For the monolithic multiple-junction solar cell, the direct wafer bonding process known from microelectronics production is used. In a first step, the III-V layers are deposited on a gallium arsenide substrate. The surfaces are then deoxidized in a chamber under high vacuum with the aid of an ion beam and pressed together under pressure. The atoms of the III-V semiconductor layers form bonds with the silicon and form a unit. The subcells of GaInP, AlGaAs and silicon stacked on top of each other are interconnected by tunnel diodes. Subsequently, the GaAs substrate is wet-chemically removed and a nanostructured back contact as well as an anti-reflective coating and a contact grid are applied to the front.
Compared to earlier experimental arrangements, the deposition conditions were improved and a new cell structure was introduced for the uppermost gallium-indium phospid subcell, which better converts the visible light. "At 34.1%, the cell demonstrates the enormous potential of this technology," explains Dr. Frank Dimroth, Head of Department III-V Photovoltaics and Concentrator Technology at Fraunhofer ISE. The previous world record for this cell class was 33.3%.
Another possibility for the realization of multiple solar cells is the direct deposition of III-V semiconductor layers (GaInP/GaAs) onto the silicon solar cell. This process requires significantly fewer process steps than wafer bonding and avoids the use of the more expensive GaAs substrate, which is why it is advantageous for industrial implementation of the technology. However, the atomic structure must be very well controlled so that the gallium and phosphorus atoms at the interface to silicon occupy the correct lattice locations. Defects in the semiconductor layers can also impair the efficiency of the solar cells. "Here we were able to make important progress - the current generation in the three subcells hardly suffers from these defects, so that we were able to achieve an efficiency of 24.3 % for this technology for the first time worldwide," says Dimroth. "The potential corresponds to that of wafer-bonded cells, and here we still have some development work ahead of us in the coming years to demonstrate this. In December 2018, Fraunhofer ISE presented such a solar cell with an efficiency record of 22.3 %.