Halogenated OPV Materials - A Blessing and a Curse?

Solar energy conversion via photovoltaics is one of the main clean and renewable options for the replacement of fossil fuels as energy source and thereby very important for tackling the climate change. Organic solar cells are emerging as a highly promising photovoltaic technology, which can be fabricated by coating and printing techniques in a resource- and cost-efficient way combined with scalability and a low carbon footprint. In addition, organic solar cells possess unique properties, such as flexibility, light weight, or semitransparency, which make them attractive for a variety of applications, where conventional crystalline silicon or thin film solar cells cannot be used. Moreover, in the last years the power conversion efficiency of organic solar cells experienced a remarkable increase to values above 19%, which was mainly driven by new absorber materials. In the development of these improved organic absorbers, the integration of halogen atoms into their chemical structure played a key role. However, the stability of these halogenated materials in the solar cells is still hardly investigated and there are indications that fluorine is accumulated at the interfaces to interlayers and electrodes, probably due to a partial dehalogenation. Currently, there is a lack of knowledge about this phenomenon in organic photovoltaics and its implications on the solar cell performance and stability. Thus, in order to explore the full potential of organic solar cells, there is still a number of scientific challenges regarding material properties and their integration in device architectures to be overcome. Using an interdisciplinary approach, this research project will cover aspects of materials chemistry, device physics, advanced analytical electron microscopy and X-ray spectroscopic techniques in order to analyze (i) the halogen accumulation itself, involved materials, reactions and processes as well as (ii) the impact on the absorber materials, interfaces and solar cell properties. These investigations will be the basis for a comprehensive understanding of possible reactions of halogenated absorber materials in various conditions, which are experienced by a solar cell under operation, and will allow to elaborate mitigation strategies and design principles for next generation organic absorber materials. The results of this research could also lead to the selection of novel interlayer materials for organic solar cells and thereby to a further enhancement of their power conversion efficiency and long-term stability.