Magnetoconductance effect in organic light-emitting devices

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 B-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 B-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 B-mediated triplet-charge annihilation (TQA) process at room temperature, but those of Dev. 2 at 20 K present the B-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 (EX1 ← EX3) in Dev.1. Contrarily, the lower triplet exciton energy of the CBP donor causes the triplet exciplex to experience an energy loss process (EX3 → T1, 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.

Investigations of microscopic mechanisms in exciplex-based devices with isomers of mCBP and CBP as donors via magneto-electroluminescence

Ya-Ru¹,

Zhao²,

Tang³

et al. 2022

Reasons for “disappearance” phenomenon of both intersystem crossing of polaron-pair states and reverse intersystem crossing of high-lying triplet excitons in pure Rubrene-based OLEDs

Wang¹,

Ya-Ru²,

Feng-Jiao³

et al. 2022

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, S1,Rub ← T2, Rub) of high-lying triplet excitons (T2, Rub) 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 (S1, Rub+S0, Rub → T1, Rub+T1, Rub) 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, PP1 → PP3) 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 B-mediated ISC of polaron-pair just completely cancel out the negative Lorentzian MEL curves induced from B-mediated HL-RISC process of T2, Rub 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 2T1 and S1 and the energy approach between T2 and S1).

Negative magnetic efficiency induced by Dexter energy transfer in coexistence system of exciplex and electroplex

Wu¹,

Zhu²,

Wei³

et al. 2022

Exciplex-type organic light-emitting diodes (OLEDs) are research focus at present because of the high-efficiency luminescence at low cost due to the reverse intersystem crossing (RISC, EX1 ← EX3). Their microscopic processes usually exhibit intersystem crossing (ISC, PP1 → PP3) process dominated by polar pairs, resulting in the magneto-electroluminescence[MEL, MEL=(DEL)/EL×100%] effect values and the magneto-conductance[MC, MC=(DI)/I×100%] effect values are all positive, and the amplitude of MEL is greater than that of MC at the same current; the corresponding magnetic efficiency[Mη, Mη=(Dη)/η×100%] values are also positive due to the linear relationship EL ∝ η×I within general current (I) range. Surprisingly, although the MEL values of the device coexisting with exciplex and electroplex are also greater than the MC values at low current, MEL values are less than MC values at high current. In other words, Mη values of this device undergo a conversion from positive to negative with increasing current. In this work, to find out the reason why Mη values of exciplex-type OLED formed by TAPC and TPBi show negative under high current and study the micro-dynamic evolution mechanisms of spin-pair states in this device, three OLEDs were fabricated and their luminescence spectra and organic magnetic field effects were measured. The results indicate that the electroplex is produced in the exciplex-type OLED formed by TAPC and TPBi. Since the triplet exciton energies of monomer TAPC and TPBi are higher than the energy of triplet charge-transfer states of exciplex (CT3ex), and the CT3ex energy is greater than the energy of triplet charge-transfer states of electroplex (CT3el), the CT3ex energy can only be transferred to CT3el through Dexter energy transfer (DET) process without other loss channels. The electroluminescence (EL) spectrum of this device shows the luminescence intensity of exciplex is greater than that of electroplex, which indicates that the amount of exciplex is more than that of electroplex. Besides, EL spectra at different currents prove that the formation rate of exciplex is faster than that of electroplex with increasing current. Less quantity of exciplex at low current, so the DET process from CT3ex to CT3el is too weak to facilitate the RISC process of charge-transfer states of electroplex (CTel). Therefore, the low field amplitude of Mη curve is positive at low current. The number of spin-pair states of exciplex increases with increasing current, which enhances the DET process. These processes of direct charge carriers trapped and energy transfer critically increases the number of CT3el at high current, which strongly boosts the RISC process of CTel. Therefore, the low field amplitude of Mη curve changes from positive to negative with increasing current. Furthermore, the Mη curves of this device were measured when only exciplex exist and only electroplex exist employing filter, respectively. As expected, the results confirm the accuracy of the mechanisms of the negative value of the total Mη for this device. Obviously, this work contributes to the comprehension of the internal micro-physical mechanisms in OLEDs and the law of interactions of excited states.