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

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 SiO₂/a-Si:H and Al₂O₃/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 SiO₂ thicknesses of ∼1.55nm, achieve the best carrier-selectivity producing a contact resistivity ρ_c of ∼3 mΩ cm² and a recombination current density J_0c of ∼40 fA/cm². These characteristics are shown to be stable at temperatures up to 350°C. 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.