The exciplex-forming systems that can perform thermally activated delayed fluorescence (TADF) characters are emerging as appealing research due to their versatile applications in organic light-emitting devices (OLEDs). However, the lack of data to interpret the detailed D-A interactions impedes the further deep insight understanding of the exciplex excited state. This demand motivated us to establish an exciplex-enabling supramolecular system with X-ray structure analyses revealing the detailed donor-acceptor interactions, the thermodynamic parameters of complex formation, the photophysical property to verify the exciplex characters and the characteristics of the solution-processed OLED device employing this exciplex-forming supramolecular complex as an emitter. This unprecedented and tunable system opens a new platform for further exciplex studies. The detailed investigation of exciplex-forming systems achieved by this supramolecular approach will be presented.
Organic materials that exhibit thermally activated delayed fluorescence (TADF) are attractive materials for realizing high efficiency OLEDs. A tremendous amount of tailor-made TADF molecules have been reported based on the manipulation of the intramolecular charge transfer as well as the HOMO-LUMO overlap. Beyond this strategy, there is an emerging approach of using exciplex that simply involves intermolecular charge transfer between physically blended electron donor and acceptor molecules. This is because the exciplex-based systems can realize relatively small ΔEST since the electron and hole are located on two different molecules, thereby giving small exchange energy. Consequently, exciplex-based OLEDs have the possibility to maximize the TADF contribution and achieve theoretical 100% internal quantum efficiency. However, 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, the developments of exciplex-forming systems for achieving high-efficiency and high stability OLED devices are prospective. In this conference, our updated results of new efficient exciplex systems, and exciplex-hosted fluorescent and phosphorescent OLEDs with high efficiency and high stability will be reported.
Organic materials that display thermally activated delayed fluorescence (TADF) are a striking class of functional materials that have witnessed a booming progress in recent years. The small ΔEST in TADF-based systems prompts highly efficient RISC from T1 to S1 states, and consequently both singlet and triplet excitons can be harvested for light emission. For the last five years, 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, there is an emerging approach that simply involves intermolecular charge transfer between physically blended electron donor and acceptor molecules for high efficiency TADF-based OLEDs (via exciplex formation). This is because the exciplex-based systems can realize relatively small ΔEST (0–0.05 eV) much more easily 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 theoretical 100% internal quantum efficiency and solve the challenging issue of achieving small ΔEST in organic systems. However, 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, we will report our updated results of new efficient exciplex systems, and exciplex-hosted fluorescent and phosphorescent OLEDs with high efficiency and high stability.
A class of small molecule donors configured in a donor-acceptor-acceptor’ (d-a-a’) structure have been studied for vacuum-deposited OPVs. They consist of an electron-donating (d) functional unit connected to two consecutive electron-accepting (a, a’) groups. The rigid and rod-like molecular backbones with strong push-pull interactions between the ‘d’ and ‘a’ units result in a large ground state dipole moment along the backbone axis. This leads to antiparallel π-π stacking that favors intermolecular charge transfer. In this work we synthesized two vacuum-deposited small molecules that are modified from previously reported donors with similar structures[1]. All molecules studied have the same molecular backbone with different side chains attaching to an asymmetric heterotetracene donor block. Single crystal analysis and thin film grazing incidence x-ray diffraction are performed. The donor with a shorter branched side chain yields the highest single crystal packing density, corresponding to the largest absorption coefficient and short circuit current (JSC) among the three molecules studied. The preferred face-on stacking arrangement that facilitates charge transport in the vertical direction also leads to a higher fill factor (FF). A power conversion efficiency of 9.3% is achieved with JSC = 16.5 mA/cm2, VOC = 0.94 V and FF = 0.60, which is one of the highest performance single junction OPVs grown by vacuum thermal evaporation. By relating the side chain shape with the crystal packing habit and the device performance, we provide a means of molecular structure modification leading to significant performance improvements.
[1] X. Che, C.-L. Chung, X. Liu, S.-H. Chou, Y.-H. Liu, K.-T. Wong, S. R. Forrest, Advanced Materials 2016, 28, 8248.
Quantitative optical analyses were conducted on the mechanisms of impressively high electroluminescence (EL) efficiency (external quantum efficiency of up to 37%) achieved in previously reported blue organic light-emitting devices (OLEDs) using thermally activated delayed fluorescence emitters based on acridine–triazine hybrids. In addition to high photoluminescence quantum yields and preferentially horizontal emitting dipoles, optical simulation shows that the use of both low-index hole-transport layers (HTLs) and electron-transport layers (ETLs) also substantially contribute to enhanced optical outcoupling efficiencies and EL efficiencies of these devices. Further analyses on optical mode distributions and partitions in devices reveal significantly different optical outcoupling enhancement mechanisms for adopting low-index HTLs (i.e., reduced overall waveguided modes and enhanced microcavity effect) or adopting low-index ETLs (i.e., reduced surface plasmon and transverse magnetic waveguided modes), and their effects are combined to give even larger enhancement when reducing refractive indexes of both. Results of this work clearly indicate that optical properties of carrier-transport layers, in addition to their electrical properties, are critical factors and should also be carefully considered for future development of high-efficiency OLEDs.
Materials with thermally activated delayed fluorescence (TADF) have recently emerged as new fluorescent emitters for highly efficient organic light-emitting diodes (OLEDs). Molecule with TADF behavior needs to have a small singlet–triplet energy difference (ΔES-T) that allows the up-conversion from nonradiative triplet state (T1) to radiative singlet state (S1) via reverse intersystem crossing (RISC) process. Generally, molecules with small ΔES-T can be obtained via carefully manipulate the degree of “intramolecular” charge transfer (ICT) between electron-donating and -accepting components, such that the electron exchange energy that contributes to ΔES-T, can be minimized. Alternatively, excited state with small ΔES-T can be feasibly realized via “intermolecular” charge transfer occurring at the interface between spatially separating donor (D) and acceptor (A) molecules. Because the exchange energy decreases as the HOMO-LUMO separation distance increases, theoretically, the intermolecular D/A charge transfer state (or exciplex) should have rather small ΔES-T, leading to efficient TADF. However, it is still a challenge to access highly efficient exciplex systems. This is mainly because exciplex formation is commonly accompanied with a large red shift of emission spectra and long radiative lifetime, which tend to diminish photoluminescence quantum yield (PLQY) as well as electroluminescence (EL) performance. Until now, exciplex-based OLEDs with external quantum efficiency (EQE) above 10% are still limited. By judicious selection of donor and acceptor, the formation of efficient exciplex can be feasibly achieved. In this conference, our recent efforts on highly efficient exciplexes using C3-symmetry triazine acceptors and various donors, and their device characteristics will be presented.
We report on the characteristics of a host-guest lasing system obtained by co-evaporation of an oligo(9,9-diarylfluorene)
derivative named T3 with the red-emitter
4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye
(DCM). We demonstrate that the ambipolar semiconductor T3 can be implemented as active matrix in the realization of a
host-guest system in which an efficient energy transfer takes place from T3 matrix to the lasing DCM molecules. We
performed a spectroscopic study on the system by systematically varying the DCM concentration in the T3 matrix.
Measurements of steady-state photoluminescence (PL), PL quantum yield (PLQY) and amplified spontaneous emission
(ASE) threshold are used to optimize the acceptor concentration at which the ASE from DCM molecules takes place
with the lowest threshold.
Organic light-emitting diodes (OLEDs) implementing the DCM:T3
host-guest system as recombination layer are
fabricated for verifying the optical properties of the optimised blend in real working devices.
Indeed, the very low ASE threshold of T3:DCM makes the investigated blend an appealing system for use as active layer
in lasing devices. In particular, the ambipolar charge transport properties of the T3 matrix and its field-effect
characteristics make the host-guest system presented here an ideal candidate for the realization of electrically-pumped
organic lasers.
The dynamics of triplet recombination in fluorene trimers have been studied using steady state photoinduced absorption
(PA) spectroscopy. We investigated two type of oligomeric films, deposited by different techniques: thermal evaporation
and spincoating. The different molecular arrangement in both films is manifested in a red-shift of the absorption, PL and
T1-Tn triplet PA spectra of the sublimated film relative to the spincoated one. Triplet recombination dynamics follow a
dispersive bimolecular recombination model away from the trap filling regime.
Moreover we report on the characteristics of a host-guest lasing system obtained by co-evaporation of the most
promising oligofluorene derivative (T3) with the red-emitter
4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye (DCM). The blend satisfies the necessary condition for an efficient Förster energy transfer to take place
from T3 matrix to DCM molecules.
Thin films of organic semiconductors have been subjected to extensive studies in the last two decades due to
applications in photonics and optoelectronics. In this paper, we investigate wavelength tuning phenomena based on
fluorene-cored oligomers (T3). We studied the optical properties of oligofluorene vacuum-deposited thin films and
found that they show high optical anisotropy but become optical isotropic after annealing at a temperature around the
glass-transition temperature. The results indicate the molecular reorientation in thin films after annealing. Using this
property, we investigated the influence of molecular orientation on stimulated emission properties of organic thin films.
Employing such properties, we have also demonstrated continuous tuning of the stimulated emission wavelength of a
slab waveguide within one sample. Finally, we employed T3 thin film for laser application. We have also demonstrated
the wavelength tuning of the organic laser with DFB structures.
The synthesis and properties of 3,6-bis(triphenylsilyl)carbazole as an effective host material for blue electrophosphorescence are reported. The electrochemically active C3 and C6 sites of carbazole are non-conjugated blocked with the steric, bulky and large-gap triphenylsilyls, the resulting new compound retains the large triplet energy of carbazole yet exhibits much enhanced morphological stability and electrochemical stability in comparison with previous carbazole-based host materials. Using this new host material, blue phosphorescent OLEDs having high efficiencies up to 16%, 30.6 cd/A and 26.7 lm/W are demonstrated.
Fluorene-based materials, such as polyfluorenes or oligofluorenes, are conventionally well known as efficient blue emitting materials. Intriguingly, it is recently unveiled that ter(9,9-diarylfluorene)s also exhibit efficient unusual nondispersive bipolar carrier-transport characteristics and high carrier mobilities of >10-3 cm2/Vs for both holes and electrons in the amorphous state. Making use of convenient substitution on C9 of fluorenes and corresponding variety in molecular structures and physical properties, in this work we systematically investigate influences of molecular structures, such as dialkyl-substitution vs. diaryl-substitution and oligomer length etc., on charge transport of oligofluorenes. Furthermore, utilizing liquid-crystalline properties of oligofluorenes with appropriate substitutions, comparative studies of carrier transport of oligofluornes in both vacuum-deposited amorphous states and aligned liquid-crystal glass states are also performed. Insights on intermolecular charge transport derived from the experimental observations and theoretical examinations are then discussed accordingly.
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