Ag, Cu, and Ni metallizations were inkjet printed with near vacuum deposition quality. The approach developed can be easily extended to other conductors such as Pt, Pd, Au, etc. Thick highly conducting lines of Ag and Cu demonstrating good adhesion to glass, Si, and printed circuit board (PCB) have been printed at 100-200 deg C in air and N2 respectively. Ag grids were inkjet-printed on Si solar cells and fired through the silicon nitride AR layer at 850 deg C, resulting in 8% cells. Next generation inks, including an ink that etches silicon nitride, have now been developed. Multi-layer inkjet printing of the etching ink followed by Ag ink produced contacts under milder conditions and gave solar cells with efficiencies as high as 12%

Curtis, C. J.

Ginley, D. S.

Kaydanova, T.

Miedaner, A.

Smith, L.

van Hest, M.

UNT Digital Library

 A national laboratory of the U.S. Department of EnergyOffice of Energy Efficiency & Renewable EnergyNational Renewable Energy Laboratory Innovation for Our Energy Future Multi-Layer Inkjet Printed Contacts to Si C.J. Curtis, M. van Hest, A. Miedaner,  T. Kaydanova, L. Smith, and D.S. Ginley  Presented at the 2005 DOE Solar Energy Technologies  Program Review Meeting November 7–10, 2005 Denver, Colorado Conference Paper NREL/CP-520-38943 November 2005 NREL is operated by Midwest Research Institute ● Battelle     Contract No. DE-AC36-99-GO10337  NOTICE The submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), a contractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the US Government and MRI retain a nonexclusive royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for US Government purposes. This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.  Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Available electronically at http://www.osti.gov/bridgeAvailable for a processing fee to U.S. Department of Energy and its contractors, in paper, from: U.S. Department of Energy Office of Scientific and Technical Information P.O. Box 62 Oak Ridge, TN 37831-0062 phone:  865.576.8401 fax: 865.576.5728 email:  mailto:reports@adonis.osti.govAvailable for sale to the public, in paper, from: U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 phone:  800.553.6847 fax:  703.605.6900 email: orders@ntis.fedworld.gov online ordering:  http://www.ntis.gov/ordering.htmPrinted on paper containing at least 50% wastepaper, including 20% postconsumer waste  Multi-Layer Inkjet Printed Contacts to Si Calvin J. Curtis, Maikel van Hest, Alex Miedaner, Tanya Kaydanova, Lee Smith, and David S. Ginley National Renewable Energy Laboratory, Golden, Colorado, calvin_curtis@nrel.gov ABSTRACT Ag, Cu, and Ni metallizations were inkjet printed with near vacuum deposition quality. The approach developed can be easily extended to other conductors such as Pt, Pd, Au, etc. Thick highly conducting lines of Ag and Cu demonstrating good adhesion to glass, Si, and printed circuit board (PCB) have been printed at 100-200oC in air and N2 respectively. Ag grids were inkjet-printed on Si solar cells and fired through the silicon nitride AR layer at 850oC, resulting in 8% cells. Next generation inks, including an ink that etches silicon nitride, have now been developed.  Multi-layer inkjet printing of the etching ink followed by Ag ink produced contacts under milder conditions and gave solar cells with efficiencies as high as 12%. 1. Objectives Inkjet printing is rapidly becoming a viable alternative to the existing deposition approaches for a variety of inorganic and organic electronic materials1. For metallizations, with appropriate inks, it can replace vacuum deposition, screen printing, and electroplating. The NREL research effort is focused on inkjet printed metallic conductors and other materials necessary to form high-quality contacts.  The advantage of inkjet printing is that it is an atmospheric process capable of resolution higher than in screen printing (features as small as 5µm have been produced using an inkjet). It is a non-contact, potentially 3D deposition approach, which makes it ideally suited to processing thin and fragile substrates.  The composition of the inks can easily be tailored by the addition of elements such as adhesion promoters and doping compounds to optimize mechanical and electronic properties of the subsequently processed contact. In addition, inkjet printing is inherently suited for printing multilayer/multi­component structures.  Development of inkjet printing for deposition of high-quality solar cell contacts contributes directly to achievement of the objectives of the Solar Energy Program for improving performance and reducing the cost of PV devices. 2. Technical Approach Metal-organic compounds of Ag, Cu, and Ni dissolved in organic solvents (proprietary compositions) were used as precursor inks for inkjet and spray printing of metallic layers and patterns. These inks were printed in air or N2 on a variety of substrates including glass, Si, and PCB.  Ag contact grids were inkjet printed onto SiNx-coated Si ribbon p/n junctions supplied by Evergreen Solar, Inc. The inkjet printing apparatus is pictured in Fig. 1. It consists of a stationary drop-on-demand piezoelectric inkjet print head from Microfab Technologies with a 50­micron orifice.  A resistive substrate heater plate positioned on an X-Y stage directly under the inkjet serves to provide heating and x-y positioning to 1 µm. The printed pattern, substrate position and translation speed, as well as the print head driving parameters are controlled through a LabView interface.  Fig. 1. Inkjet print head positioned over a glass substrate. Additional inks (proprietary compositions) were developed to provide an etching agent to facilitate penetration of the SiNx AR coating during processing and promote contact formation.  These inks were printed on Evergreen substrates and annealed at various temperatures to optimize the etching rate and profile. 3. Results and Accomplishments 3.1 Printed Metal Inks Thick (up to 15µm), highly conducting lines of Ag and Cu were printed on a variety of substrates, demonstrating good adhesion to glass, Si, and PCB. The inkjet parameters for Ni printing have not yet been optimized.  In general, we found that the best Cu deposits were obtained in an inert atmosphere (N2 or Ar). However, pure Cu coatings were also obtained in air using rapid thermal processing. Typical resistivities for the metallic coatings were: 2 µOhm*cm for Ag, 7 µOhm*cm for Cu and 100 µOhm*cm for Ni. The resistivity of the Ag layer is essentially that of the bulk metal; the Cu lines are about five times more resistive than bulk, while spray-printed Ni layers show approximately an order of magnitude higher resistivity 1than the bulk value. Improving the conductivities of printed Cu and Ni metallizations is an area of active investigation. 3.2 Inkjet Printed Ag Grids on Si Solar Cells Silver lines 250 µm wide and 10 µm thick were inkjet printed on SiNx-coated Si ribbon p/n junctions provided by Evergreen Solar. Al back contacts 1µm thick were deposited by e-beam evaporation. The two contacts were co-fired in a single annealing step at 850oC for 10 min in air, forming a solar cell with 8% efficiency, Voc=0.529V, Jsc= 22.67mA and a fill factor of 0.65. In this experiment, the ohmic contact between Ag and Si was formed through the SiNx layer without the use of glass frits. The high temperature and long time required for the penetration of the Ag through the AR coating2 can be detrimental for the junction. Facilitating the process of burning through the AR coating is desirable to lower the temperature and time of annealing for inkjet printed contacts.   Table 1 summarizes the processing and performance for Si cells with inkjet printed contact grids.  The first generation is the cell just described, the second generation used Ag ink modified for better line width, and the third used the etching under layer described below. Table 1.  Summary of line dimensions, processing temperatures and efficiencies for Si cells with inkjet printed contacts. Generation 1st 2nd 3rd Line thickness 10 μm 15 μm 15 μm Line width 400 μm 250 μm 220 μm Deposition temp 180°C 180°C 180°C Annealing temp 850°C 850°C 750°C Efficiency 8% 8% 10% 3.3 Ag Grids with Etching Under Layer A significant advantage of inkjet printing is that it allows multi-layer printing so that separate writing of the contact formation layer and then the metal forming layer is possible, leading to more control of the contact formation process and improved conductivity of the conductor lines. Next generation multi-component inks (including surface modifying agents) have been developed to obtain improved fire-through contacts. These proprietary inks greatly improve the burn through and contact formation process.  Fig. 2 depicts a 1µm deep, 70µm wide etch pattern obtained by inkjet printing an ink containing a proprietary etching agent on a SiNx coated Si substrate, followed by thermal processing at 750oC for 10 min.  Complete penetration of the SiNx layer was observed at temperatures as low as 500oC. Fig. 2. AFM image of the 1µm deep, 70µm wide etch pattern produced by the inkjet-printed “fire-through” agent on an AR- coated Si wafer. Experimental solar cells have now been fabricated using this process. These cells were formed by sequential printing of the etching agent layer followed by the deposition of the Ag lines from organometallic precursors as described above. Back contacts were screen-printed using Al paste. However, annealing of the structure has proven to be more difficult than anticipated. Short (40 sec) anneals at 550°C in our lamp RTP furnace have yielded poor results due to non-uniform overheating and penetration of the contact layer too deep into the Si substrate. Short anneals in a conventional furnace have given better results (Table 1, 3rd generation), but the lack of good control of the time-at-temperature has limited the performance of these cells.  However, cell efficiencies as high as 12% have now been observed.  Further development of the ink composition and optimization of the annealing process are underway and should result in improved efficiencies. 4. Conclusions An atmospheric, direct-write deposition technique has been developed for metals including Ag, Cu, and Ni. Line widths, conductivities and thicknesses are comparable to or better than those produced by screen printing. It has also been demonstrated that new inks and multi-layer printing can improve the contacting process for Si photovoltaics and lead to better performance. ACKNOWLEDGEMENTS This work was supported by the U.S. Department of Energy under contract DE-AC36-99-GO10337. REFERENCES 1 D. B. Wallace, W. R. Cox, and D. J. Hayes, in DirectWirte Technologies for Rapid Prototyping Applications, edited by A. Pique and D. B. Chrisey (Academic Press, 2002), p. 177.2 T. Rivkin, C. Curtis, A. Miedaner, J. Perkins, J. Alleman, and D. Ginley. Direct Write Processing for Photovoltaic Cells. Paper presented at IEEE 29th Photovoltaics Specialists Conference, New Orleans, Louisiana, 2002. 2 REPORT DOCUMENTATION PAGE Form Approved OMB No. 0704-0188 The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION. 1. REPORT DATE (DD-MM-YYYY) November 2005 2. REPORT TYPE Conference Paper 3. DATES COVERED (From - To)       5a. CONTRACT NUMBER DE-AC36-99-GO10337 5b. GRANT NUMBER  4. TITLE AND SUBTITLE Multi-Layer Inkjet Printed Contacts to Si 5c. PROGRAM ELEMENT NUMBER  5d. PROJECT NUMBER NREL/CP-520-38943 5e. TASK NUMBER PVA6.4201 6. AUTHOR(S) C.J. Curtis, M. van Hest, A. Miedaner, T. Kaydanova, L. Smith,  and D.S. Ginley 5f. WORK UNIT NUMBER  7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) National Renewable Energy Laboratory 1617 Cole Blvd. Golden, CO 80401-3393 8. PERFORMING ORGANIZATION REPORT NUMBER NREL/CP-520-38943 10. SPONSOR/MONITOR'S ACRONYM(S) NREL 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)  11. SPONSORING/MONITORING AGENCY REPORT NUMBER  12. DISTRIBUTION AVAILABILITY STATEMENT National Technical Information Service U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 13. SUPPLEMENTARY NOTES  14. ABSTRACT (Maximum 200 Words) Ag, Cu, and Ni metallizations were inkjet printed with near vacuum deposition quality. The approach developed can be easily extended to other conductors such as Pt, Pd, Au, etc. Thick highly conducting lines of Ag and Cu demonstrating good adhesion to glass, Si, and printed circuit board (PCB) have been printed at 100-200oC in air and N2 respectively. Ag grids were inkjet-printed on Si solar cells and fired through the silicon nitride AR layer at 850oC, resulting in 8% cells. Next generation inks, including an ink that etches silicon nitride, have now been developed.  Multi-layer inkjet printing of the etching ink followed by Ag ink produced contacts under milder conditions and gave solar cells with efficiencies as high as 12%. 15. SUBJECT TERMS Photovoltaics; solar; inkjet printing; PV; NREL 16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON  a. REPORT Unclassified b. ABSTRACT Unclassified c. THIS PAGE Unclassified 17. LIMITATION OF ABSTRACTUL 18. NUMBER OF PAGES  19b. TELEPONE NUMBER (Include area code)  Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18 

Multi-Layer Inkjet Printed Contacts to Si

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Multi-Layer Inkjet Printed Contacts to Si

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