Conjugated Polymers: Relationship Between Morphology and Optical Properties

In this Chapter we will start by briefly summarising the basic concepts of the electronic structure of conjugated polymers. This will enable the discussion of the relevant descriptions of the dielectric function. We will relate these descriptions to the model parameterisations which are used in advanced ellipsometric analysis of thin films such as those used in devices for organic photovoltaics (OPVs) and light emitting diodes (OLEDs). Amongst other things, such parametric descriptions are useful to deal with structural changes in conjugated polymer thin films. Once the models are presented, we will provide representative examples of the nexus between morphology and optical constants, and how the latter can be employed to infer aspects of the former. First, we will discuss how chain conformation affects the optical properties. Then, we will explain the anisotropic behaviour of conjugated polymer films due to their intrinsic molecular anisotropy and review different cases (f. i., oriented films or semicrystalline polymers). We will also describe structural changes that occur upon blending polymers with fullerenes and concomitant variations of the optical properties. Here we will focus on state of the art low band gap polymers mixed with fullerenes. Finally, real-time ellipsometric experiments in which these structure-property relationships can be exploited will be presented.The authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness through grant MAT2015-70850-P and the Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496).Peer reviewe