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New molecules for cheap solar power

Researchers fabricate highly efficient organic solar cells with novel easy to synthesize and highly absorbing acceptor molecules

Jülich, 21.11.2016 – An international team of scientists from Imperial College London, King Abdullah University of Science and Technology (Saudi Arabia), Stanford University, Friedrich-Alexander Universität Erlangen-Nürnberg, Hongkong University of Science and Technology and Forschungszentrum Jülich have developed organic solar cells with increased photocurrent and open-circuit voltage. These solar cells use novel acceptor molecules that are not based on the traditional fullerene molecules that have been prevalent for the last 20 years in organic photovoltaic research. The new acceptor molecules have a range of advantages relative to fullerenes: They absorb light more strongly are easier to synthesize, their energy levels can be fine-tuned easily and their stability is strongly enhanced relative to fullerene based solar cells. The research on non-fullerene acceptors was recently published in the Journals Nature Materials (DOI: 10.1038/NMAT4797) and Energy and Environmental Science (DOI: 10.1039/c6ee02598f).

The main target in photovoltaics materials research is to improve efficiency, stability and cost of the solar cells. One approach to reduce is cost is to use printable materials such as organic molecules. While this technology has the potential for low cost, the stability and efficiency is still lower as compared to photovoltaics based on inorganic materials, where higher temperatures are used to create the functional layers of the solar cell. Organic solar cells could be made extremely thin (~ 100 nm) due to the high absorption of the used molecules. However, in order to extract electrons from the organic absorber layer, fullerenes had to be mixed in. Fullerenes are spherical or ellipsoidal molecules consisting of 60 or 70 carbon atoms arranged like the outside of a soccer ball. While these molecules conduct electrical current reasonably well, they hardly absorb visible light. Therefore, efforts have been devoted to synthesizing alternative molecules that have the advantages of fullerenes in terms of extracting charge carriers but overcome the disadvantages such as low absorption, moderate stability, difficulty to fine-tune the energy levels and a fairly energy intensive synthesis compared with other molecules.

Dr. Derya Baran, postdoctoral research fellow in Jülich, is the lead author of two new papers showing how novel non-fullerene acceptors can improve efficiency, stability and cost of organic solar cells at the same time. The first paper in Nature Materials presents the implementation of ternary blend solar cells, where one polymer is mixed with two different non-fullerene acceptors. Interestingly, the more complex ternary blends outperform binary blends with either of the two acceptors. In addition, the authors show that the non-fullerene acceptors allow the use of highly stable and easy-to-synthesize polymers like polythiophene (P3HT) in solar cells with efficiencies of 7.7 %, which is much better than the values that can be reached with fullerene based acceptors. When blending the non-fullerene acceptors with more complicated polymers, efficiencies of 11% were reached.

The second paper in Energy and Environmental Science focusses on the possibility of a substantially enhanced open-circuit voltage in organic solar cells with non-fullerene acceptors. One of the key parameters in solar cells is the amount of energy per extracted charge carrier that has to be as high as possible. As a figure of merit for the extracted energy, typically the voltages at open circuit are compared with the minimum photon energy that leads to absorption (the band gap). In fullerene based solar cells, the difference between the energy per extracted charge and the energy of the incoming photons is relatively high as compared to other photovoltaic technologies. In the new devices with non-fullerene acceptors, this energy loss is reduced substantially which is a promising result for further development of organic solar cells.

Funded by the Helmholtz-Association

Dr. Barans Research in Jülich and London was funded by the international postdoc fellowship program of the Helmholtz-Association of German Research Centres. The fellowship program allows young scientist with excellent results during their PhD to pursue their research for three years to establish a reputation in their respective field and to extend their scientific competences.

Fullerenefreie Solarzelle

Comparison between the structure of fullerene containing and fullerene free organic solar cells. Example of the current voltage curves published in DOI: 10.1039/c6ee02598f .

Original Publications:

"Reducing the efficiency–stability–cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells" by Derya Baran et al., Nature Materials
DOI: 10.1038/NMAT4797

"Reduced voltage losses yield 10% efficient fullerene free organic solar cells with >1 V open circuit voltages" by Derya Baran et al., Energy & Environmental Science
DOI: 10.1039/c6ee02598f
Further information
Institut für Energie- und Klimaforschung, Photovoltaik (IEK-5)

Organische und Hybridsolarzellen

Presseinformation zum internationalen Postdoktorandenprogramm der Helmholtz-Gemeinschaft vom 21. November 2014
Scientific Contacts:

Dr. Derya Baran
Institut für Energie- und Klimaforschung, Photovoltaik (IEK-5)
Telefon: 02461 61-96500

Prof. Thomas Kirchartz
Institut für Energie- und Klimaforschung, Photovoltaik (IEK-5)
Telefon: 02461 61-96500

Press Contact:

Dr. Regine Panknin
Telefon: 02461 61-9054