A study on the improved growth rate and morphology of chemically deposited ZnS thin film buffer layer for thin film solar cells in acidic medium

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Solar Energy 85 (2011) 2903–2911 www.elsevier.com/locate/solener

A study on the improved growth rate and morphology of chemically deposited ZnS thin film buffer layer for thin film solar cells in acidic medium Seung Wook Shin a, So Ra Kang b, K.V. Gurav b, Jae Ho Yun c, Jong-Ha Moon b, Jeong Yong Lee a,⇑, Jin Hyeok Kim b,⇑ b

a Department of Materials Science and Engineering, KAIST, Daejeon 305-701, South Korea Photonics Technology Research Institute, Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong-Dong, Puk-Gu, Gwangju 500-757, South Korea c Photovoltaic Research Group, Korea Institute of Energy Research, 71-2 Jang-Dong, Yuseong-Gu, Daejeon 305-343, South Korea

Received 27 April 2011; received in revised form 19 August 2011; accepted 21 August 2011 Available online 15 September 2011 Communicated by: Associate Editor Nicola Romeo

Abstract Zinc sulfide (ZnS) thin films have been prepared by chemical bath deposition method with improving growth rate and morphology using the mixed complexing agents of ethylenediamine tetra-acetate disodium salt (Na2EDTA) and hexamethylenetetramine (HMTA). The effects of HMTA quantity on the morphological, compositional, optical, structural and electrical properties of ZnS thin films with fixed Na2EDTA concentration have been investigated. ZnS thin films were deposited on glass substrates using aqueous solutions containing zinc acetate dehydrate and thioacetamide in acidic medium (pH 4). Field emission scanning electron microscopy results show that the morphology of a deposited ZnS thin film using HMTA as a complexing agent is rough. However, very uniform and smooth ZnS thin films are obtained using mixed complexing agents of Na2EDTA and HMTA. The growth rate and root mean square of ZnS thin films are improved with increasing HMTA quantities. X-ray diffraction patterns show that all the ZnS thin films are grown as a hexagonal structure without secondary phase (ZnO) regardless of HMTA quantity. Optical band gap energy of ZnS thin films deposited using mixed complexing agents increase from 3.75 to 3.87 eV with increasing quantity of HMTA. Ó 2011 Elsevier Ltd. All rights reserved. Keywords: ZnS thin films; Chemical bath deposition; Less toxic complexing agent; Growth rate; Acidic medium

1. Introduction ZnS has been widely used in optoelectronic device application including optical switching device, photo catalysts and optical sensors because of its outstanding properties such as wide band gap (3.65 eV), high refractive index (2.35) and high dielectric constant (Goudarzi et al., 2008; ⇑ Corresponding authors. Tel.: +82 62 530 1709; fax: +82 62 530 1699 (J.H. Kim), Tel.: +82 42 350 4216; fax: +82 42 350 3310 (J.Y. Lee). E-mail addresses: [email protected] (J.Y. Lee), jinhyeok@chonnam. ac.kr (J.H. Kim).

0038-092X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2011.08.030

Lee et al., 2003). Recently, Cu(In,Ga)Se2 (CIGS)-based thin film solar cells using ZnS buffer layer has been achieved high conversion efficiency of 18.5% (Bhattacharya et al., 2004). Although that using chemical bath deposition CdS (CBD-CdS) buffer layer has the best efficiency record of 19.9% (Repinsl et al., 2008), Cd causes serious environmental problems due to the large amount of Cd compound wastes during deposition processing (Hariskos et al., 2005; Siebentritt, 2004; Bhattacharya et al., 2004; Bhattacharya and Rammanathan, 2004). In addition, the conversion efficiency of thin film solar cells using CBD-CdS buffer layer decreases due to the reduced absorption resulting

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from the narrow band gap energy of CdS buffer layer (2.4 eV) (Sartale et al., 2005; Johnston et al., 2002; Raviprakash et al., 2009). These problems have stimulated researches developing a Cd-free buffer layer to overcome these difficulties. ZnS thin film has been considered as a promising material used as an alternative buffer layer in CIGS-based solar cells because it is less toxic and costeffective (Gangopadhyay et al., 2004; Kushiya, 2004). ZnS buffer layers have been prepared by several methods such as sputtering (Gayou et al., 2010), chemical vapor deposition (Zhang et al., 2006), CBD (Goudarzi et al., 2008; Lee et al., 2003; Sartale et al., 2005; Johnston et al., 2002), electro-deposition (Kassim et al., 2010) and pulsed-laser deposition (Hiramatsu et al., 2002). Among these, CBD method has been widely used because of several advantages such as convenient, simple and cheap process as compared to the other methods and it can be easily applied to a large scale deposition area (Goudarzi et al., 2008; Gangopadhyay et al., 2004; Siebentritt, 2004). Practically, the CIGS-based thin film solar cells with chemically deposited buffer layer showed the best conversion efficiency because there were smaller grain size and lower defect density at the interface between CIGS absorber and buffer layer and larger inter-diffusion distance of elements than other methods (Hariskos et al., 2005). CBD-ZnS buffer layer needs to have uniform morphologies, pinhole free, good interfacial adhesion and high growth rate for its application in thin film solar cells and it is resulted in the improved conversion efficiency (Lee et al., 2003; Repinsl et al., 2008; Qi et al., 2008; Kushiya, 2004). It has been well known that the morphological property of a CBD-ZnS buffer layer is strongly related to the complexing agents, which control Zn2+ ion concentration during deposition processing (Sartale et al., 2005; Asenjo et al., 2008). Hydrazine and ammonia are commonly used as complexing agents. Although the crystal quality and growth rate of CBD-ZnS thin films are improved using hydrazine as a complexing agent, it is flammable, carcinogenic and toxic materials (Goudarzi et al., 2008; Johnston et al., 2002). In order to solve these problems, several researches have examined a ZnS buffer layer deposited using less toxic complexing agents. Recently, Alireza et al., reported that the improved crystal quality of ZnS thin film could be grown using Na2EDTA as a less toxic complexing agent in acidic medium (Goudarzi et al., 2008). Our previous paper, ZnS thin films with uniform and continuous morphology were deposited in acidic medium (Kang et al., 2010). Although ZnS thin films were successfully grown with the improved crystal quality and uniform morphology using Na2EDTA as a less toxic complexing agent in acidic medium, the thickness of ZnS thin films grown for 4 h was below 100 nm. This long deposition time for a CBD-ZnS buffer layer was resulted in the low conversion efficiency of thin film solar cells due to the easy formation of Zn(OH)2 in ZnS buffer layer (Lee et al., 2003; Repinsl et al., 2008; Kushiya, 2004). Table 1 shows the deposition condition and growth rate of chemically deposited

ZnS thin films using less toxic complexing agents reported in the literature as compared with our results. Therefore, it is necessary to improve the growth rate for ZnS thin film by introducing a proper complexing agent during the CBD process. It is well known that hexamethylenetetramine (HMTA) easily forms Zn2HMTA in hot reaction solution, which results in increasing the ZnS formation probability by releasing Zn2+ ions with reaction proceeds (Zhang et al., 2000; Hoffmann et al., 2003; Anand et al., 2009). This characteristic suggested that the growth rate of ZnS thin film could be improved with keeping smooth and dense microstructure by proper use of mixed complexing agents. The purpose of this study is to improve the growth rate of ZnS thin films by CBD method in acidic medium at 80 °C without changing uniform morphology by using mixed complexing agents of Na2EDTA and HMTA. The effects of HMTA quantity on the morphological, chemical, optical, structural and electrical properties of ZnS thin films have been reported. 2. Experimental details ZnS thin films were prepared on glass substrates by CBD method from aqueous solution bath containing zinc acetate dehydrate (Zn(CH3COO)2
2H2O) and thioacetamide (C2H5NS, TAA) in acidic medium. The tetra-acetate disodium salt (C10H14N2Na2O8
2H2O, Na2EDTA) and hexamethylenetetramine (C6H12N4, HMTA) were used as complexing agents. The reaction bath solutions were prepared using 40 ml of 0.2 M Zn(CH3COO)2
2H2O, 40 ml of 0.055 M Na2EDTA and 80 ml of 0.4 M TAA. The different solutions were prepared by adding the concentration of 0.5 M HMTA quantity from 0 ml to 30 ml. The de-ionized water was added to make a 200 ml solution in the reaction beaker. The pH of the solution was adjusted at 4 by adding HCl. Prior to deposition, the substrates were cleaned ultrasonically using acetone and de-ionized water for 10 min. The cleaned substrates were dried using N2 blower before they were introduced into the bath and then fixed vertically. The cleaned glass substrates were immersed vertically in the bath. The deposition was carried out at 80 °C for desired times without stirring the reaction solution. After deposition, the substrates were taken out of the reaction bath, rinsed with distilled water, dried in air at room temperature and preserved in an airtight plastic container. The deposited thin films were well adherent to the substrates. The thin films deposited with 0, 10, 20 and 30 ml of HMTA quantity designated as HMTA 0, HMTA 10, HMTA 20 and HMTA 30, respectively. The surface morphology and roughness were investigated by field emission scanning electron microscopy (FE-SEM, Model: JSM-6701F, Tokyo, Japan) and atomic force microscopy (AFM, Digital Instrument, nanoscope III, USA), respectively. The optical transmittance of the deposited thin films was measured by UV–visible spectroscopy (Cary 100, Varian, Mulgrave, Australia) in the wavelength from 300 nm to 800 nm. The structural properties of

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Table 1 The deposition condition and growth rate of chemically deposited ZnS thin films using less toxic complexing agents reported in the literature as compared with our results. Zn source

S source

Complexing agent

pH

Growth temperature (°C)

Growth rate (nm/ h)

Refs.

ZnSO4 (0.01–0.03 M)

TU (0.01– 0.05 M) TU (0.15 M)

10

70

90

9.5–10.5

82–86

50

Johnston et al. (2002) Ladar et al. (2007)

TU (0.2 M)

Tri-sodium citrate (0.15– 0.7 M) Tri-sodium citrate (0.075– 0.6 M) Tri-sodium citrate (0.2 M)

10

80

33

Shin et al. (2010)

TAA (0.4 M)

Na2EDTA (0.2 M)

6

70–73

40

TAA (0.2 M) TAA

Urea (0.5 M) Acetic acid

3.8 2–2.5

80 70

50 77.4

TAA (0.4 M)

Na2EDTA (0.055 M)

5–6.5

80

24

Goudarzi et al. (2008) Bayer et al. (2002) Makhova et al. (2005) Kang et al. (2010)

TAA (0.4 M)

Na2EDTA (0.005 M)

4

80

15

Present

TAA (0.4 M)

HMTA (0.5 M)

4

80

150

Present

TAA (0.4 M)

HMTA 0

4

80

37

Present

TAA (0.4 M)

HMTA 10

4

80

77

Present

TAA (0.4 M)

HMTA 20

4

80

105

Present

TAA (0.4 M)

HMTA 30

4

80

135

Present

Zn(CH2COO)2 (0.015 M) Zn(CH2COO)2 (0.02 M) Zn(CH2COO)2 (1 M) ZnCl2 (0.02 M) Zn(CH2COO)2 (0.02 M) Zn(CH2COO)2 (0.02 M) Zn(CH2COO)2 (0.02 M) Zn(CH2COO)2 (0.02 M) Zn(CH2COO)2 (0.02 M) Zn(CH2COO)2 (0.02 M) Zn(CH2COO)2 (0.02 M) Zn(CH2COO)2 (0.02 M)

TU: Thiourea (CS(NH2)2) and TAA: Thioacetamide: (CH3C(S)NH2).

the deposited thin films were determined by using high-resolution X-ray diffraction (XRD, X’pert PRO, Philips, Eindhoven, Netherlands) using Ni- filtered Cu Ka1 radia˚ ] operated using a grazing incidence diftion [k = 1.54056 A fraction mode. The microstructures of the deposited thin films were analyzed by using conventional transmission electron microscope (HRTEM, TECNAI G2F30 ST, FEI, Netherlands). The electrical properties of the deposited thin films were characterized using a Hall measurement (M/N #7707_LVWR, LAKE SHORE CRYOTRONICS INC., USA) at room temperature. 3. Results and discussion Fig. 1 shows the cross-sectional tilted-view FE-SEM images of ZnS thin films deposited only using Na2EDTA as a complexing agent (a) and only HMTA as a complexing agent (b) in aqueous solutions at pH 4 for 2 h. A thin (30 nm) and a thick (300 nm) ZnS thin films are observed in Fig. 1a and b. ZnS thin films deposited using only Na2EDTA is consisted of only nano-sized grains while that only using HMTA is consisted of nano-sized grains as well as particle shaped grains. In particular, the growth rate of ZnS thin film having rough morphology deposited using a HMTA dramatically increased as compared to that using a Na2EDTA as a complexing agent. Fig. 2 shows the cross-sectional tilt and surface view FESEM images (a–h) and thickness (i) of ZnS thin films deposited using different HMTA quantity in aqueous solu-

tions. The grain size of ZnS thin films decrease with increasing HMTA quantity from 100 nm to 30 nm. ZnS thin films deposited using HMTA 0, HMTA 10 and HMTA 30 are consisted of only nano-sized grains while that using HMTA 20 is consisted of nano-sized grains as well as particle shaped grains. In addition, ZnS thin films deposited using HMTA 0 and HMTA 10 solutions have some voids and cracks in surface. However, no voids and cracks solutions are observed at ZnS thin film deposited using HMTA 20 and HMTA 30. The thickness of ZnS thin films increase with increasing HMTA quantity from 37 nm to 135 nm (cross sectional results are not shown here). Fig. 3 shows the AFM images (a–d) and root mean square (RMS) values (e) of ZnS thin films deposited using different quantity of HMTA. The RMS values of ZnS thin films dramatically decrease with increasing HMTA quantity from 10 nm to 2.6 nm. It is also observed that the grain size of ZnS thin films decrease with increasing HMTA quantity. From AFM images of ZnS thin films deposited using HMTA 0 and HMTA 10 solution, some voids also are observed and it is consistent with FE-SEM results. Noticeable information was obtained from abovementioned FE-SEM and AFM experimental results (Figs. 1– 3). Our experimental results show two different characteristics of ZnS thin films deposited using Na2EDTA or HMTA as a complexing agent. These low growth rate and rough morphology are easily formatted the Zn(OH)2 in buffer layer and many defects at the interface between absorber layer and buffer layer indicating the low conver-

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(a) ZnS Glass 500nm

(b) ZnS Glass

500nm

Fig. 1. Cross-sectional tilted-view FE-SEM images of ZnS thin films deposited only using Na2EDTA as a complexing agent (a) and only HMTA as a complexing agent (b) in aqueous solutions at pH 4 for 2 h.

140

(i)

Thickness (nm)

120 100 80 60 40 20

0

10

20

30

HMTA Quantity Fig. 2. Cross-sectional tilt and surface view FE-SEM images ((a and e) HMTA 0, (b and f) HMTA 10, (c and h) HMTA 20 and (d and g) HMTA 30) and thickness (i) of ZnS thin films deposited using different quantity of HMTA in aqueous solutions at pH 4 for 1 h.

sion efficiency (Lee et al., 2003; Repinsl et al., 2008; Kushiya, 2004). Our experimental results of ZnS thin

films deposited using mixed complexing agent suggest the solution for these difficulties. The thickness of ZnS thin films deposited using the mixed complexing agents are improved without changing surface morphology. In addition, thick ZnS thin films deposited using proper HMTA quantity have uniform morphology, pinhole free and good interfacial adhesion without any agglomerated particles. These behaviors are attributed to the different coordination number of Zn2+ ions in the between Na2EDTA and HMTA. The coordination number of Na2EDTA was one while that of HMTA was two because of two positive ions of HMTA (Zhang et al., 2000; Hoffmann et al., 2003; Anand et al., 2009). There are relatively many concentrations of Zn-complexing agent ions in reaction solution using mixed complexing agents as compared to that using only Na2EDTA. Generally, the growth mechanism of ZnS thin film has been known to be that the Zn-[complexing agent]2+ ion reacts with S2 ion at the solution or substrate surface at hot solution (Sartale et al., 2005; Asenjo et al., 2008; Hubert et al., 2007). Therefore, the thickness of ZnS films deposited using mixed complexing agents were thicker than that deposited using Na2EDTA. Fig. 4 shows the compositional ratio of ZnS thin films deposited using different concentrations of HMTA in aqueous solutions. The composition ratio of ZnS thin films deposited using HMTA 0 and HMTA 10 solution are slightly poor in Zn and rich in S. However, those deposited using HMTA 20 and HMTA 30 solution are rich in Zn and poor in S. The Zn atomic ratio of ZnS thin films deposited using HMTA 0, HMTA 10, HMTA 20 and HMTA 30 solutions are 49.27%, 49.46%, 51.56% and 52.42%, respectively. The higher Zn concentrations of ZnS thin films with increasing HMTA quantity are attributed to the higher coordination number of HMTA than that of Na2EDTA. The higher coordination number is resulted in the more formation of Zn-[complexing agent]2+ and finally Zn atomic ratio of ZnS thin films are improved with increasing HMTA quantity. Fig. 5 shows UV–visible transmittance in the range from 300 nm to 800 nm (a) and (aht)2 vs. photon energy plots (b) of the ZnS thin films deposited using different quantity of HMTA in aqueous solutions. All the ZnS thin films show good transparency in the visible region and sharp absorption edges. This sharp absorption feature is attributed to the good homogeneity in the grain sharp and size and low defect concentration in the films {Kang, 2010 #46}. The sharp absorption edge of the ZnS thin films are observed near 290 nm. The transmittances of ZnS thin films deposited using HMTA 0 and 30 solutions are about 80% while that deposited using HMTA 10 and HMTA 20 solutions are about 70%. It was well known that the transmittance of thin film was strongly related to the thickness and morphology (Goudarzi et al., 2008). The thickness and smooth morphology of thin film shows the high transmittance because these characteristics are resulted in the low scattering as compared to that with

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(a)

ZnS Glass

500nm

(b)

ZnS Glass

500nm

(c)

ZnS Glass

500nm

(d)

ZnS Glass

500nm

Fig 2. (continued)

thick thickness and rough morphology (Goudarzi et al., 2008). The thick ZnS thin film deposited using HMTA 30 solution shows high transmittance due to low RMS value as compared to those deposited using HMTA 10 and HMTA 20 solutions. Fig. 5b shows the optical band gap energy of the ZnS thin films deposited using different

quantities of HMTA. It was obtained from the (aht)2 vs. photon energy plots by extrapolating the line portion of the curves at a = 0. The optical band gap energies are 3.75–3.87 eV and these values are wider with increasing HMTA quantity. This band gap energy of the ZnS thin films deposited using mixed complexing agents were wider

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(a)

(c)

(b)

(d)

12

(e)

RMS (nm)

10

8

6

4

2

10

0

20

30

HMTA Quantity Fig. 3. AFM images ((a) HMTA 0, (b) HMTA 10, (c) HMTA 20 and (d) HMTA 30) and root mean square (RMS) values (e) of ZnS thin films deposited using different quantity of HMTA in aqueous solutions at pH 4 for 1 h.

than the typical value of the bulk ZnS (3.65 eV) (Sartale et al., 2005).

ZnS thin films consisted of cubic (zinc blende-type structure) and hexagonal (wurtzite-type structure) phases. The

S.W. Shin et al. / Solar Energy 85 (2011) 2903–2911

60

Composition ratio (%)

S Zn

55

50

45

40 0

10

20

30

HMTA Quantity Fig. 4. Compositional ratio of ZnS thin films deposited using different quantity of HMTA in aqueous solutions at pH 4 for 1 h.

(a)

100

Transmittance (%)

80

60

40

Bare glass HMDA 0 HMDA 10 HMDA 20 HMDA 30

20

0

200 300 400 500 600 700 800

2909

cubic phase of ZnS thin film was more stable than hexagonal phase at room temperature while the hexagonal phase was stable at high temperatures (Goudarzi et al., 2008; Hariskos et al., 2005; Bhattacharya et al., 2004; Bhattacharya and Rammanathan, 2004). In addition, the band gap energy of hexagonal ZnS thin film (3.8–3.9 eV) is wider than that of cubic phase (3.6–3.7 eV). Fig. 6 shows the XRD patterns of ZnS thin films deposited using different quantities of HMTA in aqueous solutions. X-ray patterns of ZnS thin films show a broad peak around 28° corresponding to the cubic (1 1 1) plane or hexagonal (0 0 6) plane [JCPDS data file No.: 65-1691 (ZnS/Cub.) and 720163 (ZnS/Hex.)]. They cannot be distinguished clearly into cubic and hexagonal planes because their similar angle position results have been reported by others (Kang et al., 2010). From our optical property of ZnS thin films, it is suggested that the crystal structure of ZnS thin films are hexagonal phase because the band gap energy of hexagonal ZnS thin film is about 3.8–3.9 eV. The intensities of diffraction peaks for the ZnS thin films deposited using mixed Na2EDTA and HMTA were slightly shaper than that deposited using Na2EDTA. It is also observed that the peaks of the ZnS thin films were enhanced with increasing HMTA quantity. This improved crystallinity of ZnS thin film was attributed to increase thickness of deposited thin film with increasing HMTA quantity. The electrical properties of the ZnS thin films deposited using quantity of HMTA 30 were examined by Hall measurements system at room temperature. When electrical properties of ZnS thin films measured, 200 nm thick Au deposited by electron beam evaporation technique to improve the ohmic condition. Hall measurements results of ZnS thin films indicated that the ZnS thin film was n-type semiconductor characteristics. The electrical resistivity, carrier concentration and mobility of the ZnS thin films were 2.4  104 Xcm, 6.09  1010 cm 3, and

(αhυ) (arb.unit)

2

(110) Hex.

(006) Hex.

(103)Hex.

HMDA 0 HMDA 10 HMDA 20 HMDA 30

Intensity (arb.unit)

(b)

(101) Hex.

Wavelength (nm)

HMTA 30 HMTA 20 HMTA 10 HMTA 0

2.5

3.0

3.5

4.0

4.5

Photon energy (eV) Fig. 5. UV–visible transmittance in the range from 300 nm to 800 nm (a) and (aht)2 vs. photon energy plots (b) of the ZnS thin films deposited using different quantity of HMTA in aqueous solutions at pH 4 for 1 h.

20

30

40

50

60

2θ (°) Fig. 6. XRD patterns of ZnS thin films deposited using different quantity of HMTA in aqueous solutions at pH 4 for 1 h.

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4.28 cm2 V 1 S 1, respectively. The resistivity values were similar to earlier report (Ashour et al., 1994). Fig. 7 shows cross-sectional FE-SEM images (a) and typical cross-sectional high-resolution bright field TEM images (b) of the ZnS thin film deposited using mixed complexing agents of HMTA 30 for 30 min on CIGS absorber. ZnS thin film on CIGS absorber shows a continuous and uniform morphology with 70 nm-thick and small grains were fully covered on the CIGS absorber (Fig. 7a). From Fig. 7b, it is observed that the interfaces between the ZnS buffer layer and the CIGS absorber layer are very sharp without any indication of interfacial reaction or any formation of interfacial compounds. This result suggests that 70 nm-thick and uniform ZnS buffer layer were successfully grown on the CIGS absorber for 30 min. It is believed that the efficiency of CIGS thin film solar cells using CBD-ZnS buffer can be improved by introducing HMTA as a new complexing agent.

4. Conclusions Thick ZnS thin films were successfully prepared by chemical bath deposition method in acidic medium at 80 °C changing uniform morphology by introducing a new less toxic complexing agent i.e.; HMTA. The morphology of deposited ZnS thin film using HMTA as a complexing agent was rough as compared to that deposited using Na2EDTA. However, the uniform and smooth morphology of ZnS thin films were obtained using mixed complexing agents and growth rate increased with increasing HMTA quantity. X-ray diffraction results showed that all the deposited thin films are hexagonal phase regardless of quantity of HMTA. The optical band gap energies of ZnS thin films increased with increasing concentration of HMTA. The electrical resistivity of ZnS thin film deposited using HMTA 30 solution are about 2.4  104 Xcm. ZnS buffer layer deposited using HMTA 30 solution on the CIGS absorber showed very continuous and uniform morphology. Future works on ZnS buffer layer deposited using HMTA as a less complexing agent for thin film solar cells application are being carried out. Acknowledgements This research was partially supported by a Grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy (10037233) and partially supported Ho-Nam Leading industry office through the Leading industry Development for Economic Region and partially by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (No. 2011-0001002).

500nm

200nm Fig. 7. Cross-sectional FE-SEM images (a) and typical cross-sectional high-resolution bright field TEM images (b) of the ZnS thin film deposited using mixed complexing agents of HMTA 30 for 30 min on CIGS absorber.

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