Extrafluorescent electroluminescence in organic light-emitting devices

Segal, Michael; Singh, Madhusudan; Rivoire, Kelley; Difley, Seth; Voorhis, Troy Van; Baldo, Marc A.

doi:10.1038/nmat1885

Cited by 191 publications

(119 citation statements)

References 23 publications

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“…Another possible mechanism of one-to-one conversion of triplets into singlets was proposed by Baldo and co-workers [18]. Rather than employing molecules with a small energy gap between the lowest molecular excited states of triplet and singlet multiplicity, this approach exploited the possibility of an intersystem crossing step prior to the formation of molecular excited states.…”

Section: Evolution Of Efficiencies Of Fluorescent Organic Light-emittmentioning

confidence: 99%

Triplet–triplet annihilation in highly efficient fluorescent organic light-emitting diodes: current state and future outlook

Kondakov

2015

Phil. Trans. R. Soc. A.

128

119

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Studies of delayed electroluminescence in highly efficient fluorescent organic light-emitting diodes (OLEDs) of many dissimilar architectures indicate that the triplet–triplet annihilation (TTA) significantly increases yield of excited singlet states—emitting molecules in this type of device thereby contributes substantially to their efficiency. Towards the end of the 2000s, the essential role of TTA in realizing highly efficient fluorescent devices was widely recognized. Analysis of a diverse set of fluorescent OLEDs shows that high efficiencies are often cor-related to TTA extents. It is therefore likely that it is the long-term empirical optimization of OLED efficiencies that has resulted in fortuitous emergence of TTA as a large and ubiquitous contributor to efficiency. TTA contributions as high as 20–30% are common in the state-of-the-art OLEDs, and even become dominant in special cases, where TTA is shown to substantially exceed the spin-statistical limit. The fundamental features of OLED efficiency enhancement via TTA—molecular structure-dependent contributions, current density-dependent intensities in practical devices and frequently observed antagonistic relationships between TTA extent and OLED lifetime—came to be understood over the course of the next few years. More recently, however, there was much less reported progress with respect to all-important quantitative details of the TTA mechanism. It should be emphasized that, to this day and despite the decades of work on improving blue phosphorescent OLEDs as well as the recent advent of thermally activated delayed fluorescence OLEDs, the majority of practical blue OLEDs still rely on TTA. Considering such practical importance of fluorescent blue OLEDs, the design of blue OLED-compatible materials capable of substantially exceeding the spin-statistical limit in TTA, elimination of the antagonistic relationship between TTA-related efficiency gains and lifetime losses, and designing devices with an extended range of current densities producing near-maximum TTA electroluminescence are the areas where future improvements would be most beneficial.

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Section: Evolution Of Efficiencies Of Fluorescent Organic Light-emittmentioning

confidence: 99%

Triplet–triplet annihilation in highly efficient fluorescent organic light-emitting diodes: current state and future outlook

Kondakov

2015

Phil. Trans. R. Soc. A.

128

119

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“…However, this limit only arises if the recombination process is independent of spin. If at any stage the recombination is effected directly or indirectly by the spin configuration of the intermediate CT states [53] or the polaron capture is affected by their spins [54], then it can follow that the 25% limit is broken and possibly more singlets could be produced [21].…”

Section: Exciton Formation In An Oledmentioning

confidence: 99%

Singlet Generation from Triplet Excitons in Fluorescent Organic Light-Emitting Diodes

Monkman

2013

ISRN Materials Science

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A potential major drawback with organic light-emitting devices, (OLEDs) is the limit of 25% singlet exciton production through spin-dependent charge recombination. Recent device results, however, show that this limit does not hold and far higher efficiencies can be achieved in purely fluorescent-based systems (Wohlgenannt et al. (2012)). Here, the various routes by which triplet excitons can generate singlet states are discussed and their relative contributions to the overall electroluminescence yield are given. The materials requirements to obtain maximum singlet production from triplet states are discussed. These triplet contributions can give very high device yields for fluorescent emitters, which in the case of blue devices can be highly advantageous. Further, new devices architectures open up which are simple and have intrinsically low turn on voltages, ideal for large-area OLED lighting applications.

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“…As a consequence, more delocalized charge transfer states such as polaron-pairs show a reduced singlet-triplet splitting. [37,38] While in conjugated molecules such states are usually ascribed to intermolecular excitations, [26,38,39] in organometallic complexes they can have an intramolecular nature, due to the peculiar transitions involving the d and p orbitals of the metal and the ligands, respectively. The degree of the d metal orbital's participation in the transition is usually referred to as the metal to ligand charge transfer (MLCT) character.…”

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confidence: 99%

Controlling the Radiative Rate of Deep‐Blue Electrophosphorescent Organometallic Complexes by Singlet‐Triplet Gap Engineering

et al. 2008

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Recently, solid-state lighting has received considerable attention in both academic and industrial research. [1,2] Of particular interest, for the replacement of the existing light sources, are organic light-emitting diodes (OLEDs) based on phosphorescent molecules. [3][4][5][6] The advantage of using these materials lies in the possibility to internally convert all the spin uncorrelated injected charges into light. Indeed, an internal quantum efficiency of nearly 100% has been achieved in devices based on the green-emitting organometallic complex Ir(ppy) 3 .[7]However, many unresolved issues are the subject of current research in order to implement efficient white light sources and expand the number of applications. In particular, the origin of the efficiency roll-off at high voltages, [8][9][10] the light outcoupling, [11,12] the long-term stability [13,14] and the generation of white light with an all-phosphor device [6,15] are subjects under intense investigation. White light generation is a key issue because of the wide range of applications involving full-color displays and lighting. [1,2] Among the different approaches, solution processed devices based on white light emitting molecules [16] have been demonstrated as well as thermally evaporated red, green and blue (RGB) blends [15] or stacks. To date, white light OLEDs (WOLEDs) with long term operational lifetimes have been obtained mainly with a combination of a blue fluorescent emitter [6] and phosphors for the other colors. Such an elegant approach relies on a well engineered harvesting of singlet and triplet excitons and requires therefore a precise doping of the RGB emitting dyes in the transporting hosts. In contrast, efficient WOLEDs based on blue phosphors can be obtained with all the emitters in one single layer, [17] simplifying the processing. Generally, however, blue phosphors have in the past turned out to be rather unstable. While a physical explanation for blue phosphor based device instability is still lacking, a shorter phosphorescence lifetime, eventually approaching the sub-microsecond time regime, would decrease the residence time of potentially unstable excited states. Moreover, processes detrimental to the efficiency, such as exciton charge-carrier quenching [8] or triplet-triplet annihilation, [9,10] could be strongly reduced with a faster exciton recombination. A shorter phosphorescence lifetime while maintaining high quantum efficiencies requires a large radiative rate. For organometallic complexes this rate is directly proportional to the spin-orbit coupling (SOC) matrix element involving the emitting triplet and the perturbing singlet state and inversely proportional to the degree of mixing between them, i.e., the singlet-triplet splitting (DE ST ). [18][19][20] Photophysical studies of the role of SOC and DE ST in tuning the radiative rate are still sparse, mainly because the large intersystem crossing (ISC) rates ($10 13 s À1 ) of such phosphors, [21] which makes detection (and therefore direct measurement of DE ST ) rather cha...

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Extrafluorescent electroluminescence in organic light-emitting devices

Cited by 191 publications

References 23 publications

Triplet–triplet annihilation in highly efficient fluorescent organic light-emitting diodes: current state and future outlook

Triplet–triplet annihilation in highly efficient fluorescent organic light-emitting diodes: current state and future outlook

Singlet Generation from Triplet Excitons in Fluorescent Organic Light-Emitting Diodes

Controlling the Radiative Rate of Deep‐Blue Electrophosphorescent Organometallic Complexes by Singlet‐Triplet Gap Engineering

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