Study On Tandem Organic Light Emitting Diode With Undoped CGL Structures

Tandem organic light emiting diode(TOLED) owns higher luminance, higher efficiency and longer lifetime at low current density, which have attracted much attention in scientific community and industry. By analyzing the current problems of TOLED in scientific area, we fabricated monochromatic flurescent and phosphorescent TOLED based on different CGLs and analyzed the work mechanics of different CGLs. The main research content includes three sections as follows:Firstly, based on emitting unit Alq3and undoped CGL LiF/Al/MoO3/NPB, we fabricated high power efficiency TOLED and simplified the fabrication technology. The capacity of CGL generating charges was decided by the thicknesses of Al and MoO3. The most optimized CGL structure was LiF(1nm)/Al(5nm)/MoO3(10nm)/NPB(10nm), and then we fabricated TOLED. With adding an electron barrier layer TCTA we can improve the power efficiency of TOLED. At a current density of50mA/cm2, the current efficiency of the tandem OLED with two standard NPB/TCTA/Alq3emitting units is8.5cd/A, which is5.6times greater than that of the single EL device. Besides, the power efficiency was also enhanced from0.91to2.53lm/W.Secondly, An efficient TOLED was demonstrated by employing LiF/Al/F4-TCNQ/NPB as CGL. It is found that the inverted device with CGL in which the thicknesses of8nm F4-TCNQ and5nm Al exihibit the best current density. Then, we fabricated the tandem OLED with the emission layer of CBP:6%Ir(ppy)3. By comparing the TOLED and conventional single OLED, it was found that the turn-on voltage of tandem OLED with the CGL of LiF/Al/F4-TCNQ/NPB was increased only1.4times relative to that of single OLED, however, was two times lower than that of single OLED, which can improve the power efficiency. The maximum current efficiency of52.6cd/A was obtained, which was about two times higher than that of single OLED. The power efficiency was improved to28.4lm/W. It is indicated that LiF/Al/F4-TCNQ/NPB is an effective CGL for highly efficient tandem OLED.Finally, white TOLED was demonstrated by employing an orange phosphorescent material Hex-Ir(phq)3and conventional blue material FIrpic. By putting orange emitter and blue emitter into two independent emitting units, the EL spectra was steady with the voltage increasing. Otherwise, the device showed about100%improvement in efficiency compared to conventional single white OLED, while there was only a slight change of CIE(x,y) coordinate from (0.42,0.46) to (0.43,0.46) as the luminance increased from1000cd/m2to20000cd/m2. It is believed that the new design of white TOLED can fine tune the contribution of primary emission even the combination zone shifted with applied bias changing, resulting in high quality phosphorescent WOLED.