White Organic Light Emitting Diodes Based On The Interface Luminescence Of NPB/Zn(4-TfmBTZ)₂

In recent years, as a new type of solid-state light source and display technique white OLED (White organic light-emitting diode, WOLED) has attracted more and more attention due to its low production cost, short response time, high brightness, wide viewing angle and low driving voltage. Using electroplex or/and exciplex to modulate the emission spectrum of the white diode is considered as an important way, which can simplify the preparation process of WOLED. However, as is known to all, electroplex or exciplex luminescence has some drawbacks such as low efficient, color instability. In this paper, we fabricated a series of white light electroluminescent diodes based on the electroplex and exciplex of NPB/Zn(4-TfmBTZ)2 interface. On this basis, we study the performances of these diodes and the influence of device structure on performance.Firstly, we studied the effect of thickness of NPB and Zn (4-TfmBTZ) 2 on device performance. Two groups of EL devices with the structure of ITO/NPB/Zn(4-TfmBTZ)2/LiF/Al were produced. The production method of the first group is fixing the Zn (4-TfmBTZ) 2 thickness at 50nm and changing the NPB thickness from 20nm to 70nm (20nm, 40nm, 50nm, 70nm).The other group is produced by fixing the NPB thickness at 50nm and changing the Zn (4-TfmBTZ) 2 thickness from 20nm to 40nm (20nm, 30nm, 40nm). We found that the device shows the best performance with the maximum brightness of 391cd/m2, the maximum efficiency of 0.64 cd/A, the best color coordinates of (0.27, 0.32), and the highest color rendering index is 83.6, when the thickness of NPB and Zn (4-TfmBTZ) 2 is 40 and 30nm, respectively.Secondly, we studied the effect of doping NPB in Zn (4-TfmBTZ)2 layer on the device performance. The doping devices with the structure of ITO/NPB/ Zn(4-TfmBTZ)2:NPB/LiF/Al, where NPB concentration in Zn (4-TfmBTZ) 2 layer is 0%, 2.5%, 5%, 10%, 20%, respectively, were made. The device with high NPB concentration has strong exciplex and electroplex emission. The device shows the best performance with the maximum brightness of 683cd/m2, the maximum efficiency of 0.87 cd/A, the best color coordinates of (0.31, 0.35), and the highest color rendering index is 91.2, when the doping concentration is 5%.Then we introduced Alq3 as electron transport layer to the devices, which have the structure of ITO/NPB/Zn(4-TfmBTZ)2 /Alq3/LiF/Al. The performance of the devices with Alq3 thickness of 10nm,20nm,30nm,40nm are tested, respectively. When the thickness of Alq3 is 30nm the device shows the best performance with the maximum brightness of 2445cd/m2, the maximum efficiency of 1.39 cd/A, the best color coordinates of (0.31, 0.35), and the highest color rendering index is 88.2.Finally, we produced a group of hetero-layer quantum well structure devices :ITO/NPB/Zn(4-TfmBTZ)2(3.0nm)/NPB(4.0nm)/Zn(4-TfmBTZ)2/Alq3/LiF/Al. The quantum well structure devices with the cycle number of 1 and 2 were studied, respectively. We found that the introduction of the hetero-structure quantum well structure can increase the intensity of exciplex and electroplex emission and stabilize the color coordination of the device. On this basis, we get a white electroluminescent device with high color purity, high brightness and color stability by adjusting the thickness of NPB layer, of which the color coordinates is (0.324, 0.337), the maxmum of brightness is 2013cd/m2, the maximum efficiency of 1.03 cd/A, and the highest color rendering index is 92.5 which is very close to the maximum color index (100), and the device emission spectrum shows high color stability when the driving voltage varied from 10V to 14V.