Emitter Orientation Research Articles

Orientation distributions of vacuum-deposited organic emitters revealed by single-molecule microscopy

The orientation of luminescent molecules in organic light-emitting diodes strongly influences device performance. However, our understanding of the factors controlling emitter orientation is limited as current measurements only provide ensemble-averaged orientation values. Here, we use single-molecule imaging to measure the transition dipole orientation of individual emitter molecules in a state-of-the-art thermally evaporated host and thereby obtain complete orientation distributions of the hyperfluorescence-terminal emitter C545T. We achieve this by realizing ultra-low doping concentrations (10−6 wt%) of C545T and minimising background levels to reliably measure its photoluminescence. This approach yields the orientation distributions of >1000 individual emitter molecules in a system relevant to vacuum-processed devices. Analysis of solution- and vacuum-processed systems reveals that the orientation distributions strongly depend on the nanoscale environment of the emitter. This work opens the door to attaining unprecedented information on the factors that determine emitter orientation in current and future material systems for organic light-emitting devices.

Mesogenic Groups Control the Emitter Orientation in Multi‐Resonance TADF Emitter Films**

AbstractThe use of thermally activated delayed fluorescence (TADF) emitters and emitters that show preferential horizontal orientation of their transition dipole moment (TDM) are two emerging strategies to enhance the efficiency of OLEDs. We present the first example of a liquid crystalline multi‐resonance TADF (MR‐TADF) emitter, DiKTa‐LC. The compound possesses a nematic liquid crystalline phase between 80 °C and 110 °C. Importantly, the TDM of the spin‐coated film shows preferential horizontal orientation, with an anisotropy factor, a, of 0.28, which is preserved in doped poly(vinylcarbazole) films. Green‐emitting (λEL=492 nm) solution‐processed OLEDs based on DiKTa‐LC showed an EQEmax of 13.6 %. We thus demonstrate for the first time how self‐assembly of a liquid crystalline TADF emitter can lead to the so‐far elusive control of the orientation of the transition dipole in solution‐processed films, which will be of relevance for high‐performance solution‐processed OLEDs.

Mesogenic Groups Control the Emitter Orientation in Multi-Resonance TADF Emitter Films.

The use of thermally activated delayed fluorescence (TADF) emitters and emitters that show preferential horizontal orientation of their transition dipole moment (TDM) are two emerging strategies to enhance the efficiency of OLEDs. We present the first example of a liquid crystalline multi-resonance TADF (MR-TADF) emitter, DiKTa-LC. The compound possesses a nematic liquid crystalline phase between 80 °C and 110 °C. Importantly, the TDM of the spin-coated film shows preferential horizontal orientation, with an anisotropy factor, a, of 0.28, which is preserved in doped poly(vinylcarbazole) films. Green-emitting (λEL =492 nm) solution-processed OLEDs based on DiKTa-LC showed an EQEmax of 13.6 %. We thus demonstrate for the first time how self-assembly of a liquid crystalline TADF emitter can lead to the so-far elusive control of the orientation of the transition dipole in solution-processed films, which will be of relevance for high-performance solution-processed OLEDs.

Tailoring the directional dependent emitter interaction based plasmonic antenna character of Ag–Au heterodimer

Based on the concept of optical nanoantennas, light-matter interaction between metal heterodimer (Ag and Au spherical nanoparticles) and single dipole emitter system are numerically simulated utilizing a MATLAB toolbox MNPBEM17 based on boundary element method (BEM). The precise position and orientation of the dipole emitter systematically controls not only the (non)radiative losses, but also the efficiency of excitation (absorption) and respective emission processes in the vicinity of the plasmonic optical antenna system. The overall (non)radiative decay rates and associated surface charge distributions help estimate the spatio-directional dependency of dipole emitter emission, analogous to that of the SERS enhancement. Furthermore, the Poynting vector analysis of our present hybrid plasmonic antenna system effectively assesses the flow of electromagnetic energy, thus accounting for improvement in both near- and far-field propagation modes. The calculation of Purcell factor and coupling strength along with quantum yield evaluate the detailed optical response of a nanoantenna characteristics simultaneously based on the dipole emitter position and orientation, offering better scope for understanding the underlying mechanisms governing such intriguing interactions in complex geometries as well.

From Molecule to Device: Prediction and Validation of the Optical Orientation of Iridium Phosphors in Organic Light-Emitting Diodes

Due to their thin amourphous structure, unique electrical properties, and the associated variety of possible applications, OLEDs can now be found in smartphones, TVs, laptops, and wearables. While already big steps have been made in optimizing and understanding the properties influencing the external quantum efficiency (EQE), there is still room for improvement, especially when it comes to finding design principles for new emitter complexes. One contributer to the EQE here is the molecular orientation of the emitter in a given host matrix. In this work we study the viability of using molecular modeling approaches in sampling these emitter orientations for a set of already published homoleptic Ir carbene emitters and a set of emitter materials synthesized at Merck KGaA, Darmstadt, Germany, comprising both homoleptic and heteroleptic Ir(ppy)3 derivatives. We combine these simulations with different measurements for the orientation parameter and EQE, all performed with the same material stack under the same conditions. We observe a good agreement between simulation and experiment and find that the horizontal orientation of emitter molecules seems to be the main factor contributing to a higher EQE.

Deep ultraviolet spontaneous emission enhanced by layer dependent black phosphorus plasmonics.

Although graphene has been the primary material of interest recently for spontaneous emission engineering through the Purcell effect, it features isotropic and thickness-independent optical properties. In contrast, the optical properties of black Phosphorus (BP) are in-plane anisotropic; which supports plasmonic modes and are thickness-dependent, offering an additional degree of freedom for control. Here we investigate how the anisotropy and thickness of BP affect spontaneous emission from a Hydrogenic emitter. We find that the spontaneous emission enhancement rate i.e. Purcell factor (PF) depends on emitter orientation, and PF at a particular frequency and distance can be controlled by BP thickness. At lower frequencies, PF increases with increasing thickness due to infrared (IR) plasmons, which then enhances visible and UV far-field spectra, even at energies greater than 10 eV. By leveraging the thickness and distance-dependent PF, deep UV emission can be switched between 103 nm or 122 nm wavelength from a Hydrogenic emitter. Additionally, we find that doping can significantly tune the PF near BP and this alteration depends on the thickness of the BP. Our work shows that BP is a promising platform for studying strong plasmon-induced light-matter interactions tunable by varying doping levels, emitter orientation, and thickness.

Long-Term Stability of Emitter Orientation in Organic Light-Emitting Diodes at Temperatures in the Range of the Active Layer Glass Transition

Long-Term Stability of Emitter Orientation in Organic Light-Emitting Diodes at Temperatures in the Range of the Active Layer Glass Transition

Dripper clogging by soil particles entering lateral lines directly during irrigation network assembly in the field

Emitter clogging is a problem that occurs with high frequency in drip irrigation systems, and it is usually caused by physical, chemical or biological factors, isolated or associated. There are situations in which clogging can occur from contaminants entering the irrigation system during assembly or repair of broken pipes in the field. Studies on the processes of emitter clogging under these conditions by pulses of soil particle directly in the lateral lines have not been reported in the literature yet, although they occur with relative frequency in the field. Thus, the objective of this research was to evaluate the clogging of emitters by pulses of soil particles that entered the pipes downstream of the filtering system as a function of the discharge from the emitters, the particle size, concentration of the contaminating particles, and orientation of installation of the emitters in the lateral lines. The experiment was carried out on a test bench specially developed for this study. Two non-pressure compensating (NPC) driplines with integrated flat emitters were evaluated. The tested emitters had nominal discharges of 1.6 L·h−1 (emitter A) and 2.3 L·h−1 (emitter B). For each emitter model, 30 treatments were performed, consisting of combinations of three factors: emitter orientation, soil particle size and amount of soil per pulse (concentration). In addition, three more blank treatments were tested. The tested emitters showed different responses when exposed to pulses of soil particles of different sizes and amounts. Emitter A had a mean value of relative discharge (Dra) of 73% in all treatments while the Dra of emitter B was 91%, indicating that emitter A was more sensitive to clogging than emitter B. The particle size of the soil was a dominant factor in the physical clogging of these emitters. Treatments involving particles with diameters ranging from 0.26 to 0.50 mm resulted in mean Dra values of 67% for emitter A and 87% for B. The position of the emitter was a factor that contributed to the increase in susceptibility to clogging by pulses of soil particle, and emitter position facing to the side presented the best performance; it should be noted that this dripper orientation has not been used in subsurface drip irrigated areas to date, because traditional burial implements are not able to install this emitter orientation in the field. Treatments that involved the sideways position resulted in mean Dra values of 75% for emitter A and 94% for B.

Simulating the growth of amorphous organic thin films

Physical vapor deposition is used widely to produce organic electronics devices, in particular for OLEDs in smartphones, TVs and other display applications. Despite its long history, recent years have seen a number of surprising observations, such as deposition-induced emitter orientation and built-in electrostatic potentials in thin organic films. Modeling the details of these effects is complicated by the long time scales involved in the underlying processes. In this paper, we compare two different modeling approaches, which both aim to simulate the physical vapor deposition (PVD) process for small organic molecules. We compare a molecular dynamics approach, based on a classical bead-spring force field and time integration, with a Metropolis Monte Carlo approach, where the intramolecular degrees are limited to the torsional rotations. To analyze the resulting structures, we calculate the density and radial distribution functions (RDF) of all films. We observe a good agreement for the RDFs, but an approximately 10% higher density for the films generated by the molecular dynamics approach. Additionally, we investigate the anisotropic nature of such morphologies by calculating the ordinary and extra-ordinary refractive index for each material. Finally, we calculate electron and hole mobilities with an Kinetic Monte Carlo protocol.

End-fire all-anisotropic transition metal dichalcogenide nanoantennas

We demonstrate that all-anisotropic transition metal dichalcogenide (TMDC) nanostructures can support advanced optical nanoantenna functionalities, such as high end-fire directivity and directionality inversion. The designed chain structures, driven by a dipole emitter, are composed of negative uniaxial anisotropic tungsten disulfide $({\mathrm{WS}}_{2})$ TMDC nanoparticles featuring ordinary/extraordinary refractive indices. The ordinary index, whose values are much higher than those of Si (Si is typically used in all-dielectric nanostructures), is also much higher than the extraordinary index, thus allowing for a strong material birefringence. We find that anisotropy of this type makes it possible to achieve high end-fire directivity, and also offers an extra degree of freedom that allows for an orientation-based coupling between a free-to-rotate dipole emitter and fixed ${\mathrm{WS}}_{2}$ elements. Furthermore, appropriately rotating half the array elements achieves end-fire directional inversion (at a fixed operating wavelength) simply by adjusting the emitter orientation. This unique feature is characteristic of the high anisotropy exhibited by ${\mathrm{WS}}_{2}$ in the optical-near infrared regime, and cannot be readily realized with isotropic all-dielectric or plasmonic spherical structures due to their material symmetry. The presented material-anisotropy-based designs comprise a nanophotonics platform that allows for unique functionalities at the nanoscale, as well as for the implementation of tunable nanophotonic devices.

Role of emitter position and orientation on silicon nanoparticle-enhanced fluorescence

High-index spherical dielectric nanoparticles are explored as templates for tailoring the fluorescence of nearby electric point-dipole-like emitters. The role of emitter orientation and position around the nanosphere on the modification of both its excitation and its emission rate is studied rigorously through derivation of appropriate analytic solutions. It is shown that dielectric nanoparticles, which support a richness of optical modes of electric or magnetic character and thus a variety of mechanisms for near-field enhancement, provide moderate fluorescence enhancement factors—as compared, e.g., to their plasmonic counterparts—which survive, however, for a wide range of emitter positions and orientations. Understanding the physical mechanisms governing this behaviour, which offers a promising route towards efficient control of the emission properties of randomly arranged emitters, is widely facilitated by the existence of analytic expressions such as the ones derived here. Further enhancement can be achieved by designing nanoparticle-aggregate antennas, or by combining dielectric and plasmonic components.

Necessity of submonolayer LiF anode interlayers for improved device performance in blue phosphorescent OLEDs

Lithium fluoride (LiF) is a widely used interlayer in organic optoelectronic devices, at both top and bottom electrodes, with various mechanisms proposed for its effectiveness at each interface. Here we examine the influence of LiF at the ITO electrode as a function of surface coverage. Both thermally evaporated LiF and LiF nanoparticles deposited from solution using di-block copolymer reverse micelles were used to probe the effect of island coverage on device characteristics. From hole-only devices (HOD) with a deep highest occupied molecular orbital (HOMO) level, capacitance–voltage and current–voltage characteristics show that LiF is effective only in the submonolayer range. Injection of holes is maximized at $$\sim 12\%$$ coverage, and decreases with increasing coverage of LiF. Above a critical surface coverage ( $$\sim 50\%$$ ), the barrier to injection becomes greater than that of bare ITO surface, but is mediated by dipole-induced interfacial trap states. Eventually, the barrier becomes so high that charge carriers cannot be effectively injected and the device does not operate. Using this insight, we fabricated blue phosphorescent organic light-emitting diode (PHOLED) using the optimal coverage of LiF nanoparticles, and observed maximum luminous and quantum efficiency that were improved by 30%. The effect of an array of LiF nanoparticles on emitter orientation was observed by angular-dependent photoluminescence spectroscopy. With this deeper understanding of how LiF operates at the ITO surface, it will be possible to further tune electrode interfaces to accommodate organic transport layers with deeper HOMO levels and larger bandgaps for next-generation devices.

What Controls the Orientation of TADF Emitters?

Thermally-activated delayed fluorescence (TADF) emitters—just like phosphorescent ones—can in principle allow for 100% internal quantum efficiency of organic light-emitting diodes (OLEDs), because the initially formed electron-hole pairs in the non-emissive triplet state can be efficiently converted into emissive singlets by reverse intersystem crossing. However, as compared to phosphorescent emitter complexes with their bulky—often close to spherical—molecular structures, TADF emitters offer the advantage to align them such that their optical transition dipole moments (TDMs) lie preferentially in the film plane. In this report, we address the question which factors control the orientation of TADF emitters. Specifically, we discuss how guest-host interactions may be used to influence this parameter and propose an interplay of different factors being responsible. We infer that emitter orientation is mainly governed by the molecular shape of the TADF molecule itself and by the physical properties of the host—foremost, its glass transition temperature Tg and its tendency for alignment being expressed, e.g., as birefringence or the formation of a giant surface potential of the host. Electrostatic dipole-dipole interactions between host and emitter are not found to play an important role.

Effect of emitter orientation on the outcoupling efficiency of perovskite light-emitting diodes.

Metal halide perovskite light-emitting diodes (PeLEDs) have experienced a rapid advancement in the last several years with the external quantum efficiencies (EQEs) reaching over 20%, comparable to the state-of-the-art organic LEDs and quantum dot LEDs. The photoluminescence quantum yields of perovskite films have also been approaching 100%. Therefore, the next step to improving the EQE of PeLEDs should be focused on boosting light extraction. In this Letter, we demonstrate the emitter dipole orientation as a key parameter in determining the outcoupling efficiency of PeLEDs. We find that the CsPbBr3 emitter has a slightly preferred orientation with the horizontal-to-vertical dipole ratio of 0.41:0.59, as compared to 0.33:0.67 in the isotropic case. A theoretical analysis predicts that a purely anisotropic perovskite emitter may result in a maximum EQE of 36%.

Getting the Right Twist: Influence of Donor–Acceptor Dihedral Angle on Exciton Kinetics and Singlet–Triplet Gap in Deep Blue Thermally Activated Delayed Fluorescence Emitter

Here, a novel deep blue emitter SBABz4 for use in organic light-emitting diodes (OLED) is investigated. The molecular design of the emitter enables thermally activated delayed fluorescence (TADF), which we examine by temperature-dependent time-resolved electroluminescence (trEL) and photoluminescence (trPL). We show that the dihedral angle between donor and acceptor strongly affects the oscillator strength of the charge transfer state alongside the singlet-triplet gap. The angular dependence of the singlet-triplet gap is calculated by time-dependent density functional theory (TD-DFT). A gap of 15 meV is calculated for the relaxed ground state configuration of SBABz4 with a dihedral angle between the donor and acceptor moieties of 86{\deg}. Surprisingly, an experimentally obtained energy gap of 72+/-5 meV can only be explained by torsion angles in the range of 70{\deg}-75{\deg}. Molecular dynamics (MD) simulations showed that SBABz4 evaporated at high temperature acquires a distribution of torsion angles, which immediately leads to the experimentally obtained energy gap. Moreover, the emitter orientation anisotropy in a host matrix shows an 80% ratio of horizontally oriented dipoles, which is highly desirable for efficient light outcoupling. Understanding intramolecular donor-acceptor geometry in evaporated films is crucial for OLED applications, because it affects oscillator strength and TADF efficiency.

26‐1: Invited Paper: Thermally Activated Delayed Fluorescence Organic Light‐Emitting Diodes Comprising Ultrastable Glass Layers

In our previous work, we investigated the effects of the formation of ultrastable glasses within monochrome phosphorescent organic light‐emitting diodes.1 The ultrastable glasses are fabricated by heating the glass substrate to about 85% of the glass transition temperature while evaporation of the material. We observed significant enhancements of the external quantum efficiency (in the range of 20%) and device lifetime (up to 4x). These improvements are attributed to a denser packaging of the organic molecules resulting in reduced non‐radiative rates of the emitters. The change in the nano‐morphology can also have impact on the charge transport, the radiative rates of the emitter, and emitter orientation. For those properties, we have not identified a significant impact on the device performance. In this paper, we apply our concept to a green thermally activated delayed fluorescent emitter in a proof of principle device series. We find that insufficient energy transfer to the emitter holds back our devices from achieving state‐of‐the‐art efficiencies.

Highly-symmetrical plasmonic nanoantenna for fluorescence enhancement and polarization preservation of arbitrarily oriented fluorophore

This theoretical work illustrates the robustness of a highly-symmetrical cross nanoantenna in manipulating the fluorescence emission characteristics of a randomly oriented fluorophore. Owing to the highly symmetrical feature, the cross nanoantenna is able to generate stable excitation rates and quantum yields over all the emitter orientations. As a result, the cross structure produces a consistent fluorescence enhancement factor for any arbitrarily oriented fluorophore, which is more than twice that of a dimer counterpart. In addition, the cross nanoantenna preserves the original orientation information of the emitter at the far field, which is generally unachievable with a dimer configuration.

High-Efficiency Deep-Blue-Emitting Organic Light-Emitting Diodes Based on Iridium(III) Carbene Complexes.

High-efficiency pure blue phosphorescent organic light-emitting diodes (OLEDs) remain one of the grand challenges, principally because the emissive complexes employed either do not possess sufficiently high photoluminescence quantum yields or exhibit unsatisfactory Commission International de l'Éclairage (CIE) coordinates. Here two deep-blue-emitting homoleptic iridium(III) complexes are reported and OLEDs are demonstrated with CIE coordinates of (0.15, 0.05) and maximum external quantum efficiency of 13.4%, which decreases slightly to 12.5% at 100 cd m-2 . They represent examples of the most efficient OLEDs surpassing the CIEy requirement of the National Television System Committee (NTSC) and the European Broadcasting Union (EBU). Emitter orientation contributes to the excellent device performance.

Emitter Orientation as a Key Parameter in Organic Light-Emitting Diodes

Neatness counts, even in optoelectronics. Controlling the orientation of dye molecules in an organic light-emitting diode (OLEDs) is a powerful tool to improve its light outcoupling. By aligning transition-dipole-moment vectors parallel to the substrate plane, up to 40% quantum efficiency can be achieved. The authors give a comprehensive review of the physics behind this phenomenon, and highlight prospects for future device designs.

Aluminum Nanotripods for Light‐Matter Coupling Robust to Nanoemitter Orientation

AbstractNanoantennas enable the concentration and manipulation of light at the (sub‐)nanoscale. This ability offers novel strategies to strengthen light‐matter interactions in a controlled fashion. However, most nanoantennas are highly sensitive to light polarization and emitter orientation, which is disadvantageous for many applications (e.g., Raman and fluorescence spectroscopy depend strongly on molecular symmetry and orientation, as well as on the local optical field gradient). It is also unfortunate that analytical descriptions, essential to bridge experimental observations to knowledge and future design guidelines, have lagged behind. Here, resorting to conformal transformation, aluminum nanotripods excited by a nanoemitter of arbitrary orientation are studied analytically. Our results, corroborated with full‐wave simulations, show that aluminum nanotripods are robust not only to emitter orientation, but also to its position. Hence, this work exemplifies the effectiveness and efficiency of transformation optics to analytically describe and optimize light‐matter interaction in complex plasmonic nanoantennas.