Numerical Device Model for Organic Light‐Emitting Diodes Based on Thermally Activated Delayed Fluorescence

Abstract

have the drawback of using expensive, environmentally unfriendly heavy-metal complexes. In 2012, a new class of fully organic OLED materials emerged using thermally activated delayed fluorescence (TADF). [3,4] Here, the small singlet-triplet energy gap allows for efficient conversion from triplets to singlets on the same molecule, i.e., efficient reverse intersystem crossing (rISC) prompted by thermal energy. Therefore, while retaining the theoretically achievable 100% IQE, the necessity of an added toxic heavy-metal element was eliminated. Consequently, TADF materials are being treated as promising candidates for highly efficient OLEDs and have therefore gained considerable scientific attention in recent years. [5][6][7][8] High efficiencies have been demonstrated for TADF devices employing host-guest systems in multilayer architectures, similar to phosphorescent OLEDs. However, due to the many variables induced by multilayer architectures, such as the individual transport properties of the layers and illdefined barriers at the heterojunctions, a quantitative understanding of the device operation of TADF OLEDs is hampered. For this reason, a comprehensive device model for TADF OLEDs is presently lacking. In contrast, singlelayer polymer LEDs (PLEDs) have been successfully modeled with a well-established numerical drift-diffusion simulation program. [9][10][11][12] Also for TADF OLEDs, a simplified device structure would strongly benefit the development of a quantitative device model for TADF OLEDs.Recently, an efficient TADF OLED (EQE ≈ 19% at 500 cd A −1 ) was demonstrated based on a single TADF emitting layer of pristine 9,10-bis(4-(9H-carbazol-9-yl)−2,6-dimethylphenyl)−9,10diboraanthracene (CzDBA), [13] of which the chemical structure is shown in Figure 1a. A photoluminescence quantum yield (PLQY) of >90% in the neat film, low electron-and hole-trap densities combined with balanced bipolar transport [14] and efficient charge injection via Ohmic electron and hole contacts, [15] make these single-layer CzDBA OLEDs an ideal model system to uncover the device physics of OLEDs based on TADF emitters. In a recent study, the efficiency decrease at high voltage (rolloff) of a single-layer CzDBA OLED was studied using analytical formulas, [16] and triplet-triplet annihilation (TTA) was identified to be the origin of the roll-off in EQE at high voltages. However, in this analytical approach a number of assumptions were made by neglecting the following features: the effect of intersystem crossing (ISC), the effect that TTA generates singlets, In organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF), non-emissive triplet excitons are converted to emissive singlet excitons via reverse intersystem crossing (rISC). To model the operation of TADF-based OLEDs, quantification of the triplet population is therefore a prerequisite. A numerical drift-diffusion model is presented for TADF OLEDs that next to singlet and triplet generation also includes the positional dependence of intersyste...

Enhanced Operational Stability by Cavity Control of Single‐Layer Organic Light‐Emitting Diodes Based on Thermally Activated Delayed Fluorescence

“…The EQE gradually increases to its maximum at a few hundreds of cd m −2 , then starts to roll off because of triplet–triplet annihilation. [ 32 ] The maximum EQE for the device with the first‐order cavity ( L = 85 nm) is ≈19%, while it is as high as 16–17% for devices with the second‐order cavity ( L = 300 nm). Thus, only a marginal EQE reduction is induced when extending the optical cavity, though the luminous efficacy has a larger decrease because of increased driving voltages, as shown in Figure S3 (Supporting Information).…”

Reduction of Non‐Radiative Losses in Trap‐Free Organic Light‐Emitting Diodes by Dilution with A Large Bandgap Host

“…To analyze the EQE versus voltage characteristics in more detail, we have simulated the OLEDs with a recently developed drift‐diffusion model for TADF OLEDs. [ 41 ] First, the J – V characteristics are simulated based on the electron and hole mobility obtained from the charge‐transport measurements. The drift‐diffusion simulations then solve the recombination rate, and therefore exciton density, as a function of position in the device.…”

Trap‐Assisted Charge Generation and Recombination in State‐of‐the‐Art Organic Photodetectors

“…[78] Further characterization including quantum-chemical studies suggested that water-oxygen complexes cause electron trapping. [78][79][80][81] In its neutral state, this complex is only weakly bound. Charge detrapping effectively deactivates the trap.…”