Impact of Selective BSF on Performance of Bifacial nPERT Cells with Ni/Ag Co-Plated Contacts

Abstract

In this work, we evaluate the impact of implementing a selective back surface field (BSF) on the performance of bifacial n-type Passivated Emitter and Rear Totally diffused (PERT) cells with Ni/Ag co-plated contacts. For this, we compare bifacial nPERT cells featuring a homogeneous BSF etched-back to ~100 Ω/sq and patterned by laser ablation to cells featuring an identical front side design and a selective BSF obtained by means of laser doping. In the first part of this paper, we demonstrate using test structures that selective BSF cells benefit from drastically lower area weighted recombination current density (J0) contributions at the BSF side resulting in estimated Voc improvement from ~657 mV to ~ 681 mV. In the second part of this paper, we confirm at cell level that implementing a selective BSF results in gains in Voc ~ 30 mV, jsc~ 0.2 mA/cm2, and Fill Factors FF ~ 0.9%abs. resulting in average cell efficiencies improving from 20.7±0.4 % to 22.0±0.2 %. This clearly confirms that a selective BSF is required in nPERT cells with Ni/Ag co-plated contacts to maximize cell efficiencies. Finally, we fabricated a 9-cells bifacial glass-glass laminate for proof-of-concept and discuss several possible improvements to bring 60-cells module front maximum power values under standard test conditions (STC) above 340 Wp.