11:10 - 11:35InvitedID: 236
MATERIALS FOR HIGH EFFICIENCY EXCIPLEX-HOSTED OLED
National Taiwan University, Taipei, Taiwan
Organic materials that display thermally activated delayed fluorescence (TADF) are useful materials for OLEDs. A tremendous amount of TADF molecules have been reported based on the manipulation of the intramolecular charge transfer as well as the HOMO-LUMO overlap. Beyond this strategy, the approach that simply involves intermolecular charge transfer between physically blended electron donor and acceptor molecules for high efficiency TADF-based OLEDs (via exciplex formation) received great attentions. The exciplex-based systems can realize relatively small ΔEST since the electron and hole are positioned on two different molecules, thereby giving small exchange energy. Consequently, exciplex-based OLEDs have the possibility to maximize the TADF contribution and achieve high internal quantum efficiency. The research on exciplex-forming materials is still at a growing stage, and consequently, new molecules with remarkable electro and or photo-physical property are still being explored. Thus, by focusing on the development of exciplex systems, we shall have the prospective of achieving the demands for high-efficiency and high stability OLED devices. In this conference, our updated results of new efficient exciplex systems, and exciplex-hosted high efficiency fluorescent and phosphorescent OLEDs will be reported.
11:35 - 11:50ID: 169
LESS IS MORE; DILUTION ENHANCEMENT OF TADF EXCIPLEXES
Durham University, Durham, United Kingdom
A surprising yet highly practical approach to improve the performance of a TADF exciplex blend is reported. Using TSBPA donor and PO-T2T acceptor to form an exciplex we are able to blueshift emission, increase PLQY from 58% to 80%, and increase device EQE from 14.8% to 19.2% by simply diluting the exciplex with an inert high triplet energy host material – here either UGH-3 or DPEPO.
These effects are explained in terms of an increasing donor-acceptor distance and associated charge separation, while different behaviors observed in the different hosts are attributed to different energy barriers to electron transfer. We have find that the observed blueshift with dilution is general to different exciplex blends, while the effiiciency enhancement is not. This presentation will discuss recent advances in identifying the mechanism of enhancement and how it relates to the structural properties of the individual donor and accetptor molecules.
Ultimately, we expect that using this discovery to optimize the electrical properties of exciplex emission layers will allow them to match and exceed the performance of molecular TADF materials.
11:50 - 12:05ID: 177
USING TRANSIENT ABSORPTION TO PROBE EXCITED STATE DYNAMICS IN ORGANIC THERMALLY ACTIVATED DELAYED FLUORESCENCE EMITTING SMALL MOLECULES
University of Durham, Durham, United Kingdom
Thermally Activated Delayed Fluorescence (TADF) serves as a useful mechanism for enhancing output efficiency in Organic Light Emitting Diodes (OLEDs): In organic emitters of TADF, non-emissive triplet states are harvested through undergoing reverse intersystem crossing (RISC) back to the emissive singlet state with assistance from thermal energy. Research has shown in donor-acceptor TADF molecules, the rate of RISC between singlet and triplet charge transfer (CT) excited states is determined by spin-orbit coupling between the CT triplet state and a locally excited (LE) state which mediates the process.1 Transient Absorption Spectroscopy has proved to be effective in measuring excited state dynamics, particularly in direct detection of intermediate states during transitions in addition to singlet and triplets formed within the material. 2-3
Here transient absorption measurements are undertaken to further investigate how efficient RISC can be achieved during TADF. The transient absorption measurements are undertaken using a pump-probe spectroscopic setup, with an actinic laser as pump and a white light continuum as probe. The investigation is carried out on a series of TADF emitting small molecules which demonstrate tunable emission wavelengths and high photoluminescence efficiency. Transient absorption spectra were recorded to monitor the absorption at various time delays. The spectra were used to determine the evolution of excited states, which plays a fundamental role in optimising the RISC rate and hereby in maximising TADF efficiency within the materials.
1. Etherington, M. K.; Gibson, J.; Higginbotham, H. F.; Penfold, T. J.; Monkman, A. P., Nature Communications 2016, 7, 13680.
2. Hosokai, T.; Matsuzaki, H.; Furube, A.; Tokumaru, K.; Tsutsui, T.; Nakanotani, H.; Yahiro, M.; Adachi, C., SPIE: 2016; Vol. 9941, p 6.
3. Hosokai, T.; Matsuzaki, H.; Furube, A.; Tokumaru, K.; Tsutsui, T.; Nakanotani, H.; Yahiro, M.; Adachi, C., 58-2. SID Symposium Digest of Technical Papers 2016, 47 (1), 786-789.
12:05 - 12:20ID: 176
SOLVATION PHENOMENA IN TADF AND SYMMETRY BREAKING
Parma University, Parma, Italy
Weak intramolecular charge transfer (CT), leading to almost degenerate singlet and triplet state, is the key to TADF. Here we will discuss the basic physics of TADF and the highly non-trivial effect of the environment exploiting essential state models, a family of parametric Hamiltonians that applies to CT dyes. Essential state models have been extensively validated on different families of CT dyes and allow a reliable calculation of their steady-state and time-resolved linear and non-linear optical spectra, accounting for vibrational coupling and for polar solvation and its dynamics [see e.g. Terenziani & Painelli, Phys.Chem.Chem.Phys. (2015) 17, 13074]. Different families of dyes: polar ( DA), quadrupolar (DAD or ADA), octupolar (DA3 or AD3) will be discussed and parametrized against experiment and TD-DFT calculations. A careful analysis of the effect of the medium polarity on the relative energies of the excited states will be presented, highlighting the pros and cons of the different approaches, towards a general understanding. A thorough discussion of the different behaviour of polar (DA) vs quadrupolar (DAD or ADA) and octupolar dyes (DA3 or AD3) will be presented.
Symmetry breaking in the excited state of quadrupolar dyes will be addressed exploiting a newly derived multistate Redfield-Smoluchowski equation. Specifically, the complex spectral evolution of time-resolved fluorescence in dyes undergoing symmetry-breaking will be simulated in different environments. The subtle interplay among non-adiabatic molecular vibrations (driving the molecular geometrical relaxation) and the solvent dynamics will be explicitly addressed. The qualitatively different behaviour of the same molecular specie in an environment with similar polarity but different dynamics (liquid vs frozen environment) will be quite naturally addressed and rationalized.