Synthesis And Electro-optical Proprety Investigation Of Bipolar Host Materials Based On Diphenylphosphine Oxide Unit

Organic electroluminescent diodes?OLEDs?have attracted more and more attention due to its broad viewing angle,low driving voltage,high luminous efficiency,short response time,high contrast ratio and so on.Therefore,OLEDs have shown broad prospects in display and lighting technologies.In recent years,solution-processed OLEDs have attracted great attention,because they can realize large-area lighting,flexible display,especially simple production process and low cost.As the phosphorescent device can fully utilize the singlet and triplet excitons,the internal quantum efficiency of the OLEDs can approach 100%.However,due to the existence of serious triplet-triplet state annihilation phenomenon in phosphorescent OLEDs,it is necessary to doping in the appropriate host material to achieve high efficiency of light emission.In addition,the thermally activated delayed fluorescence host materials?TADF?can transform the triplet excitons into singlet excitons through reverse intersystem crossing,and to achieve 100%internal quantum efficiency.In particular,a TADF molecule as the host material doped a guest TADF molecule in the preparation of devices,which will be able to effectively improve the efficiency of the device.Currently,either in terms of efficiency or stability,green and red OLEDs have made great strides.However,as an important part of solid-state lighting and full-color displays,the performance of blue PhOLEDs is still far below the level of other primary colors.Therefore,the performance of blue light devices needs to be further improved.To achieve highly efficient blue OLEDs,a relatively simple and effective way is to design a suitable host material.The main research work of this paper is as follows:?1?A novel bipolar host material BCz-BPO based on diphenylphosphine oxide and carbazole units was designed and synthesized.BCz-BPO has a high triplet level,excellent thermal stability and film-forming properties.Density functional theory?DFT?calculations and experimental results showed that increasing the number of diphenylphosphine oxide groups not only increased the electron transport and injection capacity of the bipolar host material,but also enabled the BCz-BPO based device to have a more balanced charge flux and higher exciton recombination efficiency.As a result,the blue solution-processed electrophosphorescent device using BCz-BPO as the host showed the maximum luminance efficiency of 30.1 cd A^-1,which was comparable to BCz-PO based vacuum deposited device and increased about three times in compare with the corresponding device based on BCz-PO.?2?A thermally activated delayed fluorescence bipolar host material POCz-CzCN based on diphenylphosphine oxide groups was designed and synthesized.The molecule was a dendrimer and consists of the inner emissive core and the peripheral carbazole and diphenylphosphine groups,which were linked by a non-conjugated alkyl chain.It was the host material for the TADF luminescent guest,and it also had the characteristics of TADF,which could fully ensure the complete transfer and utilization of energy so as to realize high device efficiency.The TADF dendrimer POCz-CzCN was studied by experimental tests.POCz-CzCN had a large molecular weight and spatial configuration,and therefore had good thermal stability and film-forming properties.The molecule had a higher triplet energy level and a smaller?E_ST.Alkyl chain as a bridge increased the solubility of the molecule,which was conducive to the wet preparation of devices.Diphenylphosphine oxide units in the peripheral branches could balance the carrier transport and improve the device efficiency.Using POCz-CzCN as the host,solution-processed blue OLEDs achieved a high EQE of 7.1%,and a maximum current efficiency of 19.5 cd A^-1.