Organic light-emitting diodes (OLEDs) have been the subject of intense research by academics and industry for many years due to their great potential for lighting and full-color display. Researchers have long discovered that adding a number of functional layers such as a hole transport layer, an electron transport layer, an exciton blocking layer and the like between the electrode of the OLED and the light-emitting layer will help significantly improve the light-emitting efficiency of the OLED. It can be said that the high-efficiency OLED devices reported today all contain multilayer structures without exception. However, although the multilayer structure is conducive to the realization of the ultimate high efficiency, the adoption of such a structure also brings the problems of high cost, low reliability, complicated process and low production efficiency to actual commercial production, severely restricting OLED production of low-cost and large-scale. Professor SU Shi-Jian, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, led the team to solve this problem by developing a new type of blue fluorescent material and producing a high-efficiency monolayer OLED device. The results are published in "Advanced Functional Materials" (Adv. Funct. Mater., 2015, 25, 5190, DOI: 10.1002 / adfm.201502163).

To achieve high efficiency single-layer structure of the device, the characteristics of the light-emitting material has more stringent requirements. The material must have both high fluorescence quantum efficiency, good electron and hole transport properties, and easy injection of carriers from the cathode and anode. From the perspective of constructing bipolar molecules, a new type of blue fluorescent material, NI-1-PhTPA and NI, was synthesized by combining the traditional hole-transporting unit triphenylamine and the newly developed electron-transporting unit naphthoimidazole. -2-PhTPA. The only difference between NI-1-PhTPA and NI-2-PhTPA is that it contains a different configuration of naphthoimidazole units. But otherwise, their molecular orbital energy levels and fluorescence quantum efficiencies are very close. Surprisingly, however, the performance of these two materials in a single-layer device is quite different. The single-layer device based on NI-1-PhTPA has a turn-on voltage of 3.0 V and a maximum external quantum efficiency of 0.52%. The single-layer device based on NI-2-PhTPA has a turn-on voltage of only 2.7 V, a maximum brightness of over 10,000 cd m-2 and a maximum external quantum efficiency of 4.37%. This performance even exceeds the efficiency of many of the previously reported multilayer structure fluorescent devices. Using X-ray and ultraviolet photoelectron spectroscopy, it can be found that the exposed nitrogen atom contained in the naphthaleneimidazole unit in NI-2-PhTPA can have a coordination function with the cathode and reduce the electron injection barrier from the cathode to obtain Good carrier balance, while NI-1-PhTPA does not have this feature. In addition, the use of these two materials as the undoped light-emitting layer and the introduction of the electron transport layer and the hole transport layer to prepare the multilayered device resulted in both devices exhibiting very good electron and hole injection characteristics due to the similar electron and hole injection characteristics of the multilayered device The similar driving voltage, efficiency and external quantum efficiency reach 6.08% and 5.95%, respectively, and the emission colors are the deep blue light near the NTSC standard blue chromaticity coordinates.

The work on the differences between the emission of isomers and the injection of carriers is more important in the field of organic electroluminescence.

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