News: Photovoltaics
29 June 2021
Fraunhofer ISE reports record 68.9% efficiency for GaAs thin-film PV cell under laser light
In addition to the classical applications for solar cells on roofs and open spaces, photovoltaic devices can also be used with laser light for efficient power transmission. At the 48th IEEE Photovoltaic Specialists Conference, researchers from the Fraunhofer Institute for Solar Energy Systems ISE of Freiburg, Germany presented how they were able to achieve a record conversion efficiency of 68.9% with a photovoltaic cell under monochromatic laser light. For this, the team used a very thin photovoltaic cell based on gallium arsenide (GaAs) and applied a highly reflective, conductive mirror on its backside.
Picture: Fraunhofer ISE’s new GaAs-based thin-film photovoltaic cell. (©Fraunhofer ISE / picture: Henning Helmers).
The photovoltaic effect is particularly efficient when the energy of the incident light lies slightly above the semiconductor material’s bandgap energy. Very high efficiencies are therefore theoretically possible when a monochromatic laser as light source is matched with a suitable compound semiconductor material.
In this ‘power by light’ energy transfer, the laser energy is delivered either through the air or via an optical fiber to a photovoltaic cell whose properties match the power and the wavelength of the monochromatic laser light. Compared with conventional power transmission via copper wires, power-by-light systems are especially beneficial for applications that require a galvanically isolated power supply, lightning or explosion protection, electromagnetic compatibility, or completely wireless power transmission, for example.
Fraunhofer ISE has achieved a record conversion efficiency of 68.9% for a III-V semiconductor photovoltaic cell based on GaAs exposed to 858nm-wavelength laser light. This is reckoned to be the highest efficiency achieved to date for the conversion of light into electricity.
This was made possible with thin-film technology in which the solar cell layers are first grown on a GaAs substrate that is then subsequently removed. A conductive, highly reflective mirror is applied to the back surface of the remaining semiconductor structure, which is only a few microns thick.
“This thin-film approach has two distinct advantages for the efficiency,” says physicist Dr Henning Helmers, head of the research team. “First of all, photons are trapped in the cell and the absorption is maximized for photon energies close to the bandgap, which simultaneously minimizes thermalization and transmission losses, making the cell more efficient. Secondly, the photons additionally generated internally by radiative recombination become trapped and effectively recycled. This extends the effective carrier lifetime, thus additionally increasing the voltage.”
The research group investigated thin-film photovoltaic cells with back-surface reflectors made of gold and an optically optimized combination of ceramic and silver, with the latter showing the best results. An n-GaAs/p-AlGaAs heterostructure was developed as absorber, which shows particularly low charge carrier losses due to recombination.
“This is an impressive result that shows the potential of photovoltaics for industrial applications beyond solar power generation," comments Fraunhofer ISE’s director professor Andreas Bett. Optical power transmission has manifold applications. Examples are the structural monitoring of wind turbines; the monitoring of high-voltage lines, fuel sensors in aircraft tanks, or passive optical networks; the optical supply of implants from outside the body; or a wireless power supply for applications in the Internet of Things.
