From PERC to Tandem: POLO- and p+/n+ Poly-Si Tunneling Junction as Interface Between Bottom and Top Cell

We present a novel cell concept that combines the tandem cell approach with the passivated emitter and rear cells (PERC) mainstream technology. As an interface between Si bottom and top cell, we utilize passivating n ⁺ -type polysilicon on oxide (POLO) contacts and a p ⁺ poly-Si/n ⁺ poly-Si tunneling junction. Our full area PERC+ Si bottom cells are fabricated within a typical industrial process sequence where the POCl3 diffusion and SiNx deposition are replaced by the POLO junction formation processes. The implied open-circuit voltage iVoc that is measured on these devices reaches up to 708 mV (684 mV) under 1 sun (under filtered spectrum to simulated top cell absorption). On sister cells with planar front side, the respective iVoc values are 718 mV (696 mV). In order to understand the device physics of our ultra-abrupt p ⁺ poly-Si/n ⁺ poly-Si tunneling junction, we determined the carrier lifetime in the poly-Si by time-resolved photoluminescence. The extracted lifetimes of 42-54 ps enter as input parameter for numerical Sentaurus Device simulations. These simulations reveal the importance of band-to-band and trap-assisted tunneling for a low tunneling junction resistivity of 2.95 mΩ·cm ² . Experimentally, an upper limit for the combined junction resistance of the p ⁺ poly-Si/n ⁺ poly-Si/SiOx stack of 100 mΩ·cm ² is determined.