Screen Printable Doped Self-Aligned Metallization for Solar Cells
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SCREEN PRINTABLE DOPED SELF-ALIGNED METALLIZATION FOR SOLAR CELLS Ernest A. Addo1, Ismat Shah1,2, Robert Opila1, Allen. M. Barnett3, Kevin Allison3, Oleg Sulima3 1. 2. 3.
Department of Material Science and Engineering, University of Delaware, Newark, DE. 19716 Department of Physics and Astronomy, University of Delaware, Newark, DE. 19716 AstroPower Inc, Newark DE. 19716
ABSTRACT Alloyed metal contacts to and p-type silicon (Si) were investigated using doped self-aligning metallization techniques [1]. The contact allows the formation of a pnjunction and provides metallization for photovoltaic applications via thick film technology [2]. A formulated screen-printable thick film was annealed above Ag/Si eutectic temperature of 830oC. The annealing process resulted in a junction depth of 0.3-1.2 µm and improved adhesion by the reduction of native oxide layer through use of a wetting agent. The technique inhibits shunts (high conductivity paths through the solar cell pn-junction caused by excessive metallization penetration). The technique also reduces the interfacial resistance due to dissimilar materials in contact – a parasitic resistance that also limit solar cell performance. The use of metallic Mg as a wetting agent additive in the thick film silver matrix was explored. Studies were carried out on screen printable thick films and the reducing effects of Mg were investigated using both air and reducing atmosphere (H2) annealing furnaces. We observed a correlation between increased alloying with reduced series resistance. INTRODUCTION Metallization is one of the major cost determining and efficiency limiting steps in solar cell processing. In industrial processing of crystalline silicon, the applicability of any metallization scheme depends on low resistivity, ease of formation, stability in high temperature oxidizing conditions, mechanical stability, good adherence, negligible reaction with final metallic interconnects, low contact resistance, minimal junction penetration, and low electromigration. The purpose of this work is to develop an understanding of phosphorus-doped, silver self-aligning metallization process for high performance, environmentally friendly and low-cost solar cell center frame manufacturing using doped Ag paste. One of the important considerations in this method is the mutual solubilities of Ag and Si. The binary alloy phase diagram of Ag-Si [3] shows a single eutectic at 830oC, with the invariant point at 96.9 % by weight of Ag. The individual melting points of elemental Ag and Si are 1415oC and 950oC, respectively. The maximum equilibrium solid Thickness [nm] solubility limit of Si in Ag is 0.93 atom percent at 830° 14 -3 and of Ag in Si is 7x10 cm [4]. While annealing, the dopant source which is an Fig.1. Junction Depth by depth organic based-phosphorus compound, decomposes and profiling using Anodic Oxidation diffuses into the substrate forming a n+ layer underneath the contacts that could be 0.3-1.2 µm deep, depending on time and temperature of annealing above the eutectic temper
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