Fraunhofer Institute for Solar Energy Systems ISE
III‑V semiconductors have unique properties that make them ideal for efficiently converting the visible and near-infrared regions of the solar spectrum into electricity. When paired with silicon, these materials enable the creation of highly efficient tandem solar cells, incorporating either two or three subcells. Thanks to the high absorption coefficients of III‑V materials, their layers can remain just a few micrometers thick. Integrating these materials with silicon opens promising pathways toward cost-effective manufacturing techniques.
At Fraunhofer ISE, our research is centered on developing monolithic 2-terminal solar cell concepts, where III‑V layers are either grown directly on silicon or transferred from gallium arsenide (GaAs) substrates through advanced wafer bonding or adhesive technologies. We pursue two main approaches:
1. Direct Epitaxy: III‑V semiconductor layers are deposited directly on silicon using cutting-edge MOVPE reactors from Aixtron (2800G4R with 8x6” configuration and CRIUS CCS with 7x4”). This process begins with growing a GaP nucleation layer on silicon, followed by increasing the lattice constant using ternary GaAsP buffer layers to finally enable absorber layers with bandgaps ranging from 1.4 eV (GaAs) to 1.9 eV (GaInP). Recent advancements in growth optimization have reduced defect densities in GaAs-on-silicon layers to below 10⁷ cm⁻². Additionally, we are working on developing advanced absorber layers, barriers, and tunnel diodes.
2. Wafer Bonding/Adhesive Transfer: III‑V layers are first grown on GaAs substrates and then transferred onto partially processed silicon subcells. Connections between the layers are established using direct wafer bonding or transparent conductive adhesives. The resulting structures form monolithic 2-terminal devices. This approach requires cost-effective processes for III‑V layer growth, thin-layer transfer, and GaAs substrate recycling.