MehrSi project achieves record 22.3% efficiency for tandem solar cell with III-Vs grown directly on silicon

In cooperation with partners, Fraunhofer Institute for Solar Energy Systems ISE of Freiburg, Germany has achieved a new energy conversion efficiency record of 22.3% for a multi-junction solar cell made of silicon and III-V semiconductor materials. The III-V layers were directly grown on the silicon.

Silicon solar cells dominate the photovoltaic market today but the technology approaches the theoretical maximum efficiency that can be achieved with silicon as the only absorber material. In contrast, tandem solar cells combine several absorber materials, enabling better energetic use of the solar irradiance spectrum and hence higher efficiency.

By combining different semiconductor materials, researchers are attempting to surpass the theoretical efficiency limit of 29.4% for a single-junction silicon solar cell.

A promising approach is the combination of silicon material with III-V semiconductor compounds like gallium arsenide (GaAs), for example by first depositing the III-V solar cell structures on GaAs substrates then transferring to a silicon solar cell using semiconductor bonding technology and lastly etching away the GaAs substrate.

However, a less costly approach is to directly grow the III-V layers on the silicon solar cell. The atomic structure must be controlled extremely well during growth so that the gallium and phosphorous atoms arrange on the correct lattice sites at the interface to the silicon material. Also, the distance between the atoms in the crystal lattice must be increased in order to produce the GaAs material.

Researchers have been working on these challenges for over ten years. Now they have been able to greatly reduce the defect density in the III-V layers on the silicon and have successfully produced a III-V/Si tandem solar cell with a new efficiency record of 22.3% using this direct-growth approach.

Graphic: Current-voltage characteristic of new III-V/Si tandem solar cell with 22.3% efficiency. III-V layers were directly deposited on the silicon bottom cell in an epitaxial process. To match the atomic bonding between atoms in the crystal structures, a nucleation layer of GaP and a metamorphic buffer was introduced between the Si and GaAs. Optimizing these transition layers was a main challenge in the project.

Direct growth of III-Vs on silicon is “an important research approach for tandem solar cells,” says Fraunhofer ISE director Dr Andreas Bett. “We are presently building a new research center for high-efficiency solar cells. Our work on tandem cells will be carried out in the new facilities upon its completion in 2020,” he adds. “With the improved technical infrastructure, we expect the developments in multi-junction solar cells based on silicon to accelerate rapidly.”

Within the MehrSi project over the past few years, the junction between the silicon crystal and the first III-V semiconductor layer of gallium phosphide (GaP) was investigated and continuously optimized in close cooperation with the research groups of professor Thomas Hannappel at TU Ilmenau and professor Kerstin Volz at the Philipps University Marburg. The defects in the crystalline structure were first made visible and then reduced step by step. “The record efficiency of our III-V/Si tandem solar cell demonstrates that we have achieved a very good understanding of the materials,” says MehrSi project coordinator Dr Frank Dimroth. “With the successful direct growth of III-V layers on silicon, we can avoid using expensive III-V substrates for epitaxy,” he adds. “This approach is, therefore, a key technology for the cost-effective manufacture of high-efficiency tandem solar cells in the future.”

The MehrSi project, in which the record multi-junction solar cell on silicon was created, was financed by the German Federal Ministry for Education and Research (BMBF). The project partners were TU Ilmenau, the Philipps University of Marburg and metal-organic chemical vapor deposition (MOCVD) system maker Aixtron SE.

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Fraunhofer ISE demonstrates two-terminal GaInP/GaAs/Si solar cell with efficiency of 33.3%

Tags: Fraunhofer ISE III-V multi-junction solar cells

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