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Ellipsometry and polymer semiconductors for OLEDs and OPVs: From delocalisation of excitons to molecular dynamics

Nanoscience Seminar by M. Campoy-Quiles, Imperial College London.

Abstract
Conjugated polymers are currently attracting widespread attention due to their potential as cost-effective and easy-to-process materials for large area applications such as displays, thin film electronics and organic photovoltaic devices (OPVs). In this talk, we will show how ellipsometry can be applied to retrieve different types of information for thin organic polymer films. First, we will demonstrate how a physically meaningful characterization of the dielectric function of these materials can be made, based on the nature of their electronic excitations. The analysis of ellipsometry data suggests that the optical constants of materials with highly localized excitations can be properly described using zero-dimensional critical point (exciton) lineshapes. These materials include common saturated polymers such as polymethylmethacrylate and polyimide, as well as small molecules, such as C60 and pyrromethene. However, the dielectric function of conjugated polymer thin films needs to take into account the semi-delocalized nature of excitons in these materials. We will show how a combination of zero- and one-dimensional critical points can be used for this purpose. We will present a series of examples for polyfluorene copolymers, commonly used in organic light-emitting diodes (OLEDs), gain media, and organic lasers, as well as polythiophenes, which are used in organic field effect transistors and solar cells. Different morphologies will also be discussed in connection with their optical dielectric function. In addition, we will show how real time in-situ ellipsometry can be employed to study morphological changes in thin polymeric films. We will discuss the determination of the glass and crystalline transition temperatures in conjugated polymer thin films, and present their film thickness dependence. Other effects such as heat induced morphology stabilization and thermal memory will also be introduced. Finally, we will demonstrate the use of in-situ ellipsometry for monitoring molecular self assembly and diffusion in polymer/fullerene blends for organic solar cells.