Highly efficient solar cells: Researchers of the LZN develop new structures to increase efficiency of solar cells

In order to develop more efficient solar cells, researchers from the LZN have worked together in order to produce new forms of cell structures. The Bragg-structure with its integrated high converters makes the conversion of currently useless photons into useful photons possible. Within the framework of the pilot project, the team made up of researchers from the University of Freiburg and from Fraunhofer ISE was able to produce such a structure. With the help from various experiments and simulations, an optimized structure could be created, in order to not only improve solar technology but also to contribute to the current alternative energy research.

Solar energy is currently one of the most promising alternative sustainable energy sources. In order to capitulate on its full potential however, more research needs to be conducted. For example, the efficiency of materials, which are used to produce the solar cells themselves, needs to be increased. There are diverse materials that are currently used for solar cells, but silicon is the most prevalent, as it is not only inexpensive but also efficient in terms of its degree of efficiency. However, its level of efficiency still has room for improvement. A main reason for its impeded efficiency is that a part of the solar spectrum is not absorbed by silicon. In order to rectify this impediment, researchers from the Albert-Ludwigs-Universität Freiburg and the Fraunhofer-Institute for Solar Energy Systems (ISE) set the goal of converting the useless, low-energy photons into useable, high-energy photons via high conversion. “The theoretical efficiency limit of a silicon solar cell would thereby be increased from 30% to 40%,” explains Clarissa Hofmann from Fraunhofer ISE. The team consisted of researchers from Fraunhofer ISE and from the Institute of Physics of the University of Freiburg as well as the Department of Microsystems Engineering (IMTEK). Hofmann and her colleagues collaborated as part of the pilot project of the Sustainability Center Freiburg “NaLuWiLeS: Luminescence enhancing Nanostructures for LEDs and Solar Cells with increased Efficiency.”

In order to reach the proposed efficiency improvement, the researchers did not wish to modify the silicon material itself but instead, the structure of the cells. In other words, they wanted to add a supporting material to silicon, which would increase its efficiency. Built upon previous extensive work by the Fraunhofer ISE researchers, in which various combinations of silicon and other materials were produced and characterized, the team decided upon a promising alternating layer-structure, a so-called Bragg-Structure. This consisted of titanium dioxide (TiO2) and high converted nanoparticles embedded in Plexiglass (PMMA + NP). Bragg-Structures with seven to nine layers were developed via a highly precise process. These structures were then inspected for their high conversion characteristics. Researchers were able to determine the overall effect plus the angle dependency of the structure in an experiment with an infrared laser.

This comprehensive experimental work was only part of the project. The structure still needed to be described and optimized based upon simulations. To achieve this, a rate equation model was developed by the team for the comprehensive description of the high conversion process. Additionally, the two main effects of the Bragg-Structure were taken into account in this model: in what way the efficiency is influenced by an increased local irradiation and by a change in the photonic density. In the next step, the material was characterized in the laboratory. The team could thereby determine that the “signal” which is reflected by the Bragg-Structure was significantly higher than that of the examined reference structure. They thereby reached their goal of increasing the efficiency of the high-conversion process with help from an optimized Bragg-Structure, by making additional photons useable.

In the fourth research point, an angle analysis was carried out. As sunlight hits solar panels from different angles, it was important for researchers to be able to describe if and how dependent the efficiency was on the incidence angle. This relationship was thoroughly examined in the laboratory of Fraunhofer ISE: the manufactured samples were irradiated by lasers from varying angles. These examinations are still undergoing. The researchers have delved deeper into this research area and are additionally examining the effect of the beam angle. Initial results have proven successful and could be verified by simulations.

“We would love to continue this work with our colleagues in the future,” says Dr. Jan Goldschmidt, also from Fraunhofer ISE. “An EU-project would be an excellent opportunity for us to further develop our ideas.” Their proposed solution is not quite market-ready however; the process is still currently too expensive and time-consuming. However, the long-term potential of such a solution exists which has prompted the team to consider contacting industries. Such potential partnerships could help expedite the development of their modified solar cell structure into a market-ready product.