New solar cell record

Published on: August 10, 2020 / Updated on: August 17, 2020 – Author: Konrad Wolfenstein

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Higher efficiencies in tandem – new solar cell record

Photovoltaic research is working diligently to continuously increase the efficiency of solar cells. Tandem photovoltaics is increasingly coming into focus, where high-performance solar cell materials are combined in various ways to utilize the solar spectrum even more efficiently in the conversion of light into electrical energy. Fraunhofer ISE has now recorded a new record efficiency of 25.9 percent for a III-V/Si tandem solar cell grown directly on silicon. This was the first cell to be produced on a cost-effective silicon substrate – an important milestone on the path to economically viable solutions for tandem photovoltaics.

Several III-V tandem solar cells on a silicon substrate with a 10 cm diameter – © Fraunhofer ISE – Photo: Markus Feifel

The Fraunhofer Institute for Solar Energy Systems ISE has been working for many years on multi-junction solar cells, in which two or three sub-cells are stacked on top of each other to convert different wavelengths of sunlight into electricity. Silicon is suitable as an absorber for the infrared portion of the spectrum, and thin layers of III-V semiconductors, just a few micrometers thick, are applied over it. These materials are from groups III and V of the periodic table and convert ultraviolet, visible, and near-infrared light into electricity more efficiently. Pure III-V semiconductor solar cells are already used in space and in concentrator photovoltaics. More cost-effective processes, combined with silicon as the bottom sub-cell, are intended to make this tandem technology accessible for widespread photovoltaics in the future. However, there is still a long way to go.

25.9 percent for directly silicon-grown III-V/Si tandem solar cell

There are different approaches to manufacturing combinations of III-V and silicon solar cells. Since 2019, the Fraunhofer ISE has held the world record efficiency of 34.1 percent (now 34.5 percent) for a tandem solar cell in which the III-V semiconductor layers are transferred from a gallium arsenide substrate to silicon, with the layers being bonded together using a wafer bond. This technology is efficient but expensive. Therefore, the Fraunhofer ISE has been working for many years on more direct manufacturing processes in which the III-V layers are deposited or epitaxially bonded onto a silicon solar cell. Maintaining high crystal quality in all layers is crucial here – a major challenge. A new world record efficiency of 25.9 percent has now been achieved for such a III-V/Si tandem solar cell grown directly on silicon. Fraunhofer ISE scientist Markus Feifel recently presented his success at the 47th IEEE Photovoltaic Specialists Conference, which, like many conferences these days, is being held online, and was honored with the Student Award in the Hybrid Tandem Solar Cells category. "The complex internal structure of the cell is not visible from the outside, as all absorbers are interconnected and electrically wired by further crystal layers," explains the young solar cell researcher, who was thus able to improve the efficiency of his work from 24.3 to 25.9 percent in less than a year. "This success was achieved by replacing a single thin layer within the multi-cell," he continues. "A careful analysis of our cells revealed that this layer was acting as a barrier to electrical conduction."

Since 2007, Fraunhofer researchers, together with the Technical University of Ilmenau, the Philipps University of Marburg, and the company Aixtron, have been gradually developing the technology, building specialized epitaxy systems and examining each individual layer of the structure. These developments were funded by the German Federal Ministry of Education and Research (BMBF) within the framework of the "III-V-Si" and "MehrSi" projects. A particular highlight of the new tandem solar cell is that the III-V layers were not grown on a chemically and mechanically polished substrate, as was previously the norm, but rather on a silicon wafer. After the crystal was sawn, the wafer was treated using a simple, cost-effective grinding and etching process. The Danish company Topsil developed these silicon wafers within the framework of the European "SiTaSol" project, thus taking an important step towards the economical production of the new multi-junction solar cells. In the future, the focus will be on further increasing efficiency and making the deposition of the layers even faster, with higher throughput and therefore more cost-effective, with the aim that tandem photovoltaics can make an important contribution to the photovoltaic expansion necessary for the energy transition.

Key technology for the energy transition

Electricity from solar cells is currently the most cost-effective form of energy generation in many parts of the world. "European photovoltaic research is working on numerous concepts to further improve the efficiency of this key technology for the energy transition," says Prof. Dr. Stefan Glunz, Head of Photovoltaics Research. "We are not only working to make the production of silicon solar cells even more sustainable and cost-effective, but are also exploring new avenues to achieve even higher efficiencies with proven silicon in combination with other semiconductor materials. We are achieving this with tandem photovoltaics." Tandem photovoltaics not only paves the way for the future of electricity generation, but these solar cells are also ideally suited – due to their higher voltage – for electrolysis, the direct splitting of water into hydrogen and oxygen. This technology thus also contributes to the production of hydrogen as an energy storage medium and an important building block for the energy transition.

Layer structure of the III-V/Si multi-junction solar cell – © Fraunhofer ISE

Layer structure of the III-V/Si multi-junction solar cell, quantum efficiency and IV characteristic under AM 1.5g spectral conditions

Higher efficiencies in tandem – new solar cell record

Photovoltaics research is working hard to continually increase the efficiency of solar cells. Increasingly, the focus is on tandem photovoltaics, in which high-performance solar cell materials are brought together in various combinations in order to use the solar spectrum even more efficiently in converting light into electrical energy. Fraunhofer ISE is now reporting a new record efficiency of 25.9 percent for a III-V/Si tandem solar cell grown directly on silicon. For the first time, this was produced on a low-cost silicon substrate – an important milestone on the way to economical solutions for tandem photovoltaics.

Several III-V tandem solar cells on a silicon substrate with 10 cm diameter – © Fraunhofer ISE – Photo: Markus Feifel

The Fraunhofer Institute for Solar Energy Systems ISE has been working for many years on multi-junction solar cells in which two or three partial cells are arranged one above the other to convert different wavelengths of sunlight into electricity. Silicon is suitable as an absorber for the infrared part of the spectrum, and layers of III-V semiconductors, materials from groups III and V of the periodic table, which convert the ultraviolet, visible and near-infrared light more efficiently into electricity, are deposited on top of it. Pure III-V semiconductor solar cells are already used in space and in concentrator photovoltaics. Through more cost-effective processes in combination with silicon as the lowest sub-cell, the tandem technology is to be made accessible for broad-based photovoltaics in the future. However, there is still a long way to go until then.

25.9 percent for III-V/Si tandem solar cells grown directly on silicon

There are different approaches to produce combinations of III-V and silicon solar cells. For example, since 2019, Fraunhofer ISE has held the world record of 34.1 percent efficiency (now 34.5 percent) for a tandem solar cell in which the III-V semiconductor layers are transferred from a gallium arsenide substrate to silicon, with the layers being connected by a so-called wafer bond. This technology is efficient but expensive. For this reason, Fraunhofer ISE has been working for many years on more direct manufacturing processes in which the III-V layers are deposited or epitaxied onto a silicon solar cell. Here, it is crucial to maintain a high crystal quality of all layers – a major challenge. A new world record efficiency of 25.9 percent has now been achieved for such a III-V/Si tandem solar cell grown directly on silicon. Fraunhofer ISE scientist Markus Feifel was recently able to present his success at the 47th IEEE Photovoltaic Specialists Conference, which, like many conferences at present, is held online, and was honored with the Student Award in the category Hybrid Tandem Solar Cells. “From the outside, the complex internal structure of the cell is not visible, since all absorbers are connected to each other by additional crystal layers and electrically wired,” explains the young solar cell researcher, who was thus able to improve the result of his work from 24.3 to 25.9 percent in less than a year. “This success was achieved by replacing a single thin layer within the multiple cell,” he continues. “A careful analysis of our cells revealed that this layer created a barrier to the power line.”

In small steps, the Fraunhofer researchers have been developing the technology further since 2007 in collaboration with the Technical University of Ilmenau, Philipps Univ. Marburg and the company Aixtron, setting up special epitaxy equipment and examining every single layer of the structure. These developments were funded by the German Federal Ministry of Education and Research (BMBF) as part of the “III-V-Si” and “MehrSi” projects. A particular highlight of the new tandem solar cell is that the III-V layers were not grown on a chemically-mechanically polished substrate as was previously the case, but on a silicon wafer which, after sawing the crystal, was treated in a simple process using only inexpensive grinding and etching processes. Within the European project “SiTaSol”, the Danish company Topsil had developed these silicon wafers and thus realized an important step towards an economic production of the new multi-junction solar cells. In the future, the aim will be to increase efficiency even further and also to realize the deposition of the layers even faster, with higher throughput and thus more cost-effectively, with the aim that tandem photovoltaics can make an important contribution to the photovoltaic expansion necessary for the energy turnaround.

Key technology for energy system transformation

In many parts of the world today, electricity from solar cells is the cheapest form of energy generation. “European photovoltaic research is working on numerous concepts to further develop the efficiency of this key technology for the energy turnaround,” says Prof. Dr. Stefan Glunz, head of the Photovoltaics Research Division. "We are not only working on making the production of silicon solar cells even more sustainable and cost-effective, but at the same time we are also breaking new ground in order to lead the proven silicon in combination with other semiconductor materials to even higher efficiencies. We are achieving this with tandem photovoltaics. Tandem photovoltaics not only opens the way into the future of power generation, these solar cells - due to their higher voltage - are also ideally suited for electrolysis, the direct decomposition of water into hydrogen and oxygen. This technology thus also contributes to the production of hydrogen as an energy storage medium and an important building block for the energy turnaround.

Layer structure of the III-V/Si multi-junction solar cell – © Fraunhofer ISE

Layer structure of III-V/Si multi-junction solar cell, quantum efficiency and IV characteristics under AM 1.5g spectral conditions