Optical characterization of organic materials for optoelectronic applications

Abstract

To deal with the rapidly growing global energy demand and deteriorating environment, people are in urgent need to develop sustainable green energy. One of the promising energy sources is solar energy. Polymer solar cells are of particular interest due to their relatively lower fabrication cost, mechanical flexibility, light weight and capabilities of vacuum-free fabrication process as compared to conventional solar cells. However, their low power conversion efficiencies are still the main obstacles for them to compete with conventional inorganic solar cells.

This thesis starts with the introduction of solar energy and the basic theory of organic solar cells. Related characterization methods will also be discussed. To facilitate systematic research as well as optimization of organic solar cells, computer simulation on solar cell performance is applied by scientist and engineers worldwide. Optical properties of the active layer in solar cell simulation are experimentally obtained, and their accuracies are critical to the validity of the simulation result. However, these optical properties are often characterized by simple absorption measurement which ignores the effect of internal reflections and interferences within the thin films. Therefore, Spectroscopic Ellipsometry (SE), which is for obtaining accurate optical properties of thin films, will be introduced in detail.

The performances of organic solar cells rely heavily on the nanoscale features and the optical properties of the active layers. These are very sensitive to fabrication conditions and source material properties. Based on a common type of polymer obtained from different batches and from different commercial suppliers, the influences of source material properties on photovoltaic performance are investigated in detail using various characterization methods. The investigation covers effects related to optical properties, cathode interface, nanomorphology and charge transport properties of the active layers. The active layers of the samples exhibiting good performance have more electron acceptors near the cathode interface, but this is not determined by the molecular weights of the source materials. Good performance samples are also showing small domain size in nanomorphology and small roughness, but the relation between nanomorphology and molecular weight of source materials is not clear. It is found that molecular weight as well as the synthesis and purification procedure can affect the resulting photovoltaic performance. Higher molecular weight can lead to higher power conversion efficiency, which is mainly attributed to the resulting higher mobilities.