.

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337 

J.D. Perkins, M.F.A.M. van Hest, C.W. Teplin, 

J.L. Alleman, M.S. Dabney, L.M. Gedvilas, B.M. 

Keyes, B. To, and D.S. Ginley 

National Renewable Energy Laboratory

M.P. Taylor and D.W. Readey 

Colorado School of Mines

A.E. Delahoy and S. Guo 

Energy Photovoltaics, Inc.

Prepared for the 31st IEEE Photovoltaics Specialists 

Conference and Exhibition 

Lake Buena Vista, Florida 

January 3-7, 2005 

February 2005 • NREL/CP-520-37420 

Combinatorial Optimization of 

Transparent Conducting Oxides 

(TCOs) for PV 
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 
COMBINATORIAL OPTIMIZATION OF TRANSPARENT 

CONDUCTING OXIDES (TCOS) FOR PV 

J.D. Perkins1

, M.P. Taylor2, M.F.A.M. van Hest1, C.W. Teplin1, J.L. Alleman1, M.S. Dabney1, L.M. Gedvilas1, 

B.M. Keyes

1

, B. To1, D.W. Readey2, A.E. Delahoy3, S. Guo3 and D.S. Ginley1

1

National Renewable Energy Laboratory, Golden, CO 80401 2

Colorado School of Mines, Golden, CO 80401 3

 Energy Photovoltaics, Inc., Princeton, NJ 08543 

ABSTRACT 

Fig. 1. Electrical conductivity (a), carrier concentration (b) 

and mobility (c) for In-Zn-O (IZO) deposited at 100 °C. 

Solid symbols are for data taken using automated combinatorial tools and open symbols are for data taken using a 

conventional Hall effect probe on individual cut out pieces. 

Transparent conducting oxides (TCOs) can serve a 

variety of important functions in thin film photovoltaics 

such as transparent electrical contacts, antireflection coatings and chemical barriers. Two areas of particular interest are TCOs that can be deposited at low temperatures 

and TCOs with high carrier mobilities. We have employed 

combinatorial high-throughput approaches to investigate 

both these areas. Conductivities of σ = 2500 Ω-1-cm-1

have been obtained for In-Zn-O (IZO) films deposited at 

100 °C and σ > 5000 Ω-1-cm-1 for In-Ti-O (ITiO) and In-MoO (IMO) films deposited at 550 °C. The highest mobility

obtained was 83 cm2

/V-sec for ITiO deposited at 550 °C. 

INTRODUCTION

Transparent conducting oxides (TCOs) can serve a 

variety of important functions in thin film photovoltaics 

such as transparent electrical contacts, antireflection coatings and chemical barriers [1]. Two areas of particular 

interest are TCOs that can be deposited at low temperatures and TCOs with high carrier mobilities. In this study, 

we report on new high performance In2O3-based materials 

substituted separately with Zn, Mo, and Ti [2-6]. We have 

employed combinatorial high-throughput approaches to 

investigate these materials [2,3,7]. 

EXPERIMENTAL APPROACH

Compositionally graded samples (“libraries”) are deposited by co-sputtering onto 2”x2” glass substrates [8]. 

Three to five libraries are generally required to cover the 

full composition range for a binary tie-line, such as from 

In2O3 to ZnO. After deposition and, in some cases, additional controlled atmosphere annealing, the libraries are 

characterized by a variety of automated combinatorial 

mapping tools. At present, these include EPMA for metals 

stoichiometry, linear 4-point probe for sheet resistance [2], 

UV/VIS/NIR (200 - 2000 nm) reflection and transmission, 

FTIR optical reflection and transmission (1.8 – 25 μm) [9] 

and x-ray diffraction (XRD) using a large-area 2D detector 

[3]. For selected libraries, smaller samples are cut out for 

Hall effect measurements to determine the carrier concentration and mobility.

RESULTS AND DISCUSSION 

In-Zn-O (IZO) 

For In-Zn-O (IZO) libraries deposited from ceramic oxide targets at 100 °C in Ar with no post-deposition annealing, a broad maximum in the conductivity with σ ≈ 2500 

Ω-1-cm-1 is found for x ~ 0.55 to 0.75 in Zn1-xInxOy (Fig. 

1a). This roughly correlates with the composition range 

found to be amorphous by the XRD mapping. For higher 

In content, the carrier concentration decreases (Fig 1b) 

and for lower In content, the mobility decreases (Fig 1c). 

In Fig 1b, the combinatorially measured carrier concentration (solid symbols) is determined from the measured optical reflection and transmission spectra as described below. For samples with a composition in the amorphous 

region, the conductivity is unchanged by annealing for one 

1
Fig. 4. Electrical conductivity (a), carrier concentration (b, 

left) and mobility (b, right) for In-Ti-O (ITiO) samples deposited at 550 °C. 

hour in air at 200 °C. Figure 2 shows the optical reflectivity from 0.3 to 25 μm for an IZO library compositionally

centered on the conductivity maximum. These samples 

are transparent in the visible region as evident from the 

spectra shown in the top panel and, in both panels, the 

overlaid open circles show the approximate plasma wavelength (λp). For most TCOs, the infrared optical properties 

are well described by a simple free-carrier (Drude) model 

[9] in which 

Fig. 2. Optical reflectance spectra for In-Zn-O (IZO) deposited at 100 °C. Top panel: Example spectra for one 

spot. Bottom panel: Reflectivity spectra for 11 compositions displayed using gray scale. The overlaid open white 

circles show the wavelength for Rmin which approximately

corresponds to λp. 

λ p ∝ m*

N (1) 

where m* is the electron effective mass and N is the electron carrier concentration. Hence, from the composition 

dependence of λp (Fig. 2, bottom panel) one can determine the composition dependence of the carrier concentration across an entire library without having to cut the 

library into pieces for individual Hall measurements (Fig 

1b.). 

Fig. 3. Electrical conductivity (a), carrier concentration (b, 

left) and mobility (b, right) for In-Mo-O (IMO) for samples 

deposited at 350 °C and 550 °C. 

In-Mo-O (IMO) 

For In-Mo-O (IMO) libraries deposited at 350 °C, a 

maximum in the conductivity with σ ≈ 1000 Ω-1-cm-1 is 

found for ~ 6% Mo in place of In (Fig. 3a). Increasing the 

deposition temperature to 550 °C results in a five fold increase in the maximum conductivity to σ ≈ 5000 Ω-1-cm-1. 

This is due to increases in both the carrier concentration 

and the mobility (Fig. 3b). In particular, for IMO samples 

grown at 550 °C, the maximum mobility obtained is 65 

cm2

/V-sec at ~ 4% Mo and the maximum carrier concentration is 6.6 x 1020/cm3

 at ~ 8.5% Mo. 

In-Ti-O (ITiO) 

In addition, In-Ti-O (ITiO) has been investigated for 

use as a high mobility TCO (Fig. 4). For ITiO libraries 

deposited at TS = 550 °C, σ ≈ 5000 Ω-1-cm-1 for ~ 3 to 4 % 

Ti in place of In with a maximum mobility of 83 cm2

/V-sec 

at ~ 2 % Ti. For the sputtered ITiO samples, there is a 

2
linear increase in the carrier concentration of 3.4 x 

1020/cm3

 per %Ti from ~ 1.5 to 3 % Ti. This corresponds 

to 1.06 electrons / Ti indicating that for these growth conditions, Ti is a very effective dopant for In2O3 contributing 

the expected 1 electron per dopant atom for Ti doping of 

In2O3. For comparison, in IMO films grown by pulsed laser 

deposition (PLD), Mo-doping yields ~ 0.2 – 0.3 electrons / 

Mo [5]. 

SUMMARY 

We have employed combinatorial high-throughput approaches to investigate both these areas. Conductivities 

of σ = 2500 Ω-1-cm-1 have been obtained for In-Zn-O 

(IZO) films deposited at 100 °C and σ > 5000 Ω-1-cm-1 for 

In-Ti-O (ITiO) and In-Mo-O (IMO) films deposited at 550 

°C. The highest mobility obtained was 83 cm2

/V-sec for 

ITiO deposited at 550 °C. Hence, IZO films should work 

well for applications requiring low-temperature deposition 

and processing such as TCO coatings onto flexible polymer substrates. Whereas, either IMO or ITiO films should 

work well for applications requiring a high carrier mobility

such as devices requiring optical transparency in the infrared. 

REFERENCES

[1] D. S. Ginley and C. Bright, "Transparent conducting 

oxides", MRS Bulletin 25, 2000, pp. 15-18. 

[2] M. P. Taylor, D. W. Readey, C. W. Teplin, M. van 

Hest, J. L. Alleman, M. S. Dabney, L. M. Gedvilas, B. M. 

Keyes, B. To, P. A. Parilla, J. D. Perkins, and D. S. Ginley, 

"Combinatorial growth and analysis of the transparent 

conducting oxide ZnO/In (IZO)", Macromolecular Rapid 

Communications 25, 2004, pp. 344-347. 

[3] M. P. Taylor, D. W. Readey, C. W. Teplin, M. F. A. M. 

van Hest, J. L. Alleman, M. S. Dabney, L. M. Gedvilas, B. 

M. Keyes, B. To, J. D. Perkins, and D. S. Ginley, "The 

electrical, optical and structural properties of InxZn1-xOy 

(0 ≤ x ≤ 1) thin films by combinatorial techniques." Measurement Science and Technology 16, 2005, pp. 90-94. 

[4] Y. Yoshida, D. M. Wood, T. A. Gessert, and T. J. 

Coutts, "High-mobility, sputtered films of indium oxide 

doped with molybdenum", Applied Physics Letters 84, 

2004, pp. 2097-2099. 

[5] C. Warmsingh, Y. Yoshida, D. W. Readey, C. W. Teplin, J. D. Perkins, P. A. Parilla, L. M. Gedvilas, B. M. 

Keyes, and D. S. Ginley, "High-mobility transparent conducting Mo-doped In2O3 thin films by pulsed laser deposition", Journal of Applied Physics 95, 2004, pp. 3831-3833. 

[6] A. E. Delahoy, L. Chen, B. Sang, S. Y. Guo, J. Cambridge, F. Ziobro, R. Govindarajan, S. Kleindienst, and M. 

Akhtar, "CIGS Devices, Modules, and Processing Using a 

Hybrid Evaporation/Sputtering Process", Proc. of the 19th 

European PVSEC, Paris, June 7-11, 2004. 

[7] J. D. Perkins, J. A. del Cueto, J. L. Alleman, C. 

Warmsingh, B. M. Keyes, L. M. Gedvilas, P. A. Parilla, B. 

To, D. W. Readey, and D. S. Ginley, "Combinatorial studies of Zn-Al-O and Zn-Sn-O transparent conducting oxide 

thin films", Thin Solid Films 411, 2002, pp. 152. 

[8] C. W. Teplin, M. van Hest, M. Dabney, C. L. Perkins, 

L. M. Gedvillas, B. To, P. A. Parilla, B. M. Keyes, J. D. 

Perkins, D. S. Ginley, Y. P. Lin, and Y. W. Lu, "Combinatorial study of reactively sputtered Cr-Ti-N", Applied Surface 

Science 223, 2004, pp. 253-258. 

[9] J. D. Perkins, C. W. Teplin, M. van Hest, J. L. Alleman, X. Li, M. S. Dabney, B. M. Keyes, L. M. Gedvilas, D. 

S. Ginley, Y. Lin, and Y. Lu, "Optical analysis of thin film 

combinatorial libraries", Applied Surface Science 223, 

2004, pp. 124-132. 

3
F1147-E(12/2004) 

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)

February 2005 

2. REPORT TYPE

Conference Paper 

3. DATES COVERED (From - To)

3-7 January 2005 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Combinatorial Optimization of Transparent Conducting Oxides 

(TCOs) for PV 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/CP-520-37420 

5e. TASK NUMBER

PVA54201 

6. AUTHOR(S) 

J.D. Perkins, M.P. Taylor, M.F.A.M. van Hest, C.W. Teplin, 

J.L. Alleman, M.S. Dabney, L.M. Gedvilas, B.M. Keyes, B. To, 

D.W. Readey, A.E. Delahoy, S. Guo, 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 

 Colorado School of Mines, Golden, CO 80401 

 Energy Photovoltaics, Inc., Princeton, NJ 08543 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

NREL/CP-520-37420 

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)

Transparent conducting oxides (TCOs) can serve a variety of important functions in thin-film photovoltaics such as 

transparent electrical contacts, antireflection coatings, and chemical barriers. Two areas of particular interest are TCOs 

that can be deposited at low temperatures and TCOs with high carrier mobilities. We have employed combinatorial highthroughput approaches to investigate both these areas. Conductivities of σ = 2500 Ω-1-cm-1 have been obtained for InZn-O (IZO) films deposited at 100°C and σ > 5000 Ω-1-cm-1 for In-Ti-O (ITiO) and In-Mo-O (IMO) films deposited at 

550°C. The highest mobility obtained was 83 cm2

/V-s for ITiO deposited at 550°C. 

15. SUBJECT TERMS

PV; transparent conducting oxides (TCOs); thin film; transparent electrical contacts; antireflection coatings; chemical 

barriers; polymer substrate; 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified

17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

19b. TELEPHONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18