High-performance Organic Light-emitting Diodes Based On Phosphorescent Materials

Organic light emitting diodes(OLEDs)have drawn widespread attention,because of their appealing advantages of fast response speed,light weight,flexibility and excellent light quality,etc.Phosphorescent materials can simultaneously use singlet and triplet excitons,which can make the upper limit of internal quantum efficiency of devices reach100%,while the traditional fluorescent materials can only make the upper limit of internal quantum efficiency of devices reach 25%;through the molecular design,the light color of the phosphorescent material can be regulated within the entire visible light range.Therefore,phosphorescent OLEDs(PHOLEDs)are capable of achieving both high luminous efficiency and color purity.Based on the advantages of phosphorescent materials,this thesis designs,prepares,and characterizes high performance PHOLEDs,and also discusses relevant carrier transport and recombination mechanisms.The research work is summarized as follows:(1)The exciton formation zone of PHOLEDs is studied.The exciton formation zone is detected by the emission spectra of devices and the width of exciton formation zone is calculated based on linear fitting.Besides,the recombination current generated in the exciton formation zone is explained by equivalent circuit diagram and the Shockley ideal equation.It is found that the width of exciton formation zone increases with the thickness of emissive layer increasing;with the width of exciton formation zone increasing,the current efficiency increases firstly and then becomes saturated.The properly managed the width of exciton formation zone is important to reduce the ohmic loss of devices.(2)The efficient warm white OLEDs(WOLEDs)are fabricated.High performance warm WOLEDs consisting of four light-emitting layers(EMLs)were fabricated by doping three primary color phosphors PQ₂Ir(dpm),Ir(mppy)₃ and FIrpic into host materials with matching energy levels as red,green and blue guest materials,respectively.Experimental results demonstrate that energy transfer between dopants,energy transfer between host and guest,direct carriers'capture and main recombination zone are all important factors in determining the performances of obtained devices.The optimized warm WOLED obtained the highest external quantum efficiency of 20.63%,current efficiency of 69.00 cd/A and power efficiency of 60.19 lm/W.The Commission Internationale de l'Eclairage(CIE)coordinates of(0.388,0.383)is located in the warm white area.(3)The efficient pure WOLEDs are fabricated.High performance three EMLs pure white OLEDs were constructed by doping PQ₂Ir(dpm),Ir(mppy)₃ and FIrpic into host materials as red,green and blue guest materials,respectively.By precisely managing dopants'concentrations and EMLs'thicknesses,the optimal pure white device exhibited the highest current efficiency of 60.48 cd/A,power efficiency of 51.32 lm/W,external quantum efficiency of 16.90%,color rendering index of 72.03,respectively.The CIE coordinates of(0.331,0.385)is located in the pure white area.