This PDF file contains the front matter associated with SPIE Proceedings Volume 13122, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Perhaps the single most important problem confronting the development of OLED displays and lighting today is how to achieve sufficiently long triplet-controlled emission device lifetime to prevent rapid color change during operation, while achieving 100% internal emission efficiency. It has been shown1 that bimolecular (e.g. triplet-polaron, triplet-triplet) annihilation provides a source of energy sufficient to destroy the blue triplet chromophore (whether a phosphor or a TADF molecule) or its host. Since that time, many materials, structures and strategies to extend blue emission lifetime based on this understanding have been demonstrated. Furthermore, various molecular fragments have been identified whose presence leads to the observed luminance loss. Unfortunately, a fully satisfactory solution has not been shown where blue triplet emitter lifetime is sufficient to meet the standards of high performance displays, although white OLED illumination sources may now have adequate lifetime to meet industry standards. In this talk I will discuss progress in extending blue phosphorescent OLED (PHOLED) lifetime, and in understanding of the limitations to extending the lifetime of blue triplet emitters. In particular, I will focus on the relationship between radiative state lifetime, exciton density, and the longevity of the PHOLED. I will review efforts that have resulted in increasing the deep blue phosphorescent longevity by at least 14 X via emitter design, polaritons, and optical cavity engineering. Prospects for future advances will be discussed. 1. “Intrinsic luminance loss in phosphorescent small-molecule organic light emitting devices due to bimolecular annihilation reactions”. N.C. Giebink, B.W. D’Andrade, M.S. Weaver, P.B. Mackenzie, J.J. Brown, M.E. Thompson, and S.R. Forrest, J. Appl. Phys., 103, 044509 (2008).

Commercial carbazole (Cz) has been widely used to synthesize organic functional materials, which are entwined with recent breakthroughs in ultralong organic phosphorescence, thermally activated delayed fluorescence, organic luminescent radicals, and organic semiconductor lasers. Recently, we discovered that different from commercial Cz, the fluorescence of lab-synthesized-Cz (Lab-Cz) is blue-shifted by 54 nm and the well-known room-temperature ultralong phosphorescence almost disappears. Detailed studies reveal the presence of a Cz isomer as the impurity, which is widespread in commercial Cz sources with <0.5 mol%. Ten representative Cz derivatives were resynthesized from the Lab-Cz and all failed to show the reported ultralong phosphorescence in the same crystal states. However, even 0.1 mol% isomer doping can recover the reported ultralong phosphorescence. The presence of the isomer in commercial carbazole triggers us to re-examine the structure-property of many optically active materials with important discoveries.

The vast amount of biological mysteries and biomedical challenges faced by humans provide a prominent drive for seamlessly merging electronics with biological living systems (e.g. human bodies) to achieve long-term stable functions. Towards this trend, one of the key requirements for electronics is to possess biomimetic form factors in various aspects for achieving long-term biocompatibility. To enable such paradigm-shifting requirements, polymer-based electronics are uniquely promising for combining advanced electronic functionalities with biomimetic properties. Among all the functional materials, stretchable light-emitting materials are the key components for realizing skin-like displays and optical bio-stimulation. In this talk, I will mainly introduce our research in imparting stretchability onto “third-generation” OLEDs that can harness all the excitons through thermally activated delayed fluorescence (TADF), thereby with a theoretical near-unity quantum yield and high OLED efficiency. Our developments of fully stretchable OLEDs show the promise of achieving all the desired EL and mechanical characteristics, including high efficiency, brightness, stretchability.

Stretchable light-emitting materials that can mimic skin are key for wearable displays that feel natural, but until now, they could mainly produce a greenish-yellow light due to limitations in the materials available, particularly those in the super yellow series. To create wearable displays that can show the full spectrum of colors, it is vital to have materials that can emit red, green, and blue (RGB) light and are also stretchable. In my presentation, I will discuss a novel method for creating highly stretchable light-emitting films. This method involves mixing traditional RGB light-emitting polymers with a nonpolar elastomer to form blend films. These films are made up of light-emitting polymer nanodomains interconnected within an elastomer matrix, enabling efficient light emission when stretched. The RGB blend films we developed show a brightness of over 1000 cd/m2, require a low activation voltage (less than 5 Von), and can maintain consistent light emission up to a 100% strain, even after being stretched and released 1000 times.

The study presents innovative strategies for intrinsically-stretchable (is-) quantum dot light-emitting diodes (QLEDs), crucial for future user-friendly displays. It introduces a mechanically soft and stretchable is-EML composed of colloidal quantum dots (QDs), elastomeric polymer, and charge transport polymer. Despite high stretchability, the inter-QD spacing remains minimally altered under ~50% strain, ensuring stable device operation. A polymer-rich charge transport layer facilitates efficient hole transport to QDs within the elastomer matrix. Fully-optimized is-QLEDs, comprising an is-EML, charge transport layers, and stretchable electrodes, demonstrate impressive performance, including a low turn-on voltage of 3.2 V and a maximum luminance of 15,170 cd m-2 at 6.2 V, maintaining brightness even under 50% strain. These advancements enable full-color is-passive-matrix QLED arrays, showcasing potential for fully-stretchable QD displays.

Organic light-emitting diodes (OLEDs) offer a multitude of properties that render them particularly attractive for wearable and implantable applications in biomedicine. In this contribution, I will present our progress towards high brightness, patterned, flexible OLEDs for neuronal stimulation and imaging including stacked OLEDs for multicolor neuronal control, use of smartphone displays for optogenetics, and development of narrowband OLEDs for fluorescence imaging. Furthermore, I will show how the devices may be combined with organic photodiodes to enable simultaneous control and read-out of neuronal signals from an integrated, flexible device.

Hybrid metal halide perovskite has emerged as a remarkable light-emitting material for photonic devices such as lasers, LEDs, and light sources in general. Hyperbolic metamaterial (HMM) is a special class of anisotropic material that exhibits metal and dielectric features at the same time. This unique feature allows HMMs to be used in applications such as super-resolution imaging, spontaneous emission enhancement, and topological photonics. However, due to the inherent loss from the metal constituent, HMM’s insertion into these applications is hindered. In order to overcome this hurdle, we show a gain-assisted HMM that consists of MAPbI3 perovskite and Au in a metasurface configuration, wherein the dielectric constituent is fully composed of perovskite gain, thus compensating for the metal loss. We employ this HMM as a luminescent metasurface that emits highly linearly polarized light. In addition to linear polarization, there is an increasing need to generate chiral emission with well-defined spin angular momentum, i.e., circular polarization. We show a perovskite metasurface light source that can emit tailored circular polarization and radiation patterns on a silicon platform.

Inkjet printing can be applied for a variety of functional layer and structure fabrication such as microscale pixel patterning of organic light emitting diode (OLED), quantum dot-OLED, and other functional layers. However, overcoming the several drawbacks, such as resolution, reliability during the process, and issue of device stability, is still required. In this work, effect of solvent formulation was characterized both for drop jetting reliability and relevant printing quality of patterning. Condition for stable droplet formation and a homogeneity of printed pixel quality was surface tension lower than 36mN/m and viscosity > 5cP. Designing of experiments for solvent mixture was accompanied with the evaluation of the composition-property relationship between jetting flow and pattern formation. Further consideration for QD/light diffusing nanoparticle mixture inkjet (color-converter from blue OLED) as well as a pixel-aligned low index lens array on top of OLED transparent electrode will be discussed.

Despite three decades of research, colloidal quantum dot (QD) lasers are still at the stage of exploratory devices rather than a practical technology. A primary complication is nonradiative Auger recombination of gain-active multicarrier states. To overcome this challenge, we introduce type-(I+II) QDs that feature a trion-like optical-gain state with strongly suppressed Auger recombination. Using a single QD sample, we achieve stable lasing tunable 634 nm (red) to 590 nm (orange-yellow). These results point towards the feasibility of technologically viable QD lasers that combine wide dye-laser-like spectral tunability with high operational stability and unparallel diversity of optical characteristics of zero-dimensional semiconductors.

Halide perovskites are exciting new semiconductors that show a great promise in low cost and high-performance optoelectronics devices. However, the poor stability is limiting their practical use. In this talk, I will present a molecular approach to the synthesis of a new family of hybrid material – Organic Semiconductor-incorporated Perovskite (OSiP). Energy transfer and charge transfer between adjacent organic and inorganic layers are extremely fast and efficient, owing to the atomically flat interface and short interlayer distance. In addition, the rigid conjugated ligand design dramatically enhances their chemical stability, suppresses solid-state ion diffusion, and modulates electron-phonon coupling, making them useful in many applications, particularly solid-state lighting. Using these stable hybrid materials, we demonstrate efficient light emission and amplification in single crystalline nanostructures, epitaxial heterostructures, and polycrystalline thin films.

We are giving an overview of our work on ultrabright perovskite LEDs with the ultimate goal of injection lasing. Starting from the analysis of perovskite films in standard LED configurations, we can already identify beneficial operation conditions as cryogenic cooling and short electrical pulsing. By downscaling the active area of these devices, we reach current densities multiple kA/cm2 approaching the range expected to reach inversion in the recombination zone. In experiments combining nanosecond laser pulses of optical pumping and sub-microsecond electrical pulses of high voltages, we have been able to demonstrate the contribution of injected carriers to the overall net gain in a PeLED. In parallel, we have optimized distributed feedback resonators for perovskite lasers and their possible integration in light emitting diodes. The combination of the opto-electrical analyses enables us to sketch a possible route towards a perovskite injection laser.

Halide perovskites have emerged as a novel class of revolutionary semiconductors with wide tunability of energy bandgap, low cost, and simple solution process for optoelectronic devices such as LEDs, and photodetectors. In this talk, we will discuss the effects of ligands on low-dimensional perovskites including the defects passivation, phase distribution, carrier transportation, and confinement for improving the efficiency and stability of perovskite LEDs. Meanwhile, by introducing the new approaches of double-side crystallization and passivation, we realize high-performance perovskite photodetectors with a wide detection range from UV to NIR will be described. The work contributes to paving perovskites for practical optoelectronics.

Metal halide perovskites (MHPs) are increasingly recognized as promising materials for future display technologies due to their exceptionally high color purity. In this talk, we will explore the unique benefits and approaches in utilizing MHPs for display technologies, focusing on innovative nanostructures and material strategy in precisely engineered colloidal perovskite nanocrystals (PNCs) for high luminous efficiency in perovskite light-emitting diodes (PeLEDs). This includes the addition of zero-dipole cations and bromide-incorporated molecules for effective mitigation of defect sites in PNCs. In terms of scalable manufacturing, we have developed a modified bar-coating technique that yields PeLEDs with emission efficiency comparable to those of PeLEDs with small emission area. Additionally, we'll discuss an advanced core/shell PNC synthesis method, leading to realization of simultaneously bright, efficient, and stable PeLEDs. Further, we developed a novel hybrid tandem PeLEDs with an ideal optical structure that emits light much efficiently with narrow bandwidth. These developments highlight the potential of MHPs as leading materials for self-emissive displays.

Perovskite light-emitting diodes (PeLEDs) has progressed rapidly and has bright prospects for next-generation display due to advantages of superior color gamut and high photoluminescence quantum yield (PLQY). Recently, major breakthroughs have been made in the improvements of device performance through additive engineering, ligand engineering and optimization of device architectures. As a results, PeLEDs have recorded an external quantum efficiency (EQE) of up to 30.84% for green emission, 26.3% for red emission and 23.8% for near-infrared emission, respectively. However, the performance of blue PeLEDs is unsatisfactory compared to that of red and green PeLEDs, hindering the application of perovskite devices in full-color displays and solid-state lighting. Therefore, effective strategies to achieve high-performance blue PeLEDs are urgently needed. Here, we introduce various approches with metal doping, additive engineering, ligand engineering and interface engineering for high-performance blue light-emitting devices.

This study outlines effective surface tailoring strategies for surface and grain boundaries in polycrystalline perovskite thin films by manipulating the crystal nucleation and growth during the solution processing. The successful mitigation of these defective grain boundaries results in a marked reduction in charged trap densities, enhanced environmental stability, and minimized ion migration within the perovskite thin film. Consequently, a noticeable improvement in the operational stability of both solar cells and light-emitting diodes are demonstrated, underscoring the importance of precise control over crystal nucleation and growth in optimizing the performance of metal halide perovskite-based optoelectronic devices.

Halide perovskite is a promising candidate for high-performance and ultra-compact light emitters which are the core of the next-gen display technology. Despite various µ-Perovskite-LED array design availability, achieving finely controlled emission patterns, by implementing nanopatterns on the active media (metasurfaces), is a key challenge. In this work, we demonstrate a design and fabrication procedure for perovskite metasurface LED whose optical properties are controlled by nanopatterning. The obtained results pave the way for radiation control in perovskite LEDs and micro-LEDs.

Halide perovskite quantum dots (PQDs) have attracted a lot of attention in various applications, including light-emitting diodes (LEDs), solar cells, bio-imaging, and photodetectors due to their excellent photoelectric properties. However, the poor structural stabilities of PQDs under external stimuli (e.g., moisture, heat, UV) are the weakness that needs to be addressed for commercialization. The structural degradation of PQDs is mainly caused by the formation of vacancies due to ion migration (intrinsic) and detachment of weakly binding ligands on the surface (extrinsic). In this work, the PQDs with high structural stabilities were fabricated by various methods including (1) core-shell structure (2) alkali metal doping, and (3) ligand passivation.

Linear, heteroleptic Au(I) complexes produce long-lived fluorescence with negligible contributions of prompt fluorescence, due primarily to a rapid equilibrium between their singlet and triplet states. This efficient spin mixing provides a unique principle to creating high-efficiency, roll-off-free electroluminescence devices. To maximize the electroluminescence utility, my group investigated factors that control radiative and nonradiative processes of excited-state Au complexes. A series of Au(I) complexes with different sterically encumbering substituents or amido ligands were synthesized. Our investigations revealed the key role of the d-orbital of the Au center in the excited-state equilibrium between the singlet-triplet states, despite its d10 electronic configuration. Specifically, although the fluorescence efficiency is governed by nonradiative control dictated by the bandgap energy law, it is under radiative control the extent of which increases with overlap between hole- and electron centroids in the fluorescence transition. This discovery enabled the highest efficiency electroluminescence in the near-infrared regions from Au(I) complex emitters.

A grand challenge in emitter design for OLEDs is to develop bright materials that emit at the industry-targeted deep blue color point and use translates into efficient and stable blue devices. This requires the emitter to be narrowband emissive in the deep blue, be very bright, and be able to have short exciton lifetimes to mitigate undesired biexcitonic degradation pathways like singlet-triplet and triplet-triplet annihilation. In this presentation, I will discuss our recent efforts to towards the design of multiresonant TADF emitters that meet all of these design criteria simultaneously, and demonstrate their potential in OLEDs.

Most of the electrically generated excitons in organic light emitting diodes (OLEDs) are in a non-radiative triplet state. In thermally activated delayed fluorescence (TADF) chromophores triplet excitons can be converted into bright singlet excitons via reverse intersystem crossing (RISC). This requires the energy barrier between the singlet and triplet states to be very low or, ideally, completely absent. In addition, it is desirable for the color of OLED chromophores to be sharp and tunable. Computer simulations can accelerate the discovery of materials with these properties. Time dependent density functional theory (TDDFT) is the method of choice for predicting the excited-state properties of chromophores. However, its reliability depends strongly on the exchange-correlation functional used. We benchmark the performance of TDDFT methods [Phys. Rev. Research 4, 033147 (2022)] and demonstrate their applications for the discovery of new TADF chromophores [Electronic Structure 5, 014010 (2023)] and graphitic carbon nitride flakes with inverted singlet-triplet gaps [J. Phys. Chem. Lett. 14, 10910 (2023)].

This conference presentation was prepared for SPIE Optics and Photonics, 2024.

Spontaneous orientation polarization (SOP) can lead to interfacial charge accumulation and exciton-polaron quenching in OLEDs. Well-considered is the case of electron-transport layer (ETL) SOP which results in accumulation at the emissive layer (EML)-ETL interface. This work considers two device architecture driven approaches to engineer accumulation and reduce quenching. First, the impact of EML SOP is considered using a polar host material. Second, addition of a blocking layer at the hole-transport layer-EML interface is demonstrated as a means to tune charge accumulation. These results underscore the importance of SOP in OLEDs while offering architecture-based methods for altering associated charge dynamics.

We discuss the potential of OLEDs for biomedical applications. Specifically, we present our latest research on near-infrared (NIR) OLEDs based on phosphorescent compounds. Beginning with fundamental synthetic concepts, we delve into how the design of the emitting layer influences the emission spectrum and efficiency, aiming for optimal device design. We also showcase the application of these NIR OLEDs in photobiomodulation (PBM). Following this, we share our advancements in internal PBM using OLED catheters, which incorporate ultrathin OLEDs. Additionally, we discuss the outcomes of animal experiments, focusing on rats with type-II diabetes that have been treated with duodenal OLED PBM.

The efficiency of all LEDs decreases as their light output increases, and this effect is known as efficiency roll-off. It is a significant effect in phosphorescent OLEDs, and can be severe in TADF OLEDs, leading to efficiencies at useful brightnesses far below reported maximum efficiencies. The literature suggests that the main approaches to improving the situation are to reduce the energy difference between singlet and triplet, and to increase the rate of reverse intersystem crossing. We show this is incomplete and by considering the dynamic equilibrium between singlet and triplet, propose a figure of merit to guide the development of improved materials. The possibility of extending the approach to hyperfluorescence will also be considered.

Polar organic semiconducting molecules can exhibit a preferential alignment of their permanent dipole moments, inducing a giant surface potential via spontaneous orientation polarization (SOP). Previous work has shown how blends of polar and non-polar molecules offer one approach to either engineer SOP and its deleterious effect on OLED efficiency. Here, the emphasis is instead on composite films where both components exhibit SOP. In these systems, the observed SOP follows a linear combination of neat film behaviors. This study expands the avenues for control over SOP via molecular blending that can be further exploited for applications.

Recently, we have successfully synthesized niobium oxide nanoparticles (Nb NPs) with a particle size of less than 4 nm by DMF reduction. Nb NPs dispersed in water or alcohol showed blue-pale photoluminescence. We then fabricated organic-inorganic hybrid EL devices using Nb NPs as the emitting layer. These EL devices showed emission in a wide visible region including the red region. We found that Nb NPs have emission centers in the wide visible region and that their emission color may be controlled by the density of nanoparticles due to the Förster-type resonance energy transfer.

This presentation will report our recent studies on circularly polarized luminescence and persistent luminescence enabled by spin-orbital coupling and spin-phonon coupling effects. Our experimental studies of magnetic field effects found that the circularly polarized luminescence are essentially originated from the circularly polarized orbital ordering effects when spin-orbital coupling is combined with nonlinear optical polarizable structures. Furthermore, we found that extremely slow spin-phonon coupling can be realized by combining the un-usual phonon dynamics from dipolar crystalline structures with spin-orbital coupling. Interestingly, the extremely slow spin-phonon coupling can extend the excited state dynamics from traditional time window of nano/micro-seconds to an emerging time window of seconds, leading to persistent delayed fluorescence. Therefore, spin-orbital coupling and spin-phonon coupling function as an important mechanism to generate circularly polarized luminescence persistent luminescence.

We report three highly efficient multiresonance thermally activated delayed fluorescence blue-emitter host materials that include 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene and tetraphenylsilyl groups. The new host materials doped with the conventional v-DABNA blue emitter exhibit a high PLQY greater than 0.82, a high horizontal orientation greater than 88%, and a short PL decay time of 0.96–1.93 μs. The device with TDBA-Si shows high EQE of 36.2/35.0/31.3% at maximum luminance/500 cd m−2/1,000 cd m−2. This high performance is attributed to fast energy transfer from the host to the dopant, which is enabled by the external heavy-atom effect of Si, increased spin–orbit coupling, inhibition of aggregation by the bulky tetraphenylsilyl groups, and fast RISC of the dopant. It can also be explained by a hot triplet excited-state contribution, high thermal stability, and high horizontal orientation. We achieved a high efficiency greater than 30% and a small roll-off value of 4.9% at 1,000 cd m−2 for the first time using the newly developed TDBA-Si host material. The presentation of OLED results incorporating different heteroatoms is also scheduled.

The utilization of stable tris(2,4,6-trichlorophenyl)methyl radical (TTM) doublet emitters in OLEDs is promising, as light-emitting radicals can overcome spin-statistical efficiency limitations that exist for conventional closed shell emitters. Donor-functionalized TTM derivatives show quite long fluorescence lifetimes, which are in part caused by structural reorganization of the excited state, in which the donor plane becomes more perpendicular with regards to the TTM unit. Restricting this change in the dihedral angle between donor and acceptor moieties is expected to shorten the fluorescence lifetime, potentially opening up other new applications of light-emitting radicals, for example in the field of organic lasers. I will show the data to support my claims and discuss possible pathways for non-radiative relaxation as well as strategies for further improvement of photoluminescence lifetimes and quantum yields.

Hund's multiplicity rule states that for a given electronic configuration, a higher spin state has a lower energy. This energetic ordering requires thermal activation of dark triplet excited states to bright singlet excited states to emit delayed fluorescence. Here we report an organic molecule that exhibits delayed fluorescence from energetically inverted singlet and triplet excited states.

This talk will explore a new class of organic light-emitting diode (OLED) that exhibits bistability owing to positive photonic feedback between an organic photodiode integrated in the same layer stack as a tandem OLED. These unusual devices exhibit giant hysteresis in both their current and light emission and respond sensitively to low-level external illumination, enabling optoelectronic upconversion with thousand-fold photon-to-photon gain. These devices may find use in new types of display and upconversion imaging applications as well as provide a new platform for neuromorphic optoelectronics and image recognition.

The optical and electronic properties of organic semiconductor thin films are intimately coupled to their morphology at the atomic level. Atomic level morphology of non-crystalline systems is difficult to probe experimentally and hence molecular dynamics (MD) simulations have been used to provide a means to examine the morphologies of evaporated amorphous thin films with unrivalled spatial resolution. These simulated films have been used to undertand the experimentally measured properties such as charge mobility and photoluminescence quantum yield. However, MD simulations occur on timescales much faster than that used in experiment. In this presentation we report high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with depth sectioning to reconstruct the three-dimensional distribution of the emissive layers composed of iridium(III) complexes blended into an amorphous host. We will show that the results validate the morphology of blend films created using MD simulations which mimic the evaporation film forming process and are also consistent with experimentally measured charge transport and photophysical properties.

We are currently conducting research on OLEDs based on the following four aspects: 1) high-throughput development of TADF materials and realization of highly efficient OLEDs, 2) quantitative prediction and fundamental understanding of emission processes in OLEDs based on precise quantum chemical calculations, 3) quantitative prediction and fundamental understanding of charge transport processes in OLEDs based on multiscale simulations, and 4) NMR analysis of OLEDs. Here, we will present our recent progress on these topics.

Spontaneous orientation polarization (SOP) results in excess charge accumulation at the interfaces within OLEDs. The excess charge accumulation leads to significant exciton-polaron quenching (EPQ) and correlates to the device degradation. While SOP is observed in various OLED materials, the optimized SOP configuration remains not well understood. In this study, we demonstrated the correlations of the interface charge accumulation and device characteristics, particularly focusing on EPQ and device degradation, by the simultaneous measurement technique of displacement current and photoluminescence intensity (DCM-PL).

In contrast to traditional OLED displays found in mobile phones and TVs, OLED microdisplays present distinct challenges. Despite their reduced panel size, achieving high resolution is crucial, resulting in pixel sizes of only a few micrometers and gaps between pixels less than 1 micrometer. In this presentation, we will delve into OLED device technology, focusing on achieving high luminance and a wide color gamut, while considering the unique characteristics of microdisplays. A noteworthy demonstration involved a 3000 pixels-per-inch (ppi) OLED microdisplay with a color gamut of 130.2% based on the sRGB standard. This achievement is attributed to a thoughtful combination of light-emitting materials, OLED device structure, and subpixel arrangement. This work was supported by Institute of Information & communications Technology Planning & Evaluation (IITP) grant funded by the Korea government(MSIT) (No.2022-0-00026, Near-eye light field device technology development for hyper-realistic metaverse service)

Interfacial exciton-polaron quenching (EPQ) in organic light-emitting diodes (OLED) refers to the remote and direct energy transfer from the excitons in the emission layer (EML) to the charges accumulated at the interface of the adjacent charge transport layers (CTLs). Here, interfacial EPQ is investigated by using a bilayer hole-only device (HOD), where an ultra-thin dopant layer is selectively introduced near the interface. At the heterointerface, positive-charge accumulation is induced due to the energy difference between distinct organic layers, while excitons are optically pumped in the dopants exclusively. The interfacial EPQ is characterized by measuring a shift in the photoluminescent intensity of the dopants. Such interfacial EPQ indeed depends on the interfacial energy offset and the distance between charges and excitons, and universally occurs regardless of the emission mechanism of OLEDs. We propose the device architecture to potentially reduce interfacial EPQ for achieving high-performance OLEDs.

The exciplex-forming systems that can perform thermally activated delayed fluorescence (TADF) characters are emerging as appealing research due to their versatile applications in organic light-emitting devices (OLEDs). However, the lack of data to interpret the detailed D-A interactions impedes the further deep insight understanding of the exciplex excited state. This demand motivated us to establish an exciplex-enabling supramolecular system with X-ray structure analyses revealing the detailed donor-acceptor interactions, the thermodynamic parameters of complex formation, the photophysical property to verify the exciplex characters and the characteristics of the solution-processed OLED device employing this exciplex-forming supramolecular complex as an emitter. This unprecedented and tunable system opens a new platform for further exciplex studies. The detailed investigation of exciplex-forming systems achieved by this supramolecular approach will be presented.

beeOLED, a German start-up, pioneers a solution to the well-known issue of short blue emitter lifetimes in OLED. Leveraging divalent Europium and its unique intra-metallic transition, beeOLED introduces a groundbreaking approach. Eu(II) features parity-allowed d-f transitions with a ~1 µs lifetime, comparable to widely-used phosphorescent emitters. With 100% internal quantum efficiency and a tunable deep-blue emission spectrum, beeOLED's Eu(II) complexes promise enhanced stability and longevity in OLEDs. By circumventing organic bond cleavage through single intra-metallic atom excitation, beeOLED overcomes typical issues encountered with organic blue emitters. Through rational ligand design, challenges of Eu(II) complex oxidation and sublimation are resolved. beeOLED is spearheading a new wave of robust and versatile blue OLED devices.

Among the three primary colors, blue emission in organic light-emitting diodes (OLEDs) are highly important but very difficult to develop. OLEDs have already been commercialized; however, blue OLEDs have the problem of requiring a high applied voltage due to the high-energy of blue emission. Herein, we discovered a novel combination of a blue emitter and an electron acceptor and realized upconversion emission through triplet-triplet annihilation near the interface. The OLED with the combination shows ultralow voltage turn-on at 1.47 V for blue emission with a peak wavelength at 462 nm (2.68 eV). This OLED reaches 100 cd/m2 , which is equivalent to the luminance of a typical commercial display, at 1.97 V. Blue emission from the OLED is achieved by the selective excitation of the low-energy triplet states at a low applied voltage by using the charge transfer (CT) state as a precursor and triplet-triplet annihilation, which forms one emissive high-energy singlet from two low-energy triplet excitons. The UC process of the excited states greatly reduce the turn-on voltage of OLED.

An optimization scheme to minimize angular color shifts in OLEDs is developed using a combination of optical simulations and experimental measurements of device performance. This minimization does not compromise other critical device operation parameters, such as efficiency and the angular intensity profile. By considering both bottom- and top-emitting OLEDs, this study utilizes strong feedback between simulation and experiment to identify stack architectures that have a minimum color change with viewing angle while still maintaining high power efficiency.

High-purity deep-blue perovskite light-emitting diodes (PeLEDs) are essential for next-generation displays that meet the Rec. 2020 standard. However, halide vacancies, particularly chloride, cause bandgap instability and reduced LED performance in mixed-halide perovskites. We present a chloride vacancy passivation (CVTP) strategy using sulfonate group ligands with varying chain lengths, which strongly bind to Pb(II) ions and fill the vacancies. This approach prevents phase segregation, producing color-stable deep-blue PeLEDs with a 461 nm emission peak and 2,707 cd/m2 luminance, among the highest for Rec. 2020 compliant PeLEDs. Additionally, external quantum efficiency improved to 5.68%, influenced by ligand chain length.

Homoleptic Ir(III) carbene complexes were particular suitable in serving as the OLED blue phosphors. These carbene complexes exhibit both the destabilized T1 and MC dd excited states due to the high lying π* orbital of carbene fragments and stronger Ir-C bonding interaction. Hence, tuning emission to true-blue color can be done by stabilization of T1 state via addition of electron deficient substituent(s) at the carbene fragment, resulting in the increased energy gap between the T1 excited state and metal-centered dd quenching state. The enlarged energy gap retarded the non-radiative decay, making them one of the best candidates in showing both the efficient and durable blue phosphorescence. Underlining theory and representative examples will be discussed in this presentation.

We fabricated QD EL devices with a ZnMgO layer as an ETL by RF sputtering. The deposition condition of ZnMgO layer was optimized by controlling the partial pressure of oxygen and RF power for the best EL performance. Mg doping effectively reduced the defect sites induced by oxygen vacancy and the charge balance in the EML of QD EL devices with a Zn0MgO layer deposited by RF sputtering was dramatically improved by the controlled injection of electrons, which was resulted from the upshift of the conduction band minimum. As a result, large QD EL devices with an inverted structure showed the maximum luminance and current efficiency of 136,257 cd/m2 and 20.7 cd/A, respectively.

We report on the tailoring strong-light matter coupling and polariton formation in organic semiconductors, with particular focus on the design of OLEDs with angle-independent emission and polariton lasers with reduced thresholds and fully metallic contacts. With ever more stringent colour requirements in the display industry, there is a need for OLEDs with more saturated-colour emission. The use of strong micro-cavities is a potential solution but can introduce undesired changes in colour with observation angle. By introducing an assistant absorber layer into microcavity OLEDs, we create exciton polaritons that inherit the angle insensitivity of molecular absorption and the narrow linewidth of the cavity mode. We also demonstrate that our approach can be generalized beyond OLEDs, e.g. to obtain optical filters with sharp and angle independent spectral features. In addition, we discuss work on polariton lasers that exploit conformation, molecular alignment, and the disorder introduced by micro-domains in liquid crystalline polymer films to enhance lasing performance. This then allows us to replace the dielectric mirrors of these lasers with electrically conducting metal mirrors.

Perovskites are emerging as promising materials for both solar cells and light-emitting diodes (LEDs). Despite the impressive electroluminescence efficiencies demonstrated by perovskite LEDs, their device designs still largely rely on classical optics, similar to previous organic LEDs. In this work, the focus is on the role of photon recycling in perovskite electroluminescence. A novel optical modeling approach is introduced, which accounts for the reabsorption properties of luminescent materials. This analysis reveals that photon recycling can significantly enhance the electroluminescence efficiencies of perovskite LEDs, surpassing previous limits for outcoupling. This model elucidates how recent advancements in perovskite LEDs have achieved such high efficiencies, despite the challenges posed by the high refractive index of perovskites. Moreover, it is discovered that photon recycling has an even more pronounced effect in perovskite solar cells with thicker active layers. This suggests that perovskite solar cell architectures hold potential for generating very bright electroluminescence, potentially surpassing even state-of-the-art perovskite LEDs.

Macroscopic quantum effects such as superconductivity, superfluidity, and Bose-Einstein condensation emerge due to the collective coherence of quantum particles. For electronic phase transitions in solids, thermal processes such as random electron-phonon scattering cause dephasing and limit these phenomena to low temperatures. We recently observed superfluorescence in lead halide perovskite thin films at unprecedentedly high temperatures. In superfluorescence, optically excited dipoles synchronize and reach a collectively coherent quantum state. The resulting giant dipole emits a burst of photons. Similar to other macroscopic quantum effects, superfluorescence also requires robust coherence. Hence, the observation of this collective quantum optical effect at high temperatures provides an unprecedented opportunity for the investigation of mechanisms that enable high-temperature collective quantum states in solids. In this presentation, I will discuss our analysis of system-bath interactions that stabilize macroscopic coherence in superfluorescence in perovskites. Our work provides important insights into the design and development of quantum materials for practical applications

The precise control of polarization in solid-state materials holds immense significance across various scientific disciplines, particularly optics and electronics. This control enables tailored manipulation of light-matter interactions, device functionalities, and electronic properties, leading to a host of practical applications. In this presentation, I will showcase our recent discovery of a broadband optical retardation effect in 2D organic-inorganic hybrid perovskite single crystals. We found that this remarkable optical retardation effect can be attributed to the large in-plane birefringence of 2D hybrid perovskites. We suggest that the broadband optical retardation effect arises due to the formation of mosaicity among the stacked inorganic layers, resulting in out-of-plane disorder during the self-assembly process of single crystals in the solution phase. I will discuss the design principles for tuning mosaicity and the robustness of these materials for transformative impacts.

Two-dimensional (2D) metal halide perovskites have emerged as promising materials for next-generation solar cells and light-emitting diodes thanks to their outstanding optoelectronic properties, facile tunability, and superior stability over their 3D counterparts. However, the detailed structure-property relationship of 2D perovskites underlying their optoelectronic properties has remained unclear. In this project, we design and synthesize a variety of 2D perovskite single crystals, in both Ruddlesden-Popper and Dion-Jacobson phases. We then use a combination of steady-state and time-resolved optical spectroscopy methods to characterize the exciton properties and dynamics, specifically focusing on how they are affected by changes in structural properties. Our results will help us develop a fundamental understanding of 2D perovskites and enable rational material design and development.

The challenge of developing efficient deep red emitters is addressed by the design of donor-acceptor chromophores which afford high fluorescence quantum yields through thermally activated delayed fluorescence (TADF). Alas, harmonizing TADF with the general design of deep red emitters proves difficult. One strategy to improve the (TADF) emission properties entails the design and comparison of regioisomers, i.e. systems with altered donor-acceptor configurations. In the presented work, dibenzo[a,c]phenazine-11,12-dicarbonitrile (DBPzCN) served as an acceptor template for the design of four regioisomers. These materials were computationally simulated using density functional theory (DFT). Following synthesis, time-resolved emission spectroscopy was utilized to probe the time-dependent emission in (doped) films, and to derive the TADF properties. Isomer 4-TPA-DBPzCN combines the favorable TADF characteristics of 2-TPA-DBPzCN and the higher fluorescence quantum yield of 3-TPA-DBPzCN. Via ‘isomeric modulation’, we were able to analyze the effect of geometry, orientation, and electronic interaction on the underlying emission characteristics.