Development of CZTS thin films solar cells by pulsed laser deposition: Influence of pulse repetition rate

Available online at www.sciencedirect.com

Solar Energy 85 (2011) 1354–1363 www.elsevier.com/locate/solener

Development of CZTS thin films solar cells by pulsed laser deposition: Influence of pulse repetition rate A.V. Moholkar a,b,⇑, S.S. Shinde a, A.R. Babar a, Kyu-Ung Sim b, Ye-bin Kwon b, K.Y. Rajpure a, P.S. Patil c, C.H. Bhosale a, J.H. Kim b,⇑ b

a Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong-Dong, Puk-Gu, Gwangju 500-757, South Korea c Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India

Received 7 January 2011; received in revised form 21 March 2011; accepted 23 March 2011 Available online 19 April 2011 Communicated by: Associate Editor Takhir Razykov

Abstract High-quality Cu2ZnSnS4 (CZTS) thin films were synthesized by pulsed laser deposition as a function of pulse repetition rate onto the SLG substrates. Influence of pulse repetition rate onto the structural, morphological, compositional and optical properties have been investigated for as-deposited and annealed thin films. X-ray diffraction study shows transformation of amorphous to crystalline phase after tuning pulse repetition rate and annealing of samples. FESEM images of thin films show increase in grain size upon annealing. Films are nearly stoichiometric deposited at 10 Hz repetition rate has been confirmed with the help of EDAX and XPS analysis. The direct band gap energy of the deposited CZTS thin films are in the solar energy range. The performance of solar cell based on CZTS absorber layer has been tested and the efficiency is about 2%. Ó 2011 Elsevier Ltd. All rights reserved. Keywords: CZTS; Structural; Morphological; Compositional; Optical properties; Solar cells

1. Introduction There is a growing interest in studying Cu2ZnSnS4 (CZTS) thin films, which exhibits excellent thermal, chemical, electronic, optical and mechanical properties in addition to less environmental damaging and cost effectiveness candidature. The CZTS films have emerged as a promising absorber material due to band gap energy of 1.4–1.5 eV, and large absorption coefficient over 104 cm1 (Moriya et al., 2007; Seol et al., 2003). The constituent elements of ⇑ Corresponding authors. Address: Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India. Tel.: +91 0231 2642540; fax: +91 0231 2642340 (A.V. Moholkar), tel.: +82 62 530 1709; fax: +82 62 530 1699 (J.H. Kim). E-mail addresses: [email protected] (A.V. Moholkar), [email protected] (J.H. Kim).

0038-092X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2011.03.017

CZTS are non-toxic, abundant in nature, although those of CuIn1xGaxSe2 (CIGS) are toxic (Se) and expensive rare metals (In and Ga). The crystal structure of CZTS is analogous to chalcopyrite type semiconductor of CIGS, which presently acts as a most promising absorber layer material (Tanaka et al., 2009). Katagiri’s et al. (2008) work on the CZTS seems to be the pioneering work, since the research community all over the world has been motivated towards this fascinating material which is considered as the promising one, due to increased efficiency. There are several reports based on the issues concerning the studies of different methods, process parameters and properties (Ito and Nakazawa, 1988; Johnston David, 2010; Katagiri et al., 2001; Kishore Kumar et al., 2009; Moriya et al., 2008; Oishi et al., 2008; Scragg et al., 2008a,b; Sekiguchi et al., 2006; Seol et al., 2003; Tanaka et al., 2005; Vasco and Sacedo´n, 2007; Warrender and Aziz, 2007).

A.V. Moholkar et al. / Solar Energy 85 (2011) 1354–1363

Since the microstructural changes are dependent on the precursor source, complexing agents and producing methods; a variety of physical and chemical techniques have been employed to deposit CZTS thin films which includes pulsed laser deposition (Moriya et al., 2007), atom beam sputtering (Ito and Nakazawa, 1988), hybrid sputtering (Tanaka et al., 2005), RF magnetron sputtering (Seol et al., 2003), thermal evaporation (Oishi et al., 2008), sulfurization of electron-beam-evaporated precursors (Katagiri et al., 2001), spray pyrolysis (Kishore Kumar et al., 2009), sol–gel sulfurizing method (Tanaka et al., 2009) and electrodeposition (Scragg et al., 2008a,b). Among these pulsed laser deposition (PLD) is as an important technique for preparing thin films which is process of random deposition of a number of particles on the substrate surface wherein, the particles can diffuse, aggregate and improves layerby-layer growth (Vasco and Sacedo´n, 2007). The energetic deposition in PLD is used to promote the formation of islands, thereby having a higher total island density (Warrender and Aziz, 2007). Further, the laser parameters such as laser fluencies, wavelength, pulse duration and repetition rate can be altered by tuning the preparation conditions, including target-to-substrate distance, substrate temperature, background gas and pressure, which influences growth yield of films. Besides the laser incident energy, background gas pressure and substrate temperature are the most important parameters affecting on thin film properties like phase formation, morphology and microstructures of the coatings. The preparation in inert gas atmosphere makes it an even possible to tune the film properties (stress, texture, reflectivity, magnetic properties etc.) by varying the kinetic energy of the deposited particles. Stoichiometry transfer between target and substrate is difficult to obtain with evaporation or magnetron sputtering using a single target, but PLD is proven to maintain socalled “stoichiometry transfer” required for the growth of complex systems. All this makes PLD an alternative deposition technique for the growth of high-quality thin films. Moriya et al. (2007) have reported on the fabrication of CZTS thin film solar cell on Mo-coated soda lime glass using PLD with KrF excimer laser pulses with repetition rate of 30 Hz and energy density of 1.5 J/cm2. Sekiguchi et al. (2006) have reported on the epitaxial growth of CZTS thin films on n-GaP substrates by pulsed laser deposition using a KrF excimer laser with repetition frequency of 30 Hz. Apart from these reports, the literature survey shows some studies which are based on repetition rate using KrF Excimer laser with 2 and 10 Hz (Liu et al., 2000), CeCoIn5 thin films using 2, 5, 10 and 20 Hz, ZRC thin films using 40 Hz (Craciun et al., 2009) and other films (Arias et al., 2002). But there is no report on the effect of different repetition rate and its influence on the CZTS thin films. Since the performance of the solar cell is particularly influenced by absorber layer properties which depends on many factors. The microstructure and thickness are often considered as the vital parameters, to improve the efficiency.

1355

The importance of pulse repetition rate lies in the possibility to increase the deposition rate up to a critical thickness required to improve the efficiency. This paper presents the structural, optical, compositional properties and morphology evolution of CZTS thin films subjected to different repetition rates ranging from 2–20 Hz of a 248 nm KrF excimer laser. The solar cell device properties using the best CZTS thin film have been studied. 2. Experimental Pulsed laser deposition (PLD) is a successful method for the fabrication of a large variety of thin films. It offers several promising advantages: (a) congruent transfer, (b) crystallinity enhancement due to the highly energetic species (c) virtually high density arrival of species due to the pulsed process, (d) clean deposition due to not needing an atmospheric gas and (e) simplicity and flexibility in engineering design (Yoshitake et al., 2003). The substrates were placed parallel to the target at 40 mm distance. A KrF excimer laser beam (k = 248 nm) with an optimum incident laser energy of 1.5 J/cm2 was focused through a spherical lens onto a rotating target (500 rpm) at an incidence angle of 45°. The CZTS target was prepared using the solid-state reaction of Cu2S, ZnS and SnS2 powders which was shaped into a pellet, after ball milling in 24 h and then applying the two stage pressures of 2–10 tones, further sealed inside an evacuated quartz ampoule, and kept at 750 °C for 24 h in a microprocessor controlled furnace. The ampoule was broken after room temperature cooling and the target was doubly polished before mounting in the target assembly. A deposition chamber was first evacuated to 102 Torr, using a turbo molecular pump (TMP) the deposition chamber was further evacuated to 1.5  105 Torr. The deposited films were further annealed in a furnace containing N2 + H2S (5%) atmosphere at 400 °C for 1 h. The as deposited and annealed CZTS films were characterized for their structural, morphological, compositional, chemical and optical properties. The efficiency of a typical CZTS thin film based solar cell was tested. The structural properties of the as-deposited and annealed CZTS thin films were studied using high resolution X-ray diffraction (XRD) with Ni-filtered Cu Ka radi˚ (X’pert PRO, Philips, Eindhoven, ation of 1.54056 A Netherlands). The surface morphology of films was observed by using FESEM (field emission scanning electron microscopy, Model: JSM-6701F, UK). The compositional analysis of the film was done using an energy dispersive X-ray analysis (EDAX) system attached to (JEOL, JSM-7500F, Japan). The determination of the chemical bonding was performed by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000, Thermo VG Scientific, UK). Optical absorption study of the films was carried out in the wavelength range 300–800 nm using UV–Vis– NIR spectrophotometer (Cary 100, Varian, Mulgrave, Australia). Current–voltage (I–V) characteristics were measured using a solar simulator (100 W, NEWPORT

1356

A.V. Moholkar et al. / Solar Energy 85 (2011) 1354–1363

STRATFORD, USA) at 100 mWcm2 and AM 1.5 illumination at room temperature. 3. Results and discussion Fig. 1a and b shows the XRD patterns of the as-deposited and annealed CZTS thin films deposited at various PRR ranging from 2–20 Hz. The as-deposited CZTS thin films show amorphous nature. There is an indication of crystalline phase formation along (1 1 2) plane at 2h = 28.66° above 18 Hz. Upon annealing, amorphous nature of all CZTS thin films changes into polycrystalline nature and the peak intensity of all planes increases because of the enhancement in the crystallinity (Krustok et al., 2010). The matching of standard and observed ‘d’ values (Table 1) using ASTM data card No. 26-0575 confirms the tetragonal crystal structure of annealed CZTS thin films. The films show well resolved peaks corresponding to strongest characteristic of (1 1 2) orientation. The peak intensity of (1 1 2) plane increases with pulse repetition rates. Obviously, with the elevation of pulse frequency, the film growth accomplishes the fractal-to compact island shape transformation. The main reason is more ripening time used in lower pulse frequency case. In our model, it is considered that at least two atoms can form an island, which only has a very small probability for detachment. However, if the pulse interval duration is longer, the dimmers will possibly break up into

(a)

400 200

Arb. Int. (CPS)

600

0 2 Hz 6 Hz 10 Hz 14 Hz 18 Hz 20 Hz 25

30

35

40

45

50

55

2θ (deg.)

(b) (112)

2000 1500

(312) (224)

(220)

(105)

(200)

(211)

1000 500

Arb. Int. (CPS)

2500

0 2 Hz 6Hz 10Hz 14Hz 18Hz 20Hz 25

30

35

40

45

50

55

2θ (deg.) Fig. 1. X-ray diffraction patterns of CZTS thin films (a) as deposited (b) annealed at 400 °C for 1 h.

two monomers. Therefore, according to this character of PLD film growth, we assume that the atoms can detach from the edge of stable islands. Just because the detachment of atoms considered in growth process, compact islands appear at lower frequency (Guan et al., 2008). The crystalline quality of the films is slightly degraded for higher PRR. The other weak intense peaks such as (2 0 0), (2 1 1), (2 2 0), (1 0 5), (3 1 2) and (2 2 4) have been also observed. Additional peaks corresponding to d values ˚ having Cu2SnS3 phase (JCPDS 27of 1.9612 and 1.9224 A 0198) have been observed for the CZTS thin film deposited at 10 Hz. The crystallite size of the films is calculated using the Scherrer’s equation, D¼

0:9k b cos h

ð1Þ

where D is the diameter of the crystallites forming the film, k is the wavelength of the Cu Ka line, b is the FWHM in radians and h is Bragg’s angle. The average crystallite size increases from 16 to 26 nm with increase in PRR up to 10 Hz, which then decreases. Quantitative information concerning the preferential crystallite orientation is obtained from different texture coefficient (TC) defined by the well-known relation (Barret and Massalski, 1980), TCðh k lÞ ¼

Iðh k lÞ I 0 ðh k lÞ P Iðh k lÞ 1 N I 0 ðh k lÞ N

ð2Þ

where TC is the texture coefficient of the (h k l) plane, I is the measured intensity, I0 is the ASTM standard intensity and N is the reflection number. The deviation of texture coefficient from unity implies the preferred orientation. The TC along (1 1 2) orientation is larger than (3 1 2) and (2 2 0) orientations (Fig. 2). Further, as the PRR increases the TC (1 1 2) continuously decreases while the TC (3 1 2) and TC (2 2 0) shows exactly opposite trend of each other. The TC (2 2 0) increases up to 10 Hz becomes maximum and then decreases. The observed changes can be attributed to the increase in extent of preferred orientation associated with the increased number of grains along (2 2 0) and (3 1 2). The grains may coalescence and grow in preferred direction when the interfacial energy is minimum one, which is responsible for the observed variation of TC values of different planes. 4. Morphological analysis Fig. 3 shows the variation of thickness of as-deposited and annealed CZTS thin films with respect to pulse repetition rate. Thickness of the film increases with PRR up to 18 Hz and then decreases achieving maximum thickness of about 1.985 lm for as deposited films. The annealing of film enhances the thickness of films which is 2.119 lm. The uniform, homogeneous and densely packed grains are also observed on the surface of the as-deposited films. It is reported that the efficiency of polycrystalline thin film

A.V. Moholkar et al. / Solar Energy 85 (2011) 1354–1363

1357

Table 1 Comparison of observed and standard d for different planes of CZTS thin films deposited using various PRR by PLD. ˚) ˚ ) for the sample deposited using PRR h k l plane Standard ‘d’ values (A ‘d’ Values (A 2 HZ

6 HZ

10 HZ

14 HZ

18 HZ

20 HZ

112 200 211 105 220 312

3.126 2.713 2.368 2.013 1.919 1.636

3.1309 2.7167 – – 1.9212 1.6325

3.1309 2.7167 – – 1.9212 1.6325

3.1415 2.7159 – – 1.9173 1.6376

3.1251 2.7132 2.3659 1.9588 1.9192 1.6344

3.1294 2.7099 2.3662 1.9565 1.9100 1.6310

3.1373 2.7059 – 1.9564 1.9195 1.6338

Lattice parameters/crystallite size

˚) Standard values (A

Values for the sample deposited using PRR

˚) a (A ˚) c (A

5.427 10.848 –

Csavg

6 HZ

10 HZ

14 HZ

18 HZ

20 HZ

5.44 10.81 162

5.43 10.81 181

5.43 10.81 267

5.42 10.84 227

5.43 10.81 211

5.42 10.82 200

TC (112) TC (220) TC (312)

5

28 26 24

4

22 3 20 18

2

Average crystallite size (nm)

30

6

Texture coefficient (TC)

2 HZ

16 1 0

4

8

12

16

20

24

Pulsed repitition rate (Hz) Fig. 2. The variation of TC and crystallite size with pulse repetition rate.

2.5 As deposited Annealed

Film thickness ( μm)

2.0

1.5

1.0

0.5

0.0 0

4

8

12

16

20

24

Pulse repitition rate (Hz) Fig. 3. The variation of film thickness with pulse repetition rate.

solar cell increases with increasing grain size of the absorber layer, and therefore, the larger grains are required for the fabrication of high efficiency solar cells (Caballero et al., 2005). The morphology of as-deposited films changes

into the larger grains which are strongly affected by the repetition frequency and annealing treatment. This means that the active states of the deposited spices are kept in the same time-scale as the intervals. With an increase in the PRR, the next pulsed process takes place and overlaps before the activated states are energetically released, resulting into enhancement in crystal growth. Fig. 4a–f shows the FESEM images of the annealed CZTS films at 400 °C of various PRR. Average grain size increases with PRR up to 10 Hz and then decreases for higher PRR. However, the non-uniform distribution of agglomerated particles with well-defined boundaries with few voids is also seen at some regions. Fig. 5a–f shows the cross sectional FESEM images of the annealed CZTS thin films deposited with various PRR indicating layer thickness. It is concluded that the surface morphology of CZTS thin film is strongly dependent on the PRR. As pulse frequency increases, the interval time between the sequent shots is reduced, namely, the island ripening time decreases. Therefore, at the higher pulse frequency f = 20 Hz, the interval time between two sequent pulses is reduced. In this case, the amount of particles simultaneously deposited on the surface is equal to the one in other cases, but most adatoms meet each other and easily form smaller-size islands before the next shot coming. On the contrary, for pulse frequency f = 10 Hz, the time for island ripening is relatively long, which leads to the average size of formed islands larger. At the early stage of growth, the new islands are easily formed so that island density increases rapidly. The compositional analysis of as-deposited and annealed CZTS thin films for each element is presented in Fig. 6. From EDAX study, it is concluded that CZTS films deposited at 10 Hz are nearly stoichiometric (Cu2:Zn:Sn:S4 :: 1.97:0.82:1.08:4.13, 2.04:0.8:1.0:4.16 for as deposited and annealed films respectively). 5. X-ray photoelectron spectroscopy Fig. 7a and b shows the X-ray photoelectron spectroscopy (XPS) spectra of Zn 2p core level of as-deposited

1358

A.V. Moholkar et al. / Solar Energy 85 (2011) 1354–1363

Fig. 4. (a–f) FE-SEM images of annealed CZTS thin films for pulse repetition rate from 2–20 Hz.

and annealed CZTS thin films. The pronounced splitting of the Zn 2p spectral line into the 2p1/2 and 2p3/2 core levels is observed (Fig. 7). The Zn (2p3/2) line has been shifted from the reported average binding energy position of 1021.7 eV (Shinde et al., 2010). The signs of the chemical shifts indicate electron transfer during the bonding process leading to a net change in the charges (Santos et al., 1992). The Zn (2p3/2) line slightly shifted towards higher and lower binding energy side above 10 Hz PRR for as-deposited and annealed films. Fig. 8a and b shows the narrow scan XPS spectra of Sn 3d of as-deposited and annealed CZTS thin films. The binding energies of 3d5/2 and 3d3/2 are found at 486 eV, and 495 eV for as-

deposited and annealed CZTS thin films, respectively. Because of so-called “final-state” effects, the Sn 3d region consisted of a single doublet at binding energies around 495 eV for Sn 3d3/2 and 486 eV for Sn 3d5/2, which are in good agreement with the energies reported for Sn (Babar et al., 2010). The binding energy of Sn 3d core level increases towards higher binding energy region due to formation of constituent phases due to influence of annealing (Ma et al., 2002). Fig. 9a and b shows the XPS core level spectra of Cu element of as-deposited and annealed CZTS thin films. The binding energy around 952 and 932 eV can be ascribed to the Cu 2p1/2 and Cu 2p3/2 core levels transition of Cu atoms. As we

A.V. Moholkar et al. / Solar Energy 85 (2011) 1354–1363

1359

Fig. 5. (a–f) Cross sectional FE-SEM images of annealed CZTS thin films for pulse repetition rate from 2–20 Hz.

increase the PRR from 2–20 Hz the core states enhances as compared to as-deposited films. The peak intensity increases up to 10 Hz PRR and then decreases for a while. Analysis of sulfur core states (Fig. 10a and b) for as-deposited and annealed CZTS thin films, shows the slight doublet of S 2p3/2 and S 2p1/2 core levels around binding energy of 161 and 162 eV.

"

ð1  Rk Þ at ¼ ln Tk

The optical absorption coefficient (a) was determined from the measured spectral transmittance (Tk) and reflectance (Rk) using the formula (Scragg et al., 2008a,b),

# ð3Þ

where t is the film thickness. The nature of the optical transition, whether direct or indirect and the optical band gap (Eg) of each film is obtained from the equation, a¼

6. Optical properties

2

Aðhm  Eg Þ hm

n

ð4Þ

where Eg is the band gap energy, hm is the photon energy and A is a constant. The value of n depends on the probability of transition; such as 1/2, 3/2, 2 and 3 for direct allowed, direct forbidden, indirect allowed and indirect

1360

A.V. Moholkar et al. / Solar Energy 85 (2011) 1354–1363

40

Theoretical value Target As deposited Annealed

30

20

10

0 Cu

Zn

Sn

S

Elements Fig. 6. Bar diagram of compositional analysis for optimized CZTS thin films at 10 Hz PRR.

forbidden transitions, respectively. In the present investigation, values of a are found to obey above equation for n = 1/2 indicating that the involved optical transition is direct-allowed in nature. Fig. 11 shows (ahm)1/2 and (ahm)2 against hm of the typical (PRR = 10 Hz) as-deposited and annealed CZTS thin films respectively. In case of annealed films, the absorption coefficient is larger than 104 cm1 in

(a)

the visible region, which is consistent with those reported in earlier published results (Chan et al., 2010; Kishore Kumar et al., 2009; Moriya et al., 2007), and therefore, the film is considered as a suitable material for photovoltaic solar energy conversion. Extrapolation, of the straight line potions of graphs in Fig. 11 to zero absorption coefficient (a = 0), leads to estimation of band gap energy (Eg) values of 1.38 and 1.6 eV for as deposited and annealed films respectively, which is in agreement with previous reports (Kishore Kumar et al., 2009; Moriya et al., 2007; Scragg et al., 2009). It is seen that the optical gap with respect to PRR for as deposited samples lies in the range of 1.3– 1.5 eV. While after annealing the band gap energy due to direct optical transition has been found to be in the range of 1.5–1.8 eV. Observed difference in the band gap energies is due to stoichiometric differences. Since this value is quite close to the theoretical optimal value required for a singlejunction solar cell, the CZTS films can be used as an absorber layer for thin film solar cells. 7. Device properties In order to fabricate photovoltaic cell, a CZTS/CdS/ ZnO:Al/Al structure has been fabricated on Mo-coated

(b) Zn 2p1/2

Zn 2p3/2

20 Hz Zn 2p1/2

Zn 2p3/2

20 Hz 18 Hz

Counts/s

Counts/s

18 Hz 14 Hz 10 Hz

14 Hz

10 Hz

6 Hz 6 Hz 2 Hz

2 Hz

1050 1045 1040 1035 1030 1025 1020 1015

1050 1045 1040 1035 1030 1025 1020 1015

Binding Energy (eV)

Binding Energy (eV)

Fig. 7. The Zn 2P core level spectra of CZTS thin films (a) as deposited (b) annealed at 400 °C for 1 h.

(a)

(b) Sn 3d5/2

Sn 3d3/2

20 Hz 20 Hz 18 Hz

Counts/s

18 Hz 14 Hz 10 Hz

500

495

490

485

Binding Energy (eV)

14 Hz

Counts/s

Composition (at%)

50

Sn 3d5/2

Sn 3d3/2

10 Hz

6 Hz

6 Hz

2 Hz

2 Hz

480

500

495

490

485

480

Binding Energy (eV)

Fig. 8. The Sn 3d core level spectra of CZTS thin films (a) as deposited (b) annealed at 400 °C for 1 h.

A.V. Moholkar et al. / Solar Energy 85 (2011) 1354–1363

(a)

1361

(b) Cu 2p3/2

Cu 2p1/2

Cu 2p1/2

Cu 2p3/2 20 Hz

20 Hz

14 Hz 10 Hz

960

955

950

945

940

935

18 Hz

Counts/S

Counts/S

18 Hz

14 Hz

10 Hz

6 Hz

6 Hz

2 Hz

2 Hz

930

925

960

955

Binding Energy (eV)

950

945

940

935

930

925

Binding Energy (eV)

Fig. 9. The Cu 2p core level spectra of CZTS thin films (a) as deposited (b) annealed at 400 °C for 1 h.

(a)

(b) 20 Hz

20 Hz

18 Hz 18 Hz

S 2p1/2 S 2p3/2

10 Hz

Counts/s

Counts/s

14 Hz 14 Hz

10 Hz S 2p1/2 S 2p3/2 6 Hz

6 Hz 2 Hz

2 Hz

170

168

166

164

162

160

158

156

Binding Energy (eV)

170

168

166

164

162

160

158

156

Binding Energy (eV)

Fig. 10. The core S 2p core level spectra of CZTS thin films (a) as deposited (b) annealed at 400 °C for 1 h.

1.4

500

As deposited 1.2

1.0

(eV/cm )

1/2

9

Annealed

0.6

200 0.4 100

0 1.0

2 2

(αhν)

2

0.8 300

(αhν) ,10 (eV/cm )

2 1/2

400

0.2

1.5

2.0

2.5

3.0

0.0 3.5

Energy (eV) Fig. 11. Variation of (ahm)2 as a function of photon energy (hm) for typical as-deposited and annealed CZTS thin films deposited at 10 Hz PRR.

glass substrates. A CdS buffer layer on the CZTS absorber has been deposited using a chemical bath deposition. The

ZnO:Al window layer was then deposited using rf sputtering technique. An Al top grid on the ZnO:Al window layer was deposited using a vacuum evaporation method. Fig. 12a shows the J–V characteristics of the CZTS film deposited using PRR of 10 Hz based solar cell. The CZTS thin film when used as an absorber layer in the solar cell configuration exhibits an open-circuit voltage (Voc) of 585 mV, a short-circuit current (Jsc) of 6.74 mA/cm2, a fill factor of 0.51, and a conversion efficiency of 2.02%. The efficiency is found to be enhanced as compared to reported value (Araki et al., 2008; Tanaka et al., 2009). The quantum efficiency of typical CZTS thin film (Fig. 12b) shows peak at 530 nm and then decreases monotonously with increase in the wavelength. The spectrum shows the good configuration just like a trapezoid. The etched photoelectrode yields peak quantum efficiency of 32% is observed at 530 nm. The absolute quantum efficiency in the whole wavelength region of CZTS thin film is comparable with other solar cells (Das and Damodare, 1998; Nikale et al., 2011). Further detailed investigation of the CZTS formation processes with different process parameters like deposition time, stoichiometric deviation is underway to improve the conversion efficiency of the CZTS-based solar cell.

1362

A.V. Moholkar et al. / Solar Energy 85 (2011) 1354–1363

(a)

BOYSCAST Fellowship (File No.SR/BY/P-02/2008) and University Grants Commission, New Delhi, India for the financial assistance through the minor research Project No F-47-707/2008. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (K2080200147310 B120004510).

Current Density (mA/cm 2 )

8

4

0

-4

Voc Jsc F.F. Eff

= 585 mV 2 = 6.74 mA/cm

References

= 0.51 = 2.02%

-8 0.0

0.2

0.4

0.6

Voltage (V)

(b) 0.40 Quantum Efficiency

0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 200

300

400

500

600

700

800

900

1000

Wavelength (nm) Fig. 12. (a) The J–V characteristic of typical annealed CZTS thin film deposited at 10 Hz, (b) quantum efficiency w.r.t. wavelength.

8. Conclusions CZTS films prepared by pulsed laser deposition technique with suitable physical properties for solar cell seem to be convenient way on small scale. The structural analysis shows the conversion of amorphous to polycrystalline phase due to influence of PRR as well as annealing treatment. The uniform, homogeneous and densely packed grains are observed on the surface as seen in FESEM images for annealed films. The enhancement of crystallization is apparently observed, from grain features upon annealing. The chemical composition is nearly stoichiometric as confirmed from EDAX study. Optical characteristic indicates that the CZTS film shows the required Eg 1.52 eV and can be used as an absorber layer for thin film solar cells. An approach for the improvement of the conversion efficiency of the CZTS-based thin film solar cell has been studied and the conversion efficiency 2% has been observed. Acknowledgments A.V. Moholkar is grateful to the Department of Science and Technology (DST), New Delhi for awarding the

Araki, H., Mikaduki, A., Kubo, Y., Sato, T., Jimbo, K., Maw, W.S., Katagiri, H., Yamazaki, M., Oishi, K., Takeuchi, A., 2008. Preparation of Cu2 ZnSnS4 thin films by sulfurization of stacked metallic layers. Thin Solid Films 517, 1457–1460. Arias, J.L., Mayor, M.B., Pou, J., Leon, B., Perez Amor, M., 2002. Pulse repetition rate dependence of hydroxylapatite deposition by laser ablation. Vacuum 67, 653–657. Babar, A.R., Shinde, S.S., Moholkar, A.V., Bhosale, C.H., Kim, J.H., Rajpure, K.Y., 2010. Structural and optoelectronic properties of antimony incorporated tin oxide thin films. J. Alloys Compd. 505, 416–422. Barret, C., Massalski, T.B., 1980. Structure of Metals. Petgamon, Oxford, 1923. Caballero, R., Maffiotte, C., Guillen, C., 2005. Preparation and characterization of CuIn1xGaxSe2 thin films obtained by sequential evaporations and different selenization processes. Thin Solid Films 474, 70– 76. Chan, C.P., Lam, H., Surya, C., 2010. Preparation of Cu2 ZnSnS4 films by electrodeposition using ionic liquids. Sol. Energy Mater. Sol. Cells 94, 207–211. Craciun, D., Socol, G., Stefan, N., Mihailescu, I.N., Bourne, G., Craciun, V., 2009. High-repetition rate pulsed laser deposition of ZrC thin films. Surf. Coat. Technol. 203, 1055–1058. Das, V.D., Damodare, L., 1998. Photoelectrochemical investigations on nCdSe0.5Te0.5 thin film electrode/polyiodide system. Mater. Chem. Phys. 56, 116–124. Guan, L., Zhang, D.M., Li, X., Li, Z.H., 2008. Role of pulse repetition rate in film growth of pulsed laser deposition. Nucl. Instrum. Meth. Phys. Res. B 266, 57–62. Ito, K., Nakazawa, T., 1988. Electrical and optical properties of stannitetype quaternary semiconductor thin films. Jpn. J. Appl. Phys. 27, 2094–2097. David, Johnston, 2010. Functional requirements for component films in a solar thin film photovoltaic/thermal panel. Solar Energy 84, 384–389. Katagiri, H., Jimbo, K., Yamada, S., Kamimura, T., Maw, W.S., Fukano, T., Ito, T., Motohiro, T., 2008. Enhanced conversion efficiencies of Cu2ZnSnS4-based thin film solar cells by using preferential etching technique. Appl. Phys. Express 1, 041201. Katagiri, H., Saitoh, K., Washio, T., Shinohara, H., Kurumadani, T., Miyajima, S., 2001. Development of thin film solar cell based on Cu2ZnSnS4 thin films. Sol. Energy Mater. Sol. Cells. 65, 141–148. Kishore Kumar, Y.B., Suresh Babu, G., UdayBhaskar, P., Sundara Raja, V., 2009. Preparation and characterization of spray-deposited Cu2ZnSnS4 thin films. Sol. Energy Mater. Sol. Cells 93, 1230–1237. Krustok, J., Josepson, R., Danilson, M., Meissner, D., 2010. Temperature dependence of Cu2ZnSn(SexS1x)4 monograin solar cells. Solar Energy 84, 379–383. Liu, Z.Q., Feng, Y., Yi, X.S., 2000. Coupling effects of the number of pulses, pulse repetition rate and fluence during laser PMMA ablation. Appl. Surf. Sci. 165, 303. Ma, H.L., Hao, X.T., Ma, J., Yang, Y.G., Huang, J., Zhang, D.H., Xu, X.G., 2002. Thickness dependence of properties of SnO2:Sb films deposited on flexible substrates. Appl. Surf. Sci. 191, 313–318. Moriya, K., Tanaka, K., Uchiki, H., 2007. Fabrication of Cu2ZnSnS4 thin film solar cell prepared by pulsed laser deposition. Jpn. J. Appl. Phys. 46, 5780–5781.

A.V. Moholkar et al. / Solar Energy 85 (2011) 1354–1363 Moriya, K., Tanaka, K., Uchiki, H., 2008. Cu2ZnSnS4 Thin films annealed in H2S atmosphere for solar cell absorber prepared by pulsed laser deposition. Jpn. J. Appl. Phys. 47, 602–604. Nikale, V.M., Shinde, S.S., Babar, A.R., Bhosale, C.H., Rajpure, K.Y., 2011. Photoelectrochemical performance of sprayed n-CdIn2Se4 photoanodes. Solar Energy 85, 325–333. Oishi, K., Saito, G., Ebina, K., Nagahashi, M., Jimbo, K., Maw, W.S., Katagiri, H., Yamazaki, M., Araki, H., Takeuchi, A., 2008. Growth of Cu2ZnSnS4 thin films on Si (1 0 0) substrates by multisource evaporation. Thin Solid Films 517, 1449–1452. Scragg, J.J., Dale, P.J., Peter, L.M., 2008a. Towards sustainable materials for solar energy conversion: preparation and photoelectrochemical characterization of Cu2ZnSnS4. Electrochem. Commun. 10, 639–642. Seol, J.S., Lee, S.Y., Lee, J.C., Nam, H.D., Kim, K.H., 2003. Electrical and optical properties of Cu2ZnSnS4 thin films prepared by rf magnetron sputtering process. Sol. Energy Mater. Sol. Cells 75, 155– 162. Sekiguchi, K., Tanaka, K., Moriya, K., Uchiki, H., 2006. Epitaxial growth of Cu2ZnSnS4 thin films by pulsed laser deposition. Phys. Stat. Sol. (C) 3, 2618–2621. Shinde, S.S., Shinde, P.S., Sathe, V.G., Barman, S.R., Bhosale, C.H., Rajpure, K.Y., 2010. Electron–phonon interaction and size effect study in catalyst based zinc oxide thin films. J. Mol. Struct. 984, 186–193.

1363

Santos, A.E., Leiva, E., Macagno, V.A., 1992. Characterization of TiO2 films modified by platinum doping. Thin Solids Films 219, 7–17. Scragg, J.J., Dale, P.J., Peter, L.M., Zoppi, G., Forbes, I., 2008b. New routes to sustainable photovoltaics: evaluation of Cu2ZnSnS4 as an alternative absorber material. Phys. Stat. Sol. B 245, 1772–1778. Scragg, J.J., Dale, P.J., Peter, L.M., 2009. Synthesis and characterization of Cu2ZnSnS4 absorber layers by an electrodeposition-annealing route. Thin Solid Films 517, 2481–2484. Tanaka, K., Oonuki, M., Moritake, N., Uchiki, H., 2009. Cu2ZnSnS4 thin film solar cells prepared by non-vacuum processing. Sol. Energy Mater. Sol. Cells 93, 583–587. Tanaka, T., Nagatomo, T., Kawasaki, D., Nishio, M., Guo, Q., Wakahara, A., Yoshida, A., Ogawa, H., 2005. Preparation of Cu2ZnSnS4 thin films by hybrid sputtering. J. Phys. Chem. Solids 66, 1978–1981. Vasco, E., Sacedo´n, J.L., 2007. Role of cluster transient mobility in pulsed laser deposition-type growth kinetics. Phys. Rev. Lett. 98, 036104. Warrender, J.M., Aziz, M.J., 2007. Kinetic energy effects on morphology evolution during pulsed laser deposition of metal-on-insulator films. Phys. Rev. B 75, 085433. Yoshitake, T., Hara, T., Nagayama, K., 2003. The influence of the repetition rate of laser pulses on the growth of diamond thin films by pulsed laser ablation of graphite. Diam. Relat. Mater. 12, 306–309.