Implementation and understanding of p+ fired rear hole selective tunnel oxide passivating contacts enabling >22% conversion efficiency in p-type c-Si solar cells

Passivating contacts are key enablers for high efficiency c-Si solar cells. Here, we present our latest advancement in terms of implementation and understanding in fired passivating contacts (FPC) used as rear p + passivating contact in p-type solar cells. We study three different layer configurations and show that the microstructural layer properties play a major role on the surface passivation and charge carrier transport. Upon optimization we demonstrate implied open circuit voltage of 722 mV corresponding to saturation current density of ~7 fA/cm2 and contact resistances below 10 mΩ cm2, when metallized with ITO/Ag. P-type c-Si solar cells employing a screen-printed P-diffused emitter co-fired with the FPC on the rear side with conversion efficiency up to 22.5% are demonstrated. Temperature-dependent measurements on test structures and solar cells reveal that tunneling is the main transport mechanism for FPC layers that crystallize during the firing process, whereas for more amorphous FPC layers, an additional component due to thermionic emission is also present. Finally, we present the efficiency potential of solar cells employing the developed FPC layer as hole selective rear side passivating contact.