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

Abstract

Front side recombination in tunnel oxide passivated contact solar cells is frequently described by the recombination parameters j_0e for the passivated and j_0e,met for the metallized front region. The combination of Al-free metallization pastes and current-assisted contact formation has shown significant reduction of j_0e,met. Such new metallization approaches enable shallower doping profiles and higher sheet resistances R_sheet, which in turn also reduce j_0e. 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 R_sheet of the resulting profile. The newly developed homogeneous boron emitter features j_0e = 14 fA cm⁻² at a R_sheet = 161 Ω sq⁻¹, a 62% decrease from the initial value, and an even lower j_0e = 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%.