"On the role of molecular weight and homocoupling defects in organic solar cells"
This research is situated in the field of organic photovoltaics and the general goal is to elucidate batch-to-batch variation originating from “intrinsic” material quality parameters , more specifically molecular weight distribution and the presence of homo coupled side products. This is of high importance for a better understanding of the photovoltaic performance of a given material system and especially toward robust large are a processing.
Organic solar cells have surpassed the 10% efficiency barrier, which is surely promising. However, to fully exploit the benefits associated with this technology (i.e. high production speeds with low thermal budget, aesthetic options, flexibility, power-to-mass ratio), three main challenges need to be tackled: the efficiency, lifetime, and production methods need to be improved. Although device efficiencies > 10% are becoming more common, it remains difficult to achieve consistent results in labs and production facilities. Therefore , it is important to understand the different factors which cause reproducibility issues. By reducing or eliminating batch-to-batch variations, you do not only contribute to more robust production methods, but the efficiency and lifetime can also be improved, so a manifold of problems are tackled at the same time.
The key component in an organic solar cell is the photoactive layer , comprised out of a blend of conjugated molecules or polymers . The intrinsic quality of these materials is the starting point for reproducibility issues. When buying batches of the same conjugated polymer from commercial sources , large variations c an be observed , although the material carries the same chemical name and is being sold at fairly high research prices (typically €1500 gram-1). When one understands the problems and knows what is going on, strategies can be designed to systematically solve these issues .
The main goal of this work is to indentify the related critical material parameters and elucidate how they influence the device performance and morphology via advanced electro-optical techniques for different polymer and small molecule systems. This resulted in several publications in peer-reviewed journals. It will be shown that the molecular weight (distribution) of a polymer and homocoupled side product s play a crucial role towards device performance and strongly impact the active layer morphology.
This thesis consists out of concise introductory chapters (Chapters 1 to 4) followed by a collection of papers with a more detailed description of the research methodology and results. Chapter 1 gives a background to the field of organic photovoltaics. Chapter 2 is included to provide the relation and motivation for writing the papers located in the appendix. Chapter 3 contains the general conclusions and answers to the research questions from Chapter 1. Chapter 4 provides an outlook for future experiments regarding batch-to-batch variation.