Approaches for reducing metallization-induced losses in industrial TOPCon solar cells

Minimizing carrier recombination in silicon solar cells is key to increase the conversion efficiency, as recombination affects both the fill factor and the open circuit voltage. Recombination at metal-semiconductor interfaces plays a crucial part in this, however, processing conditions which lead to low recombination, such as e.g., a low firing set temperature or the use of thick dielectrics, typically result in increased contact resistivities. Also, a too low firing set temperature leads to an incomplete hydrogenation of the interfaces. Recently, laser-enhanced contact optimization has been introduced to decouple recombination and contact properties to some extent, which allows for high fill factors and high open circuit voltages, and which explains the growing interest from manufacturers in that technology. We elucidate on the need for improved hydrogenation of interfaces, which contradicts the wish to decrease firing temperatures for reduced carrier recombination at metal-semiconductor interfaces. The implementation of an additional annealing step, e.g. in a tube furnace, after dielectric surface passivation is shown to lead to improved passivation properties so that the thermal budget during contact firing can be optimized to minimize contact resistivities. Overall, contact optimization allows for solar cell efficiencies of 24.1%, measured at an industrial cell tester, for a traditional approach without additional annealing step, and applying an AgAl front side metallization paste. A comparison of Ag and AgAl front side metallization pastes reveals a higher open circuit voltage for the Ag paste, at the drawback of an increased contact resistivity.