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Spray Deposition of High Quality 

CuInSe2 and CdTe Films 

Preprint 

C.J. Curtis, M. van Hest, A. Miedaner, P. Hersh, 

J. Leisch, and D.S. Ginley 

National Renewable Energy Laboratory 

J. Nekuda 

Colorado School of Mines

Presented at the 33rd IEEE Photovoltaic Specialists Conference 

San Diego, California 

May 11–16, 2008 

Conference Paper 

NREL/CP-590-42538 

May 2008 

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Spray Deposition of High Quality CuInSe2 and CdTe Films 

Calvin J. Curtis1

, Maikel van Hest1, Alex Miedaner1, Jennifer Nekuda2, Peter Hersh1, Jennifer Leisch1, 

and David S. Ginley1

1

National Renewable Energy Laboratory, Golden, CO 80401 2

Colorado School of Mines, Golden, CO 80401 

ABSTRACT 

A number of different ink and deposition approaches 

have been used for the deposition of CuInSe2 (CIS), 

Cu(In,Ga)Se2 (CIGS), and CdTe films. For CIS and CIGS, 

soluble precursors containing Cu, In, and Ga have been 

developed and used in two ways to produce CIS films. In 

the first, In-containing precursor films were sprayed on 

Mo-coated glass substrates and converted by rapid 

thermal processing (RTP) to In2Se3. Then a Cu-containing 

film was sprayed down on top of the In2Se3 and the 

stacked films were again thermally processed to give CIS. 

In the second approach, the Cu-, In-, and Ga-containing 

inks were combined in the proper ratio to produce a mixed 

Cu-In-Ga ink that was sprayed on substrates and 

thermally processed to give CIGS films directly. For CdTe 

deposition, ink consisting of CdTe nanoparticles dispersed 

in methanol was prepared and used to spray precursor 

films. Annealing these precursor films in the presence of 

CdCl2 produced large-grained CdTe films. The films were 

characterized by x-ray diffraction (XRD) and scanning 

electron microscopy (SEM). Optimized spray and 

processing conditions are crucial to obtain dense, 

crystalline films. 

INTRODUCTION 

The use of ink based precursors deposited and 

processed under atmospheric conditions to produce high 

quality semiconductor absorber layers, metals and TCOs 

offers an attractive alternative to conventional vacuum 

based deposition approaches in terms of process 

simplicity and cost. The key element to atmospheric 

processing is to develop and formulate precursors and 

inks that can be sprayed or printed and which decompose 

to give the desired materials with the proper composition, 

morphology, optical and electrical properties. 

Solar cells based on thin film absorber layers are 

attractive because of the reduced material cost compared 

to Si wafers. Two of the most attractive thin film absorber 

materials are CdTe and CIGS. Currently, the favored 

technology for deposition of CdTe is closed-space 

sublimation under vacuum[1], while the best large-grained 

CIGS films are produced by deposition of the metals by 

evaporation followed by selenization with Se or H2Se[2, 3]. 

This CIGS process has produced very efficient laboratory 

scale devices, but translation into a manufacturing 

environment has proven difficult, and high temperature 

selenization is hazardous. The use of ink based 

precursors that can be deposited and processed under 

atmospheric conditions could be a simple and inexpensive 

alternative method for deposition of these absorber 

materials. The key element in this approach is to develop 

precursors and formulate inks that decompose to give the 

desired materials with the proper composition, morphology 

and properties for the application. The inks must also be 

formulated to wet the substrate properly in order to obtain 

smooth, continuous films. In addition, thermal processing 

conditions that allow volatile materials to escape and lead 

to formation of dense films must be found. The objective of 

this work is to demonstrate the feasibility of depositing 

high quality absorber layers of CIGS and CdTe by spray 

deposition of precursor inks coupled with rapid thermal 

processing under atmospheric conditions. 

TECHNICAL APPROACH 

For CIS deposition, a variety of soluble Cu, In and 

Ga-containing precursors to binary selenide materials 

have been developed. These were dissolved in organic 

solvents, along with dispersants and other additives, to 

produce the proprietary inks used in this study. For CdTe 

deposition, an ink consisting of CdTe nanoparticles 

dispersed in methanol was prepared by the reaction of 

CdI2 with Na2Te in methanol at low temperature, as 

shown[4]. 

CdI2 + Na2Te nano-CdTe + 2NaI

methanol

-78oC

The CdTe nanoparticles were collected by centrifugation, 

washed with methanol to remove the NaI co-product and 

re-dispersed in methanol for spray deposition. 

The inks were deposited using an ultrasonic spray 

head (SonoTek Corp.) fed by a variable speed liquid pump 

(Fluid Metering, Inc.). A substrate heater mounted on a 

computer-controlled X-Y motion system allowed for 

movement of heated substrates under the sprayed stream. 

The thickness of the sprayed film was controlled by 

varying the ink concentration, the flow rate through the 

sprayer and the number of coats sprayed. Conditions 

were found such that smooth, uniform precursor films 

were obtained for all of the sprayed inks. 

The precursor films were converted to the desired 

materials using rapid thermal processing (RTP) in a 

controlled atmosphere. The RTP conditions were varied 

systematically to ascertain the effect of conditions on the 

film compositions and morphologies obtained. The film 

compositions were characterized by x-ray fluorescence 

(XRF), crystalline phases were identified using x-ray 

diffraction (XRD) and film morphology was examined 

using scanning electron microscopy (SEM). 

1
RESULTS 

Spray Deposition of CIS Films 

The inks developed for deposition of binary selenide 

materials have been used in two ways to produce CIS and 

CIGS films. In the first approach, an In-containing 

precursor film was first sprayed on a heated, Mo-coated 

glass substrate and processed by RTP to give In2Se3. 

Figure 1 shows XRD scans of thickened In2Se3 films 

processed under these conditions. After processing the 

first film to In2Se3, the Cu-containing ink was used to spray 

a precursor film on top of the In2Se3 film, and the stacked 

films were then processed again by RTP, which caused 

the films to react to form a CIS film. Figure 2 shows XRD 

scans of the initial In2Se3 film and the CIS film that was 

produced by reaction with the Cu-containing precursor. 

In the second approach, the Cu, In, and Gacontaining inks were mixed in the proper ratio to produce a 

Cu-In-Ga ink. This ink was sprayed on to Mo-coated glass 

substrates to produce precursor films that were then 

annealed at various temperatures to produce CIGS films. 

Figure 3 shows the XRD scans for films sprayed at low 

temperature and annealed at increasing temperatures. 

With the exception of the as-sprayed film, which was 

amorphous, all of the temperatures examined produced 

CIGS films from this mixed ink. However, films processed 

under these conditions were found to be cracked and 

somewhat porous. It was found that spraying the 

precursor film at low temperature, followed by processing 

with a slower temperature ramp to high temperature gave 

a dense film, as shown in Figure 4. This result represents 

a breakthrough for solution-based precursors.

Figure 3. XRD scans of CIGS films produced from 

the mixed Cu-In-Ga ink annealed at increasing 

temperatures. The large peak at ~40o is due to Mo. 

Figure 1. XRD scans of In2Se3 films of different 

thickness deposited using the In-containing ink. 

The large peak at ~40o is due to Mo. 

Spray Deposition of CdTe Films 

The CdTe ink consists of nanoparticles 2.5-7.5 nm in 

diameter suspended in methanol, as determined by TEM 

and XRD analysis. Ink with a concentration of about 25 

Figure 4. SEM image of a CIS film produced from 

the mixed ink sprayed at low temperature and 

processed with a slower temperature ramp. 

Figure 2. XRD scans of In2Se3 film and CIS film that 

resulted from treatment with Cu-containing ink and 

processing. The large peak at ~40o is due to Mo. 

2
mg of CdTe per ml was spray deposited on glass 

substrates coated with layers of F:SnO2 and CdS at a 

temperature of 150oC to produce precursor films 1 to 5 μm 

thick, depending on the number of coats sprayed. The 

precursor films were annealed at temperatures ranging 

from 250 to 500oC in argon and forming gas (10%H2 in N2) 

atmospheres. Sharp XRD patterns corresponding to cubic 

CdTe were observed for films annealed at all 

temperatures, with the sharpest XRD peaks exhibited by 

films annealed at 400oC and above, as shown in Figure 5. 

Little difference was observed between films annealed in 

argon or forming gas. 

As-sprayed 

100 nm grains 

2501⁄4C H2/N2

3001⁄4C H2/N2

4001⁄4C H2/N2

3001⁄4C Ar

4001⁄4C Ar

Figure 5. XRD data for CdTe films sprayed at 

150oC and annealed in forming gas or argon at 

the temperatures shown (* denotes Te). 

Annealed 400oC, 

30 min 

300 nm grains 

10% CdCI2/MeOH dip 

Annealed 400oC, 30 min 

400 nm grains 

20% CdCI2/MeOH dip 

Annealed 400oC, 30 min 

The use of CdCl2 to promote grain growth during 1400 nm grains 

annealing was also examined. Sprayed precursor films 

were dipped into CdCl2 solutions in methanol at 10% and 

20% of saturation at 50oC for 5 min, followed by annealing 

at 400oC for 30 min in argon. Figure 6 shows atomic force 

microscopy (AFM) images of sprayed films annealed with 

and without CdCl2 treatment. The CdCl2 treatment 

definitely promotes grain growth compared to untreated 

films, with grains larger than 1 μm observed after 

treatment with CdCl2 at the higher concentration. These 

films were finished into solar cells by addition of a 

standard doped carbon top contact. Cells based on 

nanoparticle-derived absorber layers have shown 

efficiencies of 6% to date. 

Figure 6. AFM images of as-sprayed and 

annealed films with and without CdCl2

treatment. 

CONCLUSION 

Inks containing Cu, In and Ga have been formulated 

from new soluble precursors and have been used to 

deposit CIS and CIGS films by two approaches. The first 

approach involved deposition of stacked layers of In- and 

Cu-containing precursor materials followed by thermal 

processing to give CIS. The second approach consisted 

3
of mixing Cu, In, and Ga inks in the proper stoichiometry REFERENCES

for CIGS. This mixed ink was then spray deposited on 

Mo-coated glass substrates and thermally processed to 

give CIGS films. A nanoparticle CdTe ink was used to 

spray deposit CdTe films. Treatment of the sprayed films 

with a CdCl2 solution followed by annealing produced 

grain growth in the CdTe film. Solar cells with conversion 

efficiencies of 6% have been produced using these films. 

For both CIGS and CdTe films, optimized spray and 

thermal processing conditions are key to obtaining films 

with the desired composition and morphology. 

1. Chopra, K.L., P.D. Paulson, and V. Dutta, "Thinfilm solar cells: An overview." Progress in 

Photovoltaics, 2004. 12, 69-92. 

2. Lundberg, O., M. Edoff, and L. Stolt, "Optimized 

growth conditions for Cu(In,Ga)Se/sub 2/ layers 

grown by co-evaporation at high deposition 

rates." Compound Semiconductor Photovoltaics. 

Symposium. Mater. Res. Soc. Symposium 

Proceedings, 763, 2003, 365-70. 

ACKNOWLEDGEMENT 

3. Ramanathan, K., G. Teeter, J.C. Keane, and R. 

Noufi, "Properties of high-efficiency CuInGaSe2 

thin film solar cells." Thin Solid Films, 2005. 480, 

499-502. 

This work was supported by the U.S. Department of 

Energy under Contract No. DE-AC36-99GO10337 with the 

National Renewable Energy Laboratory. 

4. Schulz, D.L., M. Pehnt, D.H. Rose, E. Urgiles, 

A.F. Cahill, D.W. Niles, K.M. Jones, R.J. 

Ellingson, C.J. Curtis, and D.S. Ginley, "CdTe 

Thin Films from Nanoparticle Precursors by 

Spray Deposition." Chem. Mater., 1997. 9, 889-

900. 

4
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Spray Deposition of High Quality CuInSe2 and CdTe Films: Preprint

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C.J. Curtis, M. van Hest, A. Miedaner, J. Nekuda, P. Hersh, 

J. Leisch, and D.S. Ginley 

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14. ABSTRACT (Maximum 200 Words)

A number of different ink and deposition approaches have been used for the deposition of CuInSe2 (CIS), 

Cu(In,Ga)Se2 (CIGS), and CdTe films. For CIS and CIGS, soluble precursors containing Cu, In, and Ga have been 

developed and used in two ways to produce CIS films. In the first, In-containing precursor films were sprayed on Mocoated glass substrates and converted by rapid thermal processing (RTP) to In2Se3. Then a Cu-containing film was 

sprayed down on top of the In2Se3 and the stacked films were again thermally processed to give CIS. In the second 

approach, the Cu-, In-, and Ga-containing inks were combined in the proper ratio to produce a mixed Cu-In-Ga ink 

that was sprayed on substrates and thermally processed to give CIGS films directly. For CdTe deposition, ink 

consisting of CdTe nanoparticles dispersed in methanol was prepared and used to spray precursor films. Annealing 

these precursor films in the presence of CdCl2 produced large-grained CdTe films. The films were characterized by 

x-ray diffraction (XRD) and scanning electron microscopy (SEM). Optimized spray and processing conditions are 

crucial to obtain dense, crystalline films. 

15. SUBJECT TERMS

CIGS; spray deposition; ink-based precursors; x-ray diffraction, scanning electron microscopy; crystalline film; large 

grain; 

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