Mechanism underlying the beneficial effect of forming gas annealing on screen-printed Ag contacts of crystalline Si solar cells

Forming gas annealing (FGA) has been known to be beneficial for improving the contact resistance of crystalline Si (c-Si) solar cells containing overfired front-side Ag contacts. This study examines the microstructural changes responsible for the beneficial effect of FGA through the use of ultrahigh-resolution scanning electron microscopy. The FGA process at 400°C under N2+10% H2 led to the formation of a layer of fine (<300nm) Ag particles on the glass surface of the interfacial pores, which were connected to the open channels in the porously sintered Ag bulk. Aside from the pore surfaces, FGA had little influence on the contact microstructure, and there was no noticeable formation of Ag or other metallic particles within the interfacial glass layer. The present results demonstrate that the Ag+ ions dissolved into the lead borosilicate glass during the firing process, and subsequently diffused out only to be reduced in the vicinity of the pore surfaces during FGA. This result suggests that the permeation rate of hydrogen into the lead borosilicate glass should be negligible, compared to the out-diffusion rate of the Ag+ ions during FGA. Furthermore, the conductivity measurements indicated that the pore surfaces that were decorated with a dense layer of fine Ag particles after FGA could act as an additional path for current transport across the contact interface, thereby lowering the contact resistance.