Aluminum–silicon interdiffusion in silicon heterojunction solar cells with a-Si:H(i)/a-Si:H(n/p)/Al rear contacts

We characterize a-Si:H(i)/a-Si:H(n)/Al and a-Si:H(i)/a-Si:H(p)/Al contacts implemented on the rear side of silicon heterojunction solar cells. Electrical test structures and full-area solar cells employing these contacts demonstrate promising performance. For example, a-Si:H(i)/a-Si:H(p)/Al test structures with a 40 nm thick a-Si:H(p) layer that were annealed at 180 ^◦C had contact resistivities of 48 mΩ·cm² and implied open-circuit voltage losses after metallization of only 9 mV. Similarly, solar cells with full-area rear a-Si:H(i)/a-Si:H(n)/Al contacts that were annealed at 150 ^◦C had open-circuit voltages of 717 mV and contact resistivities of 9.4 mΩ·cm². For thinner doped a-Si:H layers and higher annealing temperatures, the contacts become less stable and performance degrades. Complementary transmission electron microscopy and energy-dispersive x-ray spectroscopy analysis show the Al–Si interactions at these interfaces that explain the range of exhibited performance. This analysis leads to a better understanding of the materials properties limiting the contact stability.