Organic Light-emitting Device Performance Optimization And Charge Transport Mechanism

Since the demonstration of the first double-layer organic electroluminescent device in 1987,more and more researchers have paid attention to organic light-emitting devices(OLEDs) due to their potential application in flat-panel displays. The thesis is focused on improving the performance of OLEDs and investigating the charge generation mechanism in tandem-type OLED.The thesis consists of the following topics:1.High-contrast organic light-emitting devices.High-contrast OLEDs with low-reflection cathodes are fabricated.The cathode consists of a semi-transparent metal layer,a phase-changing(PC) layer,and a reflective metal layer.With C₆₀ as PC layer,devices exhibit the average reflectivity of the ambient light as low as about 10%.And its electrical characteristics are almost identical to that of a conventional device,although the thickness is increased by 70%.We try to use tandem-type structure to improve the low-reflection cathode.2.A new bilayer structure,Al₂O₃/F₄-TCNQ:NPB,or Al:Alq₃/F₄-TCNQ:NPB was fabricated as a charge generation unit(CGU) for tandem type organic light-emitting devices(OLEDs).Different from reported p-n junction structure CGUs,an insulating layer,Al₂O₃,or an efficient hole-blocking layer,Al-doped Alq₃,was used to replace the n-type layer.F₄-TCNQ-doped p-type layer generates both holes and electrons,and the insulating layer or the efficient hole-blocking layer helps to establish a high localized electric field across itself. This high localized electric field reduces the energy barrier for the electrons generated in F₄-TCNQ-doped film to tunnel into the emitting unit.3.We investigated the temperature dependence of OLED's current efficiency.We find that although the current is not resulted from NPB,NPB affects the temperature dependence of OLED's current efficiency.For the device without NPB,the current efficiency of the device decreases with raising of temperature monotonously.For eh device containing NPB as hole-transporting layer,the current efficiency increases initially and then decrease with raising of temperature. We explain the phenomena with the electron blocking effect of NPB changing with raising of temperature.The electron blocking effect of NPB increases with the raising of temperature.And we found that at the temperature of liquid nitrogen,the current efficiency of the device with two light-emitting unit and without charge generation unit can be twice as high as that of the device with only one light-emitting unit.