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We will discuss the photophysics and particularly the role of spin states in thermally activated delayed fluorescence (TADF) in films and organic light emitting diodes (OLED). In particular, whether the TADF process is spin-dependent and, if yes, what is the exact mechanism and what are the relevant precursor states. We perform direct spin-sensitive measurements on TADF OLED devices applying multi-frequency electroluminescence- and current-detected magnetic resonance (ELDMR, EDMR). The idea behind these experiments is that the static magnetic field applied to devices modifies only the energy levels of spin-carrying states due to Zeeman splitting, thus changing the emission rates. We observe that the resonant microwave radiation, applied to OLEDs, leads to enhancement of the EL intensity. The effect was found to be very sensitive to experimental conditions, thus modifying the resonance frequency, temperature and microwave power we were able to shed light on the underlying mechanism of the reverse intersystem crossing and the spin states involved. With temperature-dependent ELDMR, the singlet-triplet splitting ∆EST can be determined, as we reported for two different donor-acceptor systems [1]. Comparing ELDMR, EDMR and photoluminescence detected magnetic resonance (PLDMR), we revealed differences in TADF processes under optical excitation and electrical injection. Finally, we compare the mechanisms of triplet-singlet conversion in poorly emissive charge-transfer states in OPV donor-acceptor blends [2] with those in highly emissive TADF systems. The information gained from magnetic resonance experiments can potentially help to design new OLED materials as well as to further improve their performance.
[1] S. Väth, K. Tvingstedt, M. Auth, A. Sperlich, A. Dabuliene, J. V. Grazulevicius, P. Stakhira, V. Cherpak, V. Dyakonov, Adv. Opt. Mater. 5, 1600926 (2017).
[2] S. Väth, K. Tvingstedt, A. Baumann, M. C. Heiber, A. Sperlich, J. A. Love, T.-Q. Nguyen, V. Dyakonov, Adv. Energy Mater. 7, 1602016 (2017).
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