The Study On White Phosphorescent Sensitized Polymer Electroluminescence Devices

ABSTRACT:White organic light emitting devices (WOLEDs) have attracted considerable attention for their unique features such as a broad range of emitting colors and high efficiency. The adventure of the phosphorescent materials leads to the potentiality for OLED breaking through theoretic upper bound of 25%. After studying the issue in WOLEDs especially in white polymer light emitting devices (WPLEDs), the thesis presents a way to achieve high brightness and efficiency WOLEDs.The three energy transfer processes occurring in bi-dopant system of PVK: Ir(ppy)3:DCJTB were presented in the thesis by analyzing electroluminescence and photoluminescence characteristics, that is energy transfer from PVK to Ir(ppy)3, PVK to DCJTB and Ir(ppy)3 to DCJTB, and the main energy transfer of the bi-dopant system is the one from Ir(ppy)3 to DCJTB which could be enhanced by increasing the ratio of DJCTB. The spectra's blue shift of ITO/PEDOT/PVK:Ir(ppy)3:DCJTB/Al was observed as voltage increased because of increment of producing probability of triplet of excitons on Ir(ppy)3 which was cause by larger cross section of excitons on Ir(ppy)3 than that on DCJTB.On the other side, the different methods to improve white light emission from the bi-dopants system were carried out, one is inserting the blue light layer into device and the other is blending blue light material into PVK:Ir(ppy)3:DCJTB layer. White light emitting was obtained by device of ITO/PEDOT:PSS/PVK:Ir(ppy)3:DCJTB (100:5:0.4) /NPB(X nm)/BCP(10 nm)/Alq3(20 nm)/Al(X=0,5,10,20 nm) and the best characteristic was found when NPB is 5 nm and its CIE coordinates is (0.31,0.39). The better performance of device, such as lower turn-on voltage, more brightness and higher efficiency, were obtained by different approaches such as blending OXD-7 into emission layer, plasma-ozone optimizing and inserting ultra-thin LiF layer between organic layer and Al cathode layer. In experiment, it was also found that the device with ITO/PEDOT:PSS/PVK:Ir(ppy)3:DCJTB:NPB(100:5:0.4:X)/BCP(10 nm)/Alq3(20 nm)/ Al structure was unable to emit white light effectively for the reasons that the energy transfer of PVK,NPBâ†'Ir(ppy)3â†'DCJTB reduce the excitons in NPB and the doping NPB restrains the energy transfer to Ir(ppy)3, which leads to lower brightness of Ir(ppy)3 with increasing dopants of NPB. Finally the white emission was obtained in the device of ITO/PEDOT:PSS/PVK/BCP(10 nm)/Alq3(20 nm)/Al. The white emission was attributed to the electroplex emission, which is due to the electron transition from LUOM of BCP to HOMO of PVK. Its CIE coordinates shifts from (0.35,0.34) to (0.37,0.37) along with the increase of driving voltage from 18 V to 28 V.