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. 

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
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

1

Colorado School of Mines, Golden Colorado 80401 USA 2

National Renewable Energy Laboratory, Golden Colorado 80401 USA

ABSTRACT

Atmospheric-pressure solution deposition of the 

transparent conducting oxide (TCO), amorphous indiumzinc 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 αIZO 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 Ar4%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 200

C/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 

2100

C 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 Inrich 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, “SolutionProcessed 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