Abstract
The successful development of thermally activated delayed fluorescence (TADF) OLEDs relies on advances in molecular design. To guide the molecular design toward compounds with preferable properties, special care should be taken while estimating the parameters of prompt and delayed fluorescence. Mistakes made in the initial steps of analysis may lead to completely misleading conclusions. Here we show that inaccuracies usually are introduced in the very first steps while estimating the solid-state prompt and delayed fluorescence quantum yields, resulting in an overestimation of prompt fluorescence (PF) parameters and a subsequent underestimation of the delayed emission (DF) yield and rates. As a solution to the problem, a working example of a more sophisticated analysis is provided, stressing the importance of in-depth research of emission properties in both oxygen-saturated and oxygen-free surroundings.
Highlights
According to spin statistics, only 25% of excitons in the typicalOLED device are of a singlet nature
The thermal activation of triplet excitons and the subsequent reverse intersystem crossing in thermally activated delayed fluorescence (TADF) compounds allow us to utilize most of the excited states and attain efficient emission.[1−3] To enable triplet recycling, the lowestenergy singlet and triplet states should be nearly isoenergetic.[4]
The most important parameters of prompt and delayed fluorescence (e.g., the rates of intersystem crossing (ISC) and reverse intersystem crossing), radiative and nonradiative fluorescence rates are calculated starting from the simplest ones−prompt and delayed fluorescence quantum yields (ΦPF and ΦDF, respectively) and the corresponding fluorescence decay rates.[5,8,9]
Summary
OLED device are of a singlet nature. To enhance the internal quantum efficiency of a device with singlet emitters, nonemissive triplet excitons should be employed. Dense solid surrounding efficiently prevents oxygen diffusion inside the film, when the emitter molecules close to the surface are susceptible.[14−16] Typically, an evident part of only weakly quenched TADF still exists under +O2 conditions, making the direct application of eqs 1 and 2 inaccurate.[17] the unquenched part of TADF is larger for compounds with larger rISC rates since the rapid upconversion of triplet states reduces the chance of nonradiative collision with molecular oxygen,[17] especially complicating the analysis of novel TADF materials with rapid rISC. We show that fluorescence decay rates may be estimated within the 1 order of magnitude error, depending on the accuracy of the initial emission parameters Such variation of TADF rates significantly complicates the analysis and comparison of material parameters and the prediction of OLED performance. We show that reliable emission parameters can be obtained after the thorough analysis
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