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Superlattices and Microstructures 76 (2014) 339–348

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Superlattices and Microstructures

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o ca t e / s u p e r l a t t i c es

Influence of precursor thin films stacking order onthe properties of Cu2ZnSnS4 thin films fabricatedby electrochemical deposition method

http://dx.doi.org/10.1016/j.spmi.2014.10.0220749-6036/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +60 163597004.E-mail address: [email protected] (E.M. Mkawi).

E.M. Mkawi a,⇑, K. Ibrahim a, M.K.M. Ali a, M.A. Farrukh b, Nageh K. Allam c

a Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysiab Department of Chemistry, University Lahore, 54000 Lahore, Pakistanc Energy Materials Laboratory (EML), Department of Physics, School of Sciences and Engineering, The American University in Cairo,New Cairo 11835, Egypt

a r t i c l e i n f o

Article history:Received 3 July 2014Received in revised form 11 October 2014Accepted 14 October 2014Available online 25 October 2014

Keywords:Cu2ZnSnS4 (CZTS)Electrochemical depositionThin film solar cellsStacked metallic films

a b s t r a c t

We fabricated Cu2ZnSnS4 (CZTS) thin films by electrochemicallydepositing precursor stacks on Mo-coated glass in a variety oforders: Cu/Sn/Cu/Zn, Cu/Zn/Cu/Sn, Zn/Cu/Sn/Cu, and Sn/Cu/Zn/Cu.Using Raman spectroscopy and X-ray diffraction, we found thatfor all stacking orders the annealed film was composed of a singleCZTS phase with good crystallinity and strong (112) orientation.For the Cu/Sn/Cu/Zn stack, field-emission scanning electronmicroscopy revealed a homogeneous, compact surface morphologyand large columnar grains. This stack also had an opticalabsorption coefficient of >104 cm�1 and an optical band gap of1.51 eV. We fabricated a solar cell with the structure SLGsubstrate/Mo/Cu2ZnSnS4/CdS/i-ZnO Al:ZnO/Al, which achieved aconversion efficiency of 2.3%.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Kesterite compounds such as Cu2ZnSnS4 (CZTS) are promising absorber materials for thin-film solarcells, because of their low toxicity, abundant elemental constituents, and good optoelectronicproperties (e.g., band-gap energy of 1.45 eV and absorption coefficient of >104 cm�1) [1–3]. CZTS

340 E.M. Mkawi et al. / Superlattices and Microstructures 76 (2014) 339–348

can be obtained by replacing the In atoms in the chalcopyrite CuInS2 with equal amounts of Znand Sn.

To date, most CZTS-based solar cells are fabricated by evaporation or sputtering followed byannealing and sulfurization at elevated temperatures (250–600 �C). To improve the cost and efficiencyof these cells, new processing techniques should be explored, particularly solution-basedhigh-throughput electrochemical techniques. CZTS has been prepared by several methods, includingsputtering [4] or evaporation [5], spray drying [6], sol–gel [7], hydrazine deposition [4], and electrodedeposition [8]. In solar cells, the composition of the CZTS absorber layer greatly influences the cell’sperformance, the most efficient absorbers tend to have compositions that are Cu-poor(Cu/(Zn + Sn) = 0.8–0.9) and Zn-rich (Zn/Sn = 1.1–1.2). To account for the loss of volatile species duringsulfurization and annealing, it is important to control the initial metal ratios. Fernandes et al. [9]reported producing high-quality CZTS by using a nonstoichiometric initial metal composition ofCu/(Zn + Sn) = �0.7–1.1 and Zn/Sn = 1.0.

By using stacked precursor layers, one can easily control the compositional ratio of CZTS thin filmsby adjusting the thickness of each layer. The properties of the resultant CZTS-based thin film dependstrongly on the stacking order of the precursor films. Araki et al. reported how six different precursorstacks (deposited using electron-beam evaporation) influenced the properties of the resultant CZTSthin films; they reported a conversion efficiency of 1.79% [10].

Unfortunately, CZTS thin films grown using metallic stacked precursors tend to lose Zn and Sn dur-ing annealing in a sulfur atmosphere. Also, H. Katagiri found the Sn precursor layer to be quite roughcompared with the other elemental precursor thin films, leading to many voids and defects and, thus,causing their CZTS-based thin-film solar cells to have low conversion efficiency [11]. Although Zn andSn losses during sulfurization have been reduced, no reports have studied varying the stacking order ofSn- and Zn-based sulfur bindery compounds. Because CZTS is composed of four elements, many sec-ondary phases can form, such as ZnS, CuS, Cu2S, SnS2�x, and Cu2SnS3 [12]. To manufacture a CZTSabsorber, the two main approaches to electrochemically deposit the precursor layers are (i) usingstacked elemental layers [13,14] and (ii) using a single Cu–Zn–Sn co-electrodeposited layer [4,10].

In this work, we fabricated CZTS thin films by sulfurizing precursor layers (Cu/Sn/Zn) electroplatedin various orders and then annealing those layers in an Ar-filled quartz tube furnace containing S pow-der. We then characterized those layers and investigated how the stacking order of the precursor filmsaffected the structural, morphological, chemical, electrical, and optical properties of the resultant CZTSthin films.

2. Experimental

We first sputtered a 1 lm Mo layer (sheet resistance of 0.25 X/sq) on soda-lime glass substrates inan Ar atmosphere (pressure of 6–7 mTorr, power of 200 W). Pieces cut from this original sample werethen sonicated sequentially in detergent, distilled water, ethanol, and isopropanol. For electrodepos-ition, we used an Ag/AgCl reference electrode, a Pt counter electrode as an inert anode, and a 2 � 2 cm2

Mo-coated glass substrate as the working electrode. Electrodeposition was performed at roomtemperature without stirring. Prior to electrodeposition, the electrolyte solutions were bubbled withAr (99.995% purity) for 30 min. The Cu solution was composed of 1 mmol of copper (II) chloride(monohydrate, 98+%, Aldrich, USA), 3.0 M NaOH, and 0.2 M sorbitol. The depositions were carriedout at V = �0.9, �1.3V (vs. Ag/AgCl) for copper. The Zn solution was composed of 0.5 mmol zinc (II)chloride (anhydrous, 98%, Aldrich, USA), Hydrion buffer (pH 3), and 1 M KCl. The depositions werecarried out at V = �0.9 V (vs. Ag/AgCl) for zinc. The Sn solution was composed of 0.25 mmol tin (II)chloride (anhydrous, 98%, Aldrich, USA), 1.5 M NaOH, and 0.3 M sorbitol. The depositions were carriedout at V = �0.7 V (vs. Ag/AgCl) for tin. The deposited multilayer films were sulfurized in a vacuum fur-nace with three heating zones in an N2 atmosphere (99.99% purity) at a flow rate of 40 mL min�1. Onezone held S power (30 g), and the other held the stacked metallic film. The two zones were heatedsimultaneously to their target temperatures over 20 min; the S powder zone was heated to 240 �C,and the sample zone was heated to 580 �C; both zones were held at these temperatures for 2 h. Theywere then cooled naturally to room temperature. The time required for the deposition of metals in

E.M. Mkawi et al. / Superlattices and Microstructures 76 (2014) 339–348 341

order Cu/Sn/Cu/Zn was �12/17/27/32 min and in order Cu/Zn/Cu/Sn was �11/14/21/26 min and inorder Zn/Cu/Sn/Cu was �14/16/24/28 min, and in order Sn/Cu/Zn/Cu was �18/15/20/28 min. Eachsample had a metal-layer ratio of 1:1:1:1with thickness �250 nm for each layer; the thickness ofthe entire stack was 1 lm before annealing, and increased to �2 lm after annealing.

Using the CZTS film, we fabricated photovoltaic cells with the following structure: SLG substrateCZTS/CdS/ZnO/ZnO:Al/Al on a Mo-coated glass substrate. The CdS buffer layer (�70 nm) was depos-ited by chemical bath deposition with ammonium hydroxide (1.3 M), cadmium sulfate (0.02 M),and thiourea (0.75 M), mixed at room temperature. Once mixed, this solution was introduced to aheated bath at 70–75 �C containing the samples to be coated. After 15 min, the samples were removedfrom the solution, rinsed with deionized water, and dried with nitrogen gas. A ZnO P-type (�250 nm)deposited by RF sputtering. A ZnO:Al window layer was then deposited by RF sputtering. We depos-ited electrodes of 500-nm-thick Al front contact by vacuum evaporation.

Crystal structure was studied by using X-ray diffraction (XRD, PANalytical X’pert PRO MRDPW3040, Netherlands) using Cu Ka radiation (1.5406 Å). Surface morphology was characterized byusing field-emission scanning electron microscopy (FESEM; FEI Nova Nano SEM 450, Japan) with ana-lytical accuracy ±1%. Optical properties were determined by using an ultraviolet–visible–near infraredspectrophotometer (Cary 5000-UV; BROP-Agilent technologies, Australia). The electrical propertieswere characterized by Four probe Hall effect measurements at RT using the HL5500PC system-Australia). Raman spectra were obtained by using a Raman spectrometer (HR 800 UV; Jobin Yvon,France). Current–voltage (J–V) characteristics were obtained by using an I–V source meter (Keithley2400) under dark conditions as well as under illumination with AM 1.5 G radiation at 100 mW cm2

generated by a 1 sun solar simulator (SS 1000; Optical Radiation Corporation, France). Series and shuntresistance as well as saturation current were determined from the dark J–V curves.

3. Results and discussion

Fig. 1 shows the cyclic voltammograms (CVs) and FESEM images during electrochemical depositionof Cu, Zn, and Sn. The CVs were obtained at room temperature at a scan rate of 50 mV/s. Fig. 1(d)–(f)show the CV curves of the Cu, Zn, and Sn reduction peaks at �1.0 V, �0.9 V, and �0.7 V (vs. Ag/AgCl),respectively.

Fig. 2 shows FESEM images and cross-sections of annealed CZTS thin films with stacking orders ofCu/Sn/Cu/Zn (stacking A), Cu/Zn/Cu/Sn (stacking B), Zn/Cu/Sn/Cu (stacking C), and Sn/Cu/Zn/Cu (stack-ing D). The annealed CZTS thin film using stacking A consisted of closely packed grains, about 2 lm indiameter, with a uniform morphology and no voids or cracks. In contrast, the films using stackings B,C, and D had many voids and cracks on the surface as well as smaller grain sizes. In thin-film solarcells, voids in the absorber layer cause low conversion efficiency because carriers generated from irra-diation are disturbed into both grids [11]. Increasing the grain size decreases the density of grainboundaries, minimizing recombination of charge carriers. Because stacking A had the biggest grainsand the least number of voids, it appeared to be the best CZTS layer.

Table 1 shows compositional ratios of Cu/(Zn + Sn), Zn/Sn, and S/metal for the annealed CZTS thinfilms with different precursor stacking orders. For stacking A, the measured value for Cu/ (Zn + Sn) was0.93 which indicates Cu-poor (Cu/ (Zn + Sn)) 6 1 and measured value for Zn/Sn was 1.11 which indi-cates Zn-rich (Zn/Sn > 1). Stacking B likely lost some Zn because its [Zn]/[Sn] ratio was 0.98, whichshould be nominally higher than 1. Energy-dispersive X-ray spectroscopy (EDS) profiling also showeda Zn-poor film near the surface. Despite Zn loss, the sample was still Cu-rich (1.03). For stacking C, its[Cu]/([Zn] + [Sn]) ratio was 1.01, a higher value than intended. Also, its [Zn]/[Sn] ratio was 0.96, in theideal range of >1. These results show some Zn loss, although EDS profiling showed a uniform Zndistribution. Stacking D was meant to be Cu-rich and Zn-poor. Its [Cu]/([Zn] + [Sn]) ratio was 1.03and its [Zn]/[Sn] ratio was 0.96.

While the small variations of the [Cu]/[Sn] ratio likely originated from chemical, non-uniformity,they may have also been caused by sample morphology or topography because EDS quantificationis sensitive to geometric factors. The former reason would have been caused by Cu migration fromthe bottom to the upper surface during sulfurization, and the latter reason would have been caused

Fig. 1. FESEM surface micrographs and cyclic voltammograms (vs. Ag/AgCl) of (a, d) Cu, (b, e) Zn, and (c, f) Sn.


by inhibition of the Cu–Sn reaction needed to form the Cu2SnS3 phase [15]. We also examined how theorder of the precursor stack influenced the growth of the CZTS film. For this purpose, we took FESEMsurface morphology images of the CZTS films, as shown in Fig. 2. The CZTS films made from the stackwith a Zn top layer (i.e., Cu/Sn/Cu/Zn; stacking A) had a larger grain size than the stacks using Sn andCu for the top layer (stackings B, C, D).

Fig. 3 shows an FESEM cross-section of stacking A (Cu/Sn/Cu/Zn), showing fewer voids and defectsthan the other stackings as well as a larger grain size. The cross-sectional micrograph shows a compactfilm with polyhedral shaped grains, whereas the surface micrograph shows a rough surface.

Fig. 4 shows the XRD results for a Cu–Sn–Zn precursor film on Mo-coated glass, sulfurized for 2 h.The results of stacking A showed major peaks at 28.5�, 33.0�, 47.4�, and 56.5�, attributable to kesterite

Fig. 2. Surface and cross-sectional FESEM images of CZTS thin films stacked as follows: Cu/Sn/Cu/Zn (stacking A), Cu/Zn/Cu/Sn(stacking B), Zn/Cu/Sn/Cu (stacking C), and Sn/Cu/Zn/Cu (stacking D).

Table 1Chemical compositions of CZTS thin films for different precursor stacking order.

Samples Cu% Zn% Sn% S/m ([Cu]/([Zn]+[Sn]) [Zn]/[Sn]

A 22.6 12.7 11.4 0.92 0.93 1.11B 25.9 12.4 12.6 1.01 1.03 0.98C 26.5 12.8 13.3 1.02 1.01 0.96D 26.8 12.6 13.3 0.93 1.03 0.95


Cu2ZnSnS4 (JCPDS 26-0575) orientations of (112), (200), (220), and (312), respectively. These resultsagree well with the reported features of stoichiometric tetragonal CZTS [9,16]. We also foundsecondary phases of SnS2 (JCPDS 89-2028) and Cu2S (JCPDS 84-0206). The main peak was intenseand narrow, indicating good crystallinity. The intensity of the (112) diffraction peak of stacking Awas higher than those of stackings B, C, and D. The (220) and (312) CZTS peaks appeared only instackings A and B.

After annealing and sulfurizing stacking A, we found a well crystallized CZTS film, and we observedno peaks from secondary phases or impurities, even with the deviations in Cu ratios between precur-sor films. We estimated the crystallite size according to the full width at half maximum (FWHM) of thediffraction peaks using Scherrer’s formula [17]

Fig. 3. FESEM images of a broken cross-section for Cu/Sn/Cu/Zn (stacking A).

Fig. 4. X-ray diffraction patterns of the stacked precursor thin films.


D ¼ 0:9kb cos h

ð1Þ

where b is the broadening of the diffraction line measured at half the maximum intensity (in radians)and k = 1.5406 Å is the wavelength of the impinging radiation. Using the prominent peak along the(112) plane, we found the crystallite size in stackings A, B, C, and D to be 64.6, 47.8, 37.9, and30.7 nm, respectively. This result shows a significant increase in grain size for stacking A.

The crystal structures of CZTS and cubic ZnS exhibit similar lattice constants, they differ greatlyonly in their occupation of cationic lattice sites. We used Raman spectroscopy to further characterizethe CZTS absorber layer and to confirm the presence of kesterite CZTS or secondary phases such asCu2SnS3 and ZnS. Fig. 5 shows Raman spectra of annealed CZTS thin films with different precursorstacking orders, which exhibit only CZTS peaks at 287, 338, and 368 cm�1 [18].

In Fig. 5, the intense peak near 338 cm�1 and the shoulders near 288 cm�1 and 306 cm�1 [19] con-firm the presence of the CZTS phase in our films. The peaks become more distinct in samples stackingsC and D, and by sample stackings A, the peaks are sharp. There is no significant indication of phaseseparation for the Cu2SnS3 phase peaks located at 336 and 351 cm�1 (according to ICDD data

Fig. 5. Raman scattering analysis of the stacked precursor thin films.


04-010-5719 (Cu2SnS3)), and ZnS at 355 cm�1 (according to ICDD data 36-1450 (ZnS)) [20]. Thesecharacteristics confirm that the diffraction peak at 2h = 28.5� found for all stacking orders correspondsto a single kesterite CZTS phase. Raman spectra show that stacking A had a kesterite CZTS phasewithout significant amounts of secondary phases. In Fig. 5, the SnS peak at 2h = 74.2� may have beconcealed, but we found no evidence from Raman scattering for SnS, which would have appearedat 160, 190, and 220 cm�1 [21].

XRD and Raman measurements showed that all four CZTS films grew well with (112) crystallinetexture. Although we fabricated the films with different metallic precursor stacks, the results ofXRD and Raman were similar between them. From these results, we conclude that even with somecompositional deviations in the precursor films we obtained quality crystalline CZTS films.

For sulfurization in pure N2, the sample with a top layer of Zn seems a better choice than that witha top layers of Cu, Sn. Considering the growth of the Cu, Sn layers, our analyses reveal the presence of aZn-rich CZTS layer at surface as well as poor diffusion of Zn. However, we found no evidence of ZnS orCTS phases. Also for the samples which deposited with a top layer of Zn and sulfurization performed inN2, we found complete diffusion of Zn and no evidence Zn loss during processing. This growth processalso produced a sample with better crystallinity, according to the results of Raman scattering. ThisRaman shift may be related to the d-spacing or to some compressive stress in the sprayed films.Together with the peak shift found in XRD, these Raman results imply that the as-sprayed films exhib-ited some strain caused by compressive stress, which was released after sulfurization with the accom-panying d-spacing increase. We found large differences in the FWHMs of the 338 cm�1 Raman peakfrom CZTS between samples. Stacking D had the largest FWHM of 23.12, while stacking A had thesmallest, 15.55, meaning it had the best crystallinity. Stacking C, the sample with some Sn loss, hadan FWHM of 21.22, while stacking B had one of 19.93.

As shown in Fig. 6(a)–(b), the optical absorption coefficients and plots of (ahm)2 vs. photon energy(hm) for the annealed CZTS thin films with different precursor stacking orders. The absorption coeffi-cients (a) of the samples were calculated from the absorption spectra of the films grown on Mo-coatedglass substrates. The optical absorption coefficient of the annealed CZTS thin films was >104 cm�1 inthe visible region, indicating a direct band gap. The absorption coefficient is related to the opticalenergy gap Eg by the power-law behavior of Tauc’s relation [22].

ðahtÞ ¼ Bðht� EgÞm ð2Þ

where B is an energy-independent constant, Eg is the optical band-gap energy, and m is an index thatcharacterizes the optical absorption process (theoretically equal to 2 and 1/2 for indirect and directtransitions, respectively). The optical band-gap energies of stackings A, B, C and D were 1.47, 1.41,1.31 and 1.25 eV, respectively, determined by extrapolating (ahm)2 to the x-axis. We attribute the nar-rower band-gap energy of stackings D and C versus stacking A and B to the secondary phases such as

Fig. 6. (a)–(b). Optical absorption coefficients and plots of (ahm)2 vs. photon energy (hm) of the annealed CZTS thin films usingdifferent precursor stacking orders.


Table 2Electrical properties of annealed CZTS thin films with different precursor stacking orders.

Samples Carrier concentration (cm �3) Hole mobility (cm2V�1s�1) Resistivity (X cm)

A 1.46 � 1018 79.25 45.5B 3.66 � 1017 47.12 57.8C 2.86 � 1018 35.26 88.6D 5.87� 1019 21.66 116.6

Fig. 7. Illuminated and dark J–V curves of solar cells fabricated from CZTS films grown with stacking order A: Cu/Sn/Cu/Znmeasured under the irradiance of AM 1.5G full sunlight (100 mW cm�2) with a cell active area of 1.0 cm2.


Cu2S, ZnS, and SnS in the annealed thin films. The band-gap energies of the Cu2S and SnS phases are 1.3and 1.21 eV, respectively [19]. The ZnS secondary phase had a much larger band gap than did CZTS,which will form internal barriers expected to degrade the performance of the solar cell.

The electrical properties of the annealed CZTS thin films were characterized by Four probe Halleffect measurements at RT. Table 2 shows the electrical resistivity, carrier concentration, and mobilityof the annealed CZTS thin films with different precursor stacking orders. The annealed CZTS films werep-type. The annealed CZTS thin film using stacking A show best Carrier concentration bout5.87 � 1019 cm�3. We found the mobility of the annealed CZTS films to depend strongly on stackingorder, which increased from 21.66 to 79.25 (cm2 V�1 s�1), a dependence that we attribute to differ-ences in microstructure and the presence of secondary phase.

We analyzed the performance of the solar cell (area of 0.1 cm2) by measuring current density vs.voltage (I–V) curves in the dark and while illuminated, as shown in Fig. 7 and Table 3 lists the photo-voltaic properties of the CZTS solar cells, where, JSC is the short-circuit current density, FF is the fillfactor, and g is the conversion efficiency. Cell A had a high short-circuit current Isc of 10.96 mA/cm2, but only a low open voltage Voc of 0.38 V and low fill factor (FF) of 0.55% and a conversion effi-ciency of 2.5%, regardless of deviations from the ideal compositional ratios (Cu/(Zn + Sn) = �0.93 andZn/Sn = �1.11). The decrease in Jsc and FF may have been caused by the smaller grain size and

Table 3A comparison of the photovoltaic parameters of the CZTS solar cells using different precursor stacking orders.

Samples Voc (V) JSC (mA/cm2) Jm (mA/cm2) V m (V) FF g (%)

A 0.380 10.96 7.88 293 0.55 2.30B 0.389 10.68 7.80 290 0.54 2.24C 0.400 10.0 7.18 496 0.52 2.08D 0.403 9.00 7.00 496 0.56 2.03


accumulation of ZnS in the back absorber region; these may have deteriorated the transport chargeproperties, particularly the carrier diffusion length. Reduced Voc has been widely observed in high-band-gap chalcogenide solar cells, even without phase separation. Other likely contributions to thelow Voc may include grain boundary chemistry [20] and interface recombination at the front of theCdS–CZTS interface. In addition, phase separation of a lower band-gap compound such as Cu2SnS3

(band gap of 0.95 eV) embedded in the absorber layer near the back contact can reduce theopen-circuit voltage of a device.

4. Conclusion

We prepared CZTS thin films by depositing stacked Cu–Zn–Sn layers with different depositionsequences, and then annealing and sulfurizing the stacked films into CZTS films. We characterizedthese by using XRD, scanning electron microscopy, energy dispersive X-ray spectroscopy, and Ramanscattering. Results of XRD, Raman, and EDS showed that the annealed CZTS thin film using a stacking Ahad a single kesterite crystal structure without secondary phases, whereas stackings B, C, and D had akesterite phase with secondary phases such as Cu2xS, SnS2, and SnS. Stacking A had a very dense mor-phology without voids, whereas stackings B and C contained volcano-shaped voids. We concluded thatelectrochemical deposition and sulfurization is a nontoxic, effective way to produce high-quality,homogeneous CZTS thin-film absorbers for solar cells. Using our best CZTS film, we built a solar cellthat exhibited a conversion efficiency of 2.3%, a Voc of 0.38 V, a JSC of 10.96 mA/cm2, and a FF of0.55%, despite the non-ideal elemental composition in the absorber layer.

Acknowledgement

This work was supported by the Nano-optoelectronics Research Laboratory, School of Physics, Uni-versiti Sains Malaysia under Grant No. 203/PSF-6721001.

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