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Pneumatically sprayed Cu2ZnSnS4 films under Ar and Ar–H2 atmosphere

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2014 J. Phys. D: Appl. Phys. 47 245101

(http://iopscience.iop.org/0022-3727/47/24/245101)

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Journal of Physics D: Applied Physics

J. Phys. D: Appl. Phys. 47 (2014) 245101 (8pp) doi:10.1088/0022-3727/47/24/245101

Pneumatically sprayed Cu2ZnSnS4 filmsunder Ar and Ar–H2 atmosphere

M Espindola Rodriguez1,2, D Sylla1, Y Sanchez1, S Lopez-Marino1,X Fontane1, J Lopez-Garcıa1, M Placidi1, A Perez-Rodrıguez1,3,O Vigil-Galan2 and E Saucedo1

1 Catalonia Institute for Energy Research, IREC. Jadins de les Dones de Negre 1, 08930 Sant Adria deBesos, Barcelona, Spain2 Escuela Superior de Fısica y Matematicas-Instituto Politecnico Nacional (IPN), 07738 Mexico DF3 IN2UB, Departament d’Electronica, Universitat de Barcelona, C. Martı i Franques 1, 08028 Barcelona,Spain

E-mail: esaucedo@irec.cat

Received 13 February 2014, revised 6 April 2014Accepted for publication 15 April 2014Published 15 May 2014

AbstractCu2ZnSnS4 (CZTS) thin films have been deposited by pneumatic spray pyrolysis ontosoda-lime glass and Mo-coated glass substrates by using a special reactor designed forworking under controlled atmospheres. The films were grown using both Ar and Ar–H2 as thecarrier gas and atmosphere, aiming to eliminate the oxygen-containing atmosphere from theclassical spray pyrolysis configuration that uses air as the carrier gas. Thin films with differentcompositions were prepared and thermally annealed to investigate its compositional andphysical properties and its performance as absorber layers in solar cells. To know the impactof the thermal annealing, two different annealing temperatures were used: 550 and 580 ◦C,both for 30 min. The last temperature showed a great improvement in the crystallinity andallowed a better incorporation of Sn into the films. The optical band gap of the annealed filmswas found to be around 1.45 eV. Cu–S and ZnO were found as contaminant phases in theas-grown Ar-sprayed samples, but were effectively transformed after annealing. ZnS wasdetected in both the as-grown and annealed films as the main secondary phase. Solar cells freeof scarce elements were fabricated showing an improved conversion efficiency of 1.4%,evidencing the higher potential of this approach for the synthesis of device-grade films.

Keywords: spray pyrolysis, chalcogenides, CZTS thin films, solar cells

(Some figures may appear in colour only in the online journal)

1. Introduction

In the search for novel eco-friendly materials, Cu2ZnSnS4

(CZTS) has recently arisen as a promising candidate forthin-film photovoltaic applications, due to its interestingoptoelectronic characteristics, together with its relatively lowtoxicity and the earth abundance of its constituent elements [1].However, the greatest efficiency for a pure sulphur kesterite-based solar cell is 8.5% [2], still far from that reported formore mature thin-film solar cell technologies such as CIGS(with 20.8%) or CdTe (with 19.6%) [2]. The developmentof cost-effective and eco-friendly methods for the preparationof kesterite is a major challenge for the future deployment of

this incipient technology. Among them, spray pyrolysis [3–5],sol–gel [6, 7], spin- [8, 9] and dip-coating [10] are chemicaldeposition techniques based on precursors’ dissolution, witha high potentiality for cost-reduction and that usually do notinvolve either toxic or dangerous chemical compounds. Inparticular, spray pyrolysis is a conceptually simple technique,being very attractive for the preparation of photovoltaic-gradethin films.

Pneumatic spray pyrolysis has been demonstrated to bea suitable technique for the deposition of Cu2ZnSnS4 films(spray-CZTS) [3]. It can be used to obtain films over largeareas, guaranteeing a relatively high deposition rate, whichis an important factor in industrial applications. Different

0022-3727/14/245101+08$33.00 1 © 2014 IOP Publishing Ltd Printed in the UK

J. Phys. D: Appl. Phys. 47 (2014) 245101 M Espindola Rodriguez et al

works studying some spray parameters have been published;these have investigated the type of dissolvent [11], substratetemperature [3, 12], precursor concentrations [13–16], post-thermal treatments [11, 13, 16] and dissolution pH [17]. Thosespray parameters come from the typical set-up of a spraypyrolysis system consisting mainly of three parts: a nozzle,a substrate heater and a temperature controller; all of themmounted in a fume hood to protect the user from inhalingthe resulting gases. Using this configuration, some researchusing air or N2 as the carrier gas had been reported [3, 18];however, no such work is found in the literature concerningthe spray-deposited CZTS as-grown films’ oxidation and itsadverse effects on the final solar cells, given that an oxygen-containing carrier gas is used in some cases, and in all of themthe growing process has been reported to be in air ambientunless otherwise stated. In this work we want to add afourth system part: the spraying reactor. This is a metallicenclosed chamber capable of containing pressures from 0.8 Pato 130 kPa, within which all of the spraying process takes place,and which brings to light an unexamined spray parameter, thespraying atmosphere. Using this configuration, it is possibleto guarantee that the films were grown under a controlledatmosphere and additionally that they were also cooled downin a controlled oxygen-free atmosphere; as far as we know, thecooling down process is a carefree time for the spray-depositedfilms, or at least is not reported in detail. However, thecooling process is not a less crucial part of the process, becausewhen the growing time has finished, the sample temperaturecould increase (if gas flux is stopped) to reacting temperatures(∼450 ◦C) due to thermal inertia, inherent in the commonlyused metallic substrate holder. There are few reports onsolar cells prepared by spray-CZTS films, and this is directlyrelated to the complexity of managing all of the parametersinvolved in the pyrolysis process to obtain device-grade films;nevertheless, interest in this technique is increasing. For spray-CZTS, a maximum efficiency of 1.85% has been reported [19]using In2S3 as the buffer layer. We have shown, using airas the carrier gas, the importance of controlling the precursorconcentration [15]. It was observed that the presence of oxygenin the atmosphere during the spraying process hinders theincorporation of sulphur, contaminating the as-grown filmswith oxide phases. The presence of these oxides has beenshown to be detrimental for the solar cell’s characteristics.Nevertheless, p–n heterojunctions with a photovoltaic effectwere reported in terms of solar cell efficiency, demonstratingthe potential of the spray-CZTS films to function as absorbersin heterojunction substrate configuration solar cells. Inthis work, we present the development of a spray pyrolysismethod using an inert Ar atmosphere and a reducing Ar–H2 atmosphere, with the aim of avoiding the indiscriminatepresence of oxygen when air is used. For this purpose, aspecial chamber was built, making it possible to use Ar andAr–H2 as the carrier gases and atmosphere; in these conditionswe grew CZTS films with different compositions. Our resultsshow the importance of developing a completely inert routeto increase the efficiencies of the devices prepared with thistechnology, avoiding the contamination with oxygen that takesplace when using the standard configuration that involves anoxygen-containing atmosphere.

Table 1. The molar concentration of CC and ZA used in the differentdissolutions. The concentrations of SC and TH were kept constant.

Concentration

Dissolution [CC] mM [ZA] mM

Stq 9 4.5Cu1–Zn1 8.1 4.95Cu2–Zn1 7.2 4.95Cu1–Zn2 8.1 5.4Cu2–Zn2 7.2 5.4

2. Experimental details

2.1. Sample preparation

CZTS films were deposited onto 3 × 3 cm2 soda-lime glass(SLG) and onto 3 × 3 cm2 Mo/SLG substrates (Mo wasdeposited by dc magnetron sputtering: 500±7 nm in thicknessand an Rsh of 0.25 �/�). As the precursors we usedCuCl2.2H2O (CC) (Sigma-Aldrich �99.0%), Zn(CH3CO2)2

(ZA) (Alfa-Aesar 98.0–101.0%), SnCl4.2H2O (SC) (Alfa-Aesar 98%) and CH4N5S (TH) (Sigma-Aldrich � 99.0%).One of the main advantages of this deposition technique is thatthe rheology of the solution does not need to be optimized,as in other solution-based technologies where the correctrheology is usually provided by carbon-containing solventsand binding agents. This opens the possibility of using carbon-free solvents such as water, eliminating the possibility ofcarbon contamination. In our case, the only sources of carbonare the Zn and S precursors, but it was shown that the depositiontemperature (224 ◦C) promotes the decomposition reaction ofthe precursors with the formation of volatile CO2 [5], resultingin no carbon contamination in our as-grown samples.

Because of the effect of an inert carrier gas in the films,their composition was still unknown. We decided to spraythe dissolutions with different compositions favouring Cu-poor and Zn-rich conditions, characteristic of device-gradeabsorbers. The use of Cu-poor and Zn-rich compositionsallows us to minimize the occurrence of ternary Cu–Sn–Sphases in solution-based routes [20] that are known to bedetrimental for device cell efficiency. Cu-poor dissolutions(Cu1, Cu2) were obtained by reducing the molar concentrationof CC, and Zn-rich solutions (Zn1, Zn2) were prepared byincreasing the concentration of ZA, as shown in table 1.

According to this table, Cu2 dissolutions are poorer in Cuthan Cu1, and Zn2 dissolutions are richer in Zn than Zn1. Theconcentrations of SC (4.4 mM) and TH (50.0 mM) were keptconstant. In growing all of the samples, as is normally donewith this technique, we have used an excess of the sulphursource to compensate for what is lost during the sprayingprocess. We also added a few drops of high purity HCl(35–37% V/V) to the dissolution to maintain an acidic media(pH < 3) in order to avoid the precipitation of the precursors.All the dissolution compositions stated in table 1 were sprayedusing Ar as the carrier gas, and then two selected compositionswere sprayed using Ar–H2 as the carrier gas.

In order to grow a film, the spray chamber was evacuatedand filled with the desired gas. After that, the temperature

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J. Phys. D: Appl. Phys. 47 (2014) 245101 M Espindola Rodriguez et al

controller was powered on to allow the substrate heater to heatthe substrate up in a controlled atmosphere. The temperatureof the samples was directly measured on the top of the substrateusing a mineral insulated type K thermocouple at in ituconditions. During deposition, the substrate temperature forall of the samples was kept at 220 ± 4 ◦C thanks to ourspecially designed double concentric zone iron-made heater.A pressure of 200 kPa of compressed Ar or Ar–H2 (H2 5% byvolume) was used as the carrier gas, using a dissolution flowrate of 3 ml min−1, controlled by a pre-calibrated Masterflexperistaltic pump; the spraying time lasted for 10 min in allcases. The carrier gas used was the same as the one used to fillthe chamber. When the growing time was over, to protect thestill-hot films from possible oxidation or adsorption, sampleswere kept inside the spray chamber under a fixed atmosphereof Ar or Ar–H2 until the sample temperature was below 70 ◦C.

After deposition, the samples were submitted to a reactivethermal annealing, using a three-zone tubular furnace andsemi-closed graphite boxes (with a free volume of 23.5 cm3),under Argon atmosphere (100 kPa) containing 50 mg ofsulphur (Alfa-Aesar, 99.995%) and 5 mg of tin (Alfa-Aesar,99.995%), at two different temperatures (550 and 580 ◦C) for30 min, in conditions similar than those reported elsewhere[21, 22]. These correspond to the annealing conditionsinvestigated for the synthesis of device-grade films usingmulti-stacked metallic precursors obtained by physical vapourdeposition (PVD) techniques. At those temperatures, thereaction mechanism is found to form the kesterite phase inmetallic stack precursors [22] or elemental powders [23].

2.2. Sample characterization

The compositional ratios of the CZTS films were measuredby x-ray fluorescence (XRF Fisherscope XVD), whichhad been previously calibrated with ICP-OES (inductivelycoupled plasma optical emission spectroscopy, Perkin ElmerOptima 3200RL). The absorption spectra were obtained fromspectral transmittance and reflectance measurements, whichwere recorded in the wavelength range 400–2500 nm, usinga Perkin Elmer Lambda 950 UV/VIS Spectrometer. Thesurface morphology and films’ grain size were analyzed byscanning electron microscopy (SEM, FEI NovaTM NanoSEM230) using a work distance of 5 mm and an operating voltageof 5 kV. The phases present in the films were identified byx-ray diffraction analysis on an Advance D8 diffractometer(Bruker) using CuKα radiation (λ = 1.5406 Å, 40 kV, 40 mA).To this end, typical θ–2θ measurements were performed from15◦ to 70◦ with a step size of 0.010◦. The CZTS films werealso characterized by Raman spectroscopy in a backscatteringconfiguration with a T64000 Horiba-Jobin Yvon spectrometerusing the 514.5 nm excitation wavelength of an Ar+ laser, andwith a LabRam HR800-UV Horiba Jobin-Yvon system with a325 nm (UV) excitation wavelength.

To complete the solar cell devices, the samples wereetched in 1% KCN for 2 min at room temperature in orderto remove the potential presence of Cu–S secondary phasesat the surface region of the absorbers. Immediately after,a CdS film (∼60 nm) was deposited by chemical bath

deposition, and intrinsic ZnO (50 nm) and ZnO : Al (450 nm,25 �/� films were grown by pulsed dc magnetron sputteringdeposition (CT100 Alliance Concepts). For optoelectroniccharacterization, 3 × 3 mm cells were scribed using amicro-diamond scriber (MR200 OEG), thus avoiding thenecessity of metallic grid deposition onto the ZnO : Al surface.Measurement of the optoelectronic properties was carried outusing a pre-calibrated Sun 3000 Class AAA solar simulatorfrom Abet Technologies (with a uniform illumination area of15 × 15 cm, AM 1.5 filter), and finally the external quantumefficiency (EQE) of the devices was also measured, using aBentham PVE300 system.

3. Results and discussion

It was observed that the molar composition of the dissolutionshas a direct effect on the atomic composition of the films,as expected. We have also seen that even a concentrationdeviation of only 5% of any of the metallic precursors inthe dissolution can be detected by XRF in as-grown sprayedfilms. Samples were named after the gas and the dissolutioncomposition used to spray them, as shown in table 2. Acomparison between the metallic ratios (deduced from themolar concentration of the different species) in the startingdissolutions, and the same ratios calculated with the measuredatomic composition by XRF in the as-grown and annealedfilms, is reported in table 2.

In the Ar-sprayed films, the Cu/(Zn+Sn) and Zn/Sn ratiosare higher than those corresponding to the starting dissolutions.This suggests that some Sn is lost during the spraying processbecause the Cu/Zn ratio of the as-grown samples are veryclose to those of the starting dissolutions, meaning the sameproportion of Cu and Zn added in the dissolution was thendeposited into the film. This signifies that the amount of copperand zinc are well preserved during the Ar-spraying process.

Concerning to use of Ar–H2 as the carrier gas, we firstobserved that the behaviour of the composition when usingAr–H2 as the carrier gas is different than when using Ar.Table 2 shows that even when using the same compositionof dissolution. if the carrier gas is changed, then a differentcomposition results. It shows that the carrier gas andatmosphere are in fact spray parameters because they defineand determine the performance of the spray system, having adirect effect on the main characteristic of the sprayed films:its composition. As a parameter, the compositional influenceof the carrier gas has to be subjected to further optimization(in terms of the concentration of the dissolution used) to makeit possible to reach the target ratios (Cu/(Zn + Sn) = 0.8 andZn/Sn ∼1.1 were reported as the optimums) [1].

Table 2 also shows the compositional ratios of the filmsannealed at 580 ◦C. In this case, the initial variations of Cuand Zn from the starting precursors are well preserved afterannealing. From this table, it is clear that in order to reach thevalues of the target ratios, both Cu-poor and Zn-rich startingdissolution conditions are required and specific concentrationsof Cu and Zn in dissolution are needed for each carrier gas. Inthe case of Ar sprayed films, the best approach to the targetratios was obtained when using the Cu2–Zn2 composition (the

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J. Phys. D: Appl. Phys. 47 (2014) 245101 M Espindola Rodriguez et al

Table 2. Compositional ratios from the sprayed dissolution, as-grown and annealed (at 580 ◦C) films. The samples were named after boththe carrier gas and the dissolution used.

Sprayed dissolution As grown Annealed at 580 ◦C

Sample name CCZA +SC

ZASC

CCZA

CuSn+Zn

ZnSn

CuZn

SM

CuSn+Zn

ZnSn

CuZn

SM

Ar–Stq 1.00 1.00 2.00 1.12 1.15 2.07 0.86 1.03 1.06 2.02 1.30Ar–Cu1–Zn1 0.86 1.10 1.64 0.88 1.16 1.64 0.88 0.79 1.01 1.59 1.08Ar–Cu2–Zn1 0.76 1.10 1.45 0.75 1.17 1.42 0.80 0.75 1.20 1.38 1.12Ar–Cu1–Zn2 0.82 1.20 1.50 0.83 1.33 1.46 0.77 0.85 1.22 1.57 1.16ArH–Cu1–Zn2 0.84 1.22 1.53 1.01 0.87 1.28 1.54 1.19Ar–Cu2–Zn2 0.73 1.20 1.33 0.78 1.34 1.37 0.80 0.80 1.17 1.49 1.12ArH–Cu2–Zn2 0.68 1.58 1.12 1.08 0.64 1.49 1.08 1.15

Figure 1. SEM images of (a) typical as grown sample, (b) Ar–Cu2–Zn2 annealed sample at 580 ◦C and (c) ArH–Cu1–Zn2 annealed sampleat 580 ◦C. Insets show cross section images respectively.

Ar–Cu2–Zn2 sample), and in the case of ArH it was whenusing the Cu1–Zn2 composition (the ArH–Cu1–Zn2 sample),demonstrating that there is a no real improvement with the useof Ar–H2 as carrier gas at all.

One improvement to note after the use of Ar as the carriergas and atmosphere is the better incorporation of sulphur inthe as-grown films with S/M = 0.8 (Ar–Cu2–Zn2), whichcorresponds to a sulphur incorporation about 13% higher thanthose previously reported on our as-grown films using air as thecarrier gas [15]. In this sense, the use of Ar–H2 improves theincorporation of sulphur in the as-grown samples even more,resulting in an S/M = 1.01 (ArH–Cu1–Zn2). However, thetop morphology of the as-grown films, as seen in figure 1(a),remains almost like the one obtained using air as the carriergas, showing a near-amorphous nature independent of thecomposition of the starting dissolution and the carrier gas used.The insert in figure 1(a) shows a typical cross-section of anas-grown sample where no grains are distinguished, showingsome voids that disappear after a one-step thermal annealing.

During annealing in a S + Sn containing atmosphere, asulphur incorporation process occurs and the sulphur contentafterwards is about 50 at.%; at the same time, a noticeableimprovement in crystallinity and grain size is observed,provided there is the presence of S and Sn. When onlyS powder is used to anneal the samples (not shown here),voids disappear, but only small grains are formed surroundedby amorphous-like zones. Figure 1(b) corresponds to a topview of the Ar–Cu2–Zn2 sample annealed at 580 ◦C in thepresence of elemental powders of S and Sn. At this annealingtemperature, a compact layer results formed by facets showinggrains of up to ∼200 nm in size and free of amorphous-likezones, revealed by the cross-section insert in the same figure.

In the case of the ArH–Cu1–Zn2 sample (figure 1(c)), compactfilms are also obtained showing less sharpened grains, smallerin size than those of the Ar sprayed films. For both carriergases, the grain size is still lower than that generally acceptedas suitable for high efficiency devices, in the order of microns.A major problem to overcome (evident in the SEM pictures)inherent to this deposition technique is the surface roughness,which is higher for the spray pyrolysis technique than forPVD deposition. Roughness is detrimental to the p–n junctionformation adding an additional challenge to the kesterite filmsobtained by this deposition technique.

The use of a controlled atmosphere enhances sulphurincorporation, decreasing the formation probability of volatileS–O species like SO2. The next step could be to reduce theoxygen presence to a minimum by using other polar solventsinstead of water, with a minimum content of oxygen atoms,with a lower boiling point and with as short a carbon chain aspossible, such as CH3–OH or CH3–CH2–OH, to push possiblecarbon contamination to the minimum. The enhanced sulphurincorporation is the first evidence of the positive effect ofspraying in a controlled atmosphere, favouring the kesteriteformation right from the spraying period and not only duringthe annealing time. The second is the bigger grain size obtainedafter annealing using the same conditions used for the air-sprayed films previously reported [15]. Further optimizationof the recrystallization processes and their adaptation to theintrinsic characteristics of the as-grown sprayed precursors topromote the formation of larger grains and smoother surfacesis needed.

In order to evaluate the band gap energy, the spectraltransmittance (Tλ) and reflectance (Rλ) of the samples grownon glass substrates were measured to determine the absorption

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Figure 2. The (αhν)2 versus photon energy of as-grown films(squares) and annealed ones at 580 ◦C in S + Sn containingatmosphere (circles), (a) shows the optical band gap of theAr–Cu2–Zn2 sample, as grown and annealed and (b) shows theoptical band gap of the ArH–Cu1–Zn2 sample, as grown andannealed.

coefficient (α) as a function of the photon energy, hv [24].The optical band gap energy can be estimated from the linearextrapolation of the plot (αhv)2 versus hv with the hv-axis.

Figures 2(a) and (b) show the plots of (αhv)2 versus hv ofAr–Cu2–Zn2 and ArH–Cu1–Zn2 samples, respectively. Theas-grown samples (squares) show an optical band gap valueof 1.68 eV (the Ar–Cu2–Zn2 sample) and 1.66 eV (the ArH–Cu1–Zn2 sample), which agrees with the direct optical bandgap of CZTS thin films as reported by Katagiri et al [1] andKumar et al [13] (in the last one the films were also obtainedby spray pyrolysis).

After annealing (circles in figure 2), both films showedan improved optical band gap value; 1.45 eV for the Ar–Cu2–Zn2 sample and 1.55 eV for the ArH–Cu1–Zn2 sample. Thefirst well matches the pursued bad gap value for absorbers, thesecond has a reasonably adequate value taking into accountthat the composition of the Ar–H2 sprayed films need to beimproved. ThroughTλ andRλ it is not possible to determine thepresence or not (or the amount) of the binary or ternary phasesformed during the spraying or annealing process, but it isuseful to determine the global behaviour of absorber materialsin the part of the electromagnetic spectrum of interest. Thepresence of CuxS in as-grown samples will be disclosed laterby Raman scattering measurements, giving the first evidence ofthe formation of CuxS (Eg = 1.2 to −1.7 eV, typically p-type)secondary phases in the as-grown samples and by XRD in theannealed ones, whose presence is enhanced in the ArH–Cu1–Zn2 samples that were sprayed with a bigger concentrationof Cu, even when its composition is Cu-poor-like, suggestingan incomplete reaction of the precursors because the substratetemperature is not sufficient for a complete reaction of the

Figure 3. XRD pattern of Ar–Cu2–Zn2 (top) and ArH–Cu1–Zn2(bottom) after annealing. The peak identification corresponds toboth samples.

precursors. The presence of ZnS (Eg = 3.7–3.8 eV, typicallyn-type) will be also evidenced by UV Raman scatteringshowing a reduced intensity after annealing, suggesting thatthe annealing process assists the reaction between CuS(s),ZnS(s) and SnS2(g) (from the annealing atmosphere), leadingto a decrease in the presence of secondary phases in theannealed films. This all is globally measured by the Tλ

and Rλ experiments; which would help us to understandthe high Eg values for the as-grown samples and for thosewith compositions favouring some of these unwanted phases.The error in the estimation of the optical band gap energyis ±0.03 eV.

In order to investigate the presence of possible secondaryor ternary compounds in the 580 ◦C annealed films, the θ–2θ

XRD pattern was recorded and is shown in figure 3. Bothlayers exhibit the same diffraction pattern with comparableintensity (the labels identifying the different phases wereindistinctively placed in the top or bottom spectra just forclarity). Only the (0 0 2), (1 0 1) and (1 0 3) diffraction planesof kesterite [25] were identified in both samples demonstratingthe presence of kesterite. However, the x-ray diffraction planesof most the compounds in figure 3 are overlapped, making theiridentification difficult or even impossible using this technique.Presumably ZnS, SnS, Cu2S, Cu7S4 and MoS2 [25] are presentin the films, but these peaks are rather discernible with this typeof characterization.

To better assess the secondary phase identification,complementary Raman spectroscopy was used in the samesamples presented before. In figure 4, the Raman spectra,measured with an 514 nm excitation wavelength on Ar-sprayedand ArH-sprayed samples, are shown. The spectra of both as-grown samples is characterized by the presence of a broadpeak at about 337 cm−1, which has been identified with themain A symmetry peak from the kesterite CZTS phase [26].The annealing of the film leads to a significant decreaseof the full width at half maximum (FWHM) of the mainA peak, 6.4 and 5.6 cm−1 for Ar–Cu2–Zn2 and ArH–Cu1–Zn2, respectively, and a second peak also appears at around287 cm−1, which is attributed to a second A symmetry modecharacteristic of the kesterite phase [26], confirming the XRDresults. Fitting the spectra with Lorentzian curves allowsthe observation of additional weaker contributions in the

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J. Phys. D: Appl. Phys. 47 (2014) 245101 M Espindola Rodriguez et al

Figure 4. Normalized Raman spectra measured with a 514 nmexcitation wavelength. ◦ symbol corresponds to as grown samplesand the * symbol to annealed samples in S + Sn containingatmosphere at 580 ◦C. (a) Ar–Cu2–Zn2 sample and(b) ArH–Cu1–Zn2 sample.

annealed films at 123, 166, 252, 272, 353 and 366 cm−1, whichcorrespond to E or B symmetry modes from the CZTS kesteritephase [27]. This agrees with the significant improvement inthe crystallinity of the samples after the annealing process, asrevealed by SEM. In addition, no ternary phases on the surfacewere conclusively detected from the Raman spectra. However,by Raman scattering, the CuxS phases (x � 1.8), which arecharacterized by a main peak at 475 cm−1, are present only inthe surface of the as-grown samples.

The analysis of the spectral features from the main Ramankesterite peak at about 337 cm−1 measured in the samplesgrown with different starting Cu-poor and Zn-rich dissolutionsand annealed at different conditions shows a decrease ofthe FWHM (from 6 cm−1 down to 5 cm−1) for the samplesannealed at 580 ◦C under a S+Sn atmosphere. This suggests animprovement in the structural quality of the films recrystallizedat the highest temperature with higher Zn/Sn content ratios(in the range between 1.17 and 1.22). In contrast, the annealingof the film under a S atmosphere leads to a drastic increase(up to 8–9 cm−1) of the FWHM of the peak, which correlatesto a significant worsening of the structural quality of thekesterite phase resulting from a Sn deficiency in the samples.Figure 5 shows the UV Raman spectra measured from the samesamples from figure 4. By using UV excitation conditions(at a wavelength of 325 nm), pre-resonant excitation of themain Zn–S vibrational modes are reached, greatly enhancingthe sensitivity of the measurement for the detection of thisphase [28].

Figure 5. Normalized Raman spectra measured using 325 nm (UV)as the excitation wavelength. (a) Ar–Cu2–Zn2 sample and(b) ArH–Cu1–Zn2 sample.

As shown previously by XRD, in figure 5 the presenceof characteristic intense ZnS first-second-order peaks of thisbinary compound is observed at about 346 and 694 cm−1,which confirms the presence of this secondary phase in both theas-grown and annealed samples. The presence of ternary Cu–Sn–S compounds cannot be confirmed nor discarded, becausetheir main Raman peaks at 290, 317 and 352 cm−1 are closeto those characteristic of both CZTS and ZnS phases [27]. Onthe other hand, and in spite of the imposed inert atmosphereused during the spraying process, the spectrum measured fromthe as-grown sample shows a small peak at about 565 cm−1,which has been identified with the main Raman peak fromZnO [29]. Nevertheless, after annealing, and given the pre-resonant excitation used, no evidence of ZnO is observed inany of the films, no matter what the composition or the gasused to spray them, supporting the XRD results suggestingthe complete elimination of this detrimental phase under theapplied thermal conditions. This contrasts with the behaviourobserved in films grown using air as the carrier gas, wherethe presence of ZnO persists even after reactive annealing,as reported previously [15]. This shows the effectiveness ofthe imposed inert atmosphere and the reactive Sn+S annealingprocesses to minimize the presence of detrimental oxidizedspecies.

The relations between the deconvolutioned intensity ofthe first-order ZnS Raman peak at 346 cm−1 and the mainCZTS peak at about 337 cm−1 are 2.5 and 3 for Ar–Cu2–Zn2and ArH–Cu1–Zn2 samples, respectively. That means that forthe ArH-sprayed sample, the deconvolutioned intensity of the

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J. Phys. D: Appl. Phys. 47 (2014) 245101 M Espindola Rodriguez et al

Figure 6. Optoelectronic characteristics of the devices fabricatedwith Ar–Cu2–Zn2 and ArH–Cu1–Zn2 absorbers, respectively. (a)Illuminated J-V response and (b) external quantum efficiency of therespective cells.

main ZnS peak is three times higher than the intensity of themain kesterite peak for the specific wavelength used. In otherwords, the most intense ZnS peak corresponds to the samplewith the bigger Zn/Sn ratio. We are currently working on thereduction of this binary and surface roughness by means ofchemical and physical methods.

Solar cells were fabricated using an Ar–Cu2–Zn2 absorberannealed at 580 ◦C showing an efficiency η = 1.4%,FF = 33.0%, Voc = 407 mV and Jsc = 10.7 mA cm−2

(figure 6). Compared to our previous work [15], thisimprovement is mainly due to an increase of the shuntresistance (Rsh), Voc and FF. This is likely related to an increasein the average grain size and a better passivation of the grainboundaries by using an Ar atmosphere, where oxidation effectsare minimized. The Rsh and Voc improvements observed inthis work when using Ar are still below the optimal reportedvalues [2, 22, 24], and they could be caused by the presenceof low band-gap materials such as CuxS or SnSx that reducethe open circuit voltage and decrease the shunt resistance [30].

Referring to the use of ArH2 as the carrier gas and from theoptoelectronic point of view, a great improvement was alsoachieved with respect to the use of air as the carrier gas,as evidenced by a power conversion efficiency η = 0.65%,FF = 31%, Voc = 305 mV and Jsc = 6.9 mA cm−2. Thisefficiency would increase if the compositions of the ArH2

sprayed films were closer to the target ratios; however welimited this work to show the effect of changing only the carriergas on the films and devices.

A further improvement on the Voc values can be obtainedby optimizing the thermal processes to increase the grainsize in the absorbers, thus reducing the density of grainboundaries, which in turn would help to decrease the densityof recombination centres. Also, the low short circuit currentsof the solar cells could be related to the high roughness ofthe absorbers. Figure 6(b) shows the EQE of the mentionedsolar cells. The linear extrapolation of the long wavelengthregion that corresponds to the contribution of the bulk of theabsorber to the EQE measurement gives a band gap value of1.44 ± 0.01 eV and 1.45 ± 0.01 eV, respectively, for Ar–Cu2–Zn2 and ArH–Cu1–Zn2, in close agreement with the acceptedvalue for CZTS [1, 21, 22], and with a maximum EQE of 50.5%and 42.7% at 550 nm, respectively. The value of the EQE fallsrapidly while going to the long wavelength region, showingthat the absorber and the absorber–buffer interface need to beimproved. Considering that the composition of absorbers isclose to those reported for the best CZTS-based solar cells [1,21, 22], and the minimization or even absence of oxidizeddetrimental phases, the main limitations of our devices arelikely to be related to the morphology (small grain size, highsurface roughness) of the absorbers. The improvement ofthe thermal processes, making them better adapted for thecharacteristics of the sprayed precursors, aiming to obtainlarger grains with smoother surfaces, could help to increasethe conversion efficiency. Nevertheless, the results presentedin this report show the potential of this modification ofthe spray pyrolysis method to improve the optoelectroniccharacteristics of device-grade films synthesized using acost-effective process compatible with its possible industrialimplementation.

4. Conclusions

CZTS thin films were deposited by a modified pneumatic spraypyrolysis method under a controlled atmosphere using Ar orAr–H2 as the carrier gas. Using this approach, we have triedand verified two ideas: first that the composition of the startingdissolution has a direct influence on the composition of thefinal film, and second, that the carrier gas is a real sprayparameter modifying the final properties of the films. Byout-of-stoichiometry variations in the starting dissolution, itis possible to modify the elemental Cu/(Zn + Sn) and Zn/Snratios, thus obtaining the desired compositional values, i.e.Cu-poor and Zn-rich films. CuxS and ZnS are detected asthe main secondary phases in the as-grown samples, evenunder the imposed Cu-poor conditions. A small oxidationamount is also observed, evidenced by the presence of a

7

J. Phys. D: Appl. Phys. 47 (2014) 245101 M Espindola Rodriguez et al

weak Raman contribution from a ZnO phase. After high-temperature (580 ◦C) thermal annealing, the crystallinity andgrain size of the as-grown films was improved. The resultsshow that the use of a S + Sn reactive atmosphere is necessaryduring the annealing process to improve the crystalline qualityand composition of the films. Annealing also leads to thereaction of some of the secondary phases detected in the as-grown films: CuxS by reaction with ZnS and SnS2 formsCZTS. The use of Ar as the carrier gas faces one of themain problems with the classical spray pyrolysis technique:the formation of oxide compounds. Nevertheless, the grainsize and roughness of the surface are still far from the optimalmorphology of CZTS films grown by other techniques. In afirst optimization, we report a 1.4% efficiency device that isfree of scarce elements. Improvement of the optoelectronicproperties of these absorbers is linked to the optimization ofthe reactive annealing to achieve larger grains and smothersurfaces for better interfaces.

Acknowledgments

This research was partially supported by Ministerio deEconomıa y Competitividad, project KEST-PV (ref ENE2010-121541-C03-1), by the Framework 7 program under theprojects PVICOKEST (PIRSES-GA-2009-269167) and byEuropean Regional Development Founds (ERDF, FEDERPrograma Competitivitat de Catalunya 2007–2013). Authorsfrom IREC and the University of Barcelona belong tothe M-2E (Electronic Materials for Energy) ConsolidatedResearch Group and the XaRMAE Network of Excellenceon Materials for Energy of the ‘Generalitat de Catalunya’.M.E-R. for the FPI-MINECO (BES-2011-045774), ES thanksthe Government of Spain for the ‘Ramon y Cajal’ fellowship(RYC-2011-09212), VI for the ‘Juan de la Cierva’ fellowship(JCI-2011-10782), and DS for the PTA fellowship (PTA2011-5985-I). O. Vigil-Galan acknowledges support from SENER-Conacyt Program 117891 and COFAA and EDI of IPN.

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