An Overlooked Charge‐Transfer Interaction in the Interfacial Triplet–Triplet Upconversion Process in Blue Organic Light‐Emitting Diodes

Abstract

collide, alongside the formation of singlets and quintets, highly reactive "hot" triplets (>5 eV) would unwarrantedly be formed, leading to the rapid degradation of OLEDs. [5] Therefore, the control of overcrowded high-energy triplets has been the most critical issue for OLED research over the last 20 years. [4,[6][7][8] An alternative triplet-excitonic pathway in blue OLEDs is a triplet-triplet upconversion (TTU), which is a well-established solution for practical applications. [9,10] Anthracene derivatives that are a wellknown TTU material for blue OLEDs have the lowest-excited triplet energy level (T 1 ) of around 1.6 eV that is close to half of the singlet excited energy. The collision of the two low-energy triplets produces just enough energy for a singlet generation rather than the formation of a hot triplet that partially decays nonradiatively via a dissociation potential curve. For this reason, blue TTU-OLEDs provide a practical solution. Although the IQE in TTU-OLEDs falls short of 100%, TTU-OLEDs can theoretically gain the high singlet exciton production yield up to 62.5%.Despite the practical use of pure-blue TTU-OLEDs, TTU's underlying mechanism still remains debated with varied viewpoints. First, from the aspect of the exciton formation process, Monkman's group proposed a family of anthracene derivatives featuring high triplet levels (T n ) exceedingly twice the T 1 levels, leading to nearly 62.5% of IQE in OLEDs in 2013. [11,12] Recently, our group has proposed that spin-orbit coupling induced by intramolecular charge-transfer (CT) interaction in anthracene derivatives could facilitate TTU events. [13] Second, from the aspect of device engineering, in 2015, Xiang et al. revealed that the carrier recombination zone in a blue TTU-OLED exists in the emitter layer (EML) close to the interface between the EML and the hole-transport layer (HTL), [14] indicating the benefit from a dense triplet accumulation at the interface. Alternatively, an origin of the sub-bandgap EL in rubrene-based OLEDs was explained as the CT-sensitized TTU generating at the rubrene/ fullerene interface. [14,15] Further, B.-Y. Lin et al. proposed the CT-sensitized blue TTU-OLEDs to reduce a driving voltage. [16] Although the IQE from these CT-sensitized TTU-OLEDs was far from the theoretical limit, it has been suggested that the CT interface can facilitate TTU events at the interface according to the dense triplet accumulation. [14,[16][17][18][19] However, the interfacial CT formation has been claimed as the main reason for Two low-energy triplets can generate one singlet via triplet-triplet upconversion (TTU), and result in an exciton production yield exceeding 25% in conventional fluorescence-based organic light-emitting diodes (OLEDs). In most cases, since such low-energy triplets induce no serious OLED degradation, TTU-OLEDs are the only commercialized blue OLEDs so far. Herein, it is clarified that the charge-transfer (CT) interaction at a hole-transport/ emitter-layer interface is an overlooked pathway to enhance TTU yield s...

Anthracene derivatives with strong spin–orbit coupling and efficient high-lying reverse intersystem crossing beyond the El-Sayed rule

“…Recently, Nguyen et al reported that the charge transfer (CT) complexes generated at the interface between the electron blocking layer (EBL) and anthracene hosts could have a significant impact on efficiency enhancement. 39 Therefore, it is necessary to distinguish whether the high efficiency of the 2MIQ-NPA device is due to the hot exciton channel or the formation of CT complexes. In order to investigate the formation of CT complexes, the PL and time-resolved PL (TrPL) data of a single film of the EBL material (BPADBF) and mixed films with TTF-Phen and 2MIQ-NPA were carried out.…”

Organoboron-based multiple-resonance emitters: synthesis, structure–property correlations, and prospects

“…As a result, DABNA derivatives are now widely commercialised as blue emitters in triplet–triplet annihilation (TTA)-driven fluorescent devices for smartphones and televisions. 87–91 However, MR emitters have not been implemented as TADF emitters because of their rapid degradation through the long-lived T 1 state in TADF-OLEDs. 87 One of the most effective ways of realising reliable TADF devices is boosting the RISC of MR emitters.…”

Enhancing Triplet–Triplet Upconversion Efficiency and Operational Lifetime in Blue Organic Light-Emitting Diodes by Utilizing Thermally Activated Delayed Fluorescence Materials

“…Indeed, an external quantum efficiency (EQE) of ∼12% in pure blue TTU-OLEDs has been demonstrated, and it is reported that device optimization enables an ultralong operational lifetime (1000,000 h) for blue TTU-OLEDs. ,, Because the probability of a TTU event should depend on triplet density inside an active layer, a design of a device architecture to enable the dense accumulation is essential. For instance, a double emitting layer (EML) system has been proposed to ensure the highly dense triplet generation in the EML. , Further, our previous studies have demonstrated that the TTU process at the hole-transporting layer (HTL) and EML interfaces is highly efficient if there is a charge-transfer (CT) interaction present at the interface, leading to a dense accumulation of triplet that trigged TTU events at the interface . However, despite the theoretical upper limit of the TTU contribution yield being expected to be 60%, , blue OLEDs with maximum TTU contribution are still scarce.…”

“…27,33 Further, our previous studies have demonstrated that the TTU process at the hole-transporting layer (HTL) and EML interfaces is highly efficient if there is a charge-transfer (CT) interaction present at the interface, leading to a dense accumulation of triplet that trigged TTU events at the interface. 34 However, despite the theoretical upper limit of the TTU contribution yield being expected to be 60%, 27,35 blue OLEDs with maximum TTU contribution are still scarce.…”