Optimization of boron depletion for boron-doped emitter of N-type TOPCon solar cells

Abstract

During the preparation of boron-doped emitters for TOPCon solar cells, boron atoms accumulate, forming a boron-rich layer (BRL). Oxidation, during the boron diffusion process, can eliminate the BRL. Prolonged oxidation which forms a SiO2 layer can remove the BRL, also act as a protective mask for the front surface. Nevertheless, during high-temperature thermal oxidation, boron will be redistributed. Boron reaches a higher equilibrium concentration in SiO2 than in the silicon wafer, and the SiO2/Si interface shifts inward during SiO2 formation. Simultaneously, the high temperature during oxidation prompts the boron inside the silicon wafer to diffuse deeper. The dual effects of oxidation and diffusion cause boron to be depleted near the surface of the silicon wafer. Boron depletion leads to a reverse bending of the energy band, which hampers carrier collection and impedes further enhancement of cell power conversion efficiency. In this paper, the oxidation process was mainly adjusted to reduce surface boron depletion, resulting in approximately 0.09%abs. increase in power conversion efficiency compared to pre-optimization.