title: What Controls Thermally Activated Delayed Fluorescence in Guest-Host Systems for Organic Light-Emitting Diodes?
Creators: name: Duda, Eimantas
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: TADF
subject: guest-host system
subject: emitter
subject: photophysics
subject: OLED
subject: photoluminescence
subject: spectroscopy
subject: enhance TADF
subject: control TADF
subject: high efficiency
subject: extended donor
subject: dendron
subject: para-connection
subject: meta-connection
subject: excited states
subject: intermediate triplet states
subject: higher-lying triplet state
subject: multi-resonance
subject: narrow emission
subject: full width at half maximum
subject: FWHM
subject: blue emitter
subject: structure-property relationship
subject: time-resolved photoluminescence
subject: photoluminescence decay
subject: absorption
subject: iCCD
subject: singlet
subject: triplet
subject: singlet-triplet gap
subject: internal quantum efficiency
subject: external quantum efficiency
subject: photoluminescence quantum yield
subject: concentration quenching
subject: radiative decay
subject: non-radiative decay
subject: intersystem crossing
subject: reverse intersystem crossing
subject: ISC, RISC, TADF parameters
subject: 500
subject: 530
subject: 540

description: Thermally activated delayed fluorescence (TADF) enables the production of heavy-atom-free OLEDs that are of comparable or higher efficiency compared to currently commercialised phosphorescent emitters. My research work investigates the factors controlling TADF in guest-host systems for OLEDs. I explored four different aspects: the role of the donor group, including its linkage position to the acceptor core, the size of the oligomer length, the role of the host and, finally, a new type of TADF emitters that are based on the multi-resonant molecular structures. I evaluated the guest-host systems presented in this work using the absorption, steady-state photoluminescence (PL) emission and time-resolved PL emission techniques. First, I studied how TADF-related properties change based on the electron-hole wavefunction overlap that was controlled by the donor extension strength. The reference compound selected was based on the dicyanobenzene unit as the acceptor and carbazole units as donors. From the comparison of donor extensions consisting of carbazole, diphenylamine and phenoxazine-based units, I found that the optimal performance in terms of the singlet-triplet gap (ΔEST), TADF and photoluminescence quantum yield (PLQY) is reached for the molecule owning carbazole-based donor extensions. I then studied the molecules owning carbazole-based donor extensions in terms of their linkage position to the acceptor core via a phenylene bridge (para-, meta- and a synergistic meta- and para-connection). I discovered that while the para-connection owns the largest radiative decay rate, the meta-based connection shows the smallest ΔEST gap and largest TADF. Moreover, I revealed that the synergistic meta- and para-connection allows both a high radiative decay rate and a small ΔEST gap for efficient TADF. Furthermore, I found that the compound with the donor groups connected to the acceptor core synergistically in the meta- and para-positions possess almost no concentration quenching, allowing to use this emitter host-free in the emissive layer of the OLED. Second, I investigated the TADF-related parameters as a function of the oligomer length. Oligomers owning multiple donor-acceptor units were expected to create a larger number of quasi-degenerate states, enhancing the TADF-related properties. The oligomers investigated were based on the benzonitrile as acceptors and amines as donors. My photophysical study revealed that the intersystem crossing and the reverse intersystem crossing rates are increasing with increasing oligomer size. I attributed this to the increasing number of intermediate triplet states along the series. Furthermore, I found that the enhancement of TADF-related properties can be achieved with no redshift of photoluminescence emission wavelength with the increasing oligomer size. Third, I explored the role of the host molecule on the TADF-related properties of the guest-host systems. Host materials of different ΔEST gaps were selected, including a small ΔEST gap (&lt;200 meV) host showing TADF by itself. My photophysical study revealed that the guest-host systems with a host that demonstrates TADF by itself have a stronger TADF character as a whole, especially at lower emitter concentrations. My electroluminescence study of the organic light-emitting diodes (OLEDs) with different host materials in the emissive layer demonstrated that the highest external quantum efficiency (EQE) is present with the host showing TADF by itself, with the most significant difference at lower emitter concentrations. I ascribed this to both stronger TADF of the guest-host system and the bipolar nature of the host. Finally, I investigated a new type of TADF emitters that are based on multi-resonant structures. While such emitters possess very narrow emission spectra and high EQE, the major problem is a strong concentration quenching, induced by a rigid and planar molecular structure of such emitters. As a possible solution to a strong concentration quenching of TADF emitters based on multi-resonant structures, the bulky substituents can be introduced. To study how such modification reflects on the TADF performance of the compound, I compared the TADF-related properties for both compounds with and without bulky substituents. I found that TADF-related properties are comparable in both cases, yet the compound with bulky substituents shows a strong reduction in concentration quenching. This implies that concentration quenching in TADF emitters based on resonant structures can be successfully reduced without suppressing TADF. Overall, my photophysical study presented in this thesis successfully demonstrates several possible ways to enhance or preserve TADF for efficient emitters.