Low resistance screen-printed Ag contacts to POCl<inf>3</inf> emitters with low saturation current density for high efficiency Si solar cells

Abstract

The silicon (Si) PV industry recognizes the value of phosphorus (P) emitters with low saturation current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> ) for ability to produce high final cell open circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> ). However, emitters of such quality, which usually display low surface phosphorus concentration ([P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">surface</sub> ]) are notoriously difficult to contact using traditional screen-printed silver (Ag) thick film pastes. Here, we tailored P emitter profiles via POCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> diffusion to create solar cell emitters displaying low J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> values of 67 - 148 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and variable electrically active [P <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">surface</sup> ] of 0.5E20 - 2.0E20 atoms/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">surface</sub> ] emitters, we show fill factor (FF) as high as 80% while maintaining V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> as high as 637 mV on tailored emitters with low J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> . This results in average solar cell efficiencies of 18.6% with a best efficiency of 18.8%. Series resistance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SERIES</sub> ) analysis revealed that contact resistance was the major resistive component dictating final R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SERIES</sub> 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0e</sub> and low [P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">surface</sub> ].