Characteristics of CuIn1�xGaxS2 thin films synthesized by chemicalspray pyrolysis

Mejda Ajili a,n, Michel Castagné b, Najoua Kamoun Turki a

a Laboratoire de Physique de la Matière Condensée, Faculté des Sciences de Tunis, Tunis El Manar 2092, Tunisiab Institut d’Electronique du Sud, Université de Montpellier II, Sciences et Techniques du Languedoc, case courrier 083. Place Eugène BATAILLON,34 095 Montpellier cedex 05, France

a r t i c l e i n f o

Article history:Received 26 July 2013Received in revised form11 December 2013Accepted 29 December 2013Available online 31 January 2014

Keywords:CuIn1�xGaxS2Thin filmsSpray pyrolysisSolar cell

a b s t r a c t

CuIn1�xGaxS2 multi-component semiconductors thin films were prepared by chemical spray pyrolysis onglass substrates using different concentrations of gallium in the spray solutions (y¼([Ga3þ]/[In3þ])varying from 0 to 20 at% by a step of 5 at%). Samples were characterized using X-ray diffraction, Ramanspectroscopy, Atomic Force Microscopy, photoluminescence spectroscopy, spectrophotometric and Halleffect measurements. The X-ray spectra reveal that the CuIn1�xGaxS2 thin films are of chalcopyritecrystalline phase with a highly (1 1 2) preferential orientation. The best crystallinity is obtained for 10 at%Ga incorporation since the maximum (1 1 2) peak intensity and grain size are obtained at this Gaincorporation rate. The level of the residual microstrain and dislocation network seems to be reducedrespectively to the values 0.09% and 4�108 lines mm�2 for an optimum y¼10 at% for which thecrystallinity of CuIn1�xGaxS2 thin layers is the best one. Raman spectra indicate that the sprayed thinfilms are grown only with CH-ordering. Optical analysis by means of transmission T(λ) and reflection R(λ)measurements allow us to determine the direct band gap energy value which increases by increasing theGa content and it is in the range 1.39–1.53 eV, indicating that CuIn1�xGaxS2 compound has an absorbingproperty favorable for applications in solar cell devices. Photoluminescence measurements areperformed on CuIn1�xGaxS2 crystals and the analysis reveals that the emission is mainly due todonor–acceptor pair transitions. The film resistivity (ρ) and Hall mobility (μ) are strongly affected byGa incorporation rate. The lowest resistivity (ρ¼0.1 Ω cm) and maximum value of Hall mobility(μ¼0.5 cm2 V�1 s�1) are also obtained for the thin layers prepared with y¼10 at%. Finally, we reportedtwo new structures for CuInS2/β-In2�xAlxS2/ZnO:Al and CuIn1�xGaxS2 (y¼10 at%)/β-In2-xAlxS2/ZnO:Alsolar cells to investigate the effect of gallium incorporation on the photovoltaic parameters. We foundthat the Ga-containing cell shows conversion efficiency (η¼1.6%) higher than the Ga-free reference celldue to higher open-circuit voltage (Voc¼540 mV) and short-circuit current density (Jsc¼10 mA cm�2).

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Chalcopyrite compounds, CuXY2 (X¼ In, Ga, Al, Y¼Se, S), havebeen studied extensively for application of high efficiency thin filmsolar cells, light-emitting diodes, optical electronic devices andphotocatalytic reactions. Among these ternaries, the copper andindium disulfide CuInS2 (CIS) material presents a particular interestdue to their suitable energy band gaps at room temperature beingclose to the optimal theoretical value (Eg¼1.5 eV), high opticalabsorption coefficient (α) which is in the range 5–18�104 cm�1, inthe visible light region for considerable conversion efficiency andeasy conversion of n/p type [1–4]. This material has a high chemicaland thermal stability and generally crystallized in chalcopyrite

structure with lattice parameters a¼b¼5.5 Å and c¼11 Å [5,6].CIS thin films have been prepared by various methods, includingsol–gel dip coating [6], pulse galvonostatic [1], paste coating [2],reactive magnetron co-sputtering [7], spray pyrolysis [3–5], eva-poration [8–12] and graphite box annealing of stacked elementallayers [13]. For economical reasons, it will be useful to prepare thinfilms using a low cost deposition technique. One of such methods isa spray pyrolysis technique, which allows to obtain large-area filmsat extremely low cost. Ease with which doping can be done byincorporating dopants in spray solution is the other advantage ofthis technique. In several studies it was shown that the physicalproperties of CIS thin films could be improved by optimizeddeposition conditions [1,2,5,6] and doping such as aluminum (Al)[14,15], bismuth Bi [4], tin (Sn) [8], antimony (Sb) [9], sodium (Na)[10], zinc Zn [11], iron (Fe) [16] and gallium (Ga) [7,12,13,17,18]. Inthis work, the interest was focused on Ga-incorporation in CISthin films. We chose Ga as the dopant because according to the

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Journal of Luminescence

0022-2313/$ - see front matter & 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jlumin.2013.12.059

n Corresponding author. Tel.: þ00216 53 285 494.E-mail address: ajili.mejda@yahoo.fr (M. Ajili).

Journal of Luminescence 150 (2014) 1–7

literature, we have known that the incorporation of gallium in theCuInS2 thin films prepared by the method of evaporation improvesthe photovoltaic parameters of solar cells based on the CuIn1�x-

GaxS2 (CIGS) absorber material and hence increases their perfor-mance (Voc¼0.766 V, Jsc¼19.46 mA cm�2, FF¼67.85% and η¼10.12%) [17]. Another considerable effect of Ga-incorporation inthe evaporated CIS thin layers is the increase of the optical band-gap energy [12]. In our knowledge, it was not found in thebibliography a study on the effect of Ga incorporation on physicalproperties of CIS thin films synthesized by the chemical spraypyrolysis technique. Furthermore, in our laboratory the gap Eg ofsprayed CIS compound is in the order of 1.38 eV [3]. With the aim toreach Eg to the optimum value for the conversion of solar energyinto electrical energy we introduced different compositions ofgallium in the CIS materials. For this purpose and in order to geta deeper understanding of the beneficial effects on Ga incorpora-tion, CIGS thin films with various Ga concentrations were preparedby the chemical spray pyrolysis method. The dependence ofstructural, electrical and optical properties on Ga concentrationwas studied. Our attempt was to investigate the effect of Ga-incorporation on the electronic properties of CIGS solar cell suchas Al/CIGS/β-In2�xAlxS2/ZnO:Al/SnO2:F, where Al and SnO2:F wereused as an ohmic contact, β-In2�xAlxS2 and ZnO:Al as a buffer layerand an optical window, respectively.

2. Experimental procedure

2.1. Preparation and characterization of CIGS thin films

CIGS thin films were prepared by the pulverization technique inliquid phase (spray). The experimental setup used to spray CIS thinlayers involves a heating system for the substrate, a nozzle fixedon a two-dimensional moving table allowing to pulverize thewhole isothermal zone containing the heated substrates [19]. Theaqueous solutions used for pulverization contain the precursors ofthe CIS material, i.e. 3.3�10�2 M of CuCl2 for the copper,3�10�2 M of InCl3 for the indium and 12�10�2 M of SC(NH2)2for the sulfur. The CIS films were formed on heated substrates at320 1C by the following reaction [20]:

CuCl þ InCl3 þ2SC NH2ð Þ2þ 4H2O-CuInS2 þ 2CO2þ4NH4Cl ð1ÞThe gallium trichloride GaCl3 is added to the starting solution toobtain CIGS thin films. During the deposition run, respectively, therate of spray and the distance between the substrate and thenozzle are maintained constant and are equal to 3 ml min�1 and30 cm. The spray run lasts for 4 min. After deposition, the thick-ness of the films (e) was evaluated using the weight differencemethod with the following relation:

e¼ mρA

ð2Þ

wherem is the mass of the film deposited on the substrate, ρ is thedensity of the deposited material in the bulk form and A (in cm2) isthe effective area on which the film was deposited. The crystal-linity of the films was examined by Raman spectrometer (RS)(Jobin Yvon) excited by He–Ne laser with wavelength of 632.81 nmand X-ray diffraction (XRD) which are recorded with an automatedBruker D8 advance X-ray diffractometer with CuKα radiations for2θ values over 20–601. The wavelength, accelerating voltage andcurrent are respectively 1.5418 Å, 40 kV and 20 mA respectively.The grain size was estimated from the full width at half maximum(β) of the diffraction peaks using the Scherrer formula [21]:

D¼ 0:94λ

cos θffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiβ2�β20

q ð3Þ

where β0 is the width of the corresponding peak due to theinstrumental expansion, which is about 0.1251 and θ is the Bragg'sangle. Using grain size values, the dislocation density (δ), defined asthe length of dislocation lines per unit volume of the crystal has beencalculated by using the Williamson and Smallman's formula [22]:

δ¼ 1

D2 ð4Þ

The strain (ε), defined as a measure of the change in the size or shapeof a body referred to its original size or shape, developed in the filmscould be indirectly measured from the change of the lattice spacing,which induces a change in the position of diffraction peaks. The strain(ε) in thin films affects the opto-electronic properties of the layers dueto the distorted lattice and was assessed using the relation [23]

ε¼ dhkl�dhkl0

dhkl0

ð5Þ

where dhkl is the lattice spacing obtained from the Bragg angle θhkl anddhkl0 is the strain free lattice spacing of the (hkl) lattice planes. Thesurface morphology of CIGS thin layers were investigated by theatomic force microscopy (AFM, standard veeco Dimension 3100, usedin tapping mode). Resistivity, carrier concentration and mobility weredetermined from Hall effect measurements in the Van der Paw-configuration. The optical properties were studied according to UV–vis–NIR spectrum with Perkin-Elmer Lambda 950 spectrophotometerin the wavelength range of 250–2500 nm at room temperature, takingair as reference. Finally, photoluminescence (PL) spectra of the filmswere recorded using Perkin-Elmer LS 55 Fluorescence spectrometerwith excitation wavelength of 485 nm.

2.2. Preparation and characterization of CIGS (y¼0 and 10 at%)based solar cells

Solar cells were elaborated by sprayed CuIn1�xGaxS2 (y¼0 and10at %) absorber material on β-In2�xAlxS3/ZnO:Al/SnO2:F thinfilms.

The buffer layers of indium sulfide β-In2-xAlxS3 ([Al3þ]/[In3þ]¼20 at% Al), were deposited onto the substrate ZnO:Al/SnO2:F/glass,kept at a temperature of about 340 1C with a distance samples-nozzle of 30 cm and a spray rate of 3 ml min�1, during 60 minspray time, using an aqueous solution containing of 0.01 M of InCl3for indium and 0.02 M of SC(NH2)2 for sulfur by the followingreaction [20]:

2InCl3þ3SC NH2ð Þ2þ6H2O-In2S3þ3CO2þ6NH4Cl ð6Þ

The addition of aluminum to binary compound β-In2S3 leads to anincrease of the band gap making them very promising candidatesfor the use as buffer layer in photovoltaic devices [24].

The optical window layers of ZnO:Al ([Al3þ]/[Zn2þ]¼1 at% Al)were sprayed onto the substrate SnO2:F/glass kept at a temperature of440 1C with a distance samples-nozzle of 27 cm and a spray rate of25 ml min�1, during 40 min spray time. The spray solution consistedof zinc acetate, H2O and propanol-2. ZnOmaterial was doped with 1 at% Al to enhance its optical and electrical properties [25].

The source of aluminum incorporated in the β-In2S3 and ZnOspray solution is aluminum chloride (AlCl3).

The thin films of SnO2:F were prepared using SnCl4, H2O andmethanol. Fluorine doping was achieved by adding fluoride(NH4F). SnO2:F thin films are formed on glass substrate held at asubstrate temperature of about 440 1C with a distance samples-nozzle of 27 cm and a solution spray rate of about 20 ml min�1,during 10 min spray time by the following reaction [20]:

SnCl4 þ2H2O-SnOxþ4HClþ2�x2

O2 ð7Þ

M. Ajili et al. / Journal of Luminescence 150 (2014) 1–72

Top electrode was Al of thickness 100 nm. This was depositedusing the physical vapor deposition.

The J–V characteristics of the solar cells were measured withthe help of the Keithley source measure unit (SMU, 2601A) and theMetric's interactive characterization software (ICS). The cell wasilluminated using a lamp having intensity 100 mW cm�2, on thesubstrate surface.

3. Results and discussion

3.1. Structural properties

In order to study the effect of Ga concentration on the micro-structural properties of CIGS thin layers deposited on glass sub-strates, different values of Ga concentration in the spray solutionwere studied (y¼[Ga3þ]/[In3þ]¼0, 5, 10, 15 and 20 at%). The XRDpatterns of five films for different y values were shown in Fig. 1. Ahighly (1 1 2) preferential orientation is observed. The (2 0 0),(2 0 4) and (3 1 2) diffraction peaks are also detected. Therefore,the prepared CIGS thin films crystallize in the tetragonal structure(PDF#65-1572). It may also be interesting to note that the relativeintensity of the (1 1 2) diffraction peak increases by increasing theinclusion rate of gallium until y¼10 at%.

In Table 1 are listed the various ratios I(hkl)/I(1 1 2) calculatedfrom the XRD spectra. From this table it is clear that the minimumratios are obtained for y¼10 at% and all ratios of relative inten-sities are much less than 1. Thus, we can confirm that CIGS thinfilms were preferentially oriented towards the direction (1 1 2).Similar result was found for CuIn1�xAlxS2 thin films [14].

The grain sizes calculated from Sherrer's formula are in the range12–50 nm as shown in Fig. 2. The maximum value is obtained forCIGS thin films deposited with y¼10 at%. Similar result was reportedearlier for zinc doped CIS thin films grown by the vacuum evapora-tion method [11]. The decrease of the crystallites'size beyondy¼10 at% can be explained by the existence of an optimum of y,for which the crystallinity of the CIGS thin films is the best one. Thisresult confirmed the XRD analysis which shows an improvement ofthe crystallinity at this Ga incorporation rate. In fact, for an inclusionrate of gallium y¼5 and 10 at% in the spray solution of CuInS2 thinlayers, the gallium atoms are substituted with indium atoms oroccupy vacant indium sites because in our laboratory we found thatfor CuInS2 thin layers, the ratio Cu/In¼1.35 [26]. In the stoichiometricCIS material, Cu/In is equal to 1, but for our CIS thin films grown byspray this ratio is more than 1 so there is a lack of indium atoms. Thisresult leads to an improvement of crystallinity for 5 and 10 at%.However, we can say that after saturation of indium sites by galliumatoms (for y410 at%), the gallium atoms occupy the interstitial sites,which leads to degradation in crystallinity.

To confirm the results obtained previously we used thetechnique of Raman spectroscopy which can determine the vibra-tion modes of the thin layers (Fig. 3). For all the films, the samephase was identified. As previously mentioned, CIS film isobserved to be grown with two different structures CH-orderingand CA-ordering mixed [27,28]. In our case, only A1 mode for CH-ordering is reported to be observed as the biggest peak, whichappear at 295 cm�1 (space group I42d). The other peak at340 cm�1 (E1LO) represents Raman modes for CH-ordering [24].

AFM analysis allows us to get microscopic information on thesurface structure and to plot topographies representing the surfacerelief. In this work, we use this technique to visualize the surfacerelief, specify the growth and determine the RMS (root meansquare) roughness. AFM images of CuIn1�xGaxS2 thin layers areshown in Fig. 4. These images show that the surface morphologiesof the layers are dependent on the Ga concentrations. We remarkthat the surface of the layers is dense and practically covered withequal sized spherical grains that are distributed. When y increasesfrom 0 to 10 at% the grain size increases slightly and when itexceeds 10 at% the grain size decreases. These results are consis-tent with the X-ray diffraction analysis. A significant improvementof RMS roughness with Ga incorporation can be noticed (Fig. 5).Thus, for 20 at% Ga incorporation rate CIGS thin films exhibit thesmallest RMS roughness (3.18 nm).

Fig. 2 shows the variation of the dislocation density (δ) as afunction of Ga content. It is clear that δ decreases slightly from6.5�108 to 4�108 lines mm�2 when the concentration of Gaincreases until y¼10 at%. Beyond this Ga concentration (y¼10 at%),the dislocation density increases again but strongly to 64.8�108 line-s mm�2 for y¼20 at%. This behavior can be explained by the changeof the particles’ size (D) with y. Indeed, the smaller crystallites allowdeposition in relatively large numbers and possibly the appearance ofsome linear defect (dislocation) and their development throughout thegrowing structure. Yahmadi et al. also reported similar results for In2S3thin films deposited on glass by the CBD technique [29].Fig. 1. X-ray diffraction of CIGS sprayed thin films.

Table 1Relative intensity of XRD peaks of CIGS sprayed thin films.

y (%) I(2 0 0)/I(1 1 2) I(2 0 4)/I(1 1 2) I(3 1 2)/I(1 1 2)

0 0.15 0.17 0.125 0.18 0.21 0.14

10 0.13 0.16 0.0915 0.22 0.24 0.1620 – 0.32 0.23

Fig. 2. Variation of the grain size D and the dislocation density (δ) of CIGS sprayedthin films as a function of Ga content.

M. Ajili et al. / Journal of Luminescence 150 (2014) 1–7 3

The variation of lattice strain (ε) with the Ga incorporation inthe spray solution is shown in Fig. 6 which clearly shows that thestrain in the films is tensile and decreased with the increase of y to10 at%. Beyond this Ga concentration, the strain decreases furtherand the nature of the strain was changed to compressive, whichindicates that the films would tend to be compressed parallel tothe substrate surface. In fact, the Ga3þ whose ionic radius (0.83 Å)is smaller than that of In3þ (0.92 Å), was incorporated into thelattice of CuInS2 samples. This result leads to the decrease in thed-spacing, which may explain the decrease of the micro-strain.The level of the residual microstrain and dislocation networkseems to be reduced to the values 0.09% and 4�108 lines mm�2

respectively for an optimum y of about 10 at% for which thecrystallinity of CIGS thin films is the best one. This result is in goodagreement with the X-ray analysis.

Fig. 7 depicts the evolution of the film thickness of CIGS thinlayers as a function of the Ga incorporation rate. Apparently, twodifferent regions are displayed: an initial region (yr10 at%) alsocalled the growth phase, the film thickness increases linearly withy. This phase is followed by a saturation region called the terminalphase, at which the film ceases to grow. At y¼0, the film thickness

Fig. 3. Raman scattering measurements of CIGS sprayed thin films.

Fig. 4. AFM images of CIGS sprayed thin films: (a) (y¼0); (b) (y¼5 at%); (c) (y¼10 at%); (d) (y¼15 at%); and (e) (y¼20 at%).

Fig. 5. Variation of the RMS roughness of CIGS sprayed thin films as a function ofGa content.

Fig. 6. Variation of the lattice strain (ε) of CIGS sprayed thin films as a function ofGa content.

M. Ajili et al. / Journal of Luminescence 150 (2014) 1–74

is about 300 nm and then it increases approximately by 135 nmwhen we increase the Ga incorporation in the spray solution by astep of 5 at% in the linear region and the estimated final thicknessis about 600 nm. This value is comparable to that reported in [4].

3.2. Optical properties

Transmission measurements at near-normal incidence areperformed in order to investigate the effect of the variation ofgallium inclusion rate (y¼0, 5, 10, 15 and 20 at%) on the opticalperformance of CuInS2 thin films. Transmission spectra are pre-sented in Fig. 8. An increase of transmission in the near-infraredspectral region with increasing Ga concentration is observed.The detailed mechanism to explain the Ga effect on the increaseof the transmission is not yet clear. First, we assume that Ga atomsare easily diffused in the volume during deposition and so affectthe optical properties. Second, the increase in the transmissionmay be due to the decrease in the RMS roughness detected fromthe AFM analysis. Indeed, if the roughness decreases, the diffusionof the incident light arriving at the air-film interface decreases andconsequently, the refracted light increases and this may explainthe increase of T (λ). CIGS thin films exhibit a high intrinsicabsorption in the visible domain and all the transmission spectrashow a sharp fall of transmittance at the band edge when theincident photon energy is above the gap of the semiconductor. Infact, this photon is absorbed by the material and thus thetransmission energy loss to cancel which leads to the onset ofthe intrinsic interband absorption in the CuIn1�xGaxS2 thin layers

in the visible region (400–800 nm). This property permits toCuInS2 compound to have an absorbing character favorable forits use in the photodetection of the light in the visible domain.

An analysis of the optical band-gap Eg of these films has beenmade using the optical absorption coefficient α given in the meanabsorption domain by the approximate formula [30].

α¼ � 1eln

T

ð1�RÞ2

" #ð8Þ

The Eg values corresponding to direct band gap transitions werededuced from the (αhν)2 versus the photon energy hν plots (Fig. 9)extrapolating the straight line from the relatively high absorptionregion conforming to the well-known Tauc law [31]:

ðαhνÞ2 ¼ Aðhν�EgÞ ð9Þ

where A is a constant.Without Ga incorporation, CuInS2 thin films have a band gap Eg

equals to 1.39 eV. After Ga incorporation, Eg increases. We remarkthat for y varying from 10 to 20 at%, Eg is in the interval 1.46–1.53 eV which is in suitable range for sunlight absorption andclosely agrees with the values reported for CuInS2:Sn thin filmselaborated by the double source thermal evaporation method [8].The observed band gap widening is due to the formation of aquaternary CuIn1�xGaxS2 alloy caused by inter diffusion of Ga andIn from the ternary compound [12].

3.3. Photoluminescence (PL)

Photoluminescence (abbreviated as PL) describes the phenom-enon of light emission from any form of matter after the absorptionof photons (electromagnetic radiation). Fig. 10 shows the emissionspectra of CIGS thin layers. Photoluminescence spectra have beenrecorded at room temperature with an excitation wavelength of485 nm for all samples. A strong and broad emission peak isobserved in the wavelength range 500–550 nm centered at 520 nm(maximum emission) when y¼10 at%. This feature corresponds todonor–acceptor pair transition (DAP) between a sulfur vacancy andan indium vacancy or band–band luminescence of CIGS crystal[32,33] or probably Ga on an indium site. Further increase in thegallium incorporation in the spray solution, the peak at which themaximum emission occurs slightly shifts towards shorter wavelengthregion (higher energy positions), this result may be related to theincrease in the band gap energy which is in agreement with theoptical analysis. Similar result was found by GukPark et al. for Cu(In,Ga)Se2 thin films [34]. The high intensity of the band-edge emission

Fig. 7. Variation of the CIGS sprayed thin film thickness as a function of Ga content.

Fig. 8. Transmission spectra of CIGS sprayed thin films with different Ga concen-trations.

Fig. 9. Relationship between (αhν)2 and photon energy (hν) of CIGS sprayed thinfilms with different Ga concentrations.

M. Ajili et al. / Journal of Luminescence 150 (2014) 1–7 5

at y¼10 at% confirms the high state of crystallinity with fewelectronic defects and good dispersity of nanomaterials [35], thisresult also is in good agreement with the X-ray analysis.

3.4. Electrical properties

The variation of electrical resistivity (ρ) at room temperaturemeasured by the Vander Paw technique as a function of thegallium inclusion rate (y) is reported in Fig. 11. The resistivity isfound to decrease significantly with increasing y, reaching amaximum in the order of 0.1 Ω cm for CIGS thin layers grown aty¼10 at%. This result can be explained by the good crystallizationof CIGS thin layers as shown in XRD results at this Ga concentra-tion. However, when y exceeds 10 at% ρ increases strongly to12 Ω cm for higher Ga concentration (y¼20 at%). These values aresmaller than the reported values which are in the range4.3–5.3�102 Ω cm [9]. It is apparent also from Fig. 11 that the Hallmobility (μ) shows a gradual increase with the increase of the Gaconcentration until y¼10 at%, reaching a value of 0.5 cm2 V�1 s�1,above 10 at% it decreases. The increase of the Hall mobility untily¼10 at%, can be attributed also to the improvement of crystallinity.Moreover, the positive value of Hall coefficient indicates that allsamples are p-type conductive semiconductors.

The effect of gallium concentration on surface and volumecarrier concentrations (Ns and Nv) is shown in Fig. 12. Ns and Nv ofthe films exhibit a slight increase when y increases from 0 to 10 at%, reaching respectively 5�1015 cm�2 and 10.4�1019 cm�3.

Then it is clear from Fig. 12, that Ns and Nv show a significantdecrease for yZ15 at%.

Thus, we can say that an increase of gallium inclusion rate until10 at%, improves the crystallinity, reinforces contact at the grainboundaries generating more free electrons, improves the Hallmobility and increases the band gap energy. In fact, from thesestudies, we are able to optimize the process in order to produceCIGS thin layers suitable for optical absorbers in solar cells.

3.5. Analysis of the J–V characteristics

The J–V characteristics and solar cell parameters obtained fromthese J–V curves are shown in Fig. 13 and Table 2, respectively. Animprovement of photovoltaic parameters was obtained for the cellprepared using CIGS (y¼10 at%) as an absorber material (C2). Infact, the efficiency (η) increases from 0.23% for the cell elaboratedusing CIS as an absorber material (C1) to 1.6% for C2. This would bedue to better structural, optical and electrical properties of CIGSmaterial at y¼10 at%. These resulted in relatively low seriesresistance (Rs) derived from C2, thus leading relatively largeshort-circuit current density (Jsc¼10 mA cm�2) in C2 than in C1(Jsc¼2 mA cm�2) and high fill factor in C2 (FF¼55%) comparedwith the one in C1 (FF¼38%) which due to the better carriergeneration in the solar cell (C2) [1,36,37].

Another difference in cell parameters between C1 and C2summarized in Table 2 is the open circuit voltage (Voc) whichincreases from 300 mV for C1 to 540 mV for C2. The increase of the

Fig. 10. Photoluminescence spectra of CIGS sprayed thin films.

Fig. 11. Variation of the resistivity (ρ) and Hall mobility (μ) of CIGS sprayed thinfilms as a function of Ga content.

Fig. 12. Variation of the surface and volume carrier concentrations (Ns and Nv) ofCIGS sprayed thin films as a function of Ga content.

Fig. 13. Current–voltage (J–V) curves of Al/CuInS2/β-In2�xAlxS3/ZnO:Al/SnO2:F (C1)and Al/CuIn1�xGaxS2 (y¼10 at%)/β-In2�xAlxS3/ZnO:Al/SnO2:F (C2) solar cells.

M. Ajili et al. / Journal of Luminescence 150 (2014) 1–76

Voc can be caused by a change of the interface recombination andan increase of the band gap of the absorber material [38,39]. Thisresult is in good agreement with our optical analysis whichshowed that after 10 at% Ga incorporation in the spray solutionof CuInS2 material, the band gap Eg increases from 1.39 to 1.46 eVwhich is more closed to 1.5 eV, the optimum theoretical value forthe photovoltaic conversion of solar energy.

4. Conclusions

CIGS thin films were prepared on ordinary glass substrates bythe chemical spray pyrolysis technique. It is particularly observedthat well crystallized CIGS thin films were obtained at y¼10 at% Ga.The increase of Ga incorporation rate in the spray solution is relatedby decreasing the RMS roughness and increasing the averagetransmittance and the optical band gap energy (EgE1.5 eV). Weattributed the band gap widening to the formation of quaternaryCuIn1�xGaxS2 alloy caused by inter diffusion of Ga and In from theternary compounds. All CIGS thin films exhibit p-type conductivity.The best electrical properties such as minimum resistivity, max-imum Hall mobility and charge carrier concentration were obtainedat y¼10 at%, which is in good agreement with the XRD analysis. Weinvestigated the electrical properties of the heterostructures CuInS2/β-In2�xAlxS2/ZnO:Al and CuIn1�xGaxS2 (y¼10 at%)/β-In2�xAlxS2/ZnO:Al. The solar cell based on the CIGS (y¼10 at%) absorber thinlayer showed good rectification with significant improvements inall of the photovoltaic parameters compared to those of the solarcell based on CIS absorber thin layer due to the band gap widening,better structural and electrical properties of CIGS than those of CISthin films.

At the end, we can say that in this work the optimization of theGa concentration added in the spray solution of CIS material leadsto slight improvement of the photovoltaic parameters of the solarcells based on the CIGS thin absorber layers. In perspective toimprove the properties of the solar cells elaborated by CIGS(y¼10 at%) absorber thin films, the annealing under vacuum ofCIGS thin films will be done.

Acknowledgments

The authors wish to thank the Comité Mixte de CooperationUniversitaire (Tunisia–France) for financial support under theProject number 07S1304, as well as Egide France under the projectHubert Curien – Utique Number 15385QG. They would like also tothank Dr. C. Guash from Institut d'Electronique du Sud, Universitéde Montpellier II for help.

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Table 2Photovoltaic parameters of Al/CuInS2/β-In2�xAlxS3/ZnO:Al/SnO2:F (C1) and Al/CuIn1�xGaxS2 (y¼10 at%)/β-In2�xAlxS3/ZnO:Al/SnO2:F (C2) solar cells.

Solar cell Voc (mV) Jsc (mA cm�2) Rs (Ω) FF (%) η (%)

(C1) 300 2 120 38 0.23(C2) 540 10 40 55 1.6

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