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Cu2-xS quantum dot-sensitized solar cells
Electrochemistry Communications 13 (2011) 1376–1378
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Cu2-xS quantum dot-sensitized solar cells Mei-Chia Lin, 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 16 June 2011 Received in revised form 4 August 2011 Accepted 6 August 2011 Available online 12 August 2011 Keywords: Quantum dots Solar cells Copper sulfide Successive ionic layer adsorption and reaction Sensitizer
a b s t r a c t In this study we report on the photovoltaic performance of Cu2-xS (x = 1, 0.03) quantum-dot (QD) sensitized liquid-junction solar cells. The QDs were grown with successive ionic layer adsorption and reaction. The CuS phase in the as-grown QDs was transformed into Cu1.97S after annealing. The assembled Cu1.97S cells yielded a best power conversion efficiency η of 0.90% under illumination of 100 mW/cm2. The CuS cell had an η ~ 75% lower than that of Cu1.97S. The external quantum efficiency (EQE) spectrum of Cu1.97S covers the spectral range of 350–1100 nm with a maximal EQE of 63% at λ = 500 nm and an average EQE of 47% over the entire spectral range. This spectral range is also two times broader than that of N3 dye. The results show that Cu2-xS QDs can be used as highly efficient broadband sensitizers for solar cells. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSSCs) are a promising low-cost alternative to silicon-based renewable photovoltaics. The key component of a DSSC is a photoelectrode comprising of a nanoporous, 10 μm-thick TiO2 film sintered to fluorine-doped tin oxide (FTO) glass. A monolayer of organic dye is coated on the TiO2 surface for light harvesting. The three-dimensional nanoporous structure greatly increases the surface of dye adsorption, resulting in an impressive power conversion efficiency of ~ 11% [1]. Further improvement in efficiency has been hampered by the incomplete overlap of the dye absorption spectrum with the solar spectrum. The solar spectrum covers the spectral range of 350–2500 nm. However, the most commonly used organic dyes, N3 and N719, have strong absorption in the visible range of 350–700 nm but only weak absorption in the IR. The search for sensitizers with broader absorption ranges is crucial for enhancing DSSC efficiency. A successful candidate is sensitizers based on inorganic semiconductor nanomaterials. Extremely thin absorbers (ETA) semiconductors have been successfully used as sensitizers for DSSCs [2,3]. Semiconductor quantum-dots (QDs) (CdS, CdSe, PbS, and PbSe) have also been successfully employed [4–7]. QD sensitizers have three advantages over organic dyes, tunable absorption bands [8], large extinction coefficients and multiple electron-hole pair generation [4,9]. Copper sulfides (CuxS) are important materials for applications in p-type semiconductors and optoelectronics [10]. CdS/Cu2S heterojunctions have been used as solar absorbers [11]. There are five stable
⁎ Corresponding author. Tel.: + 886 422852783; fax: + 886 22862534. E-mail address: [email protected] (M.-W. Lee). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.08.013
CuxS phases for 1 ≤ x ≤ 2: CuS, Cu1.75S, Cu1.8S, Cu1.97S, and Cu2S. The band gap of CuxS depends on the oxidation state of Cu (Eg = 2.0, 1.5 and 1.2 eV for CuS, Cu1.8S and Cu1.97S, respectively) [12–14]. The most attractive phase is Cu1.97S, whose Eg is close to the optimum Eg (1.1 eV) for a photovoltaic device [15], suggesting that Cu1.97S can be an ideal absorber material. Recently Page et al. demonstrated an ETA solar cell using Cu2-xS as a light absorber [3], achieving a power conversion efficiency of 0.06%. Here we demonstrate liquid-junction solar cells sensitized with Cu2-xS QDs. We study the effects of the SILAR number, passivation coating and counterelectrode on the photovoltaic properties. The Cu2-xS DSSCs are found to exhibit a broad absorption range and yield efficiencies much higher than that of the ETA solar cells. 2. Experimental Nanoporous TiO2 films were prepared by spreading a TiO2 paste (Dyesol DSL-18NR-T, particle size ~20 nm) onto an FTO glass substrate (15 Ω/□) using the procedure described previously [16]. The TiO2 film had a three-layer structure: a blocking layer (thickness~ 90 nm), a TiO2 film (thickness ~10–12 μm) for QD coating, and a TiO2 scattering layer (3-μm thick, 400 nm in size) on the top [16]. The blocking layer, prepared using a spin-coated titanium isopropoxide ethanol solution, reduced the carrier recombination at the FTO/TiO2 interface. The scattering layer increased the light scattering and harvesting. The thickness was determined from cross-sectional transmission electron microscopic (TEM) images. Cu2-xS QDs were synthesized using the successive ionic layer adsorption and reaction (SILAR) process. A TiO2 film was first dipped into the 25 °C, 0.02 M Cu(NO3)2 ethanol solution for 1 min, before being rinsed with ethanol, then heated at 75 °C. The film was then
M.-C. Lin, M.-W. Lee / Electrochemistry Communications 13 (2011) 1376–1378
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dipped into the 25 °C, 0.02 M Na2S methanol solution for 1 min, washed with methanol, then heated at 75 °C. This two-step dipping process formed one SILAR cycle. Samples that went through n SILAR cycles are referred to as Cu2-xS(n). The QD-coated TiO2 electrode was assembled with an Au-coated FTO counterelectrode and sealed with a 190 μm parafilm spacer [16]. The Au counterelectrode was prepared by sputtering a 50 nm-thick Au film on FTO glass. The polysulfide electrolyte consisted of 0.25 M Na2S, 1 M S, 0.1 M KI and 0.2 M KCl in methanol/water (7:3 by volume). The absorption spectra were then recorded using a Hitachi 2800A spectrophotometer. The current–voltage (I-V) curves were recorded with a Keithley 2400 source meter with an Oriel 150 W Xe lamp. An Oriel band pass filter simulated the AM 1.5 solar spectrum. The external quantum efficiency (EQE) spectra were measured using an Acton monochromator with a 250 W tungsten–halogen lamp. The active area of the cell was 3 mm× 3 mm. A metal mask of 3.5 mm× 3.5 mm was placed above the cell during measurements. 3. Results and discussion Fig. 1(a) displays a TEM image of a Cu2-xS(11)-QD coated TiO2 film. Many QDs can be seen to deposit randomly over the TiO2 surface. The average diameter of the QDs is ~ 3–4 nm. No change in QD size is observed after annealing. Fig. 1(b) shows the absorption spectra of Cu2-xS QD-coated TiO2 electrodes with various SILAR cycles. The spectra exhibit two absorption bands: the first at ~400 nm and the second (near IR-NIR) at ~ 800–1100 nm. The absorption intensity increases with the SILAR number n. The increasing absorption indicates an increasing amount of QDs coating on the TiO2 surface as n is increased. The first band is attributed to the fundamental transition (valence band to conduction band-CB) of Cu2-xS. The NIR band is attributed to an impurity state [17], or a valence-band free-
Fig. 2. (a) Absorption spectra, and (b) XRD patterns, of QDs before (CuS phase) and after (Cu1.97S phase) annealing at 400 °C.
carrier absorption [18]. When the sample was annealed at 400 °C for three min, as Fig. 2(a) shows, the second absorption band disappeared. The color also changed from dark green to light brown (inset). The absorption spectra for various Cu2-xS phases have been carefully studied by Zhao et al. [18]. Based on their results, we assign the spectrum before annealing to the CuS phase and the spectrum after annealing to the Cu1.97S phase. To confirm the assignments, look at Fig. 2(b) which shows X-ray diffraction (XRD) patterns of the QD samples before and after annealing. The diffraction peaks are assigned to the CuS (JCPDS78-0880) and Cu1.97S (JCPDS 84-0209) phases, respectively. The stoichiometric phase Cu2S is thermodynamically unstable and difficult to synthesize under ambient reaction conditions [19]. The absorption and XRD results clearly show the phase transformation after annealing. For photovoltaic experiments, we first studied the CuS QD (the unannealed phase) cells. Table 1 lists the photovoltaic parameters of various QD cells. Fig. 3(a) shows some representative I-V curves of QD cells under 100 mW/cm2 illumination. The data for CuS cells (Sample nos. 1–3) indicate that the short-circuit current density Jsc, opencircuit voltage Voc and power conversion efficiency η all increase with the SILAR number n. The best η is 0.29% for CuS(11). When n ≥ 12 (not shown), η starts to decrease again. The results show that the optimum SILAR number is 11. The annealed Cu1.97S(11) cell (Sample no. 4) yields a significantly larger η of 0.51%, which is a 75% enhancement Table 1 Photovoltaic performance of Cu2 - xS-QD sensitized solar cells (x = 1, 0.03) with different SILAR cycles, ZnS coating and Pt counterelectrode.
Fig. 1. (a) TEM micrograph of a CuS(11)-QD-coated TiO2 film. (b) Optical absorption spectra of TiO2 electrodes with various CuS QDs. The number next to the curve denotes the SILAR number n.
Sample no.
Electrodes
Jsc (mA/cm2)
Voc (V)
FF (%)
η (%)
1 2 3 4 5 6
CuS(8) CuS(9) CuS(11) Cu1.97S(11) Cu1.97S(11)/ZnS Cu1.97S (11)/Pt
10.5 12.7 12.9 21.4 28.1 22.9
0.10 0.11 0.12 0.14 0.17 0.14
15.9 16.0 18.9 17.0 18.9 20.2
0.17 0.22 0.29 0.51 0.90 0.65
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of ~ 60% over the visible range of 400–600 nm, ~ 50% over the NIR range of 650–900 nm, and ~ 47% over the entire spectral range. The broad EQE range of 350–1100 nm is the most important result of this work. This range is two times broader than that of N3 dye (350– 700 nm). It is also much broader than that of the CdS (400–550 nm) and CdSe (400–700 nm) systems. The difference in η between the Cu1.97S(11) (Sample no. 4) and CuS(11) (Sample no. 3) cells can also be attributed to the difference in the spectral range. Cu1.97S (Eg = 1.2 eV) can absorb the solar range of 350–1100 nm whereas CuS (2 eV) can absorb 350–600 nm. The larger-range Cu1.97S cell yields a larger η. It is interesting to compare Cu1.97S to another semiconductor sensitizer, Ag2S [16], which has an Eg of 1.1 eV. Ag2S DSSCs have an η = 0.98%, close to that of Cu1.97S, but the characteristics are different. Ag2S has Jsc ~ 6 mA/cm 2 and Voc = 0.37 V, the Jsc is smaller and Voc is larger than the respective values of Cu1.97S. This difference is probably due to the different CB energy levels of the two systems. 4. Conclusion We have demonstrated Cu2-xS QD-sensitized liquid-junction solar cells, prepared by SILAR of Cu2-xS on TiO2 electrodes. The cells show respectable efficiencies over a broad spectral range covering visible and NIR. Cu1.97S yields better photovoltaic performance than CuS. Acknowledgment
Fig. 3. (a) Current–voltage curves of various Cu2-xS cells. (b) EQE spectra of Cu1.97S(11) cells before and after ZnS coating.
The authors are grateful for the financial support received from the National Science Council of the Republic of China. References
over the CuS(11) cell. The enhancement arises primarily from Jsc, which increases from 12.9 to 21.4 mA/cm 2, an increase of 66%. The coating of a ZnS layer produces a potential barrier at the QD surface, which blocks the photoelectrons in the CB of a QD from recombination with holes in the electrolyte. The addition of a ZnS coating to the Cu1.97S(11) cell (Sample no. 5) demonstrates the highest-η cell in this work. The best cell yields Jsc = 28.1 mA/cm 2, Voc = 0.17 V and η = 0.90%. Compared to the uncoated sample (Sample no. 4), the coating produces a 76% enhancement in η. This result indicates that the passivation treatment is effective in reducing recombination in Cu2-xS-QD DSSCs. The best η (0.90%) achieved herein is 15 times larger than that (0.06%) from ETA Cu2-xS cells [3]. We also studied the performance of cells using a Pt counterelectrode. When Au was replaced by Pt (Sample no. 6), the η decreased to 0.65%, significantly lower than that of the best cell. Fig. 3(b) shows the EQE spectra of Cu1.97S(11) cells with and without a ZnS coating (Sample nos. 4 and 5). The spectra cover the spectral range of 350–1100 nm. The cutoff at 1100 nm (1.1 eV) is consistent with Eg = 1.2 eV. The ZnS coating produces significant EQE enhancements, especially in the long-wavelength range (600– 1100 nm). From the onset of enhancement (600 nm), the ZnS barrier height is calculated to be ~ 0.8 eV. The best EQE has an average value
[1] M.K. Nazeeruddin, F.D. Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru, M. Grätzel, J. Am. Chem. Soc. 127 (2005) 16835. [2] A. Belaidi, T. Dittrich, D. Kieven, J. Tornow, K. Schwarzburg, M. Lux-Steiner, Phys. Status Solidi. (RRL) 2 (2008) 172. [3] M. Page, O. Niitsoo, Y. Itzhaik, D. Cahen, G. Hodes, Energy Environ. Sci. 2 (2009) 220. [4] R.D. Shaller, V.I. Klimov, Phys. Rev. Lett. 92 (2004) 186601. [5] L.M. Peter, D.J. Riley, E.Z. Tull, K.G.U. Wijayantha, Chem. Commun. (2002) 1030 (Cambridge). [6] I. Robel, V. Subramanian, M. Kuno, P.V. Kamat, J. Am. Chem. Soc. 128 (2006) 2385. [7] R. Plass, P. Serge, J. Krüger, M. Grätzel, J. Phys. Chem. B 106 (2002) 7578. [8] S. Gorer, G. Hodes, J. Phys. Chem. 98 (1994) 5338. [9] I. Moreels, K. Lambert, D. De Muynck, F. Vanhaecke, D. Poelman, J.C. Martins, G. Allan, Z. Hens, Chem. Mater. 19 (2007) 6101. [10] L.D. Partain, P.S. Mcleod, J.A. Duisman, T.M. Peterson, D.E. Sawyer, C.S. Dean, J. Appl. Phys. 54 (1983) 6708. [11] E. Vanhoecke, M. Burgelman, L. Anaf, Thin Solid Films 144 (1986) 223. [12] M.T.S. Nair, L. Guerrero, P.K. Nair, Semicond. Sci. Technol. 13 (1998) 1164. [13] L. Reijnen, B. Meester, A. Goossens, J. Schoonman, Chem. Vapor. Depos. 9 (2003) 15. [14] C. Nascu, I. Pop, V. Ionescu, E. Indrea, I. Bratu, Mater. Lett. 32 (1997) 73. [15] A. Martí, G.L. Araújo, Sol. Energy. Mater. Sol. Cells. 43 (1996) 203. [16] A. Tubtimtae, K.L. Wu, H.Y. Tung, M.W. Lee, G.J. Wang, Electrochem. Commun. 12 (2010) 1158. [17] T. Kuzuya, K. Itoh, K. Sumiyama, J. Colloid. Interf. Sci. 319 (2008) 565. [18] Y. Zhao, H. Pan, Y. Lou, X. Qiu, J. Zhu, C. Burda, J. Am. Chem. Soc. 131 (2009) 4253. [19] P. Lukashev, W.R.L. Lambrecht, T. Kotani, M. van Schilfgaarde, Phys. Rev. B 76 (2007) 195202.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Cu2-xS quantum dot-sensitized solar cells Mei-Chia Lin, 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 16 June 2011 Received in revised form 4 August 2011 Accepted 6 August 2011 Available online 12 August 2011 Keywords: Quantum dots Solar cells Copper sulfide Successive ionic layer adsorption and reaction Sensitizer
a b s t r a c t In this study we report on the photovoltaic performance of Cu2-xS (x = 1, 0.03) quantum-dot (QD) sensitized liquid-junction solar cells. The QDs were grown with successive ionic layer adsorption and reaction. The CuS phase in the as-grown QDs was transformed into Cu1.97S after annealing. The assembled Cu1.97S cells yielded a best power conversion efficiency η of 0.90% under illumination of 100 mW/cm2. The CuS cell had an η ~ 75% lower than that of Cu1.97S. The external quantum efficiency (EQE) spectrum of Cu1.97S covers the spectral range of 350–1100 nm with a maximal EQE of 63% at λ = 500 nm and an average EQE of 47% over the entire spectral range. This spectral range is also two times broader than that of N3 dye. The results show that Cu2-xS QDs can be used as highly efficient broadband sensitizers for solar cells. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSSCs) are a promising low-cost alternative to silicon-based renewable photovoltaics. The key component of a DSSC is a photoelectrode comprising of a nanoporous, 10 μm-thick TiO2 film sintered to fluorine-doped tin oxide (FTO) glass. A monolayer of organic dye is coated on the TiO2 surface for light harvesting. The three-dimensional nanoporous structure greatly increases the surface of dye adsorption, resulting in an impressive power conversion efficiency of ~ 11% [1]. Further improvement in efficiency has been hampered by the incomplete overlap of the dye absorption spectrum with the solar spectrum. The solar spectrum covers the spectral range of 350–2500 nm. However, the most commonly used organic dyes, N3 and N719, have strong absorption in the visible range of 350–700 nm but only weak absorption in the IR. The search for sensitizers with broader absorption ranges is crucial for enhancing DSSC efficiency. A successful candidate is sensitizers based on inorganic semiconductor nanomaterials. Extremely thin absorbers (ETA) semiconductors have been successfully used as sensitizers for DSSCs [2,3]. Semiconductor quantum-dots (QDs) (CdS, CdSe, PbS, and PbSe) have also been successfully employed [4–7]. QD sensitizers have three advantages over organic dyes, tunable absorption bands [8], large extinction coefficients and multiple electron-hole pair generation [4,9]. Copper sulfides (CuxS) are important materials for applications in p-type semiconductors and optoelectronics [10]. CdS/Cu2S heterojunctions have been used as solar absorbers [11]. There are five stable
⁎ Corresponding author. Tel.: + 886 422852783; fax: + 886 22862534. E-mail address: [email protected] (M.-W. Lee). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.08.013
CuxS phases for 1 ≤ x ≤ 2: CuS, Cu1.75S, Cu1.8S, Cu1.97S, and Cu2S. The band gap of CuxS depends on the oxidation state of Cu (Eg = 2.0, 1.5 and 1.2 eV for CuS, Cu1.8S and Cu1.97S, respectively) [12–14]. The most attractive phase is Cu1.97S, whose Eg is close to the optimum Eg (1.1 eV) for a photovoltaic device [15], suggesting that Cu1.97S can be an ideal absorber material. Recently Page et al. demonstrated an ETA solar cell using Cu2-xS as a light absorber [3], achieving a power conversion efficiency of 0.06%. Here we demonstrate liquid-junction solar cells sensitized with Cu2-xS QDs. We study the effects of the SILAR number, passivation coating and counterelectrode on the photovoltaic properties. The Cu2-xS DSSCs are found to exhibit a broad absorption range and yield efficiencies much higher than that of the ETA solar cells. 2. Experimental Nanoporous TiO2 films were prepared by spreading a TiO2 paste (Dyesol DSL-18NR-T, particle size ~20 nm) onto an FTO glass substrate (15 Ω/□) using the procedure described previously [16]. The TiO2 film had a three-layer structure: a blocking layer (thickness~ 90 nm), a TiO2 film (thickness ~10–12 μm) for QD coating, and a TiO2 scattering layer (3-μm thick, 400 nm in size) on the top [16]. The blocking layer, prepared using a spin-coated titanium isopropoxide ethanol solution, reduced the carrier recombination at the FTO/TiO2 interface. The scattering layer increased the light scattering and harvesting. The thickness was determined from cross-sectional transmission electron microscopic (TEM) images. Cu2-xS QDs were synthesized using the successive ionic layer adsorption and reaction (SILAR) process. A TiO2 film was first dipped into the 25 °C, 0.02 M Cu(NO3)2 ethanol solution for 1 min, before being rinsed with ethanol, then heated at 75 °C. The film was then
M.-C. Lin, M.-W. Lee / Electrochemistry Communications 13 (2011) 1376–1378
1377
dipped into the 25 °C, 0.02 M Na2S methanol solution for 1 min, washed with methanol, then heated at 75 °C. This two-step dipping process formed one SILAR cycle. Samples that went through n SILAR cycles are referred to as Cu2-xS(n). The QD-coated TiO2 electrode was assembled with an Au-coated FTO counterelectrode and sealed with a 190 μm parafilm spacer [16]. The Au counterelectrode was prepared by sputtering a 50 nm-thick Au film on FTO glass. The polysulfide electrolyte consisted of 0.25 M Na2S, 1 M S, 0.1 M KI and 0.2 M KCl in methanol/water (7:3 by volume). The absorption spectra were then recorded using a Hitachi 2800A spectrophotometer. The current–voltage (I-V) curves were recorded with a Keithley 2400 source meter with an Oriel 150 W Xe lamp. An Oriel band pass filter simulated the AM 1.5 solar spectrum. The external quantum efficiency (EQE) spectra were measured using an Acton monochromator with a 250 W tungsten–halogen lamp. The active area of the cell was 3 mm× 3 mm. A metal mask of 3.5 mm× 3.5 mm was placed above the cell during measurements. 3. Results and discussion Fig. 1(a) displays a TEM image of a Cu2-xS(11)-QD coated TiO2 film. Many QDs can be seen to deposit randomly over the TiO2 surface. The average diameter of the QDs is ~ 3–4 nm. No change in QD size is observed after annealing. Fig. 1(b) shows the absorption spectra of Cu2-xS QD-coated TiO2 electrodes with various SILAR cycles. The spectra exhibit two absorption bands: the first at ~400 nm and the second (near IR-NIR) at ~ 800–1100 nm. The absorption intensity increases with the SILAR number n. The increasing absorption indicates an increasing amount of QDs coating on the TiO2 surface as n is increased. The first band is attributed to the fundamental transition (valence band to conduction band-CB) of Cu2-xS. The NIR band is attributed to an impurity state [17], or a valence-band free-
Fig. 2. (a) Absorption spectra, and (b) XRD patterns, of QDs before (CuS phase) and after (Cu1.97S phase) annealing at 400 °C.
carrier absorption [18]. When the sample was annealed at 400 °C for three min, as Fig. 2(a) shows, the second absorption band disappeared. The color also changed from dark green to light brown (inset). The absorption spectra for various Cu2-xS phases have been carefully studied by Zhao et al. [18]. Based on their results, we assign the spectrum before annealing to the CuS phase and the spectrum after annealing to the Cu1.97S phase. To confirm the assignments, look at Fig. 2(b) which shows X-ray diffraction (XRD) patterns of the QD samples before and after annealing. The diffraction peaks are assigned to the CuS (JCPDS78-0880) and Cu1.97S (JCPDS 84-0209) phases, respectively. The stoichiometric phase Cu2S is thermodynamically unstable and difficult to synthesize under ambient reaction conditions [19]. The absorption and XRD results clearly show the phase transformation after annealing. For photovoltaic experiments, we first studied the CuS QD (the unannealed phase) cells. Table 1 lists the photovoltaic parameters of various QD cells. Fig. 3(a) shows some representative I-V curves of QD cells under 100 mW/cm2 illumination. The data for CuS cells (Sample nos. 1–3) indicate that the short-circuit current density Jsc, opencircuit voltage Voc and power conversion efficiency η all increase with the SILAR number n. The best η is 0.29% for CuS(11). When n ≥ 12 (not shown), η starts to decrease again. The results show that the optimum SILAR number is 11. The annealed Cu1.97S(11) cell (Sample no. 4) yields a significantly larger η of 0.51%, which is a 75% enhancement Table 1 Photovoltaic performance of Cu2 - xS-QD sensitized solar cells (x = 1, 0.03) with different SILAR cycles, ZnS coating and Pt counterelectrode.
Fig. 1. (a) TEM micrograph of a CuS(11)-QD-coated TiO2 film. (b) Optical absorption spectra of TiO2 electrodes with various CuS QDs. The number next to the curve denotes the SILAR number n.
Sample no.
Electrodes
Jsc (mA/cm2)
Voc (V)
FF (%)
η (%)
1 2 3 4 5 6
CuS(8) CuS(9) CuS(11) Cu1.97S(11) Cu1.97S(11)/ZnS Cu1.97S (11)/Pt
10.5 12.7 12.9 21.4 28.1 22.9
0.10 0.11 0.12 0.14 0.17 0.14
15.9 16.0 18.9 17.0 18.9 20.2
0.17 0.22 0.29 0.51 0.90 0.65
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M.-C. Lin, M.-W. Lee / Electrochemistry Communications 13 (2011) 1376–1378
of ~ 60% over the visible range of 400–600 nm, ~ 50% over the NIR range of 650–900 nm, and ~ 47% over the entire spectral range. The broad EQE range of 350–1100 nm is the most important result of this work. This range is two times broader than that of N3 dye (350– 700 nm). It is also much broader than that of the CdS (400–550 nm) and CdSe (400–700 nm) systems. The difference in η between the Cu1.97S(11) (Sample no. 4) and CuS(11) (Sample no. 3) cells can also be attributed to the difference in the spectral range. Cu1.97S (Eg = 1.2 eV) can absorb the solar range of 350–1100 nm whereas CuS (2 eV) can absorb 350–600 nm. The larger-range Cu1.97S cell yields a larger η. It is interesting to compare Cu1.97S to another semiconductor sensitizer, Ag2S [16], which has an Eg of 1.1 eV. Ag2S DSSCs have an η = 0.98%, close to that of Cu1.97S, but the characteristics are different. Ag2S has Jsc ~ 6 mA/cm 2 and Voc = 0.37 V, the Jsc is smaller and Voc is larger than the respective values of Cu1.97S. This difference is probably due to the different CB energy levels of the two systems. 4. Conclusion We have demonstrated Cu2-xS QD-sensitized liquid-junction solar cells, prepared by SILAR of Cu2-xS on TiO2 electrodes. The cells show respectable efficiencies over a broad spectral range covering visible and NIR. Cu1.97S yields better photovoltaic performance than CuS. Acknowledgment
Fig. 3. (a) Current–voltage curves of various Cu2-xS cells. (b) EQE spectra of Cu1.97S(11) cells before and after ZnS coating.
The authors are grateful for the financial support received from the National Science Council of the Republic of China. References
over the CuS(11) cell. The enhancement arises primarily from Jsc, which increases from 12.9 to 21.4 mA/cm 2, an increase of 66%. The coating of a ZnS layer produces a potential barrier at the QD surface, which blocks the photoelectrons in the CB of a QD from recombination with holes in the electrolyte. The addition of a ZnS coating to the Cu1.97S(11) cell (Sample no. 5) demonstrates the highest-η cell in this work. The best cell yields Jsc = 28.1 mA/cm 2, Voc = 0.17 V and η = 0.90%. Compared to the uncoated sample (Sample no. 4), the coating produces a 76% enhancement in η. This result indicates that the passivation treatment is effective in reducing recombination in Cu2-xS-QD DSSCs. The best η (0.90%) achieved herein is 15 times larger than that (0.06%) from ETA Cu2-xS cells [3]. We also studied the performance of cells using a Pt counterelectrode. When Au was replaced by Pt (Sample no. 6), the η decreased to 0.65%, significantly lower than that of the best cell. Fig. 3(b) shows the EQE spectra of Cu1.97S(11) cells with and without a ZnS coating (Sample nos. 4 and 5). The spectra cover the spectral range of 350–1100 nm. The cutoff at 1100 nm (1.1 eV) is consistent with Eg = 1.2 eV. The ZnS coating produces significant EQE enhancements, especially in the long-wavelength range (600– 1100 nm). From the onset of enhancement (600 nm), the ZnS barrier height is calculated to be ~ 0.8 eV. The best EQE has an average value
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