Subsequent projects
Prof. Dr. Christoph Brabec
Friedrich-Alexander-University of
Erlangen-Nuremberg
Lehrstuhl für Werkstoffwissenschaften (Werkst. d. Elektronik u. Energietechnik)
Prof. Dr. Mike McGehee
Stanford University
Department of Materials Science & Engineering
Understanding degradation mechanisms in highly efficient solution processable photovoltaic devices: polymer and perovskite solar cells
Organic solar cells represent a next generation photovoltaic technology that enables flexible, light weight and low cost power generating devices. Collaboration between Stanford and Erlangen revealed the critical influence of the donor polymer morphology on the stability of organic solar cells. Investigations on the role of acceptor materials, such as fullerenes and also solution processable small molecules are the next steps in this project that studies the relations between microstructural changes during device operation and their influence on device performance. With a strong background in reliability of solution processable photovoltaics, studies on novel hybrid organic-inorganic perovskite materials for photovoltaic application are expected to result in important contributions to this emerging field. Perovskite solar cells have recently drawn enormous attention in the scientific community, but substantial improvements of device stability will be critical for a success of this technology.
Primary project: Understanding degradation mechanisms in highly efficient organic solar cells
Final Report
After a critical influence of the morphology of the donor material on the stability of organic solar cells was observed in the first funding period, a deeper understanding of these relationships was achieved in the follow-up project. In addition, the morphological characterizations at the Stanford Synchrotron Radiation Lightsource were extended to further investigate the influence of the morphology of the acceptor material and the oxygen stability of the materials.
By precise investigation of the charge carrier densities in fresh and aged solar cells it could be shown that the increased stability of solar cells with crystalline donor materials is due to an increased charge carrier density [1]. The charge carrier density is so large that electronic impurities occurring during aging do not influence the open-circuit voltage. In amorphous materials, however, a clear loss of open-circuit voltage due to additional impurities can be observed and also theoretically described. The gained expertise also led to cooperation with other groups at the partner university [2].
However, systems with crystalline donor materials showed a tendency to fullerene dimerization, which is another important degradation mechanism [3]. Due to the high segregation of donor and acceptor, dimerization can occur in pure fullerene domains, which can also be reduced by increasing the crystallinity of the fullerene regions. Alternatively, in systems with more amorphous donor materials, good intermixing of donor and acceptor prevents fullerene dimerization.
Finally, it has been shown that an increased crystallinity of the active layer not only provides increased stability against intrinsic photochemical reactions, but also against photo-oxidation [4]. An application of the know-how from the collaboration to perovskite solar cells has also been successful [5].
[1] T. Heumueller, T. M. Burke, W. R. Mateker, I. T. Sachs-Quintana, K. Vandewal, C. J. Brabec, and M. D. McGehee, “Disorder-Induced Open-Circuit Voltage Losses in Organic Solar Cells During Photoinduced Burn-In,” Adv. Energy Mater., vol. 5, no. 14, Jul. 2015.
[2] Z. Shang, T. Heumueller, R. Prasanna, G. F. Burkhard, B. D. Naab, Z. Bao, M. D. McGehee, and A. Salleo, “Trade-Off between Trap Filling, Trap Creation, and Charge Recombination Results in Performance Increase at Ultralow Doping Levels in Bulk Heterojunction Solar Cells,” Adv. Energy Mater., vol. 6, no. 24, p. 1601149, Dec. 2016.
[3] T. Heumueller, W. R. Mateker, A. Distler, U. F. Fritze, R. Cheacharoen, W. H. Nguyen, M. Biele, M. Salvador, M. von Delius, H. Egelhaaf, M. D. McGehee, and C. J. Brabec, “Morphological and electrical control of fullerene dimerization determines organic photovoltaic stability,” Energy Environ. Sci., vol. 9, no. 1, pp. 247–256, 2016.
[4] W. R. Mateker, T. Heumueller, R. Cheacharoen, I. T. Sachs-Quintana, and M. D. McGehee, “Molecular Packing and Arrangement Govern the Photo-Oxidative Stability of Organic Photovoltaic Materials,” Chem. Mater., vol. 27, no. 18, pp. 6345–6353, Sep. 2015.
[5] Y. Hou, W. Chen, D. Baran, T. Stubhan, N. A. Luechinger, B. Hartmeier, M. Richter, J. Min, S. Chen, C. O. R. Quiroz, N. Li, H. Zhang, T. Heumueller, G. J. Matt, A. Osvet, K. Forberich, Z.-G. Zhang, Y. Li, B. Winter, P. Schweizer, E. Spiecker, and C. J. Brabec, “Overcoming the Interface Losses in Planar Heterojunction Perovskite-Based Solar Cells.,” Adv. Mater., pp. 5112–5120, 2016.