Silicon Solar Cells
Silicon solar cells dominate the photovoltaic market today. Research and industry are continually working to optimize the cell efficiency and are moving increasingly closer to the theoretical efficiency limit of the material. Fraunhofer ISE holds the current world record of 22.3 percent for multicrystalline silicon solar cells. The key to this achievement was the holistic approach, enabling the optimization of all steps from the crystallization up to the singular solar cell processing. An optimized plasma texture and a so-called Tunnel Oxide Passivated Contact Technology (TOPCon) developed at Fraunhofer ISE for back side contacting played an essential role. With the TOPCon technology the electrical contacts are applied over the entire rear surface of the cell without patterning. This reduces charge-carrier losses and leads to much higher conversion efficiencies.
For monocrystalline silicon solar cells, Fraunhofer solar cells achieved a first-time efficiency of 25.7 percent of a both-sides contacted silicon solar cell, due to the TOPCon technology.
III-V Solar Cells
Fraunhofer ISE also holds the international record for the highest photovoltaic efficiency overall of 46.1 percent for its III-V concentrator multi-junction solar cells. This record cell is based on a combination of different III-V semiconductor compounds, e.g., gallium-indium-phosphide (GaInP), gallium-indium-arsenide (GaInAs) or germanium (Ge). Each semiconductor converts a different wavelength range of sunlight into electricity. Working together the sub-cells achieve an extremely high cell efficiency. Originally III-V multi-junction solar cells were used in space for supplying power to satellites, an application that Fraunhofer ISE is still working on today. On Earth, large concentrator power plants, which combine III-V solar cells with concentrating optics are a typical application.
Tandem Solar Cells
To create even more efficient solar cells, researchers at Fraunhofer ISE forged new paths in combining their expertise in silicon and III-V materials research. They succeeded in creating a silicon-based multi-junction solar cell with 33.3 percent efficiency with a simple front and rear contact. The high conversion efficiency of the silicon-based multi-junction solar cell was achieved with extremely thin 0.002 mm semiconductor layers of III-V compound semiconductors, bonding them to a silicon solar cell. The visible sunlight is absorbed in a gallium-indium-phosphide (GaInP) top cell, the near infrared light in gallium-arsenide (GaAs) and the longer wavelengths in the silicon subcell. In this way, the efficiency of silicon-based solar cells can be significantly increased.
The photovoltaic researchers used a well-known process from the microelectronics industry called “direct wafer bonding” to successfully transfer III-V semiconductor layers, of only 1.9 micrometers thick, to silicon. After plasma activation under high vacuum, the surfaces were bonded together under pressure. The atoms on the surface of the III-V sub-cell form bonds with the silicon atoms, creating a monolithic device. The complexity of its inner structure is not evident from its outer appearance: the cell has a simple front and rear contact just as a conventional silicon solar cell and therefore can be integrated into photovoltaic modules in the same manner.