Thin Solid Films 518 (2010) 5515–5519

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Effect of triethanolamine and sodium dodecyl sulfate on the formation of CuInSe2thin films by electrodeposition

Rui Yu a,b, Tong Ren a, Can Li a,⁎a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian, 116023, Chinab Graduate University of Chinese Academy of Sciences, Beijing 100049, China

⁎ Corresponding author. Tel.: +86 411 84379070; faxE-mail address: canli@dicp.ac.cn (C. Li).

0040-6090/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.tsf.2010.04.042

a b s t r a c t

a r t i c l e i n f o

Article history:Received 24 September 2009Received in revised form 1 April 2010Accepted 17 April 2010Available online 25 April 2010

Keywords:ElectrodepositionCopper indium selenideRaman spectroscopyScanning electron microscopySodium dodecyl sulfateTriethanolamineThin filmsSolar cells

In this paper, we describe the development of a bath comprising triethanolamine and sodium dodecyl sulfatefor electrodeposition of CuInSe2 thin films, by which long-term bath stability was found to be improved andnear-stoichiometric CuInSe2 films with smooth surface morphology were obtained. Scanning electronmicroscopy studies reveal a dramatic improvement of the crystalline quality of CuInSe2 films with theaddition of sodium dodecyl sulfate. X-ray diffraction results and Raman spectra confirm that theimprovement of the film growth is attributed to the synergistic effect of triethanolamine and sodiumdodecyl sulfate. The addition of anionic surfactant sodium dodecyl sulfate can significantly improve theadherence between the CuInSe2 layer and the substrate.

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1. Introduction

In recent years, copper indium diselenide (CuInSe2) with a I–III–VI2 ternary chalcopyrite structure has received extensive attentiondue to its desirable physical properties such as a direct band gap at∼1 eV and thus a high absorption coefficient of N105 cm−1 at photonenergies above the band gap, whichmake it ideal for the fabrication ofhigh efficiency polycrystalline thin film photovoltaic devices [1].There are multiple techniques currently available for the preparationof CuInSe2 thin films, i.e. co-evaporation [2], sputtering [3], spraypyrolysis [4], electrodeposition [5,6], molecular beam epitaxy [7], etc.Among these, electrodeposition is considered to be a promisingapproach from the viewpoint of non-vacuum large area thin filmproduction [8]. Up to now, laboratory cell efficiency of more than 11%CuInSe2 devices has been already developed using electrodepositedfilms in Prof. Lincot's group [9].

Nevertheless, to extend this technique to practical processes, onecritical problem in the development of CuInSe2-based solar cell is thecontrol of the composition. It has been proved that the filmcomposition should be very close to Cu: In:Se=1:1:2 to obtainhigher efficiency in thin film solar cell. This stoichiometric relation

between the Cu, In and Se atoms is directly related to the electrode-position conditions and concentrations. According to the Nernstequation, the electrode potentials for selenium and copper will, ofcourse, precede the deposition of indium. The standard reductionreactions for Cu, In and Se ions are as following (vs. normal hydrogenelectrode) [10]:

C u2+ + 2e−⇔Cu; E = 0:34 + 0:0295 log αCu2+ =αCu� �

In3+ + 3e−⇔In; E = −0:34 + 0:0197 log αIn3+ =αIn� �

HSeOþ + 4Hþ + 4e− + OH−⇔H2SeO3 + 4Hþ + 4e−⇔Se + 3H2O

E = 0:74 + 0:0148 log αHSeO−2=αSe

� �−0:0443pH

where E, refers to chemical potential, R, ideal gas constant, F, Faradayconstant, αCu, αCu2+, αIn, αIn3+, αSe, αHSeO2

+, aH+ refer to activityof Cu, Cu2+, In, In3+, Se, HSeO2

+ and H+, respectively. From theequations above, it is evident that the single step of electrodepositionof CuInSe2 from aqueous solution containing CuSO4, In2(SO4)3 andH2SeO3 is difficult since the range of the reduction potential of thesemetal ions is considerably large. In order to bring depositionpotentials of metal species closer for a better co-deposition environ-ment, complexing agents were used to shift copper ions in thenegative direction of potential. This problem has been overcome byusing a complexing agent (such as triethanolamine [10], citric acid[11], tartaric acid [12] and potassium thiocyanate [13], etc.) in order toshift the copper deposition potential in the negative direction

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bringing it closer to the In deposition potential [10]. It is seen in theliterature that both the dissolution and deposition reactions of Cu areinhibited by triethanolamine adsorption [10]. It is also observed thatwith an excess of In3+ in the solution, the ratio of Se4+ and Cu2+

fluxes is the key parameter setting the composition [14]. Lowerconcentrations of In3+ (less than 10−3 M) involve the deposition ofelemental selenium. In this case, the electrodeposition process islimited by the diffusion of three species [9].

Another important problem to be overcome is the instability ofthe electrodeposition bath over a long term, because it has a tendencyto form metal oxides/hydroxides precipitated out of solution dueto changes of the bath solution pH. The use of pH buffer agentcontaining potassium biphthalate and sulphamic acid has beenproposed by Bhattacharya and Fernandez [15]. In this work, weemployed an environmental-friendly system consisting of bothtriethanolamine (TEA) and anionic surfactant sodium dodecyl sulfate(SDS) to keep a stable condition for one-step electrodeposition ofCuInSe2 thin films. The addition of SDS not only allows the electrode-positing of more indium in the precursor films, but also producesmooth and compact films without pinholes by reducing the inter-facial tension between water and solid substrate, which is favorableto remove H2(g) bubbles. This electrodeposition bath provides animproved deposition process for producing stoichiometric CuInSe2precursor thin films.

The properties of thin films deposited from this buffer systemwereinvestigated by various techniques, including X-ray diffraction (XRD),scanning electron microscopy (SEM) and Raman spectra, and theanalysis results are discussed in the following section.

2. Experimental details

All chemicals, copper (II) sulfate pentahydrate (CuSO4·5H2O,99%), indium (III) sulfate (In2(SO4)3, 98%), selenious acid (H2SeO3,98%), triethanolamine (TEA), and sodium dodecyl sulfate (SDS), wereused as received. Electrodeposition of CuInSe2 was carried out usingacidic aqueous baths containing 4.5 mM CuSO4, 4.5 mM In2(SO4)3,7.5 mM H2SeO3, 0.1–2 M TEA, and 1–50 mM SDS. The pH values of allsolutions were adjusted to 2 by adding dilute sulfuric acid before theaddition of SDS because the presence of SDS in the solutioncomplicates reliable PH readings. Nitrogen gas was bubbled into thedeposition solution for 10 min before each electrodeposition process.To maintain an acidic solution during bath preparation and preventprecipitation of In hydroxides, baths were always prepared by mixingthe Cu2+, In3+, and H2SeO3 solutions, before adding to a solution ofdissolved buffer and diluting to a volume of 500 cm3. With theaddition of the buffer species, baths are stable over a time period ofweeks, with no precipitation of metal hydroxides observed duringstorage.

A three-electrode electrodeposition setup was used, employinga Pt counter-electrode and a saturated calomel electrode (SCE)reference electrode. All potentials are reported with respect to SCE.The working electrodes were 1 in.×1 in. fluorine-doped tin oxide(FTO) coated glass. The substrates were cleaned ultrasonically withisopropanol, acetone, and ethanol, each for 15 min, then rinsed withdeionized water and dried in an Ar (g) stream prior to deposition.Depositions were carried out using a Princeton Applied Research 263Apotentiostat. All depositions were carried out at 40 °C without stirringthe electrolyte solution. Deposition of CuInSe2 was generally carriedout at −0.65 V for 60–90 min.

X-ray diffraction (XRD) was carried out using Rigaku MiniFlexdiffractometer with a CuKα radiation source. Scanning electronmicroscopy (SEM) was carried out using a Quanta 200 FEG scanningelectron microscope at 20 kV attached with an energy dispersivespectroscopy (EDS) to analyze the composition. Raman spectra wererecorded on a home-made Raman spectrometer equipped with a 532-nm laser source.

3. Results and discussion

Fig. 1a–f shows the SEM images of CuInSe2 film deposited at−0.65 V with the addition of TEA. At the beginning, the buffer agentTEA added into the solution suppressed the reduction of Cu ionsduring co-deposition [11]. Deposition of CuInSe2 with the addition ofTEA was found to consistently deposit dark and powdery films,containing large porous grains of 1–5 μm in size, as shown in Fig. 1a–c.Formation of bubbles was also observed on the electrode and growingfilm during deposition from this bath. Films grown from theseconditions always exhibited cracking and contained significantsecondary phases, resembling cauliflower-like florets 1–2 μm in size,embedded in the film surface. The surface morphology of the filmsgets smoother and the size of the particle gets smaller (∼1 μm) whenthe concentration of TEA increases up to 1 M, as shown in Fig. 1c–d.When the concentration of TEA is higher than 1 M, the surfacemorphology of the films becomes rough and there are white clusters,as shown in Fig. 1e–f. Furthermore, it was found that there was asparse distribution and a cloud-like precipitation on the surface of theCuInSe2 film. This result is due to the high TEA concentration whichcauses a colloid state in the electrolyte. The precipitation of the colloidstate electrolyte adheres to the surface of the substrate during co-deposition. Thus, the quality of the electrodeposited-CuInSe2 thinfilms degraded.

Fig. 2 shows plots of composition of as-deposited CuInSe2 films as afunction of TEA concentration detected by EDS. It is shown that thestoichiometry of Cu:In:Se does not reach 1:1:2 no matter what con-centration of TEA was added, and about 15–17 atom% Owas normallydetected in most of the films.

The porous structure of the film is likely due to the high bathconcentration, which produces excessive current density and allowsthe reduction of H3O+ ions to H2(g) in the acidic bath (Reaction 1),which competes with CuInSe2 growth at this potential, to dominatethe electrode reaction. This is confirmed by the formation of H2(g)bubbles during the deposition, which disrupt the film growth andstructure and produce a porous deposit.

2H3Oþ aqð Þ + 2e−→H2 gð Þ2H2O lð Þ ð1Þ

Huang et al. [16] have reported that variation in the pH of CuInSe2baths resulted in H2 evolution and precipitation of indium hydroxideon the film surface during growth, which was avoided by the additionof buffer (Na3-citrate) to the baths. M. Estela Calixto, followed theapproach of Bhattacharya and Fernandez [17], pH 3 pHydrion buffer, apotassium biphthalate/sulfamic acid mixture, was added to stabilizethe solution chemistry and film growth. Besides, in our work, wefound that the quality of the interface between the CuInSe2 layer andthe substrate was not good when only TEA was present. However, ithas been shown that the adherence between the CuInSe2 layer andthe substrate can be significantly improved in the presence of anionicsurfactant SDS. This is a major concern for the final properties of thephotovoltaic cell. So in the current study, we use anionic surfactantSDS to stabilize the solution chemistry and improve the film growth.Surfactants are commonly used in zinc electrodeposition to controlthe metallic crystal shape and size, in order to produce smooth andbright deposits [18].

Fig. 3 show the SEM images of CuInSe2 films deposited in thesolution containing both SDS and TEA, with the concentration ofSDS varying from 1 to 50 mM while keeping the concentration ofTEA constant at 1 M. Deposition from buffered baths of TEA andSDS mixture resulted in growth of smooth and compact, silvery-gray films without formation of pinholes. The adherence of thefilms to the FTO substrate is improved tested by the paste/scratchmethod. The particles get closer and cracks decrease with pinholesdistinguishing. The surfacemorphology gets smoothwith the additionof SDS.

Fig. 1. SEM images of CuInSe2 films with the concentration of TEA at (a) 0.1 M, (b) 0.25 M, (c) 0.5 M, (d) 1 M, (e) 1.5 M, and (f) 2 M.

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With the increase of the concentration of SDS in Fig. 3, the amountof the dodecyl sulfate anion dissolving from the SDS increased, whichresults in more In content precipitation in the thin film. Thus, the ratioof Cu/In approaches to 1.0 with the increase of the concentration ofthe SDS up to 5 mM, see Fig. 4. It must be noted that the precipitationdisappeared when 20 mM SDS was added to the bath. All the aboveresults show that the quality of electrodeposited-CuInSe2 thin filmscan be improved by adding 20 mM SDS. If the pH of the growthsolution is stable during the growth process, then it is favored todeposit indium. This is due to the fact that the dodecyl sulfate anionproduces a charge transfer reaction, which lowers the deposition rate

Fig. 2. Composition of the CuInSe2 films detected by EDS as a function of TEA con-centration. Error bars represent standard deviation from 10 samples and threemeasurements in different areas per sample.

and consequently improves the quality of the thin films [16]. As isknown to all that the variation of the pH value for the bath solutionresulted in hydrogen evolution on the surface of the thin film, whichled to poor quality films and produced indium hydroxide In(OH)3precipitation on the surface of the thin film [16]. In this study, it isfound that anionic surfactant SDS added into the growth solutioncan inhibit the precipitation of indium hydroxide thus increasingIn content in the as-deposited films. Besides, SDS can efficientlyproduce smooth and compact films without pinholes by reducing theinterfacial tension between water and solid substrate, which isfavorable to remove H2(g) bubbles. The exact mechanism of theimprovement of the film growth with the addition of SDS is not fullyunderstood. However, evidence suggests that SDS in the electrode-position bath greatly facilitates the growth of higher quality CuInSe2thin films.

To get structural information, the deposited films were analyzedby X-ray diffraction. Fig. 5 shows the XRD patterns of CuInSe2 filmsdeposited in the solution containing (a) SDS 20 mM, (b)–(f) bothSDS and TEA, with the concentration of SDS at (b) 1 mM, (c) 5 mM,(d) 10 mM, (e) 20 mM, and (f) 50 mM while keeping the concentra-tion of TEA at 1 M. Fig. 5a shows that when only SDS is present, thepeaks from CuInSe2 are extremely weak while the peak from FTOsubstrate is obvious, indicating that CuInSe2 can hardly be produced inthe electrodeposition baths containing just SDS. When the bathscontained both SDS and TEA (Fig. 5b–f), the peaks characteristic ofchalcopyrite CuInSe2 structure, (112), (204)/(220), and (116)/(312)are observed. On the basis of the results presented above, theproduction of CuInSe2 thin films could be attributed to the synergisticeffect of SDS and TEA, but not just from SDS.

It is found that the intensity and the full width at half maximum ofthe CuInSe2 (112) peaks in Fig. 5b–f are weak and large, respectively,which reflect a poorly crystallized material. Besides, the breadth andposition of the XRD peaks did not allow for a clear identification of thesecondary phases. Due to the nanocrystalline and/or amorphous

Fig. 3. SEM images of CuInSe2 films deposited in the solution containing both SDS and TEA, with the concentration of SDS at (a) 1 mM, (b) 5 mM, (c) 10 mM, (d) 20 mM, (e) 30 mM,and (f) 50 mM while keeping the concentration of TEA at 1 M.

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structure of the polyphasic materials obtained, additional analysismethods are required to identify phases in addition to conventionalX-ray diffraction method. Raman spectroscopy is a suitable tech-nique for this task. It is a widely used technique in polycrystallinesilicon and chalcopyrite monitoring the process in the solar cellsindustry.

Fig. 6 presents Raman spectra as the further evidence for the phaseand purity for the same sample. The main part of this spectrum issituated between 100 cm−1 and 400 cm−1. The most intense peaksituated at 177 cm−1 is associated with the A1 mode of the

Fig. 4. Composition of the CuInSe2 films detected by EDS with the concentration of SDSvarying from 1 to 50 mM while keeping the concentration of TEA at 1 M.

chalcopyrite lattice vibration according to Tanino [19], while the240 cm−1 peak is attributed to Se [20]. When only SDS is present(Fig. 6a), the peaks arising from CuInSe2 (177 cm−1) are weak whilethe peak from Se is strong, indicating that CuInSe2 can hardly beproduced in the electrodeposition baths containing just SDS.When only TEA is present (Fig. 6b), the peaks arising from CuInSe2(177 cm−1) are strong while the peak from Se is absent. In addition, aweak peak at 258 cm−1 related to the presence of a high-conductiveCuxSe phase leading to shunting of devices appears, indicating thatTEA can produce both the CuInSe2 phase and the CuxSe phases. When

Fig. 5. XRD patterns of CuInSe2 films deposited in the solution containing (a) SDS20 mM, (b)–(f) both SDS and TEA, with the concentration of SDS at (b) 1 mM, (c) 5 mM,(d) 10 mM, (e) 20 mM, (f) 50 mM while keeping the concentration of TEA at 1 M.Reflections generated by the FTO substrate are marked as *.

Fig. 6. Raman spectra of CuInSe2 films deposited in the solution containing: (a) SDS20 mM, (b) TEA 1 M, (c)–(g) both SDS and TEA, with the concentration of SDS at(c) 1 mM, (d) 5 mM, (e) 10 mM, (f) 20 mM and (g) 50 mM while keeping theconcentration of TEA at 1 M.

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both SDS and TEA are present in the bath (Fig. 6c–g), the peakscharacteristic of CuInSe2 and Se are observed, while the peak relatedto CuxSe phase disappeared, which indicates the film is suitable for theproduction of the efficient solar cells. These are in accordancewith theresults of XRD analysis above, confirming that the improvement offilm growth could be attributed to the synergistic effect of SDS andTEA, but not just from SDS. The films prepared in both TEA and SDScan be treated at room temperature with sodium sulfide (Na2S)solution for elementary Se removal [20].

4. Conclusion

This work co-electrodeposited CuInSe2 thin films on FTO substrate.During the preparation process, the reduction of Cu ions could besuppressed by triethanolamine adsorption. The addition of SDS notonly allows the electrodepositing of more indium in the precursor

films, but also produce smooth and compact filmswithout pinholes byreducing the interfacial tension between water and solid substrate,which is favorable to remove H2(g) bubbles. The improvement of thefilm growth is attributed to the synergistic effect of triethanolamineand sodium dodecyl sulfate. The composition ratio of the annealedfilm was 25.57% Cu, 25.01% In and 49.42% Se, which is closelyapproaching the stoichiometry 1:1:2. It is demonstrated that theelectrodeposited-CuInSe2 thin film could be prepared on a FTOsubstrate for CuInSe2-based solar cell.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (20673112) and the ProgrammeStrategic Scientific Alliances between China and the Netherlands(2008DFB50130).

References

[1] A. Luque, S. Hegedus (Eds.), Handbook of Photovoltaic Science and Engineering,Wiley, New York, 2006.

[2] B.E. McCandless, R.W. Birkmire, 20th IEEE Photovoltaic Specialists Conference, 2,1988, p. 1510.

[3] G.P. Vassilev, P. Docheva, N. Nancheva, B. Arnaudov, I. Dermendjiev, Mater. Chem.Phys. 82 (2003) 905.

[4] Clayton W. Bates, Masakaza Uekita, Kim F. Nelson, Cammy R. Abernathy, John B.Mooney, Appl. Phys. Lett. 43 (9) (1983) 851.

[5] J.-F. Guillemoles, P. Cowache, S. Massaccesi, L. Thouin, S. Sanchez, D. Lincot, J.Vedel, Adv. Mater. 6 (5) (1994) 379.

[6] N.B. Chaure, A.P. Samantilleke, R.P. Burton, J. Young, I.M. Dharmadasa, Thin SolidFilms 472 (2005) 212.

[7] S. Niki, A. Yamada, R. Hunger, P.J. Fons, K. Iwata, K. Matsubara, A. Nishio, H.Nakanishi, J. Cryst. Growth 237–239 (2002) 1993.

[8] D. Lincot, Thin Solid Films 487 (2005) 40.[9] D. Lincot, J.F. Guillemoles, S. Taunier, D. Guimard, J. Sicx-Kurdi, A. Chaumont, O.

Roussel, O. Ramdani, C. Hubert, J.P. Fauvarque, N. Bodereau, L. Parissi, P.Panheleux, P. Fanouillere, N. Naghavi, P.P. Grand, M. Benfarah, P. Mogensen, O.Kerrec, Sol. Energy 77 (2004) 725.

[10] T.-J. Whang, M.-T. Hsieh, Y.-C. Kao, S.-J. Lee, Appl. Surf. Sci. 255 (2009) 4600.[11] M.C.F. Oliveira, M. Azevedo, A. Cunha, Thin Solid Films 405 (2002) 129.[12] T. Yukawa, K. Koumoto, K. Kuwabara, Thin Solid Films 280 (1996) 160.[13] M. Benaicha, N. Benouattas, C. Benazzouz, L. Ouahab, Sol. Energy Mater. Sol. Cells

93 (2009) 262.[14] J. Kois, S. Bereznev, E. Mellikov, A. Opik, Thin Solid Films 511–512 (2006) 420.[15] R.N. Bhattacharya, A.M. Fernandez, Sol. Energy Mater. Sol. Cells 76 (2003) 331.[16] C.J. Huang, T.H. Meen, M.Y. Lai, W.R. Chen, Sol. Energy Mater. Sol. Cells 82 (2004)

553.[17] M. Estela Calixto, K.D. Dobson, B.E. McCandless, R.W. Birkmire, J. Electrochem. Soc.

153 (6) (2006) G521.[18] A. Gomes, M.I. da Silva Pereira, Electrochim. Acta 52 (2006) 863.[19] H. Tanino, Phys. Rev. B: Condens. Matter 45 (1992) 13323.[20] O. Ramdani, J.F. Guillemoles, D. Lincot, P.P. Grand, E. Chassaing, O. Kerrec, E.

Rzepka, Thin Solid Films 515 (2007) 5909.