Amorphous silicon enhanced metal-insulator-semiconductor contacts for silicon solar cells

摘要

Carrier recombination at the metal-semiconductor contacts has become a significant obstacle to the further advancement of high-efficiency diffused-junction silicon solar cells. This paper provides the proof-of-concept of a procedure to reduce contact recombination by means of enhanced metal-insulator-semiconductor (MIS) structures. Lightly diffused n~+ and p~+ surfaces are passivated with SiO_2/a-Si:H and Al_2O_3/a-Si:H stacks, respectively, before the MIS contacts are formed by a thermally activated alloying process between the a-Si:H layer and an overlying aluminum film. Transmission/scanning transmission electron microscopy (TEM/STEM) and energy dispersive x-ray spectroscopy are used to ascertain the nature of the alloy. Idealized solar cell simulations reveal that MIS(n~+) contacts, with SiO_2 thicknesses of ～1.55nm, achieve the best carrier-selectivity producing a contact resistivity ρ_c of ～3 mΩ cm~2 and a recombination current density J_(0_c_ of ～40 fA/cm~2. These characteristics are shown to be stable at temperatures up to 350 ℃. The MIS(p~+) contacts fail to achieve equivalent results both in terms of thermal stability and contact characteristics but may still offer advantages over directly metallized contacts in terms of manufacturing simplicity.