Synthesis, Structural and Optical Studies of Yb Doped CuGaS2 Thin Films Prepared By Facile Chemical

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Synthesis, Structural and Optical Studies of Yb Doped CuGaS2 Thin Films Prepared By Facile Chemical Spray Pyrolysis Technique 1 S. Kalainathan1,2, N. Ahsan2, T. Hoshii2, Y. Okada2, T. Logu3, K. Sethuraman3 1 – Centre for Crystal Growth, School of Advanced Sciences, VIT University, Vellore, India 2 – Research Center for Advanced Science and Technology (RCAST), The University of Tokyo, Japan 3– School of Physics, Madurai Kamaraj University, Madurai, Tamil Nadu, India DOI 10.2412/mmse.66.48.915 provided by Seo4U.link

Keywords: thin films, intermediate band solar cells, spray pyrolysis, CuGaS2, optical properties.

ABSTRACT. Pristine and Ytterbium (Yb) doped (1-4%) chalcopyrite CuGaS2 (CGS) thin films were successfully prepared by facile homebuilt chemical spray pyrolysis technique and annealed in vacuum, nitrogen and argon atmospheres. X-ray diffraction characterization confirmed that all the prepared films are in tetragonal chalcopyrite structure with polycrystalline nature. The structural characterization of the thin films confirmed the formation of CGS without any presence of secondary phases in X-ray diffraction analysis. The optical band gaps of pristine and Yb doped CGS thin films were obtained from UV absorption spectra. The pristine CGS film shows a band gap of 2.40 eV. It is found that the band gap values decreases from 2.40 to 2.20 and 2.10 eV for 1 and 2 wt% Yb doping, and further widen from 2.4 to 2.2.47 and 2.61 eV for 3 and 4wt% of Yb. Fascinatingly, 1 and 2wt% Yb doped CGS thin films gives two band gaps 2.2- 1.1 eV and 2.1-1.0 eV, and this can be due to the formation of sub-band gap below the conduction band after doping. The presence of Yb in the host CGS thin film was confirmed by X-ray photoelectron spectroscopy studies.

The photoelectric response of the sample has also been studied which shows significant photo current for the 1 wt% Yb doped CGS thin films.

Introduction. The incorporation of an impurity band within the semiconductor band gap can allow the absorption of low energy photons and thus can increase the efficiency of intermediate band (IB) solar cells [], [2], [3]. For a traditional photovoltaic semiconductor the electrons are excited directly from the valence band (VB) to the conduction band (CB) by absorbing photons, whereas in the case of IB semiconductors three photon transitions from VB to IB, IB to CB and VB to CB occurs due to the insertion of partially filled IB into the forbidden band gap which results in the enhancement of photocurrent without affecting the photo voltage. The percentage of upper limit efficiency was calculated to be 65.1% which was greater than the conventional Schokley-Queisser single junction solar cell whose efficiency was about 40.7%, and by increasing more the number of IBs will result in the increase of efficiency upto 80% [1], [2], [3]. Various IB materials such as thin films of highly mismatched alloys III-V dilute nitrides [4, 5], deep impurity doped hosts [6], and nanostructures using quantum dots [7], quantum rings [8], quantum wells [9], etc. makes the IB material to be easily fabricated and also its high density enhances absorption [6].

Ternary chalcopyrite semiconductor copper gallium sulphide (CuGaS2/CGS) attracts research interests for the optoelectronic and photovoltaic solar cell device applications due to the direct band gap of 2.49eV in the green region of the visible spectrum at room temperature [10]. Doping of transition metals in CGS has been found to be a potential candidate for IB solar cells [6, 11]. Earlier reports for doping of transition metals such as Fe [12], [13], [14], V [15], Mn [16], [17], [18], Cr [1921], Zn [22], [23], Ti [24] to the CGS hosts have been predicted for the creation of IB. Theoretical insights and experimental verifications have also been reported for transition metals doped CGS [21].

The valency match and the less distortion in lattice make the transition and rare earth elements to a 1

© 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

suitable dopant for a chalcopyrite lattice. This motivated us to dope rare earth element Ytterbium (Yb) in our chalcopyrite CGS and to study the influence of Yb on the structure and optical properties of CGS. In this paper we report the preparation of Yb doped CGS by spray pyrolysis method which seems to be better method due to its cost effectiveness and large scale production.

Experimental details In archetypal synthesis of Pristine and Yb doped chalcopyrite CGS thin films, 0.1 M of copper acetate, gallium chloride and of thiourea were dissolved using deionised water and excess thiourea was added then stirred for a 30 minutes at ambient temperature to obtain a homogeneous transparent solution.

For the synthesis of Yb doped CGS thin films, the concentrations of dopant of ytterbium chloride were varied between (1-4 wt%). The follow-on mixture solution was then used for the deposition of CGS thin films by the chemical spray pyrolysis technique. In each deposition, the nozzle to substrate distance maintained at 23 cm and 45 mL of precursor solution sprayed at a rate of 3 mL/min on ultrasonically cleaned glass substrate maintained at an optimized substrate temperature of 250 ºC. These CGS thin films were annealed in vacuum, nitrogen and argon atmosphere. Obtained thin films thickness is in the range of ~ 650 nm. The final films were characterized by X-ray diffractometer (Bruker D8 Advance model, Germany), optical absorption studies were carried out using UV-Vis instrument (Hamamatsu, Japan).The band-gap of the samples was estimated using the Tauc plot.

Scanning electron microscope (JEOL, JCM-6000) was used to examine particle size and the surface morphology. X-ray Photoelectron Spectroscopy analysis (XPS, Shimadzu ESCA – 3400) was performed to investigate the elemental states of prepared samples. The electrical properties of the films were studied using the Hall measurement setup in Vander Pauw configuration (Ecopia HMS3000).

Results and discussions X-ray Diffraction (XRD) studies

Fig. 1. XRD patterns of (a) as deposited CGS and (1-4% Yb doped) CGS thin films (b) annealed in vacuum atmosphere, (c) annealed in nitrogen atmosphere, (d) annealed in argon atmosphere.

MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

The XRD patterns were recorded using Powder X-ray Diffractometer (Bruker, Germany) D8 Advance model in the 2θ angle range of 10-80o. The XRD patterns of the CGS thin films for prisitine and various Yb doping concentrations annealed in vacuum, nitrogen and argon are shown in figures 1(a-d). The formation of chalcopyrite structure of CGS (JCPDS Card No.:65-1571) is confirmed by XRD. No secondary phases were observed in all doping concentrations. This proved the presence of single phase CGS in the prepared thin films. The average crystallite size is calculated using Scherrer equation and dislocation density obtained using Williamson and Smallman’s formula and tabulated in Table 1.

Optical Properties Optical absorption properties of the prepared pristine and doped CGS thin films were analyzed by UV-Vis-NIR absorption spectroscopy in the wavelength range between 300-1500nm. The absorption spectra for pristine and Ytterbium doped CGS thin films are shown in Fig. 2(a), there is a strong absorption in the visible region between 400 to 500 nm for the pristine and ytterbium doped CGS thin films. The fluctuation in the film thickness may be the origin for the oscillation in the absorption spectra [25].

Fig. 2. (a) Absorption spectra and (b) Tauc Plot of pristine and Yb doped (1, 2, 3 & 4 wt%) CGS thin films annealed in argon atmosphere.

Ytterbium doping has influenced the absorbance value in the visible region, and the absorbance value increased for the increase in ytterbium concentration. The direct optical band gap of the prepared thin films can be determined by extrapolation of the linear region to the photon energy (hν) axis vs. (αhν)2.

The Tauc’s plot is shown in Fig.2 (b).The pristine CGS film shows a band gap of 2.41 eV. It is found that the band gap values decreases from 2.40 to 2.20 and 2.10 eV for 1 and 2 wt% Yb doping, and further widen from 2.4 to 2.47 and 2.61 eV for 3 and 4wt% of Yb. Fascinatingly (Fig. 3), 1 and 2wt% Yb doped thin films gives two band gaps 2.2- 1.1 eV and 2.1-1.0 eV, and this can be due to the formation of sub-band gap below the conduction band after doping [38, 39]. The change of optical band gap values of the CGS film for increasing Yb doping levels can be explained by the Burstein– Moss effect [40].

MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Fig. 3. Schematic diagram of bandgap and IB bandgap of pristine and Yb doped (1& 2 wt%) CGS thin films annealed in argon atmosphere.

X-ray photoelectron spectroscopy (XPS) Studies XPS studies were carried out for prisitine and Yb (1% and 4%) doped CGS thin films in order to evaluate the presence of elements in the thin films. The wide scan XPS spectra is shown in Fig. 4(a) which shows the presence of Ga, S, C and Cu ions in different states within the prepared thin films.

The clear distinction of peaks with a separation of around 27eV for Ga2p1/2 and Ga2p3/2 proves that Ga exists in trivalent state [29]. As shown in Fig.4 (b), the XPS spectra for the thin film with 4% of Yb clarify the presence of Yb4d state, and prove the existence of Yb ions in the prepared CGS thin films [30].

Fig. 4. (a) Wide range XPS and (b) selective region spectra of pristine, Yb (1% doped) and Yb (4% doped) CGS thin films.

Hall measurements The electrical property is an essential parameter for high-quality absorber material. The electrical properties of pristine and Yb doped CGS thin films were characterized by Hall Effect measurements.

The progression of Yb doped CGS thin film resistivity, conductivity, carrier concentration and mobility as a function of doping concentration is shown in fig.5 and in table 1.

MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Fig. 5. The dependence of resistivity, Hall mobility, carrier concentration on Yb doping level in CGS thin films.

Table 1. The dependence of average crystallite size, average dislocation density band gap, type of conduction, carrier concentration, Hall mobility and resistivity on Yb doping level in CGS films.

Sample Name

Avg. Crystallite size (nm)

Avg. Dislocation density

Bandgap (eV)

Type

Carrier Concentration × 1016 (cm-3)

Mobility (cm2/Vs)

Resistivity (Ω cm)

1015 lines/m2 CGS

12

6.944

2.40

p

1.102

188.2

2.792

CGS: 1Yb

17

3.460

2.20, 1.10

p

5.498

309.2

0.253

CGS: 2Yb

16

3.906

2.10, 1.00

p

5.601

161.3

0.681

CGS: 3Yb

9

12.345

2.47

n

6.435

305.8

1.193

CGS: 4Yb

6

27.777

2.61

n

423.0

78.2

2.251

It is found that samples up to 2 wt% have p-type conductivity then it changed to n type. There is a decrease in resistivity for 1 wt% of Yb doped CGS from 2.792  cm to 0.253  cm with respect to pristine CGS thin film. The variation in the electrical resistivity is attributed to the change in carrier concentration which is 5.498×1016(cm-3). As expected, electron mobility drops significantly with the increase in Yb doping concentration. This is primarily due to the rise in the ionized impurity scattering with increasing Yb doping in the CGS thin films. Introducing Yb increases free electron density by substituting host Ga ions with Yb ions thereby giving free electron. The carrier concentration has risen with increasing Yb dopant concentration. This has caused higher amount of formation of conduction electrons. The resistivity then started increasing with doping concentration to 2.251 Ω cm at 4wt% doping. The 1 wt% of Yb is a suitable donor dopant for the fabrication of low resistance P type CGS thin films.

Photo-response study The photo response property was investigated for the pristine and 1 wt % Yb doped CGS thin films.

As shown in the fig.6, it was observed that the photo response property was improved for 1 wt% of Yb dopant CGS film compared to pristine CGS thin film.

MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Fig. 6. Photo response for (b) Pristine and (b) 1 wt% Yb doped CGS thin films.

From absorption spectra, we could observe the absorption edge and absorption area increases as we increase the dopant concentration. Intermediated band is also predominant for 1 wt of Yb doped CGS film. Consequently, the large absorption area and the presence of intermediate band in 1wt% Yb doped CGS thin films have a direct correlation with the photo response. Here the visible light photons produce excitation of the electrons from the valence band to the conduction band with the help of intermediate band, thus the generation of majority carriers in the presence of light enhances the film conductivity. This property may be employed for the generation of photocurrent and potential application of Yb doped CGS thin film in solar energy conversion devices.

Summary. Pristine and Yb doped CGS thin films were prepared by facile chemical spray pyrolysis technique. Then the films were annealed in vacuum, nitrogen and argon atmospheres. The single phase formation of CGS was confirmed by XRD and XPS techniques. The absorption in the visible region was found to be increasing as the concentration of Yb is increased which is likely due to the presence of intermediate bands due to the Yb species, which absorbs the photon energy. The absorbance due to the intermediate bands could be controlled by changing the doping concentration of Yb. Elemental analysis and presence of Yb ions were proved by X-ray photoelectron spectroscopy.

The doping of Yb is found to decrease the film resistivity and increase the electron carrier concentration in the films. The photoelectric response of the sample has also been studied which shows significant photo current for the 1 wt% Yb doped CGS thin films. By considering above reports, we suggest that 1 wt% Yb is the superior choice for use as a donor dopant to formulate CGS based solar cells.

Acknowledgement. The authors would like to thank VIT University for their constant support and encouragement. This work was performed under the JSPS fellowship for research program in Japan.

References [1] Y.Okada, N.J.Ekins-Daukes, T.Kita, R.Tamaki, M.Yoshida, A.Pusch,O.Hess,C.C.Phillips, D.J.Farrell, K.Yoshida, N.Ahsan, Y.Shoji, T.Sogabe and J.F.Guillemoles, Intermediate band solar cells: Recent progress and future directions, Appl. Phys Rev 2,21-302 (2015).

[2] Ping Chen, Mingsheng Qin, Haijie Chen, Chongying Yang, Yaoming Wang, and Fuqiang Huang, Cr incorporation in CuGaS2 chalcopyrite: A new intermediate-band photovoltaic material with widespectrum solar absorption, Phys. Status Solidi A, 210 (2013), 1098-1102.

[3] Miaomiao Han, Xiaoli Zhang and Z. Zeng, The investigation of transition metal doped CuGaS2 for promising intermediate band materials, RSC Adv., 4 (2014), 62380-62386.

[4] N. Ahsan, N. Miyashita, M. M. Islam, K. M. Yu, W. Walukiewicz, and Y. Okada, Two-photon excitation in an intermediate band solar cell structure, Appl. Phys. Lett. 100 (2012), 72-111.

MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

[5] N. López, A. Reichertz, K. M. Yu, K. Campman, and W. Walukiewicz, Phys. Rev. Lett., Engineering the Electronic Band Structure for Multiband Solar Cells, 106, (2011), 028701.

[6] J. Wu, D. Shao, Zh. Li, M. O. Manasreh, V. P. Kunets, Zh. M. Wang, and G. J. Salamo, Intermediate-band material based on GaAs quantum rings for solar cells, Appl. Phys. Lett. 95 (2009), 071-908.

[7] Q. Shao, A. A. Balandin, A. I. Fedoseyev, and M. Turowski, Intermediate-band solar cells based on quantum dot supracrystals, Appl. Phys. Lett. 91, (2007), 163503-163503-3.

[8] D. C. Johnson, I. M. Ballard, K. W. J. Barnham, J. P.Connolly, M. Mazzer, A. Bessie`re, C. Calder, G. Hill, and J. S. Roberts, Observation of photon recycling in strain-balanced quantum well solar cells Appl. Phys. Lett. 90 (2007), 213-505.

[9] Antonio Martí, David Fuertes Marrón and Antonio Luque, Evaluation of the efficiency potential of intermediate band solar cells based on thin-film chalcopyrite materials, Journal of Applied Physics 103 (2008), 073706.

[10] Woon-Jo Jeong, Gye-Choon Park, Structural and electrical properties of CuGaS2 thin films by electron beam evaporation, Solar Energy Materials & Solar Cells 75 (2003) 93–100.

[11] Zhao Zongyan, Zhou Dacheng, and Yi Juan, Journal of Semiconductors, Analysis of the electronic structures of 3d transition metals doped CuGaS2 based on DFT calculations, 35 (2014), 1.

[12] Teranishi T, Sato K, Kondo K, Optical Properties of a Magnetic Semiconductor: Chalcopyrite CuFeS2: I. Absorption Spectra of CuFeS2 and Fe-Doped CuAlS2 and CuGaS2, J Phys Soc Japan, 36 (1974), 1618-1624.

[13] Von Bardeleben H J, Goltzene A, Meyer B, Effects of iron content and stoichiometry on the coloration of CuGaS2, Phys Status Solidi A, 48 (1978), 145.

[14] Sato K, Teranishi T. Effect of delocalization of d-electrons on the optical reflectivity spectra of CuGa1-xFexS2 and CuAl1-xFexS2 systems. Jpn J Appl Phys, 19S3 (1980) (Supplement 19-3), 101.

[15] Palacios P, Sánchez K, Conesa J C, Theoretical modelling of intermediate band solar cell materials based on metal-doped chalcopyrite compounds, Thin Solid Films, 515 (2007), 6280-6284.

[16] Zhao Y J, Zunger A, Electronic structure and ferromagnetism of Mn substituted CuAlS2, CuGaS2, CuInS2, CuGaSe2, and CuGaTe2, Phy. Rev B, 69 (2004) 104-422.

[17] Zhao Y J, Zunger A, Site preference for Mn substitution in spintronic CuMIIIX2VIchalcopyrite semiconductors, Phys Rev B, 69 (2004), 075208 [18] Picozzi S, Zhao Y J, Freeman A J, Mn-doped CuGaS2 chalcopyrites: An ab initio study of ferromagnetic semiconductors, Phys Rev B, 66(2002), 205-206.

[19] Palacios P, Aguilera I, Wahnon P, et al. Thermodynamics of the Formation of Ti- and Cr-doped CuGaS2 Intermediate-band Photovoltaic Materials, J Phys Chem C, 112 (25) (2008), 9525-9529.

[20] Palacios P, Aguilera I, Wahnón P, Ab-initio vibrational properties of transition metal chalcopyrite alloys determined as high-efficiency intermediate-band photovoltaic materials, Thin Solid Films, 516 (2008) 7070-7074.

[21] Aguilera I, Palacios P, Wahnón P, Optical properties of chalcopyrite-type intermediate transition metal band materials from first principles, Thin Solid Films, 516 ( 2008), 7055-7059.

[22] Seminóvski Y, Palacios P, Wahnón P, Intermediate band position modulated by Zn addition in Ti doped CuGaS2, Thin Solid Films, 519(2011) 7517-7521.

[23] Seminóvski Y, Palacios P, Conesa J C, Thermodynamics of zinc insertion in CuGaS2:Ti, used as a modulator agent in an intermediate-band photovoltaic material, Computational and Theoretical Chemistry, 975(2011) 134-137.

MMSE Journal. Open Access www.mmse.xyz

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

[24] W-J Jeong, G-C Park, Structural and electrical properties of CuGaS2 thin films by electron beam evaporation, Sol Energ Mat Sol C 75 (2003), 93-100.

[25] Dong C, PowderX: Windows-95-based program for powder X-ray diffraction data processing, Journal of Applied Crystallography 32 (1999) 4.

[26] Arham S. Ahmed, Shafeeq M. Muhameda , M.L. Singla, SartajTabassum , Alim H. Naqvi , Ameer Azam, Band gap narrowing and fluorescence properties of nickel doped SnO2 nanoparticles, Journal of Luminescence 131 (2011) 1–6.

[27] K. Sankarasubramanian, P. Soundarrajan, T. Logu, S. Kiruthika, K. Sethuraman, R. Ramesh Babu, K. Ramamurthi, Influence of Mn doping on structural, optical and electrical properties of CdO thin films prepared by cost effective spray pyrolysis method Materials Science in Semiconductor Processing 26 (2014) 346–353.

[28] V.R. Shinde, T.P. Gujar, C.D. Lokhande, R.S. Mane, S.H. Han, Mn doped and undoped ZnO films: A comparative structural, optical and electrical properties study, Mater. Chem. Phys. 96 (2006) 326.

[29] Zhongping Liu, Qiaoyan Hao, Rui Tang, Linlin Wang and Kaibin Tang, Facile one-pot synthesis of polytypic CuGaS2 Nanoplates, Nanoscale Research Letters 8, (2013) 524.

[30] Youichi Ohno, XPS studies of the intermediate valence state of Yb in (YbS)1.25CrS2, Journal of Electron Spectroscopy and Related Phenomena 165 (2008) 1–4.

MMSE Journal. Open Access www.mmse.xyz