Direct-Write Printing of Silver Metallizations on Silicon Solar Cells
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Direct-Write Printing of Silver Metallizations on Silicon Solar Cells C. J. Curtis, T. Rivkin, A. Miedaner, J. Alleman, J. Perkins, L. Smith, and D. S. Ginley National Renewable Energy Laboratory, Golden, CO 80401 ABSTRACT Direct-write technologies offer the potential for low-cost materials-efficient deposition of contact metallizations for photovoltaics. We report on the inkjet printing of metal organic decomposition (MOD) inks with and without nanoparticle additions. Near-bulk conductivity of printed and sprayed metal films has been achieved for Ag and Ag nanocomposites. Good adhesion and ohmic contacts with a measured contact resistance of 400 µΩ•cm2 have been observed between the sprayed silver films and a heavily doped n-type layer of Si. Silver deposited using the MOD ink burns through the Si3N4 antireflection coating when annealed at 850°C to form an ohmic contact to the n-Si underneath. An active solar cell device was fabricated using a top contact that was spray printed using the Ag MOD ink. Inkjet printed films show adhesion differences as a function of the process temperature and solvent. Silver lines with good adhesion and conductivity have been printed on glass with 100 µm resolution. INTRODUCTION A key area for improvement of photovoltaic cells is the development of low-cost materialsefficient process methodologies. Atmospheric process approaches potentially offer these advantages. Inkjet printing, as a derivative of direct-write processing, offers the additional advantages of low capitalization, very high materials efficiency, elimination of photolithography, and noncontact processing [1]. Conceptually, for Si solar cells, all device elements except the Si could be directly written or sprayed, including contact metallizations (front and rear), dopants, transparent conductors, and antireflection coatings. Our initial thrust has been in the area of developing contact metallizations. As the thickness of Si cells falls below 100 µm, contact grids for the front and rear contacts can be inkjet printed, even on the rough surface of polysilicon, without contacting the thin, fragile substrates. At present, inkjets are capable of line resolutions < 20 µm, which is at least two times better than the current state of the art obtained by screen printing [2]. In addition, it is an inexpensive, atmospheric process and can be an environmentally friendly, nowaste approach. A major challenge in applying inkjet processes for direct writing is formulating suitable inks. The inks must contain the appropriate precursors and a carrier vehicle. In addition, they may contain various binders, dispersants, and adhesion promoters, depending on the nature of the precursor and the particular application. In the case of inks for metallization, the content of the metallic ink must be adjusted to provide the required resolution, with good adhesion and the desired electronic properties for the conducting lines. Ink composition is critical because it defines the way in which the ink can be jetted, the adhesion to the substrate,
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