The Organic Light Emitting Device Interface And Device Stability

With up-growing development in the organic semiconductor industry,more and more insights have been put into the fundamental physics and chemistry about the operations of the organic microelectronic devices.In those typical layer-based organic devices,including organic light-emitting diode(OLED),organic field effect transistor(OFET) and organic photovoltaic cell (OPVC),the most focused work are to improve the injection efficiency through the interfaces and the lifetime of the devices.Mostly,the major factor to control carrier injection is the interface barrier height.At metal/organic semiconductor interface,the electron(hole) injection barrier depends on the position of the LUMO(HOMO) with respect to the metal Fermi level under the Schottky-Mott rule.Most of the actual metal/organic or some of the organic/organic interfaces, however,deviate from the rule due to the existence of interface dipoles.Therefore,the studies of energy level alignment and dipoles at organic interfaces would play a key role on the optimization of organic electronic devices,it has been demonstrated that photoemission(UPS/XPS) is one of the most successful experimental methods to research the interface energy alignment and chemistry.In this thesis,a series of interfaces,including organic/metal,organic/organic and organic/inorganic semiconductors,were investigated systematically to elucidate the interface formations and electronic structures by synchrotron radiation photoemission(SRPES) method.All the molecules or metal atoms were deposited step-wised in situ onto substrate under ultra high vacuum system.In addition,we also study the feature of the device operated at constant voltage and control the transportation of hole of the OLEDs.The detail subject and results are showed as below:1 We have employed the high-resolution synchrotron radiation photoemission to investigate the electronic structure of the interfaces of LiF/AI/Alq,AI/LiF and LiF/Al,the detailed process and product of the reaction between LiF and Al was investigated clearly.The chemical reaction model of the role of LiF at Al/Alq interface was well demonstrated. 2 The synchrotron radiation photoemission spectroscopy study showed that the energy level of the interface between the hole transport material NPB and Al could be varied by adjusting the thickness of pre-coved C₆₀ on the metal substrate.The interface dipole induced shifts of the highest occupied molecular orbital(HOMO) level and the cut-off of the NPB layer by inserting thin C₆₀ layer were investigated and lower the barrier height for hole injecting from Al to NPB by as much as 0.98eV.The optimize thickness of the inserted C₆₀ layer was found to be 8-12(?) which could increase the work function of the Al from 4.22eV to 4.92eV.3 We have studied the turn-on stability of aging OLEDs by constant voltage measurement.The characteristics of OLEDs which operated by constant voltage was discussed.In addition,we have studied the different hole block effect of different metal(Au,Ag,Al,Li) doped into hole transport layer NPB.We proposed that the hole blocking ability depends on the metal's ionizability. Transient photo-voltage spectra shows the different injection ability of holes of different ITO\NPB:metal interfaces.Utility of the hole blocking effect of the metal doped system in the electron transport layer to reduced the leakage of holes across the Alq layer to the cathode and gained the more stability than the device that without this layer.