Diffusion profiles beneath silicon heterojunction contacts reduce contact resistivity and increase efficiency

Silicon heterojunction solar cells have historically had high open-circuit voltages due to the passivation provided by the intrinsic amorphous silicon layer, yet this same layer can also limit the fill factor of these devices. In comparison, diffused-junction solar cells have traditionally had higher fill factors than silicon heterojunction solar cells due to the low contact resistivity between the metal and doped surface of the wafer. By combining these two device architectures, it is possible to increase the fill factor - through a reduction in contact resistivity - while also maintaining a high open-circuit voltage with the passivating contact. In particular, we show through simulation and experiment that adding a diffusion under an amorphous silicon heterojunction contact reduces contact resistivity by approximately 0.04\ \Omega\text{cm}^{2}, and, in contrast to standard silicon heterojunction devices, the contact resistivity does not increase with the intrinsic amorphous silicon thickness. In addition, this contact allows for an efficiency boost of 0.56-0.85% absolute over our standard device structure.