Investigation of the Screen-printable Ag/Cu Contact for Si Solar Cells Using Microstructural, Optical and Electrical Ana
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.438
Investigation of the Screen-printable Ag/Cu Contact for Si Solar Cells Using Microstructural, Optical and Electrical Analyses Keming Ren and Abasifreke Ebong University of North Carolina at Charlotte, 9201 University City Blvd, Charlotte, NC 28223, U.S.A.
ABSTRACT
In a bid to further reduce the cost of the front Ag contact metallization in Si solar cells, Cu is the potential alternative to replace the Ag in the Ag paste. However, this requires an understanding of the contact mechanism of screen-printable Ag/Cu paste in Si solar cell through rapid thermal process. The pastes with different weight percent of Cu (0 wt%, 25 wt% and 50 wt%) were used and the Voc of the cells was reduced with the increasing weight percent of Cu. This is because the presence of Cu in the paste changed the microstructure of the Ag/Cu/Si contact through Cu doping of the glass frits and hence increasing the Tg of the glass. The increased Tg of the glass impeded the uniform spreading of the molten glass and resulted in poor wetting and etching of the SiN x, which impacted the contact as evident in ideality factor of less than unity. This also led to the formation of agglomerated Ag crystallites with features of 700 nm in length and 200 nm in depth, which is close to the p-n junction, of which depth is ~300 nm. However, the interface glass layer acted as an effective diffusion barrier layer to prevent Cu atoms from diffusing into the Si emitter, which is quite remarkable for Cu not to diffuse into silicon at high temperature. Further investigation of the Ag/Cu contacts with the conductive AFM in conjunction with the SEM and STEM analyses revealed that the growth of Ag crystallites in the Si emitter is responsible for carrier conduction the gridlines as with the pure Ag paste. INTRODUCTION The front contact metallization of commercial Si solar cell is dominated by Ag metal due to its high conductivity and chemical stability. However, because of the high cost of Ag metal, metallization is the second most expensive part of solar cell fabrication after silicon. In order to reduce the cost of the solar cell, the amount of Ag in the screen-
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printable paste should be replaced with low-cost metal materials. Among metal candidates, Cu is considered a more viable alternative because of its conductivity (1.7 μΩ-cm) is quite close to that of Ag (1.6 μΩ-cm), but 100 times less costly [1-3]. The major challenge of using Cu for Si solar cell is the fast diffusion of Cu into the bulk Si at high temperature. The diffusion coefficient of Cu in Si at 500oC can be 2.0×10-5 cm2/s, which is 15 orders of magnitude higher than that of Al. The diffused Cu atoms in the Si form a defect band close to the middle of the Si bandgap, providing extremely efficient mi
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