Triplet annihilation exceeding spin statistical limit in highly efficient fluorescent organic light-emitting diodes
TL;DR: In this article, it was shown that the triplet-triplet annihilation (TTA) contribution in combination with the remarkably high total efficiencies [>11% external quantum efficiency (EQE)] indicates that the absolute amount of electroluminescence attributable to TTA substantially exceeds the limit imposed by spin statistics.
Abstract: We have demonstrated that the exemplary red fluorescent organic light-emitting diodes (OLEDs) gain as much as half of their electroluminescence from annihilation of triplet states generated by recombining charge carriers. The magnitude of triplet-triplet annihilation (TTA) contribution in combination with the remarkably high total efficiencies [>11% external quantum efficiency (EQE)] indicates that the absolute amount of electroluminescence attributable to TTA substantially exceeds the limit imposed by spin statistics, which was independently confirmed by studying magnetic field effects on delayed luminescence. We determined the value of 1.3 for the ratio of the rate constants of singlet and triplet channels of annihilation, which is indeed substantially higher than the value of 0.33 expected for a purely statistical annihilation process. It is, however, in an excellent quantitative agreement with the extent of the experimental contribution of delayed luminescence to steady-state electroluminescence. The ...
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TL;DR: In this article, a blue organic light-emitting diodes that harness thermally activated delayed fluorescence was realized with an external quantum efficiency of 19.5% and reduced roll-off at high luminance.
Abstract: Blue organic light-emitting diodes that harness thermally activated delayed fluorescence are realized with an external quantum efficiency of 19.5% and reduced roll-off at high luminance.
1,909 citations
TL;DR: An overview of the quick development in TADF mechanisms, materials, and applications is presented, with a particular emphasis on their different types of metal-organic complexes, D-A molecules, and fullerenes.
Abstract: The design and characterization of thermally activated delayed fluorescence (TADF) materials for optoelectronic applications represents an active area of recent research in organoelectronics. Noble metal-free TADF molecules offer unique optical and electronic properties arising from the efficient transition and interconversion between the lowest singlet (S1) and triplet (T1) excited states. Their ability to harvest triplet excitons for fluorescence through facilitated reverse intersystem crossing (T1→S1) could directly impact their properties and performances, which is attractive for a wide variety of low-cost optoelectronic devices. TADF-based organic light-emitting diodes, oxygen, and temperature sensors show significantly upgraded device performances that are comparable to the ones of traditional rare-metal complexes. Here we present an overview of the quick development in TADF mechanisms, materials, and applications. Fundamental principles on design strategies of TADF materials and the common relationship between the molecular structures and optoelectronic properties for diverse research topics and a survey of recent progress in the development of TADF materials, with a particular emphasis on their different types of metal-organic complexes, D-A molecules, and fullerenes, are highlighted. The success in the breakthrough of the theoretical and technical challenges that arise in developing high-performance TADF materials may pave the way to shape the future of organoelectronics.
1,473 citations
TL;DR: In this article, high efficiency fluorescent organic light-emitting diodes have been realized by employing custom-designed molecules that make it possible to convert non-radiative triplet states into radiative singlet states.
Abstract: High-efficiency fluorescent organic light-emitting diodes have been realized by employing custom-designed molecules that make it possible to convert non-radiative triplet states into radiative singlet states.
1,270 citations
TL;DR: It is shown that a large delocalization of the highest occupied molecular orbital and lowest unoccupied molecular orbital in these charge-transfer compounds enhances the rate of radiative decay considerably by inducing a large oscillator strength even when there is a small overlap between the two wavefunctions.
Abstract: Organic compounds that exhibit highly efficient, stable blue emission are required to realize inexpensive organic light-emitting diodes for future displays and lighting applications. Here, we define the design rules for increasing the electroluminescence efficiency of blue-emitting organic molecules that exhibit thermally activated delayed fluorescence. We show that a large delocalization of the highest occupied molecular orbital and lowest unoccupied molecular orbital in these charge-transfer compounds enhances the rate of radiative decay considerably by inducing a large oscillator strength even when there is a small overlap between the two wavefunctions. A compound based on our design principles exhibited a high rate of fluorescence decay and efficient up-conversion of triplet excitons into singlet excited states, leading to both photoluminescence and internal electroluminescence quantum yields of nearly 100%.
1,007 citations
TL;DR: In this article, a material possessing a very small energy gap between its singlet and triplet excited states, ΔE1−3, which allows efficient up-conversion of triplet excitons into a singlet state and leads to efficient thermally activated delayed fluorescence (TADF), is reported.
Abstract: A material possessing a very small energy gap between its singlet and triplet excited states, ΔE1−3, which allows efficient up-conversion of triplet excitons into a singlet state and leads to efficient thermally activated delayed fluorescence (TADF), is reported. The compound, 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)-1,3,5-triazine, breaks the restriction of a large energy gap, with a ΔE1−3 of just 0.11 eV, while maintaining a high fluorescent radiative decay rate (kr∼107). The intense TADF provides a pathway for highly efficient electroluminescence.
906 citations
References
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Book•
01 Jan 1973
TL;DR: Hammett et al. as discussed by the authors proposed a triplet-state diffusion-controlled triplet state diffusion-controlled rate for photolysis of organic molecules in solution, where triplets are derived from triplets of the triplets.
Abstract: . Photophysics of Organic Molecules in Solution Triplet-State Energies: Ordered Flash Photolysis: Designing Experiments Low Temperature Photophysics of Organic Molecules Ground-State Absorption Spectra ESR and ODMR Parameters of the Triplet State Diffusion-Controlled Rate Constants Rate Constants of Singlet-State Quenching Rate Constants of Triplet-State Quenching Ionization Energies, Electron Affinities, and Redox Potentials of Organic Compounds Bond Dissociation Energies Solvent Properties Chemical Actinometry Transmission Characteristics of Light Filters and Glasses Spectral Distribution of Photochemical Sources Spin-Orbit Coupling Fundamental Constants and Conversion Factors Hammett (T Constants Bibliography and Indexes References Compound Name Index Molecular Formula Index
2,513 citations
Book•
02 Dec 1999
TL;DR: The first edition of the Electronic Processes of Organic Crystals, published in 1982, became the classic reference in the field and has been expanded to cover the major theoretical and experimental advances over the last fifteen years.
Abstract: The first edition of Pope and Swenberg's Electronic Processes of Organic Crystals, published in 1982, became the classic reference in the field. It provides a tutorial on the experimental and related theoretical properties of aromatic hydrocarbon crystals and includes emerging work on polymers and superconductivity. This new edition has been expanded to cover the major theoretical and experimental advances over the last fifteen years. It contains a unified description of what is known in almost every aspect of the field. The basic phenomena covered in the first edition included fluorescence, exciton and charge carrier generation, transport, recombination, and photoemission; the new edition adds solitons, polarons, bipolarons, spin waves, and charge density waves. It provides in-depth coverage of such model polymers such as polyacetylene, polydiacetylene, poly (phenylene-vinylene), polyanilines, polysilanes, and fullerenes. It also provides detailed treatments of the expanding areas of electroluminescence, non-linear optics, organic magnets, organic superconductors, and Langmuir-Blodgett films. In addition, it contains a chapter on major applications, including LED's, photocopiers, photoconductors, batteries, transistors, liquid crystals, photorefractive devices, and sensors. As in the first volume, the authors take informed positions in controversial areas. This book will be an essential reference for organic material scientists, whether they are experienced researchers or just entering the field. It will also be a reliable guide to anyone interested in this rapidly growing field
1,733 citations
TL;DR: In this article, a multilayer device structure consisting of a hole-injecting layer, a hole transport layer, and an electron transport layer was employed in order to reduce the driving voltage as well as to maximize carrier recombination efficiency.
Abstract: Efficient organic electroluminescent (EL) devices were fabricated by using a highly fluorescent aluminum complex, tris(4-methyl-8-quinolinolato)aluminum(III) (Almq3), as an emitter layer. In addition to using this complex, a multilayer device structure, consisting of a hole-injecting layer, a hole transport layer, a dye-doped Almq3 emitting layer, and an electron transport layer, was employed in order to reduce the driving voltage as well as to maximize carrier recombination efficiency. From this device, a maximum luminance of over 140 000 cd/m2 and an external quantum efficiency of 7.1% was observed, which is the highest efficiency ever reported for organic EL devices.
352 citations
TL;DR: In this paper, a detailed measurement of the magnetic field dependence of the rate of mutual annihilation of triplet excitons in anthracene crystals at room temperature is presented, and the amplitude of the field dependence is highly anisotropic.
Abstract: Detailed measurements of the magnetic field dependence of the rate of mutual annihilation of triplet excitons in anthracene crystals at room temperature are presented. The field dependence consists of an increase at low fields, with a maximum at ca. 350 Oe, followed by a decrease at higher fields to less than the zero-field annihilation rate. For most field directions, a second maximum occurs at ca. 600 Oe. The amplitude of the field dependence is highly anisotropic. For fields g2000 Oe, resonances in the annihilation rate are found at +76\ifmmode^\circ\else\textdegree\fi{} and -17\ifmmode^\circ\else\textdegree\fi{} with respect to the $a$ axis for fields in the $\mathrm{ac}$ plane and at \ifmmode\pm\else\textpm\fi{}23.5\ifmmode^\circ\else\textdegree\fi{} with respect to the $b$ axis in the $\mathrm{ab}$ plane. For fields 500 Oe, a second set of resonances occurs at directions bisecting the high-field resonances and at intermediate fields the two sets coexist. These results are discussed in terms of a density-matrix description of the spin states of the interacting triplet pair and of the annihilation process. The field dependence is accounted for on the basis of the field dependence of the pair spin states together with the postulate that annihilation is spin-allowed. The observed resonances result from level crossings among the pair spin states. All of the structure in the field dependence and anisotropy data is satisfactorily reproduced by calculations based on the model, although complete quantitative agreement is not achieved.
312 citations
TL;DR: In this paper, the authors find that the decreasing luminance efficiency is linearly correlated with an accumulation of immobile positive charge at the HTL|ETL interface, and the magnitude of the charge is comparable to the total charge at that interface when an unaged device is operated.
Abstract: Organic light-emitting diodes (OLEDs) are attractive for display applications because of their high brightness, low driving voltage, and tunable color. Their operating lifetimes, hundreds or thousands of hours, are sufficient for only a limited range of applications. The luminance efficiency decreases gradually as the device is operated (electrically aged), for reasons that are poorly understood. A prototypical OLED has the structure anode|HTL|ETL|cathode, where the HTL and ETL are hole- and electron-transporting layers, and the recombination and emission occur at or near the HTL|ETL interface. We find that the decreasing luminance efficiency is linearly correlated with an accumulation of immobile positive charge at the HTL|ETL interface, and the magnitude of the charge is comparable to the total charge at that interface when an unaged device is operated. A natural explanation of the connection between the two phenomena is that electrical aging either generates hole traps (and trapped holes) or drives met...
273 citations