Study on selective emitter fabrication through an innovative pre-diffusion process for enhanced efficiency in TOPCon solar cells

Abstract

TOPCon (tunnel oxide passivated contact) solar cell is the mainstream high-efficiency crystalline silicon solar cell structure. However, the lack of efficient passivation contact mechanisms on the front surface restricts the electrical performance ability to improve further. Selective emitter (SE) technology, considered a potential solution, needs to be more mature. This work provides a unique thermal pre-diffusion approach combined with laser treatment and post-oxidation annealing to create SE structures in TOPCon solar cells. Times for the high-temperature process are equivalent to those for a traditional homogenous emitter. The innovative thermal pre-diffusion process created a unique boron doping profile, achieving a high surface concentration of nearly 1 × 10²⁰ cm⁻³ with a shallow junction depth of approximately 0.25 μm. Laser treatment further activated boron and facilitated its diffusion, influenced by the boron silicate glass layer and surface boron atoms. Adjustments were made to improve the pre-diffusion recipe, including an additional boron deposition step, increasing non-activated boron atoms. Introducing larger pyramidal microstructures also improved the junction depth and surface concentration in the heavily doped region. Compared to homogeneous emitters, the SE structures exhibited lower surface concentration in the lightly doped region, reducing the recombination current density in the passivation region J_0,pass values. The SE structures achieved higher junction depths, limiting metal atom diffusion and reducing the current recombination density in the metal contact region J_0,metal values. The contact resistivity between metal and silicon was also decreased. Overall, introducing SE structures resulted in a batch-average efficiency improvement of 0.26%, reaching an average efficiency of 25.22% for TOPCon solar cells, and has industrial mass-producible.