Using direct-write approaches in photovoltaics for metallization and contact formation can significantly reduce the cost per watt of producing photovoltaic devices. Inks have been developed for various materials, such as Ag, Cu, Ni and Al, which can be used to inkjet print metallizations for various kinds of photovoltaic devices. Use of these inks results in metallization with resistivities close to those of bulk materials. By means of inkjet printing a metallization grid can be printed with better resolution, i.e. smaller lines, than screen-printing. Also inks have been developed to deposit transparent conductive oxide films by means of ultrasonic spraying

Curtis, C. J.

Ginley, D. S.

Hersh, P.

Kaydonova, T.

Miedaner, A.

Pasquarelli, R. M.

van Hest, M. F. A. M.

van Hest, M.

Curtis, C.

Pasquarelli, R.

Kaydanova, T.

Ginley, D.

UNT Digital Library

Direct-Write Contacts: Metallization and Contact FormationMaikel van Hest, Calvin Curtis, Alex Miedaner, Robert Pasquarelli, Tanya Kaydanova, Peter Hersh, and David GinleyNational Renewable Energy Laboratory  •  Golden, CO  80401Goal: Low cost Photovoltaics Deposition Tools Ink Requirements for Materials PrintingSi Solar Cell Contact Formation Metal InksSummaryPrinted Silver and NickelPrinted Silver Contacts on Silicon Printed Silver Lines on P3HT/PCBMMetal Contacts for CIGSDirect-writeHow to reduce cost? Eliminate expensive processing steps• Vacuum based thin film deposition steps• Steps that involve patterningUse less expensive materials • Other substratesReduce the amount of material used• Reduce thickness of absorber• Reduce or eliminate wasteCritical parameters to be optimized:•Electrical Properties•Resolution•Deposition Rate•Adhesion•Quality of interfaceOptimization of ink composition and processing parameters(Application specific)byPrintable: • Proper physical characteristics• Clean formation of desired materialFujifilm Dimatix Materials Printer Trident inkjet system  Sono-TekUltrasonic spray systemAll systems:• fully automated substrate control (X-Y)• substrate heating Si3N4 - Anti-Reflection coatingSi p-n junctionAg fingers npAl back contactnpRapid, High-Temperature Contact formationLines printed: • at ~180ºC-220ºC• in air• no post printing processingInk: MO Ag complex + solvent + surfactant ~25% Ag loadingSilver500 micronsNickel GridNickelInk: MO Ni complex + solvent + surfactant ~20% Ni loadingAg on SiNx AR Coated SiAg on Ni Lines on GlassAg on Ni FoilNi on Glass• Best resistivity obtained for silver: 2.1 µΩ⋅cm• Nickel can be improved by changing atmosphere• Also printed copper with resistivity close to bulk Line thickness: 10 µm 15 µm   10 µm      15 µmLine width: 400 µm 250µm  80µm    <100µmDep. temperature : 180°C 180°C   180°C 180°C Ann. temperature: 850°C   850°C  750°C   <750°CCell efficiency            8% 8% 13%+ 15%+1st generation 2nd generation1st 2nd GoalBestBest195ºC, 50 passes175ºC, 10 passes175ºC, 50 passesResistivity: • at 195 ºC good ~2x Bulk• Increases with decreasing printing temperature• at 160 ºC <10x Bulk Lines:• ~5µm thick @ 50 passes• Good resolution <100 µm • Changed wetting vs SiBurn-Through materialNi Line Target: • Uniform thin and narrow lines• Good adhesion to substrateAg Line Target:• thicker, narrow line, near bulk-material conductivity• adhesion to Ni lineCIGS absorber layerMo back contactGlass substrateZnO (TCO) / CdSNi adhesion layerAg contact gridThe information contained in this poster is subject to a government license.Presented at the 33rd IEEE Photovoltaic Specialists Conference •  San Diego, CA  • May 11–16, 2008  •  NREL/PO-520-43307 • Line width independent of number of layers• Continuous thickness increase• Lines <50µm wide obtained• Silver drops and lines on SiNx coated silicon• Drops 30 - 40 µm diameter• Lines 30 -40 µm wide @ 10 µm thick  Demonstrated inkjet printing of silver and nickel• Resistivity close to bulk• Resolution comparable or better than screen-printing• Low deposition temperature (<200 ºC)• No post processing needed• Can be printed on various substratesAlso inks available for copper and aluminumInks have also been developed for:• Semiconductors:  CdTe and Cu(In,Ga)Se2• Metal oxides:  BaxSr1-xTiO3 (BST), ZnO, In2O3, IZO, SnO2 and SiOxNo nanoparticles!

Direct-Write Contacts: Metallization and Contact Formation (Poster)

 A national laboratory of the U.S. Department of EnergyOffice of Energy Efficiency & Renewable EnergyNational Renewable Energy Laboratory Innovation for Our Energy Future Direct-Write Contacts: Metallization and Contact Formation  Preprint  M.F.A.M. van Hest, C.J. Curtis, A. Miedaner, R.M. Pasquarelli, T. Kaydanova, P. Hersh,  and D.S. Ginley  National Renewable Energy Laboratory Presented at the 33rd IEEE Photovoltaic Specialists Conference San Diego, California May 11–16, 2008 Conference Paper NREL/CP-520-42535 May 2008 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/bridge Available 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.gov Available 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.htm Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste DIRECT-WRITE CONTACTS:  METALLIZATION AND CONTACT FORMATION   Maikel F.A.M. van Hest, Calvin J. Curtis, Alex Miedaner, Robert M. Pasquarelli, Tanya Kaydanova, Peter Hersh, and David S. Ginley National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 USA   ABSTRACT  Using direct-write approaches in photovoltaics for metallization and contact formation can significantly reduce the cost per watt of producing photovoltaic devices. Inks have been developed for various materials, such as Ag, Cu, Ni and Al, which can be used to inkjet print metallizations for various kinds of photovoltaic devices. Use of these inks results in metallization with resistivities close to those of bulk materials. By means of inkjet printing a metallization grid can be printed with better resolution, i.e. smaller lines, than screen-printing. Also inks have been developed to deposit transparent conductive oxide films by means of ultrasonic spraying.   INTRODUCTION   In an effort to reduce the cost and improve efficiency of photovoltaics, alternatives to conventional processing techniques are becoming a necessity. One of the most attractive alternative approaches is the application of direct-write approaches for depositing contact metallization. Using inkjet printing as the direct-write technique has multiple benefits over the currently used screen-printing or photolithographic techniques. The first benefit is that inkjet printing is a non-contact technique. As wafer and ribbon Si cells get thinner this is especially attractive allowing higher process yield. This is also potentially useful for textured or thin films on plastics where conformal coating is difficult by contact methods. The second benefit of using inkjet printing is that the resolution of the contact grid lines can be improved compared to screen-printing. Where typical line widths for screen printing are around 100 µm, line widths using inkjet printing can be reduced to less than 50 µm, which significantly reduces the shadow losses by the contact grid and therefore increasing the overall cell efficiency.  APPROACH  Multiple inks have been developed that can be used to inkjet print metal grids. Silver, copper, nickel, and aluminum metal organic decomposition (MOD) inks have been developed that will form pure metal at low temperature (<200ºC) with near bulk conductivity [1,2]. A modified Fujifilm-Dimatix material printer (see fig. 1) has been used to deposit line patterns. For initial testing of ink an ultrasonic spray system is used to print uniform films (see fig. 1).     Fig. 1: Fujifilm-Dimatix Materials Printer (top), inkjet head and ink cartridge (center) and ultrasonic spray nozzle (bottom).  1 Ultrasonic spraying can also be used to deposit large area contacts, such as back contacts. Metal grid patterns are printed on a hot substrate in order to obtain the best conductivity. Printing on a hot substrate (T<200ºC) has the benefit of not needing a secondary anneal step in order to obtain pure and conductive material [3]. This is for example suitable for contact to TCOs and organic photovoltaics directly.  RESULTS  Metalization   Fig. 2 shows the line dimensions for printed silver and nickel as a function of the number of layers printed. It can be seen that the line width is almost independent of the number of layers, therefore making it very easy to thicken the material to the desired thickness. In this example the line width for nickel and silver are 80 µm and 110 µm respectively.     Fig. 2: Line width, thickness (top) and resistivity (bottom) as function of the number of layers printed using the nickel (A) and silver (B) inks.  Also it can be seen in Fig. 2 that the thickness increases continuously with the number of layers. Even though in the shown example that the thickness per pass is of the order of 50 nm increasing the MOD precursor loading concentration in the ink and decreasing the drop spacing can increase this number to about 1µm per pass. By optimizing the printing conditions the line width can be significantly reduced to our currently best of around 30-40 µm (see fig. 3). This number can be improved even more by changing the nozzle diameter on the inkjet system. This however would reduce the line thickness per pass due to a reduction in droplet diameter and volume.   The resistivity of the lines as function of the number of passes is also shown in fig. 2. It can be seen that for silver a resistivity of 3x10-6 Ωcm is obtained, which is close to that of bulk silver (1.6x10-6 Ωcm). Optimizing the printing conditions has obtained resistivity as low as 2x10-6 Ωcm for silver. The resistivity of inkjet printed silver lines is significantly better than that of screen printed silver lines, which are typically in the range of 5 to 10 times bulk resistivity. In the case of printed nickel the resistivity does not reach number as close to bulk as for silver. This is most likely due to the formation of small amounts of nickel oxide. Using the nickel ink in an inert environment will likely reduce the resistivity significantly.    Fig. 3: Micrograph images of inkjet printed silver lines using a metal-organic decomposition precursor. Lines and individual drops printed on SiNx-coated ribbon silicon.  Screen-printing pastes contain frit material that facilitates the burn-through process of the printed grid lines through the silicon nitride anti-reflection coating typically present on top of a silicon photovoltaic device. An ink to replace this frit material has been developed that can be printed in between the silicon nitride and the silver grid lines (see fig. 4) to facilitate burn-through. This ink will etch through the silicon nitride at relatively low temperatures (<750ºC). Using the burn-through ink in combination with the silver ink has produced silicon-based cells with efficiencies similar to or better than those produced using conventional screen-printed contacts. The best contact resistance for inkjet printed contacts using inkjet printed burn-through material and inkjet printed silver was found to be 7 mΩcm2 which is below that of screen-printed contacts. Using the burn-through layer approach also reduces the amount of  2silver that is needed since the inkjet printed silver has better conductivity than the screen-printed alternative.     Fig. 4: Schematic of using a separate burn-through material in combination with pure silver to produce contacts for silicon photovoltaic devices with SiNx anti-reflection coating.  Printed contacts are nor limited to silicon-based photovoltaics, but they can also be used for contacts to CuInSe2 and organic thin film photovoltaic devices. Also it should be possible to replace silver with a less costly material such as copper. If copper is used for metallization on silicon photovoltaic devices a buffer layer of some kind needs to be introduced to prevent copper from diffusion into the silicon, which would negatively affect the efficiency. A candidate buffer material would be nickel. Changing metallization material from silver to a cheaper material would contribute to a reduction in production cost of photovoltaic devices.  Transparent Conductive Oxides  Another field for which inks have been developed are the deposition of transparent conductive oxides (TCOs). TCO layers are used in thin film photovoltaic devices, such as CIGS, CdTe and organic photovoltaics. TCOs are also used in heterojunction wafer silicon devices and as an alternative to the SiNx anti-reflection coating. The later has the potential to reduce the number of grid lines and therefore reducing the shadow loses on a device. Using a non-vacuum technique for the deposition of TCOs will contribute to a reduction in production cost. Inks have been developed for a variety of TCOs, e.g. amorphous indium zinc oxide and doped zinc oxides. Initial testing is underway and the results are very promising. Conductivities comparable to vacuum deposition processes can be obtained [4,5].   CONCLUSIONS  Using direct-write approaches in photovoltaics for metallization and contact formation can significantly reduce the cost per watt of producing photovoltaic devices. This is accomplished in two ways. First the direct write deposition techniques can be used under atmospheric conditions, which eliminates the need of costly vacuum deposition techniques. Second better resolution in comparison to screen-printing can be obtained, resulting in higher efficiency devices.   This work was supported by the U.S. Department of Energy under Contract No. DE-AC36-99GO10337 with the National Renewable Energy Laboratory.  REFERENCES  [1] T. Rivkin, C.J. Curtis., A. Miedaner, J. Alleman, D. L. Schulz and D. S. Ginley, “Copper and silver inks for inkjet printing.”, 198th International Symposium of the Electrochemical Society., 2000.  [2] T. Rivkin, C.J. Curtis., A. Miedaner, J. Perkins, J. Alleman, and D.S. Ginley, “Direct Write Processing  for Photovoltaic Cells.”, IEEE 29th Photovoltaics  Specialists Conference. 2002 [3] A.L. Dearden, P. J. Smith,, D.-Y. Shin, N. Reis, and P. O’Brein, “A Low Curing Temperature Silver Ink for Use in Ink-Jet Printing and Subsequent Production of Conductive Tracks”, Macromol. Rapid Commun, 2005, 26,  pp. 315-318 [4] G.G. Choi, S.-J. Seo, and B.-S. Bae, “Solution-Processed Indium-Zinc-Oxide Transparent Thin-Film Transistors”, Electrochem. Solid State Lett., 2008, 11 pp. H7-H9 [5] M.L. de la Olvera, H, Gomez, and A. Maldonado, “Doping, vacuum annealing, and thickness effect on the physical properties of zinc oxide films deposited by spray pyrolysis”, Solar Energy Mater. and Solar Cells, 2007, 91, pp. 1449-1453   3F1147-E(09/2007) 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) May 2008 2. REPORT TYPE Conference Paper 3. DATES COVERED (From - To) 11-16 May 2008 5a. CONTRACT NUMBER DE-AC36-99-GO10337 5b. GRANT NUMBER  4. TITLE AND SUBTITLE Direct-Write Contacts: Metallization and Contact Formation; Preprint5c. PROGRAM ELEMENT NUMBER  5d. PROJECT NUMBER NREL/CP-520-42535 5e. TASK NUMBER PVA74201 6. AUTHOR(S) M.F.A.M. van Hest, C.J. Curtis, A. Miedaner, R.M. Pasquarelli,  T. Kaydanova, P. Hersh, 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-520-42535 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) Using direct-write approaches in photovoltaics for metallization and contact formation can significantly reduce the cost per watt of producing photovoltaic devices. Inks have been developed for various materials, such as Ag, Cu, Ni and Al, which can be used to inkjet print metallizations for various kinds of photovoltaic devices. Use of these inks results in metallization with resistivities close to those of bulk materials. By means of inkjet printing a metallization grid can be printed with better resolution, i.e. smaller lines, than screen-printing. Also inks have been developed to deposit transparent conductive oxide films by means of ultrasonic spraying. 15. SUBJECT TERMS PV; direct-wire contact; inkjet printing; transparent conductive oxide; ultrasonic spraying; metal organic decomposition; metallization  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. TELEPHONE NUMBER (Include area code)  Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18 

Direct-Write Contacts: Metallization and Contact Formation; Preprint

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Direct-Write Contacts: Metallization and Contact Formation; Preprint

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