007) 5957–5960www.elsevier.com/locate/tsf

Thin Solid Films 515 (2

The influence of the precursor concentration on CuSbS2 thin filmsdeposited from aqueous solutions

Simona Manolache a, Anca Duta a,⁎, Luminita Isac a, Marian Nanu b,Albert Goossens b, Joop Schoonman b

a Transilvania University of Brasov, The Centre: Product Design for Sustainable Development, Eroilor 29, 500036, Brasov, Romaniab Delft Institute for Sustainable Energy, TU Delft, Julianalaan 136, 2628 Delft, The Netherlands

Available online 30 January 2007

Abstract

The paper discusses the influence of precursor concentration on the morphology and the structure of CuSbS2 thin films obtained from aqueoussolutions and used as absorber for three-dimensional (3D) solar cells. CuSbS2 films are obtained by Spray Pyrolysis Deposition, varying theprecursor weight ratio (CuCl2d 2H2O: H2NCSNH2: (CH3COO)3Sb) between 2.57: 1: 5.71–6.86: 1: 5.71, at 240 °C. The films were analyzed byXRD, I–V dark measurements and SEM. Enriching the films in antimony proved to be a control method of the films morphology and structure.© 2007 Elsevier B.V. All rights reserved.

Keywords: Precursor concentration; Spray Pyrolysis Deposition; CuSbS2 absorber; 3D solar cell

1. Introduction

An important aim in the solar cells field is to obtain a devicewith low production cost, low payback time, high efficiency,easy to process and able to replace the silicon solar cells. The3D solar cells represent a new alternative to this problem, in thetrend firstly stated by Graetzel in 1991 [1] and M. Nanu in 2004[2].

The 3D solar cell is formed when an n-type nanoporous wideband gap semiconductor is infiltratedwith a p-type semiconductoron a nanometer scale. Themost efficient 3D cell, (4% [2]), has thestructure: TCO (transparent conducting oxide)/dense TiO2

anatase/nanoporous TiO2 anatase (n-type transparent semicon-ductor)/CuInS2 absorber (p-type semiconductor)/Au, Fig. 1.

The absorber CuInS2 (CIS) is part of the I-III-VI2 group ofsemiconductors, with chalcopyrite structure. It is suitable forphotovoltaic applications, because it is stable, non-toxic,inexpensive, has a high absorption coefficient (more than104 cm−1) with a direct band of 1.5.eV. Pure CIS is a p-typesemiconductor, but n-type conduction can be obtained bydoping [3]. Still, the main topic in the cell research is related to

⁎ Corresponding author. Tel./fax: +40 268 416 308.E-mail address: a.duta@unitbv.ro (A. Duta).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.12.046

find new p-type inorganic semiconducting materials as opticalabsorbers.

An alternative to replace the CuInS2 is CuSbS2; it is part ofthe same I-III-VI2 group of semiconductor with chalcopyritestructure and the ionic radius of indium and antimony are almostequal. CuSbS2 is a direct semiconductor, with the band gap1.52 eV, and its properties match with the requirement for thephotovoltaic materials [4,5]. Another reason is the price ofantimony which is lower than for indium.

So far, the literature mentions the deposition of CuSbS2 thinfilms only through annealing chemically deposited Sb2S3–CuSthin films [4].

The paper presents CuSbS2 as a new absorber for the 3Dcell, obtained from aqueous solutions by Spray PyrolysisDeposition (SPD) and discusses the influence of theprecursor weight ratio on the CuSbS2 morphology andstructure. The proposed structure for the 3D cell is TCO/dense TiO2 (anatase)/nanoporous TiO2 (anatase)/CuSbS2(absorber)/Au.

2. Experimental

The CuSbS2 thin films are deposited on two differentsubstrates: (1) TCO, and (2) TCO/dense TiO2/nanoporous TiO2

substrates.

Fig. 1. The structure of a 3D solar cell.

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(1) TCO glass (F doped SnO2 coated glass, Libbey OwensFord, 5×5 cm2), cleaned by successive immersion inethanol and acetone in an ultrasonic bath and dried in anitrogen flow.

(2) The dense TiO2 film is deposited by SPD [6], usingabsolute ethanol (J.T. Baker) solutions of titaniumte-traisopropoxid (TTIP 99.99%, Sigma–Aldrich), andacetylacetonate (AcAc, 99+%, Aldrich) in a volumetricratio of 1:1.5:22.5. The deposition is done on the heatedTCO substrate at 350 °C in open atmosphere. Afterspraying, the samples are annealed in air, at 450 °C, for2 h, and cooled, on the heater, down to the roomtemperature. The nanoporous TiO2 film is obtained bydoctor blade technique [7], on top of dense TiO2/TCOsubstrate, using an aqueous colloid paste (ECN). Thefilms are dried and annealed in air for 12 h at 500 °C.

CuSbS2 films are deposited using as precursors copper (II)–chloride dehydrate, CuCl2d 2H2O, antimony (III) acetate (CH3-

COO)3Sb, 99.99%, and thiourea H2NCSNH2, 99% (both assulphur source and as complexing agent) in aqueous solutions.Small amounts of HCl are used to increase the solubility ofantimony acetate. In order to obtain CuSbS2 films thedeposition temperatures and the precursor weight ratio werevaried, as presented in Table 1.

During spraying, the pressure of the carrier gas (Ar) was 1.2bars and the distance between the spraying nozzle and the heaterwas fixed at 27 cm.

The CuSbS2 films were analyzed using X-Ray Diffraction(XRD, Bruker D8 Advance Diffractometer), Scanning ElectronMicroscopy (SEM, Jeol JSM-5800LV), and current–voltage(I–V) measurement recorded in dark (DC Source Meter,Keithley, model 2400) and using graphite paste as contact.

Table 1The parameters varied in the deposition of CuSbS2 layers by SPD

Tests Substrate CuCl2. 2H2O: (CH3COO)3Sb: H2NCSNH2 T(°C)

(A) TCO 1: 2.57: 5.71 a 200 – 240 a– 350(B) 1: 3.43: 5.71 240(C) 1: 4.29: 5.71 240(D) 1: 6: 5.71 240(E) 1: 6.86: 5.71 240a Optimized condition.

3. Results and discussions

The deposition temperature for CuSbS2 films was fixed at240 °C when black, homogenous films are formed. Attemperatures below 200 °C the reaction rate are too low andfilms are not formed. Temperatures higher than 280 °C preventthe deposition of the CuSbS2 films due to the instability of thecopper-compounds that are sublimated from the substrate,resulting just different compounds of antimony.

The films were deposited using different precursor weightratio in the sprayed solution, in order to obtain CuSbS2 films,Table 1.

The XRD patterns, Fig. 2 identified the formation oforthorhombic CuSbS2 films according to JCPDS: 24–0347,with the representative peaks at 2Θ=28.713, 47.331, 56.439,corresponding to the (040), (160), and (002) directions.Temperature optimization may improve the crystalline structureof the thin film.

The crystallites grain sizes were calculated for the A and Esamples, using the Scherrer formula, for the (0 4 0) peak,(2Θ=28.713):

D ¼ 0:9kbcosh

; ð1Þ

where:

λ The wavelength=0.179021 nmβ The full-width at half-maximum of the peak, in radianΘ, The Bragg angle

The calculated values are D=49, 68 nm (A sample) andD=88.31 nm (E sample).

Aggregates of crystallites are formed, as the comparisonbetween the crystallites grain size calculated from XRD and thevalues estimated from the SEM pictures shows; increasing theantimony amount, the films are transforming from grainaggregates with a fibber texture (A sample) in films withincreasing density (E sample), Fig. 3.

Electrical measurements, I–V curves, recorded in dark,prove that pinholes are resulting in the A film, Fig. 4, and free of

Fig. 2. XRD analysis of CuSbS2 thin films deposited on TCO substrate atdifferent precursor weight ratio.

Fig. 3. SEM pictures at different precursor weight ratio.

Fig. 4. I–V dark curve of CuSbS2 film (sample A). Fig. 5. I–V dark curve of Sb-rich film (sample E).

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pinholes films result in the E sample, Fig. 5. The I–V darkresponse of the E sample can be associated to the high amountof antimony that increases the films’ density.

The E-type films were further deposited on nanoporousTiO2/dense TiO2/TCO structure in order to investigate itsefficiency as absorber in a 3D solar cell. The I–V curverecorded under illumination gives reasons to continue theresearch in this direction.

4. Conclusions

CuSbS2 films were obtained by varying the depositionparameters: the substrate temperature (200–350 °C) and theprecursors weight ratio CuCl2

. 2H2O: (CH3COO)3Sb:H2NCSNH2, in the range 1: 1.7: 2.6–1: 6.8: 5.7.

The XRD patterns identify the formation of orthorhombicCuSbS2 and with a crystallite size increasing from 49.68 nm to88.31 nm, while increasing the antimony amount. Grainaggregates with a fibber texture are obtained from precursorsaqueous solutions with a weight ratio close to stoichiometry.

Free of pinholes films are obtained from precursors with ahigher amount of antimony (Sb-rich film). Thus, enriching the

films in antimony can be a control method of the filmsmorphology and structure.

Acknowledgements

This work was supported by the Romanian National Councilfor Research in High Education according with the TD. 290grant.

References

[1] B. O'Reagan, M. Gratzel, Nature 353 (1991) 737.[2] M. Nanu, J. Schoonman, A. Goossens, Adv. Mater. 16 (5) (2004) 453.[3] B. Mahrov, Uppsala University, 2004.[4] Y. R. Lazcano, M.T.S. Nair, P.K. Nair, J. Cryst. Growth 223 (2001) 399.[5] J. Nelson, The Physics of Solar Cells, Imperial College Press, 2003.[6] A. Duta, I. Visa, M. Nanu, ISES World Congress, Goteborg, Sweden, CD

proceeding, 2003.[7] V. Shkolover, M.K. Nazeeruddin, S.M. Zakeeruddin, C. Barbe, A. Key, T.

Haibach, R. Steurer, H.U. Hermann, Nissen, M. Gratzel, Chem. Mater. 9(1997) 430.