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Accepted Manuscript This article can be cited before page numbers have been issued, to do this please use: N. S. Arul, D. Y. Yun, D. U. Lee and T. W. Kim, Nanoscale, 2013, DOI: 10.1039/C3NR03892K.
Volume 2 | Number 1 | 2010
This is an Accepted Manuscript, which has been through the RSC Publishing peer review process and has been accepted for publication.
www.rsc.org/nanoscale
Volume 2 | Number 1 | January 2010 | Pages 1–156
Nanoscale
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ISSN 2040-3364
Pages 1–156
COVER ARTICLE Graham et al. Mixed metal nanoparticle assembly and the effect on surface-enhanced Raman scattering
REVIEW Lin et al. Progress of nanocrystalline growth kinetics based on oriented attachment
2040-3364(2010)2:1;1-T
Please note that technical editing may introduce minor changes to the text and/or graphics contained in the manuscript submitted by the author(s) which may alter content, and that the standard Terms & Conditions and the ethical guidelines that apply to the journal are still applicable. In no event shall the RSC be held responsible for any errors or omissions in these Accepted Manuscript manuscripts or any consequences arising from the use of any information contained in them.
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DOI: 10.1039/C3NR03892K
Strong quantum confinement effects in kesterite Cu2ZnSnS4 nanospheres for
Narayanasamy Sabari Arul, Dong Yeol Yun, Dea Uk Lee, and Tae Whan Kim∗
Department of Electronics and Computer Engineering, Hanyang University, Seoul 133-791, Korea
Abstract X-ray photoelectron spectroscopy spectra, X-ray diffraction patterns, scanning electron microscopy images, and high-resolution transmission electron microscopy images showed that the as-prepared samples were Cu2ZnSnS4 (CZTS) nanospheres with a kesterite phase. Ultraviolet-visible absorption spectra for the CZTS nanospheres with an average crystallite size of 3.26 nm showed that the absorption edge corresponding to the energy gap shifted to the higher energy side due to the quantum confinement within the CZTS nanoparticles. Current-density measurements showed that the power conversion efficiency (0.952%) of the organic photovoltaic cells with CZTS nanospheres was much higher than that (0.120%) of the cells without CZTS nanospheres.
Keywords: CZTS, solvothermal synthesis, structural property, solar energy material, quantum confinement.
∗
Corresponding author’s e-mail: [email protected]
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organic photovoltaics cells
Nanoscale
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DOI: 10.1039/C3NR03892K
Rapid advancements in synthesis technologies of the high-quality quaternary semiconductors have made possible the fabrication of photovoltaic (PV) devices with high
attracted as a great deal of interest because of low-cost alternatives to conventional absorber materials in PV devices [3]. Among the various kinds of copper-based quaternary chalcogenides, Cu2ZnSnS4 (CZTS) semiconductors have attracted as potential photon absorbing layers for nextgeneration thin film PV devices due to their excellent advantages of near optimal direct band gap of approximately 1.5 eV, a large optical absorption coefficient of 104 cm-1 in a visible spectrum range, composed materials of nontoxic and earth abundance elements [4-6]. CZTS semiconductors have existed in two crystallographic structures as stannite and kesterite phases with a tetragonal crystal cell. However, CZTS materials have appeared as a kesterite phase because of thermodynamically more stable than the stannite type [7]. Some studies on the structural and optical properties of CZTS nanocrystals with a kesterite or a stannite phase have been performed to improve the PV efficiencies [8-11]. Quantum dots (QDs) provide promising opportunities for applications in high-efficiency third-generation PV devices because of their tunable band gap, efficient optical absorption, and multiple exciton generation [12, 13]. Furthermore, Shockley-Queisser photon balance calculations hav3e shown that the theoretical single junction power-conversion limit of CZTS materials is 32.2%, which exceeds through the multiple exciton generation or hot-electron extraction [14, 15]. Recently, solution processed CZTS absorber layers have improved the power conversion efficiencies (PCE) up to 10.1%, which is much higher than those obtained by vacuum-deposition techniques [16]. A solution processed technique is cost effective and controlled morphology with a tunable band gap for the fabrication of PV films than the traditional physical methods,
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characteristic performance [1, 2]. Copper-based quaternary chalcogenides have particularly
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benefiting from the progress in synthesizing high quality semiconductor nanocrystals [17]. Even though some investigations concerning the formation and physical properties of the CZTS thin
CZTS nanospheres for organic photovoltaic (OPV) cells [18] have not been conducted yet. This letter reports the data for strong quantum confinement effects in kesterite Cu2ZnSnS4 synthesized by using a solvothermal method for OPV cells. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) measurements were performed to investigate the structural properties and stoichiometries of the CZTS nanospheres. Ultraviolet-visible (UV-vis) and current-voltage (I-V) measurements were carried out in order to investigate the optical and photovoltaic properties of the CZTS nanospheres. All the chemicals used in this work are of analytical grade and used without further purification. In a typical synthesis, 2 mmol copper (II) chloride dehydrate (CuCl2·2H2O), 1 mmol zinc (II) chloride (ZnCl2), 1 mmol tin (IV) chloride tetrahydrate (SnCl4·4H2O), and 8 mmol of thiourea SC(NH2)2 were dissolved in 40 ml ethylene glycol under magnetic stirring. Then, after the mixtures were loaded into a teflon-lined stainless-steel autoclave of 50 ml capacity, they were maintained at 180oC for 6 h and subsequently were cooled at room temperature. The precipitate was centrifuged and washed with de-ionized water and ethanol several times to remove by-products. Finally, the CZTS nanoparticles were dried in a vacuum oven at 80oC for 4 h. XRD patterns were obtained by using a Rigaku D/MAX-2500 diffraction meter with Cu Kα radiation, which was operated at a scanning rate of 5o/min for a 2θ range between 20o and 80o. Field emission SEM (FESEM) images were measured by using a Nova Nano SEM 200 and 3
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films have been performed [8-11], studies about strong quantum confinement effects in kesterite
Nanoscale
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an FEI system operating at 30 kV. High-resolution TEM (HRTEM) and selected area diffraction pattern measurements were performed by using a JEM 2100F system operating at 200 kV. The absorption
spectra
were
measured
by
using
a
SHIMADZU
UV-2401PC
spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements were performed by using a Thermo Electron (U.K.) model equipped with an Mg Kα X-ray as an excitation source. Figure 1 shows the morphology and microstructural properties of the CZTS nanoparticles. Figure 1(a) shows the SEM image of the CZTS nanoparticles. Figures 1(b), 1(c), and 1(d) show the HRTEM images of the CZTS nanospheres composed of nanoparticles as well as triangle geometry. HRTEM images shown in Fig. 1(d) revealed that CZTS nanospheres assigned to the (112) lattice plane of kesterite phase [19]. The selected area diffraction pattern of CZTS nanoparticles confirmed the presence of the kesterite phase, as shown in Fig. 1(e). Figure 1(f) shows the XRD pattern of the CZTS nanospheres with three broad peaks at 28.5o, 47.6o and 56.3o corresponding to the diffraction peak of (112), (220), and (312) planes of the tetragonal CZTS crystal with JCPDS, Card No. 26-0575. The average crystallite size of the CZTS nanoparticles in the nanospheres, as determined from the most prominent (112) reflections by using a Scherrer formula, was 3.26 ± 0.2 nm (Fig. 1(f)). High-resolution XPS spectra of Zn 2p, Cu 2p, Sn 3d and S 2p were measured to determine the stoichiometries and the oxidation states of the elements in the formed samples, as shown in Fig. 2. The peak of Cu 2p split into 931. 6 (2p3/2) and 950.9 eV (2p1/2) with a splitting energy of 19.3 eV was assigned to the value of Cu(I). The peak of Zn 2p appeared at binding energies of 1021.5 (2p3/2) and 1044.3 eV (2p1/2) with a splitting energy of 22.8 eV, which was consistent with the value of Zn(II). The peak of Sn 3d split into 486.2 (2p5/2) and 494.6 eV (2p3/2) with a splitting energy of 8.4 eV was attributed to Sn(IV). The peak of S 2p exhibited at 161. 2
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optical
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(2p3/2) and 163.1 eV (2p3/2) with an energy difference of 1.9 eV was consistent with the S range between 160 and 164 eV in the sulfide phase. XPS results indicate that the formed samples are
The optical absorption spectra of CZTS nanospheres dispersed in water are shown in Fig. 3. The optical band gap of the CZTS nanospheres was calculated by using the formula [6]:
(α hν ) n = A( h ν − E g ) ,
(1)
where α is the absorption coefficient, A is a constant, n (n = 2 for the direct band gap) is the characteristic number of the transition process, Eg is the band gap of the material, ν is the frequency of the incident radiation, and h is the Planck’s constant. The inset of Fig. 3 discloses a (αhν)2 versus hν plot for determining the optical band gap of CZTS nanospheres by using Eq. (1). The extrapolation of the linear part shows a direct band gap of 1.84 eV, which is larger than the band gap of the bulk CZTS (1.4 - 1.5 eV) [21, 22]. The band gap of the CZTS nanospheres with average crystallite size of 3.26 nm shifts to the higher energy of 1.84 eV. The strong blue shift of the optical band gap is attributed to the presence of quantum confinement within the CZTS nanocrystals [23]. Furthermore, the exciton peak of the nanocrystals with a short-wavelength infrared band-gap shift toward a blue wavelength side due to an optical quantum confinement effect by ensuring that the dimension of the nanocrystals is smaller than the order of the Bohr exciton radius characteristics of their constituent semiconductor material [24]. The exciton Bohr radius for the CZTS nanoparticles can be calculated by using the following equation [25];
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CZTS nanospheres, which is in reasonable agreement with the reported literatures [20].
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E g ( nano ) = E g ( bulk ) +
h 2π 2 1 1 1 .8e 2 , [ + ]− 2 2 R me m h εR
(2)
(1.45 eV) of the bulk CZTS, ħ is the Planck constant, R is the nanoparticle radius, µ = [
1 1 + ] me mh
is the reduced mass, where me and mh are the effective masses of electron (0.18 mo) in a conduction band and holes (0.71 mo) in a valance band, respectively, 1.8 is a constant value, e is the electron charge, and ε (6.7) is the high frequency dielectric constant [26]. The estimated exciton Bohr radius of synthesized CZTS nanoparticles is between 2.5 and 3.4 nm [27], indicative of strong quantum confinement effects, which is in reasonable agreement with experimental results. The OPV cells used in this study were fabricated on indium-tin-oxide (ITO)-coated glass substrates, and the measured sheet resistance of the ITO thin film was approximately 10 Ω/sq. Initially, the ITO substrates were cleaned in acetone and isopropanol by using an ultrasonic cleaner with sonication amplitude of 135 W and a frequency of 42 kHz for 10 min and the substrates were introduced into a glove box with a high-purity N2 atmosphere. The poly(3,4ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) solution was spin-coated onto the ITO-coated glass substrates by spin coating at 4500 rpm for 41 s in the glove box. Then, the photoactive layer comprised regioregular poly (3-hexylthiophene) (P3HT) and 1-(3methoxycarbonyl1) propyl-1-pheny [6, 6] methanofullerene (PCBM) with a weight ratio of 1:1 was mixed into 1, 2-dichlorobenzene solution. The mixed P3HT:PCBM layer was spin coated on the PEDOT:PSS at a spin rate of 2000 rpm 60 s. Successively, the synthesized CZTS nanospheres were mixed in ethanol to form a colloidal solution. The colloidal CZTS solution
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where Eg(nano) is the optical band gap (1.84 eV) of CZTS nanoparticles, Eg(bulk) is the band gap
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was spin-coated onto the P3HT:PCBM layer at 2000 rpm for 30 s and was annealed at 145oC for 10 min. The OPV devices were completed by deposition of a cathode buffer layer LiF and Al
× 2 mm. Current-density (J-V) measurements were measured in the dark and under illumination by using a Keithley-2400 source meter. The photovoltaic characteristics were measured by using a Xenon lamp under AM 1.5 stimulated illumination at an intensity of 100 mW/cm2. All measurements were carried out at room temperature (27oC) under ambient air with relative humidity of 60%-70%. Figure 4(a) displays the energy diagram of fabricated OPV cells with CZTS nanospheres. Figure 4(b) shows the J-V characteristics of the OPV cells in dark, cells with and without CZTS nanoparticles. The inset of Fig. 4(b) depicts the schematic illustration of the device structure, ITO/PEDOT:PSS/P3HT:PCBM-CZTS/LiF/Al. The open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and power conversion efficiency (PCE) of the fabricated OPV cells with and without CZTS nanospheres are summarized in Table.1. The device prepared without CZTS nanospheres exhibited an Voc of 0.208 V, a Jsc of 2.74 mA/cm2, and a FF of 0.17, resulting in a PCE of 0.120%. After the addition of CZTS nanospheres into the photoactive layer exhibited a Voc of 0.52 V, a Jsc of 4.90 mA/cm2, and a FF of 0.36, resulting in the PCE of 0.952%. Currentdensity measurements showed that the OPV cells with CZTS nanospheres exhibited higher PCE than those of cells without CZTS nanospheres due to their large optical absorption from visible to near IR region (Fig. 3) [2]. Our results showed that the PCE of OPV cells with CZTS nanospheres is quite low, but it is noteworthy to mention that our study is an initial effort towards demonstrating the potential of nontoxic CZTS as a light-absorbing material for the next generation PV cells.
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front contacts with thicknesses of 10 and 100 nm; the active area of the fabricated cell was 2 mm
Nanoscale
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In summary, Kesterite CZTS nanospheres were synthesized by using a solvothermal method. The structural and optical properties confirmed the phase purity of the as-prepared
average crystallite size of 3.26 nm exhibited that the energy gap of the CZTS nanospheres shifted towards a higher energy with a band gap of 1.84 eV. The estimated exciton Bohr radius suggested that the CZTS nanoparticles displayed a strong quantum confinement effects. Currentdensity measurements showed that the PCE (0.952%) of the OPV cells employing CZTS nanospheres was much higher than that (0.120%) of the cells without CZTS nanospheres. These results indicate that the CZTS nanospheres formed by using a solvothermal method hold promise for potential applications in third generation PV devices with high photovoltaic performance due to their excellent optoelectronic properties.
Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013-016467).
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kesterite CZTS nanospheres. UV-vis absorption spectra of the CZTS nanospheres with an
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References [1] J.Y. Chane- Ching, A. Gillorin, O. Zaberca, A. Balocchi, and X. Marie, Chem. Commun.,
[2] J. Wang, X. Xin, and Z. Lin, Nanoscale, 2011, 3, 3040-3048. [3] D. B. Mitzi, O. Gunawan, T. K. Todorov, K. Wang, and S. Guha, Sol. Energy Mater. Sol. Cells, 2011, 95, 1421-1440. [4] M. Graetzel, R. A. J. Janssen, D. B. Mitzi, and E. H. Sargent, Nature, 2012, 488, 304-312. [5] A. S. R. Chesman, N. W. Duffy, S. P. L. Waddington, N. A. S. Webster, and J. J. Jasieniak, RSC Adv., 2013, 3, 1017-1020. [6] M. Wei, Q. Du, D. Wang, W. Liu, G. Jiang, and C. Zhu, Mater. Lett., 2012, 79, 177-179. [7] D. S. Su and S-H. Wei, Appl. Phys. Lett. 1999, 74, 2483-2485. [8] Q. Guo, H. W. Hillhouse, and R. Agrawal, J. Am. Chem. Soc., 2009, 131, 11672-11673. [9] S. C. Riha, B. A. Parkinson, and A. L. Prieto, J. Am. Chem. Soc., 2009, 131,12054-12055. [10] A. Shavel, D. Cadavid, M. Ibanez, A. Carrete, and A. Cabot, J. Am. Chem. Soc., 2012, 134, 1438-1441. [11] B. Shin, O. Gunawan, Y. Shu, N. A. Bojarczuk, S. J. Chey, and S. Guha, Prog. Photovolt. Res. Appl., 2013, 21, 72-76. [12] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, and Y. Yang, Nat. Mater., 2005, 4, 864-868. [13] C. Steinhagen, M. G. Panthani, V. Akhavan, B. Goodfellow, B. Koo, and B. A. Korgel, J. Am. Chem. Soc., 2009, 131, 12554-12555. [14] O. E. Semonin, J. M. Luther, S. Choi, H. Y. Chen, J. B. Gao, A. J. Nozik, and M. C. Beard, Science, 2011, 334, 1530-1533.
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2011, 47, 5229-5231.
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[15] J. B. Sambur, T. Novet, and B. A. Parkinson, Science, 2010, 330, 63-66. [16] W. A. Tisdale, K. J. Williams, B. A. Timp, D. J. Norris, E. S. Aydil, and X. Y. Zhu, Science,
[17] D. A. R. Barkhouse, O. Gunawan, T. Gokmen, T. K.Todorov, and D. B. Mitzi, Prog. Photovolt. Res. Appl., 2012, 20, 6-11. [18] K. Wang, O. Gunawan, T. Todorov, B. Shin, S. J. Chey, N. A. Bojarczuk, D. Mitzi, and S. Guha, Appl. Phys. Lett., 2010, 97, 143508-143510. [19] C. Zou, L. Zhang, D. Lin Y. Yang, Q. Li, X. Xu, Chen, and S. Huang, Cryst. Eng. Comm., 2011, 13, 3310-3313. [20] H. C. Jiang, P. C. Dai, Z. Y. Feng, W. L. Fan, and J. H. Zhan, J. Mater. Chem., 2012, 22, 7502-7506. [21] A. Singh, H. Geaney, F. Laffir, and K. M. Ryan, J. Am. Chem. Soc., 2012, 134, 2910-2913. [22] H. Yang, L. A. Jauregui, G. Zhang, Y. P. Chen, and Y. Wu, Nano Lett., 2012, 12, 540-545. [23] A. M. Smith and S. Nie, Acc. Chem. Res., 2009, 43, 190-200. [24] W. C. Liu, B. L. Guo, X. S. F. M. Zhang, C. L. Mak, and K. H. Wong, J. Mater. Chem. A, 2013, 1, 3182-3186. [25] F. Zhang, Q. Jin, and S.W. Chan, J. Appl. Phys., 2004, 95, 4319-4326. [26] C. Persson, J. Appl. Phys., 2010, 107, 053710-1 - 053710-7. [27] A. Khare, A. W. Wills, L. M. Annerman, D. J. Norris, and E. S. Aydil, Chem. Commun., 2011, 47, 11721-11723.
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2010, 328, 1543-1547.
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Table 1. Device performances parameters of OPV cells with and without CZTS nanospheres. Jsc (mA/cm2)
Voc (V)
FF
ߟ (%)
ITO/PEDOT:PSS/P3HT:PCBM-CZTS/LiF/Al
4.9062
0.52042
0.3689
0.9520
ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al
2.7428
0.20811
0.17848
0.120
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Device Structure
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Figure Captions Fig. 1. (a) Scanning electron microscopy image, (b, c, d) high-resolution transmission electron
prepared CZTS nanospheres.
Fig. 2. X-ray spectroscopy of the as-prepared CZTS nanospheres. Fig. 3. Ultraviolet-visible absorption spectra of the CZTS nanospheres. Inset represents the plot of (αhν)2 as a function of hν for estimating the optical band gap.
Fig. 4. (a) Schematic energy band diagram and (b) current-voltage curves of the OPV cells measured in dark, cells with and without CZTS layer. The inset of Fig. 4(b) displays the device structure of fabricated OPV cells with CZTS layer.
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microscopy images, (e) selected area diffraction pattern, and (f) XRD spectrum of the as-
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(a) Scanning electron microscopy image, (b, c, d) high-resolution transmission electron microscopy images, (e) selected area diffraction pattern, and (f) XRD spectrum of the as-prepared CZTS nanospheres. 126x184mm (300 x 300 DPI)
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X-ray spectroscopy of the as-prepared CZTS nanospheres. 69x56mm (300 x 300 DPI)
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Ultraviolet-visible absorption spectra of the CZTS nanospheres. Inset represents the plot of (αhν)2 as a function of hν for estimating the optical band gap. 68x54mm (300 x 300 DPI)
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(a) Schematic energy band diagram and (b) current-voltage curves of the OPV cells measured in dark, cells with and without CZTS layer. The inset of Fig. 4(b) displays the device structure of fabricated OPV cells with CZTS layer. 119x165mm (300 x 300 DPI)
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Accepted Manuscript This article can be cited before page numbers have been issued, to do this please use: N. S. Arul, D. Y. Yun, D. U. Lee and T. W. Kim, Nanoscale, 2013, DOI: 10.1039/C3NR03892K.
Volume 2 | Number 1 | 2010
This is an Accepted Manuscript, which has been through the RSC Publishing peer review process and has been accepted for publication.
www.rsc.org/nanoscale
Volume 2 | Number 1 | January 2010 | Pages 1–156
Nanoscale
Accepted Manuscripts are published online shortly after acceptance, which is prior to technical editing, formatting and proof reading. This free service from RSC Publishing allows authors to make their results available to the community, in citable form, before publication of the edited article. This Accepted Manuscript will be replaced by the edited and formatted Advance Article as soon as this is available. To cite this manuscript please use its permanent Digital Object Identifier (DOI®), which is identical for all formats of publication. More information about Accepted Manuscripts can be found in the Information for Authors.
ISSN 2040-3364
Pages 1–156
COVER ARTICLE Graham et al. Mixed metal nanoparticle assembly and the effect on surface-enhanced Raman scattering
REVIEW Lin et al. Progress of nanocrystalline growth kinetics based on oriented attachment
2040-3364(2010)2:1;1-T
Please note that technical editing may introduce minor changes to the text and/or graphics contained in the manuscript submitted by the author(s) which may alter content, and that the standard Terms & Conditions and the ethical guidelines that apply to the journal are still applicable. In no event shall the RSC be held responsible for any errors or omissions in these Accepted Manuscript manuscripts or any consequences arising from the use of any information contained in them.
www.rsc.org/nanoscale Registered Charity Number 207890
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DOI: 10.1039/C3NR03892K
Strong quantum confinement effects in kesterite Cu2ZnSnS4 nanospheres for
Narayanasamy Sabari Arul, Dong Yeol Yun, Dea Uk Lee, and Tae Whan Kim∗
Department of Electronics and Computer Engineering, Hanyang University, Seoul 133-791, Korea
Abstract X-ray photoelectron spectroscopy spectra, X-ray diffraction patterns, scanning electron microscopy images, and high-resolution transmission electron microscopy images showed that the as-prepared samples were Cu2ZnSnS4 (CZTS) nanospheres with a kesterite phase. Ultraviolet-visible absorption spectra for the CZTS nanospheres with an average crystallite size of 3.26 nm showed that the absorption edge corresponding to the energy gap shifted to the higher energy side due to the quantum confinement within the CZTS nanoparticles. Current-density measurements showed that the power conversion efficiency (0.952%) of the organic photovoltaic cells with CZTS nanospheres was much higher than that (0.120%) of the cells without CZTS nanospheres.
Keywords: CZTS, solvothermal synthesis, structural property, solar energy material, quantum confinement.
∗
Corresponding author’s e-mail: [email protected]
1
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organic photovoltaics cells
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Rapid advancements in synthesis technologies of the high-quality quaternary semiconductors have made possible the fabrication of photovoltaic (PV) devices with high
attracted as a great deal of interest because of low-cost alternatives to conventional absorber materials in PV devices [3]. Among the various kinds of copper-based quaternary chalcogenides, Cu2ZnSnS4 (CZTS) semiconductors have attracted as potential photon absorbing layers for nextgeneration thin film PV devices due to their excellent advantages of near optimal direct band gap of approximately 1.5 eV, a large optical absorption coefficient of 104 cm-1 in a visible spectrum range, composed materials of nontoxic and earth abundance elements [4-6]. CZTS semiconductors have existed in two crystallographic structures as stannite and kesterite phases with a tetragonal crystal cell. However, CZTS materials have appeared as a kesterite phase because of thermodynamically more stable than the stannite type [7]. Some studies on the structural and optical properties of CZTS nanocrystals with a kesterite or a stannite phase have been performed to improve the PV efficiencies [8-11]. Quantum dots (QDs) provide promising opportunities for applications in high-efficiency third-generation PV devices because of their tunable band gap, efficient optical absorption, and multiple exciton generation [12, 13]. Furthermore, Shockley-Queisser photon balance calculations hav3e shown that the theoretical single junction power-conversion limit of CZTS materials is 32.2%, which exceeds through the multiple exciton generation or hot-electron extraction [14, 15]. Recently, solution processed CZTS absorber layers have improved the power conversion efficiencies (PCE) up to 10.1%, which is much higher than those obtained by vacuum-deposition techniques [16]. A solution processed technique is cost effective and controlled morphology with a tunable band gap for the fabrication of PV films than the traditional physical methods,
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characteristic performance [1, 2]. Copper-based quaternary chalcogenides have particularly
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benefiting from the progress in synthesizing high quality semiconductor nanocrystals [17]. Even though some investigations concerning the formation and physical properties of the CZTS thin
CZTS nanospheres for organic photovoltaic (OPV) cells [18] have not been conducted yet. This letter reports the data for strong quantum confinement effects in kesterite Cu2ZnSnS4 synthesized by using a solvothermal method for OPV cells. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) measurements were performed to investigate the structural properties and stoichiometries of the CZTS nanospheres. Ultraviolet-visible (UV-vis) and current-voltage (I-V) measurements were carried out in order to investigate the optical and photovoltaic properties of the CZTS nanospheres. All the chemicals used in this work are of analytical grade and used without further purification. In a typical synthesis, 2 mmol copper (II) chloride dehydrate (CuCl2·2H2O), 1 mmol zinc (II) chloride (ZnCl2), 1 mmol tin (IV) chloride tetrahydrate (SnCl4·4H2O), and 8 mmol of thiourea SC(NH2)2 were dissolved in 40 ml ethylene glycol under magnetic stirring. Then, after the mixtures were loaded into a teflon-lined stainless-steel autoclave of 50 ml capacity, they were maintained at 180oC for 6 h and subsequently were cooled at room temperature. The precipitate was centrifuged and washed with de-ionized water and ethanol several times to remove by-products. Finally, the CZTS nanoparticles were dried in a vacuum oven at 80oC for 4 h. XRD patterns were obtained by using a Rigaku D/MAX-2500 diffraction meter with Cu Kα radiation, which was operated at a scanning rate of 5o/min for a 2θ range between 20o and 80o. Field emission SEM (FESEM) images were measured by using a Nova Nano SEM 200 and 3
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films have been performed [8-11], studies about strong quantum confinement effects in kesterite
Nanoscale
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an FEI system operating at 30 kV. High-resolution TEM (HRTEM) and selected area diffraction pattern measurements were performed by using a JEM 2100F system operating at 200 kV. The absorption
spectra
were
measured
by
using
a
SHIMADZU
UV-2401PC
spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements were performed by using a Thermo Electron (U.K.) model equipped with an Mg Kα X-ray as an excitation source. Figure 1 shows the morphology and microstructural properties of the CZTS nanoparticles. Figure 1(a) shows the SEM image of the CZTS nanoparticles. Figures 1(b), 1(c), and 1(d) show the HRTEM images of the CZTS nanospheres composed of nanoparticles as well as triangle geometry. HRTEM images shown in Fig. 1(d) revealed that CZTS nanospheres assigned to the (112) lattice plane of kesterite phase [19]. The selected area diffraction pattern of CZTS nanoparticles confirmed the presence of the kesterite phase, as shown in Fig. 1(e). Figure 1(f) shows the XRD pattern of the CZTS nanospheres with three broad peaks at 28.5o, 47.6o and 56.3o corresponding to the diffraction peak of (112), (220), and (312) planes of the tetragonal CZTS crystal with JCPDS, Card No. 26-0575. The average crystallite size of the CZTS nanoparticles in the nanospheres, as determined from the most prominent (112) reflections by using a Scherrer formula, was 3.26 ± 0.2 nm (Fig. 1(f)). High-resolution XPS spectra of Zn 2p, Cu 2p, Sn 3d and S 2p were measured to determine the stoichiometries and the oxidation states of the elements in the formed samples, as shown in Fig. 2. The peak of Cu 2p split into 931. 6 (2p3/2) and 950.9 eV (2p1/2) with a splitting energy of 19.3 eV was assigned to the value of Cu(I). The peak of Zn 2p appeared at binding energies of 1021.5 (2p3/2) and 1044.3 eV (2p1/2) with a splitting energy of 22.8 eV, which was consistent with the value of Zn(II). The peak of Sn 3d split into 486.2 (2p5/2) and 494.6 eV (2p3/2) with a splitting energy of 8.4 eV was attributed to Sn(IV). The peak of S 2p exhibited at 161. 2
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optical
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(2p3/2) and 163.1 eV (2p3/2) with an energy difference of 1.9 eV was consistent with the S range between 160 and 164 eV in the sulfide phase. XPS results indicate that the formed samples are
The optical absorption spectra of CZTS nanospheres dispersed in water are shown in Fig. 3. The optical band gap of the CZTS nanospheres was calculated by using the formula [6]:
(α hν ) n = A( h ν − E g ) ,
(1)
where α is the absorption coefficient, A is a constant, n (n = 2 for the direct band gap) is the characteristic number of the transition process, Eg is the band gap of the material, ν is the frequency of the incident radiation, and h is the Planck’s constant. The inset of Fig. 3 discloses a (αhν)2 versus hν plot for determining the optical band gap of CZTS nanospheres by using Eq. (1). The extrapolation of the linear part shows a direct band gap of 1.84 eV, which is larger than the band gap of the bulk CZTS (1.4 - 1.5 eV) [21, 22]. The band gap of the CZTS nanospheres with average crystallite size of 3.26 nm shifts to the higher energy of 1.84 eV. The strong blue shift of the optical band gap is attributed to the presence of quantum confinement within the CZTS nanocrystals [23]. Furthermore, the exciton peak of the nanocrystals with a short-wavelength infrared band-gap shift toward a blue wavelength side due to an optical quantum confinement effect by ensuring that the dimension of the nanocrystals is smaller than the order of the Bohr exciton radius characteristics of their constituent semiconductor material [24]. The exciton Bohr radius for the CZTS nanoparticles can be calculated by using the following equation [25];
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CZTS nanospheres, which is in reasonable agreement with the reported literatures [20].
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E g ( nano ) = E g ( bulk ) +
h 2π 2 1 1 1 .8e 2 , [ + ]− 2 2 R me m h εR
(2)
(1.45 eV) of the bulk CZTS, ħ is the Planck constant, R is the nanoparticle radius, µ = [
1 1 + ] me mh
is the reduced mass, where me and mh are the effective masses of electron (0.18 mo) in a conduction band and holes (0.71 mo) in a valance band, respectively, 1.8 is a constant value, e is the electron charge, and ε (6.7) is the high frequency dielectric constant [26]. The estimated exciton Bohr radius of synthesized CZTS nanoparticles is between 2.5 and 3.4 nm [27], indicative of strong quantum confinement effects, which is in reasonable agreement with experimental results. The OPV cells used in this study were fabricated on indium-tin-oxide (ITO)-coated glass substrates, and the measured sheet resistance of the ITO thin film was approximately 10 Ω/sq. Initially, the ITO substrates were cleaned in acetone and isopropanol by using an ultrasonic cleaner with sonication amplitude of 135 W and a frequency of 42 kHz for 10 min and the substrates were introduced into a glove box with a high-purity N2 atmosphere. The poly(3,4ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) solution was spin-coated onto the ITO-coated glass substrates by spin coating at 4500 rpm for 41 s in the glove box. Then, the photoactive layer comprised regioregular poly (3-hexylthiophene) (P3HT) and 1-(3methoxycarbonyl1) propyl-1-pheny [6, 6] methanofullerene (PCBM) with a weight ratio of 1:1 was mixed into 1, 2-dichlorobenzene solution. The mixed P3HT:PCBM layer was spin coated on the PEDOT:PSS at a spin rate of 2000 rpm 60 s. Successively, the synthesized CZTS nanospheres were mixed in ethanol to form a colloidal solution. The colloidal CZTS solution
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where Eg(nano) is the optical band gap (1.84 eV) of CZTS nanoparticles, Eg(bulk) is the band gap
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was spin-coated onto the P3HT:PCBM layer at 2000 rpm for 30 s and was annealed at 145oC for 10 min. The OPV devices were completed by deposition of a cathode buffer layer LiF and Al
× 2 mm. Current-density (J-V) measurements were measured in the dark and under illumination by using a Keithley-2400 source meter. The photovoltaic characteristics were measured by using a Xenon lamp under AM 1.5 stimulated illumination at an intensity of 100 mW/cm2. All measurements were carried out at room temperature (27oC) under ambient air with relative humidity of 60%-70%. Figure 4(a) displays the energy diagram of fabricated OPV cells with CZTS nanospheres. Figure 4(b) shows the J-V characteristics of the OPV cells in dark, cells with and without CZTS nanoparticles. The inset of Fig. 4(b) depicts the schematic illustration of the device structure, ITO/PEDOT:PSS/P3HT:PCBM-CZTS/LiF/Al. The open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and power conversion efficiency (PCE) of the fabricated OPV cells with and without CZTS nanospheres are summarized in Table.1. The device prepared without CZTS nanospheres exhibited an Voc of 0.208 V, a Jsc of 2.74 mA/cm2, and a FF of 0.17, resulting in a PCE of 0.120%. After the addition of CZTS nanospheres into the photoactive layer exhibited a Voc of 0.52 V, a Jsc of 4.90 mA/cm2, and a FF of 0.36, resulting in the PCE of 0.952%. Currentdensity measurements showed that the OPV cells with CZTS nanospheres exhibited higher PCE than those of cells without CZTS nanospheres due to their large optical absorption from visible to near IR region (Fig. 3) [2]. Our results showed that the PCE of OPV cells with CZTS nanospheres is quite low, but it is noteworthy to mention that our study is an initial effort towards demonstrating the potential of nontoxic CZTS as a light-absorbing material for the next generation PV cells.
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front contacts with thicknesses of 10 and 100 nm; the active area of the fabricated cell was 2 mm
Nanoscale
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In summary, Kesterite CZTS nanospheres were synthesized by using a solvothermal method. The structural and optical properties confirmed the phase purity of the as-prepared
average crystallite size of 3.26 nm exhibited that the energy gap of the CZTS nanospheres shifted towards a higher energy with a band gap of 1.84 eV. The estimated exciton Bohr radius suggested that the CZTS nanoparticles displayed a strong quantum confinement effects. Currentdensity measurements showed that the PCE (0.952%) of the OPV cells employing CZTS nanospheres was much higher than that (0.120%) of the cells without CZTS nanospheres. These results indicate that the CZTS nanospheres formed by using a solvothermal method hold promise for potential applications in third generation PV devices with high photovoltaic performance due to their excellent optoelectronic properties.
Acknowledgments This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013-016467).
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kesterite CZTS nanospheres. UV-vis absorption spectra of the CZTS nanospheres with an
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References [1] J.Y. Chane- Ching, A. Gillorin, O. Zaberca, A. Balocchi, and X. Marie, Chem. Commun.,
[2] J. Wang, X. Xin, and Z. Lin, Nanoscale, 2011, 3, 3040-3048. [3] D. B. Mitzi, O. Gunawan, T. K. Todorov, K. Wang, and S. Guha, Sol. Energy Mater. Sol. Cells, 2011, 95, 1421-1440. [4] M. Graetzel, R. A. J. Janssen, D. B. Mitzi, and E. H. Sargent, Nature, 2012, 488, 304-312. [5] A. S. R. Chesman, N. W. Duffy, S. P. L. Waddington, N. A. S. Webster, and J. J. Jasieniak, RSC Adv., 2013, 3, 1017-1020. [6] M. Wei, Q. Du, D. Wang, W. Liu, G. Jiang, and C. Zhu, Mater. Lett., 2012, 79, 177-179. [7] D. S. Su and S-H. Wei, Appl. Phys. Lett. 1999, 74, 2483-2485. [8] Q. Guo, H. W. Hillhouse, and R. Agrawal, J. Am. Chem. Soc., 2009, 131, 11672-11673. [9] S. C. Riha, B. A. Parkinson, and A. L. Prieto, J. Am. Chem. Soc., 2009, 131,12054-12055. [10] A. Shavel, D. Cadavid, M. Ibanez, A. Carrete, and A. Cabot, J. Am. Chem. Soc., 2012, 134, 1438-1441. [11] B. Shin, O. Gunawan, Y. Shu, N. A. Bojarczuk, S. J. Chey, and S. Guha, Prog. Photovolt. Res. Appl., 2013, 21, 72-76. [12] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, and Y. Yang, Nat. Mater., 2005, 4, 864-868. [13] C. Steinhagen, M. G. Panthani, V. Akhavan, B. Goodfellow, B. Koo, and B. A. Korgel, J. Am. Chem. Soc., 2009, 131, 12554-12555. [14] O. E. Semonin, J. M. Luther, S. Choi, H. Y. Chen, J. B. Gao, A. J. Nozik, and M. C. Beard, Science, 2011, 334, 1530-1533.
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2011, 47, 5229-5231.
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[15] J. B. Sambur, T. Novet, and B. A. Parkinson, Science, 2010, 330, 63-66. [16] W. A. Tisdale, K. J. Williams, B. A. Timp, D. J. Norris, E. S. Aydil, and X. Y. Zhu, Science,
[17] D. A. R. Barkhouse, O. Gunawan, T. Gokmen, T. K.Todorov, and D. B. Mitzi, Prog. Photovolt. Res. Appl., 2012, 20, 6-11. [18] K. Wang, O. Gunawan, T. Todorov, B. Shin, S. J. Chey, N. A. Bojarczuk, D. Mitzi, and S. Guha, Appl. Phys. Lett., 2010, 97, 143508-143510. [19] C. Zou, L. Zhang, D. Lin Y. Yang, Q. Li, X. Xu, Chen, and S. Huang, Cryst. Eng. Comm., 2011, 13, 3310-3313. [20] H. C. Jiang, P. C. Dai, Z. Y. Feng, W. L. Fan, and J. H. Zhan, J. Mater. Chem., 2012, 22, 7502-7506. [21] A. Singh, H. Geaney, F. Laffir, and K. M. Ryan, J. Am. Chem. Soc., 2012, 134, 2910-2913. [22] H. Yang, L. A. Jauregui, G. Zhang, Y. P. Chen, and Y. Wu, Nano Lett., 2012, 12, 540-545. [23] A. M. Smith and S. Nie, Acc. Chem. Res., 2009, 43, 190-200. [24] W. C. Liu, B. L. Guo, X. S. F. M. Zhang, C. L. Mak, and K. H. Wong, J. Mater. Chem. A, 2013, 1, 3182-3186. [25] F. Zhang, Q. Jin, and S.W. Chan, J. Appl. Phys., 2004, 95, 4319-4326. [26] C. Persson, J. Appl. Phys., 2010, 107, 053710-1 - 053710-7. [27] A. Khare, A. W. Wills, L. M. Annerman, D. J. Norris, and E. S. Aydil, Chem. Commun., 2011, 47, 11721-11723.
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2010, 328, 1543-1547.
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Table 1. Device performances parameters of OPV cells with and without CZTS nanospheres. Jsc (mA/cm2)
Voc (V)
FF
ߟ (%)
ITO/PEDOT:PSS/P3HT:PCBM-CZTS/LiF/Al
4.9062
0.52042
0.3689
0.9520
ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al
2.7428
0.20811
0.17848
0.120
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Device Structure
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Figure Captions Fig. 1. (a) Scanning electron microscopy image, (b, c, d) high-resolution transmission electron
prepared CZTS nanospheres.
Fig. 2. X-ray spectroscopy of the as-prepared CZTS nanospheres. Fig. 3. Ultraviolet-visible absorption spectra of the CZTS nanospheres. Inset represents the plot of (αhν)2 as a function of hν for estimating the optical band gap.
Fig. 4. (a) Schematic energy band diagram and (b) current-voltage curves of the OPV cells measured in dark, cells with and without CZTS layer. The inset of Fig. 4(b) displays the device structure of fabricated OPV cells with CZTS layer.
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microscopy images, (e) selected area diffraction pattern, and (f) XRD spectrum of the as-
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DOI: 10.1039/C3NR03892K
(a) Scanning electron microscopy image, (b, c, d) high-resolution transmission electron microscopy images, (e) selected area diffraction pattern, and (f) XRD spectrum of the as-prepared CZTS nanospheres. 126x184mm (300 x 300 DPI)
Nanoscale
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X-ray spectroscopy of the as-prepared CZTS nanospheres. 69x56mm (300 x 300 DPI)
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Ultraviolet-visible absorption spectra of the CZTS nanospheres. Inset represents the plot of (αhν)2 as a function of hν for estimating the optical band gap. 68x54mm (300 x 300 DPI)
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DOI: 10.1039/C3NR03892K
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DOI: 10.1039/C3NR03892K
(a) Schematic energy band diagram and (b) current-voltage curves of the OPV cells measured in dark, cells with and without CZTS layer. The inset of Fig. 4(b) displays the device structure of fabricated OPV cells with CZTS layer. 119x165mm (300 x 300 DPI)
