New efficiency records for the evolving solar cell technology are set almost every month. However, most of these achievements are bound to some exotic materials or configurations. This time, researchers have spiraled up the efficiency for silicon-based multi-junction cells.

What makes the result so extraordinary? They exceeded the theoretical limit of silicon solar cells.

For this achievement, researchers from the Fraunhofer Institute for Solar Energy ISE and semiconductor process company EV Group (St. Florian, Austria) used a direct wafer bonding process to transfer a few micrometres thin layer of III-V semiconductor material to silicon. After plasma activation, the subcell surfaces are bonded together in vacuum by applying pressure. The atoms on the surface of the III-V subcell form bonds with the silicon atoms, creating a monolithic device.

The efficiency achieved by the researchers presents a first-time result for this type of fully integrated silicon-based multi-junction solar cell. 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. A conversion efficiency of 30.2% for the III-V / Si multi-junction solar cell of 4cm2 was measured at Fraunhofer ISE’s calibration laboratory. In comparison, the highest efficiency measured to date for a pure silicon solar cell is 26.3%, and the theoretical efficiency limit is 29.4%.

The III-V/Si multi-junction solar cell consists of a sequence of subcells stacked on top of each other. Tunnel diodes internally connect the three subcells made of gallium-indium-phosphide (GaInP), gallium-arsenide (GaAs) and silicon (Si), which span the absorption range of the sun’s spectrum. The GaInP top cell absorbs radiation between 300nm and 670nm. The middle GaAs subcell absorbs radiation between 500nm and 890nm and the bottom Si subcell between 650nm and 1180nm, respectively. The III-V layers are first epitaxially deposited on a GaAs substrate and then bonded to a silicon solar cell structure. Subsequently the GaAs substrate is removed, and a front and rear contact as well as an antireflection coating are applied.

“Key to the success was to find a manufacturing process for silicon solar cells that produces a smooth and highly doped surface which is suitable for wafer bonding as well as accounts for the different needs of silicon and the applied III-V semiconductors,” explained Jan Benick, research team leader at Fraunhofer ISE.

Despite the high efficiency, this technology is still too expensive to compete against other solar cells. Therefore, the researchers will work ways to reduce the manufacturing costs. According to Fraunhofer ISE, the ultimate objective is making high efficiency solar PV modules with efficiencies beyond the 30% mark possible.