New Terrylene-based Acceptors for Organic Solar Cells

Photovoltaics is one major pillar on the way to clean and renewable energy production. After reports on the first silicon based solar cells in the 1950ies the field has been steadily growing and many new types of solar cells have been developed. Lately, organic solar cells have attracted increased research interest due to their attractive features, such as strong and tunable light absorption, mechanical flexibility, reduced manufacturing costs and ecological advantages. In these cells, the light absorbing layer is made from a blend of two different organic materials, called donor and acceptor materials, which are based on organic molecules (or polymers). Regardless of the solar cell type, the central research goal - increased efficiency and reduced manufacturing costs, has remained unchanged since the earliest solar cells. In the current project this goal will be addressed in a systematic way by investigation of the structure-properties relationship of novel terrylene-based acceptor molecules. Terrylenes belong to the class of rylene dyes. Perylene derivatives, which are also member of this class, are already well established acceptor materials for organic solar cells. In comparison to perylenes, terrylenes have improved spectral properties, while being synthetically just as accessible, making them interesting for application in solar cells. However, due to their increased molecular size, terrylenes are more prone to a formation of intermolecular aggregates. Such aggregate formation decreases the solubility and, as a result, solar cell fabrication becomes difficult. Furthermore, aggregation of the acceptor molecules decreases the efficiency of electron transfer in a blend with donor materials, which has a direct, negative impact on the solar cells performance. In this project we will address this problem. That will be done by preparing an ensemble of terrylene derivatives. The optical and electronical properties of these compounds then will be characterized. Computational methods will be used to aid the interpretation of the results. Furthermore, blends with electron donors will be prepared and their structural properties characterized using such methods as X-ray scattering measurements and atomic force microscopy. These blends will also be used to build solar cells, whose efficiency then will be determined. From the data acquired the structure-properties relationship for terrylene derivatives will be elucidated. Understanding of this relationship will enable production of terrylene based acceptor materials for solar cells with improved efficiency. These new terrylene dyes and the results on the structure-properties relationship is also interesting to other fields such as material science (e.g. organic field-effect transistors) or analytical chemistry (e.g. new fluorescent dyes).

One day in the near future, we might be looking through the window of a building and not even realize that this window is producing electricity at that very moment. This vision could become reality with the help of organic solar cells. How? Because they are very thin, their color can be adjusted and the production can be done by roll-to-roll printing. These factors allow us to envision a thin, semitransparent organic solar cell which covers the entire surface of our window, converting sun light to electricity during day time, and recycling the indoor light at night. Organic solar cells are similar to a cake consisting of multiple layers - substrate, electrodes, inter- and light absorbing layers. Each of them consists of different chemicals. In case of the light absorbing layer, it is made from a mixture of two substances, called a donor and an acceptor. These names come from the fact that with the help of light (photons!) one of them gives its electrons to the other one - and a charge is generated. After a short travel through the before mentioned interlayers, the charge is extracted at the electrodes, which means that light gets converted to electricity. In this project, improved acceptors were developed. To that end, chemicals from the class called terrylene diimides were used. They are special due to their very strong light absorption, as well as photochemical stability. Multiple new terrylene diimides were prepared and used in the construction of organic solar cells, followed by the characterization of the solar cell efficiencies. A trend on how the structural changes in the terrylene diimide influence their performance in solar cells was discovered. These conclusions could be applied in the preparation of further, improved terrylene diimides. To that end, closely related chemicals consisting of two perylen monoimide molecules connected by a phenylene "bridge" were prepared. All together they also form a diimide structure. Delightfully, their performance in solar cells was even better than that of the terrylene diimides. Furthermore, computations were used to assist with the experimental work. Here, a computational method previously rarely used for the research of new acceptors for organic solar cells could be successfully applied. The results obtained in this study have provided new, unknown before acceptor molecules for organic solar cells. The results have been summarized in a scientific publication, as well as in multiple bachelor and master theses, performed at the Graz University of Technology. Furthermore, a comprehensive scientific literature review on various (over 700) acceptors was prepared and published. This review will be a valuable asset for researchers around the world in their work on new acceptor materials.