Development of tunnel junctions with high peak tunneling currents for InP-based multi-junction solar cells

In this work we developed lattice-matched InAlGaAs tunnel junctions for an InP-based multi-junction cell. We discuss the design and modeling of the device architecture and present results from the growth and characterization of a range of test structures. The impact of dopant diffusion during the overgrowth of subcells on the performance of the tunnel junctions was characterized using both electrical measurements and SIMS. We investigated different structures to mitigate the effects of dopant diffusion and maximize the peak tunnel current, achieving a peak tunnel current of 28.9 A/cm² using a bulk, double heterostructure design, compared to the baseline bulk device with a peak tunnel current of 2.3 A/cm². We have also investigated a structure incorporating two InGaAs quantum wells to facilitate the interband tunneling via the confined electron and hole states in the quantum wells. This improved the tunneling current to 113 A/cm² with only a small impact on the transparency of the tunnel junction.