Researchers Observe Molecular Processes of Organic Solar Cells

Scientists at the Technical University of Munich (TUM) successfully observe the unique molecular processes that occur during the production of organic solar cells. in real time.

The processes that occur on the molecular scale during the production of organic solar calls is not entirely clear. The findings, published in the specialist journal Advanced Energy Materials, could improve efficiency of organic solar cells.

Plastic-based solar cells can be easily created with the help of a printer. These solar cells are lightweight and can be installed easily. However, during the development of organic solar cells, certain molecular processes occur that have not been fully established.

Many homes have already installed the semiconductor, silicon-based solar modules on their roof tops. Though, such modules tend to be rather heavy, and prove expensive for roof installation, they also don't blend in easily with their surroundings. An alternative option to silicon-based solar modules is organic solar cells that contain organic molecules, similar to a cling film or plastic bags. Such solar cells are also soluble, and can be created by means of a printer. Organic solar cells are also lightweight and have an ultra-thin design, this makes them easy to install in various locations. Additionally, it is also easy to adjust the shape and color of the solar cells. However, a major drawback of organic photovoltaics is that their efficiency level is less than the silicon solar cells.

One key parameter in harvesting more energy from the flexible solar cells is the way in which the material’s molecular components are arranged. Important for the energy conversion, this creates free electrions, much like the "classic" solar cells. In order to do this two types of material need to be used for organic solar cells, one material that accepts electrons and one that donates them. To change light into electricity, the interface between both of these materials should be kept as large as possible. Until now, it was not entirely clear as to how molecular alignment occurs throughout the printing process, and how the crystals formed by the molecules grow during the course of the drying process. These molecules are initially contained in a solution, similar to the pigments found in printer ink.

In order to be able to control the arrangement of the components, we need to understand what happens at the molecular level during the drying process.

Dr. Eva M. Herzig, Munich School of Engineering (MSE) at TUM

This presented an experimental challenge to the researchers, who wanted to resolve the small structures inside a drying film with sufficient time resolution.

MSE doctoral candidate, Stephan Pröller, in collaboration with the Lawrence Berkeley National Laboratory in the USA, utilized X-rays to create the molecules and their related processes visible during the printing of a plastic film. Pröller was able to detect a range of different phases which occurred during the film’s drying process.

At first, the other materials remain in solution while the solvent evaporates. This results in better concentration of the plastic molecules present in the wet film, until the time the electron donor begins to crystallize and the electron acceptor begins to form aggregates. A rapid crystallization process follows, which forces the electron acceptor aggregates more closely together. At this point, the space between the interfaces of both materials is defined, which is closely correlated to efficiency. This stage of the printing process has to be controlled in order to systematically enhance the solar cells. In the final stage, optimizing processes within individual materials occurs, similar to optimizing the packing process of crystals.

The production speed also plays an important role.

Stephan Pröller, MSE Doctoral Candidate

While faster drying processes are used to preserve this pattern, both the crystals and aggregates formed by the materials, affect the remaining process of structure formation. This way, the slower formation of structures has a better effect on the ultimate efficiency.

In the future, the scientists are planning to apply these molecular processes to achieve better control over the material’s arrangement using other parameters. The results from this study could be applied to industrial production and help optimize it.