Bausi, F; (2016) Innovative solutions and applications for polymer light-emitting diodes. Doctoral thesis , UCL (University College London).

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Abstract

This work focuses on the development of new technical solutions for polymer light-emitting diodes (PLEDs). Treatments were developed to use innovative materials, such as graphene and carbon nanotubes, as hole-transporting layers and surface modifiers on top of ITO. A processing treatment for the preparation of the polymeric emissive layer of PLEDs was also investigated which allowed the improvement of the on/off switching speed of the devices thus enabling their employment in novel applications for visible light communications. Graphene-based transparent thin films obtained via the exfoliation of graphite in the liquid phase were produced with a view of using them as surface modifiers of indium tin oxide (ITO) or as transparent electrodes in organic optoelectronic devices. As-deposited films are relatively resistive, but the sheet resistance was decreased by up to three orders of magnitude by thermal treatment down to values of 10E5 Ohm/square . The films were also chemically doped via the physisorption of the electron-withdrawing molecule (CF3SO2)2NH. This resulted in an increase of the work function by up to 0.5 eV , to yield a value of 5.3 eV comparable to what can be achieved with poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). Thin films of liquid-phase exfoliated graphene and sorted single-walled carbon nanotubes (SWNTs) were deposited on thin ITO layers ( 100 nm). The work function values of these films measured in air using the Kelvin probe were compared with the work function values measured via electroabsorption when the film is inside a device and in contact with the emissive polymer of the device’s active layer. The data shows that the latter appear to be 0.1/0.2 eV higher than those measured in air, whereas this difference is not present for ITO alone. This suggests a charge transfer at the interface of graphene films with the polymer layer. Finally, the thermal processing of the active layer of PLEDs was reported to increase the on/off speed of encapsulated devices by more than double to reach a cut-off frequency of approximately 260 kHz. The underlying mechanism was investigated. Unexpectedly, the increased speed of the device corresponds to the formation of crystalline domains that decrease the thin-film transistor mobility. The data suggest suggests an increase in the charge injection inside the active layer via a trap-assisted injection mechanism. Thanks to such optimisation processing a maximum data rate up to 55 Mbit/s can be envisaged by employing wavelength multiplexing with a high-performance artificial neural network (ANN) equalizer [1].

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