NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Amorphous Indium-Zinc-Oxide 

Transparent Conductors for 

Thin Film PV

Preprint

J. Perkins, T. Gennett, M. Galante, D. Gillaspie, 

and D. Ginley

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50822

July 2011
NOTICE

The submitted manuscript has been offered by an employee of the Alliance for Sustainable Energy, LLC 

(Alliance), a contractor of the US Government under Contract No. DE-AC36-08GO28308. Accordingly, the US 

Government and Alliance 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/help/ordermethods.aspx

Cover Photos: (left to right) PIX 16416, PIX 17423, PIX 16560, PIX 17613, PIX 17436, PIX 17721

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.
1

AMORPHOUS INDIUM-ZINC-OXIDE TRANSPARENT 

CONDUCTORS FOR THIN FILM PV

J. Perkins, T. Gennett, M. Galante, D. Gillaspie, and D. Ginley

National Renewable Energy Laboratory, Golden, CO, USA

ABSTRACT

Amorphous InZnO’s (a-IZO) basic PV applicability has 

now been demonstrated in prototype CIGS, Si 

Heterojunction (SiHJ) and organic photovoltaics (OPV). 

However, to move beyond initial demonstration devices,

improved TCO properties and processibility of the a-IZO 

films are needed. Here, RF-superimposed DC sputtering 

was used to improve the reliable deposition of a-IZO with 

conductivity σ > 3000 S/cm.

INTRODUCTION

Amorphous InZnO (a-IZO) films sputter deposited at 

ambient temperature have excellent opto-electronic 

properties and smoothness (RRMS < 0.5 nm) [1]. Further, 

the optical transparency and electrical conductivity of 

optimized a-IZO do not change in damp heat at 85°C, 

85%RH [2]. In addition, a-IZO’s basic PV applicability has 

now been demonstrated in prototype CIGS, Si 

Heterojunction (SiHJ) and organic photovoltaics (OPV) [2].

However, to move beyond initial demonstration devices, 

improved TCO properties and processibility of the a-IZO 

films are needed. For example, for DC sputtering, the % 

of O2 in the Ar sputter gas which yields optimized a-IZO for 

a 200 nm thick film, yields grey 1000 nm thick films due to 

oxygen depletion from the target during film growth. The 

use of RF-superimposed DC sputtering [3] eliminates this 

problem and a-In0.8Zn0.2Ox (87 wt.% In2O3 / 13 wt.% ZnO) 

films with RS < 4 Ω/sq. and TVIS = 88 % (vs glass) were 

grown.

EXPERIMENTAL APPROACH

Amorphous InZnO (a-IZO) thin films were deposited at 

ambient temperature onto 2”x2” Eagle 2000 glass 

substrates by RF-superimposed DC sputtering from a 

In2O3/ZnO ceramic target in varying Ar/O2 gas mixtures. 

Three different In:Zn target compositions were used 

(70:30, 80:20, 87:13 In2O3:ZnO wt%). Thickness was 

determined by ellipsometry. Typical film thicknesses were 

300 nm. Optical reflection and transmission spectra were 

measured from 300 – 1100 nm using fiber optically 

coupled CCD array spectrometers. Sheet resistance was 

measured with a linear 4-pt probe method. Amorphous 

structure of the films was confirmed by X-ray diffraction 

(XRD). For each metals composition, the electrical 

conductivity was optimized with respect to variation of the 

%O2 in the sputter gas and the RF:DC power ratio.

RESULTS AND DISCUSSION

Figure 1 shows the electrical conductivity for amorphous 

In0.8Zn0.2Ox (87:13 In2O3/ZnO wt%) films deposited by RFsuperimposed DC sputtering with equal RF and DC power 

as a function of the %O2 in the Argon sputter gas. As the 

total power (RF+DC) is increased from 50W to 100W to 

150W, the %O2 necessary for maximum conductivity 

increases. Here, the %O2 is the measured gas 

composition during deposition and hence oxygen sourced 

from the target sets a lower limit on %O2 even when no 

additional oxygen is added. A common maximum 

conductivity of ~ 3200 S/cm is found for both the 100W 

depositions (red) and the 150W depositions (black) albeit 

at different %O2. For the 50W depositions (blue), the 

conductivity increases monotonically with decreasing %O2

but does not reach a maximum even at the lowest %O2, ~ 

0.3%. This suggests that at this lower power, 50W, the 

residual oxygen sourced from the InZnO sputter target 

was already too high to achieve the optimized 

conductivity.

To determine if the film properties were constant with 

changing thickness, two films with differing thickness were 

grown by changing only the total deposition time. The 

conductivity for these two films is shown in Figure 1 using 

an open red circle for the 1200 nm thick film and a solid 

red circle for the standard 300 nm thick film. The near 

overlap of the two data points in Figure 1 shows that the 

conductivity of the films deposited by RF-superimposed 

DC sputtering is not changing with film thickness. This 

Figure 1: Conductivity of a-IZO vs %O2 films 

sputtered using RF-superimposed DC
2

was not the case with pure DC sputtering due to gradual 

reduction of the InZnO sputter target surface with 

increasing deposition time. This improved deposition 

stability achieved by RF-superimposed DC sputtering is 

important in that it now allows a-IZO film thickness to be 

treated as a device engineering variable without having to

re-optimize the deposition parameters for each different 

thickness of interest.

Figure 2 compares the optical transmission spectra 

normalized to a bare glass substrate for the 300 nm thick 

(blue spectra) and 1200 nm thick (red spectra) a-IZO films 

just discussed above. With the improved deposition 

stability of RF-superimposed DC sputtering, the 1200 nm 

thick film has a low sheet resistance of RS = 3.2 Ω/sq. 

coupled with a high average transparency of ~ 85% across 

the visible and near-IR.

Figure 3 compares the maximum conductivity achieved 

with optimized RF-superimposed DC deposited a-IZO 

films (red triangles) with best obtained by DC-only 

deposition (black circles) [4]. For the RF-superimposed 

this corresponds to optimization across both the RF:DC 

mixture and the %O2 but for a fixed total deposition power 

(RF+DC) of 100W. The RF-superimposed DC sputtering 

process yields about a 20% increase in conductivity for the 

three different compositions tested. In addition, the 

optimized conductivity increases monotonically with 

indium content from about σ = 2500 S/cm at 58 at % In to 

3200 S/cm at 80 %. To make clearer how the conductivity 

scales with In content, Figure 4 shows the maximum 

conductivity for RF-superimposed DC deposited a-IZO 

films normalized to their relative In content, showing that 

the conductivity/In-content is nearly constant at ~ 40 

(S/cm)/(at.% In). This suggests that a-IZO may be 

essentially diluted and structurally stabilized amorphous 

indium oxide which is itself a good TCO but is challenging 

to deposit and crystallizes easily at 180 °C [5]. In contrast, 

a-IZO containing 20 – 40 at.% Zn does not crystallize until

500 °C or higher [1].

SUMMARY

In summary, a-IZO thin film transparent conductors with 

excellent TCO properties can be easily deposited at 

ambient and near-ambient temperatures. By using RFsuperimposed DC sputtering, conductivities of σ = 3200 

S/cm can be reliably achieved for In0.8Zn0.2Ox films. 

Further, prior work has shown that a-IZO is optically and 

electrically inert for up to 40 days in damp heat (85°C, 

85% RH) and also is an effective water vapor transport 

barrier making it a very promising TCO for thin film PV 

applications.

Figure 2: Optical transmission of 300 and 1200 

nm thick a-IZO thin films.

Figure 3: Comparison of conductivity vs 

indium content for differing levels of process 

optimization.

Figure 4: Maximum conductivity normalized to 

indium content for optimized a-IZO.
3

ACKNOWLEDGEMENT

Employees of the Alliance for Sustainable Energy, LLC, 

authored this work. This work was supported by the U.S. 

Department of Energy under Contract No. DE-AC36-08-

GO28308 with the National Renewable Energy 

Laboratory. The United States Government and the 

publisher, by accepting the article for publication, 

acknowledge that the United States Government retains a 

non-exclusive, paid-up, irrevocable, worldwide license to 

publish or reproduce the published form of this work, or 

allow others to do so, for United States Government 

purposes.

REFERENCES

[1] M.P. Taylor et al., "The Remarkable Thermal Stability 

of Amorphous In-Zn-O Transparent Conductors", 

Advanced Functional Materials 18, 2008, pp. 3169-3178.

[2] J.D. Perkins et al., "Amorphous Transparent 

Conductors for PV Applications", Proceedings of 35th 

IEEE Photovoltaic Specialists Conference, 2010.

[3] M. Stowell et al., "RF-superimposed DC and pulsed 

DC sputtering for deposition of transparent conductive 

oxides", Thin Solid Films 515, 2007, pp. 7654-7657.

[4] J.D. Perkins et al., "Conductivity and transparency in 

amorphous In-Zn-O transparent conductors", International 

Journal of Nanotechnology 6, 2009, pp. 850-859.

[5] C.W. Ow-Yang, D. Spinner, Y. Shigesato and D.C. 

Paine, "A time-resolved reflectivity study of the 

amorphous-to-crystalline transformation kinetics in dcmagnetron sputtered indium tin oxide", Journal of Applied 

Physics 83, 1998, pp. 145-154.