Conjugated Luminescent Molecular Materials : An Experimental and Theoretical Study of Electronic, Optical and Chemical Properties of some Conjugated Molecules, Polymers and Interfaces

Abstract: In recent years intensive studies of conjugated polymers and molecules have been carried out all over the world. The most promising use for these materials appears at the moment to be as the active luminescent component in flat panel displays. For these purposes, research on conjugated polymers some eight years ago when PPV was found display electroluminescence when sandwiched between two electrodes. Interest in these materials is based on the possibilities of "simple processing", comparing with comparable uses based upon inorganic compounds, in the manufacturing of devices. As a matter of fact, sometime during this year products, based on this technology, will appear on the market. Spurred by the success in making ordinary electroluminescent devices, considerable effort is now put into taking the process a little further and make electronically driven organic lasers.The understanding of the electronic and chemical structure in these materials is a basic requirement for understanding the physics and chemistry of the devices. With this motivation, the basic electronic structure of both conjugated molecules and polymers have been studied using a combination of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, spectroscopic ellipsometry, Raman spectroscopy, and quantum chemical calculations. In some instances, optical absorption and luminescence spectroscopy were performed.The behavior of the interface between metals and conjugated polymers is one of the primary factors determining the suitability of using certain electrode/conjugated polymer combinations in devices applications. For light emitting devices, a low work function metal is used as the electron injecting electrode, while optically-transparent ITO (indium-tin-oxide) is used as the hole injecting electrode. In this context, the early stages of interface formation between a ladder polymer for poly (P-phenylene) and two possible candidates for electrodes, potassium and rubidium, have been studied. Both metals are found two diffuse into the polymer, however, but on totally different time scales. Potassium diffuses readily into the polymer, doping the polymer n-type, leading to the formation of bipolarons, which appear in the form of two filled energy states in the middle of the otherwise-forbidden electron energy gap. On the other hand, rubidium diffuses slowly in to the polymer while causing the elimination of a hydrogen atom from one of the side groups. This results in the formation of one state in the otherwise forbidden band gap.The general environmental stability is also of interest in order to provide additional insight into the doping and degradation mechanisms of operating devices, especially in unprotected environments. In this context photo-oxidation of PPV exposed to UV light in air was studied. Singlet oxygen leads to the formation of chemical groups which interrupt the conjugation of the polymer, permanently modifying the electronic structure. In addition, the chemistry taking place at the hole-injecting electrode have been studied in samples prepared by argon plasma sputtering (deposition) of thin layers of ITO on top of poly(p-phenylenevinylene), or PPV. The reactions taking place at the interface were addressed using angle dependent X-ray photoelectron spectroscopy. It was found that the interface between ITO and PPV contained three types of altered carbon species, corresponding to carbon to oxygen single-bonds, carbon-oxygen double bonds and carboxyl groups. Thus the interface appears to be approximately an oxidized layer of PPV, which may lead to adverse effects on the charge injection.A stepping stone to the realization of electronically excited lasers based on organic materials is to find polymers or molecules that are suitable candidates. In this context, the stimulated emission characteristics of a new class of molecules, the so called spiro-type molecules have been investigated by means of photo-pumping with a nitrogen laser at various excitation intensities. These molecules fulfill at least some of the basic requirements for such organic molecular lasing materials. First of all, all of the investigated molecules have the ability of forming stable amorphous films which means that one has control over, for the photoluminescence detrimental, intermolecular interactions. Secondly, the quantum efficiency is quite high. Third, and not at all least, some of the molecules investigated show a low on-set for spectral narrowing due to stimulated emission. Moreover, since the glass transition temperature is around 200 °C, thin films of these molecules retain their amorphous nature even at elevated temperatures, which is important, since the lasing efficiency decreases dramatically in crystalline films due to nearest neighbor quenching effects. In electronically excited devices, where current densities on the order of hundreds A/cm are expected to be necessary in order to obtain lasing, crystallization would apparently be a detrimental effect.Finally, steps have been taken towards the development of a method for detection and study of relatively long-lived transient species in a molecular chargetransfer system consisting of naphthalene on top of tetranitromethane. It is shown, that by doing time-dependent XPS "by hand" it is possible to detect a transient species corresponding, most likely, to the trinitromethylnaphthelenyl radical. Further efforts to study transient species are under way.