The mCBP and CBP are two kinds of isomers containing carbazole groups and often used as the device hosts for fluorescence and phosphorescence emission. However, there are little studies on the microscopic mechanisms of exciplex-type devices based on mCBP or CBP. In this paper, the isomers of mCBP and CBP are used as donors and the PO-T2T is selected as an acceptor. The two kinds of exciplex-based devices are fabricated according to a mass ratio of 1:1, which are respectively referred to as device 1 (Dev. 1) and device 2 (Dev. 2). Their magneto-electroluminescence (MEL) curves are measured at different working temperatures and various injection currents. It is found that the low field effects of the MEL curves from Dev. 1 are dominated by the <i>B</i>-mediated reverse intersystem crossing (RISC) process at room temperature, and as the operational temperature decreases, the MEL line-shapes change gradually from RISC to the intersystem crossing (ISC) process. Conversely, the low field effects of the MEL curves of Dev. 2 are governed by the <i>B</i>-mediated ISC process at room temperature, and the ISC process first weakens then strengthens with temperature decreasing. The high field effects of the MEL curves of Dev. 1 and Dev. 2 are both dominated by the <i>B</i>-mediated triplet-charge annihilation (TQA) process at room temperature, but those of Dev. 2 at 20 K present the <i>B</i>-mediated triplet-triplet annihilation (TTA) process. The completely opposite low-field line-shapes of MEL traces from Dev. 1 and Dev. 2 can be attributed to their different structures of mCBP and CBP, which lead to the higher and lower triplet state exciton energy, respectively. The higher triplet exciton energy of the mCBP donor causes the triplet exciplex energy to be confined effectively, which promotes the RISC process (EX<sub>1</sub> ← EX<sub>3</sub>) in Dev.1. Contrarily, the lower triplet exciton energy of the CBP donor causes the triplet exciplex to experience an energy loss process (EX<sub>3</sub> → T<sub>1</sub>, CBP) , resulting in the suppressed RISC process in Dev. 2. Consequently, the overlapped effects of the ISC process of polaron pairs and the RISC process of exciplex in Dev. 2 under the action of external magnetic field display the ISC-determined process at room temperature. Moreover, the temperature-dependent change in the microscopic process of Dev. 1 such as the conversion from RISC to ISC is because decreasing temperature is not conducive to the occurrence of the RISC process of exciplex states due to its endothermic property. The low-temperature TTA process occurring in Dev. 2 is due to the suppressed energy loss process of triplet exciplex via the Dexter energy transfer from the triplet exciplex to the triplet exciton of CBP donor. In addition, when the mass ratio of mCBP donor to PO-T2T acceptor varies from 1:4 to 1:1 to 4:1, the RISC process of MEL curves of devices turns stronger and stronger, which is because the devices tend more to balance, favoring the RISC process. A higher external quantum efficiency is obtained in the mCBP:PO-T2T host than in the CBP:PO-T2T host when fluorescent guest material of TBRb is used as a dopant in these two exciplex-based devices, which verifies the importance of the effective confinement of triplet exciplex energy in improving the luminescence efficiency. Note that via the MEL detection technology, the current- and temperature-dependent microscopic processes and their reasonable interpretations and device performances from exciplex-based devices with the isomers of mCBP and CBP as donors have not been reported in the literature. This work provides experimental and theoretical references for fabricating the high-efficiency exciplex-based organic light-emitting devices.
With unique advantages of high sensitivity, no-contact, and non-destructiveness, magneto-electroluminescence (MEL) is usually employed as an effective detection tool to visualize the microscopic mechanisms of excited states existed in organic light-emitting diodes (OLEDs) because their evolution channels of many spin-pair states in OLEDs have the fingerprint MEL line-shapes even with opposite signs. The recently-published MEL results have demonstrated the existence of high-level reverse intersystem crossing process (HL-RISC, S<sub>1,Rub</sub> ← T<sub>2, Rub</sub>) of high-lying triplet excitons (T<sub>2, Rub</sub>) in Rubrene when Rubrene with a typical content of several percent is doped into a host with high triplet exciton energy and there are also no any energy loss channels of triplet excitons from charge-carrier transporting layers. Furthermore, this HL-RISC process can considerably boost the efficiency and brightness of OLEDs operated at room temperature, for example, high external quantum efficiency up to 16.1% and ten thousands of brightness have been achieved in Rubrene-doped OLEDs with a co-host of exciplex. Herein, surprisingly, in the pure Rubrene-based OLEDs (i.e., the pure Rubrene film is used as an emissive layer) with no any energy loss channels of triplet excitons from charge-carrier transporting layers, only strong singlet fission (S<sub>1</sub><sub>, Rub</sub>+S<sub>0</sub><sub>, Rub</sub> → T<sub>1</sub><sub>, Rub</sub>+T<sub>1</sub><sub>, Rub</sub>) processes are detected at room temperature, but this HL-RISC process is not observed. Moreover, even the most usual evolution process of intersystem crossing of polaron-pair (ISC, PP<sub>1</sub> → PP<sub>3</sub>) cannot be observed in this pure Rubrene-based OLEDs, where the polaron-pair are generated from the recombination of the injected electrons and holes in the pure Rubrene emissive layer. To determine the cause of the underlying physical mechanism behind this abnormal and fascinating experimental phenomena, two kinds of devices with pure Rubrene and 5% Rubrene-dopant as emissive layers are fabricated and their current- and temperature- dependent MEL responses are systematically investigated. Via comparing and analyzing thes tremendously different MEL curves of these two types of devices, we find that the positive Lorentzian MEL curves induced from <i>B</i>-mediated ISC of polaron-pair just completely cancel out the negative Lorentzian MEL curves induced from <i>B</i>-mediated HL-RISC process of T<sub>2, Rub</sub> excitons. Note that such an abnormal and coincidental experiment phenomenon is the physical mechanism why ISC and HL-RISC processes cannot be observed simultaneously in the pure Rubrene-based OLEDs, and this phenomenon has never been observed in the literature. Clearly, this work has further deepened our understanding of some unique microscopic processes and physical phenomena in organic semiconductor "star" material of Rubrene (such as the energy resonance between 2T<sub>1</sub> and S<sub>1</sub> and the energy approach between T<sub>2</sub> and S<sub>1</sub>).
Intersystem crossing (ISC) and reverse ISC (RISC) between singlet and triplet polaron-pair and exciplex states are important spin-mixing processes in exciplex-based organic light-emitting diodes (EB-OLEDs). These two processes usually show normal current dependencies which weaken with increasing the bias-current. This is because the increase in the bias-current is realized by improving the device bias-voltage. When the bias-voltage rises, the electric field within the device enhances, 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 and RISC processes leading to the reduction of these two processes. 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 mA) to abnormal ISC (25-200 mA) processes, whereas those of the balanced device show conversions from normal ISC (1-5 mA) to abnormal RISC (10-50 mA) and then to normal RISC (50-150 mA) and finally to abnormal ISC (200-300 mA) processes. By fitting and decomposing the current-dependent MEL traces of the unbalanced and balanced devices, we find that the ISC and RISC processes in these two devices first enhance but then weaken as the bias-current increases. These non-monotonic current-dependent ISC and RISC processes are attributed to the competition between the increased number and the reduced lifetime of polaron-pair and exciplex states 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 to the increased number of triplet exciplex states. That is, more triplet exciplex states can convert into singlet exciplex states through the RISC process, causing a higher external quantum efficiency of the balanced device 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 insights of device physics for designing and fabricating high-efficiency EB-OLEDs.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.