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THESIS DR. TIM VANGERVEN - SYNOPSIS

"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.