Study Of Carrier Transport Property Of Fluorescence And Phosphorescence Hybrid White Organic Light Emitting Diodes

In recent years, a new type of flat panel display, organic electroluminescent diode has attracted much attention. It has some special features, such as low driving voltage, quick response, plane light source. White organic light-emitting diodes(WOLEDs) become a focus because it can not only be used as lighting but also be used to realize full color display by applying mature color filter technology.According to the principle of three basic colors, a WOLED can be realized by mixing red, green and blue colors at proper ratios. The ideal white light require mixing ratios of the three colors to be optimized. Based on the principle of three basic colors, WOLEDs are obtained by mixing three colors. We conducted the following study in this thesis.Firstly, transport property of typical bipolar material, 4,4′-N,N′-dicarbazolylbiphenyl(CBP) in electrophosphorescent diodes was studied. An ultrathin layer of fac-tris(2-phenylpyridine)iridium(Ir(ppy)3) was inserted into CBP layer at different position. By comparing the performance of different diodes, the results show that recombination of electrons and holes take place mainly near anode side, indicating that the mobility of an electron in CBP is greater than that of a hole.Secondly, a traditional green diode with a structure of ITO/Mo O3/TPD/Alq/Li F/Al was fabricated. CBP is introduced in the above diode and used as electron transporting layer and hole transporting layer, respectively. By comparing the performance of different diodes, the results show that the device using CBP as an electron transporting layer exhibited lower driving voltage than that using CBP as a hole transporting layer. This fact supports the conclusion that the mobility of an electron in CBP is greater than that of a hole.Thirdly, two different methods used to fabricate diodes: doping Ir(ppy)3 into CBP and inserting Ir(ppy)3 layer into CBP. By comparing the performance of different diodes, the results show that the diode fabricated by doping method achieved higher luminescent efficiency and lower driving voltage.Fourthly, we reported the multi-layer organic light emitting diodes based on emissive layer, in which Ir(ppy)3 and Ir(btp)2(acac) were doped into CBP host,respectively, and the fluorescent blue material CBP is employed in the OLED. By varying the thickness of the doping layers, the results show that the driving voltage rises with increasing of the thickness of CBP:Ir(btp)2(acac) layer. It has the same result when order of doping layer was changed. Another result show that the device has double luminance and lower driving voltage when CBP:Ir(ppy)3 layer wasinserted close to cathode side and CBP:Ir(btp)2(acac) layer close to anode side.Finally, the white light-emitting diode was achieved when the structure of the diodes were ITO/Mo O3/NPB/CBP/CBP:Ir(ppy)3/CBP:Ir(btp)2(acac)/CBP/Alq/Li F/Al,the thicknesses of both CBP layres are 5 nm, CBP:Ir(ppy)3 and CBP:Ir(btp)2(acac) are5 nm and 25 nm, respectively. CIE coordinates of diode were(0.32, 0.32) at a current density of 1 m A/cm2, close to balanced white emission(0.33, 0.33). The maximum luminance of the device at a current density of 500 m A/cm2 is 8477 cd/m2.