TADF materials are the third generation of organic light-emitting materials developed after fluorescent and phosphorescent materials. With a small split between singlet and triplet energy level, this kind of materials can flip triplet excitons to singlet excitons via reverse intersystem crossing (RISC) process.
However, there is usually a large twist angle between donor and acceptor units of TADF molecules. Thus, their excited states surfer from serious structural relaxations and thus Stokes shifts. As a result, OLEDs with TADF emitters usually show broad emission spectra and low color purity. Color purity can be improved by using color filter, but it will greatly reduce brightness and efficiency.
In a new paper published in Light Science & Application, teams lead by Prof. Wenfa Xie from State key Laboratory of Integrated Optoelectronics, College of Electronics Science and Engineering, Jilin University, and Prof. Chun-Sing Lee from the Center of Super-Diamond and Advanced Films (COSDAF), Department of Chemistry, City University of Hong Kong reported the investigation on thermally activated delayed fluorescence (TADF) with weak light-matter coupling. The weak coupling between TADF molecules and the local optical field of a Fabry-Pérot (FP) cavity can not only redistribute optical density of states, but also change the rate of radiative decay of TADF molecules with specific orientation. With classical electromagnetic theory and home-made simulation software OptiXLED (www.oso-jlu.com), the teams investigated the photodynamic processes of TADF molecules in weak microcavities and revealed the different influences of the Purcell effect on TADF molecules with different orientations. With weak light-matter coupling, OLEDs with high color purity were developed by using TADF emitters with broad emission. Dr. Liu and Dr. Zang from Prof. Xie's team summarize their findings of the work:
"In 2019, Kéna-Cohen et al. from École Polytechnique de Montréal investigated the influence of strong light-matter coupling on the RISC rate by placing TADF molecules in a FP microcavity consisting of two highly reflective silver films. It inspires us to consider whether the weak light-matter coupling can influence the photophysical properties of TADF as high light extraction efficiency is necessary for lighting and display applications."
"We designed a FP microcavity with Q factor less than 10 by using a semitransparent Mg:Ag alloy electrode and a highly reflective Al electrode, and then prepared top-emitting TADF OLEDs with such a FP microcavity (Fig. 1). With weak microcavity effect, the TADF molecule is coupled with the local optical field of the FP microcavity. Emission near the resonant wavelength is enhanced such that its spectral distribution narrows. Compared with the intrinsically broad emission, the color purity of TADF emitters with weak light-matter coupling is significantly improved."
"We found that the light extraction efficiency of microcavity top-emitting devices can also be enhanced by improving the horizontal orientation ratio of TADF molecules. Compared with the conventional devices, dipole emission from TADF molecule in top-emitting devices does not couple with substrate and waveguide modes, but only show interactions with surface plasmon polaritons (SPPs) (Fig. 2). This work demonstrates that the higher light extraction efficiency of TADF molecules with high horizontal orientation ratio is mainly caused by the weak coupling between the radiation of TADF molecules with horizontal orientation and the SPPs at the metal/organic interface."
"In this work, it's also found that the influences of the Purcell effect are different to TADF molecules with different molecular orientation. Our experimental and theoretical results show that TADF molecules with vertical orientation are more sensitive to the Purcell effect in a microcavity with a hundred nanometer scale (Fig. 3)."
"With weak light-matter coupling, high color-purity OLEDs can be achieved by using TADF emitters with broad emission. TADF molecules with different molecular orientation perform differently in weak microcavities. It provides the necessary photodynamic analysis to prepare high-performance thin-film LEDs with weak light-matter coupling, including OLEDs, quantum dot LEDs, and perovskite LEDs." the scientists said.