Design,Synthesis And Photoelectric Properties Of Organic Blue Light Molecules Based On Fluorene Nuclei As Nitrogen Heterocyclic Donors And Acceptors

Organic Light-Emitting Diodes(OLEDs)have gained a significant market share in the fields of flat panel displays and lighting after years of development and accumulation.However,blue light-emitting devices still suffer from inherent defects that severely limit the development of white OLEDs and the entire field of electroluminescence.Despite years of targeted development,blue light materials still exhibit low efficiency,high roll-off,and short lifetimes.To achieve good device performance,organic emissive materials must have high fluorescence quantum efficiency and balanced charge carrier mobility.This paper focuses on the design and synthesis of a series of donor-π-acceptor(D-π-A)molecules using fluorene as a conjugated bridge.By introducing different multi-nitrogen-containing donors and acceptors,the molecular conjugation and torsion are adjusted for comparative studies.Four D-π-A-type molecules were designed and synthesized:9-(7-(4,6-diphenyl-1,3,5-triazin-2-yl)-9,9-di-n-propyl-9H-fluoren-2-yl)-9H-carbazole(CZCFTAZ),9-(4-(7-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9,9-di-n-propyl-9H-fluoren-2-yl)phenyl)-9H-carbazole(9PCZCFTAZ),3-(7-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9,9-di-n-propyl-9H-fluoren-2-yl)-9Phenyl-9H-carbazole(3PCZCFTAZ),and 4-(7-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9,9-di-n-propyl-9H-fluoren-2-yl)-N,N-diphenylaniline(TPACFTAZ).Through various characterization techniques,it was demonstrated that balancing the conjugation length and torsion angle can improve the external quantum efficiency(EQE)of the materials.In addition,to explore the feasibility of blue HLCT molecules as main materials in the preparation of WOLED devices,the blue-light-emitting HLCT molecule TPACFOXZ was utilized as the main component,and a high-efficiency and low-roll-off white OLED device was fabricated through structural design of the device.The dual emission layer device achieved an EQE of up to 27.1%and maintained a high efficiency of 23.8%at a brightness of 1000 cd/m~2.In summary,firstly,by regulating the balance of conjugation and torsion,the designed molecule’s EQE was increased from 6.5%in CZCFTAZ to 8.0%in9PCZCFTAZ and 11.7%in 3PCZCFTAZ.The control of excited states is the primary reason for this efficiency improvement.Among them,the non-doped component of the3PCZCFTRZ molecule exhibits a maximum EQE of nearly 12%,with no efficiency roll-off at a brightness of 1000 cd/m~2 and an ultra-low turn-on voltage of 3.0 V.This performance is almost twice that of the initially designed CZCFTRZ molecule and is one of the best-reported performances for blue non-doping components.Secondly,by using balanced electron and hole transport host materials and controlling the arrangement of emitters in devices,a simple doped structure of yellow-blue dual-color WOLED was designed,achieving a power efficiency of 53.8 lm/W,a maximum EQE of 27.1%,and a warm white-light OLED with color coordinates(0.35,0.39)at a very high brightness of 10,000 cd/m~2.The obtained data is among the best performances known for WOLEDs with the simplest structure.This study explores the influence of material synthesis,device structure design,and device optimization on OLEDs,providing a novel and feasible approach for designing high-efficiency,high-color purity,low roll-off OLEDs and structurally simple,high-efficiency,low roll-off,and color-stable WOLEDs.