A Study On Charge Generation Unit And Interface Of Tandem Organic Light-Emitting Diode

Organic light-emitting diode(OLED) has been applied in the flat panel display field due to its excellent properties such as self emission, large contrast ratio, wide view angle, fast response time and wide operating temperature. In order to reduce the fabrication cost of display panel, enlarging panel size is currently trend of OLED technique development. For the full color display of large size OLED panel, the combination of white OLED(WOLED) and color filter is generally used. Due to the advantages of higher efficiency, longer lifetime and lower short circuit probability for tandem OLED as compared to conventional OLED, tandem OLED formed by connecting several emission units vertically with charge generation unit(CGU) has been applied as the core device for the flat panel display. The efficiency and stability of tandem OLED depend on charge generation and separation, stability, transmittance and fabrication process of the CGU, which renders it great importance of research on the material, structure, mechanism and process of CGU, and the tandem device.First, as to the problem of poor stability and complex process for the organic doped n/p CGU, the thesis demonstrates the CGU Liq/Al/NPB:F4-TCNQ with excellent comprehensive performance based on organic p-type doped layer NPB:F4-TCNQ and ultrathin electron-injecting layer(EIL) Liq/Al, and systematically investigates the charge generation and separation mechanism of CGU. The CGU possesses outstanding performance such as high charge generation and separation efficiency, high optical transmittance of over 90% within visible light region, high long term opearation stability, and simple fabrication process. Increasing evaporation rate of Al and F4-TCNQ concentration and inserting an interface layer between NPB:F4-TCNQ and Liq/Al lead to reduced energy barrier for the charge separation. Accordingly, the optimum CGU structure is Liq/Al/F4-TCNQ/NPB:F4-TCNQ, which is introduced in a WOLED based on evaporation and solution processes. White color can be tuned by adjusting the NPB:F4-TCNQ thickness of the WOLED, and the combined process facilitates fabrication cost reduction.Next, as the combination of indium gallium zinc oxide-thin film transistor and inverted OLED has been applied for large area display panel, the thesis has an investigation on efficiency and stability of the single and tandem inverted devices. The commonly used electron-injecting layer(EIL) Cs2CO3 has poor stabitlity because it is easily oxidized in air. However, it is found that Al/Cs2CO3 is an air-stable EIL due to the formation of Al-O-Cs complex, which can greatly improve the stability of the conventional inverted device. For the tandem inverted OLED, the thesis uses the nondoped p/n heterojunction Mo O3/Al/Cs2CO3 as the CGU, where the combination of Mo O3 and carbazole-based host acts as the p-type component and HTL, leading to the simplified tandem device structure. The 1 nm Al interlayer can effectively prevent the chemical reaction of Cs2CO3 and Mo O3, leading to the suppressed depletion zone. This will result in enhanced charge separation and staiblity of the CGU.Last, since the solution process has an advantage of realizing large size and flexible display panel compared to thermal evaporation process, the thesis has an exploratory research on the key points of solution process-based CGU(s-CGU) and tandem device, including precursors of solution process-hole-injecting layer(s-HIL) and s-EIL, and solution process of s-CGU. s-HIL is prepared by directly dissolving Mo O3 powder in ammonia water, followed by healing the spin coated film at 150 oC in air. The s-HIL has the nanoplate structure, and possesses the superior hole injection as compared to the evaporated Mo O3 and PEDOT:PSS. Another s-HIL is prepared by dissolving HATCN in acetonitrile solvent, and the s-HIL also has excellent hole injection property. s-EIL is prepared by reaction of Zn O powder and formic acid under catalysis of ammonia water, following by annealing the precursor at low temperature in air, and possesses strong electron injection property. The result of X-ray single crystal diffraction analysis shows that metal-organic framework(MOF) [(H3O)Zn(HCOO)3]∞ is formed in the s-EIL film. Due to the strong electron-withdrawing and electronegative property of the formate in the framework [(H3O)Zn(HCOO)3]∞, dipoles prefer to form at electrode/[(H3O)Zn(HCOO)3]∞ interface, resulting in the reduced WF of [(H3O)Zn(HCOO)3]∞-modified electrode. The s-CGU MOF:PEIE/HATCN is based on the p- and n-type components prepared from HATCN acetonitrile solution and blend of MOF [(H3O)Zn(HCOO)3]∞ and PEIE, respectively. The solution processes of charge injection layer and CGU not only reduce the fabrication cost of OLED, but also facilitate its wide application in other organic electronic devices.