Study Of Efficiency Improvement For White Phosphorescent Organic Light-emitting Devices

Organic light-emitting devices (OLEDs) have been attracting worldwide interest for their high efficiency, brightness and good colour stability. The brightness and efficiency of the devices have increased markedly with the introduction of phosphorescent materials than fluorescent materials,so this is an important direction of the development of OLED. Recently white organic light-emitting devices (WOLEDs) have drawn much attention due to their potential applications in full color displays, in backlight for LCDs, and in solid state lighting sources. Therefore the research and the development of WOLED has the special significance.This paper revolved the fabrication and performance enhancement of WOLEDs and we have done the following work:Firstly, by employing a polymer PVK as host material doped with fluorescent N-BDAVBi and phosphoresecent Ir(piq)2(acac)and Ir(ppy)₃ , the WOLED with a single light-emitting layer was fabricated successfully. Ir(ppy)₃ was used as a sensitizer. Its brightness reaches 4464cd/m² . CIE coordinates is (0.33, 0.38) .The devices show a maximum external efficiency of 0.92%which is higher than the device without Ir(ppy)₃.Then we made another type of single layer WOLED by using PVK: Rubrene:Ir(piq)₂(acac) :N-BDAVBi as light-emitting layer. The CIE coordinate of the device could be well controlled by Rubrene .Secondly, by using a phosphorescent dye Ir(piq)₂(acac) as a red emitter and TBPe as blue emitter, We fabricated a double light-emitting layers WOLED . Its structure is ITO/NPB/NPB:Ir(piq)₂(acac)/CBP /CBP:TBPe/BCP/ALq/Mg:Ag. On one hand, the introduction of red phosphorescent enhance the efficiency, on the other hand, the blue fluorescent dye employed as blue emitting dye avoided the flaw of the short lifetime for blue phosphorescen and double doping layer devices have simple facture process because of their high doping concentration comparing with single doping layer .By adjusting the proportion and thickness of the dopants,We can get admirable white light. The devices show a maximum external efficiency of 1.24% with the CIE coordinates is (0.33, 0.30) at 8 V. The brightness reaches 7500 cd/m² at 15V.Lastly,In order to improve the performance of double light-emitting device, we employ a high efficiency fluorescent dye N-BDAVBi instead of TBPe and LiF/Al is used as the cathode to enhance electrons injection.The bright and steady light emitting has been obtained.This OLED presents a maximum luminance of 25350cd/m² with CIE coordinates of (0.35, 0.35) at 22V and a maximum external quantum efficiency of 6.78% at 8.6V. The CIE coordinates vary slightly as the applied voltage changes from 6V to 22V.