Surface & Coatings Technology 212 (2012) 55–60

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Surface & Coatings Technology

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Influence of processing parameters on the preparation of CuInS2 thin film by one-stepelectrodeposition as the solar cell absorber

Lin Lu a,⁎, Yanjie Wang b, Xiaogang Li a

a Corrosion and Protection Center, University of Science and Technology Beijing, Beijing 100083, Chinab Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China

⁎ Corresponding author. Tel.: +86 10 62333931 502;E-mail address: lulin315@yahoo.com.cn (L. Lu).

0257-8972/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.surfcoat.2012.03.097

a b s t r a c t

a r t i c l e i n f o

Article history:Received 28 January 2012Accepted in revised form 19 March 2012Available online 19 September 2012

Keywords:CuInS2 thin filmSolar cellElectrodepositionAgingHeat treatment

With the application of a potential of −0.8 V, CuInS2 thin film was electrodeposited on stainless steelsubstrate by one step method in sulfurate bath, consisting of 10 mM CuSO4·5H2O, 10 mM In2(SO4)3 and200 mM Na2S2O3 with the pH value of 2.0. The influence of bath aging conditions, deposition time and theheat treatment on the morphology, stoichiometric proportion and structure of the film were studied byscanning electronic microscopy and X-ray diffraction. Also, the photoelectric performance of theas-received film was characterized by electrochemical approach and ultraviolet–visible spectrophotometer.A highly (112)-oriented chalcopyrite structure was achieved in as-deposited samples. It was proved thatthere existed a critical bath aging time of 10.5 h, an optimal deposition time of 40 min at 50 °C and anannealing temperature of 400 °C, which were favorable to produce a chalcopyrite thin film. Based on theMott–Schottky curve, the flat band potential of 0.89 V and the acceptor density of 1.01×1019 cm−3 wereobtained. The band gap of the as-deposited film was determined with the value of about 1.50 eV.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

As the most promising absorber materials, CuInS2 thin film hasbeen developed by many approaches, such as chemical vapor deposi-tion (CDV), spray pyrolysis, vacuum evaporation and sputtering.One-step electrodeposition is the most attractive approach for re-searchers due to its non-vacuum, low-energy consuming andlow-temperature process, and easy fabrication for large areas [1–11].

Theoretically, the photo properties of the CuInS2 thin film areclosely associated with its surface roughness, grain size, compositiondistribution and grain structure [12,13]. That is, the CuInS2 thin filmwith excellent morphology and ideal stoichiometric proportion isessential for the fabrication of high efficiency solar cells. In light ofit, the processing parameters play an important role in the prepara-tion procedures, because they determine the properties of the thinfilm [14,15]. At present, most studies focused on the chloride systemwhile little emphasis was put on the sulfate bath and its involvedtransformation behavior during bath aging and deposition processes[16,17]. However, during the electrodeposition process, because ofthe remarkable corrosiveness of chloride bath to the instrument,sulfate bath is more feasible and accepted in industrial practice. Thispaper tried to investigate the deposition process from the very begin-ning, i.e. the aging process of bath solution, and the deposition time,which would be helpful to establish the fundamental theory of theproperty controlling for the CuInS2 thin film.

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2. Experimental

2.1. Preparation of the CuInS2 thin films

CuInS2 film was electrodeposited potentiostatically in a three-electrode system with the applied potential of −0.8 V (vs. SCE). Astainless steel substrate with the working area of 1 cm2 was used asthe working electrode. A 5 cm×5 cm mesh electrode plated withiridium oxide and a saturated calomel electrode (SCE) were servedas counter electrode and reference electrode. The plating bathconsists of 10 mM CuSO4·5H2O, 10 mM In2(SO4)3 and 200 mMNa2S2O3, which was adjusted to pH=2.0±1 by dilute sulfuric acid.Before deposition, the bath was aged under the condition listed inTable 1, and the stainless steel substrate was polished on 2000#emery paper to remove the oxide layer, and then cleaned up withacetone and deionized water.

2.2. Thin film characterization

The surface morphology of the CuInS2 thin film was observed byscanning electron microscopy (SEM, FEI Quanta 250, USA). Thecomposition and crystallographic phase structures of the film werecharacterized by Energy Disperse Spectroscopy (EDS) and X-rayDiffractometer (XRD, Rigaku, Japan) with Cu Kα radiation (λ=1.5406A) at 40 kV and 150 mA, respectively.

In order to evaluate the parameters associated to the semiconduc-tor performance of the thin film, such as the flat band potential (Efb)and the density of carriers, AC measurements are carried out applying

Table 1Aging conditions of the bath solution.

Specimen no. a b c d e f

Aging temperature/°C 50 50 50 30 30 30Aging time/h 9 10.5 23 49 53 72

Table 2EDS results of the film shown in Fig. 1.

Sample no. Temperature (°C) Aging time (h) Bath color Cu:In:S (at.%)

a 50 9 Deep wine 1:0.887:1.829b 50 10.5 Black 1:1.018:1.931c 50 23 Black-brown 1:4.8:5.732d 30 49 Deep wine 1:0.974:1.412e 30 53 Black 1:1.692:2.441f 30 72 Black-brown 1:0.536:1.047

56 L. Lu et al. / Surface & Coatings Technology 212 (2012) 55–60

a 0.01 V sinusoidal excitation signal with a frequency of 1 kHz, usingParstat 2273 advanced electrochemical system (Princeton, USA). Theapplied bias ranges from 0 to +2.0 V, at a sweeping rate of 0.01 V/s.

The photo property was measured with an ultraviolet–visible–near‐infrared spectrophotometer (UV–vis–NIR, HITACHI, U-3900H,Japan). According to the relationship between absorption constantand band edge, the band gap can be calculated by Eq. (1) [6]

αhν ¼ A hν−Egð Þ1=2 ð1Þ

(a) (b)

(d) (e)

Fig. 1. SEM photographs of the as-deposited thin film after different bath aging condition(d) 49 hour aged at 30 °C; (e) 53 hour aged at 30 °C; (f) 72 hour aged at 30 °C.

in which α is the absorption coefficient, a constant related to theeffective mass associated with the bands; ν the frequency, h thePlanck constant, Eg the optical band gap. The absorption coefficientcan be calculated from Eq. (2):

a ¼ αd ð2Þ

where a is absorbance, d the thickness of film. Then Eq. (1) can betransformed to the following one:

ahν=Adð Þ2 ¼ hν−Eg: ð3Þ

According to Eq. (3), the film's band gap Eg can be determined atthe intercept of x-axis if hν is plotted with (ahν)2.

3. Results and discussion

3.1. Effect of aging condition of bath solution

The influence of aging time and temperature was investigatedunder different conditions. The detailed process parameters are de-scribed in Section 2.1. It was found that the color of the bath solutionwas progressively transformed into different colors, including lightyellow, light red-brown, deep wine, black to black-brown with theextending of aging time. The process involved in the color change in-dicated the reactions among the compounds in the bath. Based onEDS results of the as-deposited films under different conditions listed

(c)

(f)

s (a) 9 hour aged at 50 °C; (b) 10.5 hour aged at 50 °C; (c) 23 hour aged at 50 °C;

Fig. 2. XRD spectra of the as-deposited film under different aging conditions (a) substratematerial; (b) aged at 50 °C for 10.5 h; (c) aged at 30 °C for 50 h.

57L. Lu et al. / Surface & Coatings Technology 212 (2012) 55–60

in Table 2, the aged solution with black color was favorable towardelectrodepositing the chalcopyrite CuInS2 thin film.

Fig. 1 shows the surface morphology of the as-deposited thin filmunder different aging conditions. In Fig. 1(a) and (d), few crystal ag-gregations are observed when the aging time was 9 h. Comparably,the thin film deposited from the bath solution after longer aging pro-cess, as shown in Fig. 1(b) and (e), is covered with more aggregations.

(a) (b)

(d) (e

Fig. 3. Surface morphology of the as-deposited films deposited at the applied potential of −(c) 30 min (d) 40 min (f) 50 min and (f) 60 min.

According to the EDS results, the element ratio of the thin film forFig. 1(b) and (e) is getting more closely to the stoichiometric propor-tion of CuInS2. But, it does not mean that the longer the aging time ofthe bath solution, the closer to the stoichiometric proportion thecomposition of the film is. Based on the results in Fig. 1(c) and (f),the bath was aged too long to preserve its activity, and theas-deposited film was so thin that the substrate can be easily detectedin EDS measurement. Also, it can be seen that there is only a mono-layer in Fig. 1(f), indicating the unfavorable deposition process.

In order to determine the structure of the as-deposited film, XRDanalysis was performed on samples b and e, the compositions ofwhich were close to the stoichiometric proportion. The results areshown in Fig. 2. XRD pattern of stainless steel substrate was also mea-sured as the control to eliminate its influence on the structure charac-terization of thin films. Based on the pattern, the chalcopyrite CuInS2(PDF#85-1575) can be recognized in samples b and e. Therefore, itcan be concluded that the same structure can be obtained fromdifferentbath aging procedures. That is, as long as the bath solution experienceda certain aging process, some specific compound would be produced inthe solution which is feasible to form the chalcopyrite thin film.

Besides, from the above results, the effect of temperature onreaction rates during aging processes can be understood with theArrhenius equation [10] (Eq. (4)). That is, when temperature rose,the coefficient of reaction rate increased, which leads to the speedupof reactions.

k ¼ k0e−Ea=RT ð4Þ

(c)

) (f)

0.8 V after 10.5 hour aging at 50 °C with the deposition time of (a) 10 min (b) 20 min

Table 3EDS results of the films deposited at−0.8 V after 10.5 h of aging at 50 °C with differentdeposition times.

Sample no. Deposition time (min) Cu:In:S (at.%)

a 10 1:0.515:1.213b 20 1:0.954:1.312c 30 1:0.993:1.804d 40 1:0.899:1.969e 50 1:1.754:2.783f 60 1:1.117:1.775

10 20 30 40 50 6015

20

25

30

35

40

45

50CuInS

atom

con

tent

/ %

deposition time / min

Fig. 4. Changes in the proportion of Cu, In and S with deposition time extending.

58 L. Lu et al. / Surface & Coatings Technology 212 (2012) 55–60

in which k0 is a constant, called frequency factor; Ea is the experimen-tal activation energy, R is the gas constant and T is the temperature.Commonly, Ea is positive for most reactions.

In other words, time-temperature equivalence principle can becited to explain this phenomenon, i.e., the aging effect of the solutionat higher temperature (50 °C) for shorter time (10.5 h) is equal tothat at lower temperature (30 °C) with longer duration (53 h),which is manifested as the same black color for the solution underdifferent aging conditions. Therefore, temperature and aging timeare the key factors affecting the reaction thermodynamics and kinet-ics during aging processes and further determine the composition ofas-deposited film.

(a)

Fig. 5. Surface morphology of (a) the as-de

3.2. Effect of depositing time on the composition of thin films

The deposition time is another processing parameter relating toreaction kinetics, especially to deposition kinetics, because the struc-ture and the grain growth of the film are dependent on it. Fig. 3 showsthe morphology of these as-deposited films with different depositiontimes, which was obtained with the applied potential of −0.8 V vs.SCE after 10.5 hour aging at 50 °C. From the SEM photograph withthe magnification of 1000, it can be found that the film was thickenedwith the extending of time. In Fig. 3(a), the film is composed of tinnygrains, which were so flimsy that scratches on the substrate can beseen. With the white aggregation getting bigger, as shown inFig. 3(b)–(f), the film became thicker and the cracks resulting frominternal stress became more severe.

EDS results of these samples are shown in Table 3. According tothe obtained element ratio (Cu:In:S), the deposition time of 40 minis the favorite deposition time to fabricate the film with better stoichi-ometry. The deposition rate of the different elements is also illustrat-ed in Fig. 4. It can be seen that the deposition of copper and indiumwas behaved in the reverse way. In general, the deposited rate of cop-per inclined and that of indium increased with prolonged time. Rela-tively, sulfur presented a relatively stable deposition rate through thewhole processes.

3.3. Effect of heat treatment

In order to study the effect of heat treatment on the structure ofthe thin film, the as-deposited films were annealed in N2 atmosphereat 400 °C. Fig. 5 shows surface morphologies of the film before andafter annealing, in which grains become more homogeneous afterannealing and the white aggregations almost disappear. That is, theheat treatment improved the surface quality of the film, and the over-laid crystal aggregation became well-distributed. Further, the grainstructure of the thin film was investigated by way of XRD. Fig. 6shows the XRD patterns of the films before and after annealing. Itcan be seen that before the films were annealed, only an amorphouspeak can be found at 2θ=27.9°, indicating the formation of CuInS2.After annealing at 400 °C, the peak becomes sharper, and morepeaks attributed to CuInS2 can be observed, which verifies theexistence of chalcopyrite grain with high crystallinity. As the effectof different annealing temperatures is concerned, it was discussedin another paper [18], in which 400 °C was proved to be the optimaltemperature for the annealing of CuInS2.

According to X-ray diffraction theory, the decrease of grain sizeleads to the widening of diffraction peak when the grain size is

(b)

posited film and (b) the annealed film.

Fig. 6. XRD spectra of the as-deposited film and the annealed film.

0.0 0.5 1.0 1.5 2.00.00E+000

2.00E+011

4.00E+011

6.00E+011

8.00E+011

1.00E+012

1.20E+012

1.40E+012

Potential / V vs. SCE

(a)

-0.1 0.0 0.1 0.2 0.3 0.4 0.5

5.00E+011

6.00E+011

7.00E+011

8.00E+011

9.00E+011

1.00E+012

1.10E+012

1.20E+012

1.30E+012

Potential / V vs. SCE

experiment datalinear fit

(b)

C-2

/ F-2

cm4

C-2

/ F-2

cm4

Fig. 7. Mott–Schottky (MS) plots for the annealed CuInS2 thin film with the appliedpotential of −0.8 V (vs. SCE) for 45 min (a) original MS plot (b) linear part (0–0.5 Vvs. SCE) of MS plot.

59L. Lu et al. / Surface & Coatings Technology 212 (2012) 55–60

smaller than 100 nm. Therefore, the grain size can be calculated withthe Debye–Scherrer equation (Eq. (5)) [11], which was under theconsideration of the sample absorption effect and the influence ofstructure on diffraction patterns.

Dhkl ¼ kλ=β cosθhkl ð5Þ

In which Dhkl is the average thickness of grains in the diffractiondirection of (hkl), i.e. the diameter of the grain perpendicular to(hkl); k is the Scherrer constant (commonly using 0.94); λ is theinjected X-ray wavelength (Cu kα λ=0.15406 nm); θhkl is theBragg diffraction angle (°); β is the full width at half maximum ofthe diffraction peak (rad, noted as FWHM).

Based on the data in Fig. 6, the grain size with the orientation of(112) is obtained, as listed in Table 4. It can be concluded thatannealing is beneficial to grain growth, which finally results in theimprovement of the photo property of the thin film.

3.4. Performance of the CuInS2 thin film

Based on the results, the optimal stoichiometric proportion wasachieved for sulfurate electrodeposition bath, under the aging condi-tion of 10.5 h at 50 °C, the deposition time of 45 min and theannealing temperature of 400 °C.

AC measurements can be interpreted by means of the Mott–Schottky equation (for p-type semiconductor) as Eq. (6):

1C2sc¼ 2

εε0eNd−Eþ EFB−

KTe

� �ð6Þ

where C is the differential capacitance of the space-charge region, ε0,the permittivity of vacuum, ε the relative dielectric constant of CuInS2(ε:11) [19], N the acceptor density for a p-type semiconductor, A thesurface area of the sample, E the electrode potential, and Efb the flatband potential. Based on Mott–Schottky (MS) plots (Fig. 7), it canbe determined that the annealed film is a p-type semiconductor dueto the negative slope of the curve. According to the intercept on thepotential axis and the slope of linear part, the flat band potential of0.89 V and the acceptor density of 1.01×1019 cm−3 can be extracted.

Table 4Parameters for calculating grain size of the film.

Sample no. FWHM/rad 2θ112/° D112/nm

a 0.602 27.948 13.95b 0.099 27.568 86.30

Furthermore, the band gaps can be determined from reflectionspectra obtained by UV–vis–NIR at room temperature. As describedin Section 2.2, the band gap with the optimal process parameters

Fig. 8. Relationship between (ahν)2 and hν for the as-deposited film formed undercondition b in Fig. 2.

60 L. Lu et al. / Surface & Coatings Technology 212 (2012) 55–60

can be calculated from Fig. 8. The obtained gap of 1.459 eV was closeto 1.5 eV, the theoretical band gap of CuInS2.

4. Conclusion

(1) In this work, after the 10.5 hour age conditioning at 50 °C, thechalcopyrite CuInS2 thin film can be achieved by one-stepelectrodeposition with the applied potential of −0.8 V at50 °C for 45 min.

(2) There existed a critical time for bath aging procedure at differ-ent temperatures, which is the prerequisite for the growth ofthe chalcopyrite film. 10.5 h at 50 °C and 23 h at 30 °C wereverified to be the favorite aging times.

(3) The deposited rate of copper inclined and that of indiumincreased with prolonged time. Relatively, sulfur presented arelatively stable deposition rate through the whole processes.

(4) Annealing at 400 °C can greatly improve the surface quality ofthe as-deposited films. Also, the grain size was enlarged from14 nm to 86 nm.

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