Using X-ray imaging, scientists at the Technical University of Munich (TUM) were able to watch an organic solar cell degrade in real time. These new observations will help scientists to find new ways to increase the stability of organic solar cells.
Organic solar cells, especially those based on polymers are inexpensive to produce on a large scale. However, the efficiency of organic solar cells rapidly declines as a solar cell degrades over time—organic solar cells have a shorter service life than conventional silicon cells. The team headed by Prof. Peter Müller-Buschbaum from the TUM made the first live observations of the degradation of organic solar cells in operation using the P03 measuring station of DESY’s light source PETRA III. To do this, they lit a sample polymer solar cell using a solar simulator, which emits light that matches the spectrum and intensity of sunlight and recorded the electrical characteristics of the cell over time.
At intervals ranging from several minutes to as much as one hour, the researchers also looked inside the solar cell using the sharply focused X-ray beam from PETRA III. In this way they were able to watch how the interior structure of the active layer of the solar cell changed in the course of seven hours, while the efficiency of the cell decreased by around 25 percent.
Electricity is generated in the active layer at what is known as active domains in these solar cells. Here, light is absorbed and charge carriers are released. The diameter of these active domains increased by 17 per cent during the study, from about 70 to more than 80 nanometers (millionths of a millimeter). At the same time, the mean distance between them increased by 19 per cent from 310 nanometers to around 370 nanometers, as the X-ray measurements showed.
“This suggests that during operation small sites disappear permanently in favour of larger ones,” explains first author Christoph Schaffer, who is a PhD student in Müller-Buschbaum’s group. “Although the domains grow, they also recede from each other, this means that their total active area shrinks. This can precisely explain the observed decline in efficiency.”
“The examination explained the mechanism of degradation for the first time. It’s a first step,” says co-author Dr. Stephan Roth, the DESY scientist with responsibility for measuring station P03. “The next step involves attempting to reduce or control this growth in a targeted manner, for example, through the addition of appropriate substances. Polymer solar cells could conceivably be produced with an internal structure in which the active sites grow to their optimal size during the first hours of operation,” adds Müller-Buschbaum. “The consequence of such measures could be that industrially produced cells finally cross the economically crucial efficiency threshold also for long-term operation,” emphasises Roth.