||We report that the tandem OLEDs made of two electroluminescent (EL) units connected by the interconnecting layer. If It is compared wih the traditional OLEDs. The tandem OLEDs have higher efficiency and well lifetime. We not only used the single emitting layer WOLEDs as EL unit but also studied the effect of the interconnecting layer for whole device.|
First, we designed the interconnecting layer with Alq3：Li (1%) (n-doping layer)/MoO3 (p-doping layer), and we optimized the thickness of the interconnecting layer by using green unit cell (Alq3 for EML),
ITO/NPB(65 nm)/Alq3(30 nm)/Alq3(30 nm)/Alq3(x nm)：Li (1%)/MoO3(y nm)/NPB(65 nm)/Alq3(30 nm)/Alq3(30 nm)/LiF(0.8 nm)/Al(200 nm)
We found that the best thickness of Alq3：Li (1%) and MoO3 are 20 nm and 5 nm. In our study, we concluded that there are the best thickness to each interconnecting layer, and it keeps the charge balance between two units.
Finally, we used our single emitting layer WOLEDs as unit cell, which used 1,3,5-Tri(1-pyrenyl)benzene (TPB3) as the host, and 4-(dicyanomethylene)-2-tert-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as the guest, unit cell was
ITO(130 nm)/NPB(65 nm)/ TPB3(30 nm)：DCJTB(0.05%)/ Alq3(30 nm)/LiF(8 nm)/Al(200 nm)
Whole device was
ITO(130 nm)/NPB(65 nm)/ TPB3(30 nm)：DCJTB(0.05%)/ Alq3(30 nm)/Alq3(20 nm)：Li(1%)/MoO3(5 nm)/NPB(65 nm)/TPB3(30 nm)：DCJTB(0.05%)/Alq3(30 nm)/
LiF(0.8 nm)/Al(200 nm)
We got almost three times luminance from the tandem one at the same current density (670 cd/m2 for 2360 cd/m2 at 20 mA/cm2) and efficiency as high as 9.7 cd/A ( at 24 mA/cm2). It’s a excellent contribution for device lifetime. But the operation voltage and the power efficiency didn’t reach to our expectancy.
In order to improve the disadvantage, we changed the concentration of n-doping layer Alq3：Li (z %)，z=1%，2%，3%. It was actually improved the turn-on voltage from 10 V to 7 V. But the luminescent characteristics also degenerated. Although we enhanced the charge mobility of the n-doping layer, it also caused the degeneration of luminescent characteristics because of the unbalance of the charge transference.We got the efficiency 8.1 cd/A ( at 14 V) and almost two times luminance from the tandem one at the same current density (670 cd/m2 for 1760 cd/m2 at 20 mA/cm2), most close to the white area of CIE coordinates was (0.30 , 0.37) at 15 V. Its range of CIE coordinates was (0.35 , 0.46)~(0.28 , 0.33) at 8 V~20 V. We have already developed the tandem WOLEDs using single white emitting layer as EL units that have never be reported. It not only maintained the advantages of the tandem structure, but also had excellent stability of luminescent characteristics at wide range operation voltage. We reached our goal to improve the WOLEDs and make it more suitable for commercial applications, especially for the development of light sources.