Low-Resistance Hole Contact Stacks for Interdigitated Rear-Contact Silicon Heterojunction Solar Cells

Achieving low contact resistivity for the p contact in silicon heterojunction SHJ solar cells is challenging when classic n type transparent conductive oxides TCOs , such as indium tin oxide ITO , are used in the contact stack. Here, we report on SHJ solar cells with interdigitated back contact IBC and a direct aluminium Al metallisation applied to the p contact. We find that carefully annealing an Al a Si H p p type amorphous silicon contact at moderate temperatures leads to a specific contact resistivity that is half as low as its silver Ag ITO counterpart. This is explained by Al diffusing into a Si H p upon temperature treatment, forming a partially crystallised aluminium silicide layer. For a sufficiently high doping level in a Si H p , this enables an efficient tunnel recombination of holes from a Si H p to the Al contact. An estimate for this tunnelling dominated specific contact resistivity is calculated as a function of the interface doping density. Best fabricated IBC SHJ solar cells with Al p contact yield a fill factor of 77.5 and a power conversion efficiency of 22.3 . The main differences to devices with an Ag ITO a Si H p contact stack are a decrease in open circuit voltage by 14 mV and a slightly higher series resistance Rs . While the first aspect can be ascribed to increased interface recombination, the second one is unexpected and requires further investigation. Interestingly, omitting an intermediate TCO does not lead to current losses in devices with Al contacts, which is further investigated by optical simulations. Finally, electrical equivalent circuit simulations are conducted to describe the electrical behaviour of the investigated devices