Mitsunori Suzuki

Highly Efficient and Stable Red Phosphorescent Organic Light‐Emitting Diodes Using Platinum Complexes

There have been many reports on phosphorescent organic light-emitting diodes (PHOLEDs) because of their relatively high emission effi ciencies compared with those of conventional fl uorescent OLEDs. [ 1–3 ] In addition to their effi ciency, the driving voltage and operational stability are important factors in the application of PHOLEDs to displays and lighting. PHOLEDs often have a charge-trapping problem at the sites of dopant molecules owing to the large band gap ( E g ) difference between the host and dopant molecules. [ 4 , 5 ] An increase in the driving voltage caused by doping a phosphorescent dye is often observed, particularly in red PHOLEDs using a conventional carbazole-based host material such as 4,4 ′ -bis(9-carbazolyl)2,2 ′ -biphenyl (CBP). When a dopant with a narrow E g is doped into a host with a wide E g , the difference in the highest occupied molecular orbital (HOMO) levels and/or lowest unoccupied molecular orbital (LUMO) levels between the dopant and the host signifi cantly increases. Subsequently, the dopant becomes a deep trap for hole and/or electron transport in the emitting layer. The maximum reported power effi ciency (PE) of red PHOLEDs was about 10 lm W − 1 in the early phase of their development, where the host/dopant combination was CBP/ Ir(piq) 3 (tris[1-phenylisoquinolinato-C2,N]iridium(III)). [ 6 ]

Highly efficient polymer light-emitting devices using ambipolar phosphorescent polymers

We report on highly efficient polymer light-emitting devices (PLEDs) achieved using a phosphorescent polymer, which is a copolymer that has bis(2-phenylpyridine)iridium (acetylacetonate) [Ir(ppy)2(acac)], N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine (TPD) and 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) as a side group. The phosphorescent polymer has an ambipolar charge-transport ability. An increase in PBD unit concentration allows an improvement in the efficiency of the PLEDs. Ba and Cs were used for electron-injection layers as well as Ca, to improve the electron injection. An external quantum efficiency of 11.8% and a power efficiency of 38.6lm∕W were obtained by using Cs. The results indicate that this can be attributed to an improvement in the charge balance of electrons and holes.

Improvement of emission efficiency in polymer light-emitting devices based on phosphorescent polymers

We report improvement of emission efficiency in polymer light-emitting devices (PLEDs) employing phosphorescent polymers. A hole-blocking layer was inserted between the emissive layer and the cathode to enhance recombination efficiency for the injected holes and electrons. Aluminum(III)bis(2-methyl-8-quinolinato)4-phenylphenolato (BAlq) was used for the hole-blocking layer. The resultant PLEDs exhibited significant improvement of emission efficiency. The respective external quantum efficiencies for red, green and blue PLEDs were 6.6, 11 and 6.9%. These values are very high compared with those based on conventional fluorescent polymers.

Fabrication of 5.8-in. OTFT-driven flexible color AMOLED display using dual protection scheme for organic semiconductor patterning

— A 5.8-in. wide-QQVGA flexible color active-matrix organic light-emitting-diode (AMOLED) display consisting of organic thin-film transistors (OTFTs) and phosphorescent OLEDs was fabricated on a plastic film. To reduce the operating voltage of the OTFTs, Ta2O5 with a high dielectric constant was employed as a gate insulator. Pentacene was used for the semiconductor layer of the OTFTs. This layer was patterned by photolithography and dry-etched using a dual protection layer of poly p-xylylene and SiO2 film. Uniform transistor performance was achieved in the OTFT backplane with QQVGA pixels. The RGB emission layers of the pixels were formed by vacuum deposition of phosphorescent small molecules. The resulting display could clearly show color moving images even when it was bent and operated at a low driving voltage (below 15 V).

High-efficiency white phosphorescent polymer light-emitting devices

White phosphorescent light emission from polymer light-emitting devices (PLEDs) has been demonstrated. To fabricate the white-emitting PLED, blue phosphorescent polymer (BPP) and red phosphorescent polymer (RPP) were used for the emissive layer, and the emission color was tuned by controlling the concentration ratio of BPP to RPP. The external quantum efficiency of the white-emitting PLED, with CIE coordinates of (0.34, 0.36), was 6.0% at luminance of 100 cd/m/sup 2/. To investigate the emission mechanism in the PLED, its photoluminescence spectrum and transient decay were measured. These experimental measurements indicate that direct excitation of the iridium-complex (Ir-complex) units by carrier trapping is a major excitation process for white-emitting PLED. A 3.6-in full-color display based on the white phosphorescent PLED and color filters was demonstrated.

A 5.8‐in. phosphorescent color AMOLED display fabricated by ink‐jet printing on plastic substrate

— A flexible phosphorescent color active-matrix organic light-emitting-diode (AMOLED) display on a plastic substrate has been fabricated. Phosphorescent polymer materials are used for the emitting layer, which is patterned using ink-jet printing. A mixed solvent system with a high-viscosity solvent is used for ink formulation to obtain jetting reliability. The effects of evaporation and the baking condition on the film profile and OLED performances were investigated. An organic thin-film-transistor (OTFT) backplane, fabricated using pentacene, is used to drive the OLEDs. The OTFT exhibited a current on/off ratio of 106 and a mobility of 0.1 cm2/V-sec. Color moving images were successfully shown on the fabricated display.

16.4: Low‐Temperature Fabrication of Flexible AMOLED Displays Using Oxide TFTs with Polymer Gate Insulators

We have developed InGaZnO4 TFTs with polymer gate insulators that can be formed by spin-coating on plastic substrates at temperatures below 130 °C. A 5-inch QVGA flexible OLED display has been fabricated by means of ink-jet printing on a TFT backplane, and it has successfully displayed clear color video images while in a bent state.

Low‐temperature fabrication of 5‐in. QVGA flexible AMOLED display driven by OTFTs using olefin polymer as the gate insulator

Abstract— A 5-in. QVGA flexible AMOLED display driven by OTFTs has been fabricated at a low temperature of 130°C. A polyethylene naphthalate film was used as the flexible substrate and an olefin polymer was used as the gate insulator for the OTFT. This layer was formed by spin-coating and baking at 130°C. Pentacene was used as the organic semiconductor layer. The OTFT performance to drive the flexible display with QVGA pixels in terms of current on/off ratio, carrier mobility, and spatial uniformity on the backplane have been obtained. Phosphorescent and fluorescent OLEDs were used as light-emitting devices on a flexible display. Those layers were formed by vacuum deposition. After the flexible display was fabricated, a clear and uniform moving image was obtained on the display. The display also showed a stable moving image even when it was bent.

Improvement in image quality of a 5.8-in. OTFT-driven flexible AMOLED display

— The image quality of an OTFT-driven flexible AMOLED display has been improved by enhancing the performance of OTFTs and OLEDs. To reduce the operating voltage of OTFTs on a plastic film, Ta2O5 with a high dielectric constant was used as a gate insulator. The organic semiconductor layer of the OTFT was successfully patterned by a polymer separator, which is an isolating wall structure using an organic material. The OTFT performance, such as its current on/off ratio, carrier mobility, and spatial uniformity on the backplane, was enhanced. A highly efficient phosphorescent OLED was used as a light-emission device. A very thin molybdenum oxide film was introduced as a carrier-injection layer on a pixel electrode to reduce the operating voltage of the OLED. After an OTFT-driven flexible AMOLED display was fabricated, the luminance and uniformity on the display was improved. The fabricated display also showed clear moving images, even when it was bent at a low operating voltage.

6 inch-flexible AM-OLED moving image display

The electrical performance of an organic thin-film transistor (OTFT) based on pentacene was significantly improved through the control of the layer surfaces and device interfaces. A 6-inch flexible organic light-emitting diode (OLED) display consisting of OTFT and OLED devices on a thin plastic substrate was demonstrated. While operating the OTFT backplane at relatively low voltages below 15 V, a good average field-effect mobility of 0.1 cm2/Vs was exhibited. The flexible active-matrix organic light-emitting diode (AM-OLED) panel displayed distinct color moving images at a frame rate of 60 Hz.