Highly Efficient White Organic Light-emitting Diodes Based On Ultrathin Layer Structure

Organic light-emitting diodes(OLED)have gradually emerged in the field of display and lighting due to their high color saturation,low energy consumption,no backlight,flexible display and other advantages.With the increase of the scale of industrialization,we can already see more and more OLED display panel products on the market.However,for OLED,there are still many problems.For example,the long lifetime of triplet excitons in phosphorescent materials leads to the serious roll-off of the efficiency with the increasing voltage.Due to the unbalanced transmission of electrons and holes,the color stability of devices is poor.This problems need to be solved in further research.In order to further reduce the manufacturing cost of OLED,it is important to simplify the device structure.We first investigate the exciton recombination region of the mixed host structure by changing the position of the ultrathin layer.It is found that the main compound region of exciton with mixed host structure is located near the cathode.Then blue phosphorescent materials are doped into the mixed host structure to achieve white emission with high efficiency and good color stability.The device achieves a brightness of 14990 cd/m~2and a low opening voltage of 3.21 V.The efficiency is 25.2cd/A(19.2 lm/W)and good color stability is achieved.High performance is due to the employment of mixed host,which can transport both holes and electrons efficiently,reducing the accumulation of carriers,and making the carrier in the transmission process more balanced,which can further enhance the hole and electrons recombination effect,improve device efficiency and color stability.The use of undoped ultrathin layer structure also plays a role in simplifying the device structure.On this basis,we designed white emitting devices with double emission layers,and inserted a blue phosphorescent ultrathin layer between the two emission layers.By changing the thickness of the ultrathin layer,we explored the influence of phosphorescent materials on the device performance.We found that the insertion of ultrathin layer hindered the carrier transmission and reduced the performance of the device.The device without ultra-thin layer achieves brightness of 51720 cd/m2 and efficiency of 46.7 cd/A(37.9 lm/W).This is due to the use of mixed host structure in the emission layer,the injected holes can not only enter the blue emission layer,but also into the orange emission layer smoothly;and the injected electrons can not only enter the orange emission layer,but also smoothly into the blue emission layer,so that both emission layers can be luminescent efficiently.