Abnormal Reverse Intersystem Crossing of Polaron-Pair States and Its Conversion to Intersystem Crossing via the Regulation of Intermolecular Electron-Hole Spacing Distance

Xu, Jing; Tang, Xiantong; Zhao, Xi; Zhu, Hong Qiang; Qu, Fenlan; Xiong, Zuhong

doi:10.1103/physrevapplied.14.024011

Cited by 20 publications

(16 citation statements)

References 64 publications

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“…8 Thus, from now on, the conversion of the EX states mainly refers to the EX-RISC process, as displayed by the horizontal black arrow in Figure 2e. Note that, as is well demonstrated in the literature, 8,30 MFEs, such as MC, Mη, and MEL, are an overlapping effect when there are several Bsensitive microscopic processes simultaneously existing in the device studied. Therefore, the I-dependent MC, Mη, or MEL traces are determined by the combined effects of the Bmediated PP-ISC and EX-RISC processes in the exciplex-based OLEDs.…”

Section: I-dependent Mel MC and Mη Traces Of Devicesupporting

confidence: 61%

“…That is, the PP-ISC process is much stronger than the EX-RISC one in the single EX ED -based OLED (i.e., Device 1), as shown in Figure e. Because ISC and RISC processes have the same MFE line-shapes but with opposite signs and MFE (such as MEL, MC, or Mη) is an overlapping effect between B -mediated PP-ISC and EX-RISC processes in Device 1, , MEL, MC, and Mη traces of Device 1 only show the B -mediated PP-ISC process. Besides, as the bias-current I increases, the increased electric field inside the device can enlarge the separation distance between the electron and the hole in PP states so that PP states dissociate into free charge carriers via the Onsager process, resulting in the shortened lifetimes and weakened PP-ISC process of PP states with increasing I , that is, normal I dependence of PP-ISC process is obtained.…”

Section: Resultsmentioning

confidence: 95%

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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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Section: I-dependent Mel MC and Mη Traces Of Devicesupporting

confidence: 61%

Section: Resultsmentioning

confidence: 95%

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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“…While multiple studies have reported the polariton-enhanced rISC process within organic molecules, ,− whether such enhancement is indeed effective remains an open question. As pointed out by Yuen-Zhou and co-workers, at light–matter resonance, the rISC rate constant from a triplet to the polariton state is inversely proportional to the number of molecules coupling with the photon (denoted as N eff ), given that the polariton is delocalized across N eff singlets and only one of them can undergo coupling to a given localized triplet. , Therefore, organic microcavity systems, of which the number of coupled molecules can be as large as 10 5 to 10 6 , can hardly obtain an enhanced rISC process unless (i) the singlet–triplet mixing of the emitter is weak and (ii) the transition between triplet and singlet excitons is within the inverted Marcus regime, as theoretically demonstrated via a quantum mechanical model in ref .…”

Section: Introductionmentioning

confidence: 99%

Enhanced Reverse Intersystem Crossing Promoted by Triplet Exciton–Photon Coupling

Shao

Shuai

2021

J. Am. Chem. Soc.

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Polaritons are hybrid light−matter states formed via strong coupling between excitons and photons inside a microcavity, leading to upper and lower polariton (LP) bands splitting from the exciton. The LP has been applied to reduce the energy barrier of the reverse intersystem crossing (rISC) process from T 1 , harvesting triplet energy for fluorescence through thermally activated delayed fluorescence. The spin−orbit coupling between T 1 and the excitonic part of the LP was considered as the origin for such an rISC transition. Here we propose a mechanism, namely, rISC promoted by the light−matter coupling (LMC) between T 1 and the photonic part of LP, which is originated from the ISCinduced transition dipole moment of T 1 . This mechanism was excluded in previous studies. Our calculations demonstrate that the experimentally observed enhancement to the rISC process of the erythrosine B molecule can be effectively promoted by the LMC between T 1 and a photon. The proposed mechanism would substantially broaden the scope of the molecular design toward highly efficient cavity-promoted light-emitting materials and immediately benefit the illumination of related experimental phenomena.

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“…5c). In the context of OLED operation involving electrical excitation, where inter-CT states will be initially generated in a spin-statistical manner when the free electrons and holes come within their Coulomb capture radius 1,58 , the 25% loosely bound and long-lived dark inter-1 CT states created can act as a reservoir for the formation of the slightly higher-lying and emissive intra-1 CT exciton. The 75% of injected carriers that form inter- 3 CT states branch between: (i) conversion into inter-1 CT via HFI-ISC; (ii) dissociation into free charge carriers that are then recycled in the recombination loop 59 ; (iii) recombination into lower-lying intra-3 CT excitons.…”

Section: Discussionmentioning

confidence: 99%

Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors

et al. 2021

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Engineering a low singlet-triplet energy gap (ΔEST) is necessary for efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF) organic semiconductors but results in a small radiative rate that limits performance in LEDs. Here, we study a model DF material, BF2, that exhibits a strong optical absorption (absorption coefficient = 3.8 × 105 cm−1) and a relatively large ΔEST of 0.2 eV. In isolated BF2 molecules, intramolecular rISC is slow (delayed lifetime = 260 μs), but in aggregated films, BF2 generates intermolecular charge transfer (inter-CT) states on picosecond timescales. In contrast to the microsecond intramolecular rISC that is promoted by spin-orbit interactions in most isolated DF molecules, photoluminescence-detected magnetic resonance shows that these inter-CT states undergo rISC mediated by hyperfine interactions on a ~24 ns timescale and have an average electron-hole separation of ≥1.5 nm. Transfer back to the emissive singlet exciton then enables efficient DF and LED operation. Thus, access to these inter-CT states, which is possible even at low BF2 doping concentrations of 4 wt%, resolves the conflicting requirements of fast radiative emission and low ΔEST in organic DF emitters.

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Abnormal Reverse Intersystem Crossing of Polaron-Pair States and Its Conversion to Intersystem Crossing via the Regulation of Intermolecular Electron-Hole Spacing Distance

Cited by 20 publications

References 64 publications

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

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

Enhanced Reverse Intersystem Crossing Promoted by Triplet Exciton–Photon Coupling

Spontaneous exciton dissociation enables spin state interconversion in delayed fluorescence organic semiconductors

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