Efficiency enhanced emitter wrap-through (EWT) screen-printed solar cells with non-uniform thickness of silicon nitride passivation layer in via-holes

Emitter wrap-through (EWT) solar cells with an 18.6% cell efficiency were fabricated by etch-back with etch resist, a selective emitter (SE) with reactive ion etching (RIE) and screen printing technologies. The short-circuit current densities (Jsc) were more than 40 mA/cm2, but the cell efficiency was limited by the fill factor (FF), which was as low as 73.4%. An electroluminescence (EL) analysis showed that this low value was linked to shunts occurring in the via-holes. The doping profile difference between the wafer surface and the inside of the via-holes was statistically insignificant. However, the thickness of the deposited SiNx passivation layer in the via-holes decreased from the surface to the inner side, allowing the paste to penetrate the junction through the passivation layer and causing shunts during the firing process. By changing to Ag paste for the shallow emitter, changing the firing conditions and performing a process optimization, an efficiency of 19.5% with a FF of 77.6% was achieved using industrial 6-inch. Cz wafers under AM 1.5G.

► We fabricated EWT solar cells using etch-back and screen printing process.
► Selective emitter was introduced by reactive ion etching.
► Junction depth and SiNx thickness in the via-hole were measured by LV-SEM.
► Low FF of EWT solar cell was originated from thin SiNx layer in the via-holes.
► Cell efficiency was increased from 18.6% to 19.5% by changing Ag paste and firing condition.