K.K. Banger

Ohio Aerospace Institute, Brook Park, Ohio

J.A. Hollingsworth

Washington University, Saint Louis, Missouri

M. H.-C. Jin

Ohio Aerospace Institute, Brook Park, Ohio

J.D. Harris and S.A. Duraj

Cleveland State University, Cleveland, Ohio

M. Smith and D. Scheiman

Ohio Aerospace Institute, Brook Park, Ohio

E.W. Bohannan and J.A. Switzer

University of Missouri at Rolla, Rolla, Missouri

W.E. Buhro

Washington University, St. Louis, Missouri

A.F. Hepp

Glenn Research Center, Cleveland, Ohio

Ternary Precursors for Depositing I-III-VI2 Thin

Films for Solar Cells Via Spray CVD

NASA/TM—2002-211994

December 2002
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K.K. Banger

Ohio Aerospace Institute, Brook Park, Ohio

J.A. Hollingsworth

Washington University, Saint Louis, Missouri

M. H.-C. Jin

Ohio Aerospace Institute, Brook Park, Ohio

J.D. Harris and S.A. Duraj

Cleveland State University, Cleveland, Ohio

M. Smith and D. Scheiman

Ohio Aerospace Institute, Brook Park, Ohio

E.W. Bohannan and J.A. Switzer

University of Missouri at Rolla, Rolla, Missouri

W.E. Buhro

Washington University, St. Louis, Missouri

A.F. Hepp

Glenn Research Center, Cleveland, Ohio

Ternary Precursors for Depositing I-III-VI2 Thin

Films for Solar Cells Via Spray CVD

NASA/TM—2002-211994

December 2002

National Aeronautics and

Space Administration

Glenn Research Center
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NASA/TM—2002-211994 1 

TERNARY PRECURSORS FOR DEPOSITING I-III-VI2 THIN 

FILMS FOR SOLAR CELLS VIA SPRAY CVD

K.K. Banger 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

J.A. Hollingsworth 

Washington University 

Saint Louis, Missouri 63130 

M. H.-C. Jin 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

J.D. Harris and S.A. Duraj 

Cleveland State University 

Cleveland, Ohio 44115 

M. Smith and D. Scheiman 

Ohio Aerospace Institute 

Brook Park, Ohio 44142 

E.W. Bohannan and J.A. Switzer 

University of Missouri at Rolla 

Rolla, Missouri 65409 

W.E. Buhro 

Washington University 

Saint Louis, Missouri 63130 

A.F. Hepp 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135

Abstract 

The development of thin-film solar cells on flexible, lightweight, space-qualified substrates provides an 

attractive cost solution to fabricating solar arrays with high specific power (W/kg). Thin-film 

fabrication studies demonstrate that ternary single source precursors (SSP’s) can be used in either a hot 

or cold-wall spray chemical vapour deposition (CVD) reactor, for depositing CuInS2, CuGaS2 and 

CuGaInS2 at reduced temperatures (400 to 450 °C), which display good electrical and optical properties 

suitable for photovoltaic (PV) devices. X-ray diffraction studies, energy dispersive spectroscopy (EDS) 
NASA/TM—2002-211994 2 

and scanning electron microscopy (SEM) confirmed the formation of the single phase CIS, CGS, CIGS 

thin-films on various substrates at reduced temperatures. 

1. Introduction 

Photovoltaic modules based on I-III-VI2 ternary chalcopyrite absorber layers, have been the focus of 

intense investigation for over two decades. The use of chalcopyrite absorbers are highly appealing since 

their bandgaps correlate well to the maximum photon power density in the solar spectrum for both 

terrestrial (AM 1.5), and space applications (AM0), while displaying long term stability and excellent 

radiation tolerance [1–4]. Additionally, by adjusting the percent atomic composition of either Ga for In 

and/or S for Se, the bandgap can be tuned from 1.0 to 2.4 eV, thus permitting the fabrication of multijunction solar cells [5]. One of the key technical issues outlined in the 2001 US Photovoltaic roadmap 

is the need to develop low cost, high throughput manufacturing for high-efficiency thin film solar cells. 

Thus, an important step for device fabrication for high specific power (WKg–1) thin film solar cells, is 

deposition onto flexible lightweight substrates. A novel approach is the use of ternary I-III-VI2 single 

source precursors (SSP’s) in a spray chemical vapour deposition (CVD) process. Although, a rich and 

diverse array of binary SSP’s [6–8] are reviewed and tested, the number of known ternary SSP’s is still 

limited, as is their use in deposition processes [7]. Hence, in this paper we highlight recent advances, 

summarizing a highly promising technique for thin film growth, via molecular design of ternary SSP’s 

for use in various spray CVD processes. 

2. Experimental 

The SSP’s are synthesized based on a modification of the procedure reported by Kanatzidis [9,10]. 

Films were deposited on conventional soda-lime, corning 7059 slides, Si(100), and Si(111) substrates. 

Thin films of CuGaS2 and CuGaxInyS2 were deposited using [{PPh3}2Cu(SEt)2In(SEt)2] 1 and the new 

Ga analogue 2, which was not fully characterized [11] in a custom-made horizontal hot-wall spray 

CVD reactor [9,12,14], whereas films of CuInS2 were deposited using [{PBu3}2Cu(SEt)2In(SEt)2] 3 in 

a custom-made vertical cold wall spray CVD reactor [13]. The CuGaS2 deposition was conducted using 

a 0.011 M mixed toluene/methylene bromide solution (86 vol % toluene) (1.3 mmol; 1.10 g); substrate 

temperature of 450±5 °C with an Ar carrier-gas flow rate of 4.0 L/min [11]. The Cu(In,Ga)S2

deposition was conducted with similar deposition parameter used for CuGaS2 above except, a 0.01 M 

toluene solution of the In 1 and Ga 2 SSP’s in an attempt to achieve a ratio of In0.75Ga0.25. Films were 

characterized by optical transmission spectroscopy (Perkin Elmer, Lambda-19, Cary 5 UV-VIS-NIR 

spectrophotometer), scanning electron microscopy-EDS (Hitachi S-3000N), X-ray diffraction (Philips 

PW3710 : Cu Kα, 1.541 Å) and Van der Pauw four point probe system (Bio-Rad HL5500PC). 

Reported SEM-EDS measurements are accurate to ±3%. 

3. Results and Discussion 

We have previously reported the preparation and utility of ternary SSP’s to the semiconductor CuInS2 

in a horizontal hot wall spray CVD process [9,12,14]. We now report, the preparation of ternary SSP 

for the fabrication of thin-film CuGaS2, in addition, the versatility of ternary SSP’s in a spray CVD 

process is demonstrated by their use in either a hot and cold wall reactors of either horizontal or vertical 

configuration (Fig.1). Spray CVD studies utilizing the new Ga SSP in a toluene/CH2Br2 solution 
NASA/TM—2002-211994 3 

afforded well-adhered, visually smooth and optically transparent dense thin film exhibiting a pinkgreen surface tint. X-ray diffraction (XRD) analysis confirmed the film to be 112 oriented, tetragonal 

single phase CuGaS2 (Fig. 2). The 220/204 reflections and the 312/116 reflections were split consistent 

with the tetragonal distortion of the crystal lattice [15]. Lattice parameters a and c were calculated 

from X-ray d spacings, (1/d2

 = 1/a

2

 (h2 + k

2

) + l

2 1

/c

2

; where h, k, and l are the Miller indices of 

individual reflections [16]), Table 1. Comparison of the data collected from the CuGaS2 thin film 

shows they are in good agreement with the JCPDS reference values for single-crystal CuGaS2 and with 

those reported in literature [17].

SEM images reveal the films are dense with an average grain size of 410 nm, with a columnar grain 

structure (Fig. 3(c) to (d)). The surface microstructure consisted of faceted grains many of which 

exhibited a trigonal shape (Fig. 3(a) to (b)), which occurs as a result of close-packed intersecting (112) 

faces of the chalcopyrite lattice. These are the lowest surface-energy faces and typically control 

chalcopyrite morphology [8,18,19]. 

The resistivity for the CuGaS2 thin film samples deposited at 450 °C on fused silica were determined 

using the four-point probe method [20] and found to be 15.6(4) Ω⋅cm, which is comparable to those 

reported in literature [20,21]. The optical bandgap of the films were determined from the derivative of 

the optical transmittance data (Fig. 4), which correlates well with the reported direct band gap of 

CuGaS2, Eg = 2.43 eV [22]. 

Initial studies using a homogeneous toluene solution of the two ternary SSP’s 1 (0.0075 M) and 2

(0.0025 M) for fabrication of an alloy film Cu(In:Ga)S2 was also investigated [11]. Although a thinfilm was deposited, composition and microstructure varied along the length of the thin film. XRD 

reflections representing the 112 planes were broad and complicated by the presence of an unidentified 

reflection in that region. The 220/204 planes was represented by a single unresolved reflection that 

yielded an average grain size of ~40 nm. The relative contribution of Ga and In to the multinary 

structure was determined by comparing the 2-theta values for this reflection in the multinary pattern to 

those in the patterns of the ternary end-members, CuInS2 and CuGaS2 (Fig. 5(f)). The composition of 

each metal was assumed to vary linearly with 2-theta from 100% In to 100% Ga based on Vegard's 

Law. The tetragonal splitting was neglected in the CuGaS2 pattern by averaging the 2-theta values for 

the 220 and the 204 reflections. The atomic percent of Ga in the film was found to increase along the 

length of the film (front to rear), (Fig. 5). A uniform composition over large areas was not achieved 

and none of the compositions were close to the expected In0.75Ga0.25 ratio. The variation in film 

composition is understandable since the Ga derivative decomposed at higher temperatures then its In 

analogue. Therefore, using two SSP’s with matching thermal profiles can provide an effective means 

for depositing multi-ternary films. 

The flexibility of the SSP’s in a spray CVD has been further demonstrated by their use in a cold wall 

vertical CVD reactor [13]. The XRD data obtained from the film grown using 

[{PBu3}2Cu(SEt)2In(SEt)2] 3 in a cold-wall reactor revealed the typical tetragonal chalcopyrite CuInS2

phase with a 112 preferred orientation. The band-gap for the film grown form the liquid precursor, 

derived from a plot of (α E)2

 versus E (Fig. 6), was found to be ~1.46 eV. Although annealing the film 

showed a shift to a higher gradient band edge, it was found to have minimal effect on the observed 
NASA/TM—2002-211994 4 

bandgap. SEM-EDS analysis showed the CuInS2 thin-film to be near stoichiometric with atomic 

percents for Cu, In, and S as 26, 24, and 50 (±3%), respectively. 

4. Summary 

The versatility of the ternary SSP’s is clearly demonstrated by the preparation of various multi-ternary 

semiconductors. Spray CVD using SSP’s is a mild, simple, clean, and scalable technique for depositing 

CuME2 (E= VI, M= III) thin-films at reduced temperatures. Although tests for the deposition of the 

wide bandgap alloy Cu(Ga:In)S2, led to a non-homogenous film composition, it is evident the use of 

two SSP’s with similar thermal profiles, consistent film stoichiometry can be achieved. Spray CVD in 

conjunction with SSP design provides a proof-of-concept for a high manufacturability process. The 

work reported here on the molecular design of SSP’s for their use in a spray CVD process although still 

in its infancy, undoubtedly shows it as a mass producible, cost effective method for fabricating 

commercial thin film PV devices. 

References 

[1] M. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Hasoon, N. Rommel, 

Progress in Photovoltaics 7 (1999) 311. 

[2] D. Hoffman, T. Kerslake, A.F. Hepp, R. Raffaelle XVII Space PV Res. & Tech. Conf., Cleveland, 

OH, USA (2001). 

[3] H.W. Schock, R. Noufi, Progress in Photovoltaics 8 (2000) 151. 

[4] H.W. Schock, K. Bogus, Eds. J. Schmid, H.A. Ossenbrink, P. Helm, H. Ehmann, E.D. Dunlop, 

Proc. 2nd World Conf. On Photovolt. Energy Conf., E.C. Joint Res. Center, Luxembourg, (1998) 

3586. 

[5] D. Tarrant, J. Ermer, Proc. 23rd IEEE Photovoltaic Spec. Conf., Louisville, KY, USA, (1993) 372. 

[6] M. Krunks, V. Mikli, O. Bijakina, H. Hebane, A. Mere, T. Varema, E. Mellikov, Thin Solid 

Films 361–2 (2000) 61. 

[7] A.C. Jones, P. O’Brien, CVD of Compound Semiconductors: Precursors Synthesis, Development 

& Application VCH Press, (1997). 

[8] T.T. Kodas, M.J. Hampden-Smith, The Chemistry of Metal CVD VCH, Weinheim (1994) 475. 

[9] K.K. Banger J.E. Harris, J.E. Cowen, A.F. Hepp, Thin Solid Films 403–404 (2002) 390. 

[10] W. Hirpo, S. Dhingra, A.C. Sutorik, M.G. Kanatzidis, J. Am. Chem. Soc. 115 (1993) 1597. 

[11] J.A. Hollingsworth, Chemical routes to nanocrystallite and thin-film III-VI and I-III-VI 

semiconductors, Doctoral Thesis, Washington University (1999). 

[12] J.D. Harris, D.G. Hehemann, J. Cowen, A.F. Hepp, R.P. Raffaelle, J. Hollingsworth, Proc. 28th 

IEEE Photovoltaic Spec. Conf., Anchorage, AK, USA, (2000) 563. 

[13] M.H.-C. Jin, K.K. Banger, J.D. Harris, J. Cowen, A.F. Hepp, 29th IEEE Photovoltaic Spec. Conf., 

New Orleans, LA, USA (2002). 

[14] J.A. Hollingsworth, A.F. Hepp, W.E. Buhro, Chem. Vap. Deposition 5 (1999) 105. 

[15] D. Albin, R. Noufi, J. Tuttle, J. Goral, S.H. Risbud, J. Appl. Phys. 64 (1988) 4903. 

[16] J.W. Edington, Practical Electron Microscopy in Materials Science, Van Nostrand Reinhold, NY 

(1976). 

[17] H.S. Soliman, J. Phys. D: Appl. Phys. 31 (1998) 1516. 

[18] R. Scheer, K. Diesner, H.J. Lewerenz, Thin Solid Films 268 (1995) 130. 
NASA/TM—2002-211994 5 

[19] A.K. Jena, N.C. Chaturvedi, Phase Transformation in Materials, Prentice Hall, NJ (1992). 

[20] Resistivity determined by using the equation ρ = Rs t, (t = film thickness). B. Tell, J.L. Shay, 

H.M. Kasper, J. Appl. Phys. 43 (1972) 2469. 

[21] V. Sudarsan, S.K. Kulshreshtha, Mat. Chem. & Phys. 49 (1997) 146. 

[22] J.E. Jaffe, A. Zunger, Phys. Rev. B: Condens. Matter 28 (1983) 5176. 

Ar

Atomization 

Chamber 

Presaturation 

Bubbler 

To Cold 

Trap and

Scrubbers Ar 

Hot Zone Warm 

2.5 MHz

Nebulizer 

Solution 

Reservoir 

 

Fig. 1 Schematic of horizontal hot wall and vertical cold wall spray CVD reactors. 

Fig. 2 XRD pattern XRD of spray-CVD grown CuGaS2 film on Si(111). Reflections correspond to those reported for 

Gallite in JCPDS reference # 25-0279 
NASA/TM—2002-211994 6 

Fig. 3 SEM of CuGaS2 films deposited by spray CVD. a) surface view. b) Surface view; 30° tilt showing triangular shape 

characteristic of 112-oriented crystals. c) and d) edge views showing roughly columnar crystal-growth pattern. 

Fig. 4 a) Transmittance vs. wavelength for CuGaS2 thin film; (I = transmitted, Io= incident power; b) Plot of the 

derivative of the transmission data vs. energy. 
NASA/TM—2002-211994 7 

Fig. 5 SEM images of the alloy film showing the variation in microstructure with composition. a–c) Film deposited in the 

first centimeter: CuIn0.43Ga0.57S2. d and e) Film deposited in the last centimeter: CuIn0.27Ga0.73S2. f) XRD spectra 

highlighting the 220/204 reflections of a CuGaS2 film (bottom; Ts = 450 °C), a CuInS2 film (top; Ts = 400 °C) and alloy 

films having InxGay contents in the range: In0.43Ga0.57 – In0.27Ga0.73 (on fused silica). 

Fig. 6 A plot of (α E)2 vs. E for the film grown in the cold-wall reactor, (α is an absorption coefficient estimated from the 

optical transmittance data and E is a photon energy). 

As deposited, 

400 °C 

Annealed, 600 °C, 

10min, N2

f
NASA/TM—2002-211994 8 

Table 1. Comparison of thin-film and single-crystal CuGaS2 lattice parameters, a and c, c/a, 

and the distortion parameter x. The d spacing of the 220 reflection was used to calculate a, 

and the d spacing of the 112 reflection was used with the calculated lattice parameter a 

to determine c. (x = 2 - c/a, given that a hypothetical c/a ratio of 2 would result in the 

absence of any tetragonal distortion)

a (Å) c (Å) a/c x Thin film fabrication method 

5.353 10.495 1.9606 0.0394 Spray-CVD 

5.35 10.48 1.959 0.0410 Evaporated film [14] 

5.351 10.484 1.9593 0.0407 JCPDS card 25-0297: 

Single-crystal from the elements 
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WU–755–A4–01–00

14

Ternary Precursors for Depositing I-III-VI2 Thin Films for Solar Cells Via

Spray CVD

K.K. Banger, J.A. Hollingsworth, M. H.-C. Jin, J.D. Harris, S.A. Duraj, M. Smith,

D. Scheiman, E.W. Bohannan, J.A. Switzer, W.E. Buhro, and A.F. Hepp

Chemical vapor deposition; Chalcopyrite; Thin film technology; Solar cells

Unclassified -Unlimited

Subject Categories: 25 and 44 Distribution: Nonstandard

K.K. Banger, M. H.-C. Jin, M. Smith, and D. Scheiman, Ohio Aerospace Institute, Brook Park, Ohio 44142;

J.A. Hollingsworth and W.E. Buhro, Washington University, St. Louis, Missouri 63130; J.D. Harris and S.A. Duraj,

Cleveland State University, Cleveland, Ohio 44115; E.W. Bohannan and J.A. Switzer, University of Missouri at Rolla,

Rolla, Missouri 65404; A.F. Hepp, NASA Glenn Research Center, Cleveland, Ohio. Responsible person, A.F. Hepp,

organization code 5410, 216–433–3835.

The development of thin-film solar cells on flexible, lightweight, space-qualified substrates provides an attractive cost

solution to fabricating solar arrays with high specific power (W/kg). Thin-film fabrication studies demonstrate that

ternary single source precursors (SSP’s) can be used in either a hot or cold-wall spray chemical vapour deposition (CVD)

reactor, for depositing CuInS2, CuGaS2, and CuGaInS2 at reduced temperatures (400 to 450 °C), which display good

electrical and optical properties suitable for photovoltaic (PV) devices. X-ray diffraction studies, energy dispersive

spectroscopy (EDS), and scanning electron microscopy (SEM) confirmed the formation of the single phase CIS, CGS,

CIGS thin-films on various substrates at reduced temperatures.