Continued Progress Toward the Development of High-Performance III-V/Si Tandem Solar Cells

The monolithic, epitaxial integration of III-V compound semiconductor materials and devices with Si substrates and devices has been a ‘holy grail’ of (opto)electronics materials research for decades, especially in the area III-V/Si multijunction (“tandem”) solar cells. Such an architecture holds the potential to deliver high conversion efficiencies on par with pure III-V multijunction structures, with application to reliable power generation in both terrestrial and certain (aero)space use cases, but at a fraction of the cost. One of the most promising approaches to this end is via direct heteroepitaxial GaP/Si integration, enabling the use of active Si subcells and bandgap/lattice constant engineering for target III-V devices via compositional (metamorphic) grading over GaAs_yP_1-y(or, potentially, Ga_1-xIn_xP). Despite decades of effort, the current certified AM1.5G records for such cells remain relatively low: 23.4% for a GaAs_0.75P_0.25/Si dual-junction (or 25% for uncertified, small-area) and 25.9% for a GaInP/GaAs/Si triple-junction. However, work from the author’s group and others around the world focused toward refining the metamorphic III-V/Si platform material quality — specifically with respect to the development of heteroepitaxial processes and growth structures enabling threading dislocation densities as low as 2×10⁶ cm^-2, as well as device design approaches for optimal performance at these achievable defect densities, and characterization methods that have proven essential for both — have brought the 30% AM1.5G “threshold” within sight. To this end, we will discuss recent and ongoing work from our group that has produced 20% GaAs_0.75P_0.25 top cells and associated high-performance tunnel junctions that, when combined with state-of-the-art Si bottom cells, will substantially shrink this important performance gap.