Intensive studies of conjugated molecules and polymers are carried out all over the world with the intent of obtaining cheap and efficient organic electronic devices. The most mature application at the moment is the light-emitting diode, but also photovoltaic cells and different types of transistors shows promising results. Interest in these materials is based on possibilities of 'simple' and cheap processing techniques, comparing to inorganic compounds, in the manufacturing of devices. The understanding of the electronic and chemical structure of the surfaces and interfaces of these materials is a basic requirement for understanding the characteristics of the potential devices. Understanding the electronic structure of the pristine materials enables conclusions to be drawn concerning electrical and optical properties in these materials. The behaviour of the interface between metals and conjugated materials is one of the primary factors determining the suitability of using certain electrode/organic material combinations in device applications.
With this motivation, the electronic structure of both conjugated molecules and polymers surfaces and their interfaces to metals (and insulators) have been studied with mainly photoelectron spectroscopy (PES). In some cases complementary techniques have been needed and performed. This includes the four-point probe technique for determining surface resistance and atomic force morphology for determining surface morphology. As well as synchrotron-based techniques, such as near-edge X-ray absorption spectroscopy and resonant photoemission have been used. The main results compromised in this thesis are summarized below.
Poly(3,4-ethylenedioxythiophene)-polystyrenesulfonic acid (PEDOT-PSS) is an aqueous colloidal dispersion consisting of doped conjugated polymer PEDOT with counter ions provided by the PSS chains. PEDOT-PSS films have previously proven to have a grain-like structure in which the grains have a ~30 Å thick insulating PSS outermost layer. The conductivity of thin PEDOT-PSS films has been improved through adding some high-boiling-point solvents to the PEDOT-PSS blend. The major reason for this increase is a rearrangement of the morphology, in terms of an increase in the PEDOT-to-PSS ratio in the surface region (i.e. the insulating PPS layer is decreased for each grain).
The initial stages of interface of PEDOT-PSS with aluminum for contacting purposes has also been examined. Due to the many components in the PEDOT-PSS film its reactions with alurninum was difficult to deduce. Therefore the aluminum interfaces with model molecules of each of the components of PEDOT-PSS were investigated to discern this. Phenyl-capped EDOT-trimer was used as a model oligomer for neutral PEDOT. It has been shown that aluminum preferentially interacts and forms covalent bonds with C-S carbons that causes a rearrangement of the charge density within the oligomer and breaks then-conjugation. In PEDOT-PSS blends the PEDOT part is left intact and alurninurn preferentially reacts with the SO3-H+ and/or SO3- species of the PSS part.
A specific blend of conjugated materials used in photovoltaic cells is a one to four mixture of APFO-3 (a low band gap copolymer based on alternating fluorene and donor-acceptor-donor units) to PCBM (soluble C60 derivative). The electrode systems studied are the widely used Al and Al/LiF contacts. We demonstrate a thickness dependent effect of the LiF layer in the Al/LiF/organic structure. LiF has a protective effect for all thickness preventing formation at the Al/organic interface of Al-organic complexes that destroy the Π-conjugation. In addition to this, there are two other beneficial effects (depending on LiF thickness). Decomposition of LiF occurs for thin enough layers in which the LiF species are in contact with both the organic film and the A1 atoms. This results in Li-doping of the organic films and creates a low workfunction contact. For thicker (multi)layers, the dipole formed at the LiP/organic interface is retained as no decomposition of the LiF occurs upon Al deposition.
We have shown the occurrence of interfacial dipoles at C60/LiF/Al interfaces and confirmed interfacial dipoles at Alq3/Al, C60/Al and Alq3/LiF/Al interfaces through vacuum level shifts. There is strong interaction with the substrates in all cases. There is evidence of covalent interaction between both Alq3 and C60 films with the AI substrates. The added LiF layer (between AI substrate and the organic film) prevents the covalent bonds from forming and the LiF does not dissociate in any case, unlike what is found in literature for the reverse order of deposition. For both Alq3 and C60 there is charge transfer from the Al substrate to the organic film through the LiF layer. However, if the thickness of the LiF layer exceeds 25 Å this charge transfer is blocked. The evolution of the electronic structure upon n-doping of the first Alq3 monolayer observed here is different from previous studies of n-doping mer-Alq3, indicating that there is preferential deposition and/or formation of the unusual facial isomer of Alq3 on the LiF/Al substrate. Our results are the first reported photoemission spectra of this isomer and its n-doped state.
The electronic structure of two new low band gap polymers (APFO-3 and APFO-7) based on donor-acceptor-donor groups copolymerized with fluorine units has been characterized. The valence band of APFO-3 seems to be highly dispersed and derived from orbitals delocalized over the whole polymer chain, where as the conduction band is nearly flat as it is derived from orbitals localized on the acceptor units. The existence of a dispersed valence band would predict good hole transporting properties, where as a flat conduction band would be expected to produce poor electron transporting properties. The electronic structure of APFO-7 has similarities to APFO-7 but it is also less clear. The larger size of the acceptor unit seem to distort both the valence band and conduction band shape as compared to APFO-3, however, so further work is needed to understand the more complex APFO-7 system.
Linköping: Linköpings universitet , 2004. , 56 p.
Parker, Ian D.