Vi-En Choong

Recent progress in solution processable organic light emitting devices

Organic light emitting devices (OLEDs) have been the subject of intense research because of their potential for flat panel display and solid state lighting applications. While small molecule OLEDs with very high efficiencies have been demonstrated, solution processable devices are more desirable for large size flat panel display and solid state applications because they are compatible with low cost, large area roll-to-roll manufacturing process. In this review paper, we will present the recent progress made in solution processable OLEDs. The paper will be divided into three parts. In the first part of the paper, we will focus on the recent development of fluorescent polymer OLEDs based on conjugated polyfluorene copolymers. Specifically, we will present results of carrier transport and injection measurements, and discuss how the charge transport and injection properties affect the device performance. In the second part of the paper, we will focus on the recent progress on phosphorescent dye-dispersed nonc...

Highly efficient white organic light-emitting diode

We present a highly efficient white electroluminescence device by the combination of a solution processed blue organic phosphorescence light-emitting diode with appropriate down-conversion phosphor system. The use of this down-conversion system produced an extraordinary enhancement on device performance, resulting in a white electroluminescence device with luminance efficacy of 25lm∕W at luminance efficiency reaching 39cd∕A. The extraordinary enhancement on device performance is attributed to isotropic radiation pattern of the excited phosphor particles, leading to high light extraction properties.

Organic light-emitting diodes with a bipolar transport layer

A structure based on a bipolar transport/emitting layer is proposed and implemented for making organic light-emitting diodes. Compared to the conventional heterojunction organic light-emitting diodes, more than a factor of six improvement in device reliability (a projected operating lifetime of 70 000 h) is achieved in the structure. The significant improvement in device lifetime is attributed to the elimination of the heterointerface present in the conventional devices which greatly affects the device reliability.

Highly efficient solution processed blue organic electrophosphorescence with 14lm∕W luminous efficacy

We report highly efficient solution processed blue electrophosphorescent organic light emitting diodes (PHOLEDs) utilizing a phosphorescent dye and a nonconjugated polymer host, molecularly doped with electron transporting molecules. Based on a bilayer device architecture blue PHOLEDs with luminous efficacy of 14lm∕W at luminous efficiency reaching 22cd∕A are demonstrated. Analysis of device performance indicates that this high efficiency is achieved by a combination of improved charge balance and light outcoupling efficiency. Our results demonstrate that simple solution processed devices can have efficiencies similar to those published to date for small molecule multilayer PHOLEDs based on the same emitter.

Influence of metallic nanoparticles on the performance of organic electrophosphorescence devices

We investigate the influence of metallic nanoparticles on the performance of phosphorescence organic light emitting diodes. The metallic nanoparticles display appropriate surface plasmon resonances in the visible region of the spectrum close to the emission band of the triplet emitter to accelerate the spontaneous emission rate of the phosphorescence compound. We have incorporated gold nanoparticles in the interface between alternative buffer layers and a phosphorescence based light emitting layer. In the case of AI4083 poly(3,4-ethylenedioxy thiophene):poly(styrene sulfonate) buffer layer, the interface modification with gold nanoparticles results in a 33% improvement in the luminance efficiency of tris(2-4(4-toltyl)phenylpyridine) iridium based phosphorescence organic light emitting diodes.

Bipolar transport organic light emitting diodes with enhanced reliability by LiF doping

An electrode contact scheme based on the use of an organic LiF alloy is investigated. The performance of organic light emitting diodes (OLED) with this contact scheme in both heterojunction and bipolar transport/emitting layer (BTEL) OLED structures are compared with their counterparts with LiF buffer layers. The organic LiF contact scheme improved device reliability of BTEL OLEDs by 32% to 92 500 h while adversely affecting device reliability of heterojunction OLEDs.

General method to evaluate substrate surface modification techniques for light extraction enhancement of organic light emitting diodes

The external light output of organic light emitting diodes (OLEDs) can be increased by modifying the light emitting surface. The apparent light extraction enhancement is given by the ratio between the efficiency of the unmodified device and the efficiency of the modified device. This apparent light extraction enhancement is dependent on the OLED architecture itself and is not the correct value to judge the effectiveness of a technique to enhance light outcoupling due to substrate surface modification. We propose a general method to evaluate substrate surface modification techniques for light extraction enhancement of OLEDs independent from the device architecture. This method is experimentally demonstrated using green electrophosphorescent OLEDs with different device architectures. The substrate surface of these OLEDs was modified by applying a prismatic film to increase light outcoupling from the device stack. It was demonstrated that the conventionally measured apparent light extraction enhancement by m...

Efficient and durable organic alloys for electroluminescent displays

We present new results on a single-layer organic alloy light emitting diode. The device shows significant improvement in lifetime at room and elevated temperatures. The improvement is attributed to the elimination of the heterointerface and the minimization of the formation of unstable tris-(8-hydroxyquinoline) aluminum (Alq3 ) cations. The efficiency is comparable to those of heterojunction counterparts. Balanced bipolar carrier injection and transport are made possible by adjusting the alloy composition and doping.

Highly efficient organic electroluminescent device with modified cathode

One of the key parameters for high efficiency organic electrophosphorescent light emitting diodes is charge injection into the phosphorescence compound. By introducing a hybrid device architecture, and incorporating electron and hole interfacial layers with lowest unoccupied molecular orbital and highest occupied molecular orbital levels similar to that of the phosphorescence compound, on the cathode and the anode side of the device, respectively, charge injection properties were improved. A green electrophosphorescence device with luminous efficacy of 50lm∕W at luminance efficiency reaching 55cd∕A was demonstrated.

Light Extraction From Solution-Based Processable Electrophosphorescent Organic Light-Emitting Diodes

Molecular dye dispersed solution processable blue emitting organic light-emitting devices have been fabricated and the resulting devices exhibit efficiency as high as 25 cd/A. With down-conversion phosphors, white emitting devices have been demonstrated with peak efficiency of 38 cd/A and luminous efficiency of 25 lm/W. The high efficiencies have been a product of proper tuning of carrier transport, optimization of the location of the carrier recombination zone and, hence, microcavity effect, efficient down-conversion from blue to white light, and scattering/isotropic remission due to phosphor particles. An optical model has been developed to investigate all these effects. In contrast to the common misunderstanding that light out-coupling efficiency is about 22% and independent of device architecture, our device data and optical modeling results clearly demonstrated that the light out-coupling efficiency is strongly dependent on the exact location of the recombination zone. Estimating the device internal quantum efficiencies based on external quantum efficiencies without considering the device architecture could lead to erroneous conclusions