Structural Investigation of Printed Ag/Al Contacts on Silicon and Numerical Modeling of Their Contact Recombination

Abstract

Ag/Al pastes allow for a sufficiently low contact resistivity of less than 5 mΩ cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> with boron-doped p+ emitters. A drawback of those pastes is an enlarged recombination at the silicon/metal interface below those contacts, compared with Ag pastes. For previous Ag/Al pastes from 2013, the observed recombination is even higher than theoretically expected for a fully metal-covered surface. Newly developed Ag/Al pastes allow for a significant reduction of the recombination below the contact, compared with a 2013 Ag/Al paste; for example, the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e,m et</sub> of an 92Ω/sq. p+ emitter has decreased from 3420 down to 1014 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> due to the newly developed paste. For an R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sheet</sub> of 137 Ω/sq, the J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e,met</sub> is 1399 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Structural investigations of those contacts reveal the microscopic appearance of the contacted region. There are contact spikes of metal grown into the silicon. Those spikes cover 1-1.2% of the entire printed finger area. With values for area fraction and depth of the spikes, we conduct simulations of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e,m et</sub> . With these simulations, we are able to explain the enlarged recombination at the contact interface and describe the experimentally measured J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e,m et</sub> for both Ag/Al pastes described in this paper.