My research has focused on the effects of reflective surfaces such as metals and dielectric stacks on the light emission from conjugated polymers. Conjugated polymers have semiconducting properties which arise from the delocalisation of electrons along the polymer backbone. These plastics are efficient light-emitters both in photoluminscence (PL) and Electroluminescence (EL). One of the unique features of these materials is their broad emission spectrum. Proximity to reflective surfaces alters the radiative rate of an emitter because of interference effects. This modification to the radiative rate is a function of the dielectric properties of the mirror, the orientation of the dipole, the distance between the dipole and the mirror, the emission angle, and the emission wavelength. In addition, non-radiative energy transfer from the emitter to excitations in a metal film efficiently quench luminescence at separations less than 60 nm. I have investigated the effects of interference and non-radiative energy transfer on the emission spectra and the PL and EL quantum efficiencies of conjugated polymer structures. The PL from thin films of conjugated polymers spin-coated onto Al and Au films, with and without SiO<SUB>2</SUB> spacer layers in between, was investigated. PL was found to be quenched close to the metal films. Experiments on double-layer LEDs where the distance between the emissive region can be controlled by altering the film thicknesses of the polymer layers extended those results from PL to EL. In addition, changes in the PL spectra and PL efficiency with increasing polymer-metal separation could be explained by computer-modelling the radiative power of oscillating dipoles. The results allowed design rules to be formulated which could improve the efficiency of LEDs, photovoltaic cells and microcavity devices in which the polymer film is sandwiched between two mirrors. Comparison between experiment and modelling revealed some information about the photophysics of conjugated polymers. Microcavities (Fabry-Perot resonators containing a light-emitter) modify the radiative rate of an emitter more strongly than single mirror devices. I have investigated the role of the dielectric properties of metals on the resonance wavelengths of a microcavity. It is shown that the metal film thickness influences the cavity resonances and that the angular dependence of the microcavity emission can be reduced exploiting specific properties of the metal.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:596510 |
| Date | January 1997 |
| Creators | Becker, H. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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