Doped Self-Aligned Metallization for Solar Cells
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S. Ismat Shah Department of Physics and Astronomy and Department of Material Science and Engineering, University of Delaware, Newark, Delaware 19716
Robert Opila Department of Material Science and Engineering, University of Delaware, Newark, Delaware 19716
Allen M. Barnett, Kevin Allison, and Oleg Sulima AstroPower Inc., Newark, Delaware 19716 (Received 5 June 2003; accepted 17 October 2003)
Metal contacts using doped self-aligning metallization to 〈100〉 and 〈111〉 p-type silicon were investigated. Contacts formed in this manner allow the formation of a pn-junction and provide front metallization for photovoltaic applications. Formulated screen-printable thick films were annealed above Ag/Si eutectic temperature of 830 °C. The annealing process resulted in a junction depth of 0.3–1.1 m with improved Ag/Si metal contacts due to the reduction of parasitic native oxide layer via the use of a wetting agent. The technique inhibits shunts (high conductivity paths through the solar cell pn-junction caused by excessive metal penetration) due to limited solubility of Ag in Si. The technique also reduces series resistance (a parasitic resistance due to surface states that also limit solar cell performance) due to a robust thermal processing window. The use of magnesium (Mg) as a wetting agent in the thick film Ag matrix was explored. We observed a correlation between increased wetting and improved dark saturation current J02 in the absence of a pre-existing junction. I. INTRODUCTION
Metallization is one of the major cost determining and efficiency limiting steps in solar cell processing. In industrial processing of crystalline silicon, any metallization scheme depends on several factors. These include ease of deposition, good adherence, convenient annealing temperature, no interface reaction, minimal thermal mismatch, no surface tension effects during annealing, inertness to the ambient atmosphere, ease of defining contact geometry, good electrical and thermal behavior, and adaptability to bonding. The objective of this study is to develop an understanding of a phosphorus (P)-doped self-aligning metallization process for high performance, environmentally friendly, low-cost solar cell manufacturing. The process steps are analogous to the conventional screen printing process for forming contacts in that metal is applied in a gridline pattern and is followed by annealing. However, in the self-doping technique, the end result is different. For most conventional screen printable thick films, annealing is always carried out at temperatures below the eutectic temperature of the metal-Si system. MetallizaDOI: 10.1557/JMR.2004.0129 986
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 4, Apr 2004 Downloaded: 18 Mar 2015
tion employing a doped self-aligning technique requires the annealing/alloying step be carried out above the metal/Si eutectic temperature. In addition, the conventional screen printable thick films require a doped layer to be in place before the pastes are applied. However, the doped self-aligning met
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