Direct observation of reverse intersystem crossing from fully confined triplet exciplexes using magneto-electroluminescence

ACS Appl. Electron. Mater.

Tang

Zhu

et al. 2021

Self Cite

Room-temperature observation for reverse intersystem crossing (RISC) from triplet to singlet charge-transfer states (CT 3 → CT 1 ) and clarification of its physical mechanisms are the key requirements for designing highly efficient exciplex-based organic light-emitting diodes (OLEDs). Herein, balanced and unbalanced exciplex-based OLEDs were fabricated by employing different holeinjection layers, and RISC of CT states was directly observed via analyzing magnetoconductance (MC) and magneto-electroluminescence (MEL) traces of the balanced device at room temperature. Specifically, current-dependent MC traces of the balanced device always present B-mediated RISC features, whereas those from the unbalanced one depict the superposition of B-mediated intersystem crossing (ISC) and the dissociation of CT 3 by excessive charge carriers. Simultaneously, MEL curves of the balanced device display the conversion from ISC to RISC with lowering bias current, but those from the unbalanced one always show ISC under all of bias currents. Moreover, although all of current-dependent magneto-efficiency (Mη) traces exhibit ISC, Mη values are ∼2 times lower in the balanced device than the unbalanced one. These rich changes of magnetic-field responses demonstrate that balanced carrier injection can facilitate the occurrence of RISC by reducing the dissociation of CT 3 . Expectedly, the current efficiency of electroluminescence from the balanced device is increased by ∼2.2 times, which originates from the improvement of delayed luminescence because of the enhanced RISC. Accordingly, this work not only clarifies the prerequisite for observing RISC of CT states but also provides strategies for designing high-efficiency exciplex-based OLEDs.

Section: Introductionmentioning

confidence: 58%

“…8−15 Fortunately, in our recent work, we have observed the upright Lorentzian-type MEL traces indicating the RISC process in exciplex-based OLEDs at low temperatures. 19 However, the room-temperature RISC process in exciplex-based OLEDs is more important for their practical applications.…”

Section: Introductionmentioning

confidence: 99%

Room-Temperature Observation for Reverse Intersystem Crossing in Exciplex-Based OLEDs with Balanced Charge Injection

ACS Appl. Electron. Mater.

Tang

Zhu

et al. 2021

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“…[3][4][5][6] Moreover, it is difficult to separate ISC and RISC processes from other processes because they generally happen at the same time and influence each other. [7][8][9][10] These complex microscopic processes lead to ambiguous understandings of physical mechanisms related to ISC and RISC processes.…”

mentioning

confidence: 99%

Conversions from Normal to Abnormal Current‐Dependent ISC and from Abnormal to Normal Current‐Dependent RISC Processes in Exciplex‐Based OLEDs

Chen

Tang

et al. 2022

Adv Materials Inter

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Intersystem crossing (ISC) and reverse ISC (RISC) are important spin‐mixing processes for obtaining high quantum efficiency in exciplex‐based organic light‐emitting diodes (EB‐OLEDs) and often show normal current (I) dependencies which weaken with increasing I. Surprisingly, herein, using magneto‐conductance (MC) as a fingerprint probing tool, an unreported conversion from normal to abnormal I‐dependent ISC processes is observed at 300 K from relatively‐unbalanced EB‐OLEDs with the low electron‐injection barrier and the high hole mobility. More amazingly, after improving carrier injection balance through replacing the hole‐injection layer, a conversion from abnormal to normal I‐dependent RISC processes with increasing I is observed from the relatively‐balanced EB‐OLEDs for the first time. These two conversions are reasonably analyzed by fitting and decomposing MC traces of the devices. Furthermore, transition I of the conversion from abnormal to normal I‐dependent RISC processes decreases from 50 to 5 μA as the temperature reduces from 300 to 150 K, and only the normal I‐dependent RISC process is observed at 100 and 20 K due to high driving voltages. Obviously, this work deepens the full understandings of I‐dependent ISC and RISC processes in EB‐OLEDs and has the potential applications for the achievement of high‐performance devices.

“…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%

“…That is, PP-ISC is dominant in PP states for most organic semiconductors. , For simplicity, hereafter the conversion of the PP states mainly refers to the PP-ISC process (PP 1 → PP 3 ), as shown by the horizontal red arrow in Figure e. Similar to PP 1 and PP 3 , EX 1 and EX 3 can also undergo the interconversion (i.e., EX-ISC and EX-RISC processes) under the HFI, because they are intrinsically intermolecular excited states and almost degenerate in energy. , Factually, the interconversion of the EX states is often dominated by the RISC process from EX 3 to EX 1 (EX-RISC, EX 1 ← EX 3 ) because the number of EX 3 is three times that of EX 1 and the lifetime of EX 3 (∼μs) is 3 orders of magnitude longer than that of EX 1 (∼ns) . 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 e.…”

Section: Resultsmentioning

confidence: 99%

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

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.