Molecular Design Tactics for Highly Efficient Thermally Activated Delayed Fluorescence Emitters for Organic Light Emitting Diodes

Recently, pure organic thermally activated delayed fluorescence (TADF) emitters have attracted considerable interest from the scientific community in the field of organic light emitting diodes (OLEDs) as they can theoretically realize 100 % of the internal quantum efficiency by exploiting both the singlet and triplet excitons via the reverse intersystem crossing enabled by small singlet‐triplet energy splitting. Currently, the external quantum efficiency of the TADF emitters is reaching the level of phosphorescent emitters. Therefore, the TADF approach is considered as a potential alternative to the low efficiency conventional fluorescent and expensive phosphorescent emitters. In this account, we summarized our recent development of blue and green TADF molecular designs to improve the device performances of the TADF devices.     

Doping‐Free White Organic Light‐Emitting Diodes

Doping‐free white organic light‐emitting diodes (WOLEDs) have great potential to the next‐generation solid‐state lighting and displays due to the excellent properties, such as high efficiency, bright luminance, low power consumption, simplified structure and low cost. In this account, our recent developments on doping‐free WOLEDs have been summarized. Firstly, fundamental concepts of doping‐free WOLEDs have been described. Then, the effective strategies to develop doping‐free WOLEDs have been presented. Particularly, the manipulation of charges and excitons distribution in different kinds of doping‐free WOLEDs have been highlighted, including doping‐free fluorescent/phosphorescent hybrid WOLEDs, doping‐free thermally activated delayed fluorescent WOLEDs and doping‐free phosphorescent WOLEDs. In the end, an outlook for the future development of doping‐free WOLEDs have been clarified.     

Development of Materials for Blue Organic Light Emitting Devices

The success of organic light emitting diodes (OLED) has been witnessed by the commercialization of this technology for manufacturing the vivid and colorful displays used in our daily life now. The prospective growth of OLED technology on display industry will be optimistic. Over the last three decades, many different approaches on material and device designs have been implemented for improving the efficiency and stability of OLED devices. These efforts install main cornerstones to support the great achievement of OLED technology. However, until now, the performance and stability of blue OLEDs still have some concerns. This troublesome issue should be totally conquered before the large‐scale manufactures dominated over other display technologies, particularly liquid crystal‐based displays, takes place. Though significant progress has already been made to achieve high performance and long lifetime blue OLEDs, this topic still remains as one of the hot researches in OLEDs. We have been working on this area for about two decades and made some notable contributions. Consequently, in this personal account we have outlined our efforts to obtain better performing blue OLEDs by utilizing a range of emitters based on fluorescence, phosphorescence, delayed fluorescence and exciplex systems. We have also developed some novel host materials for blue OLEDs, which are worth mentioning in this account.     

Stabilizability of uncontrollable systems via generalized delayed feedback control

In this paper, the stabilizability problem via a generalized delayed feedback control (GDFC) for uncontrollable discrete systems is addressed. For single-input uncontrollable systems, a necessary and sufficient condition of stabilizability via the GDFC is obtained, which completely describes the limitation of the GDFC. For multi-input systems, a new necessary condition is derived, which reveals the limitation of the GDFC more exactly than the odd number limitation. Moreover, for a class of nonlinear systems with uncertain parameters, a nonlinear robust GDFC is designed for the first time to stabilize the unknown fixed points associated with the uncertain parameters.     

一类不确定延迟神经网络的自适应投影同步

研究了一类参数不确定的延迟神经网络的投影同步,基于Lyapunov稳定性理论,设计了一种新型的自适应控制方法.该方法能同时实现一类参数不确定延迟神经网络的参数识别和投影同步.数值模拟证明了该方法的有效性. 关键词： 延迟神经网络 Lyapunov稳定性理论 参数识别 投影同步     

Benchmarking of Density Functionals for the Description of Optical Properties of Newly Synthesized π-Conjugated TADF Blue Emitters

Computational modeling of the optical characteristics of organic molecules with potential for thermally activated delayed fluorescence (TADF) may assist markedly the development of more efficient emitting materials for organic light-emitting diodes. Recent theoretical studies in this area employ mostly methods from density functional theory (DFT). In order to obtain accurate predictions within this approach, the choice of a proper functional is crucial. In the current study, we focus on testing the performance of a set of DFT functionals for estimation of the excitation and emission energy and the excited singlet-triplet energy gap of three newly synthesized compounds with capacity for TADF. The emitters are designed specifically to enable charge transfer by π-electron conjugation, at the same time possessing high-energy excited triplet states. The functionals chosen for testing are from various groups ranging from gradient-corrected through global hybrids to range-separated ones. The results show that the monitored optical properties are especially sensitive to how the long-range part of the exchange energy is treated within the functional. The accurate functional should also be able to provide well balanced distribution of the π-electrons among the molecular fragments. Global hybrids with moderate (less than 0.4) share of exact exchange (B3LYP, PBE0) and the meta-GGA HSE06 are outlined as the best performing methods for the systems under study. They can predict all important optical parameters correctly, both qualitatively and quantitatively.     

CN-Modified Imidazopyridine as a New Electron Accepting Unit of Thermally Activated Delayed Fluorescent Emitters

Two efficient thermally activated delayed fluorescent (TADF) emitters were developed by utilizing CN-modified imidazopyridine as an acceptor unit. The CN-modified imidazopyridine acceptor was combined with either an acridine donor or a phenoxazine donor through a phenyl linker to produce two TADF emitters, Ac-CNImPy and PXZ-CNImPy. The acridine-based Ac-CNImPy emitter exhibited sky-blue emission with a CIE coordinate of (0.18, 0.38), whereas the phenoxazine-donor-based PXZ-CNImPy showed greenish-yellow emission with a CIE coordinate of (0.32, 0.58). A high photoluminescence quantum yield of 80 % was observed for the PXZ-CNImPy emitter compared with 40 % for the Ac-CNImPy emitter. Organic light-emitting diodes based on the PXZ-CNImPy emitter demonstrated high external quantum efficiency of 17.0 %. Hence, the CN-modified imidazopyridine unit can be considered as a useful electron acceptor for the future design of highly efficient TADF emitters.     

An identification method to estimate surface shortwave fluxes on temperature and thickness observations of sea ice in CHINARE 2006

In this paper, a parameter identification method to determine surface shortwave fluxes using temperature and thickness measurements of sea ice in CHINARE 2006 is presented. Adopting a new standard for the calculation of the thermodynamic properties of seawater named TEOS‐10, the surface shortwave fluxes are calculated by the temperature and thickness observations that were measured at Nella Fjord around Zhongshan Station, Antarctica. New simulations for temperature profiles in a different measurement period are performed by three parameterization schemes including the present method, Zillman and Shine. All numerical results are compared with in situ measurements. Results show that better simulations of the surface shortwave radiations and temperature distributions are possible with the identification method than Zillman and Shine. Therefore this method is valid, and the obtained shortwave radiation function can be applied in sea ice modeling. Copyright © 2015 John Wiley & Sons, Ltd.     

Molecular Engineering of Isomeric Benzofurocarbazole Donors for Photophysical Management of Thermally Activated Delayed Fluorescence Emitters

Benzofurocarbazole moieties are commonly used donor structures in the design of thermally activated delayed fluorescence (TADF) emitters. However, only 5 H-benzofuro[3,2-c]carbazole (34BFCz) has been reported and, to the best of our knowledge, no other benzofurocarbazole derivatives have been covered in the literature. In the present study, two further benzofurocarbazole moieties, 12 H-benzofuro[3,2-a]carbazole (12BFCz) and 7 H-benzofuro[2,3-b]carbazole (23BFCz), have been synthesized to investigate the effect of the donor structure on the photophysics and device parameters of TADF emitters. Two benzofurocarbazole-derived TADF emitters, 12-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-12 H-benzofuro[3,2-a]carbazole (o12BFCzTrz) and 7-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-7 H-benzofuro[2,3-b]carbazole (o23BFCzTrz), have been compared with 5-(2-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-5 H-benzofuro[3,2-c]carbazole (oBFCzTrz). The benzofurocarbazole donor structure governs the TADF characteristics, such as charge-transfer property and emission color. The 12BFCz donor has proved to be effective in blue-shifting the emission color, and 34BFCz has proven useful for improving the external quantum efficiency (EQE). The 12BFCz-derived o12BFCzTrz showed blue-shifted color coordinates of (0.159, 0.288), compared to (0.178, 0388) for o23BFCzTrz and (0.169, 0.341) for oBFCzTrz. The 34BFCz-derived oBFCzTrz exhibited an EQE of 22.9 %, compared to 19.2 % for o12BFCzTrz and 21.1 % for o23BFCzTrz.