Christoph Zimmermann

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.

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]

Numerical Capacitance Analysis of Single-Layer OLEDs Based on the Master Equation

We present the simulations of the small-signal capacitance characteristics of organic light-emitting diodes based on the master equation for a 3-D lattice solved under the sinusoidal steady-state condition. Small-signal measurements and current-voltage characteristics by Martens et al. can be simultaneously reproduced with appropriate parameters. According to our simulations, the frequently observed effect of a negative differential capacitance at low frequencies is affected by several parameters. It becomes more pronounced with longer recombination lifetime, lower injection barrier, and higher energetic disorder.

Simulation of bipolar organic semiconductor devices based on the master equation including generation and recombination

A new model for the simulation of bipolar organic semiconductor devices by a three-dimensional master equation is proposed. A single-layer diode with a cubic lattice is investigated. Generation and recombination of electrons and holes are included consistently, where a single parameter, the lifetime, controls the rates. Current-voltage characteristics, carrier density and recombination rate profiles of symmetric and asymmetric bipolar devices are presented with Gaussian disorder of hopping site energy levels for electrons and holes.

DC, AC and noise simulation of organic semiconductor devices based on the master equation

The hopping transport in organic semiconductors produces characteristic frequency dependencies of the admittance and noise, which are calculated in this paper for the first time based on the master equation approach, where noise is evaluated by the Langevin approach and a modified Ramo-Shockley theorem. At low frequencies and low injection the non-equilibrium noise is found to be shot noise in the framework of this model.

Noise simulation of bipolar organic semiconductor devices based on the master equation

Noise analysis in bipolar organic semiconductor devices is performed via the Langevin approach for a three-dimensional master equation under the sinusoidal steady-state condition. A single-layer diode with a cubic lattice is investigated. Hopping, generation and recombination of charge carriers are all treated as Poisson processes. Calculations of the power spectral density of the terminal current show the noise dependence on frequency, bias, position, device thickness, recombination lifetime and energetic disorder.

Analysis of trap-induced noise in organic light-emitting diodes based on the master equation

The effect of traps on noise spectra in organic light-emitting diodes is investigated via a Langevin approach under steady-state conditions based on a three-dimensional bipolar master equation model. A Gaussian-type density of states is attributed to both trapped and free charge carriers. The relative power spectral density of the terminal current in the space-charge-limited-current regime is increased by a high fluctuation in the number of trapped charge carrier, which is weakened by the emergence of the field-induced detrapping.

Simulation of exciton effects in OLEDs based on the master equation

Electroluminescence in organic light-emitting diodes is simulated by the master equations for free carriers and excitons. The IV characteristics of both unipolar and bipolar devices can be well reproduced. The luminous efficacies of the phosphorescent OLEDs, which are doped with Ir(ppy)3 in the emission layer, depend on both the triplet generation zone and the triplet transfer capability. Triplet diffusion into the hole-transport layer is primarily attributed to the decline in efficiencies of OLEDs with low emitter concentrations. Higher luminous efficacies can be obtained by graded doping profiles with the merits of broad triplet distribution within and confined to the emission layer. Moreover, triplet-polaron quenching plays a more significant role in the triplet loss than triplet-triplet annihilation does according to our simulations.