Novel synthesis of kesterite Cu2ZnSnS4 nanoflakes by successive ionic layer adsorption and reaction technique: Characterization and application

Electrochimica Acta 66 (2012) 216–221

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Novel synthesis of kesterite Cu2 ZnSnS4 nanoflakes by successive ionic layer adsorption and reaction technique: Characterization and application Sawanta S. Mali a , Bharmana M. Patil a , Chirayath A. Betty b , Popatrao N. Bhosale c , Young Woo Oh d , Sandesh R. Jadkar e , Rupesh S. Devan f , Yuan-Ron Ma f , Pramod S. Patil a,∗ a

Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India Chemistry Division, Bhabha Atomic Research Centre (BARC), Mumbai, India c Department of Chemistry, Shivaji University, Kolhapur 416004, India d Department of Nano Engineering, Kyungnam University, Masan 631-701, Republic of Korea e Department of Physics, Pune University, Pune 411007, India f Department of Physics, National Dong Hwa University, Hualien 97401, Taiwan, ROC b

a r t i c l e

i n f o

Article history: Received 16 November 2011 Received in revised form 22 January 2012 Accepted 22 January 2012 Available online 1 February 2012 Keywords: Kesterite Cu2 ZnSnS4 thin films XPS 1.85% conversion efficiency

a b s t r a c t Novel nanoflakes of Cu2 ZnSnS4 (CZTS) thin film were directly deposited on fluorine doped tin oxide (FTO)-coated glass substrates by the successive ionic layer adsorption and reaction (SILAR) method. The results of energy dispersive X-ray spectroscopy (EDX) indicate that these CZTS thin films are Cu rich and S poor. The combination of X-ray diffraction (XRD) results and Fourier Transform-Raman (FT-Raman) spectroscopy reveal that these thin films exhibit a strong preferential orientation of flakes along the [1 1 2] direction and that a small Cu2−x S phase exists in CZTS thin films. Photoelectrochemical characterization revealed a p-type photo-response when the films were illuminated in an aqueous Eu3+ redox electrolyte. The total conversion power of the CZTS cell is 1.85% under 30 mW/cm2 illumination. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction As the energy crisis has become increasingly serious, research on high efficiency, low cost solar cells has become more important. Among numerous types of solar cells, the CuInx Ga1−x S(Se)2 (CIGS) thin film solar cell has attracted great interest due to its high power conversion efficiency and stability. However, the low availability of indium and gallium increases the production costs and hinders the development of CIGS thin film solar cells. For years, scientists have been attempting to find a substitute for CIGS that avoids using costly elements. CZTS which is a structural analogue to CIGS, has drawn much attention because it is composed of readily abundant materials [1,2]. CZTS is a promising p-type semiconducting material for the absorption layer in solar cells because CZTS thin films have a suitable optical band-gap energy of 1.4–1.5 eV and because all constituents of CZTS films are abundant and non-toxic. CZTS becomes an I2 –II–IV–VI4 quaternary compound semiconductor by substituting the selenium with sulfur and the rare metal indium with zinc and tin in the CIS ternary compound. Each component of CZTS is abundant in the earth’s crust (Cu: 50 ppm, Zn: 75 ppm, Sn: 2.2 ppm, S: 260 ppm) and possesses extremely low toxicity [3–6].

∗ Corresponding author. Tel.: +91 231 2609230; fax: +91 231 2691533. E-mail addresses: [email protected] (S.S. Mali), psp [email protected] (P.S. Patil). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.01.079

Katagiri et al. first reported the formation of CZTS and used inline vacuum sputtering of Cu, SnS and ZnS followed by annealing in a 5% H2 S in N2 atmosphere for 1 h at 550 ◦ C [1,5]. Thin films of CZTS have been prepared by other methods, such as electron beam evaporation [7], electrodeposition of metallic precursors followed by annealing in sulfur vapor [8,9], RF magnetron sputtering [10], a spray pyrolysis technique (SPT) [11], the sol–gel method [12,13] and a precipitation reaction in hot organic solutions [14,15]. Wada et al. developed a method to deposit the CIGS thin films by mechanochemical and screen-printing techniques under nonvacuum conditions to reduce costs. However, these techniques required post-annealing treatment [5]. Because of the expensive nature of these methods, the central theme of this work was to develop a simple and economical method using pollution-free materials. To the best of our knowledge, there are no reports on the preparation of solar cells with CZTS absorption layers deposited by the successive ionic layer adsorption and reaction (SILAR) method. This is one of the newest solution methods used for the deposition of thin film, which is also known as a modified version of chemical bath deposition. In spite of its simplicity, SILAR has a number of advantages: (a) it is relatively inexpensive, simple and convenient for large area deposition; (b) the deposition rate and the thickness of the film can be easily controlled over a wide range by changing the deposition cycles; (c) the SILAR method offers an extremely easy way to dope film with virtually any element in any proportion by merely adding it to the cationic solution; (d) SILAR technique

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2. Experimental All of the chemicals used were analytical reagent (AR) grade and used without further purification. The anionic solution contained 0.02 M CuSO4 , 0.01 M ZnSO4 , and 0.02 M SnSO4 . The cationic precursor contained 0.16 M Na2 S. Fluorine doped tin oxide (FTO)coated glass substrates were cleaned ultrasonically in detergent, acetone, methanol, isopropanol distilled water and dried in an oven. The films (2 cm2 area) were deposited onto an FTO substrate. The uniform and adherent CZTS thin films were deposited using the SILAR method by repeating a sequential immersion in the cationic (Cu, Zn, Sn) precursor solution and the anionic (Na2 S) precursor solution. The films were rinsed with water between immersions. The 30 s immersion time was kept constant for each solution for 30 cycles with the samples designated as CZTS. After deposition, the films were rinsed in doubly distilled water. The deposited CZTS thin films were dried in the oven at 60 ◦ C and were used for subsequent investigations. The structural properties of the deposited thin films were studied using high resolution X-ray diffraction (XRD) with CuK␣ ˚ The surface morphology and composiradiation (k˛ = 1.54056 A). tion of the films were analyzed using scanning electron microscopy (SEM) (Model: JEOL, JSM-6360, Japan) attached with an energydispersive X-ray analysis (EDAX) analyzer to measure the sample composition. Optical absorption studies of the films deposited on glass substrates were carried out in the wavelength range of 350–800 nm with UV–vis spectrophotometer (Shimatzu-1800, Japan). The elemental composition of the CZTS thin films was analyzed using an X-ray photoelectron spectrometer (K-Alpha, Thermo VG Scientific, UK) with a multi-channel detector, which can withstand high photonic energies from 0.1 to 3 keV. The Raman spectra of the films were recorded in the spectral range of 35–4000 cm−1 using a Raman spectrometer (Bruker MultiRAM, Germany Make) equipped with an Nd:YAG laser source with an excitation wavelength of 1064 nm and resolution 4 cm−1 . Photoelectrochemical (PEC) measurements were carried out in 3-electrode mode in 0.2 M Eu(NO3 )3 with a platinum wire counter and Ag|AgCl reference electrodes. Samples were illuminated with a 500 W tungsten filament lamp (intensity 30 mW/cm2 ). 3. Results and discussion The optical absorption spectra for all of the samples were recorded in the wavelength range of 350–800 nm at room

CZTS

(112)

(200)

Intensity (a.u.)

operating at room temperature can produce films on less robust materials; (e) unlike the closed vapor deposition method, SILAR does not require a high quality target and/or substrates, nor does it require a vacuum at any stage, which is a great advantage if the method will be used for industrial applications; (f) there are virtually no restrictions on the substrate material, including its dimensions or its surface profile; and (g) unlike high power methods such as radio frequency magnetron sputtering (RFMS), SILAR does not cause local overheating, which can be detrimental to the materials being deposited. In the present study, we report on the deposition of CZTS nanoflakes thin film by a simple wet chemistry method called successive ionic layer adsorption and reaction (SILAR), which facilitates the growth of thin films by repeating a sequential immersion of the substrate in cationic (Cu, Zn, Sn) precursor and anionic (S) precursor solutions. The structural, morphological, compositional and optical properties of the CZTS thin films have been investigated. The photoelectrochemical (PEC) properties of the resulting electrode are investigated as a function of the repeated cycles used in the CZTS synthesis.

217

(220)

Cu2 ZnSnS4: 00-044-1476

30

40

50

60

2θ (Degrees) Fig. 1. XRD pattern of deposited CZTS thin film onto glass substrate by using SILAR technique. The reference pattern is standard kesterite Cu2 ZnSnS4 (JCPDS no. 00-441476).

temperature. To confirm the nature of the optical transition in all samples, the optical data were analyzed using the classical equation, n

˛=

˛0 (h − Eg ) , h

where all constants have their usual meanings. The nature of the plots suggests a direct interband transition. Extrapolation of the straight-line portion to the X-axis allows for the estimation of the band gap energy values at ∼1.61 eV. This result is in agreement with the band gap values reported by Ito and Nakazawa [1]. Fig. 1 shows the indexed XRD pattern for the CZTS thin films. The lattice parameters (a = 0.544 nm, b = 0.542 nm, c = 1.089 nm) calculated from the X-ray diffraction pattern were matched to the JCPDS card 00-026-0575: a = b = 0.5434 nm, c = 1.0848 nm. The XRD pattern shows evidence of the presence of Cu2 S (JCPDS: 01-072-2276) and two peaks assigned to SnS2 (JCPDS: 00-005-0566) can be clearly seen. The ZnS content of the films is difficult to ascertain because the lattice parameters of ZnS (sphalerite) are almost identical to those of CZTS. SEM was used to analyze the morphological features of the different cycles for all of the samples. Fig. 2 shows an SEM image of the sample CZTS thin film with non-uniform distribution of the agglomerated particles within well-defined boundaries. The image shows the flake-like morphology. The energy dispersive spectrum (EDS) shows that the relative elemental ratios for Cu:Zn:Sn:S were consistent with the 2:1:1:4 stoichiometry (Fig. 3). The composition of the SILAR-deposited CZTS thin film is shown in Table 1. The spectrum reveals that the film is Cu poor and Zn rich, Cu/(Zn + Sn) = 0.79 and Zn/Sn = 1.4, which improves the conversion efficiency [15]. Table 1 Results of elemental analysis of Cu2 ZnSnS4 films by energy dispersive X-ray spectroscopy. Sr. no

Element

Atomic percentage (at.%)

1. 2. 3. 4. 5. 6.

Cu Zn Sn S Zn/Sn Cu/(Zn + Sn)

21.9 16.3 11.1 50.1 1.4 0.79

218

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Fig. 2. Scanning electron microscopic images of CZTS thin film at different magnification.

Fig. 3. Energy dispersive spectrum (EDS) of CZTS thin film.

Raman Intensity (a.u.)

The FT-Raman spectrum of the CZTS thin film is shown in Fig. 4. The existence of the CZTS is confirmed by the most intense peak at 334 cm−1 and the peak at 285 cm−1 , which are close to the values reported for CZTS thin films. It is noticeable that the peaks at 351 and 274 cm−1 attributed to the zinc blend ZnS do not appear, suggesting the absence of this compound [16,17].

334 cm -1

The phonon lifetime () of the CZTS thin film can be derived from the Raman spectrum via the energy time uncertainty relation [18] E 1 = 2c =  h ¯

where E is the uncertainty in the energy of the phonon mode, h ¯ is Planck’s constant, and  is the full width at half maximum (FWHM) of the Raman peak in units of cm−1 . The phonon lifetime is mainly limited by two mechanisms: (i) anharmonic decay of the phonon into two or more phonons, so that energy and momentum are conserved, with a characteristic decay time  A , and (ii) perturbation of the translational symmetry of the crystal by the presence of impurities, defects and isotopic fluctuations, with a characteristic decay time  I . The phonon lifetime deduced from the FT-Raman measurements is therefore written as 1 1 1 = +  A I

285 cm -1

100

200

300

400

500

-1

Raman shift (cm ) Fig. 4. FT-Raman spectrum of CZTS thin film.

600

(1)

(2)

Based on Eq. (2), the phonon lifetime () of the CZTS thin film is 0.144 × 10−12 s. The first observation of a core-level binding-energy shift by Xray photoelectron spectroscopy (XPS) was made almost thirty years ago in studies of the oxidation of copper. In the years since, the field has expanded to the point that core-level binding-energy shifts constitute one of the most widely used diagnostic tools for routine surface analysis in industrial laboratories. The continued growth in sales of commercial XPS instruments indicates the very real value of the information they provide. However, it is apparent from the recent literature on core-level binding-energy shifts that even more information about the chemical and physical properties of solids

S.S. Mali et al. / Electrochimica Acta 66 (2012) 216–221

a

350000

b

1.6x10 4

250000

1.4x10 4

200000

1.2x10 4

Count / s

Count / s

300000

150000 100000

219

C1s

C1s

1.0x10 4 8.0x10 3

50000 6.0x10 3

0

4.0x10 3

-50000 1200

1000

800

600

400

200

292

0

290

Bnding Energy (eV)

c

5.0x10

Cu 2p3/2

282

280

Zn 2p Zn 2p3/2

4

4.0x10 4

20.03 eV

22.93 eV 3.8x10 4

Cu 2p1/2

4

3.0x10 4 2.5x10

4.4x10 4 4.2x10

4

4.0x10 4 3.5x10

d

Count / s

Count / s

284

Bnding Energy (eV)

Cu 2p

4.5x10 4

3.6x10 4

Zn 2p1/2

3.4x10 4 3.2x10 4

4

3.0x10 4

952.05 eV

2.0x10 4

2.8x10 4

932.02 eV

1.5x10 4

1044.99 eV

1022.06

2.6x10 4 965

960

955

950

945

940

935

930

925

1050

1045

Bnding Energy (eV)

1040

1035

1030

1025

1020

1015

Bnding Energy (eV)

2.0x10 4

f

Sn 3d 1.8x10 4

1x10 4

S 2p

8.48 eV 9x10 3

1.6x10 4

Sn 3d5/2 Sn 3d3/2 495.21 eV

8x10 3

486.73 eV

Count / s

1.4x10 4

Count / s

286

6.0x10 4 5.5x10 4

e

288

1.2x10 4 1.0x10 4

7x10 3 6x10 3

8.0x10 3

5x10 3

6.0x10 3 4x10 3 4.0x10

3

500

495

490

Bnding Energy (eV)

485

480

174

172

170

168

166

164

162

160

158

156

Bnding Energy (eV)

Fig. 5. X-ray photoelectron spectroscopic analysis of CZTS nanocrystals: (a) representative XPS survey spectrum of CZTS sample, (b) curve fit of C 1s peak, (c) core-level lines of Cu 2p, (d) curve fit of Zn 2p peak, (e) curve fit of Sn 3d peak and (f) curve fit of S 2p peak.

S.S. Mali et al. / Electrochimica Acta 66 (2012) 216–221

and surfaces could be readily obtained if the fundamental principles and recent developments in the field of core-level binding-energy shifts were more widely disseminated. The elemental information of the CZTS thin films was analyzed using an X-ray photoelectron spectrometer (Thermo K-Alpha) with a multi-channel detector, which can withstand high photonic energies from 0.1 to 3 keV. A typical survey spectrum of the CZTS thin film deposited using the SILAR method is shown in Fig. 5a. The spectrum indicates the presence of Cu, Zn, Sn and S. Additionally, a small amount of adventitious carbon C1 was incorporated from the carbon in the starting material (indicated by the peak in Fig. 5b). Fig. 5c shows the core level spectra of Cu 2p, with two peaks at 932.3 and 952.3 eV, indicating a 20.0 eV split of Cu(I), which is consistent with the standard separation of 19.9 eV. The Zn 2p peaks located at 1022.4 and 1045.4 eV and a peak splitting of 23.0 eV indicate Zn(II) (Fig. 5d). The Sn(IV) is confirmed by a peak splitting of 8.5 eV of two peaks located at 486.9 and 495.4 eV, respectively. The two S 2p peaks are located at 161.5 and 162.7 eV, with a peak separation of 1.2 eV, which is also consistent with the literature value for a metal sulfide. The quantification in XPS is determined by the intensity of the photoelectron. The XPS intensity (I) of a photoelectron peak from a homogeneous solid is given, in a very simplified form, by: I = JK

(3)

where J is the photon flux,  is the concentration of the atom or ion in the solid,  is the cross-section for photoelectron production (which depends on the element and energy being considered), K is a term that covers all of the instrumental factors (0.5 for AlK␣), and is the electron attenuation length. The intensity is usually taken as the integrated area under the peak following the subtraction of a linear or S-shaped background. In the tetragonal structure of Cu2 ZnSnS4 , the zinc atoms are surrounded by tin and copper atoms instead of a simple link with sulfur, as in ZnS. Thus, the chemical condition of Zn in Cu2 ZnSnS4 is different from that in ZnS, which affects the binding energy of Zn 2p. As shown in Fig. 5d, the Zn 2p peaks, Zn 2p3/2 and Zn 2p1/2 , appear at 1021.2 and 1044.2 eV in ZnS, whereas those of CZTS appear at 1022.06 and 1044.99 eV, which is consistent with the reported values [11]. We noticed different reported binding energy values for Zn 2p3/2 and Zn 2p1/2 in ZnS, which might be caused by the calibrating reference. Even considering those differences in binding energy values, the chemical shift between ZnS prepared intentionally and CZTS may suggest that zinc sulfide (ZnS) does not exist as a part of mixture and that the as-prepared samples are Cu2 ZnSnS4 nanocrystals. ZnS is a secondary phase in the Sn- and Cu-poor and Zn-rich region. Due to the high band gap, this material could be called an insulator (3.54 eV), meaning that the presence of ZnS can both reduce the active area (i.e., the area where electron–hole pairs are produced) and inhibit current conduction in the absorber. It crystallizes in the sphalerite and the wurtzite structure and, in both cases, serves as a semiconductor with a wide band gap of 3.54 or 3.68 eV [19]. The CZTS thin film was used as photoelectrode in a threeelectrode photoelectrochemical cell with a platinum wire counter electrode and saturated Ag|AgCl reference electrode. The electrolyte contained aqueous 0.1 M Eu(III)(NO3 )3 , which served as the redox mediator. The photocurrent, produced by illuminating the films with a 30 mW tungsten filament lamp light source, was measured using a CH-Instruments potentiostat. Photoelectrochemical characterization was chosen over preparation of a solid-state device, as it allows for rapid, nondestructive evaluation of CZTS thin films and eliminates electrical shorting from a vapor-deposited metal back contact penetrating through the pores in the film to the front contact. Additionally, the conformal contact of the electrolyte with the nanoflakes in the film minimizes the distance that

5

2

Isc=3.19 mA/cm Voc=280 mV η=1.85 %

Current Density (mA/cm2)

220

3

2

0 0

100

200

300

400

E (mV vs. Ag/AgCl) Fig. 6. Photoelectrochemical performance of CZTS nanoflakes thin film. Experiments were performed in a 0.1 M Eu(NO3 )3 aqueous electrolyte at room temperature.

minority carriers (electrons) must diffuse to reduce the Eu3+ to Eu2+ before they can recombine with the photogenerated holes. Fig. 6 shows the current density–voltage (I–V) curves for the CZTS samples under darkness and 30 mW/cm2 illumination. The photoelectrochemical measurement confirms the photo-activity of the CZTS thin film prepared by the SILAR method. The generated current that can be used is never short circuit current (Isc ), as a circuit needs both current and voltage. Thus, the point on the curve that has the highest power (current times voltage) possible is determined. This operating point is called the maximum power point, and the associated voltage and current are Vmax and Imax , respectively. The ratio between the theoretical power Isc × Voc and the maximal possible power is called the fill factor (FF): FF =

Imax × Vmax Isc × Voc

(4)

where Imax is the maximum current, Vmax is the maximum voltage, Voc is the open circuit voltage, and Isc is the short circuit current. A higher fill factor is optimal. Graphically, the fill factor is the area of a rectangle within the IV-curve as determined by the maximum power point (which is represented by Vmax and Imax ). The efficiency ( ) of a solar cell, i.e., how much of the incoming light can be converted into electrical energy, is

=

Isc × Voc × FF Pin

(5)

where Voc is the open circuit voltage, Isc is the short circuit current, and Pin is the power of the incident light. Solar cell parameters are listed in Table 2. An equivalent circuit has frequently been used to describe the electric behavior of a photovoltaic device (Fig. 7).

Fig. 7. Equivalent circuit of a CZTS solar cell. It includes next to the diode and a current source I, a series resistance Rs and a parallel resistance Rsh .

S.S. Mali et al. / Electrochimica Acta 66 (2012) 216–221

221

Table 2 Performance parameters of CZTS thin film. Rs is the series resistance and Rsh is the shunt resistance. Sample

Isc a (mA)

Voc (mV)

Imax (mA)

Vmax (mV)

Rs ()

Rsh ()

Ideality factor, nd

FF

Efficiency, (%)

CZTS

3.19

0.280

2.89

0.190

198

5623

1.56

0.62

1.85

a

2

Sample was recorded at 30 mW/cm illumination at room temperature.

The current–voltage characteristics are largely dependent on the series (Rs ) and parallel (Rsh ) resistance. A lower Rs means that higher current will flow through the device, and high Rsh corresponds to few shorts or leaks in the device. The ideal cell would have an Rs approaching zero and an Rsh approaching infinity. The Rs can be estimated from the inverse slope at a positive voltage where the I–V curves become linear. The Rsh can be derived by taking the inverse slope of the I–V curves around zero (0) voltage. The series resistance (Rs ) and the shunt resistance (Rsh ) were analyzed from the I–V curves of the CZTS film using the relation ((4) and (5) respectively)

 dI  dV

 dI  dV

I=0

V =0

=

1

=

Rs

 1  Rsh

(Note that Rs  Rsh )

(6)

In conclusion, we have successfully synthesized novel kesterite Cu2 ZnSnS4 (CZTS) nanoflake thin films by a simple and cost effective SILAR method. To optimize the synthesis conditions of these films, the dipping time was 30 s in each solution. The synthesized CZTS thin films were characterized by various techniques. This analysis demonstrates that the SILAR methodology is able to control the growth of the CZTS at the nanometer scale. The optical absorption study shows the presence of direct transition with a band gap energy of 1.50 eV. The formation of PEC cells with the CZTS thin films showed that films are photoactive with a 1.85% conversion efficiency under 30 mW/cm2 illumination.

(7)

Acknowledgment

Important parameters that are obtained from the power output are listed in Table 2. A slight reduction of the shunt resistance was observed due to shorts or leaks in the device. The series resistance can be expressed as the sum of the bulk and interfacial resistance. It is likely that the two interfaces that have been introduced in the (FTO/CZTS/0.2 M Eu(NO3 )3 /Pt) layer offer a much lower magnitude of series resistance. Increasing the thickness of the CZTS thin film decreases the series resistance in the device and thereby increases the current. As shown in Fig. 6, the cathodic photocurrent was observed at a more negative potential than +0.3 V vs. Ag/AgCl, and its magnitude increased with the negative shift of the electrode potential. This observation reveals that the synthesized CZTS thin films behaved as p-type semiconductor photoelectrodes. The ideality factor ‘nd ’ of prepared CZTS films is determined from the following diode equation: I = I0 (eqV/nd kT − 1)

4. Conclusion

(8)

where I0 is the reverse saturation current, V is the forward bias voltage, k is Boltzmann’s constant, T is the ambient temperature in Kelvin and nd is the ideality factor [20]. The ideality factor is determined under forward bias and is normally found to be between 1 and 2, depending on the relation between the diffusion current and the recombination current. When the diffusion current is more than the recombination current, the ideality factor becomes 1; it becomes 2 in the opposite case. The CZTS sample had an FF of 0.62, a Voc of 0.280 V and an Isc of 3.19 mA/cm2 . This yields an efficiency of 1.85%. Further investigation of the deposition time, film thickness, and the effect of annealing on the photocurrent is underway in our laboratory.

SSM wishes to acknowledge the DAE-BRNS Mumbai for financial support through DAE-BRNS project no. 2008/37/8/BRNS/1489 during 2008–2012. References [1] K. Ito, T. Nakazawa, Jpn. J. Appl. Phys. 27 (1988) 2094. [2] T. Tanaka, T. Nagatomo, D. Kawasaki, M. Nishio, Q. Guo, A. Wakahara, A. Yoshida, H. Ogawa, J. Phys. Chem. Solids 66 (2005) 1978. [3] J.S. Seol, S.Y. Lee, J.C. Lee, H.D. Nam, K.H. Kim, Sol. Energy Mater. Sol. Cells 75 (2003) 155. [4] H. Katagiri, N. Ishigaki, T. Ishida, K. Saito, Jpn. J. Appl. Phys. Part 1 40 (2001) 500. [5] H. Katagiri, K. Saitoh, T. Washio, H. Shinohara, T. Kurumadani, S. Miyajima, Sol. Energy Mater. Sol. Cells 65 (2001) 141. [6] T. Wada, Y. Matsuno, S. Nomura, Y. Nakayama, A. Miyamura, Y. Chiba, A. Yamada, M. Konagai, Phys. Status Solidi A 203 (2006) 2559. [7] H. Katagiri, K. Jimbo, W.S. Maw, K. Oishi, M. Yamazaki, H. Araki, A. Takeuchi, Thin Solid Films 517 (2009) 2455. [8] I. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, R. Noufi, Prog. Photovoltaics 16 (2008) 235. [9] H. Araki, A. Mikaduki, Y. Kubo, T. Sato, K. Jimbo, W.S. Maw, H. Katagiri, M. Yamazaki, K. Oishi, A. Takeuchi, Thin Solid Films 517 (2008) 1457. [10] J.J. Scragg, P.J. Dale, L.M. Peter, Electrochem. Commun. 10 (2008) 639. [11] S.M. Pawar, B.S. Pawar, A.V. Moholkar, D.S. Choi, J.H. Yun, J.H. Moon, S.S. Kolekar, J.H. Kim, Electrochim. Acta 55 (2010) 4057. [12] R.A. Wibowo, W.S. Kim, E.S. Lee, B. Munir, K.H. Kim, J. Phys. Chem. Solids 68 (2007) 1908. [13] N. Kamoun, H. Bouzouita, B. Rezig, Thin Solid Films 515 (2007) 5949. [14] K. Tanaka, N. Moritake, H. Uchiki, Sol. Energy Mater. Sol. Cells 91 (2007) 1199. [15] K. Tanaka, M. Oonuki, N. Moritake, H. Uchik, Sol. Energy Mater. Sol. Cells 93 (2009) 583. [16] S.C. Riha, B.A. Parkinson, A.L. Prieto, J. Am. Chem. Soc. 131 (2009) 12054. [17] T. Kobayashi, K. Jimbo, K. Tsuchida, S. Shinoda, T. Oyanagi, H. Katagiri, Jpn. J. Appl. Phys. 44 (2005) 783. [18] S.S. Mali, C.A. Betty, P.N. Bhosale, P.S. Patil, CrystEngComm 13 (2011) 6349. [19] D.R. Lide (Ed.), Handbook of Chemistry and Physics, 79th ed., CRC Press, 1998–1999. [20] S.M. Sze, Physics of semiconductor devices, Science 258 (1992) 1474.