Diethylenetriamine assisted synthesis and characterization of stannite quaternary semiconductor Cu2ZnSnSe4 nanorods by self-assembly

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Diethylenetriamine assisted synthesis and characterization of stannite quaternary semiconductor Cu2ZnSnSe4 nanorods by selfassembly Lin-Jer Chen, Yu-Ju Chuang

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S0022-0248(13)00275-3 http://dx.doi.org/10.1016/j.jcrysgro.2013.04.026 CRYS21527

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Journal of Crystal Growth

Received date: 15 November 2012 Revised date: 25 March 2013 Accepted date: 10 April 2013 Cite this article as: Lin-Jer Chen, Yu-Ju Chuang, Diethylenetriamine assisted synthesis and characterization of stannite quaternary semiconductor Cu2ZnSnSe4 nanorods by self-assembly, Journal of Crystal Growth, http://dx.doi. org/10.1016/j.jcrysgro.2013.04.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Diethylenetriamine assisted synthesis and characterization of stannite quaternary semiconductor Cu2ZnSnSe4 nanorods by self-assembly Lin-Jer Chena, *, Yu-Ju Chuangb

a. Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701, Taiwan b. Department of Materials Science and Engineering, National Cheng Kung University, Tainan, 701, Taiwan.

*Corresponding author: Tel: +886-9-22640510 E-mail address:[email protected] (L.J. Chen)

1

Abstract Stannites are important contenders among thin-film solar cells due to their direct band gap and higher absorption coefficient. I2-II-IV-VI4 nanocrystals of Cu2ZnSnSe4 are stannite material with a near-optimum band gap of ~1.5 eV. Semiconductor Cu2ZnSnSe4 nanorods were successfully prepared via a relatively simple and convenient solvothermal route. The device parameters for a single junction Cu2ZnSnSe4 solar cell under AM1.5G are as follows: open circuit voltage of 308 mV, short-circuit current of 29.9 mA/cm2, fill factor of 27 %, and a power conversion efficiency of 2.48 %. Based on a series of comparative experiments under various reaction conditions, the probable formation mechanism of crystals Cu2ZnSnSe4 nanorods is proposed.

Keywords: A1. Crystal structure, A2. Hydrothermal crystal growth, B1. Nanomaterials, B2. Semiconducting quarternary alloys, B3. Solar cells

1. Introduction Semiconductor nanoparticles have recently attracted much attention due to their light-harvesting properties and tunable electronic energy structure depending on their size. Low-cost and high-efficiency solar cells call for new materials and manufacturing approaches, especially in the field of nanocrystal synthesis [1–4]. The crystal structures and potential optoelectronic properties of CZTSe are similar to those of CuInGaSe2 (CIGS), an important photovoltaic material [5-7]. In order to promote the use of photovoltaic devices, it is necessary to develop the solar cells with low cost, high efficiency, and less environmental damaging. Cu2ZnSnSe4 (CZTS) is an emerging solar cell material 2

that contains earth-abundant elements and has a near-optimum direct band gap energy of ~1.5 eV and a large absorption coefficient (>104 cm-1) [8-10]. The quaternary Cu2–II–IV–VI4 compounds are considered to be a novel material for alternative thin film solar cells owing to their p-type material with band gap energies ranging from 0.96 to 1.63 eV [11]. On the other hand, the development of novel thin film solar cells has been intensively studied to construct cost-effective highly-efficient photovoltaic systems [12,13]. These techniques, however, still require relatively demanding processing conditions (e.g., high-temperature reactive sintering or the use of harsh chemicals such as hydrazine) to reach reasonable efficiencies [14-16]. In particular, photovoltaic devices based on several semiconductor nanocrystals have recently been demonstrated, including Cu2S [17], CdTe [18], PbSe [19], Pb(Sx,Se1-x) [20], CuInSe2, Cu(In,Ga)Se2 [21-23], and Cu(In1-x,Gax)S2 [24]. However, the vacuum equipments are often expensive and only applicable to high-value niche markets. It is, therefore, important to develop low-cost processes to grow CZTS films. The solvothermal technique may significantly lower the cost of the materials as it does not involve establishment of high vacuum systems. The discovery of efficient multiple exciton generation by single photons in semiconductor nanoparticles has initiated intensive research to fabricate highly efficient solar cells (quantum dot solar cells) in which the efficiency of solar light energy conversion theoretically reaches ca. 60% [25], which is much higher than that of conventional thin film solar cells. CZTS nanoparticles seem to be a promising candidate as the light absorber in quantum dot solar cells. Furthermore, tunable band gaps are also required for materials to maximize solar absorption, so that the resulting solar cells would 3

fully use the energy of photons and have remarkable energy conversion efficiencies [26–28]. Here, we have been interested in the use of solvothermal process, which are carried out at low temperatures and do not require toxic precursors, to prepare I2-II-IV-VI4 nanocrystalline with a wide range of optical and electronic properties accessible in the nanoscale and demonstrate their use in the fabrication of solar cells.

Experimental Section Chemicals. All chemicals were used as received without further purification. Copper (ൖ) chloride

(CuCl2; 99.99%), elemental selenium powder (99%) from Sigma Aldrich; zinc(ൖ) chloride (ZnCl2;

99.5%) from Alfa, tin(ൖ) chloride (SnCl2; 99.5%) from Mallinckrodt; ethanol absolute and hydrochloric acid were from PA Panreac; diethylenetriamine, oleylamine and ethylenediamine were from Osaka Japan. Preparation of the quaternary Cu2ZnSnSe4 Nanorods by solvothermal. In a typical experimental procedure, the product Cu2ZnSnSe4 can be prepared from a stoichiometric mixture of Copper (ൖ) Chloride ( 0.002 mol), zinc(ൖ) chloride ( 0.001 mol), tin(ൖ) chloride ( 0.001 mol) and elemental selenium powder ( 0.004 mol) with magnetically stirred in a nitrogen-filled glovebox. The reagents were loaded into a 50 mL Teflon lined autoclave, which was then filled with anhydrous diethylenetriamine up to 80% of the total volume. The autoclave was sealed and was maintained at 180 oC for 35 h, and then allowed to cool to room temperature naturally. The black precipitate was filtered and washed with dilute hydrochloric acid, distilled water and absolute ethanol several times to remove the by-products. Finally, the product was dried under vacuum at 60 oC several hours and 4

collected for its characterization. Structural, Optical, and Electrical Characterization of the Cu2ZnSnSe4 Nanorods. The as-prepared samples were characterized by X-ray diffraction (XRD), and transmission electron microscopy (TEM), respectively. XRD was carried out on a D/MAX-500 X-ray powder diffraction system with Cu K radiation (=1.5418 Å). A scanning rate of 0.02 s-1 was applied to record the patterns in the 2 Theta range of 15–85o. TEM characterization was conducted on a JEM-2000EX system using an acceleration voltage of 160 kV. Vis-NIR absorption spectra of the nanomaterials in toluene were recorded at room temperature using a PerkinElmer᧨Lambda 750 UV/vis/near-IR spectrophotometer. Current-voltage characteristics were acquired using an Agilent 4155C semiconductor parameter analyzer, its scanning voltage tuned from 0 to 0.7 V. X-ray photoelectron spectra (XPS) were recorded on a VG Microtech Multilab ESCA 3000 spectrometer with a non-monochromatized Mg Ka X-rays as the excitation source. .

Results and Discussion Crystalline Structure and Morphology of the Quaternary Cu2ZnSnSe4 Nanorods XRD patterns of the samples prepared in diethylenetriamine are shown in Fig. 1. The average crystal domain size of the nanocrystals calculated using Scherrer’s equation based on the (112) peak is 90 nm (D = K/ ( cos ); K = 0.89,  =0.15418 nm, = FWHM,  = diffraction angle). The major diffraction peaks observed at 17.24, 27.13, 46.06, 53.4, 65.54, 72.47, and 83.12 °2 can be indexed to the (101), (112), (204), (312), (400), (316), and (424) of the unique stannite crystal structure (JCPDS 5

52-0868). According to the Vegard’s law, the lattice constants measured for the samples were a = 5.682 ± 0.002 Å and c = 11.354 ± 0.002 Å, with the c/2a ratio of 1.0008±0.001. The observed lattice parameters and c/2a ratio agree very well with the reported values for the stannite phase [29-31]. Further structural information of the sample on macroscopic scale is obtained from Raman spectrum (Figure 2). Figure 2 shown a strong peak at 196 cm-1 and a weak peak at 231 cm-1 and agreeing well with those of bulk CZTSe [32]. On the other hand, the characteristic Raman peak of zinc selenium ZnSe at 255 cm-1 is absent in the spectrum [33], suggesting that the XRD diffractions should not result from such a phase. Figure 3 shows the TEM image of the stannite quaternary Cu2ZnSnSe4 nanorods prepared in diethylenetriamine for 35 h. The sample of Cu2ZnSnSe4 nanorods synthesized in diethylenetriamine appears to display rod-like morphology with widths of 90±5 nm, which is in agreement with the XRD result of 90 nm calculated from the Scherrer formula. The image is taken along the [ 110] zone axis with the (2 2 0), (0 1 2), and (0 2 4) crystallographic planes indicated by white lines. Spacing measurements based on 10 planes indicated d-spacings of 2.14 ± 0.02 Å for the (2 2 0) and 2.08 ± 0.02 Å for the (0 12) and (0 2 4). The measured angle between the (2 2 0) and (0 2 4) planes is 58.2 ± 0.2° and is 66.2 ± 0.2° between (0 12) and (0 2 4) planes. If the structure were sphalerite, the measured angles should be 60° between all three planes, and the measured d-spacings should be 2.02Å. The zone axis of the SAED pattern was determined to be [ 1 10], and the pattern is typical for nanocrystals from this same synthesis procedure. We also observed additional diffraction spots, circled in white in Figure 3b. These extra spots correspond to 2/3 the distance of the ( 11 4) or (21 2 ) fundamental reflections 6

and have a d-spacing of 3.2 Å. Both high-resolution TEM (Figure 3c) and XRD (Figure 1) confirmed that the nanocrystals are crystalline with stannite quaternary structure. To the best of the authors’ knowledge, this is the reported solvothermal synthesis of stannite quaternary Cu2ZnSnSe4 nanorods. In this work, the solvent plays an important role in the formation of the quaternary Cu2ZnSnSe4 nanorods. Diethylenetriamine was selected as the solvent due to its strong basic capacity and strong chelation [34]. In the solvothermal process, a nucleophilic attack by diethylenetriamine can activate zinc(ൖ) chloride, tin(ൖ) chloride and elemental selenium to form Zn2+, Sn2+ and Se2- ions. Furthermore, in this electron transfer reaction, diethylenetriamine, as a reducing solvent, can reduce Cu2᧧ to Cu᧧ ion, then Cu᧧ complexes with diethylenetriamine to form [Cu (DIEN)2]+. In addition, diethylenetriamine plays an important role in controlling the nucleation and growth of the product. Because of its N-chelation and special structure, it can easily chelate Cu+ (reduced from Cu2+) and form a relatively stable complex [Cu (DIEN) 2] +. The formation of the complex can effectively prevent the formation of binary copper chalcogenides. To improve our understanding of the above mechanism, experiments were performed under similar conditions when diethylenetriamine was replaced by other solvents such as ethylenediamine and oleylamine. In oleylamine, many impurity diffraction peaks appeared in the XRD patterns which could be indexed to copper selenide, and zinc selenide. In ethylenediamine, a longer reaction time (at least 70 h) was required in order to obtain the product with almost the same crystalline as that in diethylenetriamine. The above results indicate that diethylenetriamine is the optimal solvent for this reaction. Uniform single-crystalline Cu2ZnSnSe4 nanorods were successfully obtained by solvothermal route 7

with diethylenetriamine at 180°C for 35 h. The detailed formation process of Cu2ZnSnSe4 nanostructures with various solvent was studied (Fig. 4a-4c) and a clear solvent-dependent morphology evolution pathway from particle to rod-like shapes could be observed. Large-scale Cu2ZnSnSe4 contained particle were obtained with ethylenediamine as solvent (Fig. 4a and 4b). The nanorods began to grow up out of the particle surfaces and became longer and longer, but in contrast, the large-scale particles gradually diminished. The products obtained with diethylenetriamine as solvent almost exhibited 1D nanostructure could be observed (Fig. 4c). The growth habits always directly determine the final crystal shape, which in turn is greatly influenced by its growth conditions. On the basis of the time-dependent morphology evolution evidence, we could hypothesize that formation of nanostructures and morphology evolution from particle to rod can be rationally expressed as a nucleation- dissolution- recrystallization mechanism [35, 36].The schematical mechanism for the semiconductor Cu2ZnSnSe4 nanorods obtained during different solvothermal stages is illustrated in Figure 5. The detailed formation process of Cu2ZnSnSe4 nanostructures with various reaction times was studied (Fig. 5a-5c). At the very beginning, when the redox reaction was carried out in the solvothermal system at 180oC for 15 h, the Cu2ZnSnSe4 nanorods with irregular shapes were formed in the solution through a homogeneous nucleation process. During the solvothermal process, large quantity of Cu2ZnSnSe4 nuclei were first formed due to the decomposition of a part of ZnSe, SnSe and CuSe precipitates, as shown in Figure 5. On the other hand, certain ZnSe, SnSe and CuSe precipitates transformed into the growth units of (ZnSe)-, (SnSe)- and [Cu (DIEN)2]+ under alkaline condition. As a consequence, the rod-like Cu2ZnSnSe4 crystals were 8

formed. In our experiment, when using diethylenetriamine as the capping molecules, most of (ZnSe)-, (SnSe)- and [Cu (DIEN)2]+ ions were precipitated into Cu2ZnSnSe4. Under solvothermal conditions, the obtained Cu2ZnSnSe4 nanoparticles could then agglomerate and self assemble to give the well-defined nanorods. So, the dissolved Cu2ZnSnSe4 in the solution might nucleate onto the active sites of the small protuberances, grow along 1D direction and recrystallize into Cu2ZnSnSe4 nanorods until the Cu2ZnSnSe4 particles almost completely dissolved. The recrystallization of Cu2ZnSnSe4 nanostructure as nanorod might be due to system alkalinity and the inter-reaction between certain crystal planes and alkaline ions. Alkaline ions have been proved to have structural-direction effect in other nanostructure formation [37]. In Figure 6, high-resolution XPS data were introduced. In Figure 6(a), the single Cu 2p centered on 2p 3/2 (932.37 eV) and 2p 1/2 (952.53 eV) was resulted. In Figure 6(b), the Zn core-level spectrum was presented with the binding energies of 1022.8 and 1045.8 eV corresponding to 2p 1/2 and 2p 3/2, which are very similar to the previous report [31]. In Figure 6(c), the Sn 3d core-level spectrum was presented at the binding energy of 486.3 eV and 494.7 eV corresponding to 3d

3/2

and 3d

5/2

[31]. In

Figure 6(d), the Se 3d core-level spectrum was presented with an optimal curve fitting for the distinct peak at 2p

2/3 (54.81

eV). The samples are exposed to air, therefore, C, O-containing species on the

surface is possible detected by XPS [38]. To reduce this effect, the samples were purposely cleaned by Ar plasma for 2 min prior to the measurements. The quantitative information from XPS survey was to calculate the film composition using the CasaXPS software. By analyzing XPS data, the composition of the thin film was regarded as Cu2ZnSnSe4 by taking Se as the reference. Relative atomic 9

concentrations of these Cu2ZnSnSe4 samples within the probing depth (5 nm for the taking off angle of 53o) of XPS were summarized in Table 1. The average composition of the nanocrystals in the sample has a relative atomic concentrations Cu/Zn/Sn/Se ratio close to 2:1:1:4 with a variation from particle to particle less than the experimental error of ca. ± 4.2 atom %. All the four elements, i.e., copper, zinc, tin, and selenium were distributed over the surface and through the bulk of the film. To the best of the authors’ knowledge, this is the reported solvothermal synthesis of the stannite quaternary Cu2ZnSnSe4 nanorods. Optical and Electrical Characterization of the quaternary Cu2ZnSnSe4 nanorods Figure 7 shows UV/vis absorption spectra and optical micrographs of the quaternary Cu2ZnSnSe4 nanorods synthesized by a solvothermal process at 180 oC for 35 h. With increasing the reaction time (at 180 oC for 15, 25 and 35 h, respectively.), the nanocrystals dispersed in toluene change their color from light to dark. The band gap energy determined from absorbance spectra (Figure 7) of optically clear dispersions of nanocrystals was found to be 1.49 eV (832 nm), which is in good agreement with the reported in literature [39]. The Cu2ZnSnSe4 absorber layer may be formed simply by drop-casting on molybdenum coated soda-lime glass substrate. The films are annealed under flow of Ar at 500 °C for 3 h to remove the organic capping molecules. We fabricated the batch of thin film solar cells in our laboratory using these films following chemical bath deposition of CdS layer, and RF sputtering of 60 nm intrinsic zinc oxide and 200 nm of ITO layers. The corresponding I-V characteristic and device structure of the solar cell are shown in Figure 8. The final devices are scribed into small areas of 0.12 cm2 with a small dap 10

of silver paint applied to form the top contact. The device parameters for a single junction Cu2ZnSnSe4 solar cell under AM1.5G are as follows: open circuit voltage of 308 mV, short-circuit current of 29.9 mA/cm2, fill factor of 27 %, and a power conversion efficiency of 2.48 %. Device efficiencies might be improved by increasing the Cu2ZnSnSe4 film thickness to absorb more photons. However, lower fill factor due to high series resistance causes lower performance of the present device. Overall, the fill factor of the Cu2ZnSnSe4 devices are lower than those typically reported for high-efficiency CIGSe solar cells, partly due to the large amount of dark areas from the silver contacts and shadow from the probe [38, 40].

Conclusions In summary, we have successfully synthesized stannite quaternary Cu2ZnSnSe4 nanorods via a relatively mild and convenient solvothermal process. The solvent plays an important role in the formation of the product and diethylenetriamine is the optimal solvent for this reaction. The size of the resulting nanorods could be controlled under the temperature at 180 oC for 35 h. The Cu2ZnSnSe4 nanorods with diameters in the range of 90-95 nm were obtained. Active area cell efficiencies up to 2.48 % (Voc= 308 mV, Jsc= 29.9 mA/cm2, FF= 0.27) have been demonstrated using the I2-II-IV-VI4 based absorber layer fabricated. However, we believe that synthesize process played a very important role in the result, because only solvothermal with diethylenetriamine as a solvent was found to be effective to form chalcopyrite nanocrystals. The optical band gap and lattice parameters of the alloyed nanocrystals are composition-dependent, following the equation and Vegard’s law. These nanocrystals have good solubility in common solvents and may be potentially used as absorber and window 11

materials in multijunction photovoltaic cells and other applications.

Acknowledgments Present study has been supported in by Top 100 University Advancement and the collaboration of Center for Micro/Nano Science and Technology of National Cheng Kung University.

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Table 1 Measured CZTSe Nanocrystal Composition after annealing Precursor atomic ratio

Measured by EDSa

Measured by XPSb

Cu:Zn:Sn:Se

Cu:Zn:Sn:Se

Cu:Zn:Sn:Se

2:1:1:4

2:0.94:0.89:3.82

2:0.88:0.92:3.92

2:1.5:0.5:4

2:1.32:0.43:3.93

2:1.46:0.47:3.85

2:0.5:1.5:4

2:0.46:1.42:3.74

2:0.41:1.42:3.94

a

EDS measurements have an error of ca. ± 2 atom %. XPS measurement have an error of ± 0.2 atom

% for Zn, ± 0.1 atom % for Sn, ± 0.5 atom % for Se.

15

Figure captions FIGURE 1. XRD patterns of quaternary Cu2ZnSnSe4 nanorods synthesized by the solvothermal process for 15, 25 and 35 h, respectively. FIGURE 2. Raman spectrum of the Cu2ZnSnSe4 nanorods by the solvothermal process. FIGURE 3. TEM images of (a) as-synthesized Cu2ZnSnSe4 nanorods and (b) SAED pattern and (c) high-resolution TEM of Cu2ZnSnSe4 nanostructures recorded along [ 110]. FIGURE 4. SEM images of the Cu2ZnSnSe4 by solvothermal treated at (a) 180°C for 25 h with ethylenediamine, (b) 180°C for 35 h with ethylenediamine, and (c) 180°C for 35 h with diethylenetriamine, respectively. FIGURE 5. Growth schematic diagrams of the Cu2ZnSnSe4 nanorods prepared by solvothermal process. Inset of SEM images of the Cu2ZnSnSe4 by solvothermal treated at 180°C with diethylenetriamine for (a) 15 h, (b) 25 h, and (c) 35 h, respectively. FIGURE 6. Survey X-ray photoelectron spectra of Cu 2p, Zn 2p, Sn 3d, Se 3d regions for the as-deposited Cu2ZnSnSe4 film. FIGURE 7. UV-vis absorption spectrum of the as-synthesized Cu2ZnSnSe4 nanorods. The band gap of the nanocrystals is approximated using the direct band gap method by plotting the absorbance squared versus energy, and extrapolating to zero as shown in the inset. FIGURE 8. Device structure and electrical characteristics. (a) Schematic of device structure, with approximate layer thicknesses indicated. (b) Current-voltage characteristics by the AM 1.5 irradiation (100 mW/cm2). Table 1 Measured CZTSe Nanocrystal Composition after annealing

Highlights

z

Semiconductor nanorods were successfully prepared via a relatively simple and convenient solvothermal route.

z z z

The solvothermal technique may significantly lower the cost of the materials. CZTS nanoparticles seem to be a promising candidate as the light absorber in solar cells. Solvothermal process are carried out at low temperatures and do not require toxic precursors.

16

Figure(s)

Figures

(424)

(c)

(316)

(400)

(312)

(204) (101)

Relative Intensity (a.u.)

(112)

15 h 25 h 35 h

(b) (a) 20

30

40

50

60

70

80

2 theta (degree)

FIGURE 1. XRD patterns of quaternary Cu2ZnSnSe4 nanorods synthesized by the solvothermal process for 15, 25 and 35 h, respectively.

Intensity (a.u.)

198

231

150

200

250

300

350

400

-1

Raman shift (cm )

FIGURE 2. Raman spectrum of the Cu2ZnSnSe4 nanorods by the solvothermal process.

FIGURE 3. TEM images of (a) as-synthesized Cu2ZnSnSe4 nanorods and (b) SAED pattern and (c) high-resolution TEM of Cu2ZnSnSe4 nanostructures recorded along [ 1 10].

FIGURE 4. SEM images of the Cu2ZnSnSe4 by solvothermal treated at (a) 180°C for 25 h with ethylenediamine, (b) 180°C for 35 h with ethylenediamine, and (c) 180°C for 35 h with diethylenetriamine, respectively.

FIGURE 5. Growth schematic diagrams of the Cu2ZnSnSe4 nanorods prepared by solvothermal process. Inset of SEM images of the Cu2ZnSnSe4 by solvothermal treated at 180°C with diethylenetriamine for (a) 15 h, (b) 25 h, and (c) 35 h, respectively.

FIGURE 6. Survey X-ray photoelectron spectra of Cu 2p, Zn 2p, Sn 3d, Se 3d regions for the as-deposited Cu2ZnSnSe4 film.

-1 2

Abs  eVcm

2

Absorbance (a.u.)

1.49 eV 1.3

1.4

1.5

1.6

1.7

Photon Energy (eV)

400

500

600

700

800

900

1000

Wavelength (nm)

FIGURE 7. UV-vis absorption spectrum of the as-synthesized Cu2ZnSnSe4 nanorods. The band gap of the nanocrystals is approximated using the direct band gap method by plotting the absorbance squared versus energy, and extrapolating to zero as shown in the inset.

120

VOC= 308 mV

dark light

2

JSC= 29.9 mA cm

FF= 0.27 Efficiency = 2.48 %

80

2

Current Density  mA cm

100

60 40 20 0 -20 -40 -0.2

0.0

0.2

0.4

0.6

Voltage (V)

FIGURE 8. Device structure and electrical characteristics. (a) Schematic of device structure, with approximate layer thicknesses indicated. (b) Current-voltage characteristics by the AM 1.5 irradiation (100 mW/cm2).