Modeling Electron‐Transfer Degradation of Organic Light‐Emitting Devices

Abstract

advantages. This limitation originates from intrinsic degradation of the organic materials because degradation byproducts increase the driving voltage, accelerate non-emissive channels, and deteriorate the color purity of an electroluminescence device. [1,2] Thus, suppressing intrinsic degradation has been the focus of particular interest from both scientific and technological perspectives. Pioneering studies indicated that intrinsic degradation is mediated by an exciton or a polaron, which are the key elements for normal operation of OLEDs. [3-9] This unimolecular degradation becomes salient for blue-emissive devices involving high-energy excitons and for unbalanced devices that permit charge carrier leakage. Later studies suggested the occurrence of bimolecular degradation mechanisms. [10-15] An encounter of two long-lived triplet excitons may produce a highly unstable singlet exciton. The bimolecular upconversion often enables exergonic access to dissociative states where degradation occurs. Decomposition also proceeds through an annihilation between a polaron and a triplet exciton. This bimolecular annihilation can yield an excited-state polaron that is susceptible to bond cleavage or can lead to the formation of dimeric byproducts. These mechanistic understandings are valuable because they offer novel strategies for improving the operational lifetime of OLEDs. For example, decreasing the concentration and lifetime of excitons has been demonstrated to be an effective approach to extending device longevity. [11,16-20] Alternatively, balancing charge carrier densities and delocalizing an exciton-formation zone could also avoid the destructive exciton-polaron annihilation (EPA). [21-24] In a recent study, Crǎciun and co-workers reported that EPA involved excitons generated through Shockley-Read-Hall (SRH) recombination between trapped positive polarons and free negative polarons in polymer emitting layers. [25] Therefore, intrinsic degradation could be suppressed by avoiding SRH recombination or by facilitating Langevin recombination. Duan's group reached an opposing conclusion. [26,27] They found that Langevin recombination on hosts was detrimental to device stability, because dark and longlived triplet excitons of a host would be more susceptible to the aforementioned hazardous bimolecular annihilation The operational lifetime of organic light-emitting devices (OLEDs) is governed primarily by the intrinsic degradation of the materials. Therefore, a chemical model capable of predicting the operational stability is highly important. Here, a degradation model for OLEDs that exhibit thermally activated delayed fluorescence (TADF) is constructed and validated. The degradation model involves Langevin recombination of charge carriers on hosts, followed by the generation of a polaron pair through reductive electron transfer from a dopant to a host exciton as the initiation steps. The polarons undergo spontaneous decomposition, which competes with ultrafast recovery of the intact materials through charge recombination....

Triplet Exciton Upconverting Blue Exciplex Host for Deep Blue Phosphors

“…Therefore, the stability of the n-type host under exciton generation can improve the device lifetime. [6] Thirdly, the exciton stability of the n-type hosts was confirmed by monitoring the PL intensity decrease against UV exposure time (Figure 7) because the PL intensity reduction indicates UV irradiation induced PL degradation of the hosts. The degree of PL degradation of the mSiTrz host was larger than that of the mSiTrzCzCN host, indicating the poor exciton stability of the mSiTrz host due to high singlet energy of 3.56 eV compared to 3.38 eV of the mSiTrzCzCN host.…”

Highly efficient and stable deep-blue organic light-emitting diode using phosphor-sensitized thermally activated delayed fluorescence

“…Therefore, the increased current density of the device with TBE02 represents the less-effective direct hole trapping on TBE02, compared to that on other TBEs. The direct charge trapping by the TBEs could increase the triplet exciton population on the emitter and the triplet-polaron quenching (TPQ), which could explain device degradation (43)(44)(45).…”

Organic Photoredox Catalysts Exhibiting Long Excited-State Lifetimes

“…Figure 3a depicts the chemical structure of PXZ-TRZ, a representative long-lifetime organic molecule. [25][26][27][28][29] The PXZ and TRZ components are electron-rich phenoxazine and electron-deficient triazine, respectively, and they are linked via a phenyl bridge. As shown in the single-crystal structure in Figure 3b, these two moieties are orthogonally positioned because of steric hindrance exerted by the hydrogen atoms in the ortho positions.…”