Theodore X. Zhou

High operational stability of electrophosphorescent devices

Electrophosphorescent devices with fac-tris(2-phenylpyridine)iridium as the green emitting dopant have been fabricated with a variety of hole and exciton blocking materials. A device with aluminum(III)bis(2-methyl-8-quinolinato)4-phenylphenolate (BAlq) demonstrates an efficiency of 19 cd/A with a projected operational lifetime of 10 000 h, operated at an initial brightness of 500 cd/m2; or 50 000 h normalized to 100 cd/m2. An orange-red electrophosphorescent device with iridium(III) bis(2-phenylquinolyl-N,C2′)acetylacetonate as the dopant emitter and BAlq as the hole blocker demonstrates a maximum efficiency of 17.6 cd/A with a projected operational lifetime of 5000 h at an initial brightness of 300 cd/m2; or 15 000 h normalized to 100 cd/m2. The average voltage increase for both devices is <0.3 mV/h. The device operational lifetime is found to be inversely proportional to the initial brightness, typical of fluorescent organic light emitting devices.

High-efficiency top-emitting organic light-emitting devices

Based on theoretical arguments that top-emitting organic light-emitting devices (TOLEDs) can be more efficient than equivalent bottom-emitting devices, we fabricated TOLEDs comprising reflective anodes and transparent compound cathodes that emit 20.8% more photons in the forward 120° cone than equivalent bottom-emitting OLEDs. Device optimization by tuning the thickness of the top indium–tin–oxide layer according to a microcavity model is also reported.

Operating lifetime of phosphorescent organic light emitting devices

We investigate the continuous operating lifetime of organic light emitting devices (OLEDs) using the phosphorescent dopant, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum (II) as the light emitting molecule. We characterize devices based on two different electron transporting hosts: tris-(8-hydroxyquinoline) aluminum and 4,4′-N,N′-dicarbazolyl-biphenyl (CBP). The OLEDs lose ∼25% of their luminance in the first 50 h of operation, followed by extremely slow degradation with negligible growth of dark spots. The device lifetime of CBP-based phosphorescent OLEDs projected to 50% initial brightness is >107 h at a mean current density of 10 mA/cm2 under 50% duty cycle pulsed operation. These extremely long lifetimes are speculated to be an intrinsic property of electrophosphorescent OLEDs, where radiative phosphors significantly shorten the lifetime of potentially reactive triplet states in the conductive host material.

Semitransparent cathodes for organic light emitting devices

We optimize transparent organic light emitting devices (TOLEDs) using compound cathodes consisting of a thermally evaporated metal contact layer capped with indium–tin–oxide (ITO). The ITO is sputtered at rates of up to 1.6 A/s using a high power radio frequency magnetron process. With a Mg:Ag contact layer, we demonstrate a TOLED with 50% transparency and an operating voltage within 0.3 V of a device with identical organic layers and a conventional Mg:Ag cathode. The operational lifetime of the TOLED is shown to be equal to that of a similar, nontransparent device. We also study the effects of using different contact metals, including Ca, Al and LiF, on the operating characteristics of the TOLEDs. With a thin Ca contact layer, undoped TOLEDs with >80% peak transparency operating at (5.9±0.1) V at a brightness of >100 cd/m2 are demonstrated. These devices have application to transparent, head-up displays and to full color, stacked organic light emitting devices.

Stable and efficient electrophosphorescent organic light-emitting devices grown by organic vapor phase deposition

An electrophosphorescent organic light-emitting device (PHOLED™) employing fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] as the green emitting phosphor has been fabricated using a pilot-production organic vapor phase deposition (OVPD™) system. Highly controlled mass transport of the organic vapor to the substrate results in deposition rates of over 10A∕s and spatial uniformity better than ±2% across a 150mm×150mm substrate with less than ±2% run-to-run variations. The device current–voltage, luminous efficiency, and operational lifetime performances are compared to those of a similar device grown by conventional vacuum thermal evaporation (VTE). The green OVPD-grown PHOLED exhibits a maximum external quantum efficiency of (7.0±0.1)% at a luminance of 1000cd∕m2, comparable to the VTE device performance. The operational lifetime of the OVPD-grown devices was found to be comparable to or even somewhat longer than the lifetime achieved by VTE. Furthermore, PHOLEDs with emissive layers deposited at 4.8 and 10.8...

52.3: Display Properties of High-efficiency Electrophosphorescent Diodes

The efficiency-vs-current characteristics of phosphorescent organic light emitting diodes (PHOLEDs™) have been studied and compared to fluorescent small molecule organic light emitting diodes (SMOLED) and fluorescent polymer light emitting diodes (PLED). Results show that high efficiency PHOLEDs have significantly higher luminous efficiency than SMOLEDs and PLEDs at both the low and high drive current regimes required for active and passive matrix display applications. The efficiency roll-off of both phosphorescent and fluorescent devices is comparable. Long triplet exciton lifetime and the possible triplet-triplet annihilation do not explain this similarity. Other causes such as polaron-exciton annihilation and electric field induced photoluminescence quenching are provided.

27.4: Modeling and Fabrication of Organic Vapor Phase Deposition (OVPD) Equipment for OLED Display Manufacturing

The Principle of Organic Vapor Phase Deposition (OVPD) using AIXTRONs proprietary Close Coupled Showerhead technology will be presented and discussed. This alternative deposition technology enables substrate scalability and multiple layer deposition with high growth rates and high material efficiency. CFD-Simulations for the temperature distribution and the mass fraction of typically used Alq3 reveal excellent process control required for homogeneous deposition and reproducible mass production. The actual production equipment will be discussed.

33.3: Contrast Enhancement of OLED Displays

A high contrast OLED display technology has been developed using Luxells BlackLite™ technology integrated with UDCs transparent cathode structure. This structure demonstrates the potential of achieving an OLED display with low reflectivity, enhanced image clarity, reduced drive currents and enhanced display lifetime.

High-efficiency organic electrophosphorescent devices

We have fabricated saturated red, orange, yellow and green OLEDs, utilizing phosphorescent dopants. Using phosphorescence based emitters we have eliminated the inherent 25% upper limit on emission observed for traditional fluorescence based systems. The quantum efficiencies of these devices are quite good, with measured external efficiencies > 15% and > 40 lum/W (green) in the best devices. The phosphorescent dopants in these devices are heavy metal containing molecules (i.e. Pt, and Ir), prepared as both metalloporphyrins and organometallic complexes. The high level of spin orbit coupling in these metal complexes gives efficient emission from triplet states. In addition to emission from the heavy metal dopant, it is possible to transfer the exciton energy to a fluorescent dye, by Forster energy transfer. The heavy metal dopant in this case acts as a sensitizer, utilizing both singlet and triplet excitons to efficiently pump a fluorescent dye. We discuss the important parameters in designing electrophosphorescent OLEDs as well as their strengths and limitations. Accelerated aging studies, on packaged devices, have shown that phosphorescence based OLEDs can have very long device lifetimes.

Optimization of Reliable Transparent Organic Light‐Emitting Devices

The optical and electrical properties of the Mg-Ag/ITO electrode of transparent organic light emitting devices (TOLED) have been extensively studied. An optimum thickness of Mg-Ag has been chosen to achieve low operational voltage and high transparency. A more than 20-fold improvement in ITO deposition rate over that previously reported has been achieved. Reliability data comparable to those of conventional OLEDs have been obtained.