Routes for efficiency enhancement in fluorescent TADF exciplex host OLEDs gained from an electro‐optical device model

Fluorescence-based organic light-emitting diodes (OLEDs) using thermally activated delayed fluorescence (TADF) have increasingly attracted attention in research and industry. One method to implement TADF is based on an emitter layer composed of an exciplex host and a fluorescent dopant. Even though the experimental realization of this concept has demonstrated promising external quantum efficiencies, the full potential of this approach has not yet been assessed. To this end, a comprehensive electro-optical device model accounting for the full exciton dynamics including triplet harvesting and exciton quenching is presented. The model parameters are fitted to multiple output characteristics of an OLED comprising a TADF exciplex host with a fluorescent emitter, showing an external quantum efficiency of >10%. With the model at hand, an emission zone analysis and a parameter study are performed, and possible routes for further efficiency enhancement are presented

Integrated optical model for organic light-emitting devices

One of the most important parameters of organic light-emitting devices (OLEDs) in their application for illumination or displays is their efficiency. In order to maximize the efficiency, one needs to understand all loss mechanisms and effects present in these devices and properly model them. For that purpose, we introduce an integrated model for light emission from OLEDs. The model takes into account the exciton decay time change and light outcoupling. Furthermore, it shows how to calculate the external quantum efficiency, the spectral radiance and the luminous current efficacy of OLEDs. The overall theory is experimentally verified through a range of measurements done on a set of green OLED samples with an Ir-based phosphorescent emitter. From the analysis of simulations and experiments one can estimate the charge balance in the OLED stack and the radiative efficiency of the emitter. © 2011 American Institute of Physics

Highly efficient multilayer organic pure-blue-light emitting diodes with substituted carbazoles compounds in the emitting layer

Bright blue organic light-emitting diodes (OLEDs) based on 1,4,5,8,N-pentamethylcarbazole (PMC) and on dimer of N-ethylcarbazole (N,N'-diethyl-3,3'-bicarbazyl) (DEC) as emitting layers or as dopants in a 4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl (DPVBi) matrix are described. Pure blue-light with the C.I.E. coordinates x = 0.153 y = 0.100, electroluminescence efficiency \eta_{EL} of 0.4 cd/A, external quantum efficiency \eta_{ext.} of 0.6% and luminance L of 236 cd/m2 (at 60 mA/cm2) were obtained with PMC as an emitter and the 2,9-dimethyl-4,7-diphenyl-1,10-phenantroline (BCP) as a hole-blocking material in five-layer emitting devices. The highest efficiencies \eta_{EL.} of 4.7 cd/A, and \eta_{ext} = 3.3% were obtained with a four-layer structure and a DPVBi DEC-doped active layer (CIE coordinates x = 0.158, y=0.169, \lambda_{peak} = 456 nm). The \eta_{ext.} value is one the highest reported at this wavelength for blue OLEDs and is related to an internal quantum efficiency up to 20%

Synthesis and optoelectronic properties of new ethynylated pyrazine derivatives

Several diaryleneethynylpyrazine derivatives, in which the pyrazine unit is electron-deficient, have been synthesised using Sonogashira palladium-catalysed cross-coupling reactions. Compound 32, an important intermediate in the synthesis of diaryleneethynylpyrazine derivatives was made by a modified literature procedure which improved the yield. Examination of optical absorption and photoluminescence spectra of compound 37 shows that the pyrazine unit does not change the behaviour significantly compared to analogue 42, while compound 38 shows pyridine substituents have a profound effect on the photophysics of these pyrazine systems. The redox properties of representative compound 37 were studied by cyclic voltammetry, which shows that reduction of 37 to its radical anion occurs as a reversible process at high negative potentials of ca. -1.87 V. The X-ray crystal structure of 37 is also presented. Quantum mechanical calculations of the geometry and electronic structure were performed for compound 37; the known phenylene analogue 42 was calculated at the same level for comparison. The results show that the energies of both HOMO and LUMO orbitals of 37 are decreased compared to 42. The calculated value of the HOMO-LUMO gap of 37 (3.56 eV) is close to that estimated from the red edge of the longest wavelength absorption (382 nm = 3.25 eV)

A modular and interactive OLED-based lighting system

The concept of a flexible, large-area, organic light emitting diode (OLED)-based lighting system with a modular structure and built-in intelligent light management is introduced. Such a flexible, thin, portable lighting system with discreetly integrated electronics is important in order to allow the implementation of the lighting system into a variety of places, such as cars and temporary expedition areas. A modular construction of an OLED lighting panel makes it possible to control each OLED cell individually. This not only enables us to counteract aging or degradation effects in the OLED cells but it also allows individual OLED module brightness control to support human or ambient interaction based on integrated or centralized sensors. Moreover, integrating the driving electronics in the backplane of an OLED module improves the energy efficiency of operating large OLED panels. The thin, modular construction and individual, dynamic control are successfully demonstrated

Benzobisoxazole cruciforms: a tunable, cross-conjugated platform for the generation of deep blue OLED materials

Four new cross-conjugated small molecules based on a central benzo[1,2-d:4,5-d′]bisoxazole moiety possessing semi-independently tunable HOMO and LUMO levels were synthesized and the properties of these materials were evaluated experimentally and theoretically. The molecules were thermally stable with 5% weight loss occurring well above 350 °C. The cruciforms all exhibited blue emission in solution ranging from 433–450 nm. Host–guest OLEDs fabricated from various concentrations of these materials using the small molecule host 4,4′-bis(9-carbazolyl)-biphenyl (CBP) exhibited deep blue-emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.15 ≤ x ≤ 0.17, 0.05 ≤ y ≤ 0.11), and maximum luminance efficiencies as high as ∼2 cd A−1. These results demonstrate the potential of benzobisoxazole cruciforms as emitters for developing high-performance deep blue OLEDs.We would like to thank Dr Sarah Cady, Dr Kamel Harrata and Mr Steven Veysey of Iowa State University (ISU) Chemical Instrumentation Facility for compound analysis. We thank Eeshita Manna for technical assistance. We also thank the National Science Foundation (CHE-1413173) for financial support of this work. RK and JS were partially supported by Basic Energy Sciences, Division of Materials Science and Engineering, USDOE. Ames Laboratory is operated by Iowa State University for the US Department of Energy (USDOE) under Contract No. DE-AC 02-07CH11358. Computational resources were provided in part by the MERCURY consortium (http://mercuryconsortium.org/) under NSF grants CHE-0116435, CHE-0521063, CHE-0849677, and CHE-1229354. (CHE-1413173 - National Science Foundation; Basic Energy Sciences, Division of Materials Science and Engineering, USDOE; DE-AC 02-07CH11358 - Iowa State University for the US Department of Energy (USDOE); CHE-0116435 - MERCURY consortium under NSF; CHE-0521063 - MERCURY consortium under NSF; CHE-0849677 - MERCURY consortium under NSF; CHE-1229354 - MERCURY consortium under NSF)http://pubs.rsc.org/en/Content/ArticleLanding/2016/TC/C5TC03622D#!divAbstractPublished versio

Solution-Processable Graphene Oxide as an Efficient Hole Injection Layer for High Luminance Organic Light-Emitting Diodes

The application of solution-processable graphene oxide (GO) as hole injection layer in organic light-emitting diodes (OLEDs) is demonstrated. High luminance of over 53,000 cd m-2 is obtained at only 10 V. The results will unlock a route of applying GO in flexible OLEDs and other electrode applications.Comment: 14 pages, 3 figures, 1 tabl