Polymers for Application in Photovoltaics

Polymer-Metal Interfaces

Within recent years a new application for metalized polymer films has emerged in the fast developing field of organic electronics. Here, a thin metal film is used to apply electrical contact to the active layer of the electronic device. Numerous types of organic transistors (OFET), organic light emitting diodes (OLED) and organic solar cells have already been realized, and although all-organic devices are also an object of intensive research, the use of a thin metal film as electrode is still state-of-the-art.
In the typical sandwich type structures the polymer based photoactive film is in contact with metal or semiconductor electrodes. Evidently, controlling the relative positions of electronic energy levels in the polymer materials with respect to the electrodes is crucial for the creation of operational devices. Energy levels, and especially those of the valence electrons, in thin films of polymers on metal substrates, can be measured using ultraviolet photoelectron spectroscopy (UPS). Using this technique a picture of the density of states of the uppermost orbitals is obtained from which the position of the HOMO can be defined with respect to the Fermi level of the substrate metal. The optical band gap can be measured from the absorption spectrum as the onset of the transition between the HOMO and the LUMO. Other methods such as cyclic voltammetry can also be used to measure the energy difference between the HOMO and LUMO. Taken together, it is possible to construct a rough sketch of the uppermost part of the electronic structure.

Regardless of the polymer and metal used, an important concern in metal coatings are the interactions occurring at the metal-polymer interface, since characteristics like film adhesion and electrical contact properties are strongly influenced by the interface structure. Related to this is the question for the growth kinetics of the metal film on the polymer surface and how the polymer influences the metal film morphology in the initial growth stage. Due to their non-wetting behavior, vapor-deposited metals grow on most oxide and organic surfaces in the form of 3D islands rather than in a monolayer-by-monolayer mode. For several combinations of oxide surfaces and metals also a pseudo layer-by-layer growth model has been inferred, explaining the deviation from pure 3D growth by kinetic limitations. A number of parameters are involved in the growth process, such as electronic structures and surface free energies of the two materials, surface morphology, polymer chain mobility and experimental deposition conditions, determining size, shape, atomic structure of the metal clusters and the respective temporal correlations.

In turn, film morphology influences the characteristics of the interface and consequently the response of the whole system. In the initial stage of film growth, metal atoms can diffuse easily into the polymer and form embedded clusters, presupposed the deposition temperature is close to or above the glass transition temperature of the polymer. Interdiffusion at the interface is in favor to a high adhesion strength, on the contrary penetrating metal atoms may alter the electronic properties at the interface and therefore make it difficult to control the properties of an electronic device.

More complex is the behavior in the region around the edge of the gold contact (see figure 3). Using an x-ray beam with 500 nm width we are able to perform a local scattering experiment and obtain locally averaged structural information. Thus laterally heterogeneous samples, which mark an important class in the area of nanotechnology, can be probed. We use nGISAXS to investigate the local gold contact morphology on a photoactive polymer film. In applications, routinely such contacts are used as electrode of the corresponding device. The region around the edge of the gold contact turned out to be more complicated as compared to a simple geometric edge.