Study Of Charge Control Layer In Blue OLED Devices

Since the stable and efficient double-layer organic light-emitting diodes (OLEDs) wasdemonstrated in1987, OLEDs have attracted much attention in the field of flat panel displayand solid state lighting because of their low power consumption, high luminance, wideviewing angle, panel emitting and potential applications in flexible displays. At present, somegreat progress has been achieved for the green and red OLEDs. However,the efficiency, stability and other issues of blue OLEDs still need to be further improved. Inorder to realize the industrial application of OLEDs with full colour and white lightillumination, fabricating the high performance and long lifetime blue OLEDs with simplepreparation method is particularly important. By analyzing the emitting mechanism, we havedesigned different structures of light-emitting devices. Through optimizing the devices’structures to adjust the distributions of carriers and expand the exciton formation zones, wefabricate the blue light-emitting diodes with high efficiency and brightness.1. The charge control layer has been inserted between the emitting layer (EML) andelectron transport layer (ETL) to separate the carrier recombination area and accumulatedspace charges area, which can reduce the exciton quenching and improve the utilization rateof excitons. Moreover, non-radiative energy transfer from the charge control layer to DSA-phshould occur by F rster route. Device with ADN as an charge control layer shows themaximal current efficiency and luminance of7.0cd/A and26990cd/m2which are enhancedby42.9%and70%comparing to those of the control device. We attribute the performancesenhancement for this type of blue OLED to improve the hole-electron recombination rate andbroaden the exciton formation region.2. By inserting the various charge control layers in different positions of the emittinglayer. The EL performances could be improved for the blue OLEDs via the synergisticfunctions of the two charge control layers. The original emitting zone would be separated intotwo parts by inserting MADN in the first EML, and the exciton formation region could alsobe broadened accordingly. Meanwhile, holes could be effectively confined in the second EMLby inserting ADN between the EML and ETL, which could prevent the exciton’snon-radiative decay process in the ETL so as to improve the emitting performance. Themaximum current efficiency and brightness of the optimal device are5.6cd/A and23310cd/m2, which are enhanced by70%and87%comparing to those of the control device.3. Based on two fluorescent materials of DSA-ph and DNCA, a novel heterojunction ofdouble blue-emitting layers structure of OLED is constructed. By replacing singleblue-emitting layer structure with MADN:DSA-ph (3wt%,15nm)/ADN:DSA-ph (3wt%,15nm) heterojunction double blue-emitting layers structure, the exciton formation region couldalso be broaden for this type of doped device resulting in high efficiency. The maximum efficiency of the optimized device is increased from13.4cd/A to15.2cd/A. It can be foundthat the efficiency enhancement of the device is not obvious due to the ADN as a host in thesecond light-emitting layer can not transfer the energy to the guest efficiently. Then, we haveintroduced another double blue-emitting layers structure of MADN: DSA-ph (3wt%,15nm)/ADN: DNCA (6wt%,15nm). For this type of blue OLED, the two fluorescent materialsdoped into the different optimal hosts simultaneously as light-emitting layers can greatlyimprove the efficiency of the device. The maximum efficiency of device is18.2cd/A which isenhanced by35.8%comparing to those of the control device.