Effects of Charge Dynamics in the Emission Layer on the Operational Lifetimes of Blue Phosphorescent Organic Light‐Emitting Diodes

Abstract

Despite more than 20 years of research, the root cause of the impractically short lifetimes of blue phosphorescent organic light-emitting diodes (PHOLEDs) has remained unclear. To overcome this, the authors investigate how the electrical properties of the emission layer (EML) of blue PHOLEDs affect degradation of the devices. It is found that a large density of dopant carriers is the dominant factor triggering triplet-polaron annihilation (TPA), which is a major defect-generation and hence lifetime-reduction mechanism. In order to reduce the generation of the TPA-induced defects to ensure long device lifetimes, the dopant carrier density should be minimized by suppressing the spontaneous charge transfer from the host to the dopant initially and by supplying sufficient charges with opposite polarity into the EML. However, there exists another critical factor that offsets the low overall density of defects against device lifetimes-that is, the non-uniform distribution of defects leading to intense exciton quenching. These two degradation factors are predetermined, and hence can be controlled, by the charge mobilities of the PHOLED EML. Given these considerations, it is demonstrated that the long-lifetime blue PHOLEDs can be realized.

Influence of host materials on degradation of phosphorescent organic light-emitting diodes under electrical stress

“…In general, the large Δ V results from the large density of localized or trapped charges caused by degradation-induced defects. 24,25 Interestingly, the data shown in Fig. 6(c) indicate that the large defect density and thus Δ V do not necessarily lead to a large luminance degradation ratio.…”

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

“…Consequently, to harvest all electrically generated excitons as EL, various strategies for emissive materials have been studied. One approach is the utilization of highly emissive room-temperature phosphorescence materials such as an iridium or platinum complex, allowing for nearly 100% internal EL quantum efficiency (IQE) by radiatively decaying triplets to the ground state. − Another strategy is to convert all dark triplets to an excited singlet state with the help of thermal energy, known as the thermally activated delayed fluorescence (TADF) process. − Despite the fact that these OLEDs exhibit promising potential for achieving optimal IQE in blue OLEDs, they still suffer from relatively limited operational device lifetime compared to green and red OLEDs, likely due to their high excited triplet energy (∼2.8 eV) that induces unwarranted degradation events. − …”

Understanding the charge dynamics in organic light-emitting diodes using convolutional neural network

“…2,[9][10][11] As the first process modulating the factors involving the efficiency, optimization of polaron transport is an essential requirement to ensure the high-performance of devices. [12][13][14][15][16][17][18][19] First, driving voltage and power efficiency intrinsically depend on the charge carrier mobility of the charge transport layer since high mobility of the charge transport layer facilitates the transportation of a high number of polarons toward EML with a low electric field. [20][21][22][23] Second, the balance between electrons and holes, which could be divided into the balance of carrier density and mobility, is directly related to the formation ratio of excitons and consecutive inter-particle reactions.…”

“…[20][21][22][23] Second, the balance between electrons and holes, which could be divided into the balance of carrier density and mobility, is directly related to the formation ratio of excitons and consecutive inter-particle reactions. 12,18,19 Unbalance of the charges simultaneously degrades the quantum efficiency and roll-off characteristics due to the low exciton formation ratio and the interaction between excitons and polarons. Additionally, fast and balanced charge transport is an effective strategy to enhance device efficiency and operational stability.…”