Degradation in organic light-emitting diodes (OLEDs) is a complex problem. Depending upon the materials and the device architectures used, the degradation mechanism can be very different. In this Progress Report, using examples in both small molecule and polymer OLEDs, the different degradation mechanisms in two types of devices are examined. Some of the extrinsic and intrinsic degradation mechanisms in OLEDs are reviewed, and recent work on degradation studies of both small-molecule and polymer OLEDs is presented. For small-molecule OLEDs, the operational degradation of exemplary fluorescent devices is dominated by chemical transformations in the vicinity of the recombination zone. The accumulation of degradation products results in coupled phenomena of luminance-efficiency loss and operating-voltage rise. For polymer OLEDs, it is shown how the charge-transport and injection properties affect the device lifetime. Further, it is shown how the charge balance is controlled by interlayers at the anode contact, and their effects on the device lifetime are discussed.

Degradation Mechanisms in Small-Molecule and Polymer Organic Light-Emitting Diodes

A new approach to forming a gradient hole-injection layer in polymer light-emitting diodes (PLEDs) is demonstrated. Single spin-coating of hole-injecting conducting polymer compositions with a perfluorinated ionomer results in a work function gradient through the layer formed by self-organization, which leads to remarkably efficient single-layer PLEDs (ca. 21 cd A–1). The device lifetime is significantly improved (ca. 50 times) compared with the conventional hole-injection layer, poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate). These results are a good example for demonstrating that the shorter lifetime of PLEDs compared with small-molecule-based organic LEDs (SM-OLEDs) is not mainly due to the inherent degradation of the polymeric emitter itself. Hence, the results open the way to further improvements of PLEDs for real applications to large-area, high-resolution, and full-color flexible displays.

Self-Organized Gradient Hole Injection to Improve the Performance of Polymer Electroluminescent Devices

Control of surface composition of a hole-injecting conducting polymer complex, poly(3,4-ethylenedioxy thiophene) (PEDOT) doped with a polystyrene sulfonate (PSS) has been conducted in the spin-cast films. We found that the work function of the polymeric complex films formed via single spin-coating can be greatly increased up to 5.44 eV by increasing the surface concentration of the PSS dopant. As a result, we improved the device efficiency and the lifetime of green emitting polymer light-emitting diodes (PLEDs). This implies that the PSS surface layer of the films spin-cast from the conducting polymer complexes plays a key role in making high performance PLEDs.

http://phome.postech.ac.kr/user/pnel/publication/67.%20T.-W.%20Lee,%20Y.%20Chung%20,%20Advanced%20Functional%20Materials,%2018,%202246-2252%20(2008).pdf

Control of the Surface Composition of a Conducting-Polymer Complex Film to Tune the Work Function**

Highly efficient, simplified, solution-processed thermally activated delayed-fluorescence organic light-emitting diodes can be realized by using pure-organic thermally activated delayed fluorescence emitters and a multifunctional buffer hole-injection layer, in which high EQE (≈24%) and current efficiency (≈73 cd A(-1) ) are demonstrated. High-efficiency fluorescence red-emitting and blue-emitting devices can also be fabricated in this manner.

Highly Efficient, Simplified, Solution-Processed Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diodes.

Multilayer polymer light-emitting diodes fabricated by blade coating are presented. Multilayer of polymers can be easily deposited by blade coating on a hot plate. The multilayer structure is confirmed by the total thickness and the cross section view in the scanning electron microscope. The film thickness variation is only 3.3% in 10cm scale and the film roughness is about 0.3nm in the micron scale. The efficiency of single layer poly(para-phenylene vinylene) copolymer Super Yellow and poly(9,9-dioctylfluorene) (PFO, deep blue) devices are 9 and 1.7cd∕A, respectively, by blade coating. The efficiency of the PFO device is raised to 2.9cd∕A with a 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) hole-blocking layer and to 2.3cd∕A with a poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl))diphenylamine)] elec-tron-blocking layer added by blade coating.

Multilayer polymer light-emitting diodes by blade coating method

The authors report the effect of thermal treatment of hole-transporting interlayers between a polymeric hole injection layer and an emitting layer (EML) on the luminous efficiency and the lifetime performance in blue polymer light-emitting diodes. As the thermal annealing temperature of the interlayer increased, the hole mobility of the interlayer tended to decrease, which results in reducing the hole current injected into the EML in the devices. Hence, the device luminous efficiency decreased due to lower electron-hole balance. Nevertheless, the device lifetime increased, which can be attributed to the formation of the thicker interlayer and the better defined interlayer/EML interface.

http://phome.postech.ac.kr/user/pnel/publication/47.%20T.-W.%20Lee,%20M.-G.%20Kim,%20S.%20Y.%20Kim,%20S.%20H.%20Park,%20T.%20Y.%20Noh,%20T.-S.%20Oh,%20Applied%20Physics%20Letters,%2089,%20123505%20(2006)..pdf

Tae-Sik Oh

Papers

Hole-transporting interlayers for improving the device lifetime in the polymer light-emitting diodes