Cu Nanoparticle Array-Mediated III–V/Si Integration: Application in Series-Connected Tandem Solar Cells

The integration of III–V materials with crystalline Si (c-Si) is a promising pathway to design high-performance optoelectronic devices, including solar cells. We have previously reported high-efficiency III–V/Si tandem cells using our unique semiconductor bonding technique, termed smart stack. In the conventional smart stack cells, Pd nanoparticle (NP) arrays have been commonly employed as bonding mediators between III–V and c-Si; however, from an economical point of view, the use of other low-cost metals would be preferable. Therefore, this study focused on the possibility of Cu. A polystyrene-block-poly-2-vinylpyridine (PS-b-P2VP)-based block copolymer was utilized to prepare Cu NP arrays. Desired Cu NP arrays were achieved by starting with self-assembled PS-b-P2VP micelles preloaded with Cu²⁺ ions. Satisfying bonding properties (low-resistance interfaces) were confirmed when GaAs subcells were stacked on the Cu NP arrays formed on native-oxide-removed c-Si subcells. Conversion efficiencies of up to 25.9% have been demonstrated with triple-junction structures consisting of InGaP/GaAs top and c-Si bottom subcells. The long-term reliability of Cu NP array-mediated smart stack cells was also verified by the thermal cycle and damp heat tests. Hence, we have successfully confirmed that not only Pd but also Cu is available to realize high-efficiency smart stack cells.