Application of transition metal oxides in tandem organic optoelectronics: Energetics and device physics

Owing to the ease of processing and potential for inexpensive fabrication on low-cost substrates, organic optoelectronics have generated intense research efforts towards the realization of efficient organic light-emitting diodes (OLEDs) for flat-panel displays and solid-state lighting, organic photovoltaic cells (OPVs) for low-cost solar energy generation, thin-film field-effect transistors and photodetectors for large-area detector arrays. For all of these organic devices, optimization of charge injection/extraction and carrier transport is critically important towards their technological success. Efficient injection or extraction requires low energetic barriers, while competent transport demands highly conductive transport layers. During the last few years, transition metal oxides (TMOs) such as molybdenum tri-oxide (MoO3), vanadium pent-oxide (V2O5) or tungstentri-oxide (WO3) have been extensively used for improved charge injection and extraction in organic optoelectronic devices, due to their advantages of excellent electric properties, high optical transparency, and good environment stability. Generally, TMOs can realize efficient p-type doping of organic hole-transport layers, charge generation layers for tandem OLEDs, charge recombination layers for tandem OPVs because of their unique electronic properties. It is now recognized that a more complete knowledge on electronic structures of TMOs, especially the energy level alignments in TMO-organic interfaces is of pronounced importance for the resulting interpretation of their role as functional constituents in organic electronics. In this chapter, recent advances in understanding the interfacial energetics, chemical properties, and electrical behavior of TMO-based intermediate connectors used in tandem OLEDs and OPVs are reviewed, which are the typical examples of TMO applications in organic optoelectronics. Current understanding of the operating mechanisms that control and limit the charge generation or recombination process is presented. The specific properties of the resulting materials and their role as functional layers in organic devices are addressed. Recent efforts towards interface engineering of TMO/organic interfaces are also discussed. The physical insights provide guidance for identification of new materials and device architectures for high-performance devices.