This study investigates atmospheric-pressure solution deposition routes as an alternative to these traditional high-vacuum techniques

Berry, J.

Curtis, C.

Ginley, D.

Miedaner, A.

O'Hayre, R.

Pasquarelli, R.

Perkins, J.

van Hest, M.

UNT Digital Library

 Conference Paper NREL/CP-520-46016 June 2009 Solution Deposition of Amorphous IZO Films by Ultrasonic Spray Pyrolysis Preprint  R. Pasquarelli, M. van Hest, A. Miedaner,  C. Curtis, J. Perkins, J. Berry, R. O'Hayre, and  D. Ginley  To be presented at the 34th IEEE Photovoltaic Specialists Conference Philadelphia, Pennsylvania June 7–12, 2009  NOTICE The submitted manuscript has been offered by an employee of the Alliance for Sustainable Energy, LLC (ASE), a contractor of the US Government under Contract No. DE-AC36-08-GO28308. Accordingly, the US Government and ASE 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. 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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 1 SOLUTION DEPOSITION OF AMORPHOUS IZO FILMS BY ULTRASONIC SPRAY PYROLYSIS   Robert Pasquarelli1, Maikel van Hest2, Alexander Miedaner2, Calvin Curtis2, John Perkins2, Joseph Berry2, Ryan O'Hayre1, David Ginley2 1Colorado School of Mines, Golden Colorado 80401 USA 2National Renewable Energy Laboratory, Golden Colorado 80401 USA     ABSTRACT  Atmospheric-pressure solution deposition of the transparent conducting oxide (TCO), amorphous indium-zinc oxide (α-IZO), was investigated as an alternative to traditional vacuum-based physical vapor deposition techniques for photovoltaic applications. Solution processing is attractive due to its ease and potential to lower device manufacturing costs. Here we report on α-IZO films prepared by ultrasonic spray pyrolysis from solutions of an indium-zinc formate (IZF) precursor.  Thin, crack-free, amorphous IZO films with good optical transmittance (>75%) and conductivities of ~34 S/cm were produced from an IZF-HNO3-methanol ink using joint RTP and Ar-H2 annealing.    INTRODUCTION  Currently, the thin film transparent conducting oxides (TCOs) used for CIGS are deposited by vacuum-based physical vapor deposition techniques such as sputtering. This study investigates atmospheric-pressure solution deposition routes as an alternative to these traditional high-vacuum techniques.  Solution processing is attractive due to its ease and potential to lower device manufacturing costs. ZnO, typically used for the TCO layer, has poor resistance to water vapor.  Recently, it has been demonstrated that  amorphous indium-zinc oxide (α-IZO) has qualitatively better resistance to degradation at 85°C / 85% relative humidity. Sputtered IZO shows the highest conductivity and smoothness in the indium-rich region (~80 at% In) where the films are amorphous.  However, current TCOs deposited by solution routes have only focused on crystalline, zinc-rich films (3-5 at% In) [1-4]. In nearly all these cases, acetate precursors are used, but the high indium content of α-IZO is difficult to achieve due to the poor solubility of indium acetate.  Here we report on α-IZO films prepared by ultrasonic spray pyrolysis from solutions of an indium-zinc formate (IZF) precursor. Thin, smooth, crack-free amorphous IZO films with good optical transmittance (>75%) and conductivities of ~34 S/cm were produced from an IZF-HNO3-methanol ink using joint RTP and Ar-H2 annealing.  EXPERIMENTAL  Indium-zinc formate (IZF) was prepared (70:30 mol In:Zn) using a synthesis procedure developed in our laboratory.  The procedure and characterization of the precursor are subjects of a manuscript in progress.  0.1M IZF inks were prepared with 1 vol% of conc. HNO3 in both water and a methanol/water (10:1) mixture.  Inks were aged 1-3 days and filtered prior to use. Films were ultrasonically sprayed in open atmosphere onto O2-plasma cleaned glass microscope slides mounted on a substrate heater and X-Y stage. Typical deposition parameters were as follows: substrate surface temperatures 140-210°C, 7 SLPM N2 carrier flow, and a 0.25 mL/min ink flow rate. Films were aged for 2 days prior to annealing. Annealing was performed in a tube furnace under Ar-4%H2 at 300°C for 20min with a 3hr ramp. Additional samples were processed prior to anneal by rapid thermal processing (RTP) under Ar for 20min at 3400C on a Si susceptor with a 200C/sec ramp film side up and then down.  RESULTS AND DISCUSSION  Precursor Characterization and Ink Formulation  The In:Zn ratio for the prepared IZF was determined to be 70:30 at% by ICP-AES.  This composition is near the peak conductivity of 80:20 for IZO.  In order to determine the processing temperatures required to convert the IZF precursor into IZO, thermal gravimetric analysis (TGA) was performed in air.  The decomposition temperature was determined to be ~255°C.  Spraying was performed above the solvent boiling point but below the decomposition temperature.  Annealing was performed at 300°C to ensure full decomposition. The effect of additives to the ink not only influences ink properties (such as wetting) but also influences the ink chemistry.  The addition of nitric acid was required to ensure solubility of the precursor.  Nitric acid was chosen over other acids to promote oxide formation, as it is an oxidizing acid.  Film Morphology and Optical Properties  Sputtered IZO has been shown to be amorphous at a 70:30 In:Zn composition. XRD was performed to 2 determine whether spray deposited films are amorphous as well.  Fig. 1 shows that films annealed at 300ºC are amorphous.  Processing at higher temperature (500°C) resulted in crystallization during decomposition and/or phase separation of In2O3.  Deposition temperature affected the morphology of the final films as spraying at 210°C showed greater resistance to In2O3 formation.   Fig. 1.  XRD of sprayed films post-deposition at 1400C and 2100C and post-anneal at 3000C and 5000C.  Transparency is a critical property for thin film transparent conductors. Film morphology is crucial in determining the optical properties of solution deposited films as cracks, pores, and surface roughness can result in scattering. The ink solvent was found to affect the quality of the film produced.  The water-based ink (Fig. 2a) resulted in a loss of optical transparency due to film cracking.  However, the methanol/water mixed solvent (Fig. 2b) produced a crack free film with good optical properties. Little optical loss was observed after processing, with a respectable transmittance of >75% as shown in Fig. 3.    Fig. 2.  Optical micrographs of films a) post-anneal sprayed from water-based ink, b) post-anneal sprayed from methanol/water-based ink, c) SEM cross-section of film post-anneal sprayed from methanol/water-based ink. Spray depositing with 8 coats resulted in a smooth film with a thickness of 150-200 nm as seen in the SEM image in Fig. 2c.  AFM studies are in progress to quantify the surface morphology and roughness.    Fig. 3.  Transmission-Reflectance of IZO film.  Electrical Properties  Conductivity is another critical property for thin film transparent conductors.  Conductivity of the films was determined from 4-point probe sheet resistance and by Hall measurements.  In general, it was observed that films sprayed at 210ºC had higher conductivities.    Fig. 4.  Conductivity (σ) of IZO films as a function of annealing gas and processing conditions.  RTP denotes σ after RTP step, and RTP+ denotes σ after RTP and tube furnace anneal.  Parenthesized value indicates ramp rate.   Fig. 4 shows the conductivity of films as a function of thermal processing atmosphere (Ar and Ar-4%H2), ramp time, and thermal processing scheme (RTP, tube furnace anneal, and RTP followed by a tube furnace anneal). First, conductivity increases with longer anneal times and with more reducing annealing atmospheres.  This effect is attributed to the production of more carriers.  Second, 3 rapid thermal processing demonstrates promise as a quick production method for solution deposited TCOs.  Lower conductivity films were observed because the RTP could only be performed under Ar flow in the current setup.  More importantly, it was demonstrated that the conductivity could be increased by 4-5 orders of magnitude following a secondary Ar-H2 anneal. Fig. 5 compares the Hall conductivity (34 S/cm), carrier concentration (8.5 x1019 #/cm2), and mobility (2.0 cm2/Vs) of the best-sprayed IZO film to date with the typical property range for sputtered IZO (illustrated by bars).  The carrier concentration and especially mobility each need to be increased by ~1 order of magnitude to be in range of sputtered IZO.  The low mobility is attributed to porosity or small grains as seen in the SEM cross-section of the film (Fig. 2c).    Fig. 5.  Hall conductivity, carrier concentration, and mobility of best sprayed IZO film (points) compared to typical property range for sputtered IZO (bars).  CONCLUSIONS  In summary, In-Zn formate (IZF) can serve as an In-rich precursor for IZO films.  Ultrasonic spray deposited IZF-HNO3-methanol ink using joint RTP and Ar-H2 annealing results in thin, smooth, crack-free amorphous IZO films with good optical transmittance (>75%).  Current film conductivity (~34 S/cm) requires increases in both carrier concentration and mobility by ~1 order of magnitude to be comparable to sputtered IZO.  Taken together, with the potential for lower production costs that solution processing offers, these attributes make ultrasonic spray deposited α-IZO thin film TCOs an attractive choice for many PV applications.  ACKNOWLEDGEMENTS  This work was supported by the U.S. Department of Energy under Contract No. DOE-AC36-08GO28308 with the National Renewable Energy Laboratory.  REFERENCES  [1] J. Wienke and A.S. Booij, “ZnO:In Deposition by Spray Pyrolysis — Influence of the Growth Conditions on the Electrical and Optical Properties,” Thin Solid Films 516, 2008, pp. 4508-4512.  [2] S.-Y. Lee and B.-O. Park, “Electrical and Optical Properties of In2O3-ZnO Thin Films Prepared by Sol-gel Method,” Thin Solid Films 484, 2005, pp. 184-187.  [3] M. de la L. 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 Materials & Solar Cells 91, 2007, pp. 1449-1453.  [4] C.G. Choi, S.-J. Seo, and B.-S. Bae, “Solution-Processed Indium-Zinc Oxide Transparent Thin-Film Transistors,” Electrochemical and Solid-State Letters 11 (1), 2008, pp. H7-H9.   F1147-E(10/2008) 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) June 2009 2. REPORT TYPE Conference Paper 3. DATES COVERED (From - To) June 7-12, 2009 4. TITLE AND SUBTITLE Solution Deposition of Amorphous IZO Films by Ultrasonic Spray Pyrolysis: Preprint 5a. CONTRACT NUMBER DE-AC36-08-GO28308 5b. GRANT NUMBER  5c. PROGRAM ELEMENT NUMBER  6. AUTHOR(S) R. Pasquarelli, M. van Hest, A. Miedaner, C. Curtis, J. Perkins,  J. Berry, R. O'Hayre, and D. Ginley 5d. PROJECT NUMBER NREL/CP-520-46016 5e. TASK NUMBER PVA92810 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-520-46016 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)  10. SPONSOR/MONITOR'S ACRONYM(S) NREL 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) Atmospheric-pressure solution deposition of the transparent conducting oxide (TCO), amorphous indium-zinc oxide (α-IZO), was investigated as an alternative to traditional vacuum-based physical vapor deposition techniques for photovoltaic applications. Solution processing is attractive due to its ease and potential to lower device manufacturing costs. Here we report on α-IZO films prepared by ultrasonic spray pyrolysis from solutions of an indium-zinc formate (IZF) precursor.  Thin, crack-free, amorphous IZO films with good optical transmittance (>75%) and conductivities of ~34 S/cm were produced from an IZF-HNO3-methanol ink using joint RTP and Ar-H2 annealing. 15. SUBJECT TERMS PV; solution deposition; atmospheric pressure; transparent conducting oxides; amorphous indium zinc-oxide; vapor deposition; spray pyrolysis; rapid thermal processing; 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT UL 18. NUMBER OF PAGES  19a. NAME OF RESPONSIBLE PERSON  a. REPORT Unclassified b. ABSTRACT Unclassified c. THIS PAGE Unclassified 19b. TELEPHONE NUMBER (Include area code)  Standard Form 298 (Rev. 8/98) Prescribed by ANSI Std. Z39.18 

Solution Deposition of Amorphous IZO Films by Ultrasonic Spray Pyrolysis: Preprint

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Solution Deposition of Amorphous IZO Films by Ultrasonic Spray Pyrolysis: Preprint

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