Interfacial Studies On Tandem Organic Light-Emitting Diodes

Organic light-emitting diodes (OLEDs) have already attracted much attention fortheir applictions in lighting source, flat-panel display and flexible electronic field dueto their advantages of low power consumption, high brightness, long lifetime, and lowfabrication cost. In particular, white OLEDs have the potential as the next-generationsolid-state lighting source, because of their lower weight, thin thickness, and highpower efficiency. Although significant progress has been made in the past decades,how to design OLEDs with higher power efficiency, longer operational lifetime, andlarger area is still remaining the technical challenges. Therefore, it is very important forus to understand the working mechanism and physical characteristics in OLEDs for theoptimization of high-performance OLEDs.In this thesis, high-efficiency tandem OLEDs have been fabricated by usingtransition metal oxides (TMOs) in the intermediate connectors. Their workingmechanisms have been studied by characterizing thei carrier transporting process,interfacial electronic structures, and charges generation&separation process. Theelectronic structures and energy alignment on intermediate connectors are revealed byusing ultraviolet and X-ray photoemission spectroscopies (UPS and XPS).Capacitance-Voltage analysis also shows the dynamic process on charge generationand separation in the intermediate connectors. It is found that transition metal oxides(TMO) is crucial for the charge generation, while the adjacent electron and holetransporting layers are important for the extraction of electron and holes generated inTMOs.We also preliminarily studied the organic doping as an effective method to improve charge transporting property on holes only devices. We found that the sidechain groups in organic materials could adjust the doping effect due to themodification of hybridization energy, which could have higher charge carrier mobility.Finally, the light extraction technique was studied to improve the external quantumefficiency by reducing the total internal reflection at the glass-air interface. Microlensarrays with high fill factor have been fabricated by using Digital micro-mirror device(DMD) lithography technology&roll-to-roll mold transfer process. An increase of60%in light out-coupling efficiency with an optimized elliptical microlens array isachieved for organic light-emitting device (OLEDs) without affecting theelectroluminescent property. The theoretical simulation indicates that the increasedlight extraction is attributed to the fill factor of microlenses on the substrate surface.