Tunnel oxide passivating electron contacts as full-area rear emitter of high-efficiency p-type silicon solar cells

Tunnel oxide passivating contacts (TOPCon) consisting of an ultrathin tunnel oxide capped by a doped Si film exhibit excellent passivation and contact properties. The application of these contacts has so far resulted in efficiencies of up to 25.7% realized with an n-type Si solar cell featuring a front-side boron-doped p(+) emitter and n-TOPCon as full-area rear electron contact. In this work, we study the same cell structure on p-type Si. In this case, the p(+) diffusion on the front acts as a front surface field (FSF) and the n-TOPCon layer as a full-area rear emitter. One benefit of this rear-junction cell design is that the whole base contributes to the hole transport towards the local contacts on the front, which means that the lateral current transport within the FSF is less important than in the case of the front emitter of the n-type cell. To study this, we addressed the influence of the FSF lateral conductivity on the performance of these rear-junction cells theoretically (based on a simulation study) as well as experimentally (with fabricated cells). Efficiencies up to 24.3% (independently confirmed) have been achieved with this structure applying a FSF and up to 23.9% without the full-area FSF. As such, these results demonstrate a high device performance for these TOPCon rear emitter cells even without a lateral conductivity in the FSF. This bears the potential to simplify the process chain quite substantially as no full-area boron diffusion is required.