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AgSbS2 semiconductor-sensitized solar cells
Electrochemistry Communications 26 (2013) 48–51
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
AgSbS2 semiconductor-sensitized solar cells Yi-Rong Ho, Ming-Way Lee ⁎ Institute of Nanoscience and Department of Physics, National Chung Hsing University, Taichung 402, Taiwan
a r t i c l e
i n f o
Article history: Received 9 September 2012 Received in revised form 28 September 2012 Accepted 1 October 2012 Available online 10 October 2012 Keywords: Silver antimony sulfide Solar cell Semiconductor sensitizer Successive ionic layer adsorption and reaction
a b s t r a c t We present a ternary semiconductor nanoparticle sensitizer – AgSbS2 – for solar cells. AgSbS2 nanoparticles were grown using a two-stage successive ionic layer adsorption and reaction process. First, Ag2S nanoparticles were grown on the surface of a nanoporous TiO2 electrode. Secondly, a Sb–S film was coated on top of the Ag2S. The double-layered structure was transformed into AgSbS2 nanoparticles ~ 40 nm in diameter, after postdeposition heating at 350 °C. The AgSbS2-sensitized TiO2 electrodes were fabricated into liquid-junction solar cells. The best cell yielded a power conversion efficiency of 0.34% at 1 sun and 0.42% at 0.1 sun. The external quantum efficiency (EQE) spectrum covered the range of 380–680 nm with a maximal EQE of 10.5% at λ = 470 nm. The method can be applied to grow other systems of ternary semiconductor nanoparticles for solar absorbers. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor-sensitized solar cells (SSCs) are a promising candidate for the next generation of solar cells. The key component of a SSC is a photoelectrode with a thin semiconductor light absorber deposited on the surface of a nanoporous transparent oxide layer. There are two types of SSCs where the hole conductor can be solid (extremely thin absorber (ETA) cells) or liquid (commonly called as SSSCs). Nanoscale semiconductor sensitizers have several advantages such as high extinction coefficients [1], tunable absorption ranges due to the quantum size effect [2], and multiple electron-hole generation by a single photon [3]. The SSCs obtained an efficiency of 5–6% in 2012 [4–6]. Semiconductors like CdS, CdSe, PbS and Sb2S3 have been successfully employed as SSC sensitizers [7–10]. Most of these materials belong to the binary metal chalcogenide groups. Ternary semiconductors are equally important materials for solar cells and some ternary semiconductors (such as Cu–In–Se) have achieved efficiencies over 18% [11]. Ternary semiconductors CuInSe2 and CdxZn1−xS have been synthesized using the successive ionic layer adsorption and reaction (SILAR) method [12,13]. Ternary semiconductors (e.g. Cu3SbS4) can also be prepared using the chemical bath deposition method where two semiconductor layers are deposited first, followed by post annealing [14]. The ternary Ag–Sb–S semiconductor system has two phases: AgSbS2 (Eg = 1.7 eV) and Ag3SbS3 (Eg = 1.8 eV) [15]. AgSbS2 has a high absorption coefficient of α ~10 5 cm−1 that matches that of CuInSe2, making it a potential solar absorber [16]. Thin-film AgSbS2 SSCs were recently fabricated, yielding a short-circuit current density Jsc of 0.12 mA/cm 2 [17]. However, there has been no report on SSCs sensitized with AgSbS2 ⁎ Corresponding author. Tel.: +886 422852783; fax: +886 4 22862534. E-mail address: [email protected] (M.-W. Lee). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2012.10.003
nanoparticles so far. In this work we have successfully synthesized AgSbS2 nanoparticles using a two-step SILAR method. The photovoltaic performance of liquid-junction AgSbS2 SSCs is investigated. The dependences of the photovoltaic data on the number of SILAR cycles and light intensity are studied.
2. Experimental The TiO2 photoelectrode has a three-layer structure — a blocking layer (~ 80 nm-thick), a TiO2 transparent layer (12 μm-thick, particle size ~ 20 nm) and a TiO2 scattering layer (3-μm thick, particle size ~ 400 nm). The TiO2 transparent layer was prepared by spreading a TiO2 paste (Dyesol DSL-18NR-T) onto fluorine-doped tin oxide glass (FTO, 15Ω/□) using the previous procedure [18]. The blocking layer prevented shorting between the hole conductor and FTO. The scattering layer increased the light scattering and harvesting. AgSbS2 nanoparticles were grown on TiO2 electrodes through the two-stage SILAR process. First, Ag2S QDs were grown on the TiO2 electrode following the previous procedure [18]. Secondly, a Sb–S film was coated on top of the Ag2S using the Sb2S3 synthesis procedure [10,19]. The double-layered structure was then heated to obtain the AgSbS2 phase. Briefly, the TiO2 electrode was dipped into an AgNO3 ethanol solution (25 °C, 0.1 M, 1 min), rinsed with ethanol, and then dipped into a Na2S methanol solution (0.1 M, 4 min). The two-step dipping process forms one SILAR cycle. This process produced Ag2S QDs several nanometers in diameter [19]. The Ag2S-coated TiO2 electrode was then dipped into a Sb2Cl3 ethanol solution (0.1 M, 15 s), washed with ethanol, then dipped into a Na2S methanol solution (0.1 M, 1 min). The number of Sb–S SILAR cycles was twice that of Ag2S. The double-layered structure was transformed into the AgSbS2 structure by heating it to 350 °C in air for 10 min. Samples that went through
Y.-R. Ho, M.-W. Lee / Electrochemistry Communications 26 (2013) 48–51
n Ag2S SILAR cycles (and 2n Sb-S SILAR cycles) are referred to as AgSbS2(n). Solar cells were prepared by assembling the AgSbS2-sensitized TiO2 electrode with an Au counterelectrode using a 190 μm-thick parafilm spacer. The polysulfide electrolyte consisted of 0.25 M·Na2S, 1M·S, 0.2 M·KCl and 0.1 M·KI in ethanol/water (7:3 by volume). We also find that KI improves cell performance, as reported by Zhao et al. [20]. The absorption spectra were measured using a Hitachi 2800A spectrophotometer. X-ray diffraction was performed using a PANalytical X'Pert Pro MRD diffractometer. Current–voltage (I–V) curves were recorded under 100 mW/cm 2 light intensity with a Keithley 2400 source meter with a 150 W Oriel Xe lamp. The external quantum efficiency spectra (EQE) were measured using an Acton monochromator with a 250 W tungsten–halogen lamp. A metal mask (3.5 mm × 3.5 mm), placed above the cell, defined the active area of the cell to be 3.0 mm × 3.0 mm. 3. Results and discussion Fig. 1 displays TEM images of a bare TiO2 film (Fig. 1(a)) and an Ag2S(3)-coated TiO2 film (Fig. 1(b)). It can be seen that Ag2S QDs are separated and randomly distributed with an average diameter of 7–9 nm. Fig. 1(c) shows an image of the double-layered Ag2S(3)/ Sb–S(6) sample before heating treatment. The size of the QDs has increased to 10–13 nm due to the double-layered structure. After heating (Fig. 1(d)), the double-layered structure was transformed into AgSbS2. The nanoparticles have diameters ranging from 40 to 45 nm, which is about four times larger than that before heating.
49
The heating treatment caused the double-layered Ag–S/Sb–S nanoparticles to coalesce and form much larger nanoparticles. Fig. 2(a) displays the X-ray diffraction pattern of the sample. The clear diffraction peaks are assigned to the fcc AgSbS2 phase (JCPDS-17-0456) with a lattice constant of a = 5.648 Ǻ. Many TiO2 diffraction peaks, arising from the background of the TiO2 electrode, can also be been in the figure. Further support of the structure is seen in the optical spectra: (1) transmission (Fig. 2(b)), (2) absorption (αhν) 2 vs. hν (Fig. 2(c)) plot. The intercept of the plot is equal to Eg = 1.7 eV, in agreement with the bulk AgSbS2 gap [17]. The nanoparticles having the bulk Eg indicate that they do not exhibit the quantum-size effect, which can be accounted for by the large particle size (40 nm). Fig. 3(a) displays the I–V curves for AgSbS2 SSCs with various SILAR cycle n. Table 1 (part (a)) lists the photovoltaic data. Initially, the Jsc, open-circuit voltage Voc and efficiency η increased with n. The optimum performance was obtained at n = 3, which yielded Jsc = 2.42 mA/cm 2, Voc = 0.32 V and η = 0.34% under 1 sun intensity. When n > 3, η started to decrease again. The decreased η is mainly due to a significantly decreased Jsc, which reduced from 2.42 to 1.17 mA/cm 2 as n increased from 3 to 5. In contrast, Voc showed a weak dependence on n. The decreasing performance at high n is probably caused by the overloading of nanoparticles within the porous spaces of the TiO2 electrode, which reduces the pore size and impedes the flow of the electrolyte, resulting in a lower efficiency. The optimal SILAR number of n = 3 is a relatively small number for a SILAR process, indicating that the nanoparticles quickly fill up the pore in the early stage of the growth process. Obviously, this is a result of the double-layered structure, which produces a film twice as thick
Fig. 1. TEM images of: (a) a bare TiO2 film, (b) an Ag2S(3)-coated TiO2 film, (c) an Ag2S(3)/Sb–S(6) double-layered film before heating, and (d) AgSbS2(3) nanoparticles on TiO2 (after heating).
50
Y.-R. Ho, M.-W. Lee / Electrochemistry Communications 26 (2013) 48–51
Fig. 2. AgSbS2(3) nanoparticles: (a) X-ray diffraction pattern. (b) Optical transmission spectrum. (c) Optical absorption spectrum.
as that of the single-layered structure. Note that the best Jsc = 2.42 mA/cm 2 achieved herein is twenty times larger than that of (0.12 mA/cm 2) in Ref. 17. Fig. 3(b) displays the EQE spectrum for the best cell — AgSbS2(3). Two EQE bands appear: the first in the spectral range of 400–520 nm, the second at 520–650 nm. The spectrum has a maximal EQE of 10.5% at λ = 470 nm and an average EQE of 8.2% over the major spectral range of 420–620 nm. The upper cutoff wavelength is ~ 680 nm (1.82 eV). Fig. 3(c) shows the I–V curves under various light intensities. The photovoltaic parameters are listed in Table 1 (part (b)). The η increased from 0.34 to 0.42% — a 24% increase-as the light intensity was reduced from 101 to 10.6% sun. The enhanced η arises mainly from Jsc, which produces about two times Jsc per unit incident light when the light intensity is reduced. For a semiconductor with Eg = 1.7 eV, the theoretical maximal Jsc is ~ 24 mA/cm 2 under 1 sun (or 2.4 mA/cm 2 under 0.1 sun) [21], which is a factor of five larger than the Jsc = 0.47 mA/cm 2 (0.1 sun) in Table 1. The small Jsc as well as low EQE herein are attributed to the fact that only a small fraction of the TiO2 surface is covered with AgSbS2 nanoparticles because of the relatively large size of the nanoparticles (~40 nm), which is due to (1) the double-layered structure, and (2) the heating treatment. For efficient light harvesting, the TiO2 particles (size 20 nm) should be completely covered with a uniform layer of nanoparticles (preferably b10 nm). The large AgSbS2
size means that a large portion of the TiO2 surface is uncovered with absorbing materials, resulting in low light absorption. If the coverage of the TiO2 surface is improved by using smaller nanoparticles, the efficiency of the SSCs should be greatly improved. Ternary semiconductors will play an increasingly important role as light absorbers for solar cells. It is desirable to extend the categories of materials from mainly binary to ternary compounds. The present work reveals the possibility of synthesizing ternary semiconductor nanoparticles for SSCs. Much work is needed to develop synthesis methods for the growth of uniform, small-size nanoparticle ternary semiconductors. 4. Conclusion We successfully synthesized AgSbS2 nanoparticles on a nanoporous TiO2 electrode using the two-stage SILAR process. The AgSbS2 liquidjunction SSCs exhibit a promising photovoltaic performance. The synthesis method can be extended to other systems for research on ternary semiconductor solar absorbers. Acknowledgment The authors are grateful for the financial support received from the National Science Council of the Republic of China.
Y.-R. Ho, M.-W. Lee / Electrochemistry Communications 26 (2013) 48–51
51
Table 1 Dependences of photovoltaic performance of AgSbS2 sensitized solar cells on (a) SILAR cycles (b) sun intensity.
(a) SILAR cycles
(b) Sun intensity
AgSbS2(2) AgSbS2(3) AgSbS2(4) AgSbS2(5) 101.2% sun 40.2% sun 10.6% sun
Jsc (mA/cm2)
Voc (V)
FF (%)
η (%)
0.53 2.42 1.37 1.17 2.42 1.19 0.47
0.17 0.32 0.28 0.25 0.32 0.31 0.25
27.5 43.6 42.4 32.6 43.6 42.4 37.7
0.025 0.34 0.16 0.10 0.34 0.38 0.42
References [1] R.D. Shaller, V.I. Klimov, Physical Review Letters 92 (2004) 186601. [2] S. Gorer, G. Hodes, The Journal of Physical Chemistry 98 (1994) 5338. [3] I. Moreels, K. Lambert, D. De Muynck, F. Vanhaecke, D. Poelman, J.C. Martins, G. Allan, Z. Hens, Chemistry of Materials 19 (2007) 6101. [4] S.-J. Moon, Y. Itzhaik, J.-H. Yum, S.M. Zakeeruddin, G. Hodes, M. Gratzel, Journal of Physical Chemistry Letters 1 (2010) 1524. [5] J.A. Chang, J.H. Rhee, S.H. Im, Y.H. Lee, H.-j. Kim, S.I. Seok, Md.K. Nazeeruddin, M. Gratzel, Nano Letters 10 (2010) 2609. [6] P.K. Santra, P.V. Kamat, Journal of the American Chemical Society 134 (2012) 2508. [7] G. Larramona, C. Chone, A. Jacob, D. Sakakura, B. Delatouche, D. Pere, X. Cieren, M. Nagino, R. Bayon, Chemistry of Materials 18 (2006) 1688. [8] N. Guijarro, T. Lana-Villarreal, I. Mora-Seró, J. Bisquert, R. Gómez, Journal of Physical Chemistry C 113 (2009) 4208. [9] I. Robel, V. Subramanian, M. Kuno, P.V. Kamat, Journal of the American Chemical Society 128 (2006) 2385. [10] Y. Itzhaik, O. Niitsoo, M. Page, G. Hodes, Journal of Physical Chemistry C 113 (2009) 4254. [11] A. Ashour, A.A. Ramdhan, K. Abd El-Hady, A.A. Akl, Journal of Materials Science: Materials in Electronics 16 (2005) 599. [12] H.M. Pathan, C.D. Lokhande, Applied Surface Science 245 (2005) 328. [13] Y.F. Nicolau, M. Dupuy, M. Brunel, Journal of the Electrochemical Society 137 (1990) 2916. [14] M.T.S. Nair, Y. Peña, J. Campos, V.M. Garcia, P.K. Nair, Journal of the Electrochemical Society 145 (1998) 2113. [15] S.I. Boldish, W.B. White, American Mineralogist 83 (1998) 865. [16] A.M. Ibrahim, Journal of Physics. Condensed Matter 7 (1995) 5931. [17] M.J. Capistrán, M.T.S. Nair, P.K. Nair, MRS Proceedings 1447 (2012), http://dx.doi.org/ 10.1557/opl.2012.1085. [18] A. Tubtimtae, K.L. Wu, H.Y. Tung, M.W. Lee, G.J. Wang, Electrochemistry Communications 12 (2010) 1158. [19] S. Messina, M.T.S. Nair, P.K. Nair, Thin Solid Films 515 (2007) 5777. [20] J.J. Zhao, B.T. Jiang, S.Y. Zhang, H.L. Niu, B.K. Jin, Y.P. Tian, Science in China, Series B: Chemistry 52 (2009) 2213. [21] Editorial Solar Energy Materials & Solar Cells 92 (2008) 371.
Fig. 3. (a) I–V curves for AgSbS2 SSCs with various SILAR cycles. (b) EQE spectrum of an AgSbS2(3) SSC. (c) I–V curves of an AgSbS2(3) SSC under various light intensities.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
AgSbS2 semiconductor-sensitized solar cells Yi-Rong Ho, Ming-Way Lee ⁎ Institute of Nanoscience and Department of Physics, National Chung Hsing University, Taichung 402, Taiwan
a r t i c l e
i n f o
Article history: Received 9 September 2012 Received in revised form 28 September 2012 Accepted 1 October 2012 Available online 10 October 2012 Keywords: Silver antimony sulfide Solar cell Semiconductor sensitizer Successive ionic layer adsorption and reaction
a b s t r a c t We present a ternary semiconductor nanoparticle sensitizer – AgSbS2 – for solar cells. AgSbS2 nanoparticles were grown using a two-stage successive ionic layer adsorption and reaction process. First, Ag2S nanoparticles were grown on the surface of a nanoporous TiO2 electrode. Secondly, a Sb–S film was coated on top of the Ag2S. The double-layered structure was transformed into AgSbS2 nanoparticles ~ 40 nm in diameter, after postdeposition heating at 350 °C. The AgSbS2-sensitized TiO2 electrodes were fabricated into liquid-junction solar cells. The best cell yielded a power conversion efficiency of 0.34% at 1 sun and 0.42% at 0.1 sun. The external quantum efficiency (EQE) spectrum covered the range of 380–680 nm with a maximal EQE of 10.5% at λ = 470 nm. The method can be applied to grow other systems of ternary semiconductor nanoparticles for solar absorbers. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor-sensitized solar cells (SSCs) are a promising candidate for the next generation of solar cells. The key component of a SSC is a photoelectrode with a thin semiconductor light absorber deposited on the surface of a nanoporous transparent oxide layer. There are two types of SSCs where the hole conductor can be solid (extremely thin absorber (ETA) cells) or liquid (commonly called as SSSCs). Nanoscale semiconductor sensitizers have several advantages such as high extinction coefficients [1], tunable absorption ranges due to the quantum size effect [2], and multiple electron-hole generation by a single photon [3]. The SSCs obtained an efficiency of 5–6% in 2012 [4–6]. Semiconductors like CdS, CdSe, PbS and Sb2S3 have been successfully employed as SSC sensitizers [7–10]. Most of these materials belong to the binary metal chalcogenide groups. Ternary semiconductors are equally important materials for solar cells and some ternary semiconductors (such as Cu–In–Se) have achieved efficiencies over 18% [11]. Ternary semiconductors CuInSe2 and CdxZn1−xS have been synthesized using the successive ionic layer adsorption and reaction (SILAR) method [12,13]. Ternary semiconductors (e.g. Cu3SbS4) can also be prepared using the chemical bath deposition method where two semiconductor layers are deposited first, followed by post annealing [14]. The ternary Ag–Sb–S semiconductor system has two phases: AgSbS2 (Eg = 1.7 eV) and Ag3SbS3 (Eg = 1.8 eV) [15]. AgSbS2 has a high absorption coefficient of α ~10 5 cm−1 that matches that of CuInSe2, making it a potential solar absorber [16]. Thin-film AgSbS2 SSCs were recently fabricated, yielding a short-circuit current density Jsc of 0.12 mA/cm 2 [17]. However, there has been no report on SSCs sensitized with AgSbS2 ⁎ Corresponding author. Tel.: +886 422852783; fax: +886 4 22862534. E-mail address: [email protected] (M.-W. Lee). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2012.10.003
nanoparticles so far. In this work we have successfully synthesized AgSbS2 nanoparticles using a two-step SILAR method. The photovoltaic performance of liquid-junction AgSbS2 SSCs is investigated. The dependences of the photovoltaic data on the number of SILAR cycles and light intensity are studied.
2. Experimental The TiO2 photoelectrode has a three-layer structure — a blocking layer (~ 80 nm-thick), a TiO2 transparent layer (12 μm-thick, particle size ~ 20 nm) and a TiO2 scattering layer (3-μm thick, particle size ~ 400 nm). The TiO2 transparent layer was prepared by spreading a TiO2 paste (Dyesol DSL-18NR-T) onto fluorine-doped tin oxide glass (FTO, 15Ω/□) using the previous procedure [18]. The blocking layer prevented shorting between the hole conductor and FTO. The scattering layer increased the light scattering and harvesting. AgSbS2 nanoparticles were grown on TiO2 electrodes through the two-stage SILAR process. First, Ag2S QDs were grown on the TiO2 electrode following the previous procedure [18]. Secondly, a Sb–S film was coated on top of the Ag2S using the Sb2S3 synthesis procedure [10,19]. The double-layered structure was then heated to obtain the AgSbS2 phase. Briefly, the TiO2 electrode was dipped into an AgNO3 ethanol solution (25 °C, 0.1 M, 1 min), rinsed with ethanol, and then dipped into a Na2S methanol solution (0.1 M, 4 min). The two-step dipping process forms one SILAR cycle. This process produced Ag2S QDs several nanometers in diameter [19]. The Ag2S-coated TiO2 electrode was then dipped into a Sb2Cl3 ethanol solution (0.1 M, 15 s), washed with ethanol, then dipped into a Na2S methanol solution (0.1 M, 1 min). The number of Sb–S SILAR cycles was twice that of Ag2S. The double-layered structure was transformed into the AgSbS2 structure by heating it to 350 °C in air for 10 min. Samples that went through
Y.-R. Ho, M.-W. Lee / Electrochemistry Communications 26 (2013) 48–51
n Ag2S SILAR cycles (and 2n Sb-S SILAR cycles) are referred to as AgSbS2(n). Solar cells were prepared by assembling the AgSbS2-sensitized TiO2 electrode with an Au counterelectrode using a 190 μm-thick parafilm spacer. The polysulfide electrolyte consisted of 0.25 M·Na2S, 1M·S, 0.2 M·KCl and 0.1 M·KI in ethanol/water (7:3 by volume). We also find that KI improves cell performance, as reported by Zhao et al. [20]. The absorption spectra were measured using a Hitachi 2800A spectrophotometer. X-ray diffraction was performed using a PANalytical X'Pert Pro MRD diffractometer. Current–voltage (I–V) curves were recorded under 100 mW/cm 2 light intensity with a Keithley 2400 source meter with a 150 W Oriel Xe lamp. The external quantum efficiency spectra (EQE) were measured using an Acton monochromator with a 250 W tungsten–halogen lamp. A metal mask (3.5 mm × 3.5 mm), placed above the cell, defined the active area of the cell to be 3.0 mm × 3.0 mm. 3. Results and discussion Fig. 1 displays TEM images of a bare TiO2 film (Fig. 1(a)) and an Ag2S(3)-coated TiO2 film (Fig. 1(b)). It can be seen that Ag2S QDs are separated and randomly distributed with an average diameter of 7–9 nm. Fig. 1(c) shows an image of the double-layered Ag2S(3)/ Sb–S(6) sample before heating treatment. The size of the QDs has increased to 10–13 nm due to the double-layered structure. After heating (Fig. 1(d)), the double-layered structure was transformed into AgSbS2. The nanoparticles have diameters ranging from 40 to 45 nm, which is about four times larger than that before heating.
49
The heating treatment caused the double-layered Ag–S/Sb–S nanoparticles to coalesce and form much larger nanoparticles. Fig. 2(a) displays the X-ray diffraction pattern of the sample. The clear diffraction peaks are assigned to the fcc AgSbS2 phase (JCPDS-17-0456) with a lattice constant of a = 5.648 Ǻ. Many TiO2 diffraction peaks, arising from the background of the TiO2 electrode, can also be been in the figure. Further support of the structure is seen in the optical spectra: (1) transmission (Fig. 2(b)), (2) absorption (αhν) 2 vs. hν (Fig. 2(c)) plot. The intercept of the plot is equal to Eg = 1.7 eV, in agreement with the bulk AgSbS2 gap [17]. The nanoparticles having the bulk Eg indicate that they do not exhibit the quantum-size effect, which can be accounted for by the large particle size (40 nm). Fig. 3(a) displays the I–V curves for AgSbS2 SSCs with various SILAR cycle n. Table 1 (part (a)) lists the photovoltaic data. Initially, the Jsc, open-circuit voltage Voc and efficiency η increased with n. The optimum performance was obtained at n = 3, which yielded Jsc = 2.42 mA/cm 2, Voc = 0.32 V and η = 0.34% under 1 sun intensity. When n > 3, η started to decrease again. The decreased η is mainly due to a significantly decreased Jsc, which reduced from 2.42 to 1.17 mA/cm 2 as n increased from 3 to 5. In contrast, Voc showed a weak dependence on n. The decreasing performance at high n is probably caused by the overloading of nanoparticles within the porous spaces of the TiO2 electrode, which reduces the pore size and impedes the flow of the electrolyte, resulting in a lower efficiency. The optimal SILAR number of n = 3 is a relatively small number for a SILAR process, indicating that the nanoparticles quickly fill up the pore in the early stage of the growth process. Obviously, this is a result of the double-layered structure, which produces a film twice as thick
Fig. 1. TEM images of: (a) a bare TiO2 film, (b) an Ag2S(3)-coated TiO2 film, (c) an Ag2S(3)/Sb–S(6) double-layered film before heating, and (d) AgSbS2(3) nanoparticles on TiO2 (after heating).
50
Y.-R. Ho, M.-W. Lee / Electrochemistry Communications 26 (2013) 48–51
Fig. 2. AgSbS2(3) nanoparticles: (a) X-ray diffraction pattern. (b) Optical transmission spectrum. (c) Optical absorption spectrum.
as that of the single-layered structure. Note that the best Jsc = 2.42 mA/cm 2 achieved herein is twenty times larger than that of (0.12 mA/cm 2) in Ref. 17. Fig. 3(b) displays the EQE spectrum for the best cell — AgSbS2(3). Two EQE bands appear: the first in the spectral range of 400–520 nm, the second at 520–650 nm. The spectrum has a maximal EQE of 10.5% at λ = 470 nm and an average EQE of 8.2% over the major spectral range of 420–620 nm. The upper cutoff wavelength is ~ 680 nm (1.82 eV). Fig. 3(c) shows the I–V curves under various light intensities. The photovoltaic parameters are listed in Table 1 (part (b)). The η increased from 0.34 to 0.42% — a 24% increase-as the light intensity was reduced from 101 to 10.6% sun. The enhanced η arises mainly from Jsc, which produces about two times Jsc per unit incident light when the light intensity is reduced. For a semiconductor with Eg = 1.7 eV, the theoretical maximal Jsc is ~ 24 mA/cm 2 under 1 sun (or 2.4 mA/cm 2 under 0.1 sun) [21], which is a factor of five larger than the Jsc = 0.47 mA/cm 2 (0.1 sun) in Table 1. The small Jsc as well as low EQE herein are attributed to the fact that only a small fraction of the TiO2 surface is covered with AgSbS2 nanoparticles because of the relatively large size of the nanoparticles (~40 nm), which is due to (1) the double-layered structure, and (2) the heating treatment. For efficient light harvesting, the TiO2 particles (size 20 nm) should be completely covered with a uniform layer of nanoparticles (preferably b10 nm). The large AgSbS2
size means that a large portion of the TiO2 surface is uncovered with absorbing materials, resulting in low light absorption. If the coverage of the TiO2 surface is improved by using smaller nanoparticles, the efficiency of the SSCs should be greatly improved. Ternary semiconductors will play an increasingly important role as light absorbers for solar cells. It is desirable to extend the categories of materials from mainly binary to ternary compounds. The present work reveals the possibility of synthesizing ternary semiconductor nanoparticles for SSCs. Much work is needed to develop synthesis methods for the growth of uniform, small-size nanoparticle ternary semiconductors. 4. Conclusion We successfully synthesized AgSbS2 nanoparticles on a nanoporous TiO2 electrode using the two-stage SILAR process. The AgSbS2 liquidjunction SSCs exhibit a promising photovoltaic performance. The synthesis method can be extended to other systems for research on ternary semiconductor solar absorbers. Acknowledgment The authors are grateful for the financial support received from the National Science Council of the Republic of China.
Y.-R. Ho, M.-W. Lee / Electrochemistry Communications 26 (2013) 48–51
51
Table 1 Dependences of photovoltaic performance of AgSbS2 sensitized solar cells on (a) SILAR cycles (b) sun intensity.
(a) SILAR cycles
(b) Sun intensity
AgSbS2(2) AgSbS2(3) AgSbS2(4) AgSbS2(5) 101.2% sun 40.2% sun 10.6% sun
Jsc (mA/cm2)
Voc (V)
FF (%)
η (%)
0.53 2.42 1.37 1.17 2.42 1.19 0.47
0.17 0.32 0.28 0.25 0.32 0.31 0.25
27.5 43.6 42.4 32.6 43.6 42.4 37.7
0.025 0.34 0.16 0.10 0.34 0.38 0.42
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Fig. 3. (a) I–V curves for AgSbS2 SSCs with various SILAR cycles. (b) EQE spectrum of an AgSbS2(3) SSC. (c) I–V curves of an AgSbS2(3) SSC under various light intensities.