Contact Interface Morphology of Screen-Printable Front-Side Contacts for Industrial N- TOPCon Crystalline Silicon Solar Cells

High-resolution TEM/STEM was used to characterize the microstructure and chemistry of the front-side contact interface for optimally prepared screen-printed industrial n- TOPCon crystalline silicon solar cells. A front-side screen-printable paste comprising silver, metallic aluminum, and an inorganic lead-silicate frit was designed to contact boron-diffused
$p^{+}$
emitter surfaces with
$\text{SiN}_{\mathrm{x}}:\text{II}-\mathrm{A}1_{2}\mathrm{O}_{3}$
antireflection-passivation layers. The final front-side bulk silver metal region microstructure shows isolated metallic aluminum particles surrounded by solidified liquid phase within the bulk sintered silver conductor line. TEM/STEM characterization of the front-side silver
$\text{meta}1-p^{+}$
boron-diffused emitter contact region shows continuous, amorphous interfacial films decorated with silver colloids located between the bulk conductor metal and emitter surface.
$\text{STEM}/\mathrm{E}$
DS analysis shows the amorphous interfacial films were metal oxide-based ions comprised of lead, aluminum, silicon and silver. The interfacial films were relatively thick compared to interfacial films observed in front-side screen-printed contacts for p-type c-Si cells. Free energy calculations indicate aluminum ions dissolved in the interfacial film do not participate in
$\text{SiN}_{\mathrm{x}}:\mathrm{H}$
etch-through. Microcopy observations and free energy calculations suggest the function of aluminum metal additions in the paste is to increase the number density of the Ag colloids in the IF film which is necessary to reduce contact resistance in the relatively thick interfacial films.