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, phosphorus- and 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 (R-sh) in the range of 40-400 Omega/sq, along with a dark saturation current density (J(o)) of 55 fA/cm(2). Subsequently, boron diffusion from solid BSG layer leads to p(+) -doped areas with high doping levels (R-sh = 50 Omega/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 SiOx layer. Furthermore, by adjusting the gas flows during POCl3 diffusion, n+ -doped areas with Rsh in the range of 150-300 Omega/sq are achieved. The corresponding surfaces feature Jo values of 30 fA/cm(2). 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 eta = 20%.