Silicon solar cells, with silicon as the only absorbent material, are widely used in installations around the world today, but this technology is slowly reaching its limits in terms of maximum efficiency levels. Tandem solar cells, on the other hand, exhibit higher efficiency as they combine several different absorbent materials, which allows them to make better use of the solar irradiance spectrum. Now, for the first time researchers at Fraunhofer ISE have succeeded in combining these two technologies to design a multi-junction solar cell made of silicon and III-V semiconductor materials where the semiconductor materials were grown directly on the silicon.
The scientists combined various semiconductor materials in order to overcome the theoretical efficiency limit of 29.4% for single-junction silicon solar cells and produce electricity out of sunlight more efficiently than before. The semiconductors showing the most promising results were III-V semiconductor compounds, such as gallium arsenide. In their first attempt, the scientists at Fraunhofer ISE deposited the III-V solar cell structures on gallium arsenide substrates, then transferred to a silicon solar cell using a semiconductor bonding technology and finally etched away the gallium-arsenide substrate. In another less expensive approach, they grew the III-V layers directly on the silicon solar cell. During growth, the atomic structure had to be carefully monitored for the gallium and phosphorous atoms to arrange on the correct sites of the lattice at the interface to the silicon material. Furthermore, the distance between the atoms had to be increased so that they could produce the gallium arsenide material. It took several years of research to overcome the challenges presented by this direct-growth approach, but eventually scientists could reduce the defect density in the III-V semiconductor layers on the silicon, allowing them to create a III-V/Si tandem solar cell with a new efficiency record of 22.3 %.
This outstanding achievement is based on former research carried out on multi-junction solar cells in 2014. This kind of solar cell is able to convert 46% of the solar light into electricity. It uses different III-V compound semiconductor materials which can produce electric currents in response to different wavelengths of light. The most efficient cell is a four-junction cell whose sub-cells are able to adapt to wavelengths between 300 and 1750 nm. A challenge presented by this design was the exact distribution of the photons among the four sub-cells. For this reason the composition and the thicknesses of the layers had to be tuned precisely.
In 2016 research activity placed its focus on the improvement of light absorption and charge separation in the silicon. The researchers employed a “direct wafer bonding” process to transfer a few micrometres of III-V semiconductor material to silicon; pressure was then applied to the structure in a vacuum in order to bond the sub-cell surfaces together. The atoms on the surface of the III-V sub-cell formed bonds with the silicon atoms, creating a monolithic device. This was another step towards the production of high efficiency PV modules with efficiencies of over 30 %.
One of the apparent advantages of the direct growth of III-V layers on silicon is that costly III-V substrates for epitaxy can be avoided. This will enable cost-effective manufacture of tandem solar cells in the future. Also, solar cells produced with III-V semiconductor compounds have the potential for highly efficient energy production, as their compound materials have a high bandgap tunability by elemental compositions, cover a wide range of solar spectrum, exhibit higher photon absorption, higher resistivity against high-energy rays in space, and smaller efficiency degradation by heat than Si solar cells. Furthermore, they can be produced with very high crystalline and optoelectronic qualities.
Multi-junction cells, however, have certain drawbacks; the main problems consist of optical losses by front surface reflection, parasitic absorption in barrier layers, grid shading and electric losses. Also, there is the problem of lattice mismatch because of the different materials used.
Fraunhofer ISE is currently building a new research centre for high efficiency solar cells. “Our work on tandem cells will be carried out in the new facilities upon its completion in 2020. With the improved technical infrastructure, we expect the developments in multi-junction solar cells based on silicon to accelerate rapidly,” says Dr. Andreas Bett, institute director of Fraunhofer ISE.