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Although the host material has been proved to be a crucial factor affecting the occurrence of the high-level reverse intersystem crossing (HL-RISC, T2,rub → S1,rub) process in rubrene-doped organic light-emitting diodes (OLEDs), the roles of the charge-transporting layers on this amazing process are still unclear. Very interestingly, herein, we found that the efficient conversion from intersystem crossing (ISC) to HL-RISC is observed by tuning the triplet energies of hole- and electron-transporting layers (HTL and ETL) in rubrene-doped devices. Specifically, when the triplet energies of ETL, rubrene guest, and HTL satisfy the criterion of E(T1,ETL) < E(T2,rub) < E(T1,HTL), the conversion process can be observed under various current and temperature conditions, which can be reasonably explained by the weak energy-loss channel from T2,rub to T1,ETL states and the competition effects of excited states in the ETL and guest. Otherwise, ISC (HL-RISC) will dominate in devices due to the existence (non-existence) of energy-loss channels from T2,rub to T1,HTL states. Furthermore, very importantly, by inserting appropriate HTL or ETL materials with high triplet energies into the control devices with energy-loss channels, we found that the full confinement of E(T2,rub) indeed facilitates the HL-RISC process. Our work deepens the cognition of the HL-RISC process and provides an effective method to achieve the HL-RISC process in organic optoelectronic devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We fabricated quantum-well organic light-emitting diodes (QW-OLEDs) based on thermally activated delayed fluorescence (TADF) and measured their magneto-electroluminescence (MEL) and magneto-conductance (MC) curves over various magnetic field ranges. When the number (N) of QWs was 7, the MC curves showed a flat structure within the magnetic field range 5 ＜ ☋B☋ ＜ 20 mT. The flat structure vanished with the use of a higher electron mobility material, which we attribute to competition between the dissociation of triplet excitons by holes and scattering of electrons by triplet excitons in the QW-TADF-OLEDs. In addition, when N ranged from 3 to 9, the MEL curves in the range ☋B☋ ＜ 10 mT presented a W-like line-shape and the MC curves in the range ☋B☋ ＜ 10 mT showed variation of their line-shape from W-like to negative Lorentzian-type; however, the B-dependent efficiency Mη (i.e., MEL–MC) curves in the range ☋B☋ ＜ 10 mT, which were independent of N, showed a V-type line-shape. Our analysis suggests that these magnetic response behaviors depend on the combined effects of intersystem crossing of polaron pairs, reverse intersystem crossing of charge transfer excitons, and triplet-charge interactions under an external field B. This work provides insight into spin-pair dynamic mechanisms occurring within QW-TADF devices and suggests potential applications for a single OLED with integrated magnetic, optical, and electric properties. [ABSTRACT FROM AUTHOR]

An in situ heating method was used to investigate the effect of high temperature on the magneto-electroluminescence (MEL) of rubrene-based organic light-emitting diodes. The change of MEL indicated that the microscopic mechanism of the device converted from singlet fission (SF) to intersystem crossing (ISC) at 380 K. Analysis of the surface morphology, electroluminescence spectrum, and current–voltage curve of the device suggested that the SF-to-ISC conversion was caused by a large number of exciton traps generated in the organic layers of the device. These traps blocked the movement of excitons, which suppressed SF. By contrast, ISC was promoted by traps and became the main process. This work provides a detailed investigation of the dynamics of exciton evolution and deepens our understanding of electron–hole interaction mechanisms in organic devices at high temperature. [ABSTRACT FROM AUTHOR]