A boost for organic solar cells: New material guidelines promise higher efficiencies

Koen Vandewal Bernhard Siegmund Yuming Wang 2 Koen Vandewal Bernhard Siegmund Yuming Wang 2

A new large-scale study of ten institutes in Europe and China, including researchers from imo-imomec and EnergyVille, sheds light on the operating principle and optimization of organic solar cells, based on multiple polymeric and molecular light-absorbing materials. “This study provides chemists and physicists solid guidelines for the next-generation of photovoltaic materials. It’s a relevant contribution to exploit the full performance potential of organic solar cells in a much shorter time” says Dr. Yuming Wang, Dr. Bernhard Siegmund and Prof. Dr. Koen Vandewal. The study has been published in the renowned journal ‘Nature Energy’.

Building-integrated, efficient photovoltaics

Organic solar cells are an emerging generation of thin-film photovoltaics, standing out through their low weight, mechanical flexibility, ultralow CO2-footprint and short energy payback times. In contrast to conventional photovoltaics, they can be seamlessly embedded onto building facades on a large scale, without the need for mechanical reinforcement structures. Also, shaped surfaces, such as the tower of wind turbines or the exterior wall of biogas plants, can contribute in this way to the transition towards renewable energies. In addition, they are based on raw materials which can be supplied within Europe and can be selected to be environmentally friendly, making them also attractive for indoor photovoltaics. Finally, this solar cell class can also be tailored to be visually transparent for implementations into windows.

Given the large commercial potential of organic solar cells, European companies have announced or started mass-production with a yearly volume of 1 million square meters, including Armor in France (company), Heliatek in Germany (company), and Flex2Energy/COPE-Nano in Greece (European projects). Parallel to the commercialization, researchers worldwide continue to reduce the lab-scale performance gap to conventional photovoltaics. Hereby, a crucial strategy to even higher efficiencies are ternary based solar cells. Their advantage: By making use of three instead of two absorber materials, sunlight can be absorbed and converted even more efficiently. Up to now, the much-increased complexity in device physics poses a problem for the scientists, preventing a steep development curve so far. This new study, led by Linköping University (Prof. Dr. Feng Gao, Sweden), now gives a new boost to this research.

Insightful study will boost solar cell performance

While recent progress in ternary devices has been mostly driven by chemists, Wang et al. saw the need for developing a comprehensive picture on their device physics. “When starting the study back in 2018 in Linköping, we gradually realized that inserting a third absorber brings a number of new complex effects to light which haven’t been considered yet”, remembers Dr. Yuming Wang who joined UHasselt in 2021. Thanks to numerous spectroscopic experiments and quantum chemistry simulations on a larger set of solution-processed material systems, the researchers were able to predict the behaviour of the open-circuit voltage of the solar cells upon adding the third absorber. Dr. Bernhard Siegmund says “We surprisingly found that in some cases, the presence of the third absorber affects the morphology and thus the luminescent efficiencies of other absorber species”. This was not so in other cases, where the photo-voltage is hard to improve upon adding the third component.

Thanks to these fundamental insights, the European and Chinese researchers could conclude specific design rules for future absorber systems, taking into account the miscibility of the components, their emission yields and energetic structure at the interfaces between the absorbers. Prof. Dr. Koen Vandewal puts into a bigger picture: “Despite substantial progress in understanding and improving the photo-voltage of organic solar cells, it keeps on being the biggest leverage in catching up with the efficiency of inorganic solar cells. That’s why this voltage-centred study is so meaningful.”

The complete study can be found here.

Involved researchers: Prof. Dr. Koen Vandewal, Dr. Yuming Wang and Dr. Bernhard Siegmund of the OOE research group.

This project received funding from the European Research Council and the Research Foundation – Flanders (FWO).