The direct conversion of sunlight into electricity is a promising clean energy solution to answer the multiple challenges of energy supply security, competitiveness, electricity prices and climate change. Crystalline silicon, the dominant technology on the photovoltaic market, benefits from a well-established industry and record sunlight to electricity power conversion efficiency around 26%. However, mainly limited by its inherent material properties, the silicon technology has very little room for further power conversion efficiency improvement. On the other hand, much higher solar cell efficiencies have been achieved by stacking several layers of different materials, namely III-V semiconductor crystals, to create a so-called multi-junction solar cell which utilized a larger fraction of the solar energyspectrum. At present, however the III-V solar cells are restricted to specific markets (e.g. space) for cost and availability reasons. A hybrid solution combining the advantages of III-V multi-junction solar cells with the benefits of silicon, the most wide-spread photovoltaic material, offers great opportunities. Indeed, power conversion efficiencies above 46% (under 1-sun AM1.5G conditions) can be theoretically expected (see fig. 1) for conventional silicon based solar cell when combined with two additional III-V active layers to form a triple-junction solar cell (Gallium Indium Phosphide - GaInP - and Gallium Arsenide - GaAs - on top of silicon).
In the practice, however, there are still hurdles to be overcome. Combining III-V semiconductor materials with silicon is highly challenging. III-V materials have some fundamental crystallographic differences with silicon (lattice and thermal mismatch, polarity difference), and thus it is challenging to grow III-V layers with sufficient electrical quality directly on silicon. The innovative approach proposed in this project bypasses this issue and enables to have high quality III-V crystals combined with silicon, using a microelectronic technique called surface activated wafer bonding. With this approach, the III-V solar cells and the wafer based silicon solar cell are prepared separately, and combined using the surface activated wafer bonding technique. Once permanently joined, the III-V substrate is removed, resulting in 3-5 μm thin III-V solar cells on top of the silicon wafer (See fig. 2). The objective of this research project was to use the surface activated wafer bonding technique to produce such hybrid III-V/silicon triple-junction solar cells (GaInP/GaAs//Si), and achieve large area (4 cm2) devices with an efficiency exceeding 30% in 2-terminal configuration (under 1-sun AM1.5G conditions). These solar cells may serve as a drop in replacement into standard flat plate photovoltaic modules which facilitates market introduction.
