Complete polarization of electronic spins in OLEDs

Scharff, Tobias; Ratzke, Wolfram; Zipfel, Jonas; Klemm, Philippe; Bange, Sebastian; Lupton, John M.

doi:10.1038/s41467-021-22191-3

Cited by 14 publications

(9 citation statements)

References 69 publications

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“…Recently, magnetoelectroluminescence (MEL) has been used as an effective and sensitive detection tool to visualize the spin-related behavior of microscopic processes in various OLEDs. − For example, Peng et al found high-level reverse intersystem crossing (HL-RISC) from high-lying triplet to S 1 states in N , N -diphenyl-4-(9-phenylnaphtho-[2,3- c ][1,2,5]thiadiazol-4-yl)aniline-based OLEDs through MEL measurements . Scharff et al probed the complete polarization of electronic spins in OLEDs via analyzing temperature- and current-dependent MEL responses . Guo et al used MEL measurements to unravel the important role of the spin conversion process between high-lying triplet and S 1 states in aggregation-induced, emission-based OLEDs .…”

Section: Results and Discussionmentioning

confidence: 99%

Realization of H-Type Aggregation in Rubrene-Doped OLEDs and Its Induced Enhancement of Delayed Fluorescence

Tang

Zhao

et al. 2022

J. Phys. Chem. C

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The molecular aggregation effect on the photophysical properties of many organic semiconductors has been well studied, but this behavior of rubrene molecules is still vague in rubrene-doped organic light-emitting diodes. Surprisingly, via recording temperature-dependent electroluminescence and photoluminescence spectra, an intriguing H-type aggregation of rubrene molecules is realized from rubrene-doped devices and films only when 4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (m-MTDATA) or 4,4′,4″-tris(N-(2-naphthyl)-N-phenyl-amino)triphenylamine (2TNATA) is selected as the host; otherwise, the J-type aggregation of rubrene molecules is solely observed. We propose that this H-type aggregation is caused by the increased steric hindrance between adjacent rubrene molecules because the unique molecular structures of 12 freely rotating chemical bonds in m-MTDATA or 2TNATA are suppressed in rubrene-doped solid-state films. More importantly, we find that the H-type aggregation of rubrene molecules induces a nearly 3-fold increase in the fluorescence of rubrene-doped devices at 20 K. The increased fluorescence can be mainly attributed to the intensified triplet–triplet annihilation (TTA) process because the parallel alignment of transition dipole moments in H-aggregation can facilitate the TTA process via enlarging the TTA reaction cross-section. Thus, our findings deepen the understanding of molecular interactions in aggregation systems and pave the way for achieving highly efficient devices operated especially in a low-temperature environment such as outer space.

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Section: Results and Discussionmentioning

confidence: 99%

Realization of H-Type Aggregation in Rubrene-Doped OLEDs and Its Induced Enhancement of Delayed Fluorescence

Tang

Zhao

et al. 2022

J. Phys. Chem. C

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“…Because the rate of RISC in the TADF-based OLEDs depends on the activation energy ( E act ) of RISC, the MPL caused by the field-modulated RISC should also be activated. Figure d plots MPL versus temperature in the temperature range from 310 to 190 K. It is shown that MPL of both doped and undoped samples can be well fitted by the following Arrhenius equation: where k B is Boltzmann’s constant and T is the temperature, confirming that the field-modulated RISC channel is indeed activated. The linear fit at high temperatures gives an activation energy E act of ∼24 meV for the TADF material doped with a spin sensitizer and an E act of ∼48 meV for the TADF material without a spin sensitizer, which are of the order of k B T at room temperature (∼26 meV) and also in agreement with that (∼50 meV) obtained in a previous report .…”

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confidence: 75%

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

Jin

Zhou

Wang

et al. 2022

J. Phys. Chem. Lett.

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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.

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“…The underlying microscopic mechanisms for spin-pair states in Device 1 are schematically displayed in Figure e. Initially, electrons and holes are injected from opposite electrodes and recombine under the Coulombic attraction, then form weakly bound singlet and triplet polaron pairs (i.e., PP 1 and PP 3 ) in the emission layer. , PP 1 and PP 3 can undergo the interconversion (i.e., PP-ISC and PP-RISC processes) because they are almost degenerate in energy, and the spin flipping in the PP states can be realized under the hyperfine interaction (HFI) with the scale of several milli-Tesla. − Next, PP 1 and PP 3 will dissociate back into free carriers or form EX 1 and EX 3 with rate constants of k S and k T . Note that the interconversion of PP states is frequently dominated by the PP-ISC process (PP-ISC, PP 1 → PP 3 ) because k T is generally larger than k S .…”

Section: Resultsmentioning

confidence: 99%

Identifying the Exciplex-to-Exciplex Energy Transfer in Tricomponent Exciplex-Based OLEDs through Magnetic Field Effect Measurements

Zhao

Zhu

et al. 2022

ACS Photonics

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Tricomponent exciplex-based organic light-emitting diodes (TE-OLEDs) with high- and low-energy exciplexes have recently gained much attentions because of their already-reported high external quantum efficiency (EQE) and their exciplex-to-exciplex energy transfer (EE-ET) is considered as an important factor to influence the device performance. However, few works provide evidence to prove this EE-ET process due to the lack of the absorption band of exciplexes. Herein, the EE-ET channel from a high-energy exciplex donor (EXED) to another low-energy exciplex acceptor (EXEA) in a TE-OLED is demonstrated by probing magnetic field effects (MFEs) on the electrical and optical properties of devices including magneto-conductance (MC), magneto-efficiency (Mη), and magneto-electroluminescence (MEL), because this EE-ET can influence the evolution channels of spin-pair states in the TE-OLED which could be visualized by these featured MFE traces. Specifically, all the MC, Mη, and MEL curves of the single EXED-based OLED depict the normal bias-current (I)-dependent intersystem crossing (ISC) from singlet to triplet polaron pairs, which weakens with increasing I. Moreover, the Mη and MEL traces of the single EXEA-based OLED, respectively, present the abnormal and normal I-dependent ISC, while its MC curves show the conversion from reverse ISC (RISC) of exciplexes to ISC of polaron pairs with increasing I. However, both MEL and Mη traces of the TE-OLED with simultaneous EXED and EXEA show the abnormal I-dependent RISC from triplet to singlet exciplexes, which enhances with increasing I, while its MC traces display normal I-dependent RISC. These RISC behaviors have seldom been observed previously in the literature, which are induced by the EE-ET process from EXED to EXEA that facilitate the RISC channel of EXEA via increasing the quantity of triplet exciplex states. Moreover, higher EQE is obtained in the TE-OLED with this EE-ET channel from EXED to EXEA than the single EXEA-based OLED. Thus, these MFE measurements provide new strategies for recognizing the EE-ET process in OLEDs based on multiple exciplex emitter systems and pave the way for designing superior performance exciplex-based OLEDs.

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Complete polarization of electronic spins in OLEDs

Cited by 14 publications

References 69 publications

Realization of H-Type Aggregation in Rubrene-Doped OLEDs and Its Induced Enhancement of Delayed Fluorescence

Realization of H-Type Aggregation in Rubrene-Doped OLEDs and Its Induced Enhancement of Delayed Fluorescence

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

Identifying the Exciplex-to-Exciplex Energy Transfer in Tricomponent Exciplex-Based OLEDs through Magnetic Field Effect Measurements

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