Prof. Dr. Christoph J. Brabec
Friedrich-Alexander-University of Erlangen-Nuremberg
Institute Materials for Electronics and Energy Technology

Prof. Mike McGehee
Stanford University
Department of Materials Science & Engineering

Organic solar cells represent a next generation photovoltaic technology that enables flexible, light weight and low cost power generating devices. With organic photovoltaic efficiencies exceeding 10%, the science of stabilization and lifetime becomes a key factor for large scale application. To understand degradation mechanisms in organic solar cells, the correlation between microstructural changes during device operation and their influence on device performance is the key to rationalize failure mechanisms. The investigations focus on developing a better understanding of the origin of energetic traps, which are believed to be the main degradation mechanism in organic solar cell materials. Based on our findings, we will employ methods to modify the chemical design and to control the morphology of organic solar cells in order to improve device lifetime.

Final report:

In a fruitful collaboration between Erlangen and Stanford, we used advanced characterization techniques to study the degradation of a variety of organic photovoltaic (OPV) materials. Combining photocurrent spectroscopy measurements with unique structural analysis techniques at the Stanford Synchrotron Light Source (SSRL) revealed a clear correlation between decreased burn-in performance losses of organic solar cells with increased crystallinity of the donor polymer [1]. This lead to a general design rule for devices with increased stability which is also applicable for small molecule donors. Using advanced transient characterization techniques we identified the physical origin of the initial open circuit voltage losses as an increase of energetic disorder and contributed to the theoretical understanding of how to improve the open circuit voltage of organic solar cells [2]. Moreover, it was also demonstrated, that the glass transition temperature of the used materials is critical for device stability [3].

[1] T. Heumueller, W. R. Mateker, C. J. Brabec, and M. D. McGehee, et al. “Reducing burn-in voltage loss in polymer solar cells by increasing the polymer crystallinity,” Energy and Environmental. Science, vol. 7, no. 9, pp. 2974–2980, Aug. 2014.

[2] K. Vandewal, J. Widmer, T. Heumueller, C. J. Brabec, M. D. McGehee, K. Leo, M. Riede, and A. Salleo, “Increased open-circuit voltage of organic solar cells by reduced donor-acceptor interface area.,” Advanced Materials, vol. 26, no. 23, pp. 3839–43, Jun. 2014.

[3] I. T. Sachs-Quintana, T. Heumüller, C. J. Brabec, and M. D. McGehee, et al. “Electron Barrier Formation at the Organic-Back Contact Interface is the First Step in Thermal Degradation of Polymer Solar Cells,” Advanced Functional Materials, vol. 24, no. 25, pp. 3978–3985, Jul. 2014.