Highly dense and crystalline CuInSe2 thin films prepared by single bath electrochemical deposition

Electrochimica Acta 87 (2013) 450–456

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Highly dense and crystalline CuInSe2 thin films prepared by single bath electrochemical deposition Hana Lee a,b , Wonjoo Lee b , Jin Young Kim b , Min Jae Ko b , Kyungkon Kim b , Kyungwon Seo a , Doh-Kwon Lee b,∗ , Honggon Kim b a b

Department of Chemical Engineering, Ajou University, Suwon 442-749, Republic of Korea Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea

a r t i c l e

i n f o

Article history: Received 24 July 2012 Received in revised form 28 September 2012 Accepted 29 September 2012 Available online 8 October 2012 Keywords: CIS Electrodeposition Relative density Packing density Solar cell

a b s t r a c t Chalcopyrite CIS or CIGS have been regarded as promising absorbing materials for thin-film solar cells with widespread commercialization prospects. The most critical material properties of a CIS absorption layer that affect the overall PV performance include its microstructure and composition at a given bandgap energy. In this study, dense CISe films with high crystallinity and uniform, flat surfaces were fabricated on In2 Se3 /ITO employing single bath electrochemical deposition by adjusting the deposition parameters, such as the precursor concentration, pH, and applied potential. A simple formula is presented based on Faraday’s law to quantitatively estimate the density of the electrodeposited thin films; from this, it was found that the as-deposited films had a very high relative density of 0.73. The high green density of the asdeposited film led to the full densification of the CISe film with ca. 10 ␮m sized grains. The binary selenide phase remaining in the sintered film was subsequently etched out using a KCN solution, resulting in an overall Cu-deficient composition in the film of [Cu]/[In] = 0.95. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Cux (In1−y Gay )(Se1−z Sz )2 (CIGSeS, CIGSe or CIGS for z = 0, CISe or CIS for y, z = 0) are promising materials for highly efficient thin-film PV applications due to their high absorption coefficients (∼105 cm−1 ), tunable band gaps (1.0 eV for CISe – 2.4 eV for CGS), high material stability, etc. [1–4]. Absorbing films based on CIGS and its derivatives (CIGS henceforth) are fabricated by several methods, which can be divided into two categories: (i) the simultaneous selenization/sulfurization with elemental metal deposition on a substrate at elevated temperatures and (ii) the formation of a precursor film followed by a subsequent selenization/sulfurization. The former is most commonly performed in a vacuum-based co-evaporation technique [5,6], while the latter is carried out through vacuum-based sputtering [7] or non-vacuum methods, including electrochemical deposition (also called electrodeposition or electroplating) [8] and ink/paste coating using solution or nanoparticle-based colloidal suspension precursor materials [9,10]. Of these, the non-vacuum processes have attracted much attention in recent years as alternatives to co-evaporation and sputtering techniques due to their potential for realizing low cost photovoltaic (PV) devices.

∗ Corresponding author. Tel.: +82 2 958 6710; fax: +82 2 958 6649. E-mail address: [email protected] (D.-K. Lee). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.09.118

For a given CIGS absorbing material with specified bandgap energy, the most critical material properties influencing PV performance are its microstructure and composition. A well developed polycrystalline microstructure with a low surface roughness and a high sintered density is required for preventing losses caused by shunt path formation or the retardation of electron transport, which may take place in highly porous structures with rough surfaces and relatively small crystals. The composition of this compound semiconductor, or x, y, z, dictates the semiconducting properties, including the bandgap energy, the electronic band alignment with neighboring buffer layers, the majority carrier type and its concentration, etc. There exist important contradictory requirements for the composition, particularly for CIGS, arising from a phase stability constraint and at certain temperatures of interest, secondary liquid phase segregation. ␣-CIGS have long been known to have a solid solubility toward Cu-poor compositions, with little compositional tolerance on the other side [11]. Additionally, a Cu-poor composition is generally favored to obtain p-type semiconducting properties. On the other hand, a Cu-rich composition is believed to be essential for CIGS phase formation and grain growth in order to achieve sufficient densification with large grains. Due to the phase stability, any excess copper segregates into a binary selenide phase, e.g., Cu1−ı Se or Cu2−ı Se, which acts as a fluxing agent in the resulting liquid phase-assisted sintering [12]. Indeed, the intentional segregation of Cu-rich secondary phases during the formation of CIGS films is one of the primary rationales for the 3-stage co-evaporation technique [13].

H. Lee et al. / Electrochimica Acta 87 (2013) 450–456

It is not practical to adapt the 3-stage annealing concept to socalled 2-step processes, i.e., a precursor film formation followed by a post-annealing process. Moreover, non-vacuum depositions generally result in high porosity in the precursor films relative to sputtering, which hinders small grains from necking and growing and often leads to a rough sintered surface. Thus, this remains one of the main obstacles that must be overcome in order to achieve high efficiency PV devices. Indeed, the sintered density of CIGS films tends to be high in proportion to the packing density of the precursor film depending on the preparation methods of precursor film in 2-step processes. In general, the highest packing densities for precursor films are formed by sputtering, followed by electrochemical deposition and then by solution/colloid coating. This order is closely related to the highest efficiencies of the solar cells fabricated by each method. In order to realize the potential for cost reduction arising from a low initial investment, low equipment maintenance costs, a high material usage rate, and adaptability to large area roll-to-roll manufacturing, the precursor film must have a high packing density (green body density) along with compositional control using non-vacuum CIGS deposition methods. In this study, dense CISe films were fabricated by a single bath electrodeposition with an aqueous solution containing sulfamic acid (H3 NSO3 ) and potassium hydrogen phthalate (C8 H5 KO4 ) as pH buffering and/or complexing agents. We also examined the electrodeposition of In2 Se3 films on ITO substrates, allowing the avoidance of toxic CdS forming processes and enabling PV devices in a superstrate configuration [14–17]. An emphasis is placed on finding the optimal conditions during the electrochemical deposition in order to achieve a dense CISe film with high crystallinity and uniform, flat surfaces, which are of critical importance for PV applications. In addition, a simple quantitative method for estimating the density of the thin films prepared by electrodeposition is suggested and used to determine the relative density of the as-deposited CISe films. Adjustment of the overall CISe film composition into the Cudeficient region, which is an important criterion for high efficiency solar cells, was accomplished by using a KCN solution to etch the Cu-rich secondary phases out of the CISe film after sintering. 2. Experimental Electrochemical depositions of In2 Se3 and CuInSe2 on ITO were performed in an electrochemical cell with the following configuration:  ITO|semiconductor|electrolyte|Pt⊕

(I)

as depicted in Fig. 1. A Sn-doped In2 O3 film coated on 1 mm thick glass (ITO glass, 15 /sq., Samsung Corning Co., Korea) measuring 2.5 cm × 2.5 cm was used as the working electrode and 2.5 cm × 1.8 cm was used for electrodeposition, with the remainder of the surface being reserved for electrode deposition. A Ag/AgCl reference electrode (CH Instrument, Austin, TX) and a 2 cm × 2 cm Pt counter electrode plate were employed. The ITO glass was washed in an ultrasonic bath in IPA, acetone, and IPA in turn for 10 min each, and was then treated in an UV/ozone cleaner for 2000 s. Electrolytic solutions of 2.4 mM InCl3 (99.999%, Sigma–Aldrich), 4.8 mM SeO2 (99.5%, Daejung Chemicals Metals), and 0.24 M LiCl (98%, Junsei Chemical) dissolved in H2 O for In2 Se3 , and 2.56 mM CuCl2 (99%, Sigma–Aldrich), 2.4 mM InCl3 , 4.8 mM SeO2 , and 0.24 M KCl (99%, Sigma–Aldrich) in H2 O for CuInSe2 were prepared for electrodepositions. Complexing/buffering agents help bring the deposition potential for Cu toward the negative direction, as well as to buffer the aqueous solution at an adequate pH. Here,

451

Fig. 1. Schematic representation of the electrochemical cell for electrochemical deposition of In2 Se3 and CuInSe2 . (1) ITO glass for a working electrode; (2) Ag/AgCl for a reference electrode; (3) electrolytic solution; (4) potensiostat; (5) Pt-plate for a counter electrode.

sulfamic acid (H3 NSO3 , 98%, Sigma–Aldrich) and potassium hydrogen phthalate (C8 H5 KO4 , 99.95%, Sigma–Aldrich) [18,19] were added into the electrolytic solution as complexing/buffering agents in varying concentrations from 0 to 10 mM, in order to find the optimal concentration of each. Electrochemical depositions were carried out using a potentiostat/galvanostat (CHI 620A Electrochemical Analyzer, CH Instrument, Austin, TX) with a conventional 3-electrode setup. A constant cathodic potential was applied to the working electrode with respect to the reference electrode in the range of −1.0 < V vs. (Ag/AgCl)/V < −0.5 for depositing In2 Se3 onto ITO, and in the range of −0.6 < V vs. (Ag/AgCl) < −0.5 for depositing CuInSe2 onto In2 Se3 /ITO. A linear sweep voltammetry was employed prior to the electrochemical deposition to determine the experimental range of the cathodic potential. The electrochemical deposition and voltammetry were carried out at room temperature without stirring. A thermal treatment for the recrystallization and/or densification of the as-deposited films was carried out in a Se(g)-containing gas atmosphere at temperatures in the range of 450 < T/◦ C < 550 for 1 h. For this thermal treatment, the sample was placed into an alumina crucible and heated at 5 ◦ C/min in Ar flowing at a rate of 100 sccm. The partial pressure of the Se(g) was controlled by means of selenium pallets of 0.2 g placed at the same temperature with the as-deposited film. To better define the Se partial pressure at the sample surface during sintering, the crucible was covered with an alumina plate, but was not gas-tight. The equilibrium vapor pressures of Se at different temperatures were estimated using the Clausius–Clapeyron equation to be PSe /atm = 2.03 × 10−2 , 5.68 × 10−2 , and 1.40 × 10−1 at 450 ◦ C, 500 ◦ C, and 550 ◦ C, respectively [20]. The as-sintered CISe films were chemically etched with a 0.1 M KCN solution for 60 s to remove any Cu-containing binary selenide phases. The surface morphologies and cross-sections of the In2 Se3 and CuInSe2 films were characterized using scanning electron microscopy, and the compositions of the CISe films were determined using energy dispersive X-ray spectrometry (FE-SEM/EDS, Hitachi, S-4200). The crystal structure of the film was identified by X-ray diffraction (XRD, Philips PANalytical X’Pert Pro MRD, Netherland) using Cu-K␣ radiation ( = 0.15418 nm) in a theta–2theta scan mode. The angle between the incident X-ray and the sample surface was maintained at 2◦ .

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involving Se species. Despite the discrepancies among the reported potential values for each reduction, it has been suggested that the reduction peaks at −0.329 V together with −0.412 V are related to the formation of Cu2 Se and CuSe, and the peak at −0.489 V is associated with the subsequent formation of CuInSe2 [22,23]. The hypothesis of this sequential deposition of copper, selenium and indium, is supported by XPS measurements performed to elucidate the initial stages of electrodeposition [24]. The cathodic polarization curve in Fig. 2 implies that for a single bath electrochemical deposition of In2 Se3 or CuInSe2 , using the electrolytic solutions in the present study, one should apply a potential such that V vs. (Ag/AgCl) < −0.5 V.

0.0 -0.16

-0.18

I / mA

-0.5 -0.25 V vs. (Ag/AgCl) / V

-1.0

-0.20

3+

In /In

2+ +

Cu /Cu /Cu 4+

Se /Se -1.5 -0.7

-0.6

-0.5

-0.4

-0.3

-0.2

V vs. (Ag/AgCl) / V Fig. 2. A cathodic polarization curve of the solution containing 2.56 mM CuCl2 , 2.4 mM InCl3 , 4.8 mM SeO2 and 0.24 KCl in the presence of 10 mM H3 NSO3 and 10 mM C8 H5 KO4 as buffering/complexing agents. The potential was cathodically swept from −0.2 V vs. Ag/AgCl at a rate of 50 mV/s. The configuration of the electrochemical cell is identical with Fig. 1. The inset is the magnified cathodic current in the range of −0.29 < V vs. (Ag/AgCl)/V < −0.20.

3. Results and discussion 3.1. Voltammetry of CuCl2 , InCl3 , and SeO2 in a bath Fig. 2 shows a linear sweep voltammogram for the aqueous solution containing 2.56 mM CuCl2 , 2.4 mM InCl3 , 4.8 mM SeO2 , and 0.24 M KCl along with the buffering/complexing agents (10 mM sulfamic acid and 10 mM potassium hydrogen phthalate). Four characteristic potential values were extracted where the first derivative of the I–V curve for the cathodic polarization is zero. Three distinct reduction peaks were found on the curve at −0.329 V, −0.412 V and −0.489 V vs. Ag/AgCl. The peak at −0.329 V is assigned to the reduction of Cu+ to Cu, while a very small peak observed around −0.208 V (enlarged in inset of Fig. 2) is attributed to the Cu2+ to Cu+ [21] reduction. The In3+ /In redox couple is known to have the largest reduction potential in the Cu–In–Se precursor solution [22], and therefore the peak at −0.489 V is assigned to the reduction of In3+ to In. The peak at −0.412 V is ascribed to the reduction

3.2. Electrodeposition of In2 Se3 on ITO Fig. 3 shows the surface morphologies of In2 Se3 buffer layer films electrodeposited on ITO glass at various cathodic potentials larger than 0.5 V for 10 min in aqueous solutions containing 2.4 mM InCl3 , 4.8 mM SeO2 , and 0.24 M LiCl without buffering/complexing agents. It is recognized that the sample deposited at −0.5 V vs. Ag/AgCl (Fig. 3(a)) consists of isolated particles 300–500 nm in size. A longer deposition time at the same potential increased the density of the particles without a significant change in their size or thickness (not shown in Fig. 3). A thinner film with a flat surface was deposited by applying −0.6 V vs. Ag/AgCl. As shown in Fig. 3(b), a few openings ca. 800 nm in diameter through which the surface structure of ITO is exposed still remain; this is likely due to an insufficient deposition time. However, the continuity of the thin film became worse with a higher cathodic potential, i.e., −0.7 V (Fig. 3(c)), and an impurity phase started appearing at −0.8 V and −0.9 V (Fig. 3(d) and (e)). At a cathodic potential of −1.0 V vs. Ag/AgCl (Fig. 3(f)), a bush-like structure developed. Thus, we examined the time evolution of the In2 Se3 film structure with the deposition potential fixed at the seemingly optimal value of −0.6 V. The results are as shown in Fig. 4. As the deposition time, td , increased from 20 min, In2 Se3 covered more of the ITO surface, and full coverage was achieved by deposition for 40 min (Fig. 4(c)). The thickness of the In2 Se3 film increased from 40 nm when deposited for 10 min, to 60 nm after a deposition of 40 min. It should be noted that since more voids in the film are filled during deposition up to 40 min, the density of the film shows a corresponding increase, and

Fig. 3. Surface morphologies of In2 Se3 electrodeposited for 10 min as a function of applied potential; (a) −0.5 V, (b) −0.6 V, (c) −0.7 V, (d) −0.8 V, (e) −0.9 V and (f) −1.0 V vs. Ag/AgCl.

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the density of the microstructure. To the best of our knowledge, the microstructure exhibited in the CISe films in Fig. 5((b)–(d) and (f)–(h)) show some of the highest densities ever achieved by electrodeposition. It is not, however, easy to quantify the porosity of a thin film with a thickness in the range of a micrometer via Archimedes’ principle. Thus, we present here a quantitative way to estimate the relative density of these thin films prepared by electrodeposition. The relative density, r , and the porosity, , of a solid material are defined [26] as: r =

b , th

(1)

 = 1 − r ,

Fig. 4. Surface morphologies of In2 Se3 films on ITO electrodeposited by applying −0.6 V vs. Ag/AgCl for 20 min (a), 30 min (b), and 40 min (c), and the thickness, l of the film as a function of deposition time, td (d). The inset in (d) is the cross-section of the film deposited for 40 min.

hence the apparent thickness of the In2 Se3 film vs. deposition time (Fig. 4(d)) does not seem to follow Faraday’s law. In brief, a compact In2 Se3 film of ca. 60 nm thickness with a flat surface could be prepared by applying −0.6 V vs. Ag/AgCl for 40 min. 3.3. Electrodeposition of CuInSe2 onto In2 Se3 /ITO Fig. 5(a)–(h) shows the surface morphologies and the crosssectional images of CuInSe2 thin films electrodeposited onto In2 Se3 /ITO by applying −0.6 V ((a) and (e)) and −0.5 V ((b)–(d) and (f)–(h)) vs. Ag/AgCl in an aqueous solution containing 2.56 mM CuCl2 , 2.4 mM InCl3 , 4.8 mM SeO2 and 0.24 KCl in the presence of 10 mM H3 NSO3 and 10 mM C8 H5 KO4 as buffering/complexing agents. There is a clear difference in the morphologies of the films deposited at −0.6 V and −0.5 V. The former consists of isolated agglomerates 200–500 nm in size (Fig. 5(a) and (e)), whereas the latter exhibits continuous and compact cross-sections with a much higher planar density at the surfaces (Fig. 5(b) and (f)). A high packing density of the precursor film may be one of the most critical requirements for obtaining device-quality CISebased thin films, particularly when employing 2-step processes (precursor film formation followed by a post-annealing step). A poor density of the precursor film diminishes the number of interparticle contacts, which retards matter transport for densification during sintering. The porous microstructure of the sintered CISe film may provide a larger number of shunt paths or recombination sites, which a buffer and intrinsic ZnO layers are supposed to mitigate to some extent. Moreover, open pore channels in the precursor films allow Se-containing gaseous species to react with the Mo substrate, resulting in the formation of a thick MoSe2 layer in conventional substrate configurations, which causes a high series resistance in the PV device and consequently deteriorates the photo-conversion efficiency [25]. In this respect, a highly dense microstructure (Fig. 5(f)–(h)) in the as-deposited CISe films fabricated by non-vacuum methods may help improve the efficiency of CISe-based solar cells. Comparing the films deposited at −0.5 V for 60 min (Fig. 5(b) and (f)), 90 min (Fig. 5(c) and (g)), and 120 min (Fig. 5(d) and (h)), one can see that the thickness of the film increases with increasing deposition time, td , while maintaining

(2)

where b and th are the bulk density and the theoretical density of the film, respectively. The bulk density, b , is calculated as the mass of the solid matrix (or substance), W, divided by the total volume of the thin film, given as Vtot = lA, where l and A denote the measured thickness and the deposited area of the film, respectively. It should be noted that the total volume includes the volume occupied by the matrix, Vth , as well as that of the open and the closed pores inside the film, Vop and Vcp . One can estimate the mass of the substance electrodeposited with the aid of the Faradaic current. With the assumption that there is no side reaction in the electrolytic solution, the theoretical mass of the compound semiconductor electrodeposited, W, is proportional to the total charge transferred according to Faraday’s law. Thus, the bulk density can be written as: b =

W = Vtot



I · dtd

·

nF

M , lA

(3)

where I, td , n, F and M denote the cathodic current, the deposition time, the number of electrons transferred while depositing one molecule of the compound, the Faraday constant, and the formula weight of the compound semiconductor, respectively. By substituting Eq. (3) in (1), we readily obtain the relative density of thin film prepared by electrochemical deposition as:  r = b = th



I · dtd nF

·

M . lAth

(4)

Note that M/th in Eq. (4) can be replaced with the molar volume, Vm . The number of electrons necessary for depositing one molecule of stoichiometric CuInSe2 is given as n = 13 according to the overall reaction in the electrochemical cell: Cu2+ + In3+ + 2SeO3 2− + 12H+ + 13e− = CuInSe2 + 6H2 O.

(5)

Fig. 6 represents a typical cathodic current as a function of the deposition time during the electrodeposition of CuInSe2 on In2 Se3 /ITO at −0.5 V vs. Ag/AgCl for 90 min (for the sample  corresponding to Fig. 5(c) and (g)). The total charge transferred, I · dtd , is calculated to be 13.5 C by integrating the reduction current. The substitution of numerical values including Vm = 58.2 cm3 /mol [4], l = 1.9 ± 0.2 ␮m, and A = 4.5 ± 0.2 cm2 in Eq. (4) resulted in r = 0.73 ± 0.10. The relative uncertainty of the present results was evaluated by an error propagation calculation [27] taking into account the uncertainties from the measured quantities and found to be 14%. It should be noted, however, that the uncertainty arising from the possible side reactions or non-Faradaic current, which was not included in the above error calculation, may lead to an overestimation of the relative density as estimated by Eq. (4). The accuracy of this method, thus, can be improved by multiplying both sides of Eq. (4) by the Faradaic efficiency (FE) for the electrodeposition of CuInSe2 . For an electrochemical system where significant side reactions, such as gas evolution or water splitting, are not involved, as is the case with the present experiment, FE is found to be over 0.9 [28]. In this

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Fig. 5. Surface morphologies and cross-sections of CuInSe2 films electrodeposited by applying cathodic potential with respect to Ag/AgCl reference electrode; −0.6 V for 60 min (a) and (e), and −0.5 V for 60 min (b) and (f), for 90 min (c) and (g), and for 120 min (d) and (h). The thickness of the CISe film was plotted as a function of deposition time (i).

By combining Eqs. (3) and (6), the relative density can be rewritten as: r =

b V l = th ≈ th . th Vtot l

(7)

It should be pointed out that the final equality in Eq. (7) is based on the simplification that the planar density of the film over the electrode area is equal to unity, i.e., Ath = A. By substituting Eq. (7) in Eq. (4), the theoretical thickness of the electrodeposited film can be given by:



lth ≈

I · dtd nF

·

M , Ath

(8)

where lth is likely subject to underestimation due to the same origin with the last equality in Eq. (7). However, Eq. (8) provides a lower limit for the theoretical thickness within the uncertainty associated

CISe on In 2 Se 3 /ITO

10

at -0.5 V vs. Ag/AgCl

15000

Intensity / cps

I / mA

5

0

-5

As-deposited CISe ITO (89-4598)

(116/312)

(6)

(204/220)

W . Vth

(112)

th =

with the A value. The theoretical thickness was thus calculated by inserting the average current, I = 13.5 C/5400 s = 2.5 mA into Eq. (8), as depicted by the solid line in Fig. 5(i). A linear time dependence of the measured film thickness, l vs. td , as shown in Fig. 5(i), is a consequence of the above discussion. Fig. 7(a) shows an X-ray diffraction pattern from a CISe film electrochemically deposited on an In2 Se3 /ITO substrate at −0.5 V vs. Ag/AgCl for 90 min (for the sample corresponding to Fig. 5(c) and (g)). An XRD pattern of a porous sample of 1.86 ␮m thickness, electrodeposited employing 10 mM H3 NSO3 and 3 mM C8 H5 KO4 with all other deposition parameters identical to those used for the films in Fig. 5(c) and (g) is also presented for comparison (Fig. 7(b)). The dense CISe film was found to be highly crystalline in nature with crystallite sizes of ca. 15 nm, as estimated with the full width at half peak maximum (FWHM) by Scherrer’s equation. A closer look at Fig. 5(b)–(d) reveals that the 15 nm sized crystallites agglomerated into larger clusters ca. 50–150 nm in size. Three XRD peaks at 2 = 26.7◦ , 44.3◦ , and 52.5◦ were identified as the (1 1 2), (2 0 4/2 2 0), and (1 1 6/3 1 2) reflections of ␣-CuInSe2 with a chalcopyrite structure (JCPDS #40-1487). An additional peak at 2 = 25.3◦ has been assigned to (1 1 0) crystallographic plane of ␤In2 Se3 [14,15]. Interestingly, the reflections originating from the ITO substrate have negligible intensity compared to those from the CISe film (Fig. 7(a)). A comparison with a porous sample (Fig. 7(b)) clearly demonstrates the difference in the crystallinity of the CISe film as well as the intensity of the ITO-oriented reflections between the samples. It should be emphasized here that little detection of

-In2Se3(110)

case, a possible systemic error due to side reactions or non-Faradaic currents would be less than 10%. In brief, despite the uncertainties associated with the thickness and the deposited area of the film, l and A, as well as with the Faradaic current, I, the present analysis may present a practical way to estimate the density of the electrodeposited film. The theoretical density can be expressed with the volume occupied by the matrix, Vth , as:

10000

(a)

5000

-10

(b) 0

1000

2000

3000

4000

5000

6000

td / s Fig. 6. A typical cathodic current as a function of deposition time during electrodeposition of CuInSe2 on In2 Se3 /ITO at −0.5 V vs. Ag/AgCl for 90 min (for the sample corresponding to Fig. 5(c) and (g)).

0 10

20

30

40

50

60

70

80

2 /o Fig. 7. XRD patterns of the as-deposited CISe films on In2 Se3 /ITO at −0.5 V vs. Ag/AgCl for 90 min; (a) for the film corresponding to Fig. 5(c) and (g), (b) for a porous film prepared with 10 mM H3 NSO3 and 3 mM C8 H5 KO4 .

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455

Fig. 8. SEM micrographs of CuInSe2 films prepared by electrodeposition at −0.5 V vs. Ag/AgCl for 90 min, followed by sintering at 450 ◦ C (a) and (d), 500 ◦ C (b) and (e), and 550 ◦ C (c) and (f), respectively for 1 h. The selenium partial pressure was 2.03 × 10−2 atm (a) and (d), 5.68 × 10−2 atm (b) and (e), and 1.40 × 10−1 atm (c) and (f), respectively, as evaluated by using Clausius–Clapeyron equation [20].

100000

CuInSe2 (40-1487) CuSe (34-0171) 80000

550 C (d)

Intensity / cps

the ITO substrate by XRD under the operating conditions described in Section 2 implies that the as-deposited CISe films with 1.9 ␮m thickness (Fig. 7(a)) have a much higher density relative to the porous films (Fig. 7(b)) and even to the precursor film prepared by sputtering [29,30]. The method presented above for estimating the relative density of the thin films (Eq. (4)) cannot be applied to those prepared by sputtering or solution/colloid coating. The intensity of the X-ray diffraction peak originating from the crystalline substrate, either Mo or ITO, beneath the absorber film, may be taken as a relative measure of the porosity or density between the thin films with comparable thicknesses.

60000

500 C (c)

40000

450 C (b)

20000

As-deposited (a)

3.4. Microstructure and composition of sintered CuInSe2

0 10

Fig. 8 shows the surface morphologies and cross-sections of CuInSe2 films prepared by electrodeposition at −0.5 V vs. Ag/AgCl for 90 min, followed by sintering at 450 ◦ C ((a) and (d)), 500 ◦ C ((b) and (e)), and 550 ◦ C ((c) and (f)) for 1 h. At the surface of the film sintered at 450 ◦ C, a number of pores still remained at the grain boundaries or at triple boundary junctions. As the annealing temperature increased, however, remarkable grain growth occurred until full densification was achieved. As a result, large grains up to 10 ␮m in size were grown in films sintered at 550 ◦ C (Fig. 8(c)). The recrystallization and grain growth are also reflected in the XRD spectra as shown in Fig. 9. For a direct comparison, X-ray diffraction pattern for the as-deposited film is also shown. It is first recognized that the XRD peaks for the as-deposited films that were composed of small crystallites of 15 nm in size became much sharper with higher intensity, indicating that larger grain sizes and higher crystallinity were obtained upon sintering. In addition, the peak at 2 = 25.3◦ assigned to the (1 1 0) plane of ␤-In2 Se3 completely disappeared. However, a trace of secondary phase was detected, as depicted by the open triangles, and was indexed as CuSe (JCPDS 34-0171). The segregation of CuSe was also observed in Fig. 8(c) where CuSe platelets were found to grow vertically. These hexagonal platelets look quite similar in shape to those reported by Wan et al. who identified them as CuSe by Raman spectroscopy [31]. Apart from the deterioration of the surface morphology due to the growth of CuSe, the segregation of conductive Cu-rich binary phases such as Cu2 Se or CuSe may result in shunt paths in solar cells under operating conditions. The formation of the Cu-rich secondary phases is facilitated when the overall composition of the film becomes [Cu]/[In] > 1, as ␣-CuInSe2 has no compositional tolerance on the Cu-rich side [11]. Indeed, the as-deposited films showed Cu-rich compositions, i.e.,

20

30

40

50

60

70

80

2 Fig. 9. XRD spectra of CuInSe2 films electrodeposited at −0.5 V vs. Ag/AgCl for 90 min; as-deposited (a), after sintering at 450 ◦ C (b), 500 ◦ C (c), and 550 ◦ C (d), respectively.

[Cu]/[In] = (1.06 ± 0.12), as determined by EDS (see Table 1). In an attempt to obtain Cu-deficient CISe films, we performed a series of electrodepositions by varying the concentration of InCl3 from 2.4 mM to 9.6 mL, only to find that there was no appreciable change in the [Cu]/[In] ratio in the as-deposited films. It is common to form a copper ion complex species with a complexing agent so that the reduction potential of the Cu is brought in the negative direction, i.e., closer to those of Se and In [32–36]. We found, however, that employing complexing agents in the present electrochemical cell was not as effective in controlling the composition of the film as in obtaining a fine morphology. To overcome this challenge, 2-step electrodeposition processes in separate baths have been attempted, one of which is for depositing an In-rich layer [37,38], or In has been added by a physical vapor deposition [39]. The added complexity of these additional procedures, however, is undesirable, and we therefore employed a KCN solution [31,40] to obtain the Cu-deficient Table 1 Compositions of the CuInSe2 thin films; as-deposited, as-sintered at 500 ◦ C for 1 h, and subsequently etched in 0.1 M KCN solution for 60 s. CuInSe2

[Cu]/[In]

[Se]/([Cu] + [In])

As-deposited Sintered at 500 ◦ C Sintered at 500 ◦ C/etched with KCN etching

1.06 ± 0.12 1.13 ± 0.11 0.95 ± 0.10

1.11 ± 0.13 1.15 ± 0.11 1.10 ± 0.10

456

H. Lee et al. / Electrochimica Acta 87 (2013) 450–456

composition that is favored for p-type semiconducting properties by chemically etching any Cu-rich secondary phases out of the film. The concentration of the solution and the etching time were carefully examined to find the optimal conditions for KCN etching to induce a Cu-deficient composition in the sintered CISe films without deteriorating the adhesion of the films on the substrate. Thus, by etching the film sintered at 500 ◦ C in a 0.1 M KCN solution for 60 s, we obtained Cu-deficient CISe films with [Cu]/[In] = (0.95 ± 0.10) (see Table 1). The high sintering density together with the proper Cu-deficient composition of the CISe thin films in this study exhibit some of the common features of chalcopyrite-based solar cells showing high efficiencies. Further studies are in progress to correlate the microstructure and composition of the electrodeposited CISe films with their device performance. 4. Summary and conclusions Highly dense and crystalline CuInSe2 thin films were fabricated by a single bath electrochemical deposition process without any post-deposition treatments. Any secondary phases, such as CuSe and Cu2 Se, in the as-deposited films were successfully eliminated by adjusting the pH and concentration of an added electrolytic solution that included sulfamic acid and potassium hydrogen phthalate as buffering/complexing agents. A semi-quantitative framework was suggested and used to estimate the density of the thin films prepared by electrodeposition, showing that the CISe thin films deposited in this study have a very high relative density of 0.73. The high packing density or green density of the as-deposited films led to the full densification of the CISe films with ca. 10 ␮m sized grains by sintering at 550 ◦ C. The binary selenide phase, CuSe, remaining in the sintered films was subsequently etched out using a KCN solution, resulting in an overall Cu-deficient composition of [Cu]/[In] = 0.95. The CISe thin films in the present study are promising for solar cell applications in light of their highly dense microstructures along with their Cu-poor compositions. Acknowledgments This work was supported by the program of Korea Institute of Science and Technology (KIST) and the “National Agenda Project” program of Korea Research Council of Fundamental Science & Technology (KRCF). References [1] K. Ramanathan, G. Teeter, J.C. Keane, R. Noufi, Properties of high-efficiency CuInGaSe2 thin film solar cells, Thin Solid Films 480–481 (2005) 499. [2] A.M. Hermann, R. Westfall, R. Wind, Low-cost deposition of CuInSe2 (CIS) films for CdS/CIS solar cells, Solar Energy Materials and Solar Cells 52 (1998) 355. [3] R.N. Bhattacharya, W. Batchelor, H. Wiesner, F. Hasoon, J.E. Granata, K. Ramanathan, J. Alleman, J. Keane, A. Mason, R.J. Matson, R.N. Noufi, 14.1% CuIn1 − x Gax Se2 -based photovoltaic cells from electrodeposited precursors, Journal of the Electrochemical Society 145 (1998) 3435. [4] T. Maeda, T. Takeichi, T. Wada, Systematic studies on electronic structures of CuInSe2 and the other chalcopyrite related compounds by first principles calculations, Physica Status Solidi (A) 203 (2006) 2634. [5] I. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, R. Noufi, 19.9% - Efficient ZnO/CdS/CuInGaSe2 solar cell with 81.2% fill factor, Progress in Photovoltaics: Research and Applications 16 (2008) 235. [6] M.A. Contreras, M.J. Romero, R. Noufi, Characterization of Cu(In,Ga)Se2 materials used in record performance solar cells, Thin Solid Films 511–512 (2006) 51. [7] R. Wuerz, A. Eicke, M. Frankenfeld, F. Kessler, M. Powalla, P. Rogin, O. YazdaniAssl, CIGS thin-film solar cells on steel substrates, Thin Solid Films 517 (2009) 2415. [8] W. Lee, J. Lee, W. Yi, S.-H. Han, Electric-field enhancement of photovoltaic devices: A third reason for the increase in the efficiency of photovoltaic devices by carbon nanotubes, Advanced Materials 22 (2010) 2264. [9] M. Kaelin, D. Rudmann, F. Kurdesau, H. Zogg, T. Meyer, A.N. Tiwari, Low-cost CIGS solar cells by paste coating and selenization, Thin Solid Films 480–481 (2005) 486.

[10] C.J. Hibberd, E. Chassaing, W. Liu, D.B. Mitzi, D. Lincot, A.N. Tiwari, Non-vacuum methods for formation of Cu(In,Ga)(Se,S)2 thin film photovoltaic absorbers, Progress in Photovoltaics: Research and Applications 18 (2010) 434. [11] B.J. Stanbery, Copper indium selenides and related materials for photovoltaic devices, Critical Reviews in Solid State and Materials Sciences 27 (2002) 73. [12] J.J.M. Binsma, L.J. Giling, J. Bloem, Phase relations in the system Cu2 S-In2 S3 , Journal of Crystal Growth 50 (1980) 429. [13] M. Kemell, M. Ritala, M. Leskela, Thin film deposition methods for CuInSe2 solar cells, Critical Reviews in Solid State and Materials Sciences 30 (2005) 1. [14] A. Kampmann, P. Cowache, D. Lincot, J. Vedel, Junction formation studies of onestep electrodeposited CuInSe2 on CdS, Journal of the Electrochemical Society 146 (1999) 150. [15] V. Sittinger, J. Rechid, R. Reineke-Koch, Large area electrodeposition of Cu(In,Ga)Se2 , Thin Solid Films 361–362 (2000) 309. [16] S. Ikeda, R. Kamai, T. Yagi, M. Matsumura, Electrochemical synthesis of CuIn(Se,S)2 layer for thin-film solar cell with a superstrate configuration, Journal of the Electrochemical Society 157 (2010) B99. [17] S. Ikeda, R. Kamai, S.M. Lee, T. Yagi, T. Harada, M. Matsumura, A superstrate solar cell based on In2 (Se,S)3 and CuIn(Se,S)2 thin films fabricated by electrodeposition combined with annealing, Solar Energy Materials and Solar Cells 95 (2011) 1446. [18] R.N. Bhattacharya, A.M. Fernandez, CuIn1 − x Gax Se2 -based photovoltaic cells from electrodeposited precursor films, Solar Energy Materials and Solar Cells 76 (2003) 331. [19] M.E. Calixto, K.D. Dobson, B.E. McCandless, R.W. Birkmire, Controlling growth chemistry and morphology of single-bath electrodeposited Cu(In,Ga)Se2 thin films for photovoltaic application, Journal of the Electrochemical Society 153 (2006) G521. [20] L.S. Brooks, The vapor pressures of tellurium and selenium, Journal of the American Chemical Society 74 (1952) 227. [21] J. Liu, F. Liu, Y. Lai, Z. Zhang, J. Li, Y. Liu, Effects of sodium sulfamate on electrodeposition of Cu(In,Ga)Se2 thin film, Journal of Electroanalytical Chemistry 651 (2011) 191. [22] T.-J. Whang, M.-T. Hsieh, Y.-C. Kao, S.-J. Lee, A study of electrodeposition of CuInSe2 thin films with triethanolamine as the complexing agent, Applied Surface Science 255 (2009) 4600. [23] P. Carbonnelle, L. Lamberts, Electrochemical study of the copper-selenium system using carbon paste electrode, Electrochimica Acta 37 (1992) 1321. [24] O. Roussel, O. Ramdani, E. Chassaing, P.-P. Grand, M. Lamirand, A. Etcheberry, O. Kerrec, J.-F. Cuillemoles, D. Lincot, First stages of CuInSe2 electrodeposition from Cu(II)-In(III)-Se(IV) acidic solutions on polycrystalline Mo films, Journal of the Electrochemical Society 155 (2008) D141. [25] J. Pouzet, J.C. Bernede, MoSe2 thin films synthesized by solid state reactions between Mo and Se thin films, Revue de Physique Appliquée 25 (1990) 807. [26] R. Griffiths, C. Radford, Calculations in Ceramics, Maclaren and Sons, Ltd., London, 1965, p. 28. [27] J.R. Taylor, An Introduction to Error Analysis, University Science Books, California, 1982, p. 40. [28] N. Eliaz, K. Venkatakrishna, A.C. Hegde, Electroplating and characterization of Zn-Ni, Zn-Co and Zn-Ni-Co alloys, Surface and Coatings Technology 205 (2010) 1969. [29] J.H. Shi, Z.Q. Li, D.W. Zhang, Q.Q. Liu, Z. Sun, S.M. Huang, Fabrication of Cu(In, Ga)Se2 thin films by sputtering from a single quaternary chalcogenide target, Progress in Photovoltaics: Research and Applications 10 (2011) 160. [30] O. Volobujeva, M. Altosaar, J. Raudoja, E. Mellikov, M. Grossberg, L. Kaupmees, P. Barvinschi, SEM analysis and selenization of Cu-In alloy films produced by co-sputtering of metals, Solar Energy Materials and Solar Cells 93 (2009) 11. [31] L. Wan, Y. Cao, D. Wang, P-type CuInSe2 thin films prepared by selenization of one-step electrodeposited precursors, Journal of Materials Research 24 (2009) 2293. [32] R.N. Bhattacharya, Solution growth and electrodeposited CuInSe2 thin films, Journal of the Electrochemical Society 130 (1983) 2040. [33] F.J. Pern, J. Goral, R.J. Matson, T.A. Gessert, R.T. Noufi, Device quality thin films of CuInSe2 by a one-step electrodeposition process, Solar Cells 24 (1988) 81. [34] M. Kemell, M. Ritala, H. Saloniemi, M. Leskela, T. Sajavaara, E. Rauhala, Onestep electrodeposition of Cu2-x Se and CuInSe2 thin films by the induced co-deposition mechanism, Journal of the Electrochemical Society 147 (2000) 1080. [35] F. Chraibi, M. Fahoume, A. Ennaoui, J.L. Delplancke, Influence of citrate ions as complexing agent for electrodeposition of CuInSe2 thin films, Physica Status Solidi (A) 186 (2001) 373. [36] S. Beyhan, S. Suzer, F. Kadirgan, Complexing agent effect on the stoichiometric ratio of the electrochemically prepared CuInSe2 thin films, Solar Energy Materials and Solar Cells 91 (2007) 1922. [37] Y. Oda, M. Matsubayashi, T. Minemoto, H. Takakura, Crystallization of InSe/CuInSe2 thin-film stack by sequential electrodeposition and annealing, Journal of Crystal Growth 311 (2009) 738. [38] R.N. Bhattacharya, Electrodeposited two-layer Cu-In-Ga-Se/In-Se thin films, Journal of the Electrochemical Society 157 (2010) D406. [39] R.N. Bhattacharya, W. Batchelor, J.F. Hiltner, J.R. Sites, Thin-film Culn1 − x Gax Se2 photovoltaic cells from solution-based precursor layers, Applied Physics Letters 75 (1999) 1431. [40] T. Delsol, M.C. Simmonds, I.M. Dharmadasa, Chemical etching of Cu(In,Ga)Se2 layers for fabrication of electronic devices, Solar Energy Materials and Solar Cells 77 (2003) 331.