OLED with Carbon Nanotube as Hole Injection Layer

Organic light emitting diodes (OLEDs) have developed rapidly in recent years and shown promise as the basis for full colour, flat panel displays. However, for wide scale commercial adoption, some of the properties of OLEDs remain to be improved in order to achieve low operating voltage, high efficiency and long operating lifetime. The operating voltage and luminance efficiency of OLEDs strongly depend on the efficiency of charge carrier injection, the electron-hole balance and the carrier mobility in the organic materials. In order to achieve the lowest possible operating voltage, it is necessary to have ohmic interfaces at the electrode contacts, such that the contacts can deliver the maximum possible current density.

In this project, the aim is to study the surface interaction of solution processable carbon nanotube (e.g. o-SWCNT and o-MWCNT) films with electrode (e.g. ITO) and organic materials. The work function, energy level alignment and the interfacial electronic structure in between ITO/CNT and CNT/organic materials were studied by ultraviolet photoemission spectroscopy (UPS). The charge injection property and the injection mechanism of the CNT films were studied by the I-V characteristic of the unipolar device and compared against the different injection model in semiconductors. The effect of inserting CNT based hole injection layer were studied by comparing the OLED device performances with and without the layer.

The result from this project have shown that the thin film acid oxidised carbon nanotubes can be utilized as effective hole-injection layers in bi-layer OLEDs, reducing the drive voltage and increasing the maximum brightness as compared to standard OLEDs. The methodology proposed could open up a new era for high brightness efficient OLED production with the addition of the nano-structured ITO surface layer composed of CNT. From current-voltage analysis of hole only diodes we have shown that o-SWCNT forms an ohmic contact with TPD, whilst the contact with o-MWCNT remains injection limited. The proposed nano-structured surface layers also have potential for enhanced exciton separation and transport in photovoltaic devices.