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Illumination & Displays

Stabilizing the efficiency of phosphorescent organic light-emitting diodes

A new device architecture incorporating an exciton blocking layer is efficient enough at high luminance to be used in large displays and solid state lighting.
21 April 2008, SPIE Newsroom. DOI: 10.1117/2.1200804.1099

Organic light-emitting diodes (OLEDs) work by passing an electric current through an organic layer, resulting in the emission of photons of a wavelength specified by the type of material used. OLEDs are currently used in the displays of mobile appliances such as MP3 players, cellular phones, and portable media players due to their ability to produce excellent picture quality with low power consumption. This property is further enhanced by replacing fluorescent compounds with phosphorescent organic materials, resulting in a four-fold increase in quantum efficiency.1 However, in phosphorescent OLEDs (PHOLEDs), efficiency depends on luminance, and a sharp decrease in quantum efficiency (termed ‘efficiency roll-off’) occurs at high luminance.1 This decrease leads to an increase in power consumption, limiting the practicality of PHOLEDs in applications requiring high luminance. This limitation must be addressed and overcome if PHOLEDs are to be used in larger high-brightness displays and in solid-state lighting applications.

Figure 1. Quantum efficiency and normalized quantum efficiency of green phosphorescent organic LEDs with four different device architectures. TCTA: 4,4′,4″-tris-(N-carbazolyl)-triphenlyamine. TPBI: 1,3,5-tris-(N-phenylbenzimidazole-2-yl)-benzene.

Numerous studies have examined the possible causes of efficiency roll-off in PHOLEDs. Triplet-triplet quenching,1 triplet-polaron quenching,1 and dissociation of excitons into free carriers2 have been proposed as the main mechanisms responsible for the decrease in efficiency at high luminescence and many device architectures have been developed address efficiency roll-off in PHOLEDs. For instance, PHOLEDs with a broad exciton formation zone can achieve a significant reduction in roll-off.3 Further reductions are needed,however, before PHOLEDs are efficient enough for use in larger applications.

In a typical OLED architecture, a metallic cathode injects electrons into an electron transport layer, while a transparent anode injects electron holes (left by the removal of electrons) into a hole transport layer. Electrons and holespropagate to the interior of the device, where they recombine in an emitting layer consisting of a host material doped with a light-emitting organic material. This recombination results in a drop in the energy levels of the electrons. The excess energy is transferred to the light emitting material, which then gives off a photon. The choice of materials for the transport and emitting layers is designed to maximize the recombination process and the efficiency of the device.

We developed a novel architecture to create a PHOLED with a broad exciton-formation zone and negligible charge leakage from the emitting layer. The device has a charge-transport-controllable emitting (CTCE) layer made from two different host materials doped with a third phosphorescent material. One host material has good hole transport properties whereas the other host material has good electron transport properties. An exciton blocking layer was introduced between the hole transport layer and the light emitting layer, ensuring that the recombination takes place within the emitting layer.

We tested the new architecture using tris(2-phenylpyridine)- iridium (Ir(ppy)3), a green emitter. The hole transport and electron transport host materials, respectively, were 4,4',4''-tris(N-carbazolyl)- triphenylamine (TCTA) and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI). They can be used as host materials in PHOLEDs containing Ir(ppy)3 as the dopant material because the triplet bandgaps of TCTA and TPBI are wider than that of Ir(ppy)3. In addition, TCTA can be used as an exciton blocking layer with high triplet energy.

Four different devices were fabricated and tested for quantum efficiency. The device architecture used a stack of layers, starting from the substrate, of indium tin oxide, the hole injection layer, the hole transport layer, the emitting layer (with 5% doping), the hole blocking layer, the electron transport layer, LiF, and Al on top. One device contained only TCTA as the host material, the second contained only TPBI, and two used a combination of TCTA and TPBI in a 50:50 ratio. One of the two TCTA:TPBI(50:50) devices had TCTA inserted between the hole transport layer and the TCTA:TPBI(50:50) emitting layer to act as the exciton blocking layer.

A sharp decrease in quantum efficiency at high luminance was observed in the TCTA and TPBI devices, and overall quantum efficiency was low (see Figure 1). In contrast, both the TCTA:TPBI(50:50) and TCTA/TCTA:TPBI(50:50) devices showed high quantum efficiency and little roll-off at high luminance. In particular, the TCTA/TCTA:TPBI(50:50) device showed only a 10% reduction in quantum efficiency at 20,000cd/m2.

Figure 2. Electroluminescence spectra of two organic LEDS from Figure 1. (a) The TCTA:TPBI(50:50) device shows significant electron leakage at high driving voltage. Colors represent different driving voltages. (b) The TCTA/TCTA:TPBI(50:50) device, which includes a TCTA exciton-blocking layer, changes little with increasing voltage.

The origin of the stabilized quantum efficiency in the TCTA/-TCTA:TPBI(50:50) device was clarified by monitoring the exciton formation zone and the charge leakage from the emitting layer. The emission zone was broad at both low and high luminance and there was no charge leakage even at high luminance (see Figure 2). The TCTA:TPBI(50:50) device also showed a broad emission zone, but electron leakage from the emitting layer was evident at high driving voltage, resulting in a large drop in efficiency compared to the TCTA/TCTA:TPBI(50:50) device.

Low power consumption at high luminance in OLEDs is critical in order to expand OLED applications from small portable devices to larger displays such as monitors and TVs and to applications in solid state lighting. The use of high efficiency phosphorescent light-emitting materials is an essential component in reducing power consumption. Our work has produced a PHOLED with a broad emission zone and only a 10% drop in quantum efficiency at high luminance. Adoption of the new device will diversify OLED applications. In future, we will apply this new architecture to white OLEDs to produce a white emitter capable of maintaining high quantum efficiency at high luminance.

Jun Yeob Lee, Kyoung Soo Yook, Soon Ok Jeon, Chul Woong Joo, Hyo Suk Son
Department of Polymer Science and Engineering
Dankook University
Yongin, Korea

Jun Yeob Lee is a material and device scientist specializing in the area of organic electronics. He was educated at Seoul National University in Korea and is an expert in OLEDs. He conducts leading research on organic electronics and organic electronic devices in Korea, and has developed high-efficiency phosphorescent OLEDs with stable efficiency over a wide luminance range as well as color-stable white OLEDs with no efficiency roll-off.

Kyoung Soo Yook is a master's student at Dankook Univeristy. He is developing OLEDs with high efficiency at low driving voltages through managing the charge transport properties of organic materials.

Soon Ok Jeon is a PhD student at Dankook University. She has experience in synthesizing blue fluorescent emitting materials. Her current project is to develop high triplet charge transport materials for high efficiency phosphorescent OLEDs and organic bistable memory devices.

Chul Woong Joo is a master's student at Dankook University. He is developing polymer-based organic bistable memory and white polymer LEDs using stacked emitting structures.

Hyo Suk Son is an undergraduate student at Dankook University. He is researching the synthesis of organic electronic materials for application in OLEDs, solar cells, and organic bistable memories.

Sung Hyun Kim, Jyongsik Jang
School of Chemical and Biological Engineering
Seoul National University
Seoul, Korea

Sung Hyun Kim is a PhD student at Seoul National University. He is developing high-efficiency phosphorescent OLEDs and white OLEDs. He is also researching organic bistable memory devices on flexible substrates.

Jyongsik Jang is a professor at Seoul National University. He conducts research on the synthesis of polymer-based nanoparticles, nanofibers, and nanotubes for applications in sensors and displays. He is an expert on organic nanomaterials and organic conducting materials.