Materials for Suppressed Triplet‐Polaron Quenching in Efficient and Long‐Lifetime Phosphorescent OLEDs

The quantum efficiency of phosphorescent organic light‐emitting diodes (OLEDs) shows a decrease with increasing current density and luminance (“roll‐off”). A major contribution to the roll‐off is triplet‐polaron quenching (TPQ), upon which a triplet exciton on an emitter molecule (“donor”) is lost after exciting an electron or hole polaron on another molecule (“acceptor”), followed by non‐radiative decay. The microscopic mechanism is not well understood. Within a Förster‐type dipole‐dipole interaction model, the TPQ rate is determined by the overlap of the donor photoluminescence spectrum and the absorption spectrum of the positively or negatively charged acceptor molecules. In this work, a spectroelectrochemical method is demonstrated for measuring the absorption spectra of charged molecules, so that the interaction rate, expressed in terms of the TPQ Förster radius, can be determined. Typical values of these radii are found to be in the range of 2.5−4 nm. For often used OLED designs, in which emitter molecules (“guests”) are embedded at a small concentration in a matrix material (“host”), this study enables obtaining in a rational manner optimal host‐guest combinations. The methodology is also expected to provide an experimental benchmark for emerging open‐shell quantum‐chemical methods for quantifying TPQ.

Triplet-polaron quenching (TPQ) is a major cause of the efficiency loss at large current densities in phosphorescent organic light-emitting diodes. The nature of the interaction process is presently not well understood. In this paper, we study TPQ due to Förster-type triplet-polaron interactions in energetically disordered organic semiconductors with a Gaussian polaron density of states. A continuum theory, which neglects the spatial inhomogeneity and energetic disorder, is from a kinetic Monte Carlo approach shown to correctly predict that the effective steady-state TPQ rate coefficient kTPQ,eff depends only sensitively on the polaron diffusion in a rather narrow range of diffusion coefficients. However, in this regime, significant discrepancies between the two approaches are found, in particular for realistic values of the TPQ-Förster radius, around 3 nm, and for systems with strong energetic disorder. Both approaches show that kTPQ,eff is not constant but can depend on the polaron density and the electric field. Various methods for deducing the TPQ mechanism from experiment are discussed, including an approach which utilizes the shape of the time-dependent photoluminescence after pulsed illumination.

Hydrocarbon oligomers X9F, including S9F, D9F, and T9F as monomer, dimer, and trimer, respectively, were designed and prepared on the basis of indirect linkage and 9,9-diphenylfluorene (S9F) as repeat unit to form planar, linear, and V-shaped configurations without polarity variation and function amplification. The identical optical and electrochemical properties of X9F were achieved because of the effectively blocked intramolecualr electronic interactions by indirect linkage, including the same T1 value of 2.98 eV, high enough for hosts in blue phosphorescent organic light-emitting diodes (PHOLEDs), and the approximate FMO energy levels, which established the basis for selective investigation of independent configuration effect on the optoelectronic performance of host materials. Density function theory simulation manifested the frontier molecular orbital (FMO) location extension after oligomerization and the specific T1 locations on peripheral fluorenyls in X9F, giving rise to their different carrier-transporting abilities and host-localized triplet–triplet annihilation (TTA) and triplet–polaron quenching (TPQ) effects. As a result, D9F with the linear and locally unsymmetrical configuration revealed electron-predominant characteristics for charge balance, restrained triplet interaction for TTA suppression, and partially separated FMO and T1 locations for TPQ suppression. Consequently, the low driving voltages and the favorable maximum efficiencies, such as ∼11% for external quantum efficiency (EQE), as well as reduced roll-offs less than 8% for EQE at 1000 cd m–2, were achieved by D9F-based blue PHOLEDs as the highest performance among X9F, in which device efficiencies were improved by 50% compared to that of conventional polarized host mCP. It is conceivable that molecular configuration has significant effects on electrical properties and quenching effects of organic semiconductors with remarkable influence on intermolecular interplay and excited-state locations.

High morphology stability and ambipolar transporting host for use in blue phosphorescent single-layer organic light-emitting diodes

Phosphorescent organic light‐emitting diodes (OLEDs) hold great promise for display and lighting applications owing to their high efficiency and excellent color purity. However, their application in high‐luminance fields such as transparent displays and virtual reality is limited by severe efficiency roll‐off, primarily attributed to triplet‐triplet annihilation (TTA) and triplet‐polaron quenching (TPQ). In this study, we demonstrate that integrating a thin silver anode in close proximity to the phosphorescent emitter achieves a Purcell factor of 3.7, approximately three times that of conventional indium tin oxide–based devices. This configuration accelerates the exciton decay rate and reduces exciton density, thereby significantly suppressing both TTA and TPQ. As a result, the efficiency roll‐off is reduced from 57% to 11%. Notably, despite 64% of the energy coupling into surface plasmon polariton modes, the optimized device exhibits a 17% improvement in current efficiency at 20,000 cd/m 2 compared to conventional phosphorescent OLEDs with analogous structures, without requiring additional light extraction techniques. Moreover, the operational lifetime (LT90) is extended by five times. These findings highlight the potential of a simple plasmonic nanofilm approach for advancing high‐luminance phosphorescent OLEDs.

The use of mixed hosts has been demonstrated to be an effective way to improve the efficiency of phosphorescent organic light-emitting diodes (PhOLEDs). In this study, a macrospirocyclic oligomer (Cz-F)4 based on carbazole and fluorene is synthesized as a solution-processable matrix host material for blue PhOLEDs. The oligomer exhibits a high triplet level, good film-forming ability, solution processability, and doping compatibility. As a result, a blue solution-processed PhOLED based on a (Cz-F)4/TAPC [1,1-bis((di-4-tolylamino) phenyl) cyclohexane] mixed host achieves a maximum current efficiency of 18.8 cd/A with a turn-on voltage of 3.3 V. When a ternary mixed-host system composed of (Cz-F)4, TAPC, and OXD-7 [1,3-bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene] is employed, the device exhibits a low turn-on voltage of 2.8 V, and the driving voltages are only 3.8–4.6 V at the practical luminance of 100–1000 cd/m2. Even at 5000 and 10 000 cd/m2, the driving voltages are only 5.7 and 6.6 V, respectively. The current efficiency roll-off is only 12% from 100 to 5000 cd/m2. This study demonstrates that monodisperse oligomers can serve a matrix host for highly efficient blue solution-processed PhOLEDs with low turn-on voltage, low driving voltage, and suppressed efficiency roll-off.

Highly efficient blue and white phosphorescent organic light-emitting diodes with low-efficiency roll-off utilizing thermally activated delayed fluorescent-based co-host architecture

Phosphorescent organic light-emitting diodes (OLEDs) are suitable for display and lighting applications due to their superior luminance and efficiency. However, the strong efficiency roll-off severely hinders their potential applications in transparent displays, virtual reality, and other high-luminance-demanding fields, which is mainly attributed to severe triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ). In this study, by employing a thin Ag anode close to the phosphorescent emitter, a Purcell factor over 5 has been achieved, nearly triple that of a conventional indium tin oxide (ITO)-based device. This enhancement significantly accelerates the exciton decay rate and reduces exciton concentration, thereby considerably lowering the incidence of TTA and TPQ. Meanwhile, such devices are capable of nearly eliminating waveguide modes, with over 77% of the energy being coupled into a surface plasmon polariton (SPP). A nanoantenna array on metal (NAoM), situated on the exterior surface of the thin Ag anode, efficiently extracts the SPP when the plasmonic antenna modes resonate with the gap modes within the NAoM. This configuration yields an efficiency enhancement of 100% at 40,000 cd/m2 compared to conventional phosphorescent devices with similar structures, providing a promising avenue for high-luminance phosphorescent OLED.

Based on the one-dimensional tight-binding Su-Schrieffer-Heeger (SSH) model, and using the molecular dynamics method, we discuss the dynamics of electron and hole polarons under the influence of impurity potentials and the distance between impurities. Under an external electric field, the electron or hole polaron can move along the polymer chain with a steady velocity. When the polarons collide with impurities, the velocities of the polarons would be affected by the impurity potentials and the distance between the impurities. 1) Firstly, at a fixed impurity potential strength, the average velocities of the electron and hole polarons as a function of the distance (2-16 times the lattice constant) between impurities have been discussed in polymers. It is found that the average velocities of the electron and hole polarons increase with increasing distance between impurities. It is worth noting that the average velocities of the electron polarons are greater than those of the hole polarons, which results from the fact that the electron and hole polarons have different coulomb interactions with the impurity ions. That is to say, the coulomb repulsion is shown between the electron polarons and impurity ions, which is similar to the potential barriers; while the coulomb attraction appears between the hole polaron and impurity ions, which is similar to a potential well. However, as the distance between the impurity ions becomes large enough, the average speeds of the electron and hole polarons almost remain the same, and show just a few small oscillation. This is due to the different distances between impurity ions which generate the different superposition effects of barrier or potential well on the electron and hole polarons. 2) Next, with a fixed distance between the two impurity ions, we find that with the increase of impurity potential strength, the average velocities of the electron and hole polarons decrease. And the decrease of the average speed of the hole polaron is more obvious. It can be explained as follows: the coulomb attraction interactions between the hole polaron and impurity ions can obviously enhance the localization of the hole polaron. While the coulomb repulsion interactions between electron polaron and impurity ions can only make the electron polaron undergo a small shift in the polymer chain, so that the localization of it is almost unchanged. In view of the average speed of the polaron being closely related to the localization of the polaron, the change of the average speed of the hole polaron is more obvious. The results above may provide some theoretical basis for understanding the conduction properties in doped polymers.

Phosphorescent organic light‐emitting diodes (OLEDs) are promising for display and lighting applications due to their excellent luminance and efficiency. However, their utilization in high‐luminance‐demanding fields such as transparent displays and virtual reality is limited by severe efficiency roll‐off, primarily caused by triplet‐triplet annihilation (TTA) and triplet‐polaron quenching (TPQ). In this study, a thin silver anode positioned near the phosphorescent emitter achieves a Purcell factor of 3.7, nearly three times that of conventional indium tin oxide (ITO)‐based devices. This enhancement accelerates the exciton decay rate and reduces exciton density, significantly mitigating TTA and TPQ. Consequently, the efficiency roll‐off decreases from 57% to 11%. Despite 64% of the energy coupling into surface plasmon polariton (SPP) modes, the device still demonstrates a 17% efficiency improvement at 20,000 cd/m² compared to conventional phosphorescent devices with similar structures, without requiring additional light extraction structures. Furthermore, the operational lifetime (LT90) of the device is enhanced by a factor of five. These findings highlight the potential of a simple plasmonic nanofilm approach for advancing high‐luminance phosphorescent OLEDs.

Three new pyridine-cored alkyl-substituted carbazole derivatives of 2,6-bis(2,7-dimethyl-9H-carbazol-9-yl)pyridine (2,7-MeCzPy), 2,6-bis(3,6-dimethyl-9H-carbazol-9-yl) pyridine (3,6-MeCzPy) and 2,6-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)pyridine (3,6-tBuCzPy) were synthesized by means of connecting methyl or tert-butyl substituents on the 3,6 or 2,7 positions of carbazole with pyridine ring as the core. The influence of different alkyl and linkages mode on the thermal, photophysical, electrochemical properties and devices electrolumiescent (EL) performances of the compounds were comprehensively studied. In solution-processed blue or green phosphorescent organic light-emitting diodes (PHOLEDs) with bis[2-(4,6-difluorophenyl)-pyridinato-N,C2] picolinate iridium(III) (FIrpic) or fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) as phosphorescent dopants, EL performances follow the same sequence of 2,7-MeCzPy > 3,6-tBuCzPy > 3,6-MeCzPy, the trend is consistent with the value of triplet energies (ET). Devices hosted by 2,7-MeCzPy achieving the best EL performance, exhibited maxima 13.6 cd A−1 and 7.0 lm W−1 for current efficiency (CE) and power efficiency (PE) in blue PHOLEDs, maxima 26.2 cd A−1 and 16.2 lm W−1 for CE and PE in green PHOLEDs.

Phosphorescent organic light-emitting diodes (PHOLEDs) feature high efficiency1,2, brightness and colour tunability suitable for both display and lighting applications3. However, overcoming the short operational lifetime of blue PHOLEDs remains one of the most challenging high-value problems in the field of organic electronics. Their short lifetimes originate from the annihilation of high-energy, long-lived blue triplets that leads to molecular dissociation4-7. The Purcell effect, the enhancement of the radiative decay rate in a microcavity, can reduce the triplet density and, hence, the probability of destructive high-energy triplet-polaron annihilation (TPA)5,6 and triplet-triplet annihilation (TTA) events4,5,7,8. Here we introduce the polariton-enhanced Purcell effect in blue PHOLEDs. We find that plasmon-exciton polaritons9 (PEPs) substantially increase the strength of the Purcell effect and achieve an average Purcell factor (PF) of 2.4 ± 0.2 over a 50-nm-thick emission layer (EML) in a blue PHOLED. A 5.3-fold improvement in LT90 (the time for the PHOLED luminance to decay to 90% of its initial value) of a cyan-emitting Ir-complex device is achieved compared with its use in a conventional PHOLED. Shifting the chromaticity coordinates to (0.14, 0.14) and (0.15, 0.20) into the deep blue, the Purcell-enhanced devices achieve 10-14 times improvement over similarly deep-blue PHOLEDs, with one structure reaching the longest Ir-complex device lifetime of LT90 = 140 ± 20 h reported so far10-21. The polariton-enhanced Purcell effect and microcavity engineering provide new possibilities for extending deep-blue PHOLED lifetimes.

ABSTRACTEfficient blue and white phosphorescent organic light-emitting diodes (PhOLEDs) with low turn-on voltage were fabricated by using mixed hole and electron transporting layer materials as the co-host in the emissive layer. The optimized performance of blue PhOLEDs was achieved with the current efficiency, external quantum efficiency and power efficiency are 45.11 cd/A, 19.21% and 45.67 lm/W, respectively. Based on the blue-PhOLEDs, efficient white PhOLEDs were achieved with ultrathin green and red emitters were respectively inserted into different locations of the emitting layer. The influences of the locations on the emissive spectra of white PHOLED were investigated. The optimized white PHOLED with the maximum power efficiency of 45.22 lm/W and the peak external quantum efficiency of 16.39% were achieved.

We investigated a light emission mechanism of blue phosphorescent organic light emitting diodes (PHOLEDs), using a stepwise doping profile of 2, 8, and 14 wt.% within the emitting layer (EML). We fabricated several blue PHOLEDs with phosphorescent blue emitter iridium(III) bis[(4,6-difluorophenyl)-pyridinato-N,C2]picolinate doped in N,N'-dicarbazolyl-3,5-benzene as a p-type host material. A blue PHOLED with the highest doping concentration as part of the EML close to an electron transporting layer showed a maximum luminous efficiency of 20.74 cd/A, and a maximum external quantum efficiency of 10.52%. This can be explained by effective electron injection through a highly doped EML side. Additionally, a white OLED based on the doping profile was fabricated with two thin red EMLs within a blue EML maintaining a thickness of 30 nm for the entire EML. Keywords: Blue Phosphorescent Organic Light Emitting Diodes, Stepwise Doping Structure, Charge Trapping Effect.

We report the results of a systematic study of the interplay of triplet-polaron quenching (TPQ) and triplet-triplet annihilation (TTA) on the efficiency roll-off of organic light-emitting diodes (OLEDs) with increasing current density. First, we focus on OLEDs based on the green phosphorescent emitter tris[2-phenylpyridine]iridium(III) (Ir(ppy)3) and the red phosphorescent dye platinum octaethylporphyrin. It is found that the experimental data can be reproduced using kinetic Monte Carlo (kMC) simulations within which TPQ and TTA are due to a nearest-neighbor (NN) interaction, or due to a more long-range Förster-type process. Furthermore, we find a subtle interplay between TPQ and TTA: decreasing the contribution of one process can increase the contribution of the other process, so that the roll-off is not significantly reduced. Furthermore, we find that just analyzing the shape of the roll-off is insufficient for determining the relative role of TPQ and TTA. Subsequently, we investigate the wider validity of this picture using kMC simulations for idealized but realistic symmetric OLEDs, with an emissive layer containing a small concentration of phosphorescent dye molecules in a matrix material. Whereas for NN-interactions the roll-off can be reduced when the dye molecules act as shallow hole and electron traps, we find that such an approach becomes counterproductive for long-range TTA and TPQ. Developing well-founded OLED design rules will thus require that more quantitative information is available on the rate and detailed mechanism of the TPQ and TTA processes.