Influence of Thermal Activation on Polysilicon Tunnel Junctions for Tunnel Oxide Passivating Contacts-Based Perovskite/Silicon Tandem Solar Cells

The presented investigation focuses on different thermal activation processes and their influence on the formation of a polysilicon tunnel junction as a recombination layer in perovskite/silicon tandem solar cells. The goals of this investigation are the optimization of the polysilicon tunnel junction and the comparison of different processing pathways to enable a lean process integration. Various process routes with different annealing sequences for the polysilicon tunnel junction were examined, using a laser-based rapid thermal process (RTP) and a conventional furnace anneal. The main challenge is to control the interdiffusion of p- as well as n-type dopants at the tunnel junction interface during the thermal activation process. On pre-annealed n-tunnel oxide passivating contact (TOPCon) layers, a low process temperature for the p-polysilicon activation is beneficial without the necessity of an additional diffusion blocking layer. If the n-TOPCon and the p-polysilicon layer are annealed simultaneously in one RTP, high temperatures are required, which are enabled by a diffusion blocking SiN_x interlayer at the tunnel junction interface. Optimized tunnel junctions on TOPCon bottom cell precursors show low contact resistivity of only 30 mΩ cm² with an implied open-circuit voltage as high as 726 mV. These results demonstrate the promising properties of the developed tunnel junctions regarding tandem cell integration.