Abstract
This thesis describes research on the effects of using carbon nanotubes (CNTs) and fluoropolymer based interlayers on the performance of organic devices. Photoelectron spectroscopy showed that the insertion of acid oxidised CNTs can improve the work function of indium-tin-oxide transparent electrodes. No significant changes of the HOMO level of triphenyldiamine (TPD) indicated that the equilibrium condition was met at the interfaces. Device I-V characteristic indicated that high density of trap levels had been introduced throughout the triphenyldiamine layer. This hypothesis is supported by Atomic Force Microscopy (AFM) and by high contrast current mapping using conductive AFM measurements. The broadening and featureless spectrum for the modified triphenyldiamine provides further confirmation. The high density of traps in TPD results in a high current density with lower luminance in the organic) light-emitting diode (OLED) device due to the imbalance of charge injection from positive and negative electrodes. The insertion of CNTs into polymer LEDs shows that the direct contact between the nanotube with the electroluminescencing polymer could induce significant quenching effect of the photoluminescence at the interface. However, the insertion of a hole-transport interlayer which also acts as an electron-blocking layer between the CNT and the polymer can reduce the quenching effect by shifting the recombination zone away from the anode/CNT interface. The effects of insertion of a polytetrafluoroethylene (PTFE) interlayer deposited via thermal evaporation in OLED and organic photovoltaic (OPV) devices are studied. The PTFE interlayer can improve the work function of PEDOT:PSS and reducing the electrochemical process in-between the PEDOT:PSS/polyfluorene. A remarkable improvement on the device lifetime and colour purity are found by incorporating PTFE to prevent the de-doping of the PEDOT:PSS. In the OPV devices, the PTFE and ultraviolet (UV) treated PTFE buffer layers at the anode/organic interface can improve the short-circuit current, open-circuit voltage and power conversion efficiency due to the reduction of the hole extraction barrier.