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Growth of AgInS 2 thin lms by ultrasonic spray pyrolysis technique M. Anantha Sunil, K.G. Deepa, J. Nagaraju ,1 Energy and Health Monitoring Instrumentation Laboratory, Department of Instrumentation & Applied Physics, Indian Institute of Science, Bangalore 560012, India abstract article info Article history: Received 9 January 2013 Received in revised form 10 October 2013 Accepted 16 October 2013 Available online 22 October 2013 Keywords: AgInS 2 Chalcopyrite Ultrasonic spray pyrolysis CuAu structure Carrier density Silver Indium Di-sulde (AgInS 2 ) thin lms are deposited using ultrasonic spray pyrolysis technique and the effect of substrate temperature (T s ) on lm growth is studied by varying the temperature from 250 to 400 °C. From the structural analysis, orthorhombic AgInS 2 phase is identied with preferential orientation along (002) plane. Further analysis with Raman revealed the coexistence of CuAu ordered and chalcopyrite structures in the lms. Stoichiometric lms are obtained at T s of 300 °C. Above 300 °C, the lm conductivity changed from p to n-type and the grain size decreased. The band gap of AgInS 2 lms varied from 1.55 to 1.89 eV and absorption coefcient is found to be N 10 4 cm -1 . The lms have sheet resistance in the range of 0.05 to 1300 Ω/. Both p and n type lms are prepared through this technique without any external doping. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The demand for renewable sources such as solar energy, hydro- power, geothermal, biogas, tide energy and wind energy is increasing in high momentum. Among these sources, solar energy invites particular attention because of the huge quantity of energy which the Sun supplies to the Earth in each minute. Solar cell is one of the techniques to utilize solar energy by converting it into electricity. Thin lm technology has been widely used for developing solar cells in large scale and has gained interest of many researchers since the discovery of CdS based solar cells [1]. Over the years materials such as CdS, CdTe, Cu(In,Ga)Se 2 , CuInS 2 and Cu 2 (Zn,Sn)S 4 have been extensively studied by various methods in order to improve the efciency of the solar cell. To date, Cu (In,Ga)Se 2 based solar cell has produced a record- efciency of 20.3% [2]. AgInS 2 (AIS) is one of the potential candidates among the IIIIVI 2 alloys, as an absorber layer in solar cell. AIS has a direct band gap of 1.8 eV and high absorption coefcient [3]. The band gap of AIS is high compared to the optimum band gap of 1.5eV for a single junction thin lm solar cell. But the same property makes it particularly suitable for multi-junction or tandem solar cells. AIS could be made as a top layer in tandem solar cells. Also the band gap could be tailored by adjusting the composition as observed in the present study. AIS lms have been deposited by both physical and chemical deposition methods. Y. Akaki et al. have grown AIS lms on glass substrates by using single source thermal evaporation of Ag 2 S and In 2 S 3 powders followed by evaporation of two sources [4]. Kong-Wei Cheng et al. prepared AIS lms by sulfurization of thermally evaporated Ag and In metal precursors [5]. Arredondo et al. have prepared AIS lms by using co-evaporation method [6]. They have used Ag, In and a tantalum effusion cell for sulfur as sources to form AIS lms. S.H. You et al. have prepared AIS lms by using hot wall epitaxy method where shot type Ag, In and S were used as the sources for formation of AIS epilayers [7]. Ortega-Lopez et al. obtained n-type AIS lms with band gap between 1.87 and 2.01 eV using chemical spray pyrolysis [8]. Compared to physical methods chemical methods are simple and low cost. In addition, chemical methods consume lesser deposition time which enables the large scale manufacturing feasible in a limited period of time. In this work, we have employed chemical spray pyrolysis, which is widely used in the deposition of thin lms. A few research groups have used this technique to prepare AIS thin lms in which they have used pneumatic spray pyrolysis [813]. Here, ultrasonic spray pyrolysis (USP) technique is used. In this work, ultrasonic frequency is used to generate aerosols of the material to be sprayed. Our group has previously used this setup for the deposition of Cu 2 (Zn,Sn)S 4 lms [14]. The advantages of USP technique are narrow size distribution, uniformity of lms, faster deposition, and gas ow rate is independent of the aerosol ow rate [15]. In this paper, results of the preliminary studies conducted on AgInS 2 lms prepared at different substrate temperatures are presented. The structural, optical, electrical and morphological properties of the AgInS 2 lms are evaluated. 2. Experimental AgInS 2 thin lms are grown on soda lime glass substrates by USP technique. The substrates are ultrasonically cleaned in acetone and deionized water for 15 min followed by immediate drying of substrates using nitrogen ush. 0.01 M AgNO 3 , 0.01 M InCl 3 and 0.05 M SC(NH 2 ) 2 are taken as precursors and the total volume of the solution is kept Thin Solid Films 550 (2014) 7175 Corresponding author. E-mail address: [email protected] (J. Nagaraju). 1 Tel./fax: +91 80 22932273 (Off.). 0040-6090/$ see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.10.053 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf

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Page 1: Growth of AgInS2 thin films by ultrasonic spray pyrolysis technique

Thin Solid Films 550 (2014) 71–75

Contents lists available at ScienceDirect

Thin Solid Films

j ourna l homepage: www.e lsev ie r .com/ locate / ts f

Growth of AgInS2 thin films by ultrasonic spray pyrolysis technique

M. Anantha Sunil, K.G. Deepa, J. Nagaraju ⁎,1

Energy and Health Monitoring Instrumentation Laboratory, Department of Instrumentation & Applied Physics, Indian Institute of Science, Bangalore 560012, India

⁎ Corresponding author.E-mail address: [email protected] (J. Nagaraju).

1 Tel./fax: +91 80 22932273 (Off.).

0040-6090/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.tsf.2013.10.053

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 January 2013Received in revised form 10 October 2013Accepted 16 October 2013Available online 22 October 2013

Keywords:AgInS2ChalcopyriteUltrasonic spray pyrolysisCu–Au structureCarrier density

Silver Indium Di-sulfide (AgInS2) thin films are deposited using ultrasonic spray pyrolysis technique and theeffect of substrate temperature (Ts) on film growth is studied by varying the temperature from 250 to 400 °C.From the structural analysis, orthorhombic AgInS2 phase is identified with preferential orientation along (002)plane. Further analysis with Raman revealed the coexistence of Cu–Au ordered and chalcopyrite structures inthe films. Stoichiometric films are obtained at Ts of 300 °C. Above 300 °C, the film conductivity changed from pto n-type and the grain size decreased. The band gap of AgInS2 films varied from 1.55 to 1.89eV and absorptioncoefficient is found to be N104cm−1. The films have sheet resistance in the range of 0.05 to 1300Ω/□. Both p andn type films are prepared through this technique without any external doping.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The demand for renewable sources such as solar energy, hydro-power, geothermal, biogas, tide energy and wind energy is increasingin high momentum. Among these sources, solar energy invitesparticular attention because of the huge quantity of energy which theSun supplies to the Earth in each minute. Solar cell is one of thetechniques to utilize solar energy by converting it into electricity. Thinfilm technology has been widely used for developing solar cells inlarge scale and has gained interest of many researchers since thediscovery of CdS based solar cells [1]. Over the years materials such asCdS, CdTe, Cu(In,Ga)Se2, CuInS2 and Cu2(Zn,Sn)S4 have been extensivelystudied by variousmethods in order to improve the efficiency of the solarcell. To date, Cu (In,Ga)Se2 based solar cell has produced a record-efficiency of 20.3% [2]. AgInS2 (AIS) is one of the potential candidatesamong the I–III–VI2 alloys, as an absorber layer in solar cell. AIS has adirect band gap of 1.8 eV and high absorption coefficient [3]. The bandgapof AIS is high compared to the optimumbandgapof 1.5eV for a singlejunction thin film solar cell. But the same property makes it particularlysuitable for multi-junction or tandem solar cells. AIS could be made as atop layer in tandem solar cells. Also the band gap could be tailored byadjusting the composition as observed in the present study.

AIS films have been deposited by both physical and chemicaldeposition methods. Y. Akaki et al. have grown AIS films on glasssubstrates by using single source thermal evaporation of Ag2S andIn2S3 powders followed by evaporation of two sources [4]. Kong-WeiCheng et al. prepared AIS films by sulfurization of thermally evaporated

ghts reserved.

Ag and Inmetal precursors [5]. Arredondo et al. have prepared AIS filmsby using co-evaporation method [6]. They have used Ag, In and atantalum effusion cell for sulfur as sources to form AIS films. S.H. Youet al. have prepared AIS films by using hot wall epitaxy method whereshot type Ag, In and S were used as the sources for formation of AISepilayers [7]. Ortega-Lopez et al. obtained n-type AIS films with bandgap between 1.87 and 2.01 eV using chemical spray pyrolysis [8].

Compared to physical methods chemical methods are simple andlow cost. In addition, chemical methods consume lesser depositiontime which enables the large scale manufacturing feasible in a limitedperiod of time. In this work, we have employed chemical spraypyrolysis, which is widely used in the deposition of thin films. A fewresearch groups have used this technique to prepare AIS thin films inwhich they have used pneumatic spray pyrolysis [8–13]. Here,ultrasonic spray pyrolysis (USP) technique is used. In this work,ultrasonic frequency is used to generate aerosols of the material to besprayed. Our group has previously used this setup for the depositionof Cu2(Zn,Sn)S4 films [14]. The advantages of USP technique are narrowsize distribution, uniformity of films, faster deposition, and gas flow rateis independent of the aerosol flow rate [15]. In this paper, results of thepreliminary studies conducted on AgInS2 films prepared at differentsubstrate temperatures are presented. The structural, optical, electricaland morphological properties of the AgInS2 films are evaluated.

2. Experimental

AgInS2 thin films are grown on soda lime glass substrates by USPtechnique. The substrates are ultrasonically cleaned in acetone anddeionized water for 15min followed by immediate drying of substratesusing nitrogen flush. 0.01M AgNO3, 0.01M InCl3 and 0.05M SC(NH2)2are taken as precursors and the total volume of the solution is kept

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constant at 60ml. The precursors are dissolved separately in deionizedwater in appropriate quantities and later mixed to form AIS solution.The precursor solution is vaporized by using ultrasonic nebulizer offrequency 180 kHz. Nitrogen is used as carrier gas to transport themist from the nebulizer to the heated substrate surface. The gas flowrate and solution flow rate were kept constant at 19.6 Pa and 2ml/minrespectively. Duration of deposition is 30 min. The substrate tem-perature (Ts) is varied between 250 and 400 °C in steps of 50 and thesamples are named as AIS250, AIS300, AIS350 and AIS400 respectively.Four films of size 3.5×1.5cm2 are prepared in each set. X-ray diffraction(XRD) measurements are performed on the films using Bruker D8operated at 40 kV and 40 mA. Cu Kα line having a wavelength of1.5405 Ǻ is used as the radiation source and the diffraction pattern isrecorded from 20 to 80° in θ–2θ configuration. Raman scatteringanalysis is carried out using Jobin Yvon LabRAM HR. Argon ion laserhaving a wavelength of 514 nm is used as the excitation source andspectrum is recorded from 50 to 400 cm−1. Scanning ElectronMicroscopy (SEM) and Energy Dispersive X-ray Analysis (EDX) arecarried out using FEI ESEM Quanta-200. SEM of sample surface istaken with an acceleration potential of 25 kV and cross-sectional SEMwith 10kV. EDXmeasurement is carried out at an acceleration potentialof 30kV over an area of 5×5μm2 in a collection time of 20s. Absorptionspectra are recorded from 300 to 1000 nm using UV–VIS spectro-photometer (SPECORD S600 UV–VIS) and electrical properties aredetermined using four-point probe method (Four Dimensions probemeter model 280).

3. Results and discussions

Fig. 1 shows the XRD patterns of AgInS2 films prepared at various Tsin the range of 250–400°C. All the films exhibit orthorhombic structurewith preferential orientation along (002) plane. Films also exhibitedpeaks corresponding to (200), (201), (040), (320), (203), (400) and(004) planes of orthorhombic AIS (JCPDS No. 25-1328). AIS filmsdeposited at 300 °C showed an additional peak at 32.2° whichcorresponds to (004) plane of tetragonal AIS (JCPDS No. 25-1330).With the increase in substrate temperature, (002) peak became sharperindicating the improvement in the crystallinity of films. No secondaryphases are observed. Each diffraction pattern (Fig. 1) is a representativeof the four samples prepared in each set. This pattern is observed to beuniform in all the samples in a set apart from the small variation in theintensity.

Structural characterization is also performed by Raman scattering.Raman spectra give different vibration modes, which are characteristics

Fig. 1. X-ray diffraction patterns of AgInS2 films (a) AIS250, (b) AIS300, (c) AIS350 and(d) AIS400.

of a particular structure. The Raman activemodes of vibrations for I–III–VI2 chalcopyrite structure is represented as

Γvib ¼ A1 þ 3B1 þ 3B2 þ 6E ð1Þ

[16].A1 mode corresponds to the vibration of anions (S) in the x–y plane

with cations (Ag and In) at rest. The frequency of thismode is associatedwith the mass of anion and the bond-stretching force of anion–cationbonds. B1 mode corresponds to the frequency of vibration of cations(Ag and In) in anti-phase. B2 and E modes reflect the vibration of In–Satoms in anti-phase. B2 type modes are excited for light polarizedparallel to the optic axis and E type modes are excited only for lightpolarized perpendicular to the optic axis [17].

Raman spectrum of AIS300 (Fig. 2) has peaks at 85, 129, 160 and304 cm−1 which appears for the chalcopyrite structure. The latticestress, defects or variation in the concentrations can cause shift in thewave numbers. Peaks at 85 and 129 cm−1 correspond to the E modeof vibration and that at 160 cm−1 corresponds to B2 mode. The highestmode observed at 304cm−1 needs a discussion, since it is not identifiedas a mode from a chalcopyrite structure [18,19]. In the case of thechalcopyrite compound CuInS2, a Raman peak was observed at305 cm−1 when the film is Cu deficient or prepared at temperaturesbelow 450 °C. This is attributed to the existence of Cu–Au orderedstructure in addition to the chalcopyrite structure. In the chalcopyritestructure, the group VI atoms will be surrounded by two group I andtwo group III atoms. However, without changing the local geometry achain of polytypes can be constructed by moving a part of thechalcopyrite structure. It is demonstrated that, by shifting alternatetwo (001) planes of the chalcopyrite structure will result in the Cu–Aulike structure [20]. In this case, the Raman peak observed at 304 cm−1

is due to the Cu–Au ordered structure. In fact, all the samples showedsimilar Raman spectra with small variations in intensity which is indirect proportion to the crystallinity. In the XRD pattern of AIS300,tetragonal phase is observed together with orthorhombic phase. Theorthorhombic phase is due to the Cu–Au structure. These mixedorthorhombic and tetragonal structures are also observed in pneumaticspray pyrolysis technique [21].

EDX analysis is performed for all the samples in a set and the averagevalues for each set are given in Table 1. The atomic percentage of eachelement in each set of samples falls within a measurement error of±2%. The composition of the films varied with respect to the substratetemperature. ‘In’ concentration is almost identical in all the samples.Meanwhile Ag and S concentrations are varied with substratetemperature. AIS300 has nearly stoichiometric composition. With

Fig. 2. Raman spectra of AgInS2 thin film prepared at 300 °C.

Page 3: Growth of AgInS2 thin films by ultrasonic spray pyrolysis technique

Table 1EDX analysis of AgInS2 films.

Sample name Atomic percentage of elements

Ag (%) In (%) S (%)

AIS250 25 24 51AIS300 25 24 51AIS350 27 24 49AIS400 27 23 50

Fig. 4. Cross sectional image of AgInS2 films grown at Ts= 300 °C.

73M.A. Sunil et al. / Thin Solid Films 550 (2014) 71–75

increase in the substrate temperature, Ag concentration is found toincrease and S concentration is found to decrease. Since sulfur isvolatile, a small quantity might have got evaporated during thedeposition at higher substrate temperatures. This might also be due tore-evaporation of sulfur ions at high temperatures leading to metalrich deposits as observed in similar cases [22].

Fig. 3 shows SEM images of AgInS2 films prepared at differentsubstrate temperatures. The films deposited at the same Ts showedsimilar morphology and in Fig. 3, each SEM image represents thewhole set of samples at a particular Ts. The grain size is the highest forAIS300 which is 0.408 μm. Above this temperature the grain size isfound to decrease. This is due to variation in the stoichiometriccomposition of AgInS2 films as observed in Table 1. Films prepared at250 and 300 °C showed hexagonal particle morphology with uniformdistribution of grains. Above 350 °C, the morphology slightly changedto spherical and in the meanwhile, top surface of the films appearedas noncontinuous. All the samples are free of voids and cracks.

Fig. 3. SEM images of AgInS2 films (a) AIS250

Fig. 4 shows cross sectional image of AgInS2 film prepared at Ts =300°C. The average thickness is found to be ~0.969μmwith a standarddeviation of ±0.14μm. Since the total volume of the precursor solutionwas kept constant throughout the experiment, there was no significantvariation in the thickness of the films prepared at different substratetemperatures.

From the electrical characterizations, carrier density, sheet resistanceand conductivity type of the films are evaluated (Table 2). AIS250 and

, (b) AIS300, (c) AIS350 and (d) AIS400.

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Table 2Electrical properties of AgInS2 films.

Sample name Carrier density ions/cm2 Sheet resistanceΩ/□ Type of conductivity

AIS250 2.553 × 1018 5.12 × 10−02 pAIS300 4.184 × 1016 3.11 pAIS350 3.98× 1015 1300 nAIS400 3.584 × 1016 25.59 n

74 M.A. Sunil et al. / Thin Solid Films 550 (2014) 71–75

AIS300 showed p-type nature whereas AIS350 and AIS450 are n-type.This type of change in conductivity is attributed to the change incomposition of AgInS2. The increase in concentration of Ag may createAg interstitials which acts as donor. Sulfur concentration also showed aslight decrease which might have initiated the formation of vacancy ofsulfur (Vs) donor defects. These donors may be the reason for theobserved change in the type of conductivity. Without any externaldoping, formation of p-type AIS films through spray pyrolysis is foundto be difficult [8,21,23]. As revealed from the present work, p-typeconductivity can be achieved at lower substrate temperatures usingultrasonic spray pyrolysis. Sheet resistance of the material varies inproportion to the carrier concentration. Increase in the substratetemperature results a decrease in the sheet resistance for n-type filmswhich is due to the increase in n-type carriers.

Band gap of the AIS thin films is determined from the absorptionspectra recorded over the wavelength range of 300 to 1000 nm. Theband gap of the material is calculated using Tauc relation [23–25]given by

αhυ ¼ A hυ−Eg� �r ð1Þ

where A is a constant, Eg is the optical band gap, r = 1/2 for directallowed optical transitions and 3/2 for the direct forbidden ones [26].A graph between (αhυ)2 and hυ is plotted and the extrapolation of the

Fig. 5. Absorption spectra of AgInS2 thin films (a) A

linear region of the graph at the x-axis gives the value of the band gap(Fig. 5).

AIS250 and AIS300 showed almost identical band gaps of 1.55 and1.57 eV respectively. The band gap of these films varied slightly withvariation in composition. For Ts N 300 °C, band gap started increasingsignificantly. This may be due to the increase in the concentration ofAg and the conversion of the conductivity type from p to n. Thedecreased crystallite size also contributes to the increased band gapfor the films AIS350 and AIS400. This variation in band gap is inagreement with the values reported elsewhere [11]. The AgInS2 filmsshowed an absorption coefficient N104 cm−1.

4. Conclusions

AgInS2 films are grown using ultrasonic spray pyrolysis techniqueand the properties at different substrate temperatures are studied. Allthe films have a mixed structure of orthorhombic and tetragonalAgInS2, orthorhombic being prominent. Stoichiometric composition isobtained at a substrate temperature of 300 °C. Both p and n-type filmsare prepared by this technique. Sheet resistance of these films variedfrom 5.12 × 10−02 to 1300 Ω/□. The band gap of the material isfound to be in the range of 1.55 to 1.89 eV which makes it ideal fortandem solar cells. The AIS films showed high absorption coefficient(N104 cm−1) and the thickness of the films is found to be ~0.969 μm.All these properties make AgInS2 a potential candidate as absorberlayer for thin film solar cell application.

Acknowledgments

The authors take this opportunity to thankAFMM(Advanced Facilityfor Microscopy and Microanalysis) and MNCF (Micro and NanoCharacterization Facility) CENSe, IISc, Bangalore for providing thecharacterization facilities.

IS250, (b) AIS300, (c) AIS350 and (d) AIS400.

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