Studies On Interfacial Electronic Structures Of Transition Metal Oxide-based Intermediate Connectors In Tandem Organic Light-emitting Diodes

Tandem organic light-emitting diodes (OLEDs) have recently drawn considerable attention because of their superior current efficiency, luminance and operational lifetime compared to a conventional OLED. In a tandem OLED, multiple electroluminescent (EL) units are electrically connected in series via intermediate connectors. Under an applied electric field, the intermediate connector should be capable of charge generation, and be required to facilitate effective hole and electron injection into the adjoining EL unit. Therefore, the favorable intermediate connector is of critical importance for designing high-performance tandem OLEDs. For example, tandem structure with a transition metal oxide (TMO)-based intermediate connector has been used to achieve high-efficiency OLEDs due to TMO's high work function, optical transparency and operational stability.In this thesis, the interfacial electronic structures and energy level alignments of TMO (WO₃ and MoO₃)-based intermediate connectors were studied via ultraviolet and X-ray photoemission spectroscopies (UPS and XPS), and their working mechanisms for charge carrier generation and injection process were also explored. In addition, the impact of constituent materials and structure on the performance of intermediate connectors were discussed. The details of research work are as following:(1) The interfacial electronic structures and energy level alignments of WO₃-based intermediate connectors, including (a)NPB/WO₃/Mg:Alq₃, (b)NPB/WO₃/Alq₃, (c)NPB/Mg:Alq₃ and (d)NPB/Alq₃ were studied via UPS and XPS. The derived energy level alignments were applied to explore the working mechanism of WO₃-based intermediate connectors and to explain the performance of these corresponding tandem OLEDs. Moreover, the function of the interlayer WO₃ and Mg:Alq₃ in the intermediate connector was analyzed. (2) The interfacial electronic structures and energy level alignments of MoO₃-based intermediate connectors instead of WO₃ were also studied via UPS and XPS. The related intermediate connectors were (a)NPB/MoO₃/Mg:Bphen/Bphen, (b)NPB/MoO₃/Bphen, (c)NPB/Mg:Bphen and (d)NPB/Bphen. It was found that MoO₃-based intermediate connectors had the same working mechanism with that of WO₃-based intermediate connectors. Next, to investigate the impact of electron transport material (ETM) on the functional effectiveness of TMO-based intermediate connector, Alq₃ was used as ETM instead of Bphen. The conclusion revealed that the NPB/TMO/Mg:ETL/ETL is a high-performance intermediate connector.(3) On the basis of Mg:ETL/TMO, the intermediate connectors with double-doped structure, MoO₃:NPB/Cs₂CO₃:Bphen and MoO₃:NPB/MoO₃/Cs₂CO₃:Bphen, were studied via UPS and XPS. According to their energy level alignments, the corresponding intermediate connectors can generate carrier charge and inject into the neighboring EL unit more effectively, compared to the Mg:ETL/TMO intermediate connector.(4) Inverted device and double-insulated devices with TMO intermediate were fabricated, and their IV and capacitance properties were characterized, respectively. On the basis of the experimental result, the thermal stimulation model for TMO-based intermediate connectors was proposed.Finally, we preliminarily explored the function of TMO in the middle electrode of tandem solar cell.