Symposium FI
Materials and Technologies for Next Generation Solid State Lighting


Session FI-1 - Material Design and Processing

FI-1:IL01  Intramolecular or Intermolecular Charge Transfer Approaches for Highly Efficient TADF Materials and OLEDs
KEN-TSUNG WONG, Department of Chemistry, National Taiwan University, Taipei, Taiwan

Organic molecules with efficient thermally activated delayed fluorescence (TADF) are emerging as attractive emitters in OLEDs because of the achievable 100% internal quantum efficiency. In this talk, our recent works on acridine-based TADF materials will be presented. One of this series TADF materials exhibits nearly 100% photoluminescence quantum yield, excellent thermal stability, and a horizontal dipole ratio of 83%, leading to extremely efficient blue organic EL with external quantum efficiency (EQE) of ~37%. In addition, efficient and tunable blue-green to yellow TADF emitters capable of generating OLED EQEs of > 31% are developed adopting acridine as donor unit and CN-substituted benzene, pyridine and pyrimidine as acceptor units will also be reported. These materials permit one to systematically probe the influence of different acceptor strengths and also the influence of tunable conformations (twist angles) within the acceptor moieties through controlling the orientation of asymmetric heteroaromatic ring relative to the donor component. TADF can also be achieved by exciplex formed through intermolecular charge transfer between a hole-transporting material and an electron-transporting material. Some efficient OLEDs based on exciplex system will also be presented.

FI-1:IL04  Design of Efficient TADF Materials for OLEDs
TAKUMA YASUDA, Kyushu University, Fukuoka, Japan

To maximize the actual efficiency in organic light-emitting diodes (OLEDs), we have recently demonstrated a new viable mechanism for electroluminescence (EL), that is, thermallyactivated delayed fluorescence (TADF). In TADF processes, triplet excitons can be thermally converted to emissive singlet excitons, leading to an increase in the quantum efficiency. In this presentation, we focus on advanced molecular design of TADF materials for OLEDs. By utilizing TADF mechanism, we have successfully achieved high-efficiency OLEDs exhibiting significantly high external EL quantum efficiencies, comparable to those obtained with phosphorescence OLEDs. Using a variety of electron-accepting building blocks tethered by appropriate electron-donating aromatic segments would be a versatile and promising strategy to produce highly efficient TADF materials and OLEDs.

FI-1:IL05  Materials and Device Design for Improving the Stability of OLEDs
HIROHIKO FUKAGAWA1, YUKIKO IWASAKI1, TSUBASA SASAKI1, MUNEHIRO HASEGAWA2, KATSUYUKI MORII2, TAKAHISA SHIMIZU1, 1Japan Broadcasting Corporation (NHK), Science & Technology Research Laboratories, Setagaya-ku, Tokyo, Japan; 2Nippon Shokubai Co., Ltd., Suita, Osaka, Japan

OLEDs are key devices for realizing next-generation displays and lightings. Although the emission mechanism has been studied with the goal of harvesting all excitons as emission, it has not been uncommon to hear of devices with internal quantum efficiencies of approximately 100% that use phosphorescent or thermally activated delayed fluorescent (TADF) emitters in recent years. Thus, parameters related to stability, such as operational stability of materials and long-term stability of flexible OLED, have begun to attract much attention. Here, we report on materials and device design for improving the stability of OLEDs. First, the molecular design of the host material to obtain an operationally stable phosphorescent OLED (PHOLED) is clarified by analyzing the device characteristics of several PHOLEDs utilizing similar TADF materials as hosts. Second, we demonstrated the first example of an efficient and stable inverted OLED that does not employ reactive electron-injection materials such as alkali metals. The removal of moisture-sensitive materials has enabled us to fabricate the first ever flexible OLED display that can emit light for over one year under ambient conditions, even in the presence of trace amounts of moisture.

FI-1:IL06  Feasibility of Future GaN Large Area Light Emitting Devices
HIROSHI FUJIOKA, K. UENO, A. KOBAYASHI, Institute of Industrial Science, The University of Tokyo, Meguro-ku, Tokyo, Japan; ACCEL-JST, Chiyoda-ku, Tokyo, Japan

Recently micro-LEDs technology with group III nitrides has attracted much attention as a promising candidate for next generation display pixels. However, it is well known that nitride devices are extremely expensive because their fabrication process involves low throughput high temperature MOCVD epitaxial growth. To solve this problem and fabricate low-cost GaN devices, a highly productive crystal growth technique for nitrides has been sought after. We have recently developed a new growth technique called PSD (pulsed sputtering deposition) and found that it allows us to obtain device quality III nitrides even at room temperature. We have found that PSD is quite compatible with mass-production with large substrates because the apparatus is similar to conventional sputtering machines commonly seen in IC or LCD fabrication lines. In this technique, surface migration of the film precursors is enhanced and, therefore, the temperature for epitaxial growth is dramatically reduced. This reduction allows us to utilize various large area low cost substrates such as metal foils and graphene sheets that have never been used for growth of semiconductors due to their chemical vulnerability. In this presentation, we will demonstrate successful epitaxial growth of GaN and operation of various GaN based devices such as RGB full colar LEDs, AlGaN/GaN HEMT, and MISFETs on low-cost flexible substrates.

FI-1:L07  Recent Progress and Challenges of InN and In-rich InGaN by RF-MBE
YASUSHI NANISHI1, TOMOHIRO YAMAGUCHI2, TSUTOMU ARAKI1, 1Ritsumeikan University, Kusatsu, Sjiga, Japan; 2Kogakuin University, Hachiouji, Tokyo, Japan

In contrast to high potential of InN and In-rich InGaN , research on devices based on this material system has not made big progress due to difficulty in obtaining high quality material. We have developed a new RF-MBE growth method called DERI (Droplet Elimination by Radical Beam Irradiation) for the purpose to grow high quality InN reproducibly. This growth method consists of the two series of growth steps with In-rich growth step and consecutive nitrogen radical beam irradiation step. We have applied this DERI method also to InGaN alloy growth, where we have observed strong phase separation, with preferable capture of Ga to growing crystal from In-Ga metal cover layer on the surface and In is swept out. Using this phase separation phenomenon positively, we have successfully grown InN/ InGaN and InGaN/InGaN MQW layers. This growth technology has a potential for fabrication of IR, red and green wavelength light emitting devices based on group III nitride semiconductors. We have successfully grown uniform and thick InGaN in full compositional range by controlling Ga flux while maintaining growth surface covered by more than two mono-layers of In. This technology offers useful method for application of InGaN both to solar cells and to relaxed template for longer wave.

FI-1:IL08  New Blue Organic Emitters for OLED Lightings
JONGWOOK PARK, Department of Chemical Engineering, Kyung Hee University, Deogyeong, Giheung, Yongin, Kyunggi, South Korea

Investigations have been more actively pursued for the two-color white OLEDs whose structures are relatively simple, because low cost is regarded as important in the lighting field. In addition, since the emission spectra of organic materials generally have peaks with broader wavelengths than do the emission spectra of inorganic materials, only two types of organic emitters, in particular the emitters of sky-blue and orange light or of sky-blue and red light, need to be combined to produce white light when using OLEDs. Many studies have achieved high efficiency and long lifetime for materials that emit orange or red light, but it has been a challenge to develop materials that emit pure blue light with high efficiency because of the large difference in the energy levels of adjacent hole- and electron-transporting layers caused by the wide band gap. Five compounds were designed and synthesized. Among the synthesized molecules, the TPA-AP-TP molecule, in which triphenylamine, with its optimum electron-donating ability, was substituted into anthracene, showed excellent electroluminescence (EL) performance for OLED lighting with a current efficiency of 8.05 cd A-1, external quantum efficiency of 6.75%, and EL FWHM of 53 nm.

FI-1:L09  Transparent Spinel ceramics for White Light-Emitting Diodes Applications
M. RADWAN, J. SEDLACEK, Z. LENCES, P. SAJGALIK, Institute of Inorganic Chemistry, Slovak Academy of Sciences, Bratislava, Slovakia

The use of transparent ceramics as bulk phosphors instead of the current polymer-ceramic phosphor packages represents one of the growing research interests in the last few years in order to develop novel phosphor materials for the high-power white light-emitting diodes (w-LEDs) for the wide-area lighting applications. The spinel ceramics (aluminium oxynitride, magnesium aluminate, and magnesium aluminium oxynitride) are an important class of the transparent ceramics which form in the Mg-Al-O-N materials system. This research deals with preparation of transparent MgAl2O4 and MgAlON spinels as potential bulk phosphors for the w-LEDs applications. Aqueous processing of the initial spinel powders was studied for making stable spinel slurries for freeze granulation and drying. Transparent spinel specimens were prepared by green compaction using cold-isostatic pressing (CIP) followed by a two-steps sintering approach by using pressure-less pre-sintering and hot-isostatic pressing (HIP). The real in-line optical transmission (RIT) at λ = 632.8 nm of the polished dense spinel samples reached 76% (approx. 87% of the theoretical transmittance), and the influence of some rare-earth and transition metals dopants on optical and photoluminescent properties will be presented.

FI-1:IL10  Enhanced Performance of Luminescent Powders due to Coating of Phosphor Particles by Atomic Layer Deposition in a Fluidized Bed Reactor
H.T. HINTZEN1, O.M. TEN KATE2, Y. ZHAO2,3, L.J. YIN2,4, Z. ZHOU2,5, J.R. VAN OMMEN2, 1Luminescent Materials, Faculty of Applied Sciences, Delft University of Technology, Delft, The Netherlands; 2Product & Process Engineering, Faculty of Applied Sciences, Delft University of Technology, Delft, The Netherlands; 3College of Materials, Xiamen University, Xiamen, China; 4School of Energy Science and Engineering, University of Electronic Science and Technology of China, Chengdu, China; 5Science College of Hunan Agricultural University, Changsha, China

Phosphor ageing limits the lifetime of light-emitting devices, such as the energy-saving white LED’s for general lighting applications and electroluminescent strips for smart-highway lighting. Insufficient chemical stability with respect to attack by oxygen, water or other reagents, as influenced by heat and radiation, generally is the cause of phosphor ageing. Protection against such phosphor degradation can be provided by coating each individual phosphor particle by a closed layer. Conventional coating techniques lack sufficient control of the deposition process, and as a consequence thin layers are not completely closed, while thick closed layers may result in loss of emission intensity. To avoid the drawbacks of traditional coating techniques we are investigating the potential of alternative coating techniques such as Atomic Layer Deposition (ALD) in a Fluidized Bed (FB) reactor (FB-ALD) with the objective to coat the phosphor particles with a very thin nano-sized, but nevertheless closed and uniform, layer. Moreover, the possibility of improving the luminescence properties by adding special functionalities to the coating (such as anti-reflection, surface passivation, electric field concentration, light extraction) is studied. In the lecture an overview will be presented of the coating of phosphor powders by conventional methods in general, and of the progress and prospects of our research on FB-ALD coating of several phosphor powders in particular.

FI-1:IL11  Single-phased Eu2+-activated Phosphors with High Color Rendering for Near-UV LED Chips
PENGPENG DAI1, XINTONG ZHANG1, YICHUN LIU1, XIAOJUN WANG2, 1Key Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun, China; 2Department of Physics, Georgia Southern University, Statesboro, GA, USA

Single-phased white-light phosphors with high-color-rendering index (CRI) are highly demanded since the fast development of the near UV LED chips.  It has been a challenge to create such phosphors from a single-doped activator. Here we present a novel Eu2+-activated high CRI (Ra = 91) white-light phosphor Sr5(PO4)3-x(BO3)xCl:Eu2+ system, which is composed of Sr5(PO4)3Cl as a beginning member and Sr5(BO3)3Cl as an end-member by using the solid-solution method.  Importantly, the white-lighting phosphors display a higher R9 = 90.2 under the excitation at 365 nm, which is much higher than the R9 = 14.3 of the commercial YAG:Ce3+ phosphor for the blue LED chips [1]. In addition, we propose a general but effective method for achieving high-efficacy WLEDs using two-color phosphors with the same host, lowering the energy loss by re‑absorption. As a result, as-prepared WLEDs present a wide range for both Ra (79-90.3) and efficacy (56-28.9 lm/W). Furthermore, by employing a Mn4+-activated phosphor with a red narrow emitting line, a high CRI (Ra = 90.3, R9 = 94) WLED with an efficacy of 39.3 lm/W has been obtained [2].
[1] P.P. Dai, C. Li, X.T. Zhang, J. Xu, X. Chen, X.L. Wang, Y. Jia, X.J. Wang, and Y.C. Liu, Light: Sci. Appl. 5, 1-9 (2016). DOI: 10.1038/lsa.2016.24. [2] P.P. Dai, J. Cao, X.T. Zhang, and Y.C. Liu, J. Phys. Chem. C 120, 18713–20 (2016). DOI: 10.1021/acs.jpcc.6b03177

FI-1:L13  Design Strategies for Materials Showing Thermally Activated Delayed Fluorescence and Beyond - Towards the Fourth Generation OLED Mechanism
H. YERSIN, University of Regensburg, Regensburg, Germany

 We discuss how to design efficient organic TADF molecules [1] by Presenting four concepts: •  Donor-acceptor molecules exhibiting fluorescence. •  TADF compounds with sterical hinderance between donor and acceptor. •  Donor-acceptor molecules stabilized by two bridges resulting in short TADF decay times. •  Presentation of a new OLED mechanism beyond TADF giving sub-µs emission decays. This mechanism is illustrated by focusing on new purely organic materials that show negligible singlet S1 to triplet T1 energy separation with ∆E(S1-T1) < 1 meV. Accordingly, direct and fast intersystem crossing (ISC) between the two states can occur. Fast ISC is further enabled by interaction with energetically close lying localized triplet states. As a consequence, only prompt fluorescence without time-delaying thermal activation is occurring. This mechanism of Direct Singlet Harvesting, being beyond TADF, leads us towards a fourth generation OLED mechanism that opens the way for increase of device stability.[2]
[1] H. Yersin, ed., Highly efficient OLEDs – Materials based on TADF, Wiley-VCH, Weinheim 2018. [2] H. Yersin, L. Mataranga-Popa, R. Czerwieniec, Design of organic TADF molecules. The role of ∆E(S1-T1), XXII Intern. Krutyn Summerschool  2017, European patent 2017 and EP 17170682

Session FI-2 - Optoelectronic and Photonic Processes

FI-2:IL01  Metal Halide Perovskites Light Emitting Devices and Interface Stability
B.P. RAND, Department of Electrical Engineering and Andlinger Center for Energy and the Environment, Princeton University, Princeton, NJ, USA

Hybrid organic-inorganic perovskite materials such as methylammonium lead iodide (CH3NH3PbI3) have garnered significant interest in the thin film optoelectronics community due to their promising optoelectronic properties. However, solution processed perovskites commonly suffer from poor thin film quality, reproducibility, stability, and scalability. We have determined that the fabrication of perovskite thin films displays all of the hallmarks of sol-gel processing, an aspect that we exploit to improve the quality of spin coated thin films. In particular, we realize films with roughness on the order of 1 nanometer that consist of nanoscale crystallites, formed by incorporating a bulky organoammonium halide in addition to the stoichiometric 3D perovskite precursors. These bulky ligands passivate the 3D crystal, lead to considerably enhanced luminescence quantum yields, and increase stability. LEDs produced in this way are capable of exceeding 10% external quantum efficiency and exhibit significantly improved stability. In another aspect, we have determined that metal halide perovskites are considerably redox active, and are looking to understand how redox chemistry dictates material and device physics and degradation.

FI-2:IL02  Realization of High Performance UV Emitters by using AlGaN Materials
MOTOAKI IWAYA1, TETSUYA TAKEUCHI1, SATOSHI KAMIYAMA1, ISAMU AKASAKI1,2, 1Faculty of Science and Technology, Meijo University, Japan; 2Akasaki Research Center, Nagoya University, Japan

The nitride semiconductors are very useful materials for blue light emitting diodes (LEDs). On the other hand, the applications of these materials as the optical device are further expanding. Especially, the progress in the UV light emitting device using AlGaN is remarkable. In recent year, high performance UV LED with high external quantum efficiency of as high as 20 % at 280 nm region using AlGaN-based materials are reported and sample ships are being started. In contrast, there is a problem of expanding the realization wavelength in UV lasers. In general, it is indispensable that three factors are required: the optical gain can be obtained, the optical resonator can be formed, and the carrier injection capable of population inversion can be realized. Laser oscillation by optical pumping has already been confirmed from many research groups, and AlGaN-based materials can obtain optical gain and can form optical resonators physically. However, since it is difficult to inject carriers, the shortest wavelength of laser oscillation by current injection is 326 nm. In this presentation, we would like to discuss future direction of UV laser such as laser by electron beam excitation and carrier injection by polarization doping to solve this problem.

FI-2:IL03  Deep UV LEDs

Light emitting diodes operating in the deep ultraviolet wavelength range around and below 300nm (DUV LEDs) are gaining more and more interest due to the numerous applications which would benefit from the inherent advantages of semiconductor based light sources especially when compared to the Hg containing discharge UV lamps used nowadays. The external quantum efficiencies (EQE) of DUV LEDs based on the AlGaN material system are however still quite low when compared to InGaN based visible LEDs. In addition, the cost of DUV photon generation is still several orders of magnitude higher. Both aspects currently limit the potentially rapid growth of the DUV LED market significantly. The main reason for these issues is that the AlGaN material system poses several technical challenges, e.g. doping which gets more and more difficult with increasing Al content, as well as a still limited light extraction efficiency. In this presentation we will address the different approaches and developments in AlGaN epitaxy and chip processing, with emphasis on the requirements for mass production yet still focusing on fabricating devices of the highest attainable quality. Also an overview on the current state of the art, the potential markets and related applications will be given.

FI-2:IL04  Indirect Excitons in Group III-Nitride-Based Quantum Wells
P. LEFEBVRE, B. JOUAULT, T. GUILLET, C. BRIMONT, P. VALVIN, T. BRETAGNON, M. VLADIMIROVA, Laboratoire Charles Coulomb (L2C), CNRS, University of Montpellier, France; L. LAHOURCADE, N. GRANDJEAN, Institute of Condensed Matter Physics, EPFL, Lausanne, Switzerland; B. DAMILANO, CRHEA-CNRS, Valbonne, France

In group-III nitride quantum wells, indirect excitons (IXs) are naturally formed because the electron-hole pair is separated along the growth (0001) axis by strong internal electric fields. These IXs therefore exhibit strong permanent dipole moments and extremely long radiative lifetimes (> 10µs). Previous extensive studies of IXs in GaAs-based, biased double quantum wells, at low temperatures, have emphasized promising properties: IXs can propagate over large distances, be controlled in-situ by light and external gate voltage, form cold and dense gases of interacting bosons, and may form collective quantum states of matter. Compared to IXs in arsenide heterostrutures, IXs in nitride QWs have much larger binding energies and smaller Bohr radii. This allows exploring IX propagation up to room temperature and over a range of exciton densities larger by two orders of magnitude. By using spatially- and time-resolved photoluminescence experiments we investigate the transport of dipolar excitons in GaN-(Al,Ga)N QWs, up to room-temperature, over distances of several tens of micrometers. Variations of conditions and comparison with models allow us to discuss the relative impacts on IX transport of important factors, such as disorder and exciton-exciton interaction.

FI-2:L05  Enhancing the Electroluminescence of Organic Light-emitting Transistors by Modifying the Metal/Organic Interface with Conjugated Polar Polymers

Organic light-emitting transistors (OLETs) show, in a single device, the fascinating combination of electrical switching characteristics and light generation capability. However, to ensure an effective device operation, efficient injection of charges at the electrodes is required. In this context, the introduction of solution-processed conjugated polyelectrolytes (CPEs) films at the emissive layer/electrodes interface represents a powerful strategy to improve the electron injection process. Despite the performance of the resulting OLETs is enhanced, the presence of ionic species in CPEs layers causes complications in the device response due to charge trapping and electric field screening effects. In this sight, the use of conjugated polar polymers (CPPs) can represent a valid alternative to CPEs since the conjugated backbones of CPPs are modified with polar non-ionic side groups, thus avoiding ion-depending drawbacks. By introducing a layer of polyfluorene containing phosphonate groups (PF-EP) underneath the metal electrodes, we here demonstrate a clear improvement of the electron injection properties and an increased light-emission of p-type OLETs, with superior performance in comparison with the relative CPE-containing devices.

FI-2:IL07  Fabrication of High-quality AlN Template by High-temperature Annealing

The annealing of sputtered AlN films with different thicknesses grown on sapphire in nitrogen ambient was investigated. In the annealing, two AlN films on sapphire were overlapped “face-to-face” to suppress the thermal decomposition of the AlN films. The sputtered AlN films with small grains consisted of columnar structure were initially aligned with (0002) orientation but became slightly inclined with increasing film thickness resulting in the formation of a two-layer structure. After annealing, films became a single crystalline layer regardless of the film thickness, and their crystallinity markedly improved after annealing at 1700 oC. The full widths at half maximum of the (0002)- and (10-12)-plane X-ray rocking curves were improved to 30-50 and 200-250 arcsec, respectively.
This work was partially supported by JSPS KAKENHI Grant Numbers 15H03556 and 16H06415, and JST CREST No. 16815710, JST SICORP InRel-NPower – EU H2020 No. 720527, and JST SICORP with MOST in China.

FI-2:IL08  Simultaneous Tenfold Brightness Enhancement and Emitted-light Spectral Tunability in Transparent Ambipolar Organic Light-emitting Transistor by Integration of High-k Photonic Crystal
S. TOFFANIN, Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), Consiglio Nazionale delle Ricerche (CNR), Bologna, Italy

In organic light-emitting transistors (OLETs), the structural properties as the in-plane geometry and the lateral charge injection are the key elements that enable the monolithic integration of multiple electronic, optoelectronic, and photonic functions within the same device. In this invited lecture, we report on the realization of highly integrated multifunctional optoelectronic organic device by introducing high-capacitance photonic crystal as gate dielectric into a transparent single-layer ambipolar OLET. By engineering the photonic crystal multistack and bandgap, we show that the integration of the photonic structure has a twofold effect on the optoelectronic performance of the OLET, i.e., i) to modulate the spectral profile and outcoupling of the emitted light and ii) to enhance the transistor source-drain current by a 25-fold factor. Thus, the photonic-crystal-integrated OLET shows an order of magnitude higher emitted power and brightness with respect to the corresponding polymer-dielectric device, while presenting as-designed light spectral and spatial distribution. The results validate the efficacy of this approach that is expected to unravel the technological potential for the realization of highly integrated optoelectronic smart systems based on OLETs.

FI-2:IL09  THz-QCLs toward High Output Power near Liquid Nitrogen Temperature Operation
TSUNG-TSE LIN, HIDEKI HIRAYAMA, Center for Advanced Photonics, RIKEN, Sendai, Japan

QCLs are promising large output power semiconductor based THz sources with narrow bandwidths and wide operating frequency. However, for utilization of the real THz applications by QCLs, the performances of output power and operation temperature are both required at the same time. Here we consider a balance between operation temperature and the required output power near nitrogen temperature operation with relative compact cryogenic system. We analysis and design the temperature dependent operation of large output THz-QCLs active region by Non-Equilibrium Green's Function (NEGF) method and device fabrication. Our latest THz-QCLs recently demonstrated 350 mW peak power and 3 mW average power at 10 K, and gave the 50 mW peak power with 0.4 mW average power at 78 K. It demonstrate the QCLs as a relative compact portable sub-mW order average power semiconductor THz source unit with 78 K Dewar condenser.

FI-2:L10  Omni-friendly Low Color Temperature OLED
JWO-HUEI JOU, M. SINGH, H.-F. LIN, Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan

Improper lighting might cause damage to human eyes, suppress melatonin secretion, disrupt circadian rhythm and ecosystems, discolor artifacts, and pollute the night skies. Blue light including short wavelength emissions have been identified to be the major culprit. We will present herein a complete action spectrum of melatonin suppression, first developed by Jou, and the well established blue light hazard (or photoretinitis) function on a per lumen basis, and demonstrate how to quantify the threat of any given light on health using the corresponding indices, melatonin suppression sensitivity (MSS) and maximum permissible exposure-limit (MPE). Both functions show a comparatively safest light to be located between 600 and 620 nm. We will also present how an omni-friendly lighting source be made of candlelight OLED, which exhibits high light quality, high MPE, high power efficiency and low MSS. The blue light-less, low color temperature candlelight OLED attracts much less insects after dusk, proving which to be ecologically-friendly as well.

Session FI-3 - Electro-optical-structural Characterization

FI-3:IL01  Light-emitting Electrochemical Cells: Towards Low-cost Fabrication and High-efficiency Operation
L. EDMAN, The Organic Photonics and Electronics Group, Umeå University, Umeå, Sweden

The light-emitting electrochemical cell (LEC) is a surface-emitting device, which for the end user appears similar to the well-established organic light-emitting diode. One distinguishing feature is, however, that the active material of an LEC is electrochemically doped in-situ, so that a p-n junction doping structure can form during the initial operation. This in-situ transformation paves the way for the employment of air-stabile compounds for all of the constituent device materials (including the cathode) and for the utilization of a relatively uneven and single-layered active material. These are important characteristics, as they pave the way for a low-cost solution-based fabrication process under ambient air. Here, we present LEC devices fabricated with slot-die coating, spray-coating, and inkjet printing, and we also demonstrate such devices fabricated directly on various complex surfaces, such as copy paper, textile, and a stainless-steel fork. We further present systematic studies on the LEC operation, which has resulted in relatively good operational stability (>5000 h at >100 cd/m2 for an encapsulated LEC operating in the ambient) as well as high-efficiency operation at high luminance (27.5 % EQE at >1000 cd/m2 for a device equipped with outcoupling).

FI-3:L02  Charge Injection Investigation at the Interface between Metal Contact and Active Layer in Organic Field-effect Transistors

In order to improve the performance of organic light-emitting transistors is necessary to carefully engineer the entire device architecture. Here, we focalize our attention to the gold electrode/organic interface. It is well known that permanent dipoles, trapped states, bond interactions can affect the injection process so that the structural nature of the interface needs to be understood. We implemented as active materials a quaterthiophene derivative (NT4N) and a perylene derivative (P13). The molecular structures are similar but P13 has no sulfur atoms in the molecular core. Given that, in nanometer scale, chemical bond between metallic atom and conjugated molecular systems is possible, we investigated the structural interface formation. To unravel the real energy barriers that charge carriers have to overcome when injected into an organic layer, we build the energetic diagram taking into account the molecular nature of organic materials and the nanostructured gold electrodes. By means X-ray absorption spectroscopy we were able to inspect locally the environment of electrodes on the organic active layer during devices in real time operation. We demonstrate the existence of chemical bounds between the sulfur atoms of the NT4N small molecule and the nanostructured gold electrode.

FI-3:IL04  Inorganic Perovskite Crystals for Fast Color-conversion Applications
N. LAURAND, Institute of Photonics, Dept. of Physics, SUPA, University of Strathclyde, Glasgow, UK

Emerging applications of color-converted InGaN micro-sources for solid-state lighting include scientific instrumentation and visible light communications (VLC). A color-converted source typically consists of a blue or UV InGaN device integrated with a downconverting material. The latter absorbs photons from the InGaN device and re-emit at longer wavelengths for the production of different colors or even for the generation of white light. In these applications, conversion efficiency and color purity are important but so are the dynamics of the color conversion process. Inorganic perovskite materials, including perovskite nanocrystals, are promising for these because of their narrow emission linewidth and shorter luminescence lifetime compared to alternative color converters. Despite rapid developments, there is a need to better understand how their optical properties affect the performance of color-converted devices. Herein, we will discuss inorganic perovskite crystals for ‘fast’ color conversion of InGaN microsize light-emitting diodes (μLEDs) and laser diodes. In particular, we will discuss their optical frequency responses, which are critical for VLC, and how these might depend on the excitation density. Results of free-space VLC will be shown.

Session FI-4 - Device Architectures and System Integration

FI-4:IL01  Integration and Process Technology for Flexible OLED Lighting Systems
C. MAY, Fraunhofer Institute for Organic Electronics, Electron Beam and Plasma Technology FEP, Dresden, Germany

Beside to display technology, OLED also has unlikely potential for lighting applications. Where extremely thin, flexible or transparent light sources are desired, OLED can exploit its potential as a supplement to the LED. The improvement of flexible OLED is currently the focus of worldwide research activities. Thin substrates of primarily barrier-coated polymer films, but also metal foils or ultra-thin glass, were tested for use in OLED production and have advantages and disadvantages. OLED prototypes on polymer substrates as well as ultra-bright glass were realized by Fraunhofer FEP. The main development areas are the improvement of device stability and brightness. This includes intensive work on encapsulation as well as the development of stacked OLED architectures on flexible substrates. In future, the focus will be on the development of integration solutions in functional devices and surfaces using established technologies. Fraunhofer FEP works on processes along the entire value chain to produce flexible OLEDs in both, sheet-to-sheet and roll-to-roll processes. The requirements for the materials and technologies as well as the associated challenges will be introduced using latest results by Fraunhofer FEP. Solutions and application possibilities will be presented.

FI-4:IL02  Small Area Light Module Application, Simulation Modelling and Optimization for Architectural Lighting
N. TRIVELLIN, LightCube SRL and University of Padova, Padova, Italy; S. VENK, OSRAM SPA, Treviso, Italy

Architectural lighting is a critical field where technological aspects of Solid State Lighting are most important and exploited. With this work we report how the newest technological advancement in LED technology allow the development of new and innovative architectural lighting solutions. The run for efficiency, cost and miniaturization has differentiated the LED devices in different categories: we will present how the different state of the art LEDs compares with the applications, and in particular how the material composing the LEDs: polymers, ceramics, rare earth materials are related to their performances and reliability. We will also present different technical solutions to achieve optical control of light, in particular in applications where the glare reduction is critical. UGR control and surface uniformity of light source are often inversely related characteristics, but the use of innovative optical solutions allow for possible solutions conjugating eye protection and aesthetical appearance. Human centric lighting is becoming a major trend in architecture, its impact on circadian rhythm will be discussed and technological approaches to achieve tunable white light sources are analyzed.

FI-4:IL03  Micro-Transfer Printing for Display Applications and Interactive Solid State Lighting
A.J. TRINDADE2, E. RADAUSCHER1, S. BONAFEDE1, D. GOMEZ1, T. MOORE1, C. PREVATTE1, B. RAYMOND1, A. FECIORU2, K. GHOSAL1, M. MEITL1, C. BOWER11X-Celeprint Inc., USA; 2X-Celeprint Ltd., Cork, Ireland

Inorganic light-emitting diodes (iLEDs) are amongst the longest lived and most energy efficient light-emitters available being used to date on a wide range of applications. Emissive displays using these types of light sources have been around as an established technology (known as ‘Jumbotron’ displays) with millimeter to centimeter-sized pixels. There is a growing interest in the miniaturization of such displays with tightly packed pixels (>30 per inch) in a variety of substrates. The presented work introduces micro-transfer-printing as an innovative and disruptive technology to create the next generation of displays. With control circuits, light emitters, sensors being usually sourced from different wafer-based material systems (Si, GaN, InP, GaAs…), this introduces added complications for integration onto display panels. By use of an elastomer stamp to release and transfer the devices arrays onto non-native substrates, µTP has a proven record in iLED displays, magnetic storage, Silicon Photonics, Photovoltaics and compound semiconductor integration. Here we present small-scale display prototypes using miniaturized iLEDs as fully functional pixel emitters which can be scaled-up to fulfill large area displays and/or innovative solid state lighting applications.

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