This PDF file contains the front matter associated with SPIE Proceedings Volume 9566, including the Title Page, Copyright information, Table of Contents, Authors, and Conference Committee listing.

The creation of molecular models and the finding and understanding of structure-property relationships are the most crucial steps when developing new materials. While many great findings and inventions in the history of science and technology strongly relied on a certain degree of randomness, it becomes vital at a certain stage of development to really understand why a certain material has beneficial properties in order to create better and better materials. In in the development of organometallic light-emitting materials, scientists often use structural models based on crystallography, e.g. data obtained by the investigation of single crystal samples. Based on these models, further analyses, and comparison to known substances or so-called "chemical intuition" then leads to the proposition of modified, next-generation materials, which may or may not be realized by chemical synthesis.

While this approach has been executed with great success in the past, problems arise in cases where the initial model is too simple, inaccurate or even false. In this article, we propose an alternate approach to prevent such problems: the use of X-ray absorption spectroscopy (XAS), a long-known technique, in material science. In several case studies, we highlight problematic examples from the past and show where and how XAS was and could be used to prevent erroneous models.

Charge transports in amorphous thin films with 100 nm thickness are investigated in silico by explicitly considering organic molecules. The amorphous layer of organic molecules was constructed using molecular dynamics simulations. The rate constants for charge hopping between two organic molecules, extracted from the amorphous layers, were calculated based on quantum chemical calculations. The hopping transport in amorphous layers was simulated using a Monte Carlo method. The hole mobility was calculated to be several times larger than the electron mobility, which was consistent with the experimental results. The Monte Carlo simulation also shows that diffusion transport is dominant at low applied electric fields and that contribution of drift transport increases at high electric fields. The simulation in this study enables us to reveal molecular origin of charge transport. In the presentation, we will show the results on recently-developed new thermally activated delayed fluorescence materials and the device performances.

Simplifying the configurations of organic light-emitting diodes (OLEDs) without sacrificing device performances is of high practical importance to shorten fabrication procedures and cut down cost. In view of this, organic active materials for OLEDs are anticipated to possess multiple functions, including high solid-state emission efficiency, efficient hole- and/ or electron transport ability, etc. To realize this purpose, we designed a series of bifunctional materials consisting of a silole core and electron-transporting functional groups, such as dimesitylboryl and diphenylphosphoryl groups. These silole derivatives show aggregation-enhanced emission (AEE) characteristics and afford high emission efficiencies in the solid films. The presence of these electron-withdrawing substituents lowers the LUMO energy levels as revealed by cyclic voltammetry, and allows for efficient electron transport ability of the luminogens. The double-layer OLEDs fabricated using these silole derivatives as light-emitting and electron-transporting layers simultaneously show good electroluminescence performances, which are almost equal to those of triple-layer OLEDs with an additional electrontransporting layer (TPBi), revealing that they are excellent n-type light emitters. These results demonstrate that the combination of AEE-active luminogens with charge transport groups at molecular level is a promising design for multifunctional solid-state light emitters.

We present multi-photon OLEDs where enhanced light emission was achieved by stacking two OLEDs utilizing a regular device architecture (top cathode) and an intermediate charge carrier generation layer (CGL) for monolithic device interconnection. With respect to future printing processes for organic optoelectronic devices, all functional layers were deposited from solution. The CGL comprises a low-work function zinc oxide layer that was applied from solution under ambient conditions and at moderate processing temperatures and a high-work function interlayer that was realized from various solution processable precursor-based metal oxides, like molybdenum-, vanadium- and tungsten-oxide. Since every injected electron-hole pair generates two photons, the luminance and the current efficiency of the tandem OLED at a given device current are doubled while the power efficiency remains constant. At a given luminance, the lower operating current in the tandem device reduces electrical stress and improves the device life-time. ToF-SIMS, TEM/FIB and EDX analyses provided evidence of a distinct layer sequence without intermixing upon solution deposition.

The authors have successfully fabricated flexible OLEDs by fully R2R process, which started from film washing, gasbarrier layer deposition, planarization, vacuum depositions of core-layers and finally encapsulated with film lamination. Influence of fabrication conditions on OLEDs performance has been studied and R2R fabricated OLEDs showed good performance almost comparable with sheet to sheet (S2S) processed OLEDs by controlling fabrication conditions. This indicates that reduction of device performance caused by the fabrication process has been minimized. Other important issues such as encapsulation, protection layers for moisture permeation and R2R related mechanical stress damage have been studied.

Thin film organic lasers (TFOLs) represent a new generation of inexpensive, mechanically flexible devices with demonstrated applicability in numerous applications in the fields of spectroscopy, optical communications and sensing requiring an organic, efficient, stable, wavelength-tunable and solution-processable laser material. A distributed feedback (DFB) laser is a particularly attractive TFOL because it shows single mode emission, low pump energy, easy integration with other devices, mechanical flexibility and potentially low production cost. Here, amplified spontaneous emission (ASE) and DFB laser applications of novel high performing perylene dyes and p-phenylenevinylene (PV) oligomers, both dispersed in thermoplastic polymers, used as passive matrixes, are reported. Second-order DFB lasers based on these materials show single mode emission, wavelength tunability across the visible spectrum, operational lifetimes of >10⁵ pump pulses, larger than previously reported PV oligomers or polymers, and thresholds close to pumping requirements with light-emitting diodes.

Organic microcavities comprising the host:guest emitter system Alq₃:DCM offer an interesting playground to experimentally study the dispersion characteristics of laterally patterned microlasers due to the broad emission spectrum and large oscillator strength of the organic dye. By structuring of metallic or dielectric sublayers directly on top of the bottom mirror, we precisely manipulate the mode structure and influence the coherent emission properties of the device. Embedding silver layers into a microcavity leads to an interaction of the optical cavity-state in the organic layer and the neighboring metal which red-shifts the cavity resonance, creating a Tamm-plasmon-polariton state. A patterning of the metal can in turn be exploited to fabricate deep photonic wells of micron-size, efficiently confining light in lateral direction. In periodic arrays of silver wires, we create a Kronig-Penney-like optical potential in the cavity and in turn observe optical Bloch states spanning over several photonic wires. We modify the Kronig-Penney theory to analytically describe the full far-field emission dispersion of our cavities and show the emergence of either zero- , π-, or 2π- phase-locking in the system. By investigating periodic SiO₂ patterns, we experimentally observe stimulated emission from the ground and different excited discrete states at room temperature and are able to directly control the laser emission from both extended and confined modes of the photonic wires at room-temperature.

Exciton annihilation processes impact both the lifetime and efficiency roll-off of organic light emitting diodes (OLEDs), however it is notoriously difficult to identify the dominant mode of annihilation in operating devices (exciton-exciton vs. exciton-charge carrier) and subsequently to disentangle its magnitude from competing roll-off processes such as charge imbalance. Here, we introduce a simple analytical method to directly identify and extract OLED annihilation rates from standard light-current-voltage (LIV) measurement data. The foundation of this approach lies in a frequency domain EQE analysis and is most easily understood in analogy to impedance spectroscopy, where in this case both the current (J) and electroluminescence intensity (L) are measured using a lock-in amplifier at different harmonics of the sinusoidal dither superimposed on the DC device bias. In the presence of annihilation, the relationship between recombination current and light output (proportional to exciton density) becomes nonlinear, thereby mixing the different EQE harmonics in a manner that depends uniquely on the type and magnitude of annihilation. We derive simple expressions to extract different annihilation rate coefficients and apply this technique to a variety of OLEDs. For example, in devices dominated by triplet-triplet annihilation, the annihilation rate coefficient, K_TT, is obtained directly from the linear slope that results from plotting EQE_DC-EQE_1ω versus L_DC (2EQE_1ω-EQE_DC). We go on to show that, in certain cases it is sufficient to calculate EQE_1ω directly from the slope of the DC light versus current curve [i.e. via (dL_DC)/(dJ_DC )], thus enabling this analysis to be conducted solely from common LIV measurement data.

The limited performance stability and gradual loss in the electroluminescence efficiency of OLEDs utilizing wide band-gap materials, such as blue-emitting phosphorescent and fluorescent devices, continues to be a challenge for wider technology adoption. We recently found that interactions between excitons and polarons play an important role in the aging behavior of electroluminescent materials, and that a correlation exists between the susceptibility of these materials to this aging mode and their band-gap. This degradation mode is also found to be often associated with the emergence of new bands - at longer wavelength - in the electroluminescence spectra of the materials, that can often be detected after prolonged electrical driving. Such bands contribute to the increased spectral broadening and color purity loss often observed in these devices over time. Exciton-polaron interactions, and the associated degradation, are also found to occur most significantly in the vicinity of device inter-layer interfaces such as at the interface between the emitter layer and the electron or hole transport layers. New results obtained from investigations of these phenomena in a wide range of commonly used host and guest OLED materials will be presented.

This talk will discuss recent experiments designed to study the formation of excitons and their subsequent diffusions in OLEDs. These experimental results suggest that contrary to conventional wisdom, host singlet exciton diffusion can occur over long distances, while host triplet excitons are confined close to the exciton formation region for the archetype host and hole transport layer CBP. The exciton formation mechanism is studied and we show that the ratio of excitons formed on the host to excitons formed on the dopant varies strongly with the applied voltage. Refinements to models of efficiency roll off are discussed in light of the improved understanding of exciton formation and we suggest design guidelines to improve efficiency by engineering exciton formation.

So far self-heating has only been of concern in large-area devices where the resistive transparent anode leads to a potential drop over the device resulting in inhomogeneous current, brightness and temperature distributions. In this work, we show that even small lab devices suffer from self-heating effects originating from the organic semiconductor layer. In admittance spectroscopy of organic semiconductor devices, negative capacitance values often arise at low frequency and high voltages. In this study we demonstrate the influence of self-heating on organic semiconductor devices with the aid of a numerical 1D drift-diffusion model that is extended by Joule heating and heat conduction. Furthermore the impact of trap states on the capacitance in combination with self-heating is demonstrated. The typical signature of self-heating might be overshadowed depending on the trapping dynamics. In a next step, we compare the negative capacitance vs. frequency for uni- and bipolar devices to quantify the different processes. We emphasize the impact of self-heating and trapping on OLEDs and organic solar cells. To ease the interpretation of the results we investigate simulations in the time domain as well as in the frequency domain. We have provided clear evidence of self-heating of organic semiconductor devices and conclude that a comprehensive model requires the inclusion of heat conduction and heat generation in the drift-diffusion model.

The performance of Organic Light Emitting Diodes (OLEDs) is determined by a complex interplay of the charge transport and excitonic processes in the active layer stack. We have developed a three-dimensional kinetic Monte Carlo (kMC) OLED simulation method which includes all these processes in an integral manner. The method employs a physically transparent mechanistic approach, and is based on measurable parameters. All processes can be followed with molecular-scale spatial resolution and with sub-nanosecond time resolution, for any layer structure and any mixture of materials. In the talk, applications to the efficiency roll-off, emission color and lifetime of white and monochrome phosphorescent OLEDs [1,2] are demonstrated, and a comparison with experimental results is given. The simulations show to which extent the triplet-polaron quenching (TPQ) and triplet-triplet-annihilation (TTA) contribute to the roll-off, and how the microscopic parameters describing these processes can be deduced properly from dedicated experiments. Degradation is treated as a result of the (accelerated) conversion of emitter molecules to non-emissive sites upon a triplet-polaron quenching (TPQ) process. The degradation rate, and hence the device lifetime, is shown to depend on the emitter concentration and on the precise type of TPQ process. Results for both single-doped and co-doped OLEDs are presented, revealing that the kMC simulations enable efficient simulation-assisted layer stack development. [1] H. van Eersel et al., Appl. Phys. Lett. 105, 143303 (2014). [2] R. Coehoorn et al., Adv. Funct. Mater. (2015), publ. online (DOI: 10.1002/adfm.201402532)

We review the progress in modeling of charge transport in disordered organic semiconductors on various length-scales, from atomistic to macroscopic. This includes evaluation of charge transfer rates from first principles, parametrization of coarse-grained lattice and off-lattice models, and solving the master and drift-diffusion equations. Special attention is paid to linking the length-scales and improving the efficiency of the methods. All techniques are illustrated on an amorphous organic semiconductor, DPBIC, a hole conductor and electron blocker used in state of the art organic light emitting diodes (OLEDs). The outlined multiscale scheme can be used to predict OLED properties without fitting parameters, starting from chemical structures of compounds. Reference: Advanced Functional Materials, 2015, doi: 10.1002/adfm.201403004

The OLED is one of the key devices for realizing future flexible displays and lightings. One of the biggest challenges left for the OLED fabricated on a flexible substrate is the improvement of its resistance to oxygen and moisture. A high barrier layer [a water vapor transmission rate (WVTR) of about 10-6 g/m2/day] is proposed to be necessary for the encapsulation of conventional OLEDs. Some flexible high barrier layers have recently been demonstrated; however, such high barrier layers require a complex process, which makes flexible OLEDs expensive. If an OLED is prepared without using air-sensitive materials such as alkali metals, no stringent encapsulation is necessary for such an OLED. In this presentation, we will discuss our continuing efforts to develop an inverted OLED (iOLED) prepared without using alkali metals. iOLEDs with a bottom cathode are considered to be effective for realizing air-stable OLEDs since the electron injection layer (EIL) can be prepared by fabrication processes that might damage the organic layers, resulting in the enhanced range of materials suitable for EILs. We have demonstrated that a highly efficient and relatively air-stable iOLED can be realized by employing poly(ethyleneimine) as an EIL. Dark spot formation was not observed after 250 days in the poly(ethyleneimine)-based iOLED encapsulated by a barrier film with a WVTR of 10-4 g/m2/day. In addition, we have demonstrated the fabrication of a highly operational stable iOLED utilizing a newly developed EIL. The iOLED exhibits an expected half-lifetime of over 10,000 h from an initial luminance of 1,000 cd/m2.

In this talk, we will discuss recent innovations in organic light-emitting materials and devices. First, we will report on organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF). We will show that devices based on the emitter 4CzIPN doped in a novel ambipolar host can yield a current efficacy of 81 cd/A and a maximum external quantum efficiency of 26.5%. These devices exhibit a low turn-on voltage of 3.2 V at 10 cd/m2, as well as reduced efficiency roll-off at high current densities. The performance of these devices is comparable to that of electrophosphorescent devices based on organic-metallic compounds that contain precious metals such as Iridium. In a second part we will report on highly efficient green-emitting organic light-emitting diodes (OLEDs) fabricated on shape memory polymer (SMP) substrates for flexible electronic applications. SMPs are a class of mechanically active materials that can change and store shape upon activation by a stimulus. The combination of the unique properties of SMP substrates with the light-emitting properties of OLEDs pave to the way for new applications, including conformable smart skin devices, minimally invasive biomedical devices, and flexible lighting/display technologies. Finally, we will present OLEDs fabricated on substrates made from cellulose nanocrystals (CNC) and discuss how such substrates can reduce the environmental footprint of printable organic electronics.

The efficiency of inverted polymer light-emitting diodes (iPLEDs) were remarkably enhanced by introducing spontaneously formed ripple-shaped nanostructure of ZnO (ZnO-R) and amine-based polar solvent treatment using 2-methoxyethanol and ethanolamine (2-ME+EA) co-solvents on ZnO-R. The ripple-shape nanostructure of ZnO layer fabricated by solution process with optimal rate of annealing temperature improves the extraction of wave guide modes inside the device structure, and 2-ME+EA interlayer enhances the electron injection and hole blocking and reduces exciton quenching between polar solvent treated ZnO-R and emissive layer. As a result, our optimized iPLEDs show the luminous efficiency (LE) of 61.6 cd A-1, power efficiency (PE) of 19.4 lm W-1 and external quantum efficiency (EQE) of 17.8 %. This method provides a promising method, and opens new possibilities for not only organic light-emitting diodes (OLEDs) but also other organic optoelectronic devices such as organic photovoltaics, organic thin film transistors, and electrically driven organic diode laser.

Organic light emitting transistors (LEFETs) are an emerging class of light emitting devices that have been successfully demonstrated in single-layer [1] and mutli-layer device structures [2]. LEFETs can simultaneously execute light-emission and standard logic functions (ON/OFF) of a transistor in a single device architecture [1]. This dual functionality of LEFETs has a potential to offer a new route to simplify fabrication of display pixels. However, the key problem of existing LEFETs thus far has been their low external quantum efficiency (EQE) at high brightness, poor ON/OFF ratio, and mobility. More recently, hybrid light emitting transistors [3-4], consisting of solution processed n-type metal oxide (inorganic) as the charge transport layer and light emitting conjugated polymer (organic), have been used to achieve higher mobility, ON/OFF ratio and brightness. In this talk, I will discuss the various factors that currently influence device performance in LEFETs, and will provide insights into our recent progress in developing high-performance hybrid LEFETs. References: (1). E. B. Namdas, J. S. Swensen, P. Ledochowitsch, J. D. Yuen, D. Moses, A. J. Heeger, Adv. Mater.,20, 1321 (2008). (2). M. Ullah, K. Tandy, S. D. Yambem, M. Aljada, P. L. Burn, P. Meredith, E. B. Namdas. Adv. Mater. 25, 6213–6218 (2013). (3). B. Walker, M. Ullah, G. J. Chae, P. L Burn, S. Cho, J. Y. Kim, E. B. Namdas, J. H. Seo. Appl Phys Lett, 105, 183302 (2014). (4). K. Muhieddine, M. Ullah, B. N. Pal, P. Burn, E. B. Namdas, Adv. Mater. 26, 6410 (2014).

Large area OLEDs show pronounced Joule self-heating at high brightness. This heating induces brightness inhomogeneities, drastically increasing beyond a certain current level. We discuss this behavior considering 'S'-shaped negative differential resistance upon self-heating, even allowing for 'switched-back' regions where the luminance finally decreases (Fischer et al., Adv. Funct. Mater. 2014, 24, 3367). By using a multi-physics simulation the device characteristics can be modeled, resulting in a comprehensive understanding of the problem. Here, we present results for an OLED lighting panel considered for commercial application. It turns out that the strong electrothermal feedback in OLEDs prevents high luminance combined with a high degree of homogeneity unless new optimization strategies are considered.

We demonstrate a method for extracting waveguided light trapped in the organic and indium tin oxide layers of bottom emission organic light emitting devices (OLEDs) using a patterned planar grid layer (sub-anode grid) between the anode and the substrate. The scattering layer consists of two transparent materials with different refractive indices on a period sufficiently large to avoid diffraction and other unwanted wavelength-dependent effects. The position of the sub-anode grid outside of the OLED active region allows complete freedom in varying its dimensions and materials from which it is made without impacting the electrical characteristics of the device itself. Full wave electromagnetic simulation is used to study the efficiency dependence on refractive indices and geometric parameters of the grid. We show the fabrication process and characterization of OLEDs with two different grids: a buried sub-anode grid consisting of two dielectric materials, and an air sub-anode grid consisting of a dielectric material and gridline voids. Using a sub-anode grid, substrate plus air modes quantum efficiency of an OLED is enhanced from (33±2)% to (40±2)%, resulting in an increase in external quantum efficiency from (14±1)% to (18±1)%, with identical electrical characteristics to that of a conventional device. By varying the thickness of the electron transport layer (ETL) of sub-anode grid OLEDs, we find that all power launched into the waveguide modes is scattered into substrate. We also demonstrate a sub-anode grid combined with a thick ETL significantly reduces surface plasmon polaritons, and results in an increase in substrate plus air modes by a >50% compared with a conventional OLED. The wavelength, viewing angle and molecular orientational independence provided by this approach make this an attractive and general solution to the problem of extracting waveguided light and reducing plasmon losses in OLEDs.

Light extraction from organic light emitting diodes (OLEDs) is attracting considerable interest as being crucial for enhancing the energy efficiency in lighting applications. Light extraction can be realized by lithographically defined internal diffraction gratings or stochastic scattering centers. The former approach needs in addition an external optical layer for scrambling the angularly dependent emission spectra in order to avoid color shifts [1]. Micro lens arrays cannot only be used for fulfilling this task but they can also be used for enhancing the luminosity into a specific direction. We demonstrate recent advances towards high efficiency OLEDs with high directionality. In addition to the relevant technologies we have also developed a comprehensive simulation software for the quantitative description of the light propagation inside the devices. Here, a particular challenging task is the description of multiple and coherent optical scattering. We have recently developed a software for the exact simulation based on a scattering matrix formalism [2]. [1] T. Bocksrocker, J. B. Preinfalk, J. Asche-Tauscher, A. Pargner, C. Eschenbaum, F. Maier-Flaig and U. Lemmer, White organic light emitting diodes with enhanced internal and external outcoupling for ultra-efficient light extraction and Lambertian emission Opt. Expr. 20, A932 (2012). [2] A. Egel, U. Lemmer, Dipole emission in stratified media with multiple spherical scatterers: Enhanced outcoupling from OLEDs, Journal of Quantitative Spectroscopy and Radiative Transfer 148, 165 (2014).

We used a nanoporous polymer film prepared by cellulose acetate butyrate with ~40% of optical haze value as a diffuser. It was fabricated by a simple spin-coating process during continuous water mist supply by a humidifier. The pores were created by the elastic bouncing mechanism (rather than the thermocapillary convection mechanism) of the supplied water droplets. The shapes and sizes of the caves formed near the polymer surface are randomly distributed, with a relatively narrow pore size distribution (300–500 nm). Specifically, we focused on controlling the surface morphology to give a three-dimensional (3D) multi-stacked nanocave structure because we had already learnt that two-dimensional nanoporous structures showed serious loss of luminance in the forward direction. Using this approach, we found that the 3D nanoporous polymer film can effectively reduce the viewing angle dependency of strong microcavity OLEDs without any considerable decrease in the total intensity of the out-coupled light. We applied this nanoporous polymer film to microcavity OLEDs to investigate the possibility of using it as a diffuser layer. The resulting nanoporous polymer film effectively reduced the viewing angle dependency of the microcavity OLEDs, although a pixel blurring phenomenon occurred. Despite its negative effects, such as the drop in efficiency in the forward direction and the pixel blurring, the introduction of a nanoporous polymer film as a scattering medium on the back side of the glass substrate eliminated the viewing angle dependency. Thus, this approach is a promising method to overcome the serious drawbacks of microcavity OLEDs.

Small-molecule OLEDs, deposited by thermal evaporation, allow for precise control over layer thicknesses. This enables optimisation of the optical behaviour of the stack which ultimately determines the outcoupling efficiency. In terms of optical outcoupling there are limits to the efficiency by which the generated electromagnetic radiation can be extracted from the stack. These limitations are linked to the refractive indices of the individual layers. Values for maximum outcoupling efficiency are sometimes calculated under the implicit assumptions that the OLED stack is planar, that all layers are isotropic with a certain refractive index and that the emitters are not preferentially oriented. In reality it is known that these assumptions are not always valid, be it intentional or unintentional. In our work we transcend these limiting assumptions and look at different forms of anisotropy in OLEDs. Anisotropy in OLEDs comes in three distinct flavours; 1. Geometrical anisotropy, as for example in gratings, lenses or other internal or external scattering centres, 2. Anisotropic emitters, where the orientation significantly influences the direction in which radiation is emitted and 3. Anisotropic optical materials, where their anisotropic nature breaks the customary assumption of isotropic OLED materials. We investigate the effect of these anisotropic features on the outcoupling efficiency and ultimately, on the external quantum efficiency (EQE).

Before the market entry of organic light emitting diodes (OLEDs) into the field of general illumination can occur, limitations in lifetime, luminous efficacy and cost must be overcome. Additional requirements for OLEDs used for general illumination may be imposed by workplace glare reduction requirements, which demand limited luminance for high viewing angles. These requirements contrast with the typical lambertian emission characteristics of OLEDs, which result in the same luminance levels for all emission angles. As a consequence, without additional measures glare reduction could limit the maximum possible luminance of lambertian OLEDs to relatively low levels. However, high luminance levels are still desirable in order to obtain high light output. We are presenting solutions to overcome this dilemma.

Therefore this work is focused on light-shaping structures for OLEDs with an internal light extraction layer. Simulations of beam-shaping structures and shapes are presented, followed by experimental measurements to verify the simulations of the most promising structures. An investigation of the loss channels has been carried out and the overall optical system efficiency was evaluated for all structures. The most promising light shaping structures achieve system efficiencies up to 80%.

Finally, a general illumination application scenario has been simulated. The number of OLEDs needed to illuminate an office room has been deduced from this scenario. By using light-shaping structures for OLEDs, the number of OLEDs needed to reach the mandatory illuminance level for a workplace environment can be reduced to one third compared to lambertian OLEDs.

The elaboration of organic light-emitting diodes (OLEDs) via a solution deposition process turns out to be a cheaper alternative to the vacuum evaporation technique. However the most popular spin-coating wet deposition process mainly used in the semiconductor industry is not applicable for large mother glass substrates used in display applications. The inkjet technology addresses this drawback and appears to be a good solution to produce on a large scale wet deposited OLEDs¹. This process has been commonly used for polymer deposition and only a few examples^2–4 have demonstrated the possibility of depositing small molecules in functional devices. Deposition of small molecules from inkjet printing is supposed to be easier than polymers because monomers do not show polydispersity and consequently the viscosity of the solution containing the monomers, the ink, is easily controllable in production. This work aims at fabricating OLEDs composed of inkjet-printed hole-transporting molecules and a new class of fluorescent molecules that have been further UV-photopolymerized right after deposition.

Phosphorescent OLEDs are now being used in first commercial products, mainly in displays. Typically, such devices operate at low-to-moderate brightnes s (<500 cd m^-2), while it would be beneficial for actual lighting applications to also reach a very high luminance. However, a phenomenon called efficiency roll-off contradicts this aim. The reducing of the device efficiency with rising triplet exciton concentration due to triplet-triplet annihilation (TTA) is the most relevant factor causing roll-off for such compounds. Photophysically, this is reflected by strong concentration quenching in concentrated samples of phosphorescent materials. We present a potential solution for this issue. In this article we identify a copper(I) emitter showing thermally-activated delayed fluorescence (TADF) that seems to be much more immune to concentration quenching than conventional phosphorescent materials, even though triplet states are also populated in a similar manner.

The efficiency of organic light-emitting devices (OLEDs) is shown to significantly depend on both the thickness and roughness of the indium tin oxide (ITO) anode. The effects of changing the ITO thickness from 45 nm to 130 nm are found to be able to vary the current efficiency by 40%. The underlying mechanism is studied and revealed to be related to microcavity effects. The transmittance of the ITO substrate changes significantly with the ITO thickness, resulting in variations in microcavity, and thus light outcoupling efficiency. On the other hand, the effects of increasing the ITO roughness (rms) from 3.3 nm to 8.5 nm are found to increase light scattering at the ITO/organic interface, thus improving extraction of light trapped in the organic/ITO wave-guided mode. In addition to the enhancement in current efficiency, the device fabricated on rough ITO shows similar driving voltage to that made on smooth ITO, indicating that charge balance is not altered by ITO roughness. Contrary to common belief in the community, the lifetime of the OLED is not affected when using rough ITO. The results demonstrate the significant efficiency benefits of using ITO with optimal thicknesses and higher roughness in OLEDs.

Inverted top-emitting organic light-emitting diodes (ITOLEDs) with aluminum as cathode and semitransparent silver as anode are investigated. Comparing the blue, green and red ITOLEDs with conventional BEOLEDs based on iridium complex, it is surprising that the red ITOLED exhibits a higher efficiency nearly twice as that of the bottom-emitting counterpart, while blue and green ITOLEDs are comparable to BEOLEDs. We explain that the role of the strong microcavity effect improved the spontaneous emission of emitters in all ITOLEDs, however, only the intrinsic quantum yield of the red emitters is improved due to its comparable radiative and nonradiative decay rates.

An optical thin film was provided to address light illuminance efficiency of OLED up to 80%. A polymer material was used as a film base material which could avoid the influence of total reflection angle. One kinds of oxidized metal micro-particles was chosen to dope inside the optical thin film and to increase scattering and refractive effect.

We demonstrate the working of organic-inorganic hybrid light emitting diodes using cadmium sulphide nanoparticles doped in light emitting polymers. Cadmium sulphide nanoparticles were prepared by precipitation technique and doped in active light emitting polymers. We used blue emitting polymer PFO and green emitting polymer F8BT as active polymers. The j-L-V curves of nanoparticles based devices were compared with polymer only standard devices. Our results show enhanced performance such as increased conductivity and higher luminance, when nanoparticle are incorporated into the device.

It is well-known that hole transport layers (HTLs) in organic light emitting devices (OLEDs) are more sensitive to morphological changes than other organic layers due to the lower glass transition temperatures. A high operational temperature can alter the HTL morphology, severely impacting OLED performance and stability. Although joule heating is a known factor affecting OLED stability during operation, its effect in experimental studies is typically simulated through thermal annealing of the devices rather than applying current. In this work, a comparison of the effects of joule heating vs thermal annealing on the morphological stability of N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (NPB) and N,N′-Dicarbazolyl-4,4′-biphenyl (CBP) HTLs and the impact this has on OLED performance is investigated. While thermal annealing of an OLED can be used as an approximation of joule heating, the temperature distribution profile of the OLED is different under the two stress conditions and thus can impact the morphology of the HTL differently. However, joule heating introduces a confounding factor whereby the OLEDs experience intrinsic degradation by the flow of current, aside from thermal stress. Therefore, in this work, joule heating is studied in unipolar devices that comprise solely of the HTL. Device JVL and morphology as a function of temperature for both joule heating and thermal annealing are presented as a means to evaluate stability and performance.

The exhibition of thermally activated delayed fluorescence on triazine derivative by the introduction of a nonbonding part is demonstrated. Two molecules containing triazine core as acceptor and carbazole part as donor has been synthesized and characterized. One of these molecules bears an additional nonbonding part by the means of a phenoxy group. The results indicated that the molecule bearing the nonbonding molecular part (phenoxy) exhibit thermally activated delayed fluorescence while not on molecule free of non-bonding group. The results are supported by, photoluminescence, spectral analysis time-resolved fluorescence and time-dependent density functional estimation

We systematically investigate doping effect of cesium fluoride (CsF) on the device performance of organic light-emitting diodes (OLEDs). CsF can be used as a stable n-type dopant due to its low chemical reactivity and simple deposition process. We have observed that CsF could be employed as an effective n-type dopant in thin films of 3,3'-[5'- [3-(3-Pyridinyl)phenyl][1,1':3',1''-terphenyl]-3,3''-diyl]bispyridine (TmPyPB) through experimental studies of optical absorption spectroscopy, and X-ray photoelectron spectroscopy (XPS) with different doping concentration. In addition, we measured bulk resistance using impedance spectroscopy in an electron-only devices (EODs) with CsF-doped TmPyPB. As the doping ratio of the CsF increases, the current densities of EOD increase and the bulk resistances of the CsF-doped layer decrease. Owing to high electrical property of CsF-doped TmPyPB in EIL, green phosphorescent OLEDs showed significantly lower voltage and considerably enhanced efficiency. The device with 30 vol% CsF-doped TmPyPB showed power efficiency of 28.1 lm/W at 1000 cd/m2, whereas the device with pristine TmPyPB exhibited 13.8 lm/W. From these results, CsF-doped TmPyPB as EIL can reduce bulk resistance of EIL and improve the electron-injection and transport properties of electron-transport layer. Therefore, we can utilize CsF as an efficient n-type dopant in EIL of OLEDs.

A statistical distribution function capable of numerically characterizing the unique intensity distribution of a planar white organic light-emitting diodes (WOLED) was theoretically investigated by fitting our experimental data obtained by microscopic goniometer (MG) system associated with an energy analyzer to the normal distribution function with amplitude, average, and standard deviation as adjustable parameters. The WOLED is one of the upcoming lighting sources with planar device structure without additional optical components. The intensity characteristic of a lighting source is crucial for practical purpose. The procedure of an optical design usually requires proper numerical tools to satisfy specific application by adjusting parameters. Relatively uniform intensity distribution of a planar lighting source is needed for a specific lighting application such as back-lighting (BL) for liquid-crystal displays (LCD) in which Regular white LED's (WLED) and light-guide plate are assembled as a planar module. Our intensity measurement of a WOLED revealed a unique pattern in which the relative intensities near central area are higher than that near the edge of the emissive area. This unique intensity profile is similar to the feature of Gaussian distribution function. Our preliminary result of applying Gaussian distribution function to numerically characterize the intensity profile of a WOLED suggests that the unique intensity profile can be represented by single distribution function properly.

In this paper, two OLED device concepts are introduced. First, classical phosphorescent green carbene emitters with unsurpassed lifetime, combined with low voltage and high efficiency are presented and the associated optimized OLED stacks are explained. Second, a path towards highly efficient, long-lived deep blue systems is shown. The high efficiencies can be reached by having the charge-recombination on the phosphorescent carbene emitter while at the same time short emissive lifetimes are realized by fast energy transfer to the fluorescent emitter, which eventually allows for higher OLED stability in the deep blue.

Device architectures, materials and performance data are presented showing that carbene type emitters have the potential to outperform established phosphorescent green emitters both in terms of lifetime and efficiency. The specific class of green emitters under investigation shows distinctly larger electron affinities (2.1 to 2.5 eV) and ionization potentials (5.6 to 5.8 eV) as compared to the "standard" emitter Ir(ppy)₃ (5.0/1.6 eV). This difference in energy levels requires an adopted OLED design, in particular with respect to emitter hosts and blocking layers. Consequently, in the diode setup presented here, the emitter species is electron transporting or electron trapping.

For said green carbene emitters, the typical peak wavelength is 525 nm yielding CIE color coordinates of (x = 0.33, y = 0.62). Device data of green OLEDs are shown with EQEs of 26 %. Driving voltage at 1000 cd/m² is below 3 V. In an optimized stack, a device lifetime of LT₉₅ > 15,000 h (1000 cd/m²) has been reached, thus fulfilling AMOLED display requirements.