Low resistance screen-printed Ag contacts to POCl3 emitters with low saturation current density for high efficiency Si solar cells

The silicon (Si) PV industry recognizes the value of phosphorus (P) emitters with low saturation current density (J_0e) for ability to produce high final cell open circuit voltage (V_OC). However, emitters of such quality, which usually display low surface phosphorus concentration ([P_surface]) are notoriously difficult to contact using traditional screen-printed silver (Ag) thick film pastes. Here, we tailored P emitter profiles via POCl₃ diffusion to create solar cell emitters displaying low J_0e values of 67 - 148 fA/cm² and variable electrically active [P^surface] of 0.5E20 - 2.0E20 atoms/cm³ in order to study the conditions necessary for low resistance contact to such emitters without appreciably deviating from industrial process conditions. Using a screen-printable Ag conductor paste tailored to contact low [P_surface] emitters, we show fill factor (FF) as high as 80% while maintaining V_OC as high as 637 mV on tailored emitters with low J_0e. This results in average solar cell efficiencies of 18.6% with a best efficiency of 18.8%. Series resistance (R_SERIES) analysis revealed that contact resistance was the major resistive component dictating final R_SERIES and FF. Finally, microstructural SEM analysis of the Ag/Si contact interface suggested that thin interfacial glass films and extensive Ag precipitate/crystallite surface coverage may explain how such high FF can be attained on emitters with low J_0e and low [P_surface].