Progress of plated metallization for industrial bifacial TOPCon silicon solar cells

Industrial tunnel oxide and passivated contact (i-TOPCon) solar cells were metallized at Fraunhofer ISE using ultrashort pulse laser ablation of the passivation layers for the subsequent Ni/Cu/Ag plating process. The solar cells feature a tunnel SiOx and n-type doped polysilicon layer covered by a SiNx at the rear side, whereas the front side is made of a boron emitter passivated with a AlOx/SiNx stack. The reference i-TOPCon solar cells screen-printed at the supplier reach an efficiency of 23.46% measured by Fraunhofer ISE CalLab. The impact of the laser process on the implied open circuit voltage (iVoc) is characterized showing minor impact on the TOPCon side, while the emitter side reveals an increased iVoc loss due to laser damage. Loss analysis by simulating the plated solar cells points out the benefit of reducing the laser contact opening (LCO) area in terms of shading and contact recombination. Optimization of laser ablation and hydrofluoric acid (HF) pretreatment process result in Voc > 700 mV and FF > 82% leading to a mean efficiency 23.6% measured in-house and a champion efficiency of 23.84% measured at Fraunhofer ISE CalLab thus outperforming the references by 0.4%abs.