Study On High Performance Color Tunable Organic Light-Emitting Diodes Based On Exciplex

Organic Light-Emitting Devices((OLEDs)have the advantages of surface luminescence,bright colors,health and environmental protection,etc.,and have received widespread attention in the field of display and lighting.With the development of high-performance organic optoelectronic materials and new device structures,the performance of white OLEDs has been greatly improved.Color-tunable OLEDs have a broad application prospect in the fields of intelligent lighting,modern farming and medical treatment.However,color-tunable OLEDs still have problems that limit their development such as low efficiency,high turn-on voltage,and small color change range.The highly efficient thermally activated delayed fluorescence(TADF)phenomenon in the exciplex can achieve 100% exciton utilization,which can greatly increase the device efficiency and reduce the turn-on voltage as the host.At the same time,the exciplex has bipolar transport properties,which can be placed in the light-emitting layer to broaden the exciton recombination zone and increase the color change range.In this thesis,a new exciplex structure was proposed,and the working mechanism was initially explored to optimize the performance of yellow,green and red light-emitting devices.High-performance color-tunable white OLEDs were prepared by introducing a blue ultra-thin light-emitting layer structure.The specific work is as follows.(1)N-([1,1’-biphenyl]-2-yl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9,9’-spirobi[fluorene]-4-amine(FSF4A)was used as the donor,and 4,6-bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine(B3PYMPM)as the acceptor,and a novel structure of the exciplex was proposed.The photoluminescence and electroluminescence properties of the novel exciplex system were firstly investigated,and it was found that the exciplex photoluminescence was in the sky blue light,and the best current transfer performance was achieved at 1:3 between FSF4 A and B3 PYMPM.Then the optimal ratio of exciplex was used to optimize the monochromatic OLEDs,in which the yellow,green and red devices achieved very high efficiencies of 72.34 lm/W,106.03 lm/W and 33.09 lm/W,respectively.(2)A color-tunable white OLED was prepared by introducing a blue ultrathin light-emitting layer structure combined with a yellow exciplex light-emitting layer.Because of the higher energy level of the blue dye,an ultrathin layer structure was proposed to be inserted into the color-tunable device.The obtained devices exhibit good color tunable performance with the maximum variation of color coordinate CIE from(0.18,0.35)to(0.41,0.45)and the maximum variation of color temperature from 15213 K to 3802 K,spanning 11411 K and covering daylight color temperature.However,the efficiency of the device was not satisfactory,with a maximum of 37.34 lm/W.It is assumed that the lower efficiency is due to the extremely severe exciton quenching phenomenon effect occurring in the ultra-thin layer at high concentration.To improve the efficiency,the device was optimized using MCP doped blue dye instead of ultrathin layers,and the color change was controlled by adjusting the thickness of the two emitting layers.The device efficiency was greatly improved,up to 66.63 lm/W,and the color tunable characteristics of the device were maintained.The wide color change range is attributed to the wide exciton recombination zone provided by the exciplex and the efficient TADF process that enhances the overall device performance.