Research On Blue Fluorescent Organic Light Emitting Diodes With High Efficiency And Long-Life Time

Organic light-emitting diodes(OLEDs)exhibit numerous advantages,such as self-illumination,light weight,wide viewing angle and low operating voltage.In recent years,the commercial applications of OLED products mainly focus on mobile phone screens.The efficiency and life time are key factors for mobile phone display.The development of blue OLED is relatively slow for the full color display,device with high efficiency usually shows low life time,and vise versa.As the most promising device,blue fluorescent OLEDs have attracted much attention.How to improve the performance of blue fluorescent OLEDs have become a hotspot in the industry.In this thesis,the exciton recombination region in blue OLEDs is explored.On this basis,high efficiency utilization of exciton is realized through device structure optimization,and the device efficiency is improved.Moreover,as the target layer,the electron transporting layer and hole blocking layer are investigated to improve the lifetime of device.The specific contents are illustrated as follows:1)The exciton recombination region location was explored and the results show that the location of the exciton recombination region is mainly concentrated at the interface of EB/BH,and the distribution of excitons in the light-emitting layer is successively reduced from the near EBL side to the HBL side.Two emitting layers with gradiently doped device structures were designed.At the same brightness,the current efficiency of devices in the light-emitting layer doped with guest dye reaches 8 cd/A,which is 5%higher than that of target device.In addition,blue OLED device with double emission layers were designed to balance the carriers,whose current efficiency reaches 7.93 cd/A,which is improved by 4.5%compared with target device.2)The effects of different electron transport layers on the performance of blue fluorescent OLEDs were studied.Three devices with different electron transport materials were fabricated.The maximum brightness of devices showed a maximum of 206 cd m^-2,while the life time were different.Fluorescent OLED using high mobility electron transport layer achieved a higher device efficiency,while the device using low mobility of electron transport materials exhibited longer life time,with the LT₉₇ reaching 122 h.This is mainly because the electron transport material with low electron mobility can prevent the exciton concentration in the exciton recombination region from being too large,which improve the triplet exciton annihilation,slowing down the efficiency roll-off.The results provide a guidance for the selection of electron transport materials for high stability OLED device.3)The role of hole-blocking layer in blue OLEDs was studied.First,the blocking effect of hole blocking layer on the excitons in the emitting layer was verified by designing probe device.Then,contrast devices with or without a hole blocking layer were prepared and analyzed.Results show that the device without hole blocking layer had a significant increase in lifetime,while the efficiency was 20%lower than that of the comparison device.The blue factor of the device with/without hole blocking layer are 171 and 142,respectively.The BI value of the device with hole blocking layer is about 20%higher than that of the device without hole blocking layer.The device life of the hole blocking layer is much longer,with LT₉₇ reaching 220 h.The results show that the hole blocking layer can block the exciton leakage,enhance electron transport,and ultimately improve the performance of the device,but it will deteriorate the device life.