Tunnel junction interconnects in GaAs-based multijunction solar cells

Monolithic multijunction solar cell performance and manufacturing can be significantly affected by the scheme used to connect the component subcells. In the authors´ recent record GaInP/GaAs tandem solar cells, they developed a GaAs tunnel junction interconnect doped with carbon and selenium that had a specific resistance at zero bias of <1 mΩ-cm² after annealing at 700°C for 15 minutes, the approximate conditions for growing the upper cell in the tandem structure. The specific resistance of the unannealed tunnel junction is about an order of magnitude lower than previously reported results for GaAs tunnel junctions. When incorporated into a solar cell structure, the tunnel junction displays nonideal characteristics, including the absence of a negative resistivity region and poorer conductance for large reverse bias than for similar-magnitude forward bias. These characteristics are shown to originate in a nearby heterojunction formed when the tunnel junction is incorporated in the tandem cell device structure