Development of Boron Emitters for Tunnel Oxide Passivated Contact Solar Cells with Current-Assisted Contact Formation

Front side recombination in tunnel oxide passivated contact solar cells is frequently described by the recombination parameters j0e for the passivated and j0e,met for the metallized front region. The combination of Al-free metallization pastes and current-assisted contact formation has shown significant reduction of j0e,met. Such new metallization approaches enable shallower doping profiles and higher sheet resistances Rsheet, which in turn also reduce j0e. In this study, we provide insight into how to reduce the overall front side recombination, by tailoring the boron dopant profile from atmospheric pressure BBr3 diffusion processes. The limitation of the dopant dose in the profile leads to a higher Rsheet of the resulting profile. The newly developed homogeneous boron emitter features j0e = 14 fA cm-2 at a Rsheet = 161 Ω sq-1, a 62% decrease from the initial value, and an even lower j0e = 8 fA cm-2 is determined for an emitter with 330 Ω sq-1. An increased contact resistivity on the front side still poses a challenge for Ag front side pastes despite the use of current-assisted contact formation. Application of an alternative AgAl paste in industrial-type fabricated TOPCon solar cells results in a maximum conversion efficiency of 24.5%.