Study On Electron Injection In Inverted Bottom-emitting Organic Light-emitting Diodes

Organic light-emitting diodes(OLEDs) have been widely used in solid-state lighting and flat-panel displays due to their advantages such as wide angle, high brightness and flexible feasibility since their first demonstration by Tang and Van Slyke in 1987. However, in order to be compatible with amorphous Si(a-Si) in flat-panel displays, the inverted bottom-emitting OLEDs show more advantages compared to conventional OLEDs. Thus, this thesis is focused on the inverted bottom-emitting OLEDs. Indium tin oxide(ITO) possesses high surface work function of about 4.8 e V, while the lowest unoccupied molecular orbitals(LUMO) of most organic electron transporting layers(ETL) used in OLEDs range from 2.5 e V to 3.5 e V. It implies a very high electron injection barrier at the ITO/ETL interfaces, which forms a bottleneck to hinder the realization of high-performance inverted OLEDs. So in this thesis, we try to modify ITO to enhance electron injection and three main parts are presented:Firstly: Al is prepared simply by thermal evaporation on ITO to enhance electron injection. At first, we choose Al due to its low work function. When ITO is modified by Al, it can lower the work function of ITO and then reduce the electron injection barrier. However, the device with small-sized Al nanoparticles(NPs) showed the best performance with the lowest driving voltage and highest luminance efficiency. According to the characterization by TEM, AFM, and UPS, and based on Gauss’ s Law, we propose a physical model that local electric-field-enhanced electron emission at the Al NPs. To verify the model, we also choose Ag and Cu to modify ITO by thermal evaporation. In accordance with our expectation, the experiment results showed that both Ag and Cu NPs can enhance electron injection.Secondly: Based on the physical model mentioned above, ITO was modified by Pt(high work function metal).A series of devices with different thicknesses of Pt were presented by sputtering. Similarly, when the content of Pt is low and existed in NPs, the device shows the best performance with lowest driving voltage and highest luminance efficiency. Moreover, Au was prepared by sputtering and self-assembling to modify ITO and electron injection is also enhanced. So we conclude that the metals existing in NPs on ITO can enhance electron injection effectively and regardless of their work function.Finally: The main trend of the world today is energy saving and environment friendly. Combined with the above physical model, we choose environment friendly carbon nanostructured materials with low cost to enhance electron injection. Graphite, carbon quantum dots, carbon nanotubes and other nanostructured materials are presented by sputtering, spin-coating and thermal evaporation. Whatever the materials and preparation methods, the modified devices showed better performance when the content of modifying materials is low. In other word, the carbon nanostructured materials can enhance electron injection when they are existed in NPs on ITO.In summary, this work proposes a new approach to solve the low electron injection efficiency in inverted bottom-emitting OLEDs by modifying ITO with NPs. A physical model was proposed to clarify the mechanism and additional experiments were carried out to confirm this model. We believe the findings suggest an alternative way to enhance carrier injection in diverse organic electronic optoelectronic devices.