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We present a vertical organic transistor with high operating frequencies and high current densities based on the organic semiconductor C60. In this vertical organic triode (VOT), doped layers can easily be inserted to efficiently reduce the contact resistance without the need for additional structuring. Combined with annealing at elevated temperatures (150°C for 2 h), we achieve a current density of 1 A/cm2 at a driving voltage of 3 V together with an extremely high transconductance of 30 mS. The transistor retains a voltage gain above 1 up to 1.5 MHz in a simple inverter circuit.

We report on a theoretical framework for the efficiency analysis of complex, multi-emitter organic light emitting diodes (OLEDs). The calculation approach makes use of electromagnetic modeling to quantify the overall OLED photon outcoupling efficiency and a phenomenological description for electrical and excitonic processes. From the comparison of optical modeling results and measurements of the total external quantum efficiency, we obtain reliable estimates of internal quantum yield. As application of the model, we analyze high-efficiency stacked white OLEDs and comment on the various efficiency loss channels present in the devices.

We report on the nature of the resonant modes in organic planar microcavities featuring semi-transparent metallic layers. We theoretically demonstrate that symmetric microcavities support a total of four modes originating from the coupling of surface plasmon polaritons. For red top-emitting organic light-emitting diodes with one semi-transparent metallic electrode, we identify three coupled plasmonic modes and calculate a light outcoupling efficiency close to 34% when assuming emitters with isotropic transition dipole moment. This value is estimated to increase to 50% in the case the dipole moment is purely horizontal.

Molecular doping of organic semiconductors is a key technology for highly efficient organic light-emitting diodes. Nevertheless, the underlying fundamental mechanisms are under discussion. This is because of the complex situation of structural disorder and strong polaronic coupling in such systems. We provide for the first time a systematic study of the formation of charge depletion zones in organic pin-diodes comprising molecular doped hole and electron transport layers. Impedance spectroscopy is employed to study the capacitance of these depletion zones. In particular, we show that the voltage dependent capacitance function obeys the Mott-Schottky relation concerning the influence of doping and the effect of an additional depletion zone given by the intrinsic interlayer. From temperature dependent measurements of the depletion capacitance, we can deduce the amount of active dopant states, their activation energy, and the spatial field distribution within the junction. The measured activation energy of t...

We report on highly enhanced and controlled light outcoupling of bidirectional organic light-emitting diodes by introduction of an enhanced microcavity structure as well as an organic capping layer (OC). Combining both OC and microcavity, we find that the overall external quantum, as well as current efficiency (CE), can be greatly enhanced. Especially, the CE with an appropriate thickness of OC is almost 1.75 times larger than that of the reference device without OC. Furthermore, we also analyze our devices with a numerical optical model calculating the flux of outcoupled photons, and compare theoretical predictions with our experimental results.

We present a non-volatile organic memory device comprising a thin SiO2 layer, the organic semiconductor C60, and an organic n-type doped layer between two metallic electrodes. The memory device shows a stable hysteresis in the current-voltage characteristics with an ON/OFF ratio in the range of three or higher and reasonable switching behavior with 104 write-read-erase-read cycles. The data retention time reaches from several hours up to several days and depends on the read out frequency. We exclude a filamentary conduction mechanism as cause of the memory effect and propose that the presence of charge carrier traps at the interface of the C60 layer with the oxide causes the hysteresis of this organic non-volatile memory device.

We report on an alternating current (AC) organic electroluminance (EL) device with high brightness of over 1500 cdm−2 and very low threshold voltage for the onset of AC luminance (10 V). The device consists of organic layers enclosed in a pair of either silicon dioxide (SiO2) or hafnium dioxide (HfO2) insulating dielectric layers. We observe internal charge carrier generation and luminance due to charge carrier recombination within the device under the AC drive without charge injection. We compare AC-EL demonstrated for the two devices and show that the threshold voltage for the onset of luminance can be significantly lowered using high dielectric constant (κ) HfO2 insulating layers. Further, we discuss the limitations of internal charge carrier generation and the device performance supported with the results from time resolved measurements.

Due to their effective short channel length of only a few hundred nanometers, vertical organic triodes (VOTs) have a high potential to overcome problems of low current densities and switching speed in current organic field effect transistors (OFETs). Furthermore, VOTs are easy to build because no sub-structuring of the base contact is necessary. Nevertheless, these devices are poorly investigated. In literature, two different working mechanisms are suggested: hot carrier transport through the metallic base or transport of charge carriers through a permeable base electrode. As a strong asymmetry is expected for function principle based on hot carriers, we are able to distinguish between both mechanisms by examining the bidirectional transmission properties of the VOT consisting of electron transporting materials. We show that high transmission values (>95%) are possible for both directions, suggesting a base contact with openings forming a grid electrode.

Exciton generation and transfer processes in a multilayer organic light-emitting diode (OLED) are studied in order to realize OLEDs with warm white color coordinates and high color-rendering index (CRI). We investigate a host-guest-system containing four phosphorescent emitters and two matrix materials with different transport properties. We show, by time-resolved spectroscopy, that an energy back-transfer from the blue emitter to the matrix materials occurs, which can be used to transport excitons to the other emitter molecules. Furthermore, we investigate the excitonic and electronic transfer processes by designing suitable emission layer stacks. As a result, we obtain an OLED with Commission Internationale de lEclairage (CIE) coordinates of (0.444;0.409), a CRI of 82, and a spectrum independent of the applied current. The OLED shows an external quantum efficiency of 10% and a luminous efficacy of 17.4 lm/W at 1000 cd/m2.

We report recent results on top-emitting organic light-emitting diodes (OLEDs) using color conversion layers (CCLs) embedded into the electron transport layer of the OLED structure. The method of color conversion provides the possibility to generate a color stable emission with operating lifetime. Due to a constant ratio between absorbed blue emission and converter emission, the spectral shape remains for all time. This guarantees constant color coordinates of the OLED, which is essential for lighting applications. It is shown that OLEDs using conversion layers reach external quantum efficiencies (EQE) which can be higher than the corresponding blue top-emitting OLED. The used conversion layer thickness is below 100 nm, reaching Commission Internationale de l’Eclairage (CIE) coordinates of (0.23; 0.27) close to the Planckian locus at a maximum EQE of 3.16% using a blue fluorescent emitter system. Furthermore, we show that the excitation mechanism of the conversion layer is caused by absorption and no para...

We report on the resistive switching effect in metal/organic semiconductor/metal structures and present a general explanation of the switching mechanism in these devices. The J-V characteristics of metal/tris(8- hydroxyquinolinato)aluminum (Alq3)/metal devices will be discussed and it will be further shown that these sustain only a limited number of switching cycles. Besides Alq3, we also investigate other organic semiconductor materials and obtain a bistable behavior, which is independent of the organic material but dependent on the current injection conditions of the interface between the organic material and the metal top electrode. Further, we investigate the material independent switching effect using impedance spectroscopy and can disclose a transition from capacitive to resistive behavior at switching voltages. We propose that the switching in metal/organic semiconductor/metal structures is caused by the growth and rupture of resistive filaments in the organic semiconductor.

We report on highly enhanced light outcoupling of bi-directional red phosphorescent organic light-emitting diodes (BiOLEDs) by strong micro-cavity effects. Inserting additional metal layers between the indium tin oxide (ITO) anode and the hole transporting layer (HTL) and controlling the thickness of the counter metal electrode result in dramatic changes in electroluminescence spectra and angular distribution. We find that the external quantum efficiency (EQE) for both bottom and top emitting side can be significantly improved: the overall EQE reaches up to 24.24%.

We report on improved and controlled light outcoupling of transparent organic light-emitting diodes (TOLEDs) by inserting thin silver layers between the indium tin oxide anode and the hole transporting layer. The introduction of Ag layers influences both the bottom and top emission of the TOLEDs, and it results in dramatic changes in the electroluminescence spectra and angular distribution. We find that the overall external quantum efficiency can be increased up to 18.8%, and the ratio of bottom and top emission can be almost identical.

For improving the lifetime of organic light emitting diodes (OLEDs), it is essential to understand the chemical reaction pathways involved in the degradation process of these devices. Using the laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF-MS) technique, an OLED based on the phosphorescent emitter tris(phenylpyridine)iridium (Ir(ppy)3) was investigated. We show that the chemical dissociation reaction of the Ir(ppy)3 molecule and an additional complexation of its fragments with the used hole blocking material 4,7-diphenyl-1,10-phenanthroline is the main intrinsic degradation mechanism. This finding underlines the exciton induced dissociation mechanism of phosphorescent emitters, even at emitters without any metal-oxygen-bonds in the ligand system.

We demonstrate highly efficient small molecule organic light emitting diodes and organic solar cells based on the p-i-n-type structure using the fluorinated fullerene molecule C60F36 as p-dopant in the hole transport layer. We present synthesis, chemical analysis, and energy level investigation of the dopant as well as the conductivity of organic layers consisting of a matrix of N,N,N′,N′-tetrakis 4-methoxyphenyl-benzidine(MeO-TPD) or N,N′-[(Diphenyl-N,N′-bis)9,9,-dimethyl-fluoren-2-yl]-benzidine(BF-DPB) doped by the fullerene compound. State of the art organic p-i-n devices containing C60F36 show efficiencies comparable to devices with the commonly used p-dopant2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). The advantages of the fullerene based dopant are the low volatility and high thermal stability, which is beneficial for device operation under elevated temperature. These properties make C60F36 highly attractive for the usage as p-dopant in a broad spectrum of organic p-i-n devices like organic light emitting diodes, solar cells, memories, or transistors. © 2011 American Institute of Physics.

We present investigations of top emitting organic light emitting devices (OLED) comprising n- and p-doped organic charge transport layers. It has been found previously that in comparison to noninverted p-i-n OLEDs, inverted n-i-p OLEDs show reduced device performances after fabrication. These differences can be eliminated by subsequent thermal annealing of the whole n-i-p OLED. After this process, the n-i-p OLED exhibits a superior low driving voltage of 2.9 V at 1000 cd/m2 and shows an increase in external quantum efficiency from 11% to almost 15% which we ascribe to a modified charge balance within the intrinsic organic emission layer.

We report on red top-emitting organic light-emitting diode structures with higher order cavities. The emission zone is placed in the first, second, and third antinodes of the electric field in the cavity by increasing the hole transport layer thickness. Furthermore, the thicknesses of the cathode and the capping layer are varied to achieve high efficiencies. Using doped charge transport layers and a phosphorescent emitter, we reach up to 29%, 17%, and 12% external quantum efficiencies for first, second, and third order devices, respectively. An optical model is further used to analyze the angular dependent emission.

The external quantum efficiency of organic light-emitting diodes (OLEDs) is limited by several loss mechanisms. By applying a numerical model for the efficiency analysis of OLED devices, we analyze the distribution of the different energy loss mechanisms in bottom and top emission organic light-emitting diodes. We validate the findings by the comparison with experimental data measured on red state-of-the-art p-i-n devices containing the red phosphorescent emitting dye iridium(III)bis[2-methyldibenzo-(f, h)quinoxaline](acetylacetonate) [Ir(MDQ)2(acac)]. The model is used to design extremely efficient bottom and top emission diodes with 21% and 27% external quantum efficiencies, respectively.

We compare the degradation of organic light emitting diodes (OLEDs) by UV light and by electrical driving. We prove that the exponential dependence of the half-lifetime on the current density known from electrical aging is also valid for UV-degradation. The influence of excitons on the degradation of OLEDs is determined and we experimentally distinguish between the influence of singlet and triplet excitons. We conclude that singlet excitons are the main cause of degradation for Spiro-DPVBi(2,2′,7,7′-tetrakis(2,2-diphenylvinyl)spiro-9,9′-bifluorene)-based OLEDs by a comparison of the degradation of electrically driven and UV-excited OLEDs.

Organic blue light-emitting devices are essential for the development of future light sources and display technology. Here, we present a highly efficient host-guest system suitable for blue light emission, consisting of the wide gap host material 3,6-di(9-carbazolyl)-9-(2-ethylhexyl)carbazole (TCz1) and the phosphorescent blue emitter iridium(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic). We investigate charge carrier balance as a function of hole blocking layer thickness. For optimized structures, devices with a quantum efficiency as high as 14.3% and a luminous efficacy of 21 lm/W at a luminance of 1000 cd/m2 are realized.

In this paper, we discuss in detail a novel concept for white organic light-emitting diodes (OLEDs) comprising three different phosphorescent emitters. Its key feature, a matrix-emitter combination for the blue sub-emission layer combined with a specific arrangement of the green- and red-emitting layers, leads to very high luminous efficacies. We present the key parameters of this emission layer design that are essential for high efficiencies. Finally, we briefly discuss a second, highly efficient approach for white OLEDs: the triplet harvesting concept. It makes use of a fluorescent blue emitter having benefits for higher device stability and at the same time the potential for 100% internal quantum efficiency.

We investigate the doping behavior of the strongly electron accepting molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane coevaporated with the host molecule N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine by photoemission spectroscopy and conductivity measurements. Using interface resolved measurements, we compare the alignment on different substrates and investigate the effects of varying doping concentrations on the Fermi level position. We find that at high doping concentrations the Fermi level gets pinned at the exponentially decaying tail of the highest occupied molecular orbital and compare these results with different dopants and host molecules. The measurement of the doping dependent space charge layer thickness yields information on the amount of free charge carriers and thereby the efficiency of the doping.

Metal organic fluorescent and phosphorescent emitters are widely used in organic light emitting devices (OLEDs). Iridium-based triplet emitters are known to undergo chemical reactions with other materials during OLED aging. The material tris(8-hydroxy-quinolinato)aluminum (Alq3), which is widely used as electron transporting material and green fluorescent emitter, degrades mainly during hole transport. We investigate the chemical changes in Alq3 during device aging: using laser desorption ionization time-of-flight mass spectrometry, we study the reaction products found in degraded OLEDs. Similar to the reactions known from the phosphorescent iridium-based emitters, Alq3 undergoes a dissociation and further reaction with the hole blocking material 4,7-diphenyl-1,10-phenanthroline.

For improving the lifetime of organic light emitting devices (OLEDs), the analysis of the chemical degradation requires a deep understanding of the involved reaction pathways. We show that the dissociation reactions of phosphorescent emitters and the additional complexations with the used surrounding blocking layers are the dominant intrinsic degradation mechanisms in long living p-i-n type OLEDs. We use the laser desorption/ionization (LDI) time-of-flight mass spectrometry to correlate the laser-induced ion formation with the observed lifetime of the organic devices. The superlinear correlation between the LDI forced reactions and the lifetimes allows the prediction of the lifetime of an OLED with new materials.

We show that chemical reactions are the major intrinsic degradation mechanisms in high-performance organic light emitting devices. The resulting reaction products create deep traps, non-radiative recombination centers, and luminescence quenchers in organic devices. The main reaction pathways, induced by excitons are fragmentations, oligomerizations, and complexations. We show that laser-desorbed mass spectroscopy can be used as a predictive method to compare different materials in terms of the expected lifetime for OLEDs.

Using bottom-emitting OLEDs grown on periodically corrugated substrates, we demonstrate efficient Bragg scattering of waveguide modes into the air cone. The angular position of scattered modes is in excellent agreement with theoretical predictions.

In this paper, we discuss novel approaches for organic light-emitting diode (OLED) lighting application. In particular, we discuss OLED deposited by vacuum deposition on flexible substrates such as plastic and metal foils. We discuss the specific challenges of OLED design for such substrates, in particular when top-emitting structures are required. Furthermore, we discuss a novel vacuum roll-to-roll tool which is capable of depositing oligomer OLED with high efficiency.