New Efficiency Record for Tandem Solar Cells

Multi-junction (MJ) solar cells are type of cells with multiple P/N junctions made of different semiconductor materials. Each material’s junction produces electric current in response to different wavelengths of light, and more specifically sunlight. The use of multiple semiconducting materials allows the absorbance of a broader range of wavelengths, improving the energy conversion efficiency from the sunlight to the electricity. The simplest structure of this kind is the tandem ones where two absorbers are stacked.

An international team reached the new efficiency record with a tandem solar cell. The new value is 14 % and, thus, tops the previous record of 12.4 %, held for seventeen years by NREL, USA. The new working team is from TU Ilmenau, Helmholtz-Zentrum Berlin (HZB), the California Institute of Technology and Fraunhofer ISE. Team leader is professor Matthias May, profound academic from the Institute for solar fuels. Using a now patented photo-electrochemical process, May could modify certain surfaces of these semiconductor systems in such a way that they functioned better in water splitting, reaching better performance. The fundamental component are the tandem solar cells characterized by III-V semiconductors.

M. May

Doctor May explained that “we have electronically and chemically passivated in situ the aluminium-indium-phosphide layers in particular and thereby efficiently coupled to the catalyst layer for hydrogen generation. In this way, we were able to control the composition of the surface at sub-nanometer scales”. At the beginning, the samples only survived a few seconds before their power output collapsed. Following about a year of optimizing studies, they remain stable for over 40 hours. Further steps toward a long-term stability goal of 1000 hours are already underway. Professor Hannappel said that “forecasts indicate that the generation of hydrogen from sunlight using high-efficiency semiconductors could be economically competitive to fossil energy sources at efficiency levels of 15 % or more. This corresponds to a hydrogen price of about four US dollars per kilogram”. Professor Lewerenz said “we are nearly there. If we are successful now in reducing the charge carrier losses at the interfaces somewhat more, we might be able to chemically store more than even 17 % of the incident solar energy in the form of hydrogen using this semiconductor system”.

The solar energy is more than available on the globe surface, but unfortunately not constantly and not everywhere. One of the interesting solution for storing this energy is the artificial photosynthesis. This is what every leaf can do during the normal photosynthesis, converting sunlight to chemical energy. That can take place with artificial systems based on semiconductors as well. These use the electrical power that sunlight creates in individual semiconductor components to split water into oxygen and hydrogen. Hydrogen possesses very high energy density and can be employed in many ways. In addition, no carbon dioxide harmful to the climate is released from hydrogen during combustion, instead only water. Until now, manufacturing of solar hydrogen at the industrial level has failed due to the costs.

More information about the study can be found on May, M. M. et al. paper called Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure.