Triplet Exciton Lifetime Research Articles

Precise Regulation of the Reverse Intersystem Crossing Pathway by Hybridized Long-Short Axis Strategy for High-Performance Multi-Resonance TADF Emitters.

Multi-resonance thermally activated delayed fluorescence (MR-TADF) molecules have experienced great success in organic light-emitting diodes (OLEDs) owing to their outstanding quantum efficiencies and narrow full width at half-maximums (FWHMs). However, the reverse intersystem crossing (RISC) rates of MR-TADF emitters are usually small, which will lead to relatively long triplet exciton lifetime and severe efficiency roll-off. Here, we report an effective molecular design strategy to introduce multichannel RISC pathways and thus increase RISC rates without compromising the color fidelity and emission efficiency by the "hybridized long-short axis (HLSA)" strategy. The TPA-CN-BN shows a near-unity photoluminescence quantum yield, rapid RISC rate of 1.4 × 105 s-1, narrow FWHM of 23 nm, and small singlet-triplet energy gap (ΔEST) of 0.06 eV in solution. The non-sensitized OLED based on TPA-CN-BN exhibits a narrowband emission with the FWHM of 31 nm, in company with external quantum efficiency (EQE) of 37.9%. Notably, the device exhibits the low efficiency roll-off as the EQEs maintain 34.8% and 21.8% at 100 and 1000 cd m-2, respectively, representing the best performance for single-host OLEDs based on the BCzBN skeleton. This study provides a fresh and promising approach to realize high-performance OLEDs with high color purity and remarkable device efficiency.

Precise Regulation of the Reverse Intersystem Crossing Pathway by Hybridized Long‐Short Axis Strategy for High‐Performance Multi‐Resonance TADF Emitters

Multi‐resonance thermally activated delayed fluorescence (MR‐TADF) molecules have experienced great success in organic light‐emitting diodes (OLEDs) owing to their outstanding quantum efficiencies and narrow full width at half‐maximums (FWHMs). However, the reverse intersystem crossing (RISC) rates of MR‐TADF emitters are usually small, which will lead to relatively long triplet exciton lifetime and severe efficiency roll‐off. Here, we report an effective molecular design strategy to introduce multichannel RISC pathways and thus increase RISC rates without compromising the color fidelity and emission efficiency by the “hybridized long‐short axis (HLSA)” strategy. The TPA‐CN‐BN shows a near‐unity photoluminescence quantum yield, rapid RISC rate of 1.4 × 105 s‐1, narrow FWHM of 23 nm, and small singlet‐triplet energy gap (ΔEST) of 0.06 eV in solution. The non‐sensitized OLED based on TPA‐CN‐BN exhibits a narrowband emission with the FWHM of 31 nm, in company with external quantum efficiency (EQE) of 37.9%. Notably, the device exhibits the low efficiency roll‐off as the EQEs maintain 34.8% and 21.8% at 100 and 1000 cd m‐2, respectively, representing the best performance for single‐host OLEDs based on the BCzBN skeleton. This study provides a fresh and promising approach to realize high‐performance OLEDs with high color purity and remarkable device efficiency.

Development of an Organic Emitter Exhibiting Reverse Intersystem Crossing Faster than Intersystem Crossing

AbstractIn thermally activated delayed fluorescence (TADF)‐based organic light‐emitting diodes (OLEDs), acceleration of reverse intersystem crossing (RISC) and suppression of intersystem crossing (ISC) are demanded to shorten a lifetime of triplet excitons. As a system realizing RISC faster than ISC, inverted singlet‐triplet excited states (iST) with a negative energy difference (ΔEST) between the lowest excited singlet and the lowest triplet states have been gathering much attention recently. Here, we have focused on an asymmetric hexa‐azaphenalene (A6AP) core to obtain a new insight into iST. Based on A6AP, we have newly designed A6AP‐Cz with the calculated ΔEST of −44 meV. The experimental studies of a synthesized A6AP‐Cz revealed that the lifetime of delayed fluorescence (τDF) was only 54 ns, which was the shortest among all organic materials. The rate constant of RISC (kRISC=1.9×107 s−1) was greater than that of ISC (kISC=1.0×107 s−1). The negative ΔEST of A6AP‐Cz was experimentally confirmed from 1) the kRISC and kISC (−45 meV) and 2) the temperature‐dependent τDF. 3) The onsets of fluorescence and phosphorescence spectra at 77 K also supported the evidence of negative ΔEST (−73 meV). This study demonstrated the potential of A6AP as an iST core for the first time.

Development of an Organic Emitter Exhibiting Reverse Intersystem Crossing Faster than Intersystem Crossing.

In thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs), acceleration of reverse intersystem crossing (RISC) and suppression of intersystem crossing (ISC) are demanded to shorten a lifetime of triplet excitons. As a system realizing RISC faster than ISC, inverted singlet-triplet excited states (iST) with a negative energy difference (ΔEST) between the lowest excited singlet and the lowest triplet states have been gathering much attention recently. Here, we have focused on an asymmetric hexa-azaphenalene (A6AP) core to obtain a new insight into iST. Based on A6AP, we have newly designed A6AP-Cz with the calculated ΔEST of -44 meV. The experimental studies of a synthesized A6AP-Cz revealed that the lifetime of delayed fluorescence (τDF) was only 54 ns, which was the shortest among all organic materials. The rate constant of RISC (kRISC=1.9×107 s-1) was greater than that of ISC (kISC=1.0×107 s-1). The negative ΔEST of A6AP-Cz was experimentally confirmed from 1) the kRISC and kISC (-45 meV) and 2) the temperature-dependent τDF. 3) The onsets of fluorescence and phosphorescence spectra at 77 K also supported the evidence of negative ΔEST (-73 meV). This study demonstrated the potential of A6AP as an iST core for the first time.

Dual-Confinement and Surface-Ionization Induced Controllable Regulate Visible-Light-Activated Colorful Afterglow of Carbon Dots for Multifunctional Applications.

Low-energy visible-light-activated carbon dots (CDs)-based afterglow materials are difficult to realize due to the inherent aromatic carbon with high-energy absorption and the lack of effective regulation. Here, a new strategy for visible-light-activated CDs is proposed by combining dual-confinement and surface-ionization, which employs NaOH for additional confinement and surface ionization of CDs in a single boric acid (BA) matrix. The comparison experiments show that: i) shifting the excitation from UV-light to vis-light is realized by enhancing the low-energy surface states n→π* transition of the CDs by surface ionization of NaOH. ii) CDs are additionally protected by a more stable Na─O ionic bond after NaOH confinement, resulting in a brighter afterglow. iii) the energy gap (ΔEST) between the lowest singlet and triplet states is gradually shortened as increasing NaOH content, facilitating intersystem crossing, prolonging the lifetime of triplet excitons and efficiency. Further, vis-light-excited colorful afterglow powders are fabricated based on Förster Resonant Energy Transfer by combining the fluorescent dye 5-carboxytetramethylrhodamine. Finally, advanced white-light-activated time-resolved anti-counterfeiting and intelligent traffic flashing signs are realized. The work may shed new light on the design of low-energy-activated afterglow materials and broaden the application scenarios in the daily lives of human society.

Leveraging Compatible Iridium(III) Complexes to Boost Performance of Green Solvent‐Processed Non‐Fullerene Organic Solar Cells

AbstractIn organic solar cells (OSCs), the short exciton lifetime poses a significant limitation to exciton diffusion and dissociation. Extending exciton lifetime and suppressing recombination are crucial strategies for improving the OSC performance. Herein, an effective approach is proposed by introducing the phosphorescent emitter, tris(2‐(4‐(tert‐butyl)phenyl)‐5‐fluoropyridine)Iridium(III), with long‐lived triplet exciton lifetime in OSCs. This research reveals that the steric structure of fac‐Ir(tBufppy)3 exhibits excellent compatibility with both the donor PM6 and acceptor BTP‐eC9, maintaining efficiencies of over 90% even with a 30% third component loading. Moreover, a 10% addition of fac‐Ir(tBufppy)3 mitigates excessive aggregation in the acceptor BTP‐eC9, optimizing the active layer morphology and improving the fill factor. Transient absorption spectroscopy and transient photoluminescence measurements demonstrate that the introduction of fac‐Ir(tBufppy)3 significantly extends exciton lifetimes and suppresses recombination, which increases the short‐circuit current (JSC). Ultimately, employing the non‐halogenated solvent o‐xylene for processing, an impressive power conversion efficiency (PCE) of 18.54% is achieved in devices based on PM6:10%fac‐Ir(tBufppy)3:BTP‐eC9, surpassing the efficiency of binary PM6:BTP‐eC9 devices (17.41%). This work provides a promising approach to further improve the PCEs in binary OSCs by introducing a phosphorescent iridium(III) complex as the third component.

Exploring charge generation and separation in tandem organic light-emitting diodes based on magneto-electroluminescence

5,6,11,12-tetraphenylnaphthacene (rubrene) exhibits resonant energy properties (E S1,rub ≈ 2E T1,rub), resulting in rubrene-based organic light-emitting diode (OLED) devices that undergo the singlet fission (STT) process at room temperature. This unique process gives rise to a distinct magneto-electroluminescence (MEL) profile, differing significantly from the typical intersystem crossing (ISC) process. Therefore, in this paper, we investigate charge generation and separation in the interconnector, and the mechanism of charge transport in tandem OLEDs at room temperature using MEL tools. We fabricate tandem OLEDs comprising green (Alq3) and yellow (Alq3:rubrene) electroluminescence (EL) units using different interconnectors. The results demonstrate that all devices exhibited significant rubrene emission. However, the MEL did not exhibit an STT process with an increasing magnetic field, but rather a triplet–triplet annihilation (TTA) process. This occurrence is attributed to direct carrier trapping within doped EL units, which hinders the transport of rubrene trapped charges, consequently prolonging the lifetime of triplet excitons (T1,rub). Thus, the increased T1,rub concentration causes TTA to occur at room temperature, causing the rapid decrease of MEL in all devices under high magnetic fields. In devices where only the TTA process occurs, the TTA increases with the increasing current. Consequently, the high magnetic field of devices A–C is only related to TTA. Notably, there exists a high magnetic field TTA of device D in the Alq3/1,4,5,8,9,11-Hexaazatriphenylene-hexacarbonitrile interconnector regardless of the current. This occurs because both EL units in the device emit simultaneously, resulting in the triplet-charge annihilation process of Alq3 in the high magnetic field of the MEL. Moreover, the rapid increase in MEL at low magnetic field across all devices is attributed to the ISC between Alq3 polaron pairs. This entire process involves Förster and Dexter energy transfer. This article not only provides novel insights into charge generation and separation in the interconnector but also enhances our understanding of the microscopic mechanisms in tandem OLED devices.

Organic solar cells and their applications Shah, Razon Oda, F. Hazuan

Enhancing the power conversion efficiency (PCE) is the ma- jor task in the development of solar cells. In 1961, Willi- am Shockley and Hans J. Queisser pointed out that the highest PCE for a single-junction solar cell is limited to 31% due to the thermalization and transmission losses. Singlet fission (SF) is an appealing carrier-multiplication approach to break the Shockley-Queisser limit, since it converts one high- energy singlet exciton into two low-energy triplet excitons. Thus, one absorbed photon generates two electron–hole pairs [1]. SF phenomenon was first observed in anthracene single crystals in 1965. It attracted attention again in 2006. In organic semiconductors, a singlet exciton (S1) forms after the ab- sorption of a photon. If the energy of this singlet exciton is greater than twice of that for triplet exciton (T1), namely E(S1) > 2E(T1), then spin-allowed fission (S1 → 2T 1) could occur. The general kinetic model for SF process is S0 is the ground state, S1 is the excited singlet state, 1(TT) is the intermediate state of a correlated triplet pair, and T1 + T1 are two independent triplet states (Figure 1(a)). The application needs materials with high fission efficiency, suitable bandgap for capturing high-energy photons, appropriate triplet energy and high chemical stability. Owing to the existence of competitive pathways (i.e. S1 to S0 relaxation in picosecond–nanosecond timescale; excimer formation in femtosecond timescale; charge transfer (CT) process in femtosecond timescale, SF needs to occur in pico- second or sub-picosecond timescale. Thus, SF requires strong intermolecular coupling. The intermolecular coupling can be realized via Van der Waals force, weak non-bonded interactions and so on. The coupling distance not only affects SF rate and triplet yield, but also influences the lifetime and diffusion length of triplet excitons [2].

Improved device efficiency and lifetime of green thermally activated delayed fluorescence materials with multiple donors and cyano substitution

The challenges associated with thermally activated delayed fluorescence (TADF) materials are the reduced efficiency and lifetime because of the triplet-exciton lifetimes and thermal degradation of the devices owing to low molecular stabilities. To reduce the triplet-exciton lifetime, it is necessary to increase the rate of reverse intersystem crossing (RISC) and the intramolecular C–N bond dissociation energy (BDE) must be increased. In this study, a donor was introduced at the ortho position to reduce the energy gap between the singlet and triplet states and facilitate the expression of TADF properties. Three materials with increasing numbers of carbazole and CN− units and, consequently, increasing molecular stabilities: 4-(9H-carbazol-9-yl)-3-(4,6-diphenyl-1,3,5-triazin-2-yl)benzonitrile (Cz-mCNTrz), 3,3’-(6-phenyl-1,3,5-triazine-2,4-diyl)bis(4-(9H-carbazol-9-yl)benzonitrile) (DCz-mCNTrz), and 3,3′,3”-(1,3,5-triazine-2,4,6-triyl)tris(4-(9H-carbazol-9-yl)benzonitrile) (TCz-mCNTrz), were designed and synthesized. The best device performance of three materials, TCz-mCNTrz, exhibited 25.3% of maximum external quantum efficiency with highest photoluminescence quantum yield(PLQY) and smallest singlet-triplet energy gap. Moreover, lowest efficiency roll-off of 5.92%, as well as the longest lifetime at a 95-h at 5000 nits indicating increased anion BDE and rate of RISC after each addition of the carbazole and cyano moieties.

More than 25,000 h device lifetime in blue phosphorescent organic light-emitting diodes via fast triplet up-conversion of n-type hosts with sub μs triplet exciton lifetime

Understanding and retardation of materials degradation phenomena in blue phosphorescent organic-light emitting diodes (PhOLEDs) are crucial to replace a fluorescent device with an efficient phosphorescent device in practical applications. Among the organic functional materials in the blue PhOLEDs, the host in the emitting layer is the most populated and fragile region in view of its density of states since singlet, triplet excitons, and polarons are continuously accumulated and consumed during operation. In this work, we presented novel n-type hosts which have thermally activated delayed fluorescence (TADF) features without loss of triplet energy even in the p-n mixed host structure. Both high triplet energy of mixed host and triplet up-conversion of a new n-type host within sub μs range allowed to demonstrate highly efficient and operationally stable blue PhOLEDs. Introducing 3,3-di(9H-carbazol-9-yl)biphenyl and 3-[bis(9H-carbazol-9-yl)-1,3,5-triazin-2-yl]-5-(triphenylsilyl)benzonitrile as a mixed host, blue PhOLEDs exhibited maximum external quantum efficiency of 20.9%, color coordinate of (0.16, 0.29), and device lifetime of 13,573 h at 100 cd m−2, which was 4.2 times longer than that of state of the art blue device. Furthermore, the optimized blue PhOLEDs demonstrated device lifetime longer than 25,000 h, which is one of the longest device lifetimes of blue PhOLEDs without any sensitization or outcoupling enhancement. This result indicated that triplet exciton recycling via TADF mechanism in the n-type host is one of the effective approaches to enhance the operational stability of blue PhOLEDs.

Metal and halogen-free purely organic room temperature phosphorescence material using heavy atom effect of phenoselenazine

Development of purely organic room temperature phosphorescent (RTP) emitters is challenging and promising to various applications. However, it is very difficult to observe phosphorescence from common organic emitters. Herein, we introduced phenoselenazine (PSeZ) as a moiety to induce phosphorescence and evaluated the emission properties of (10 H -phenoselenazin-10-yl)(phenyl)methanone (BzylPSeZ) and demonstrated effects of PSeZ on room temperature phosphorescent properties in comparison with (10 H -phenothiazin-10-yl)(phenyl)methanone (BzylPTZ) which contains a phenothiazine. As a result, selenium in the PSeZ enhanced spin-orbit coupling by heavy atom effect and BzylPSeZ showed phosphorescence in crystalline state. The BzylPSeZ exhibited phosphorescence quantum yield of 7.7% and phosphorescence lifetime of 670 μs. This work proved the possibility of the PSeZ as a unit of pure organic RTP emitters. • Room temperature phosphorescence using selenium based organic units. • Strong spin-orbit coupling by heavy atom effect of selenium. • Short triplet exciton lifetime in pure organic phosphorescent emitter.

Over 30 000 h Device Lifetime in Deep Blue Organic Light‐Emitting Diodes with y Color Coordinate of 0.086 and Current Efficiency of 37.0 cd A−1

AbstractThe development of highly efficient and long‐life deep blue devices has been an ongoing challenge in the field of organic light‐emitting diodes (OLEDs) because of the short lifetime inherent to highly efficient blue devices. The efficiency and lifetime issues of the deep blue OLEDs are resolved in this work by engineering the device architecture by managing the triplet excitons of the phosphorescence and thermally activated delayed fluorescence (TADF) emitters. The CN‐modified imidazole type Ir emitter and boron‐based TADF emitter with similar emission energy are combined to develop an energy exchanging phosphorescence and TADF (EPHTADF) device. An emission mechanism inducing both forward and backward energy transfer concurrently between the Ir phosphor and TADF emitter shortens the triplet exciton lifetime of the two emitters, which dramatically enhances the device lifetime of the blue devices. Deep blue OLEDs triggering EPHTADF emission ultimately yields device performances including high current efficiency (CE) of 37.0 cd A−1, color‐coordinate corrected CE of 430.9 cd A−1, extrapolated device lifetime of more than 30 000 h, and deep blue color coordinates of (0.122, 0.086). This work demonstrates the potential of EPHTADF devices to serve as the solution to the problematic issues of the blue OLEDs.

Influence of molecular stacking pattern on excited state dynamics of copper phthalocyanine films

Photophysical processes occurring within organic semiconductors is important for designing and fabricating organic solar cells. Copper phthalocyanine (CuPc) is a typical electron acceptor. In this work, the triplet exciton lifetime is prolonged by altering the molecular stacking pattern of the CuPc film. For CuPc thin films, the excited state decays are mainly determined by the triplet-triplet annihilation process. The ultrafast transient absorption measurements indicate that the primary annihilation mechanism is one-dimensional exciton diffusion collision destruction. The decay kinetics show a clearly time-dependent annihilation rate constant with γ∝t−1/2. Annihilation rate constants are determined to be γ0 = (2.87±0.02)×10−20 cm3·s−1/2 and (1.42±0.02)×10−20 cm3·s−1/2 for upright and lying-down configurations, respectively. Compared to the CuPc thin film with an upright configuration, the thin film with a lying-down configuration shows longer exciton lifetime and higher absorbance, which are beneficial to organic solar cells. The results in this work have important implications on the design and mechanistic understanding of organic optoelectronic devices.

Photoresponsive Luminescence Switching of Metal‐Free Organic Phosphors Doped Polymer Matrices

AbstractA new type of luminescence switching behavior based on phosphorescence enhancement from a series of metal‐free organic phosphors doped polymers by UV‐irradiation is investigated. This phenomenon is observed only from pairs of organic phosphors and polymer matrices having a combination of appropriate triplet exciton lifetime and oxygen permeability. Systematic investigation reveals that the luminescence switching behavior of organic phosphors embedded in a specific polymeric matrix stems from the conversion of triplet oxygens to singlet oxygens by UV‐irradiation, leading to the unique phosphorescence enhancement of organic phosphors. Visualization of latent information by UV‐irradiation is demonstrated toward novel secure information communication applications.

Singlet relaxation dynamics and long triplet lifetimes of thiophene-coupled perylene diimides dyads: New insights for high efficiency organic solar cells

In the active layer of organic solar cells (OSCs), the lifetime of triplet excitons is one of the decisive factors in the diffusion length and therefore has important impact on the power conversion efficiency of the devices. Herein, we have investigated singlet excited state relaxation dynamics and their triplet exciton lifetimes of two thiophene-coupled perylene diimides (PDI) dyads (2PDI-Th and fused-2PDI-Th), in order to provide a unique explanation in depth on their different performances in OSC devices. From the transient absorption (TA) spectra, the singlet excitons of 2PDI-Th form excimers in the time scale of 1.5 ps. Then the excimers go into the triplet state via intersystem crossing (ISC). In fused-2PDI-Th, triplet excitons are generated directly from the singlet excited excitons via the efficient ISC. Density functional theory (DFT) calculations further support the formation of excimers. DFT results indicate that 2PDI-Th exhibits an H-typed molecular configuration which is beneficial to form the excimers, while fused-2PDI-Th gives a twisted X-shaped configuration in the optimized ground and excited state. In steady-state emission spectra, 2PDI-Th shows abroad and featureless spectral characteristics of the excimers with a decay time of 840 ps, which is much shorter than those of PDI (5.5 ns) and fused-2PDI-Th (3.3 ns). The triplet lifetime (67 μs) of fused-2PDI-Th is factor of 3 longer than that of 2PDI-Th (22 μs). These results demonstrate that ring-fused structure is an efficient strategy to eliminate excimer formation and prolong the lifetime of triplet excitons, which provides a new insight for design of optoelectronic molecules for high efficiency organic solar cells.

Triplet Acceptors with a D‐A Structure and Twisted Conformation for Efficient Organic Solar Cells

AbstractTriplet acceptors have been developed to construct high‐performance organic solar cells (OSCs) as the long lifetime and diffusion range of triplet excitons may dissociate into free charges instead of net recombination when the energy levels of the lowest triplet state (T1) are close to those of charge‐transfer states (3CT). The current triplet acceptors were designed by introducing heavy atoms to enhance the intersystem crossing, limiting their applications. Herein, two twisted acceptors without heavy atoms, analogues of Y6, constructed with large π‐conjugated core and D‐A structure, were confirmed to be triplet materials, leading to high‐performance OSCs. The mechanism of triplet excitons were investigated to show that the twisted and D‐A structures result in large spin–orbit coupling (SOC) and small energy gap between the singlet and triplet states, and thus efficient intersystem crossing. Moreover, the energy level of T1 is close to 3CT, facilitating the split of triplet exciton to free charges.

Triplet Acceptors with a D-A Structure and Twisted Conformation for Efficient Organic Solar Cells.

Triplet acceptors have been developed to construct high‐performance organic solar cells (OSCs) as the long lifetime and diffusion range of triplet excitons may dissociate into free charges instead of net recombination when the energy levels of the lowest triplet state (T1) are close to those of charge‐transfer states (3CT). The current triplet acceptors were designed by introducing heavy atoms to enhance the intersystem crossing, limiting their applications. Herein, two twisted acceptors without heavy atoms, analogues of Y6, constructed with large π‐conjugated core and D‐A structure, were confirmed to be triplet materials, leading to high‐performance OSCs. The mechanism of triplet excitons were investigated to show that the twisted and D‐A structures result in large spin–orbit coupling (SOC) and small energy gap between the singlet and triplet states, and thus efficient intersystem crossing. Moreover, the energy level of T1 is close to 3CT, facilitating the split of triplet exciton to free charges.

Кинетика замедленной флуоресценции и сенсибилизированной фосфоресценции примесных молекулярных кристаллов

A model of the kinetics of attenuation of heteroannihilation delayed fluorescence and sensitized phosphorescence of impurity organic molecular crystals is proposed. It is shown that this model makes it possible to evaluate the contribution of various mechanisms to the transformation of the energy of triplet excitations in systems when the intrinsic lifetime of nonlocalized triplet excitons of the base is much shorter than the lifetime of localized excitons.

Exciton-polaron interaction in blue fluorescent organic light-emitting diodes

OLEDs are popular as display technology nowadays, which have been widely used in commercial application. However, there are still some problems that blue light devices are not as efficient or stable as red and green light devices. Although the use of phosphorescent dyes can significantly improve the internal quantum efficiency, the high production cost and unstable performance limit the industrialization of phosphorescent OLEDs. In the development of OLEDs, the researchers found that OLEDs suffered from a decline in their efficiency at high brightness levels, a behavior known as “efficiency roll-off”. The efficiency roll-off is more pronounced in phosphorescent devices due to the longer lifetime of triplet exciton than singlet exciton, so that it has been widely investigated in recent years. Little is known, still, about fluorescent devices. Accordingly, unraveling the exciton loss mechanism in blue fluorescent OLEDs is particularly important, as it is a limiting factor for the improvement of efficiency. In this work, the efficiency roll-off in blue fluorescent OLEDs is investigated by observing the quenching of DPAVBI excitons. Firstly, the effects of electron current and hole current on photoluminescence(PL) behavior of unipolar devices are studied by steady-state and transient-state measurements, and we analyze PL spectrum and calculate the exciton quenching rate constant according to the transient PL decay curves to clarify the exciton quenching dynamics. The results show that the holes are much more efficient in quenching the excitons when the host is a hole transport material. This is different from the general understanding that exciton-polaron quenching effect with higher carrier mobility is weaker. Because the existence of bound charges produces additional charge density, and it is inferred that the exciton is mainly quenched by trapped charge rather than moving charge. We also exclude the effect of exciton–exciton annihilation and electric-field-induced dissociation on the efficiency degradation of the OLEDs. It is confirmed experimentally that exciton-polaron interaction is the dominant mechanism of the efficiency roll-off in fluorescent OLEDs. We then fabricate organic light-emitting diode devices with different doping concentrations to figure out the effect of doping concentration on exciton-polaron interaction, and obtain a blue fluorescence device with good comprehensive performance. We also summarize some feasible methods to optimize the efficiency of the OLEDs. In this paper, our findings about exciton-polaron interaction might provide a viable source for efficiency improvement by regulating charge trapping in light emitting layer.

Achieving high-performance phosphorescent organic light-emitting diodes using thermally activated delayed fluorescence with low concentration

We fabricated phosphorescent organic light-emitting diodes (PhOLEDs) using thermally activated delayed fluorescence (TADF) material 10,10′-(4,4′-sulfonylbis(4,1-phenylene)) bis(9,9-dimethyl-9,10-dihydroacridine) (DMAC-DPS) with low concentration, which showed better performance compared with 1,3-bis(carbazole-9-yl) benzene (mCP) based devices. When the concentration of DMAC-DPS was 1wt%, the driving voltage of the device was only 3.3 V at 1 000 cd/m2, and the efficiency and lifetime of the device were effectively improved compared with those of mCP based devices. The result indicated that DMAC-DPS could effectively improve the performance of phosphorescent devices. We believe that the better device performance can be attributed to the optimization of the energy transfer process in the emitter layer and lifetime of triplet excitons by DMAC-DPS. The study may provide a simple and effective strategy to achieve high-performance OLEDs.