Date: 21/10/2020

25.9% efficient solar PV from MehrSi funded project

Researchers at the Fraunhofer Institute for Solar Energy Systems ISE in close cooperation with the Technical University of Ilmenau, the Philipps University of Marburg and the epitaxy experts at AIXTRON SE develop solar PV with efficiency of 25.9 percent with a multi-junction solar cell grown directly on a silicon substrate. These members have worked as part of the funded "MehrSi" project.

This tech is called tandem PV, where different combinations of high-performance solar cell materials are layered on over the other to trap/convert energies from the complete band of wavelengths of sunlight more efficiently into flow of electrons. Silicon is good at converting infrared light from sunlight. Thin micro meter layers of different III-V compound semiconductora are topped over silicon layer to convert the ultraviolet, visible and near-infrared light more efficiently into electricity. Multi-junction solar cells grown on a silicon substrate are less costing than solar cells based on other substrates.

"For the first time we have now been able to realize a tandem solar cell based on a silicon wafer with such high efficiency," comments Dietmar Schmitz, Vice President Corporate Technology Transfer at AIXTRON SE. Until now, the production of III-V multi-junction solar cells has been based on a more expensive substrate material, for example also a compound semiconductor material.

Dietmar Schmitz emphasizes what is special about this basic research: "The gallium and phosphorus atoms must occupy the correct lattice positions at the interface with silicon. To achieve this, we have to control the atomic structure very well. This requires exceptionally high precision. In addition, to achieve the necessary high quality of the epi-wafers, it is crucial that a high crystal quality of all layers is achieved during epitaxial growth. This was achieved in the project thanks to the improved system technology developed by AIXTRON and the good cooperation with the project partners".

The group is also working on faster crystal growth to further reduce manufacturing costs and to further increase efficiency.

"Specifically, the dislocation density in the III-V solar cell layers is to be reduced from 108 cm-2 to the range of 1-5*106 cm-2 in order to increase the efficiencies to more than 30 percent. And last but not least, the cost-effectiveness of the epitaxy processes is to be further optimized," explains Prof. Dr. Michael Heuken, Vice President Corporate Research and Development at AIXTRON SE.

AIXTRON said it has already been working on such III-V multi-junction solar cells on silicon with several partners for many years. The project "SiTaSol" for the joint development of layer packages that can be produced as quickly and cheaply as possible is funded by the EU. In "SiTaSol" AIXTRON said it has built and tested a specially optimized Metal Organic Chemical Vapor Phase Epitaxy (MOVPE) system in its own laboratory.