Luminescent Polymer Films in Layered Devices : A Theoretical and Experimental Study on Light Emission from Polymer-based Devices

Sammanfattning: This thesis concerns optical modelling and characterisation of thin film layered structures, including luminescent conjugated polymers, with respect to the emission properties. Included in the study were three different types of devices: Organic light-emitting diodes, microcavities and half cavities.Important experimental methods for this study were ellipsometry for modelling, x-ray diffraction for ascribing changes in a polymer to crystal formation, and photo-induced absorption for information about the emission property in the form of stimulated emission. Lasers have been used for photoinduced absorption measurements and upon excitation of polymers in both microcavities and half cavities. Invaluable instruments have been absorption and emission spectrometers. Many other instruments have been used, of course, such as scanning force microscopy (SFM) for imaging surfaces and instruments for quantitative evaluation of organic light-emitting diodes, both electrically as well as optically.Light-emitting diodes, including single and double organic layers, have been analysed using optical modelling of the diode structure. In a double layer diode we found that the emitted light originated from the interface between the two adjacent organic films. The origin of the light was assigned an indirect optical transition between the molecule 2-(biphenylyl)-5-( 4-tert-butylphenyl)-1,2,4-oxadiazole and the polythiophene poly(3-methyl-4-octylthiophene). The transition was observed only under the influence of a strong electric field. The polythiophene was spin-coated onto an anode of indium tin oxide and the molecular layer was thermally evaporated on top to prevent holes from reaching the cathode unimpeded and, moreover, to transport the electron to the interface.The cathode consisted of calcium and aluminium.Optical modelling of a single-layer light-emitting diode, with apolythiophene between two electrodes of poly [3,4-ethylenedioxythiophene] (PEDOT) and aluminium revealed the position of the emission zone in the polymer film. The emission zone did not extend over the whole film and was, infact, rather limited in space. Moreover, the main emission was found to be generated further away from the anode than the cathode, within a narrow zone of little or no light at the cathode as a result of the quenching effect caused by the metal.Polymers often form an amorphous phase in contrast to molecules, which have a larger tendency to form crystals. One example of the opposite is the polythiopene poly[(3-dioctyl-4-phenyl)thiophene], which forms crystal phases. The substrate upon which the polymer film rested consisted of a transparent film on top of a thick aluminium mirror. This kind of device, referred to as a half cavity, improved the resolution vertically in the polymer film, changing the optical electric field and thus the absorption in the polymer film by changing the thickness of the transparent film. Using optical modelling of the device structure, simulating the emitted light generated upon photoexcitation and using x-ray diffraction, made it possible to follow a change in the polymer film and ascribe the change to crystallisation.Inserting a luminescent polymer between two highly reflective mirrors resulted in lasing upon photoexcitation by a short-pulse laser. Two different conjugated polymers, one emitting in the red and the other in the green region, were used in a so-called microcavity device. The large Stokes' shift exhibited by both polymers contributed to the low threshold for lasing. The backbone of the co-polymer consisted of two types of molecules, one of which was responsible for absorption, whereas the other took part in emission. The energy transfer between the two molecules resulted in a very large Stoke' shift (150 nm) and, thus a reduced self-absorption.