Passivated contacts to n⁺ and p⁺ silicon based on amorphous silicon and thin dielectrics

Carrier recombination at the metal contact regions has now become a critical obstacle to the advancement of high efficiency diffused junction silicon solar cells. The insertion of a thin dielectric interlayer - forming a metal-insulator-semiconductor (MIS) contact - is a known approach to reduce contact recombination. However, an insulator thickness less than 25 Å is usually required for current transport, making it difficult to simultaneously achieve good surface passivation. This paper compares standard MIS contacts to a newly developed contact structure, involving hydrogenated amorphous silicon (a-Si:H) over-layers. The contact structures are trialed on both n⁺ and p⁺ lightly diffused surfaces, with SiO₂ and Al₂O₃ insulator layers, respectively. In both cases significant improvements in the carrier-selectivity of the contacts is achieved with the addition of the a-Si:H over-layers. Simulations of idealized cell structures are used to highlight the performance and technological benefits of these carrier-selective structures over standard locally diffused contacts.