Preparation and characterization of cost effective spray pyrolyzed absorber layer for thin film solar cells

Available online at www.sciencedirect.com

Solar Energy 95 (2013) 21–29 www.elsevier.com/locate/solener

Preparation and characterization of cost effective spray pyrolyzed absorber layer for thin film solar cells Nurdan D. Sankir a,b,⇑, Erkan Aydin b, Hulya Unver b, Ezgi Uluer b, Mehmet Parlak c a

Department of Materials Science and Nanotechnology Engineering, TOBB University of Economics and Technology, Ankara, Turkey b Micro and Nanotechnology Graduate Program, TOBB University of Economics and Technology, Ankara, Turkey c Department of Physics, Middle East Technical University, Ankara, Turkey Received 11 January 2013; received in revised form 8 May 2013; accepted 29 May 2013

Communicated by: Associate Editor Takhir Razykov

Abstract In this study, highly (1 1 2) oriented crystalline copper indium disulfide (CuInS2) thin films with high mobility have been deposited via ultrasonic spray pyrolysis. Structural and electrical properties of CuInS2 thin films were examined to utilize them in solar cell applications. Various amounts of precursor solution ranging from 0.25 to 2.02 ml/cm2 were used to form CuInS2 thin films onto the soda lime glass substrates. Scanning electron microscopy (SEM) analysis revealed that all sprayed films were pin-hole and crack free. Atomic percent ratios of the Cu/In and S/In were very close to the targeted stoichiometric ratios of 1/1 and 2/1, respectively. X-ray diffraction (XRD) studies revealed that all the deposited films were polycrystalline and exhibiting the chalcopyrite structure. Optical band gap energy of the films were calculated as 2.85 eV and decreased to 1.40 eV by increasing the solution loading. Hopping mechanism could be considered as the dominant conduction mechanism in the studied temperature range. Carrier concentrations in CuInS2 films were ranging between 1015 and 1017 cm3. Mobility and the carrier concentration of the CuInS2 thin films deposited from 1.52 ml/cm2 solution loading were 40.1 cm2/V s and 1.69  1017, respectively. At last but not least, the amount of solution used in this study to form CuInS2 thin films was one of the lowest values reported in the literature. Ó 2013 Elsevier Ltd. All rights reserved. Keywords: Copper indium sulfide; Ultrasonic spray pyrolysis; Chalcopyrite film; Solar cells

1. Introduction Copper indium disulfide (CuInS2) is one of the I–III–VI2 chalcopyrite-type mixed crystal semiconductor having direct band gap around 1.5 eV and high absorption coefficient. Hence, it is possible to produce highly efficient thin film solar cells from CuInS2 used as absorber layer (Klenk et al., 2005; Siemer et al., 2001). Additionally, CuInS2 is free from the toxicity concerns arose from the

⇑ Corresponding author at: Department of Materials Science and Nanotechnology Engineering, TOBB University of Economics and Technology, Ankara, Turkey. Tel.: +90 3122924332. E-mail address: [email protected] (N.D. Sankir).

0038-092X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2013.05.024

selenium-containing chemicals, and thus it is more suitable for the large area applications. Numerous methods have been applied to deposit CuInS2 thin films such as reactive magnetron sputtering, a combination of sputtering and sulfurization processes, chemical bath deposition, flash evaporation and spray pyrolysis (Ellmer et al., 2002; Forbes et al., 2003; Bini et al., 2000; Agarwal et al., 1998; John et al., 2006). Among these methods, spray pyrolysis has been gaining more attention due to its unique advantages of very low solution consumption compare to the chemical bath and electrochemical depositions (Sebastian et al., 2009a,b; Krunks et al., 1999; Ramanathan et al., 2005). This is especially important for the usage of less indium source, which is the most expensive and limited precursor

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material to produce CuInS2 films. Moreover, spray pyrolysis offers atmospheric manufacturing, which drastically reduces the cost of solar modules (Kaelin et al., 2004; Singh et al., 2007). In terms of the spray pyrolysis of the CuInS2 thin films, there are two major issues. First one is the correlation of the electronic properties and the preparation methods. In the literature, there are very limited studies about the effect of preparation conditions on the electrical conduction mechanisms and the mobility of the spray pyrolyzed CuInS2 thin films (John et al., 2007; Sebastian et al.,2009a,b). John et al. reported the temperature dependant conductivity of the sodium incorporated spray pyrolyzed CuInS2 thin films (John et al., 2007). In another study, conductivity of the spray pyrolyzed CuInS2 thin films was explained using Mott’s variable range hopping mechanism (Sebastian et al., 2009a,b). Here, we investigated the effects of solution loading on the electrical, structural and the optical properties of the spray pyrolyzed CuInS2 thin films. According to our best knowledge this is the first study focusing on the electrical conductivity mechanisms on the spray pyrolyzed CuInS2 thin films at various solution loadings. The second important parameter for the spray pyrolysis is the amount of the precursor solution used to form thin films. Although, spray pyrolysis technique requires less amount to form thin films compare to the other solution based manufacturing methods, such as chemical bath and electrochemical deposition, the amount of the solution to form 1 lm-thick films were ranging dramatically depending on the spray pyrolysis system reported in the literature (John et al., 2005; Peza-Tapia et al., 2009; Engin et al., 2009; Sahal et al., 2009; Fujiwara et al., 2002). Moreover, the amount of the precursor solution is very important to adapt spray pyrolysis technique into the mass production. In this study, we used ultrasonic impact nozzle to form CuInS2 absorber layer on glass. Also, we obtained the best solution consumption reported in the literature. Following sections summarize the morphological, structural, optical and electrical properties of the spray pyrolyzed CuInS2 thin films, which can be used as absorber layer for thin film solar cells, and correlations of these properties with the solution loading. 2. Experimental In this study, Sono-Tek FlexiCoat USP System has been used to deposit CuInS2 thin films. Thin films have been deposited using copper (II) chloride-dehydrate (CuCl2, Sigma–Aldrich), indium (III) chloride (InCl3, Sigma– Aldrich), thiourea (NH2CSNH2, Acros Organics) as copper, indium and sulfur source, respectively. Aqueous precursor solution was sprayed on preheated soda lime glass substrate. Substrate temperature was kept constant at 350 °C during all experiments. The molar ratio of the Cu/In/S in solution and infuse rate were also kept constant for all samples at 1/1/3 and 1.5 ml/min, respectively. The crystal structure of the films was confirmed using

Panalytical, X’pert Pro MPD X-ray diffractometer. The surface morphology of the films was investigated by FEI, Quanta 200 FEG scanning electron microscopy. Chemical structures of the films were determined via both Energy Dispersive X-ray Analysis (EDX) and X-ray photoelectron spectroscopy (Thermo, K-Alpha – Monochromated high performance XPS spectrometer). The EDX microanalysis was carried out at accelerating voltage of 15 kV and spot size 3 lm. Measurements were done over large areas (300 lm  300 lm). Average of different spots have been reported. The optical transmittance was recorded in the wavelength range of 200–3300 nm using a Varian Cary 5000 UV–VIS-NIR spectrophotometer. Temperature dependent electrical conductivity measurements were carried out under 1.3  102 Pa vacuum in the temperature range 110–330 K using a Janis liquid nitrogen cryostat and monitoring the temperature with a LakeShore 331 temperature controller. Electrical contacts for electrical measurements were obtained by evaporation of silver strips, and copper wires for electrical connections were placed on the contact regions by the silver paste. The ohmic behavior of the contacts was confirmed by the linear variation of the current–voltage characteristics, which was independent from the polarity of the applied current and contact combinations for each sample. 3. Results and discussions 3.1. Film formation Conventionally, atomization of the precursor solution is done by the transferring the mechanical energy to the liquid sheet for possible disintegration in the form of droplets. Conversely, ultrasonic spray technology uses ultrasonic irradiations for inducing atomization, which results narrower droplet size distribution compare to the conventional atomization techniques. The very fine mist created via vibration of the piezomembranes can be directed on the substrates using flat jet air deflector, which reduces the over spray (Fig. 1). Amount of the precursor solution used to form thin films can be reduced dramatically compare to the other atomization techniques with the combination of the ultrasonic atomization in the nozzle and the jet air deflector. This helps to reduce the materials cost especially if it is necessary to use rare materials like indium. Moreover, almost all of the mist can be directed on the substrate via ultrasonic impact nozzles, and therefore, very little amount releases to the atmosphere. This minimizes the atmospheric contamination. Schematic structure of the ultrasonic spray pyrolysis system used in this study was illustrated in Fig. 1a. Droplets were transported by nitrogen onto a heated substrate, where the precursor thermally decomposed and formed thin films. Substrate temperature were controlled with an accuracy of ±5 °C by using PID control system. The picture of CuInS2 film deposited using 1.52 ml/cm2 loading of precursor solution can be seen in Fig. 1b. Various

N.D. Sankir et al. / Solar Energy 95 (2013) 21–29

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Fig. 1. (a) Schematic illustration of USP system, (b) picture of CuInS2 films deposited using 1.52 ml/cm2 solution loading.

amounts of precursor solution to form 1 lm-thick CuInS2 via spray pyrolysis were calculated from previous studies and values were summarized in Table 1. As seen in Table 1, solution requirement in this study is much lower than the previously reported data. Extensively reduced solution consumption reported here could be one of the key factors for later commercialization of the CuInS2-based thin film solar cells. 3.2. Structural properties Surface morphology of the CuInS2 films has been investigated via scanning electron microscopy (SEM). As seen in Fig. 2, all samples were pin-hole and crack free. Also, very smooth surfaces have been observed for the films deposited at 0.25 and 1.01 ml/cm2 solution loadings. However, when the solution loading exceeded the 1.01 ml/cm2, some agglomerated round structures have been observed. The diameter of these agglomerates increased with further increasing the solution loading. Energy Dispersive X-ray Analysis (EDX) has been used to determine the chemical structure of the CuInS2 thin films (Table 2). According to these results all sprayed films were sulfur rich as targeted, but contain some chlorine contamination up to 4.6 atomic percent. Presence of chlorine is related with the use of chloride-based precursors for deposition. Although, there were some slight deviations, targeted stoichiometry for Cu, In,

and S as 1/1/2 respectively, was successfully achieved for all our the samples. It has been also observed that the chemical composition of the flat and agglomerated regions on the films was different. Atomic percent of the Cu was higher in the island like structures compare to the flat regions. Therefore, it is possible to conclude that when the solution loading, and therefore, the production time increased, Cu atoms may diffuse through the surface and formed CuxS phases. Scheer et al. have been reported the formation of similar island like CuxS phases on the surface of co-evaporated CuInS2 films (Scheer et al., 1996). Stoichiometric composition or homogenous elemental distribution is very important in terms of photoactivity and the electrical properties of the absorber layer. Therefore, potassium cyanide (KCN) etching is generally performed to selectively remove the CuxS phases. However, this extra step does not preferred because of the well-known toxicity of KCN. Since, there was no sign of CuxS agglomeration on the surface for the solution loadings smaller than the 1.52 ml/cm2, this study propose one-step preparation and eliminates the need for extra treatments like KCN etching. It has been known that physical properties of the semiconducting thin films depend on the film thickness (Ryo et al., 2011; Revathi et al., 2009). Hence, it is important to determine the critical film thickness giving the best performances in terms of structural, optical and electrical characteristics. In this study, thickness of the CuInS2 films

Table 1 Comparison of solution consumption. Total solution (ml)

Film thickness (lm)

ba (ml)

Area (mm2)

Thin film

Deposition temperature (°C)

Ref.

375 200 50 40 75 30

0.6 1.12 0.48 0.55 1.50 1.95

625 178 104 73 50 15

– – 10  10 25  30 25  25 26  76

CuInS2 CuInS2 Zn incorporated CuO CuInS2 CuInS2 CuInS2

300 390 250 375 350 350

John et al. (2005) Peza-Tapia et al. (2009) Engin et al. (2009) Sahal et al. (2009) Fujiwara et al. (2002) This work

a

Solution required forming 1 lm-thick film.

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N.D. Sankir et al. / Solar Energy 95 (2013) 21–29 Table 2 Elemental composition from EDX of as-deposited CuInS2 films.

Fig. 2. SEM micrographs of CuInS2 thin films deposited with (a) 0.25 ml/ cm2, (b) 0.51 ml/cm2, (c) 1.01 ml/cm2, (d) 1.52 ml/cm2, and (e) 2.02 ml/cm2 solution loading.

Loading (ml/cm2)

Cu (at%)

In (at%)

S (at%)

Cl (at%)

Cu/In Ratio

S/In Ratio

0.25 0.51 1.01 1.52 2.02

28.0 22.1 23.1 23.1 24.0

21.0 24.5 22.8 22.1 21.7

45.6 48.7 50.3 50.2 50.3

– 4.7 3.9 4.6 4.0

1.33 0.90 1.01 1.05 1.11

2.17 1.99 2.21 2.27 2.32

was evaluated from cross sectional SEM studies. It was observed that thickness of the films increased by increasing the solution amount as expected (Table 3). Thin films of 280 nm were grown using 0.25 ml/cm2 solution loading. On the other hand, 2.02 ml/cm2 loading resulted in 2.02 lm-thick films. Aforementioned the CuInS2 thin films deposited from 0.25 ml/cm2 solution loading has the highest Cu/In ratio based on the EDX analysis. However, the average thickness of these films is very thin to get information from the surface via EDX. Therefore, atomic percent of the surface elements for this sample has been further controlled by the XPS analysis. As can be seen in Fig. 3, surface of the CuInS2 thin film deposited from 0.25 ml/ cm2 solution loading is In rich and Cu poor. This result supports the Cu diffusion through the surface and Cu rich phase formation for the higher solution loadings. It has been also observed that EDX and XPS analyses for the samples deposited using 0.51 ml/cm2 and higher loadings were compatible. X-ray diffraction (XRD) patterns of the CuInS2 thin films deposited at various solution loadings are shown in Fig. 4. We have obtained the typical X-ray diffraction peaks of chalcopyrite structure regardless of the solution amount (JCPDS no. 27-159). All sprayed films showed two major peaks around 27.9° (2h) and 46.3° (2h). Additionally, two minor peaks around 32.3° (2h) and 54.6° (2h) were observed for 1.52 and 2.02 ml/cm2 loadings. These two minor peaks were in good agreement with both the typical peaks of CuInS2 and CuxS (JCPDS no. 060454). Combined with the EDX data, these extra peaks could be attributed to the CuxS phase formed on the surface. The calculated tetragonal lattice parameters of the ˚ , and c = 11.060 A ˚ ) were CuInS2 films (a = b = 5.517 A ˚ , and very close to the actual values (a = b = 5.523 A ˚ , JCPDS no. 27-159) and were not affected sigc = 11.141 A nificantly from the solution loading. XRD analysis also revealed that crystallinity of the films increased with increasing the solution loading. As seen from Table 3, main (1 1 2) peak intensity revealed that our samples were very well even at solution loadings as low as 0.25 ml/cm2. It is also worth to mention here that the peak intensities obtained in this study were much higher than previously reported data (Sebastian et al., 2009a,b; Oja et al., 2005). The crystallite size of the films were calculated using the most intense diffraction peak observed around 2h = 27.9° using well known Debye–Scherrer formula;

N.D. Sankir et al. / Solar Energy 95 (2013) 21–29 Table 3 Some optical and structural properties of CuInS2 films. Loading (ml/cm2)

Film thickness (lm)

Mean crystallite size (nm)

(1 1 2) Peak intensities (a.u.)

0.25 0.51 1.01 1.52 2.02

0.28 0.68 1.27 1.95 2.25

13.63 12.70 12.37 20.77 26.72

6373 12,600 12,378 64,901 88,361

Fig. 3. Atomic percents of the surface elements of CuInS2 thin film deposited from 0.25 ml/cm2 loading obtained from XPS analysis.

25

size of the films deposited using the solution ranging between 0.25 and 1.01 ml/cm2 was comparable. On the other hand, crystallite size slightly increased when the solution amount was further increased. This was also consisted with the formation of Cu-rich structure for the precursor solution more than 1.52 ml/cm2. It is known that Cu-rich phases in CuInS2 films result large crystallite size and cause to increase in preferential orientation (Ortega-Lo´pez and Morales-Acevedo, 1998). XPS spectra have been used to further analysis of the CuInS2 films. All XPS data of the films were recorded after argon ion sputter etching for 30 s. Fig. 5 shows the XPS spectra of Cu2p3/2, In3d5/2 and S2p core levels of sprayed CuInS2 films. As can be seen in Fig. 5, peaks observed at 932.5, 952.6, 444.9, 452.5, 161.9 eV binding energies could be attributed to the Cu2p3/2, Cu2p1/2, In3d5/2, In3d3/2 and S2p, respectively. These recorded binding energies are comparable to binding energies of copper, indium and sulfur in spray deposited CuInS2 films reported in literature (Sebastian et al., 2009a,b). Moreover, peaks around 530 eV observed for all samples could be attributed to the oxygen bonded to the metal, which may indicate the formation of indium oxide in the films (Katerski et al., 2008). Also the intensity of both oxygen and indium peaks decreased with increasing the solution amount. This could be attributed to the decreasing the ratio of indium oxide in the structure with increasing the solution amount. 3.3. Optical and electrical properties Absorption coefficient and band gap values for sprayed CuInS2 films were determined from the optical transmission data. For an allowed direct band gap transition, the absorption coefficient can be related to the photon energy hm by; ðahmÞ ¼ Aðhm  Eg Þ

1=2

ð2Þ

where A is a constant, Eg is the band gap energy, h is the plank constant, m is the frequency, and a is the absorption

Fig. 4. XRD of ultrasonically sprayed CuInS2 films prepared at different solution loading.

d¼

0:89k b cos h

ð1Þ

where d is the crystallite size; k is the X-ray wavelength used; b is the angular line width of half maximum intensity; and h is the Bragg’s angle. Mean crystallite size and peak intensity of the films increased with increasing the solution loading and the calculated values of crystallite size were ranging between 12.37 and 26.72 nm (Table 3). These values were in good agreement with previously reported data (Sebastian et al., 2009a,b; Khan et al., 2012). The crystallite

Fig. 5. XPS spectra of CuInS2 thin films deposited using (a) 0.25 ml/cm2, (b) 0.51 ml/cm2, (c) 1.01 ml/cm2, (d) 1.52 ml/cm2, and (e) 2.02 ml/cm2 solution loading.

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coefficient. Band gap of the films were determined from the extrapolation of the linear region of (ahm)2 versus hm graph (Fig. 6). Eg of the CuInS2 thin films decreased dramatically when the solution loading increased. Eg of 1.45 eV has been calculated for the films deposited with 1.01 ml/cm2 loading. This value is very close to the Eg of bulk CuInS2 film. The band gap values of the films deposited from 1.52 to 2.02 ml/cm2 were 1.38 and 1.40 eV, respectively. The reason for the deviation obtained in the Eg could be the non-molecularity of the films. It is known that optical absorption coefficient and the transmittance of the thin films affects the Eg value. Hence, it is very important to find optimal thickness and the structure of the thin films to obtain maximum performance. The absorption coefficient of the CuInS2 thin films was approximately 104 cm1 for the wavelengths ranging between 980 and 1000 nm. This confirms that spray deposited CuInS2 films are promising candidates as absorber layer for photovoltaic applications. Room temperature electrical resistivity measurements of the films were performed via 2 point-probe technique. As can be seen in Table 4, bulk resistivity of the films did not change dramatically with solution loading and were ranging between 2.42  101 and 10.1  101 X m. Photosensitivity (PS) is a measure of increase in electrical conductivity under illumination and can be calculated using the following equation; PS ¼ ðI L  I D Þ=I D

ð3Þ

where IL represents the current under illumination with light and ID represents the dark current. PS measurements have been performed at room temperature by applying voltage and measuring current through 2-point contacts using Keithley 2400 IV source-measure unit. Lot Oriel low cost solar simulator equipped with 150 W xenon arc lamb has been used to perform text under illumination. PS values of the CuInS2 thin films prepared here were ranging between 0.4 and 1.5 (Table 4). Also these values were very close together, except the 0.25 ml/cm2 loading for which the Cu/In ratio was the highest compare to the other solution loadings used in this study. Hence, it is possible to

conclude that when the Cu/In ratio exceeded a critical value, photosensitivity was diminishing most probably due to the recombination processes. Tiwari et al. previously reported a similar data indicating the poor photosensitivity of the CuInS2 thin films with excess Cu (Tiwari et al., 1985). In order to further investigate the electrical conductivity, the temperature dependent measurements were performed between 120 and 330 K. Fig. 7 shows the variation of the electrical conductivity with reciprocal temperature. For all samples, conductivity decreased exponentially with decreasing the temperature and resulted twolinear regions in ln(r) versus T1 graph. The activation energies (Ea) calculated from the slopes of ln(r) versus T 1 graph at different loadings were given in Table 4. For all our samples two activation energy levels have been determined. As expected, both the shallow, Ea2, and the deeper energy level, Ea1, values increased with increasing the resistivity of the films. The highest bulk resistivity and the Ea values, which are 10.1  101 X m and 99.5 meV (Ea1) respectively, have been obtained for the solution loading at 2.02 ml/cm2. Krunks et al. reported similar activation energies for the spray pyrolyzed and annealed CuInS2 films under H2S environment. They also reported that the activation energy of the films increased with increasing the cooling rate of the samples after sulfurization (Krunks et al., 2006). The non-linear behavior of semilogarithmic plot of conductivity with inverse temperature implies that the carrier excitation to localized state with low activation energy can be provided by hopping conduction mechanism for the spray pyrolyzed CuInS2 films. Amara et al. have previously reported the hopping mechanism as a carrier transport process for co-evaporated CuInS2 and Cu(In, Ga)Se2 polycrystalline thin films (Amara et al., 2006, 2007). In variable range hopping model, the conductivity can be written as (Paul and Mitra, 1973); "   # 1=ðnþ1Þ T0 r ¼ r0 exp  ; T

ð4Þ

Fig. 6. (ahm)2 versus (hm) plots of CuInS2 thin films deposited with (a) 0.25 ml/cm2, (b) 0.51 ml/cm2, (c) 1.01 ml/cm2, (d) 1.52 ml/cm2, and (e) 2.02 ml/cm2 solution loading.

N.D. Sankir et al. / Solar Energy 95 (2013) 21–29

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Table 4 Some electrical and optical parameters of the CuInS2 films. Loading (ml/cm2)

Sheet resistivity (X/h) 5

8.63  10 1.30  106 4.40  105 1.68  105 4.48  105

0.25 0.51 1.01 1.52 2.02

Bulk resistivity (O m) 1

2.42  10 8.85  101 5.58  101 3.27  101 10.1  101

Fig. 7. Variation of the conductivity with reciprocal temperature for various loading of precursor solution.

where n shows the dimensionality. The degree of disorder T0 and the pre-exponential factor r0 can be expressed as; T0 ¼

ka3 and r0 ¼ e2 a2 tph N ðEF Þ k B N ðEF Þ

ð5Þ

where k is a dimensionless constant (18.1), a is the coefficient of exponential decay of the localized states involved in the hopping process, kB Boltzmann constant, e is the electron charge, a is the variable hopping distance, tph is the phonon frequency, and N(EF) is the density of states about Fermi energy level (Mott and Davis, 1979). The other two hopping parameters R, average hopping distance and W, average hopping energy can be written as (Singh et al., 1992); R ¼ ½9=8pak B TN ðEF Þ1=4

PS [(IL  ID)/ID]

Ea1 (meV)

Ea2 (meV)

0.4 1.0 0.9 1.5 1.2

56.3 86.7 72.2 57.7 99.5

17.9 24.0 21.2 17.6 52.2

the CuInS2 films deposited from 2.02 ml/cm2 solution, ln (rT1/2) versus T1/4 plots were linear for the temperature range 110–330 K. On the other hand, all other CuInS2 films had two different linear regions for the temperature ranges 110–210 and 220–330 K. From the slopes and intercepts of these plots, Mott’s parameters were calculated. In these calculations, a typical phonon frequency of 1013 Hz was assumed. For all our samples it has been observed that average hopping energy and the aR did not affected much from the solution loading and were about 25.9 and 2.7 meV, respectively. These values satisfied with the Mott requirements for aR  1 and W  kBT. Hence, it is possible to conclude that in the studied temperature range, the Mott variable range hopping mechanism provides the electrical conduction for the spray deposited CuInS2 films. Further characterization of the electrical properties of the CuInS2 films has been done by Hall effect measurements. Van der Pauw technique has been used for all samples to determine the d.c. Hall effect. However, we were unable to observe d.c. Hall effect for the sample deposited using 0.51 ml/cm2 precursor solution. This was most probably due to the high sheet resistivity. Previously, Tiwari et al. have been reported similar results for single-phase CuInS2 films deposited by spray pyrolysis (Tiwari et al., 1985). Table 5 summarizes the effect of solution loading on mobility and the carrier concentration of the CuInS2 films. As seen from Table 5, except the sample deposited from 0.25 ml/cm2 loading, all our films showed p-type conductivity. The maximum carrier concentration has been

ð6Þ

and W ¼ ½3=4pR3 N ðEF Þ

ð7Þ

For all our samples, the best fit of ln(r) versus T1 graph has been obtained for n = 3 indicating the three-dimensional hopping (Fig. 8). As can be seen from this figure, except the sample deposited using 2.02 ml/cm2 loading, there are two regions, which can be attributed to the polycrystalline structure of the CuInS2 films. For polycrystalline materials, localized and extended states contribute to the electrical conductivity at low and high temperatures, respectively. In order to investigate the hopping parameters, ln(rT1/2) versus T1/4 graphs were examined. For

Fig. 8. ln(rT1/2)  T1/4 variation for the CuInS2 films deposited from various loading of precursor solution.

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Table 5 Summary of Hall parameters of CuInS2 thin films. Loading (ml/ cm2)

Type

Mobility (cm2/ V s)

Carrier concentration (cm3)

0.25 1.01 1.52 2.02

n p p p

0.70 18.0 40.1 23.4

2.12  1019 7.05  1015 1.69  1017 1.13  1017

observed for the 0.25 ml/cm2 loading. On the other hand, samples deposited using 1.52 ml/cm2 loading had the maximum mobility, which is 40.1 cm2/V s. This mobility value is quite high as compared with the previously reported values for polycrystalline CuInS2 thin films (Kato et al., 2009; Qiu et al., 2005). Kato et al. have been reported the 20 cm2/V s hole mobilities and 1014 cm 3 hole concentration of the undoped CuInS2 films deposited by temperature difference method (Kato et al., 2009). Also Qiu et al. reported the mobilities of the polycrystalline CuInS2 films deposited via ion layer gas reaction technique was ranging between 1.8 and 5.6 cm2/V s depending on the preparation conditions (Qiu et al., 2005). Fluctuation in the Hall mobility and the carrier concentration of the CuInS2 films deposited in this study using various solution loadings could be explained by the deviation from stoichiometric CuInS2 structure. It has been known that the electrical properties of the CuInS2 films are affected both by non-molecularity and non-stoichiometry of the samples (Tiwari et al., 1985). 4. Conclusions CuInS2 thin films have been successfully deposited on soda lime glass substrates using very low amount of spraying solution. All our samples had a chalcopyrite CuInS2 structure as observed from XRD analysis. EDX results confirmed that Cu rich secondary phases on the surface were observed for the films deposited using solution amount more than 1.01 ml/cm2. Solution amount together with thickness of the film is critical for the film properties. By using ultrasonic impact nozzle deposition yield of CuInS2 thin films increased drastically while preserving the film quality. Besides, crystallinity of the films improved with increasing the solution amount. It should be noted that using ultrasonic impact nozzles could lower the solution consumption, which is a key for commercialization and large area application of spray deposited solar cells. According to best of our knowledge 1.52 ml/cm2 solution loading (30 ml for 26  76 mm2 area) to achieve 1.95 lm thickness obtained in this study is the lowest solution consumption reported in literature. Acknowledgements This study was supported by The Scientific and Technological Research Council of Turkey under the research Grant TBAG-110T326 and by Republic of Turkey Minis-

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