Codiffusion Sources and Barriers for the Assembly of Back-Contact Back-Junction Solar Cells

In this study, several diffusion sources are investigated, aiming at codiffusion for the fabrication of back-contact back-junction (BC-BJ) silicon solar cell. As a gaseous diffusion source, a POCl₃ -diffusion process is investigated, and for solid diffusion sources, phosphorusand boron-doped silicate glass (PSG and BSG) deposited by the means of plasma-enhanced chemical vapor deposition are considered. The n⁺ -doped areas diffused from a solid PSG layer allow for a precise adjustment of the sheet resistance (Rsh) in the range of 40-400 Ω/sq, along with a dark saturation current density (Jo) of 55 fA/cm². Subsequently, boron diffusion from solid BSG layer leads to p+ -doped areas with high doping levels (R_sh = 50 Ω/sq). However, gaseous POCl₃ diffusion in combination with solid boron diffusion from the BSG layer can only be successfully performed if the BSG layer is protected with an SiO_x layer. Furthermore, by adjusting the gas flows during POCl₃ diffusion, n⁺ -doped areas with Rsh in the range of 150- 300 Ω/sq are achieved. The corresponding surfaces feature J_o values of 30 fA/cm². The result of this study is a flexible codiffusion setup allowing for the efficient integration in advanced process chains of BC-BJ solar cells which results in the cell efficiencies well above above η = 20%.