Chemical and electronic structure of interfaces in organic-based devices

Sammanfattning: This thesis deals with some of the main physical and chemical phenomena involving organic semiconductors in (opto )electronic devices: (i) the fabrication and characteristics of organic light emitting diodes (OLEDs), (ii) the chemistry occurring at metal-polymer and polymer-metal interfaces, (iii) the nature of electronic levels induced upon doping in molecular organic semiconductors.In the first paper, fluorine tin oxide (FTO) electrodes have been studied as an alternative to indium tin oxide (ITO) for hole-injection in polymer-based light emitting devices. There appear to be several advantages in using FTO instead of ITO electrodes: (i) FTO is less expensive, (ii) there is a lower sensitivity to surface cleaning methods, and, most importantly, (iii) more light is obtained for a given voltage. This study points out that the work-function of the hole injecting electrode is not the only factor in determining hole injecting characteristics at the corresponding electrode-interface.In Paper II-IV, we show that chemical reactions occur at the metal-polymer and polymer-metal interfaces, which is expected to affect the device performance.In Paper II, chemical reactions at the interface between a precursor polymer for poly(p-phenylenevinylene) and ITO are identified using X-ray photoelectron spectroscopy (XPS). The HCl eliminated in the conversion process reacts with the surface of the ITO substrate leading to the formation of indium chloride, which then diffuses into the polymer.XPS is used to study chemical reactions at a polymer ITO interface under 100 A polymer (Paper III). This has been possible by partially replacing indium ions by gallium ions. The immediate formation of GaCl3 at the ITO/precursor- PPV interface can be seen.In OLEDs, the cathode is usually deposited on the conjugated polyme layer. In Paper IV we show that the detrimental effects arising from the physical vapor deposition of metal atoms on conjugated polymer surfaces can be controlled by using a ultra-thin protection layer of barium.For some organic semiconductor applications, as for inorganic devices, the charge mobility within the active material must be enhanced in order to achieve better devices efficiency. This can be done typically by doping the semiconductors. Paper V deals with doping of molecular organic semiconductors, composed of oligophenyls involved in propeller-shaped spiro-type molecules. The nature of electronic levels induced upon chemical doping is investigated by a joint theoretical and experimental study using UV-vis absorption spectroscopy and photoelectron spectroscopy. With lithium atoms as the dopant, bipolarons are formed, while sodium atoms give rise to polarons independent of the level of doping.