Dietrich Bertram

Highly efficient yellow organic light emitting diode based on a layer-cross faded emission layer allowing easy color tuning

An easy way to adjust the color of yellow organic light emitting diodes (OLED) is realized by basing the emission layer on a cross-fading zone of two unipolar-conducting host materials doping parts of it either with a red or green phosphorescent emitter at varying thickness ratios. At color coordinates of 0.47/0.50, a current efficacy of 42.2 cd/A (16.2% external quantum efficiency) and a power efficacy of 32.9 lm/W (1000 cd/m2) are measured without light extraction enhancement. Mixed-host emission layer OLED without cross-fading are processed for comparison. Exciton distributions are studied. The concept is suggested to be useful for white OLED.

Employing exciton transfer molecules to increase the lifetime of phosphorescent red organic light emitting diodes

The lifetime of phosphorescent red organic light emitting diodes (OLEDs) is investigated employing either N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (NPB), TMM117, or 4,4′,4″-tris(N-carbazolyl)-triphenylamine (TCTA) as hole-conducting host material (mixed with an electron conductor). All OLED (organic vapor phase deposition-processed) show similar efficiencies around 30 lm/W but strongly different lifetimes. Quickly degrading OLED based on TCTA can be stabilized by doping exciton transfer molecules [tris-(phenyl-pyridyl)-Ir (Ir(ppy)3)] to the emission layer. At a current density of 50 mA/cm2 (12 800 cd/m2), a lifetime of 387 h can be achieved. Employing exciton transfer molecules is suggested to prevent the degradation of the red emission layer in phosphorescent white OLED.

Layer Cross-Fading at Organic/Organic Interfaces in OVPD-Processed Red Phosphorescent Organic Light Emitting Diodes as a New Concept to Increase Current and Luminous Efficacy

The current and luminous efficacy of a red phosphorescent organic light emitting diode (OLED) with sharp interfaces between each of the organic layers can be increased from 18.8 cd/A and 14.1 lm/W (at 1,000 cd/m 2 ) to 36.5 cd/A (+94%, 18% EQE) and 33.7 lm/W (+139%) by the introduction of a layer cross-fading zone at the hole transport layer (HTL) to emission layer (EL) interface. Layer cross-fading describes a procedure of linearly decreasing the fraction in growth rate of an organic layer during deposition over a certain thickness while simultaneously increasing the fraction in growth rate of the following layer. For OLED processing and layer cross-fading organic vapor phase deposition (OVPD) is used. The typical observation of a roll-off in current efficacy of phosphorescent OLED to higher luminance can be reduced significantly. An interpenetrating network of a prevailing hole and a prevailing electron conducting material is created in the cross-fading zone. This broadens the recombination zone and furthermore lowers the driving voltage. The concept of layer cross-fading to increase the efficacies is suggested to be useful in multi-colored OLED stacks as well.

Hybrid white organic light-emitting diode with a mixed-host interlayer processed by organic vapor phase deposition

Abstract. Organic light-emitting diodes (OLEDs) are a key technology in solid state lighting.Withoutalong-livedphosphorescentblueemitter,ahybridconceptbasedonphosphorescentredandgreenemittersandafluorescentblueemitterinawhiteOLEDstackisapromisingapproachfor pure-white emission. Several challenges such as exciton recombination on all emitters andtripletdiffusion,aswellasquenching,havetobeovercome.Toaddresstheseissues,amixed-hostphosphorescent emission layer is employed. The mixture ratio is locally varied in the emissionlayer. An interlayer separates the phosphorescent and fluorescent emission layer. Strategies totune the color coordinates are presented. The lifetime and color stability versus luminance areinvestigated. At Commission Internationale de I’Eclairage color coordinates of 0.44/0.44, acurrent efficacy of 28.0 cd/A (at 1000 cd/m 2 ), and a luminous efficacy of 20.6 lm/W can bemeasured. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE) . [DOI: 10.1117/1.3545966]

White OLEDs for lighting applications

Efficient white OLEDs are becoming attractive as large area light sources for illumination and in future also for general lighting. We discuss device concepts for white OLEDs and their potential to achieve high efficacy and good lumen- and color-maintenance at the same time. We focus on OLEDs using a combination of fluorescent blue and phosphorescent red and green emitters (hybrid OLEDs). Hybrid OLEDs have high efficacy and lifetime in the white to warm white color region (color points B and A on the black-body-curve). Near illuminant A efficacy values of 28-29 lm/W without optical out-coupling can be achieved with a hybrid OLED. The external quantum efficiency (EQE) is 14%. A typical color rendering index (CRI) is 84. Recent results for monochrome OLEDs and for hybrid OLED stacks are presented.

66.3: Hybrid White OLEDs for General Lighting

OLEDs for general lighting require both high efficacy and good lumen maintenance. Hybrid stacks combining fluorescent blue and phosphorescent red and green emitters are a very good choice for both long lifetime and good efficacy values: 31 lm/W are demonstrated for a cold white color point. 44 lm /W are measured with improved out-coupling (ILO) using a scattering foil and 60 lm/W are demonstrated using a macro-extractor for light out-coupling. For warm white we realized 34 lm/W (with ILO 45 lm/W and 64 lm /W using a macro-extractor). LT50 lumen maintenance is for both stacks better than 30,000 hours.

Characterization of charge carrier injection in organic and hybrid organic/inorganic semiconductor devices by capacitance-voltage measurements

Organic and hybrid organic / inorganic semiconductor heterostructures offer great potential as key technology for cost-efficient electro-photonic devices. To exploit their full potential, fundamental understanding of charge carrier injection is essential. Therefore we use current-luminance-voltage (I-L-V) and capacitance-voltage (C-V) measurements to analyze the injection characteristics of monochrome OLED test structures and hybrid organic / inorganic (HOI) pentacene / n-GaN and Alq3 / n-GaN heterostructures processed by organic vapor phase deposition (OVPD) and metal organic chemical vapor deposition (MOCVD), respectively. In a first step, we fundamentally analyze the specific C-V characteristics of OLED test structures. The multilayer devices show additional features in the C-V profile as compared to the bilayer OLED investigated by Brütting et al.1,2. We attribute this behavior to the additional organic / organic interfaces and the resulting energetic barriers in multilayer devices. In addition, we compared the C-V measurements of pristine and degraded OLED test structures. Here we conclude that a deterioration of hole injection in degraded devices is dominant, whereas electron injection remains largely unaffected. Furthermore, in pristine and degraded OLED, increased temperatures generally lead to improved charge injection as well as a reduced impact of the additional barriers in multilayer stacks. By analyzing I-V and C-V measurements of HOI heterostructures, we find ambipolar currents in pentacene-based diodes with electrons from n-GaN being injected at lower bias than the onset of hole injection from the gold anode contacts. Generally, I-V measurements of both types of HOI heterostructures show an onset of charge injection at very low bias voltage. Accordingly, n-GaN offers superior electron injection characteristics recommending n-GaN as cathode contact e.g. in fully transparent OLED microdisplays.

Incoherent Light Sources

Since the invention and industrialization of incandescent lamps at the end of the 19th century electrical lighting has become a commodity in our daily life. Today, incoherent light sources are used for numerous application areas. Major improvements have been achieved over the past decades with respect to lamp efficiency (Fig. 10.1), lifetime and color properties. Open image in new window Fig. 10.1 Temporal development of the luminous efficacy of electrical light sources (LP = low pressure, HP = high pressure)

52.3: OLEDs for Lighting Applications

OLEDs for lighting applications require the combination of several properties at the same time: Large emission area, high brightness, high efficiency, long lifetime, good color stability at different brightness levels, and low cost. In order to fulfill these demands, several OLED architecture concepts are under investigation: Hybrid layered OLEDs, stacked OLEDs, pixel-OLEDs. To achieve good color stability the diode-units used for stacking have to be optimized. The talk focuses on hybrid OLEDs and their properties. In this context we demonstrate a phosphorescent yellow diode (combination of red and green emitters) which has an efficacy between 50 and 60 lm/W without improved light out-coupling (ILO) and excellent color stability. Such highly optimized OLED architectures have to be combined with suitable optical out-coupling techniques to make OLEDs ready for lighting. Optical out-coupling techniques are briefly reviewed. We demonstrate a concept for extracting more light of an OLED using low refractive index hole transport layers.

Nanocrystalline semiconductor LEDs with simple structure and high efficiency

Nanocrystalline semiconductor particles exhibit a size dependent bandgap emission, due to size quantisation effects. These particles are derived from solution chemistry and can be made monodisperse under the right synthesis conditions. Compared to organic materials, the inorganic nanoparticles show much higher stability against oxidation and degradation, which makes them an interesting candidate for LEDs and displays. So far, LEDs based on semiconductor nanoparticles typically included low stability organic materials to provide charge injection. The talk will present a new class of nanoparticle LEDs, made without sensitive organic materials. These LEDs show high efficiencies, well defined color throughout the red to green part of the visible spectrum and improved stability under ambient conditions without excessive encapsulation. Using high quality monodisperse suspensions, high color purity is achieved for the emission which paves the road to cheap, high quality displays based on inorganic semiconductor nanoparticles.