Magneto-electroluminescence Responses Research Articles

Detection of Dexter energy transfer process in interface-type OLED via utilizing the characteristic magneto-electroluminescence response of hot exciton reverse intersystem crossing

The maximum external quantum efficiency of the host-guest-type organic light-emitting diodes (OLEDs) with interface exciplex as the host has been over 36%. However, studies about the energy transfer processes occurring from the host to guest remain lacking. Herein, a strategy is proposed to probe the energy transfer processes in interface-type OLEDs by utilizing the characteristic magneto-electroluminescence (MEL) response from the hot exciton reverse intersystem crossing (T<sub>2,Rub</sub> → S<sub>1,Rub</sub>) of rubrene. Specifically, a donor/spacer/accepter (D/S/A)-type interface exciplex device and a D/spacer:<i>x</i>% Emitter/A (D/S:3% Rubrene/A)-type Rubrene-doped device are fabricated. The Förster resonance energy transfer (FRET) process occurring between the singlet state of the exciplex-host and the singlet state of Rubrene-guest is demonstrated by characterizing the photophysical properties of the donor, accepter, and guest materials. The Dexter energy transfer (DET, T<sub>1,Host</sub> → T<sub>2,Rub</sub>) process between the triplet state of the host and the triplet state of guest is visualized by the comparative studying of the current- and temperature-dependent MEL response curves of D/S/A and D/S:3% Rubrene/A devices, respectively. More importantly, the occurrence of the DET process greatly promotes the electroluminescence intensity of the D/S:3% Rubrene/A device. Furthermore, we also investigate the differences in the electroluminescence performance of devices at low temperature to demonstrate again the co-existence of FRET and DET process in the D/S:3% Rubrene/A system. Obviously, this work not only provides a promising strategy for probing the DET process in OLEDs, but also paves a new way for designing high-performance “hot exciton” type OLEDs.

Investigations on exciton recombination and annihilation in TmPyPB-ETL OLEDs using magnetic field effects.

Although the effect of the electron transport layer (ETL) material TmPyPb on the electroluminescence performance of organic light-emitting diodes (OLEDs) has been extensively studied, the process of TmPyPb regulating exciton recombination and annihilation within the device is still unclear. Here, we fabricated devices of various TmPyPb thicknesses with and without ETL. Subsequently, we measured the magneto-electroluminescence (MEL) of these devices. Specifically, at the same luminance, the triplet-charge annihilation (TQA) process is more likely to occur as the thickness of TmPyPb increases, resulting in a decrease in the maximum luminance of devices. Due to electron leakage and exciton recombination region moving towards the cathode, leading to a decrease in luminance efficiency at first and then an enhancement with an increase in the thickness of TmPyPb. Furthermore, at room temperature, the application of a large bias voltage suppresses singlet fission (SF) processes by modulating the dissociation of singlet polaron pairs (PPS) and the concentration of triplet exciton (T1). This leads to the conversion of SF to the TQA process. At low temperatures, the bias voltage and temperature can regulate the concentration and lifetime of PPS and T1. Therefore, as the temperature decreases, the transition of SF → TQA → triplet-triplet annihilation (TTA) and TQA coexistence → TTA process occurs. Moreover, MEL responses of the TmPyPb-ETL device show a W-linear pattern owing to the combined effect of the hyperfine interaction (HFI) and Zeeman splitting at 145 K. Accordingly, we explored the electroluminescence (EL) performance of TmPyPB-ETL OLEDs and investigated the evolution of SF, TQA, and TTA processes using MEL. Our study revealed the effect of exciton recombination and annihilation in OLEDs with varying thicknesses of TmPyPb.

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

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 existing 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 (Tang X T, Pan R H, Zhao X, Jia W Y, Wang Y, Ma C H 2020 <i>Adv. Funct. Mater.</i> <b>5</b> 765) 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 value of several percent in content is doped into a host with high triplet exciton energy and there are no energy loss channels of triplet excitons from charge-carrier transporting layers either. Furthermore, this HL-RISC process can considerably increase 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 energy loss channels of triplet excitons from charge-carrier transporting layers, only strong singlet fission (S<sub>1,Rub</sub>+S<sub>0,Rub</sub> → T<sub>1,Rub</sub>+T<sub>1,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 is generated through 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. By comparing and analyzing these 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 experimental phenomenon is the physical reason why the ISC process and HL-RISC process cannot be observed simultaneously in the pure Rubrene-based OLEDs, and this phenomenon has not been found in the literature. Clearly, this work further deepeneds 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>).

The role of orientation in the MEL response of OLEDs.

Magneto electroluminescence (MEL) is emerging as a powerful tool to study spin dynamics in organic light emitting diodes (OLEDs). The shape of the MEL response is typically used to draw qualitative inference on the dominant process (singlet fission or triplet fusion) in the device. In this study, we develop a quantitative model for MEL and apply it to devices based on Rubrene, and three solution processable anthradithiophene emitters. The four emitters allow us to systematically vary the film structure between highly textured, poly-crystalline to amorphous. We find significant diversity in the MEL, with the textured films giving highly structured responses. We find that the additional structure does not coincide with energy anti-crossings, but intersections in the singlet character between adjacent states. In all cases the MEL can be adequately described by an extended Merrifield model. Via the inclusion of charge injection, we are able to draw additional information on underlying physics in OLED devices.

Isotope Effect in the Magneto-Optoelectronic Response of Organic Light-Emitting Diodes Based on Donor-Acceptor Exciplexes.

The isotope effect is studied in the magneto-electroluminescence (MEL) and pulsed electrically detected magnetic resonance of organic light-emitting diodes based on thermally activated delayed fluorescence (TADF) from donor-acceptor exciplexes that are either protonated (H) or deuterated (D). It is found that at ambient temperature, the exchange of H to D has no effect on the spin-dependent current and MEL responses in the devices. However, at cryogenic temperatures, where the reverse intersystem crossing (RISC) from triplet to singlet exciplex diminishes, a pronounced isotope effect is observed. These results show that the RISC process is not governed by the hyperfine interaction as thought previously, but proceeds through spin-mixing in the triplet exciplex. The observations are corroborated by electrically detected transient spin nutation experiments that show relatively long dephasing time at ambient temperature, and interpreted in the context of a model that involves exchange and hyperfine interactions in the spin triplet exciplex. These findings deepen the understanding of the RISC process in TADF materials.

Origin of Rashba Spin-Orbit Coupling in 2D and 3D Lead Iodide Perovskites

We studied spin dynamics of charge carriers in the superlattice-like Ruddlesden-Popper hybrid lead iodide perovskite semiconductors, 2D (BA)2(MA)Pb2I7 (with MA = CH3NH3, and BA = CH3(CH2)3NH3), and 3D MAPbI3 using the magnetic field effect (MFE) on conductivity and electroluminescence in their light emitting diodes (LEDs) at cryogenic temperatures. The semiconductors with distinct structural/bulk inversion symmetry breaking, when combined with colossal intrinsic spin–orbit coupling (SOC), theoretically give rise to giant Rashba-type SOC. We found that the magneto-conductance (MC) magnitude increases monotonically with the emission intensity and saturates at ≈0.05% and 0.11% for the MAPbI3 and (BA)2(MA)Pb2I7, respectively. The magneto-electroluminescence (MEL) response with similar line shapes as the MC response has a significantly larger magnitude, and essentially stays constant at ≈0.22% and ≈0.20% for MAPbI3 and (BA)2(MA)Pb2I7, respectively. The sign and magnitude of the MC and MEL responses can be quantitatively explained in the framework of the Δg-based excitonic model using rate equations. Remarkably, the width of the MEL response in those materials linearly increases with increasing the applied electric field, where the Rashba coefficient in (BA)2(MA)Pb2I7 is estimated to be about 7 times larger than that in MAPbI3. Our studies might have significant impact on future development of electrically-controlled spin logic devices via Rashba-like effects.

Magneto-electroluminescence response in 2D and 3D hybrid organic-inorganic perovskite light emitting diodes.

We have studied the magneto-electroluminescence (MEL) response in light emitting diodes based on 2D-(PEA)2PbI4 and 3D-MAPbI3 hybrid organic-inorganic perovskites at cryogenic temperatures. We found that the MEL is negative, i.e., the EL decreases with the applied field strength, B. In addition, the MEL(B) response has a Lorentzian line shape whose width depends on the perovskite used. We interpret the MEL(B) response using the "Δg mechanism" in which the spin of the injected electron-hole (e-h) pairs oscillates between singlet and triplet configurations due to different precession frequencies of the electron and hole constituents that originate from the difference, Δg, in the electron and hole gyromagnetic constants, g. In this model, the MEL(B) linewidth is inversely proportional to the spin lifetime and Δg. The model used is validated by directly measuring the spin lifetime of photogenerated e-h pairs using the circularly polarized pump-probe transmission technique with 100 fs resolution.

Magneto-Electroluminescence Study of Fringe Field in “Magnetic” Organic Light-Emitting Diodes

Magneto-electroluminescence (MEL) represents the electroluminescence intensity change upon application of an external magnetic field. We show that the MEL field response in "magnetic" organic light-emitting diodes, where one electrode is ferromagnetic (FM), is a powerful technique for measuring the induced fringe field, B⃗F, from the FM electrode in the organic layer. We found that the in-plane fringe field, B⃗F∥, from 3 nm Co and Ni80Fe20 FM electrodes is proportional to the applied field, B⃗∥. The fringe field of the 3 nm Ni80Fe20 film was also investigated for an applied out-of-plane magnetic field, B⃗⊥. We found that the out-of-plane fringe field has two components: a component that is parallel or antiparallel to B⃗⊥ and remains unchanged with the distance, d, from the FM electrode and the other component that is highly inhomogeneous, parallel to the surface, and steeply decreases with d. We show that the obtained B⃗F is independent of the underlying mechanism for the MEL(B) response and thus may be considered universal.

Understanding the mechanisms of light emission from DCJTB charge-transfer states in OLEDs using organic magnetic field effects

Although energy transfer (ET) and direct charge trapping (DCT) are two competing mechanisms that are known to occur in dye-doped organic light-emitting diodes (OLEDs), which of these processes governs the electroluminescence is not clear. Additionally, each process influences the formation of excitons, and the interactions between excitons, differently. If ET is the dominate process, the mechanism that governs an OLED involves both the host and guest materials. In contrast, if DCT is the dominant process, the mechanism that governs an OLED will only involve the guest material. Magneto-electroluminescence (MEL) is an effective technique for exploring and understanding the mechanisms that occur within OLEDs. It exhibits sensitive fingerprint responses to intersystem crossing (ISC), reverse intersystem crossing (RISC), triplet-triplet annihilation (TTA) and triplet-charge annihilation (TQA). In this work, we fabricated three different OLEDs and measured their MEL curves. Tris-8-hydroxyquinoline aluminum (Alq3), 4,4′- N , N ′-dicarbazolebiphenyl (CBP) and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T) were used as the host materials, each of which had a different triplet exciton energy. 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4- yl -vinyl)- 4 H -pyran (DCJTB) was used as a charge-transfer (CT) dopant that had a small energy difference between the singlet and triplet excited states. By analyzing the energy level structure of the different devices, the absorption spectrum of the dopant, and the emission spectra of the host materials, we determined that ET was the dominant process when DCJTB was used as a dopant with Alq3. However, DCT dominated when DCJTB was used as a dopant with both CBP and PO-T2T. The current- and temperature-dependent MEL curves from all devices were composed of low field components (| B | ≤| B | B because both TTA and TQA were competing processes, but their low-field changes were completely different. The MEL curves from the DCJTB/Alq3 device exhibited a positive MEL because polaron-pair (PP) states can occur on the host material via ISC (PPS→PPT). Conversely, the DCJTB/CBP and DCJTB/PO-T2T devices exhibited negative MEL responses because RISC (1CT←3CT) occurred in the CT states of the dopant. However, the triplet energy levels of the host material simultaneously impacted RISC. Hence, these devices exhibited a diverse range of MEL curves. Low temperatures enhanced RISC and TTA of the triple CT state within DCJTB, which was attributed to the increased concentration and lifetime of triplet excitons at low temperature. This work serves as a reference for improving the internal quantum efficiency of fluorescent OLEDs by enhancing the use of triplet states, and importantly gives a deeper understanding of the mechanisms that govern light-emission from devices that incorporate DCJTB CT materials.

Abnormal magnetic field effects in doped organic light-emitting devices

Organic magnetic field effects (OMFEs), which primarily includes magneto-electroluminescence (MEL) and magneto-conductance (MC), is a non-contact, non-destructive and sensitive tool for probing the mechanisms that occur within in organic light-emitting diodes (OLEDs). Not only OMFEs can be used to detect charge transport, exciton evolution and luminescence mechanisms in OLEDs, and can also be used in magnetic devices, including magnetic switches, magnetic storage and magnetic sensors, etc. Stable magnetic devices are usually required for practical applications. There is a wealth of research that shows that doped devices exhibit improved stability when compared with non-doped devices, however, they always show weaker OMFEs responses. Therefore, it is important to achieve strong OMFEs responses from stable, doped devices. In this work, we have fabricated OLEDs using the thermally activated delayed fluorescence (TADF) material 2,3,5,6-tetrakis(3,6-diphenyl-carbazol-9-yl)-1,4-dicyanobenzene (4CzTPN-Ph) of different doping concentration as the dopant, and either tris-8-hydroxyquinoline aluminum (Alq3) and 4,4′- N , N ′-dicarbazole-biphenyl (CBP) as the host materials. The light-emitting layer in these OLEDs was Alq3: x % 4CzTPN-Ph (studied devices) or CBP:5% 4CzTPN-Ph (reference device). The MEL and MC curves from these devices were measured with different injection currents and doping concentrations at room temperature, and with varying temperatures at a constant injection current and doping concentration. Compared with the reduced OMFEs observed from the reference device, the studied devices exhibited increased amplitudes in their MEL and MC responses. For example, at an injection current of 150 μA at room temperature, the amplitude of the MEL from the studied device reached ~10% when exposed to a magnetic field of 300 mT, which was approximately 13 times larger than that observed from the reference device (~0.75%). The corresponding amplitude from the MC from the studied device was ~6% at 300 mT, which was approximately 50 times larger than that observed from the reference device (~0.12%). In addition, the MEL and MC responses from the studied devices were determined by the doping concentration. At a doping concentration of approximately 15%, the MEL and MC values from the studied devices at 300 mT reached maximum values of 13.4% and 9.3%, respectively. Both MEL and MC values from the studied device reduced with decreasing temperatures. Analysis of the energy levels and optical spectra from these studied devices indicated that the special energy level arrangement between the guest and host molecules resulted in a weak energy level trap. The external magnetic field suppressed the scattering of charges by triplet excitons, a channel for triplet-charge annihilation (TQA), which generated significant MEL and MC responses. Therefore, OMFEs that were stronger than those from the reference device were obtained. In addition, since TQA is affected by the concentration and mobility of triplet excitons and carriers, the doping concentration and working temperature can also mediate TQA by modifying the concentration and mobility of triplet excitons and carriers, which eventually affects the OMFEs. This work gives insight into the mechanisms that occur within 4CzTPN-Ph-based light-emitting diodes, and also importantly promotes practical applications of magnetic organic light-emitting diodes.

Manipulation of Emission Colors Based on Intrinsic and Extrinsic Magneto-Electroluminescence from Exciplex Organic Light-Emitting Diodes

Exciplex organic light-emitting diodes (XOLEDs) utilize nonemissive triplet excitons via a reverse intersystem crossing process of thermally activated delayed fluorescence. The small energy difference between the lowest singlet and triplet levels of exciplex also allows a magnetic field to manipulate their populations, thereby achieving ultralarge “intrinsic” magneto-electroluminescence (MEL) in XOLEDs. Here we incorporate it into a hybrid type of spintronic device (“hybrid spin-XOLED”), where the XOLED is connected to a magnetic tunnel junction with large magnetoresistance, to introduce an “extrinsic” MEL response that interferes with the “intrinsic” MEL. The ratio between two MEL contributions, the MEL value, and the field response were altered by changing the exciplex layer thickness or actively manipulated by adding another current source that drives the XOLED. Most importantly, by involving two XOLEDs (green and red) in the same circuit, the hybrid spin-XOLED shows a color change when sweeping the ma...

Magneto-electroluminescence responses in Rubrene-based organic light-emitting diode at ultrasmall magnetic fields

In this work, the organic light-emitting diodes based on Rubrene have been fabricated to investigate the effects at ultrasmall magnetic fields. The magneto-electroluminescences (MELs) of the devices have been measured at different temperatures and at various bias voltages. We find that the MEL responses at ultrasmall fields show two opposite sign reversals at different temperatures: (i) a sign reversal (W-shaped) in a few millitesla range around zero-field at room temperature, and (ii) a peculiar M-shaped sign reversal by applying a ultrasmall magnetic field (| B |<2 mT) under low temperatures, which has not been reported in the literatures. We conjecture that the the former is strongly dependent on hyperfine interaction and Zeeman splitting of triplet polaron pairs, and the latter is originated from the interplay of zero-field splitting and Zeeman splitting on triplet pair states which generated from triplet-triplet annihilation. The MEL responses of devices examined under different bias voltages give further experimental evidences supporting our hypothesis. This study gives new insights into magnetic phenomena in organic optoelectronic devices and provides a deeper fundamental understanding of the spin dynamics of the injected carriers.

Magnetic Field Effect in Organic Light‐Emitting Diodes Based on Electron Donor–Acceptor Exciplex Chromophores Doped with Fluorescent Emitters

A new type of organic light‐emitting diode (OLED) has emerged that shows enhanced operational stability and large internal quantum efficiency approaching 100%, which is based on thermally activated delayed fluorescence (TADF) compounds doped with fluorescent emitters. Magneto‐electroluminescence (MEL) in such TADF‐based OLEDs and magneto‐photoluminescence (MPL) in thin films based on donor–acceptor (D–A) exciplexes doped with fluorescent emitters with various concentrations are investigated. It has been found that both MEL and MPL responses are thermally activated with substantially lower activation energy compared to that in the pristine undoped D–A exciplex host blend. In addition, both MPL and MEL steeply decrease with the emitter's concentration. This indicates the existence of a loss mechanism, whereby the triplet charge‐transfer state in the exciplex host blend may directly decay to the lowest, nonemissive triplet state of the fluorescent emitter molecules.

Determining the Origin of Half-bandgap-voltage Electroluminescence in Bifunctional Rubrene/C60 Devices

Lowering the driving voltage of organic light-emitting diodes (OLEDs) is an important approach to reduce their energy consumption. We have fabricated a series of bifunctional devices (OLEDs and photovoltaics) using rubrene and fullerene (C60) as the active layer, in which the electroluminescence threshold voltage(~1.1 V) was half the value of the bandgap of rubrene. Magneto-electroluminescence (MEL) response of planner heterojunction diodes exhibited a small increase in response to a low magnetic field strength (<20 mT); however, a very large decay was observed at a high magnetic field strength (>20 mT). When a hole-transport layer with a low mobility was included in these devices, the MEL response reversed in shape, and simultaneously, the EL threshold voltage became larger than the bandgap voltage. When bulk heterojunction device was examined, the amplitude of MEL curves presented an anomalous voltage-dependence. Following an analysis of the MEL responses of these devices, we proposed that the EL of half-bandgap-voltage device originated from bimolecular triplet-triplet annihilation in the rubrene film, rather than from singlet excitons that formed via an interface auger recombination. This work provides critical insight into the mechanisms of OLED emission and will help advance the applications of bifunctional devices.

The influence of doping concentration and temperature on magnetic field effect of delayed fluorescence in organic light-emitting diode

Up to date, both reverse intersystem crossing (RISC) and triplet–triplet annihilation (TTA) are effective approaches to improve the internal efficiency of fluorescent OLEDs, because they can make the best of triplet excitons to generate delayed fluorescence. The energy difference between singlet and triplet excited states ( D E S-T) of special intramolecular charge transfer (CT) species is quite small, which make it posible for the participation of the CT species in RISC and TTA. When the RISC and TTA coexist in fluorescent OLEDs, the utilization of triplet excitons should be much better. The goal of this work was to demonstrate the feasibility of the coexistence. However, determining whether the coexistence happens in fluorescent OLEDs is not an easy task, because the general methods can hardly distinct the delayed fluorescence is originated from which approaches. Fortunately, both RISC and TTA are highly spin-dependent processes that can generate sizable magneto-electroluminescence (MEL) responses in OLEDs. Typically, the MEL response of RISC is a low-field effect, i.e., the MEL decreases rapid within the low-field regime ( 20 mT). Hence, the MEL can be used as a novel tool to discern RISC and TTA in fluorescent OLEDs. Following such strategy, in this work, we firstly fabricated the devices by doping different concentrations of intramolecular CT states specie 4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) into phosphor host material 1,3-bis(9-carbazolyl)benzene (mCP) and then measured the MEL responses of these devices at room temperature. It exhibited that the sign of low-field ( B >20 mT) but was more prominent. These results are quite similar to the theoretical MEL curves of RISC in low-field ( B >20 mT), respectively. Thus, we believe that the delayed fluorescence of DCJTB-doped devices is caused by RISC and TTA simultaneously. Moreover, the RISC and TTA rates increase as the doping concentration decreasing. Subsequently, more remarkable RISC-dominanted low-field and TTA-dominanted high-field decrease of MEL were observed with decreasing temperature from 300 to 20 K. It is attributed to the fact that low temperature can significantly enhance RISC and TTA. In summary, low doping concentration and low temperature are more favorable to the coexistence of RISC and TTA in the DCJTB-doped devices. This work has a certain reference value for the improvments of internal quantum efficiency of fluorescent OLEDs via enhancing the utilization efficiency of triplet.

Magnetic Field Enhancement of Organic Light‐Emitting Diodes Based on Electron Donor–Acceptor Exciplex

Reverse intersystem crossing (RISC) from triplet to singlet states has been recently introduced to photophysics of organic chromophores. One type of RISC occurs in donor (D)–acceptor (A) composites that form an exciplex manifold in which the energy difference, ΔEST between the lowest singlet (S1) and triplet (T1) levels of the exciplex is small (&lt;100 meV) thus allowing RISC at room temperature. This adds a delayed component to the photoluminescence emission that is widely known as thermally activated delayed fluorescence. Here, it is found that the electroluminescence in organic light‐emitting diodes (OLED) based on electron D–A exciplex is significantly enhanced (up to ≈40%) by applying magnetic field, due to the existence of an additional spin‐mixing channel between singlet and triplet states in the exciplex. The large magneto‐electroluminescence (MEL) in N,N,N′,N′‐tetrakis(4‐methoxyphenyl)benzidine: tris‐[3‐(3‐pyridyl)mesityl]borane [D–A] based OLEDs is demonstrated. These results are supported by magneto‐photoluminescence (MPL) measurements, and density functional theory calculations. Importantly, it is found that both MEL and MPL are thermally activated indicating the dominant role of the RISC process. The MEL(B) response using the Δg mechanism is analyzed, where the electron and hole g‐values are different from each other because they reside in different molecules.

The magnetic field effects caused by three-combined mechanisms in organic light emitting diodes

The Rubrene-doped organic light emitting diodes were fabricated by doping Rubrene, containing the singlet-triplet energy- resonant property ( E S≈2 E T), into phosphorescent-based host materials with different concentrations to investigate their magneto-electroluminescence (MEL). The results show that the MEL responses, including the low field component ( B ≦5 mT) and the high field component (5 mT ≦ B ≦500 mT), change significantly with the doping concentration. These MEL curves could be attributed to the hyperfine interaction, singlet excition fission and triplet-triplet annihilation three-combined processes; the triplet excition has important effect on the luminescent intensity in OLEDs through changing the proportion of singlet exciton. In addition, the influence of temperature and injection current density on the MEL further supports our suggestion.

The spin-polarized organic light emitting diode

We report magnetic field, voltage and temperature dependent studies on recently realized spin-polarized organic light emitting diode (spin-OLED) with ferromagnetic (FM) electrodes that acts as a bipolar organic spin valve (OSV). In the double-injection regime the device shows ∼1% spin-valve related magneto-electroluminescence (MEL) response that follows the FM electrode coercive fields. In stark contrast to the properties of homopolar OSV devices, both the MEL and magneto-conductivity responses in the double-injection regime are practically bias voltage independent, and their temperature dependence follows that of the FM electrode magnetization. Careful comparative studies of devices with various FM/non-FM electrodes rule out the possibility that the observed MEL is due to the effect of stray magnetic fields associated with the FM electrodes. The spin-OLED behavior is explained by the spin-aligned space charge-limited current operation upon reaching double-injection conditions during bipolar operation.

Isotope effect in the spin response of aluminum tris(8-hydroxyquinoline) based devices

We studied the spin response of various magnetic field effects and magnetotransport in both protonated and deuterated aluminum tris(8-hydroxyquinoline) [Alq${}_{3}$]-based organic light emitting diodes and spin-valve devices. Both conductivity-detected magnetic resonance in diodes and magnetoresistance in spin valves show substantial isotope dependence pointing to the importance of the hyperfine interaction (HFI) in the spin response of spin \textonehalf{} charge polarons in Alq${}_{3}$. In addition, the low field ($B$ 20 mT) magnetoelectroluminescence (MEL) response is also isotope sensitive, showing that HFI-induced spin mixing of polaron-pairs spin sublevels dominates this response too. However, the magnetoconductance (MC) response was found to be much less sensitive to isotope exchange at low fields, in agreement with previous studies. The disparity between the isotope sensitivity of MC and MEL responses in Alq${}_{3}$ indicates that the HFI in the MC response is overwhelmed by an isotope independent spin mixing mechanism. We propose that collisions of spin \textonehalf{} carriers---with triplet species such as polaron pairs may be the main spin mixing mechanism in the low field MC response in Alq${}_{3}$ diodes.

Using Magneto-Electroluminescence As a Fingerprint to Identify the Carrier-to-Photon Conversion Process in Dye-Doped OLEDs

Long-range F€ energy transfer (FET) and short-range charge trapping (CT) are two competing basic mechanisms in the carrier-to-photon processofdye-dopedorganicdiode,butlittleisknownaboutwhichofFETandCT governs the electroluminescence process for a given dye. Here we report that magneto-electroluminescence (MEL) response can serve to identify the funda- mental issue. (1) In relative high magnetic field (>20 mT), dramatic decrease in MEL response implies that CT dominates carrier-to-photon process for a common fluorescent dye of 4-(dicyano-methylene)-2-methyl-6-(p-dimethylaminostyryl)- 4H-pyran (DCM1), whereas saturating of MEL response indicates that FET is dominant for another common fluorescent dye of Coumarin 540 (C540). (2) In low field (<15 mT), overlapping between MEL responses of DCM1-doped and control OLED shows that prior to being trapped by DCM1 molecule, electron and hole have formed electrostatically bound e-h polaron pair in adjacent Alq3 molecules. Both of the observations are finally confirmed by intermolecular correlated quantum calculation.