Exciplex-based Organic Light-emitting Diodes Research Articles

Creation of Dual Thermally Activated Delayed-Fluorescence Exciplexes in a Bulk Emitting Layer and Its Interface with an Electron Transport Layer for Promoting the Performance of Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diodes Fabricated by a Solution Process.

An exciplex, which is composed of electron donor and acceptor molecules and formed by intermolecular charge transfer, is an excited-state species that is able to emit light or transfer its energy to a lower-energy emitter. In reported exciplex-based organic light-emitting diodes (OLEDs), their working mechanism is to generate exciplexes either in the bulk emitting layer (bulk exciplex) or at its interface with an electron transport layer (interface exciplex); both types give promising device performance. Here, we propose a novel strategy of creating both types of exciplexes simultaneously (dual exciplexes) for the generation of more exciplexes for better device performance as indicated in the improved photoluminescence quantum yield (PLQY). Impressively, the dual exciplex-based device with blue thermally activated delayed fluorescence (TADF) emitter 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine (DMAC-TRZ) exhibits a record-high maximum external quantum efficiency (EQEmax) of 26.7% among the solution-processed TADF blue OLEDs. By further doping with the red-emitting phosphor emitter into the EML, the white device also gives a record-high EQEmax of 24.1% among the solution-processed TADF-phosphor hybrid white OLEDs (T-P WOLEDs) with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.34, 0.42), color rendering index of 70, and correlated color temperature of 5198 K. Furthermore, both blue and white devices show an ultralow efficiency roll-off with external quantum efficiencies at a practical brightness value of 1000 cd m-2 (EQE1000) of 25.1 and 23.9%, respectively. This is the first report of employing a dual exciplex-based OLED with excellent device performance.

Highly efficient non-doped organic light-emitting diodes based on long-range coupling and efficient energy transfer

An effective approach to realize high-efficiency exciplex-based organic light-emitting diodes (OLEDs) is proposed by employing long-range coupling between electron-hole pairs and efficient energy transfer. The space-separated holes and electrons form exciplexes through charge-transfer states, and devices with such structures can reduce exciton quenching in a certain extent compared to those based on interfacial exciplexes. Remarkable external quantum efficiency about 12.6% is achieved by introducing a yellow exciplex system in a donor/spacer/acceptor structure because of the suppression of exciton quenching and the efficient energy transfer. Such a unique strategy is beneficial to better utilizing the triplet excitons and thus achieving high performance exciplex-based OLEDs.

Non-monotonic current dependence of intersystem crossing and reverse intersystem crossing processes in exciplex-based organic light-emitting diodes

Intersystem crossing (ISC) and reverse ISC (RISC) between singlet and triplet polaron-pair and exciplex state are important spin-mixing processes in exciplex-based organic light-emitting diodes (EB-OLEDs). These two processes usually show normal current dependence which weakens with the increase of bias-current. This is because the bias-current increases by improving the device bias-voltage. When the bias-voltage rises, the electric field within the device is enhanced, which facilitates the electric-field-induced dissociation of polaron-pair and exciplex states and then reduces their lifetime. That is, less polaron-pair and exciplex states participate in the ISC process and RISC process, leading these two processes to weaken. Here, magneto-electroluminescence (MEL) is used as a fingerprint probing tool to observe various current-dependent ISC and RISC processes in EB-OLEDs with different charge balances via modifying the device hole-injection layer. Interestingly, current-dependent MEL traces of the unbalanced device display a conversion from normal ISC (1–25 μA) process to abnormal ISC (25–200 μA) process, whereas those of the balanced device show conversions from normal ISC (1–5 μA) into abnormal RISC (10–50 μA) and then into normal RISC (50–150 μA) and finally into abnormal ISC (200–300 μA) process. By fitting and decomposing the current-dependent MEL traces of the unbalanced and balanced devices, we find that the ISC process and RISC process in these two devices first increase then decrease as the bias-current increases. These non-monotonic current-dependent ISC process and RISC process are attributed to the competition between the increased number and the reduced lifetime of polaron-pair state and exciplex state during improving the bias-current. Furthermore, the RISC process in the balanced device is stronger than that in the unbalanced device. This is because the balanced carrier injection can facilitate the formation of triplet exciplex states and weaken the triplet-charge annihilation (TQA) process between triplet exciplex states and excessive charge carriers, which leads the number of triplet exciplex states to increase. That is to say, more triplet exciplex states can be converted into singlet exciplex states through the RISC process, causing the external quantum efficiency of the balanced device to be higher than that of the unbalanced device. Obviously, this work not only deepens the understandings of current-dependent ISC and RISC processes in EB-OLEDs, but also provides an insight into the device physics for designing and fabricating high-efficiency EB-OLEDs.

Visualizing the exciton formation channel in exciplex-based organic light-emitting diodes

Employing exciplex to capture the triplet excitons has been proved to be an effective strategy in organic light-emitting diodes (OLEDs). However, the detailed exciton formation pathways in exciplex-based OLEDs are still ambiguous. Here, the spin-dependent exciton formation in exciplex-based OLEDs is visualized by magnetic field effect technique. The results show that when excitons are formed on donor 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) molecules, the magnetic field effect on electroluminescence (MEL) exhibits a hyperfine style line-shape with narrow half-peak width. In contrast, when excitons are directly formed on the donor-acceptor (D-A) interface in the bulk heterojunction, the MEL exhibits a Δ g style line-shape with broad half-peak width. The line-shape of MEL could be an indicator of exciton formation mechanism. Our findings bring a new method to monitor the exciton formation pathways, which offer an opportunity for linking the exciton formation to device efficiency. • Exciton formation and evolution have a great impact on the device performance. • MEL profiles could be an indicator of excion formation pathways. • The dominant Δg style indicates that excitons are directly formed across the D-A interface in the exciplex-based devices.

Conversions from Normal to Abnormal Current‐Dependent ISC and from Abnormal to Normal Current‐Dependent RISC Processes in Exciplex‐Based OLEDs

AbstractIntersystem crossing (ISC) and reverse ISC (RISC) are important spin‐mixing processes for obtaining high quantum efficiency in exciplex‐based organic light‐emitting diodes (EB‐OLEDs) and often show normal current (I) dependencies which weaken with increasing I. Surprisingly, herein, using magneto‐conductance (MC) as a fingerprint probing tool, an unreported conversion from normal to abnormal I‐dependent ISC processes is observed at 300 K from relatively‐unbalanced EB‐OLEDs with the low electron‐injection barrier and the high hole mobility. More amazingly, after improving carrier injection balance through replacing the hole‐injection layer, a conversion from abnormal to normal I‐dependent RISC processes with increasing I is observed from the relatively‐balanced EB‐OLEDs for the first time. These two conversions are reasonably analyzed by fitting and decomposing MC traces of the devices. Furthermore, transition I of the conversion from abnormal to normal I‐dependent RISC processes decreases from 50 to 5 μA as the temperature reduces from 300 to 150 K, and only the normal I‐dependent RISC process is observed at 100 and 20 K due to high driving voltages. Obviously, this work deepens the full understandings of I‐dependent ISC and RISC processes in EB‐OLEDs and has the potential applications for the achievement of high‐performance devices.

Efficiency and color-temperature-stability improvements in exciplex-based phosphorescent organic light-emitting diodes with a quantum well structure

Currently, exciplex has drawn a great deal of attention due to its potential for efficient electroluminescence and for use as a host. In this study, we used 4,4′,4"-Tris(carbazol-9-yl) triphenylamine (TCTA) and 1,3,5-Tri(m-pyridin-3-ylphenyl) benze nee (TmPyPB) to form an exciplex host, where Bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III)(FIrpic) was used as the dopant to emit blue phosphorescent light. Additional FIrpic and Bis(1phenylisoquinoline) (acetylacetonate) iridium(III) emission layers were inserted in the proposed structure to investigate how the recombination area of carriers shifts with the increase of voltage. TCTA and non-doped FIrpic layers were then inserted in both sides of the emission layer to confine the carriers, and the thickness of the emission layer was also optimized to improve the current efficiency of the proposed devices. The efficiency of the devices was increased from 56 cd/A to 63.6 cd/A with the additional quantum well structure and an emission layer thickness of 15 nm. The current efficiency reported in this paper was fairly high as compared with other published data on blue-emission exciplex-based organic light-emitting diodes. In addition, the device with the quantum well structure exhibited purer blue-light emission, and the color temperature stability was also highly improved.

Process optimization and investigation of exciton movement in high efficiency exciplex-based green organic light-emitting diodes

In this paper, we optimized exciplex-based organic light-emitting diodes (OLEDs) using single-carrier devices to improve carrier balance and identify the location of the carrier recombination region, which can enhance OLED performance. Exciplex-based OLEDs with a Indium Tin Oxide structure / 4,4′,4′'-Tris[(3-methylphenyl)phenylamino]triphenylamine (m-MTDATA) (x nm) / m-MTDATA: Bathophenanthroline (Bphen) (40 nm, 1:1) / Bphen (30 nm) / LiF (1 nm) / Al (80 nm) were fabricated with various thicknesses of the hole transport layers. We controlled the m-MTDATA:Bphen emission layer evaporation rate at 0.8 Å/s, which was the best choice for indoor application in our previous study. The maximum OLED current efficiency of 71.5 cd/A at a luminous intensity of 443 cd/m2 and emission wavelength of 552 nm was obtained, which is suitable for indoor power-saving OLED applications. Furthermore, this study used a lower-energy red-doping layer to observe how excitons move in exciplex-based OLEDs when the external electric field increases.

Microscopic processes of Rubrene-doped devices with various exciplexes as hosts

Recently, exciplex-based organic light-emitting diodes (OLEDs) have become hot topics in organic optoelectronics in terms of improving luminescent efficiency by controlling the energy transfer of excitons. This is because the formation of an exciplex can result in a small energy level difference, Δ E ST between singlet and triplet exciplex states, and with the assistance of external thermal energy, nonradiative triplet exciplex states can transform into radiative singlet ones through the so-called reverse intersystem crossing (RISC) process. Moreover, this RISC process will enhance the luminescence of OLEDs based on host–guest systems through energy transfer from exciplexes to dopants. The energy-transfer capability of OLEDs will affect the number of polaron pairs and exciplexes in host–guest systems. Indeed, it is important to find an effective probing technique for simply studying these microscopic processes. Many literature reports have demonstrated that organic magneto-electroluminescence (MEL) traces could be used for exploring and understanding the formation of and interactions between polaron pairs, excitons, and/or exciplexes. This is because MELs exhibit sensitive fingerprint responses to intersystem crossing (ISC), high-level reverse intersystem crossing (HL-RISC), singlet exciton fission (SF), and triplet exciton fusion (TF). In this study, four different OLEDs with various exciplex hosts have been fabricated, and their MEL curves have been measured at different currents and temperatures. Four exciplexes with different triplet exciton energies were used as host materials, and Rubrene was used as a fluorescent dopant. To study the exciton energy transfer and luminescence mechanism in such doping systems, we fully analyzed the emission spectra of the host materials, absorption spectrum of the guest material, and triplet exciton energy of the host and guest materials. The experimental results show that when the combined energy of the exciplex’s triplet excitons (EX3) is lower than that of the second-order triplet excitons (T2,Rub) of the Rubrene dopant, the MEL curve is dominated by the B -mediated ISC from host polaron pairs (PP1→PP3). Otherwise, the MEL curves are composed of both low-field and high-field components. The low-field components of MEL are governed by the B -mediated HL-RISC (T2,Rub→S1,Rub) process of the Rubrene molecules, while the high-field components of MEL are the result of the TF process between the triplet excitons of Rubrene (T1,Rub+T1,Rub→S1,Rub+S0,Rub) at high current densities or by the SF process of singlet excitons (S1,Rub+S0,Rub→T1,Rub+T1,Rub) at low current densities. In addition, these microprocesses will be markedly affected by the operational temperature and bias-current of the devices, because the Rubrene triplet excitons have long lifetimes (T1,Rub and T2,Rub) at low temperatures, and large quantities of Rubrene excitons are produced at high current densities. Moreover, the energy level difference between the single and triplet excitons of the host and guest will regulate the energy transfer from the former to the latter, i.e., the combined energy from the singlet excitons of the host being higher than that of the guest is the requirement of efficient Forster energy transfer where the smaller is the triplet exciton energy difference between the host and guest, the stronger is the Dexter energy transfer. Consequently, the energy-transfer mechanism will affect the utilization rate of the triplet excitons by adjusting the quantity and longevity of the singlet or triplet excitons on the Rubrene molecules, which ultimately affects the luminous intensity of the device. This work is helpful for further understanding the microscopic mechanisms of Rubrene-based devices and provides a theoretical reference for enhancing their luminescent efficiencies.

High triplet energy materials for efficient exciplex-based and full-TADF-based white OLEDs

Materials with high triplet energies have attracted great attention for their application in exciplex-based or thermally activated delayed fluorescence-based organic light-emitting diodes. To study the correlation between triplet energy and chemical structure, six compounds with typical donor-acceptor-donor structures were designed, synthesized, and characterized. Through the adjustment of molecular configuration by weakening the strengths of donors and increasing the angles between donors and acceptors, the triplet energies of the materials were gradually increased from 2.5 to 3.0 eV. Three exciplex-based organic light-emitting diodes containing one of the synthesized compounds were fabricated, which exhibited efficient blue, green-yellow, and white thermally activated delayed fluorescence. Blue and green-yellow organic light-emitting diodes exhibited the maximum external quantum efficiencies of 9.1% and 8.3%, respectively. White organic light-emitting diodes showed turn-on voltage of 4.8 V, high maximum luminance of 18474 cdm-2, and maximum external quantum efficiency of 10.6%. Maximum external quantum efficiencies of blue and white organic light-emitting diodes were close to the record values observed for all-exciplex-based blue and white organic light-emitting diodes.

The Role of Reverse Intersystem Crossing Using a TADF-Type Acceptor Molecule on the Device Stability of Exciplex-Based Organic Light-Emitting Diodes.

Exciplex system exhibiting thermally activated delayed fluorescence (TADF) holds a considerable potential to improve organic light-emitting diode (OLED) performances. However, the operational lifetime of current exciplex-based devices, unfortunately, falls far behind the requirement for commercialization. Herein, rationally choosing a TADF-type electron acceptor molecule is reported as a new strategy to enhance OLEDs' operating lifetime. A comprehensive study of the exciplex system containing 9,9',9''-triphenyl-9H,9'H,9''H-3,3':6',3''-tercarbazole (Tris-PCz) and triazine (TRZ) derivatives clarifies the relationship between unwanted carrier recombination on acceptor molecules, TADF property of acceptors, and the device degradation event. By employing a proposed "exciton recycling" strategy, a threefold increased operational lifetime can be achieved while still maintaining high-performance OLED properties. In particular, a stable blue OLED that employs this strategy is successfully demonstrated. This research provides an important step for exciplex-based devices toward the significant improvement of operational stability.

Direct observation of reverse intersystem crossing from fully confined triplet exciplexes using magneto-electroluminescence

Magneto-electroluminescence was used to observe reverse intersystem crossing in exciplex-based organic light-emitting diodes directly.

Versatile Exciplex-Forming Co-Host for Improving Efficiency and Lifetime of Fluorescent and Phosphorescent Organic Light-Emitting Diodes.

We report a new efficient exciplex-forming system consisting of a biscarbazole donor and a triazine-based acceptor. The new exciplex was characterized with a high photoluminescence quantum yield up to 68% and effective thermally activated delayed fluorescence behavior. The BCzPh:3P-T2T (2:1, 30 nm) blend was examined not only as an emitting layer (device D1) but also a reliable co-host of fluorescent and phosphorescent emitters for giving highly efficient exciplex-based organic light-emitting diodes (OLEDs) with a high maximum external quantum efficiency of 15.5 and 29.7% for devices doped with 1 wt % C545T (device D2) and 8 wt % Ir(ppy)2(acac) (device D4), respectively. More strikingly, a strongly enhanced lifetime ( T75 = 16 927 min.) of the C545T-doped device was obtained. The transient electroluminescence measurement as well as capacitance-voltage and impedance-voltage correlations were utilized to explore the factors governing the high efficiency and stability. The obtained results clearly show that the energy transfer and charge transport is highly efficient; they also show the photoelectric semiconducting characteristics of exciplex-based OLEDs, which are significantly different from those of unipolar host-based reference devices D3 (Alq3: 1 wt % C545T) and D5 (CBP: 8 wt % Ir(ppy)2(acac)). Our works have established a systematic protocol to shed light on the mechanisms behind exciplex-based devices. The combined results also confirm the bright prospect of the exciplex-forming system as the co-host for highly efficient and stable OLEDs.

Investigation of electroluminescence mechanism in deep blue exciplex devices by utilizing magneto-electroluminescence

Recently, the electroluminescence (EL) mechanism in organic-light emitting diodes (OLEDs) based on blends of NPB:TPBi for deep blue light-emission has been reported in Advanced Materials by Jankus et al. in 2013. They have recorded EL and photoluminescence transients and fluorescence decays, demonstrating that the electroluminescence in their devices arises from enhanced exciplex emission via triplet-triplet annihilation (TTA) not the reverse intersystem crossing (RISC). In recent years, a large number of researches have showed that the magneto-electroluminescence (MEL) could be used as an effective measure to study the EL mechanism in OLEDs. And as a fingerprint tool MEL is sensitive to TTA and RISC. In this paper, we report similar exciplex-based OLEDs and show that no triplet-triplet annihilation and reverse intersystem crossing are observed at room temperature. TTA only occurs at low temperature and high current. We use O3 to treat the anode or replace the cathode with lower work function material, we haven’t observed TTA at room temperature yet. In order to increase the concentration of triplet excitons to facilitate the generation of TTA, we increase the injection current of the devices by over 1 order of magnitude, but we still haven’t observed the occurrence of TTA. The magneto-current of the device and its theoretical simulation show that triplet exciton interaction with hole polaron T1( ­­ )+P+( ¯ ) ® S1( ­¯ )+P+( ­ ) may have happened in the system, namely TPI (triplet-polaron interaction) process. This study will provide a reference for understanding the EL mechanism in the NPB:TPBi blending devices with deep blue light-emission.

Large magneto-conductance and magneto-electroluminescence in exciplex-based organic light-emitting diodes at room temperature

In this work, we report on large magneto-conductance (MC) over 60% and magneto-electroluminescence (MEL) as high as 112% at room temperature in an exciplex-based organic light-emitting diode (OLED) with efficient reverse intersystem crossing (ISC). The large MC and MEL are individually confirmed by the current density-voltage characteristics and the electroluminescence spectra under various magnetic fields. We proposed that this type of magnetic field effect (MFE) is governed by the field-modulated reverse ISC between the singlet and triplet exciplex. The temperature-dependent MFEs reveal that the small activation energy of reverse ISC accounts for the large MFEs in the present exciplex-based OLEDs.

Realization of triplet–triplet annihilation in planar heterojunction exciplex-based organic light-emitting diodes

Triplet–triplet annihilation (TTA) for enhancement of luminous efficiency occurs with difficulty in exciplex-based organic light-emitting devices (OLEDs) because it is an interaction among several neighboring donor and acceptor molecules. However, TTA has been realized in our planar-heterojunction (PHJ) exciplex-based OLEDs by using a thin recombination zone to enhance the interfacial density of the triplet states. The TTA process, which is characterized by a high-field decrease (HFD) in the magneto-electroluminescence from the PHJ OLEDs, appears at approximately 150 K and becomes stronger with decreasing temperature. At a given temperature, the higher the injected current is, the stronger HFD is observed. Additionally, we find that TTA could even happens at room temperature with appropriate selection of the donor molecule, which may be attributed to the favorable electron-donating ability of the methoxy group (–OCH3) in the donor molecule and the matched overlaps of the intermolecular conformation of the donor and the acceptor.

Micromechanism of electroplex formation

In this work, we have provided a new possible explanation for the micromechanism of electroplex. The time-resolved electroluminescent spectra of light-emitting diodes based on the blend of TAPC and TpPyPB were measured. They show that when a high bias voltage is applied on the devices, the electroplex emission gradually increases over time. After the devices worked at a high bias voltage, a strong electroplex emission can be maintained at low bias voltage, but the peaks related to the electroplex are still insignificant in photoluminescence. These results may suggest that the electroplex is a charge–transfer complex with changed conformation caused by polaron-induced molecular aggregation under electric field in essence, though further investigation is needed. Using materials with morphology stability under an electric field, electroplex was greatly reduced, which may enlarge the consideration in designing exciplex-based organic light-emitting diodes (ExOLEDs).

Triplet-triplet annihilation process in exciplex-based organic light-emitting diodes

The triplet-triplet annihilation process in exciplex-based devices is hardly considered to occur at all because it is a four-intermolecular- interaction process. In this work, the annihilation process was observed in an exciplex-based device with a higher donortriplet energy and a greater energy difference between the singlet and triplet states by using the magnetic field effects (MFEs) as a new non-contact measurement approach. The red-shifted emission of the spectrum suggests that the exciplex state did indeed form. The MFE results show that the magneto-electroluminescence (Δ EL / EL ) does not decrease remarkably under higher magnetic fields ( B >40 mT) at temperatures higher than 100 K ( T >100 K); however, decreasing Δ EL / EL will only appear when the measurement temperature is reduced to the minimum (20 K) temperature. In addition, the decreasing Δ EL / EL also depends on the current density. The evolution of Δ EL / EL at different voltages and temperatures further confirmed the existence of the annihilation process. The annihilation process formation conditions for exciplex-based devices were also discussed based on the experimental results.

Identifying the efficient inter-conversion between singlet and triplet charge-transfer states by magneto-electroluminescence study

Using the magneto-electroluminescence (MEL) as a tool, we demonstrated the efficient inter-conversion between singlet and triplet charge-transfer (CT) states in exciplex-based organic light-emitting diodes (OLEDs). Results show that the MEL of exciplex-based device is larger than that of exciton-based device by a factor of 3.2. The emission of exciplex-based devices comes from the direct intermolecular electron-hole pair recombination and their spin exchange energy is much smaller, which causes the efficient inter-conversion between singlet and triplet states. This argument was supported by the consistent evolutions of the MEL and EL spectra versus applied bias and donor concentrations. Finally, the bandgap effects on the MEL as well as the external quantum efficiency of exciplex-based devices were discussed. Our findings of MEL may offer a feasible way to unravel underlying mechanisms that limit the EL efficiency in the OLEDs.