Guest concentration, bias current, and temperature-dependent sign inversion of magneto-electroluminescence in thermally activated delayed fluorescence devices

Hybridized local and charge transfer dendrimers with near-unity exciton utilization for enabling high-efficiency solution-processed hyperfluorescent OLEDs

Achieving both high emission efficiency and exciton utilization efficiency (ηS) in hot exciton materials is still a formidable task. Herein, a proof-of-concept design for improving ηS in hot exciton materials is proposed via elaborate regulation of singlet-triplet energy difference, leading to an additional thermally activated delayed fluorescence (TADF) process. Two novel dendrimers, named D-TTT-H and D-TTT-tBu, were prepared and characterized, in which diphenylamine derivatives were used as a donor moiety and tri(triazolo)triazine (TTT) as an acceptor fragment. Compounds D-TTT-H and D-TTT-tBu showed an intense green color with an emission efficiency of approximately 80% in solution. Impressively, both dendrimers simultaneously exhibited a hot exciton process and TADF characteristic in the solid state, as was demonstrated via theoretical calculation, transient photoluminescence, magneto-electroluminescence and transient electroluminescence measurements, thus achieving almost unity ηS. A solution processable organic light-emitting diode (OLED) employing the dendrimer as a dopant represents the best performance with the highest luminance of 15090 cd m-2 and a maximum external quantum efficiency (EQEmax) of 11.96%. Moreover, using D-TTT-H as a sensitizer, an EQEmax of 30.88%, 24.08% and 14.33% were achieved for green, orange and red solution-processed OLEDs, respectively. This research paves a new avenue to construct a fluorescent molecule with high ηS for efficient and stable OLEDs.

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 (ES1,rub≈ 2ET1,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,rubconcentration 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 Alq3in 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 Alq3polaron 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.

Efficient and Bright Organic Radical Light-Emitting Diodes with Low Efficiency Roll-Off

Organic radicals have been of interest due to their potential to replace nonradical-based organic emitters, especially for deep-red/near-infrared (NIR) electroluminescence (EL), based on the spin-allowed doublet fluorescence. However, the performance of the radical-based EL devices is limited by low carrier mobility which causes a large efficiency roll-off at high current densities. Here, highly efficient and bright doublet EL devices are reported by combining a thermally activated delayed fluorescence (TADF) host that supports both electron and hole transport and a tris(2,4,6-trichlorophenyl)methyl-based radical emitter. Steady-state and transient photophysical studies reveal the optical signatures of doublet luminescence mechanisms arising from both host and guest photoexcitation. The host system presented here allows balanced hole and electron currents, and a high maximum external quantum efficiency (EQE) of 17.4% at 707 nm peak emission with substantially improved efficiency roll-off is reported: over 70% of the maximum EQE (12.2%) is recorded at 10 mA cm-2 , and even at 100 mA cm-2 , nearly 50% of the maximum EQE (8.4%) is maintained. This is an important step in the practical application of organic radicals to NIR light-emitting devices.

Mediation of exciton concentration on magnetic field effects in NPB : Alq3-based heterojunction OLEDs

Organic light-emitting diodes (OLEDs) are considered one of the most promising new display technologies owing to their advantages, such as all-solid-state, high color gamut, and wide viewing angle. However, in terms of special fields, the brightness, lifetime, and stability of the devices need further improvement. Therefore, heterojunction devices with different concentrations were prepared to regulate device brightness. The brightness of the bulk heterojunction device is enhanced by 9740 cd m-2, with a growth rate of about 26.8%. The impact of various temperatures and various exciton concentrations on the device magneto-conductance (MC) and magneto-electroluminescence (MEL) was investigated. Experimental results demonstrate that the exciton concentration inside the device can be tuned to improve optoelectronic properties and organic magnetic effects. The complex spin mixing process inside the bulk heterojunction device is deeply investigated, which provides a reliable basis for the design of bulk heterojunction devices.

Spin-Enhanced Reverse Intersystem Crossing and Electroluminescence in Copper Acetate-Doped Thermally Activated Delayed Fluorescence Material

Thermally activated delayed fluorescence (TADF) materials are attractive for next-generation organic light-emitting diodes (OLEDs) because of their utilization of nonradiative triplets via reverse intersystem crossing (RISC), which requires not only a small singlet-triplet energy splitting but also the conservation of spin angular momentum. Here we use copper acetate as a spin sensitizer to facilitate RISC and thus enhance electroluminescence in TADF-exciplex OLEDs. Copper acetate is involved in the radiative decay process due to its coordination interaction with exciplex molecules having intermolecular charge-transfer characteristics, which causes significant changes in the photoluminescence intensity and lifetime. Meanwhile, magneto-photoluminescence reveals that the addition of copper acetate promotes spin conversion in the RISC process. It allows the enhancement of the electroluminescence (∼80%) from spin-sensitized OLEDs, accompanied by the suppression of magneto-electroluminescence upon the doping of copper acetate. These results illustrate that using a spin sensitizer may overcome the limitation of harvesting nonradiative triplets in organic luminescent materials and devices.

Achievement of High-Level Reverse Intersystem Crossing in Rubrene-Doped Organic Light-Emitting Diodes

Using the fingerprint magneto-electroluminescence trace, we observe a fascinating high-level reverse intersystem crossing (HL-RISC) in rubrene-doped organic light-emitting diodes (OLEDs). This HL-RISC is achieved from high-lying triplet states (T_2,rub) transferred from host triplet states by the Dexter energy transfer to the lowest singlet states (S_1,rub) in rubrene. Although HL-RISC decreases with bias current, it increases with lowering temperature. This is contrary to the temperature-dependent RISC from conventional thermally activated delayed fluorescence, because HL-RISC is an exothermic process instead. Moreover, owing to the competition of exciton energy transfer with direct charge trap, HL-RISC changes nonmonotonically with the dopant concentration and increases luminous efficiency to a maximum at 10% of rubrene, which is about ten times greater than that from the pure-rubrene device. Additionally, the HL-RISC process is not observed in bare rubrene-doped films because of the absence of T_2,rub. Our findings pave the way for designing highly efficient orange fluorescent OLEDs.

Spin-pair state-induced exceptional magnetic field responses from a thermally activated delayed fluorescence-assisted fluorescent material doping system

The thermally activated delayed fluorescence (TADF) material 2,3,5,6-tetrakis(3,6-diphenylcarbazol-9-yl)-1,4-dicyanobenzene (4CzTPN-Ph) and the conventional fluorescent dopant 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) were used to co-dope the host material 4,4'-bis(carbazol-9-yl)biphenyl (CBP) for the fabrication of TADF-assisted fluorescent organic light-emitting diodes (OLEDs). Some exceptional magnetic field effect (MFE) curves with abundant structures and four tunable components within a low magnetic field range (≤50 mT) were obtained, in sharp contrast to the maximum of two components observed in typical OLEDs. These MFE components were easily tuned by the injection current, dopant concentration, working temperature, and dopant energy gap, leading to a wide variety of MFE curve line shapes. The experimental results are attributed to the spin-pair state inter-conversions occurring in the device, including intersystem crossing (ISC) of CBP polaron pairs, ISC of 4CzTPN-Ph polaron pairs, reverse ISC (RISC) of 4CzTPN-Ph excitons, RISC of DCJTB polaron pairs, DCJTB triplet fusion, and DCJTB triplet-charge annihilation. Moreover, the exciton energy transfer processes among the host material and the guest dopants had a pronounced impact on the formation of these four components. This work gives a deeper understanding of the microscopic mechanisms of TADF-based co-doped systems for the further development of organic magnetic field effects in the extensive field of OLEDs.

Intersystem Crossing and Triplet Fusion in Singlet-Fission-Dominated Rubrene-Based OLEDs Under High Bias Current

Singlet fission is usually the only reaction channel for excited states in rubrene-based organic light-emitting diodes (OLEDs) at ambient temperature. Intriguingly, we discover that triplet fusion (TF) and intersystem crossing (ISC) within rubrene-based devices begin at moderate and high current densities (j), respectively. Both processes enhance with decreasing temperature. This behavior is discovered by analyzing the magneto-electroluminescence curves of the devices. The j-dependent magneto-conductance, measured at ambient temperature indicates that spin mixing within polaron pairs that are generated by triplet-charge annihilation (TQA) causes the occurrence of ISC, while the high concentrations of triplets are responsible for generating TF. Additionally, the reduction in exciton formation and the elevated TQA with decreasing temperature may contribute to the enhanced ISC at low temperatures. This work provides considerable insight into the different mechanisms that occur when a high density of excited states exist in rubrene and reasonable reasons for the absence of EL efficiency roll-off in rubrene-based OLEDs.