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Successive ionic layer adsorption and reaction deposited kesterite Cu2ZnSnS4 nanoflakes counter electrodes for efficient dye-sensitized solar cells
Materials Research Bulletin 59 (2014) 249–253
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Successive ionic layer adsorption and reaction deposited kesterite Cu2ZnSnS4 nanoflakes counter electrodes for efficient dye-sensitized solar cells Sawanta S. Mali, Chang Su Shim, Chang Kook Hong * Advanced Chemical Engineering Department, Chonnam National University, Gwangju 500-757, South Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 May 2014 Received in revised form 14 July 2014 Accepted 16 July 2014 Available online 19 July 2014
In this investigation, we have successfully synthesized Cu2ZnSnS4 (CZTS) nanoflakes by successive ionic layer adsorption and reaction (SILAR) method and used as a counter electrode in the hydrothermally grown TiO2 based dye sensitized solar cells (DSSCs). The prepared CZTS nanoflakes were characterized using X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), micro Raman spectroscopy and energy dispersive spectroscopic (EDS) analysis. Our DSSCs results revealed that, compared with conventional Pt/FTO counter electrode DSSCs, nanoflakes of p-type CZTS as the photocathode and n-type TiO2 thin films as the photoanode shows an increased short circuit current (13.35 mA/cm2) with 4.84% power conversion efficiency. The detailed interface properties of were analyzed by electrochemical impedance spectroscopy (EIS) measurements. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis C. X-ray diffraction D. Electrochemical properties
1. Introduction Gratzel solar cell is one of the most promising and cost effective photovoltaic technology based on photoelectrochemical cell principle [1]. The dye loaded mesoscopic nanoparticulate or 1D, 3D TiO2 film [2] or ZnO nanostructures photoelectrodes [3] catalytic platinum coated transparent conducting oxide (Pt/TCO) and iodide/triiodide (I/I3) redox couple are the key parts of dye sensitized solar cells (DSSCs). In the operation, the sunlight is absorbed by the dye molecules which excites electrons from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO), and the excited-state electrons are quickly injected into the conduction band (CB) of TiO2 or ZnO. These electrons percolate through the TiO2 film and are collected by the conducting substrate. The original state of the dye is subsequently restored by electron donation from the electrolyte, usually an organic solvent containing redox system, such as the iodide/triiodide (I/I3) redox couple. The regeneration of the sensitizer by iodide intercepts the recapture of the CB electron by the oxidized dye. The iodide is regenerated in turn by the reduction of triiodide at the counter electrode, the circuit being completed via electron migration through the external load. The voltage generated under illumination corresponds to the difference between the Fermi level of the electron in the solid and the redox potential of the electrolyte.
* Corresponding author. Tel.: +82 62 530 0635; fax: +82 62 530 1849. E-mail address: [email protected] (C.K. Hong). http://dx.doi.org/10.1016/j.materresbull.2014.07.024 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
Recently, novel approaches have been implemented in DSSCs technology like the combination of p-type DSSCs and n-type DSSCs to fabricate CZTS counter electrode based dye sensitized solar cells (DSSCs) with a theoretical efficiency limitation well beyond that of single junction DSSCs. So far, n-type DSSC are based on mesoporous TiO2 as the photoanode has a considerable 12% conversion efficiency [4]. On the other hand there is no substantial development in counter electrodes. The selection of suitable dye and semiconductor are the difficult task [5]. The earth abundant quaternary p-type semiconducting materials open a new approach in development of DSSC. The p-type Cu2ZnSnS4 (CZTS), a I2–II–IV– VI4 quaternary compound semiconductor can be used as a potential alternative to expensive CIS absorber layer in thin film solar cells. Wherein ternary CIS compound selenium (Se) is substituted with sulfur (S), the rare metal indium (In) with zinc (Zn) and tin (Sn). All the constituents required for CZTS synthesis are low-cost, abundantly available in the earth crust and are notably non-toxic. The CZTS exhibits a kesterite-type crystal structure. Because of high optical absorption coefficient (>104 cm1) and an ideal direct band gap (1.4–1.5 eV), CZTS has been regarded as one of the most promising materials for lightharvesting p-type materials in solar cells [6–8]. Narrow band gap p-type semiconducting materials opened a new way for the DSSC. Recently, Xin et al. developed low cost synthesis method of CZTS as a counter electrode for DSSC application and demonstrated 7.37% conversion efficiency [9]. Moreover, Ahmad et al. developed nanoporous poly(3,4-propylenedioxythiophene) (PProDOT) based counter electrodes were
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prepared from hydrophobic ionic liquid medium and demonstrated 9% conversion efficiency [10]. Recently, metal oxides/nitrites (NiO, ZrN, ZnO) as well as PEDOT and graphene thin films have been successfully used as counter electrodes for DSSCs [11–15]. The activated carbon [16], carbon nanotubes [17], carbon black [18] and polyvinyl pyrrolidone with platinium [19] have also been used counter electrodes. Recently different nanostructured CZTS like porous, nanofibrous CZTS was used as counter electrodes for DSSC and showed 1.23 and 3.9% conversion efficiency respectively [20,21]. In the present article, we have prepared nanostructured anatase TiO2 photoelectrodes by hydrothermal route [22,23] and used for DSSC application. The phase confirmation was carried out by X-ray diffraction (XRD) and Raman spectroscopy. The CZTS nanoflakes were synthesized by successive ionic layer adsorption and reaction (SILAR) technique [7,8]. The kesterite CZTS thin films were characterized by XRD, FESEM and energy-dispersive X-ray spectroscopy (EDX) and micro-Raman spectroscopic analysis. The photovoltaic propertied were studied with Pt/FTO and CZTS/FTO based counter electrodes. 2. Experimental
Scheme 1. Schematic representation of N3-dye/TiO2 photoanode and CZTS nanoflakes as a counter electrode.
2.1. Synthesis of the CZTS nanoflakes thin films
3. Results and discussion
The uniform and adherent CZTS nanoflakes thin films were deposited using our previous reported SILAR method with some modifications. FTO coated glass substrate was sequentially immersed in the cationic (Cu, Zn, Sn) precursor solution and the anionic (Na2S) precursor solution. The film was rinsed with water between immersions. The black colored CZTS thin film was further annealed at 450 C in nitrogen atmosphere.
Fig. 1(a) shows the FESEM image of anatase nanostructured TiO2 thin film. The FESEM image of the TiO2 at a magnification of 50 k and shows the spherical nanoflowers containing nanorods are uniform in nature. Fig. 1(b) shows FESEM image of the sample CZTS
2.2. Synthesis of the TiO2 thin film by hydrothermal process and DSSC device fabrication In a typical experiment, hydrothermal inorganic precursor solution was prepared by mixing 0.05 M TiCl4 carefully with absolute ethanol in ice-cold bath forming yellow colored titanium chloroalkoxide [TiCl2(OEt)2] solution. The transparent solution further mixed with saturated 6.8 M NaCl and transferred in sealed autoclave. The reaction was maintained at 120 C for 3 h. The deposited TiO2 sample was treated with aqueous TiCl4 before the dye and annealing at 500 C in air for 30 min. Subsequently, the TiCl4 treated TiO2 thin films were immersed in ethanolic solution of a 0.5 mM N719 dye for 24 h in dark to allow for sufficient dye adsorption. In present study we have used Di-tetrabutylammonium cis–bis (isothiocyanato) bis (2,20 -bipyridyl-4,40 -dicarboxylato) ruthenium(II) 95% (NMR) (N-719 dye, Sigma–Aldrich). The DSSC was fabricated using a standard two electrode configuration, comprising dye loaded Glass/FTO/TiO2 as the photoanode and platinum (Pt) or Cu2ZnSnS4 (CZTS) deposited on FTO coated substrates as the counter electrode, which is sealed with the working electrode using a thermoplastic (1 mm). The Pt/FTO counter electrode was prepared by commercial Pt paste (Dyesol) followed by heating at 400 C for 30 min in air. The iodide-based electrolyte (Dyesol) was used as redox electrolyte and was injected into the inter-electrode space from the counter electrode side through a pre-drilled hole. The following cell configurations were used for photovoltaic measurments: Glass=FTO=TiO2 =N719 Dye=I =I 3 =Pt=FTO
(1)
Glass=FTO=TiO2 =N719 Dye=I =I 3 =CZTS=FTO
(2)
Compact and sealed DSSCs were fabricated using above standard two electrode configurations. The device configuration of CZTS based counter electrode is shown in Scheme 1.
Fig. 1. (a) FESEM images of TiO2 sample, (b) SEM image of CZTS nanoflakes.
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Fig. 2. (a–c) Cu Ka1, Zn Ka1, Sn La1, and SK a1 elemental EELS mappings of a CZTS nanoflakes. The fright-field image and corresponding elemental analysis of CZTS nanoflakes.
thin film with uniform distribution of the nanoflakes within welldefined petals. The average size of single flake is 1.1 mm and wall thickness is 80 nm. The CZTS nanoflakes were further characterized by energy dispersive X-ray spectroscopy (EDS) mappings. Fig. 2 shows typical Cu, Zn, Sn, and S elemental mapping data of a CZTS nanoflakes. The mapping results show that Cu, Zn, Sn, and S are homogeneously distributed throughout the nanoflakes (Fig. 2 (a–d)). Fig. 2(e) shows Cu, Zn, Sn, and S elemental EDS mappings of nanoflakes indicating homogeneous distribution of the all four constituent elements throughout the samples. The EDS elemental results reveal that the atomic ratio of Cu:Zn:Sn:S samples is about 24:11:11:50, indicating copper-rich CZTS. Fig. 3 shows the indexed XRD pattern for the as synthesized CZTS nanoflakes annealed at 450 C in nitrogen atmosphere. The lattice parameters (a = b = 5.4406 Å, c = 10.8578 Å) calculated from the X-ray diffraction pattern were matched to the JCPDS card 00-026-0575: a = b = 5.4270 Å, c = 10.8480 Å. The product obtained after annealing exhibited four strong peaks at 2u = 28.5 , 47.3 , and 56.2 and four weak peaks at 2u = 32.36 , 58.02 , 69.44 and 76.79 , which were indexed to diffraction of the (11 2), (2 0 0), (2 2 0), (3 1 2), (0 0 8) and (3 3 2) planes, respectively, of the tetragonal CZTS phase with cell constants a = 5.4270 Å and c = 10.8480 Å (JCPDS 26-0575) [24].
Fig. 4 illustrates the typical Raman spectrum of the CZTS nanoflakes annealed at 450 C. From the both spectrum, one can evidently observe a single peak at about 333.5 cm1, which corresponds to quaternary CZTS. This result is in good agreement with the Raman spectra of single phase Cu2ZnSnS4 [21]. In addition, there are no additional peaks related to the presence of other compounds, which means that the single phase CZTS film was obtained. Fig. 5 shows the current density–voltage (J–V) curves of Pt/FTO and CZTS/FTO counter electrode based DSSC devices under dark and illumination. The DSSC based on Pt counter electrode produced conversion efficiency (h%) of 3.82% with short circuit current density (JSC) = 9.35 mA cm2, open circuit voltage (VOC) 0.649 V and fill factor (FF) 0.61. The DSSCs based on CZTS counter electrode shows 4.84% power conversion efficiency (PCE) (JSC = 13.35 mA cm2, VOC = 0.595 V, FF = 0.63). The TiO2 morphology is useful for the effective dye loading and can be increase the light harvesting within the electrode and thus enhance the photoncapture probability. Also it works as a ‘pathway’ for electrolyte diffusion throughout a TiO2films (Fig. 5). The solar cell parameters based of CZTS and Pt based counter electrodes are summarized in Table 1. In ordinary DSSCs the VOC is determined by the Fermi level around the CB of nanostructured TiO2 and oxidation/reduction
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S.S. Mali et al. / Materials Research Bulletin 59 (2014) 249–253 Table 1 Photovoltaic performance of DSSCs with different counter electrodes. Counter electrode
JSCa (mA/cm2)
VOC (V)
RS (V/cm2)
RSh (V/cm2)
FF
Pt CZTS
9.35 13.35
0.649 0.595
114 98
8727 5623
0.61 0.63
a
Fig. 3. XRD pattern of deposited CZTS thin film onto glass substrate by using SILAR technique.
potential of redox couple I/I3. While in case of CZTS counter electrode DSSCs the VOC is determined by the quasi-Fermi levels above the valence band (VB) of CZTS and below the CB of TiO2 [14]. Therefore, the VOC of the CZTS counter electrode DSSCs solar cell is lower than that of the traditional DSSCs solar cell, which is due to the decrease of VOC from 0.649 V to 0.595 V. In case of CZTS/FTO based counter electrode the JSC increased significantly from 9.35 mA/cm2 to 13.35 mA/cm2. From the table it is clear that CZTS/FTO based DSSC device shows lowest series resistance (RS)
Fig. 4. Micro-Raman Spectrum of CZTS nanoflakes.
h (%) 3.82 4.84
Devices were tested at room temperature with 0.25 cm2 active area.
98 V/cm2 compared to both Pt/FTO (114 V/cm2) counter electrodes. It is well known that the JSC of the DSSC depends on the RS of the devices. A smaller RS is favourable to a higher JSC and can be calculated using inverse slop of the J–V curves. Electrochemical impedance spectroscopy (EIS) has been used to study the interfaces, carrier transport, electronic and ionic processes and recombination in Pt and CZTS counter electrode based DSSCs devices. In order to confirm the effect of counter electrodes on the conversion efficiency of TiO2 thin films, EIS measurement was conducted to the Pt/FTO and CZTS/FTO based counter electrodes. Fig. 6(a) shows the Impedance spectrum of Pt/ FTO counter electrode based DSSCs. The measured impedance spectra data for TiO2–dye/electrolyte/Pt was fitted with the equivalent circuit given in inset of Fig. 6(a). The (R3, Q2) represents the electrode/electrolyte interface and R1 is the shunt resistance in parallel with the TiO2–dye structure represented by W1, R2, C1. R3 is the charge transfer resistance and Q2 is the double layer capacitance. While W1 and R2 represent the diffusion resistance and series resistance of the TiO2–dye structure, C2 represents the TiO2–dye junction capacitance [25]. Fit values indicated that R2 is lesser (and R1 higher) for CZTS/FTO counter electrode compared to Pt/FTO counter electrode. The product of the series resistance R2 and the capacitance C2 (at the TiO2/dye interface corresponds to the electron lifetime in the device [26]. There are differences in the series resistance (R2) values of both devices, where R2 of the Pt/FTO device is more than that CZTS/FTO (R2 = 238 and 195 V cm2 for devices with Pt/FTO and CZTS/FTO measured at 0.7 V, respectively). Lower R2 for CZTS/FTO electrode results in higher fill factor, thereby increasing efficiency when compared to Pt/FTO based device.
Fig. 5. J–V curves for (a) the Pt/FTO counter electrode (b) CZTS/FTO counter electrode.
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References
Fig. 6. Impedance spectra of DSSCs samples (a) Pt/FTO counter electrode and (b) CZTS/FTO counter electrode, measured in the dark under 0.70 V applied bias. Inset shows its respective equivalent circuits.
4. Conclusions In summary, we have successfully demonstrated SILAR deposited p-type CZTS quaternary materials with novel nanoflakes like morphology beneficial for efficient DSSCs. We have also demonstrated CZTS/FTO counter electrodes for TiO2 DSSCs shows good enhancement as compared to Pt/FTO. The highest solar energy conversion efficiency of 4.84% has been achieved for CZTS/ FTO based device. Such type of counter electrodes will be best alternative counter electrodes for Gratzel solar cells. Acknowledgment This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2009-0094055).
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Murata, Highperformance carbon counter electrode for dye-sensitized solar cells, Sol. Energy Mater. Sol. Cell 79 (2003) 459–469. [17] K. Suzuki, M. Yamamoto, M. Kumagai, S. Yanagida, Application of carbon nanotubes to counter electrodes of dye-sensitized solar cells, Chem Lett. 32 (2003) 28–29. [18] N. Ikeda, K. Teshima, T. Miyasaka, Conductive polymer–carbon–imidazolium composite: a simple means for constructing solid-state dye-sensitized solar cells, Chem. Commun. 2006 (2006) 1733–1735. [19] T.C. Wei, C.C. Wan, Y.Y. Wang, Poly(-vinyl-2-pyrrolidone)-capped platinum nanoclusters on indium-tin oxide glass as counterelectrode for dye-sensitized solar cells, Appl. Phys. Lett. 88 (2006) 103122. [20] P. Dai, G. Zhang, Y. Chen, H. Jiang, Z. Feng, Z. Lin, J. Zhan, Porous copper zinc tin sulfide thin film as photocathode for double junction photoelectrochemical solar cells, Chem. Commun. 48 (2012) 3006–3008. [21] S.S. Mali, P.S. Patil, C.K. Hong, Low-cost electrospun highly crystalline kesterite Cu2ZnSnS4 nanofiber counter electrodes for efficient dye-sensitized solar cells, ACS Appl. Mater. Interfaces 6 (2014) 1688–1696. [22] S.S. Mali, P.S. Shinde, C.A. Betty, P.N. Bhosale, W.J. Lee, P.S. Patil, Nanocoral architecture of TiO2 by hydrothermal process: synthesis and characterization, Appl. Surf. Sci. 257 (2011) 9737–9746. [23] S.S. Mali, C.A. Betty, P.N. Bhosale, P.S. Patil, Eosin-Y and N3-dye sensitized solar cells (DSSCs) based on novel nanocoral TiO2: a comparative study, Electrochim. Acta 59 (2012) 113–120. [24] S.S. Mali, H.J. Kim, C.S. Shim, S.P. Patil, C.K. Hong, Polyvinylpyrrolidone (PVP) assisted single-step synthesis of kesterite Cu2ZnSnS4 nanoparticles by solvothermal process, Phys. Status Solidi RRL 7 (2013) 1050–1054. [25] P. Dai, G. Zhang, Y. Chen, H. Jiang, Z. Feng, Z. Lin, J. Zhan, Porous copper zinc tin sulfide thin film as photocathode for double junction photoelectrochemical solar cells, Chem. 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Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Successive ionic layer adsorption and reaction deposited kesterite Cu2ZnSnS4 nanoflakes counter electrodes for efficient dye-sensitized solar cells Sawanta S. Mali, Chang Su Shim, Chang Kook Hong * Advanced Chemical Engineering Department, Chonnam National University, Gwangju 500-757, South Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 May 2014 Received in revised form 14 July 2014 Accepted 16 July 2014 Available online 19 July 2014
In this investigation, we have successfully synthesized Cu2ZnSnS4 (CZTS) nanoflakes by successive ionic layer adsorption and reaction (SILAR) method and used as a counter electrode in the hydrothermally grown TiO2 based dye sensitized solar cells (DSSCs). The prepared CZTS nanoflakes were characterized using X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), micro Raman spectroscopy and energy dispersive spectroscopic (EDS) analysis. Our DSSCs results revealed that, compared with conventional Pt/FTO counter electrode DSSCs, nanoflakes of p-type CZTS as the photocathode and n-type TiO2 thin films as the photoanode shows an increased short circuit current (13.35 mA/cm2) with 4.84% power conversion efficiency. The detailed interface properties of were analyzed by electrochemical impedance spectroscopy (EIS) measurements. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis C. X-ray diffraction D. Electrochemical properties
1. Introduction Gratzel solar cell is one of the most promising and cost effective photovoltaic technology based on photoelectrochemical cell principle [1]. The dye loaded mesoscopic nanoparticulate or 1D, 3D TiO2 film [2] or ZnO nanostructures photoelectrodes [3] catalytic platinum coated transparent conducting oxide (Pt/TCO) and iodide/triiodide (I/I3) redox couple are the key parts of dye sensitized solar cells (DSSCs). In the operation, the sunlight is absorbed by the dye molecules which excites electrons from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO), and the excited-state electrons are quickly injected into the conduction band (CB) of TiO2 or ZnO. These electrons percolate through the TiO2 film and are collected by the conducting substrate. The original state of the dye is subsequently restored by electron donation from the electrolyte, usually an organic solvent containing redox system, such as the iodide/triiodide (I/I3) redox couple. The regeneration of the sensitizer by iodide intercepts the recapture of the CB electron by the oxidized dye. The iodide is regenerated in turn by the reduction of triiodide at the counter electrode, the circuit being completed via electron migration through the external load. The voltage generated under illumination corresponds to the difference between the Fermi level of the electron in the solid and the redox potential of the electrolyte.
* Corresponding author. Tel.: +82 62 530 0635; fax: +82 62 530 1849. E-mail address: [email protected] (C.K. Hong). http://dx.doi.org/10.1016/j.materresbull.2014.07.024 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
Recently, novel approaches have been implemented in DSSCs technology like the combination of p-type DSSCs and n-type DSSCs to fabricate CZTS counter electrode based dye sensitized solar cells (DSSCs) with a theoretical efficiency limitation well beyond that of single junction DSSCs. So far, n-type DSSC are based on mesoporous TiO2 as the photoanode has a considerable 12% conversion efficiency [4]. On the other hand there is no substantial development in counter electrodes. The selection of suitable dye and semiconductor are the difficult task [5]. The earth abundant quaternary p-type semiconducting materials open a new approach in development of DSSC. The p-type Cu2ZnSnS4 (CZTS), a I2–II–IV– VI4 quaternary compound semiconductor can be used as a potential alternative to expensive CIS absorber layer in thin film solar cells. Wherein ternary CIS compound selenium (Se) is substituted with sulfur (S), the rare metal indium (In) with zinc (Zn) and tin (Sn). All the constituents required for CZTS synthesis are low-cost, abundantly available in the earth crust and are notably non-toxic. The CZTS exhibits a kesterite-type crystal structure. Because of high optical absorption coefficient (>104 cm1) and an ideal direct band gap (1.4–1.5 eV), CZTS has been regarded as one of the most promising materials for lightharvesting p-type materials in solar cells [6–8]. Narrow band gap p-type semiconducting materials opened a new way for the DSSC. Recently, Xin et al. developed low cost synthesis method of CZTS as a counter electrode for DSSC application and demonstrated 7.37% conversion efficiency [9]. Moreover, Ahmad et al. developed nanoporous poly(3,4-propylenedioxythiophene) (PProDOT) based counter electrodes were
250
S.S. Mali et al. / Materials Research Bulletin 59 (2014) 249–253
prepared from hydrophobic ionic liquid medium and demonstrated 9% conversion efficiency [10]. Recently, metal oxides/nitrites (NiO, ZrN, ZnO) as well as PEDOT and graphene thin films have been successfully used as counter electrodes for DSSCs [11–15]. The activated carbon [16], carbon nanotubes [17], carbon black [18] and polyvinyl pyrrolidone with platinium [19] have also been used counter electrodes. Recently different nanostructured CZTS like porous, nanofibrous CZTS was used as counter electrodes for DSSC and showed 1.23 and 3.9% conversion efficiency respectively [20,21]. In the present article, we have prepared nanostructured anatase TiO2 photoelectrodes by hydrothermal route [22,23] and used for DSSC application. The phase confirmation was carried out by X-ray diffraction (XRD) and Raman spectroscopy. The CZTS nanoflakes were synthesized by successive ionic layer adsorption and reaction (SILAR) technique [7,8]. The kesterite CZTS thin films were characterized by XRD, FESEM and energy-dispersive X-ray spectroscopy (EDX) and micro-Raman spectroscopic analysis. The photovoltaic propertied were studied with Pt/FTO and CZTS/FTO based counter electrodes. 2. Experimental
Scheme 1. Schematic representation of N3-dye/TiO2 photoanode and CZTS nanoflakes as a counter electrode.
2.1. Synthesis of the CZTS nanoflakes thin films
3. Results and discussion
The uniform and adherent CZTS nanoflakes thin films were deposited using our previous reported SILAR method with some modifications. FTO coated glass substrate was sequentially immersed in the cationic (Cu, Zn, Sn) precursor solution and the anionic (Na2S) precursor solution. The film was rinsed with water between immersions. The black colored CZTS thin film was further annealed at 450 C in nitrogen atmosphere.
Fig. 1(a) shows the FESEM image of anatase nanostructured TiO2 thin film. The FESEM image of the TiO2 at a magnification of 50 k and shows the spherical nanoflowers containing nanorods are uniform in nature. Fig. 1(b) shows FESEM image of the sample CZTS
2.2. Synthesis of the TiO2 thin film by hydrothermal process and DSSC device fabrication In a typical experiment, hydrothermal inorganic precursor solution was prepared by mixing 0.05 M TiCl4 carefully with absolute ethanol in ice-cold bath forming yellow colored titanium chloroalkoxide [TiCl2(OEt)2] solution. The transparent solution further mixed with saturated 6.8 M NaCl and transferred in sealed autoclave. The reaction was maintained at 120 C for 3 h. The deposited TiO2 sample was treated with aqueous TiCl4 before the dye and annealing at 500 C in air for 30 min. Subsequently, the TiCl4 treated TiO2 thin films were immersed in ethanolic solution of a 0.5 mM N719 dye for 24 h in dark to allow for sufficient dye adsorption. In present study we have used Di-tetrabutylammonium cis–bis (isothiocyanato) bis (2,20 -bipyridyl-4,40 -dicarboxylato) ruthenium(II) 95% (NMR) (N-719 dye, Sigma–Aldrich). The DSSC was fabricated using a standard two electrode configuration, comprising dye loaded Glass/FTO/TiO2 as the photoanode and platinum (Pt) or Cu2ZnSnS4 (CZTS) deposited on FTO coated substrates as the counter electrode, which is sealed with the working electrode using a thermoplastic (1 mm). The Pt/FTO counter electrode was prepared by commercial Pt paste (Dyesol) followed by heating at 400 C for 30 min in air. The iodide-based electrolyte (Dyesol) was used as redox electrolyte and was injected into the inter-electrode space from the counter electrode side through a pre-drilled hole. The following cell configurations were used for photovoltaic measurments: Glass=FTO=TiO2 =N719 Dye=I =I 3 =Pt=FTO
(1)
Glass=FTO=TiO2 =N719 Dye=I =I 3 =CZTS=FTO
(2)
Compact and sealed DSSCs were fabricated using above standard two electrode configurations. The device configuration of CZTS based counter electrode is shown in Scheme 1.
Fig. 1. (a) FESEM images of TiO2 sample, (b) SEM image of CZTS nanoflakes.
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Fig. 2. (a–c) Cu Ka1, Zn Ka1, Sn La1, and SK a1 elemental EELS mappings of a CZTS nanoflakes. The fright-field image and corresponding elemental analysis of CZTS nanoflakes.
thin film with uniform distribution of the nanoflakes within welldefined petals. The average size of single flake is 1.1 mm and wall thickness is 80 nm. The CZTS nanoflakes were further characterized by energy dispersive X-ray spectroscopy (EDS) mappings. Fig. 2 shows typical Cu, Zn, Sn, and S elemental mapping data of a CZTS nanoflakes. The mapping results show that Cu, Zn, Sn, and S are homogeneously distributed throughout the nanoflakes (Fig. 2 (a–d)). Fig. 2(e) shows Cu, Zn, Sn, and S elemental EDS mappings of nanoflakes indicating homogeneous distribution of the all four constituent elements throughout the samples. The EDS elemental results reveal that the atomic ratio of Cu:Zn:Sn:S samples is about 24:11:11:50, indicating copper-rich CZTS. Fig. 3 shows the indexed XRD pattern for the as synthesized CZTS nanoflakes annealed at 450 C in nitrogen atmosphere. The lattice parameters (a = b = 5.4406 Å, c = 10.8578 Å) calculated from the X-ray diffraction pattern were matched to the JCPDS card 00-026-0575: a = b = 5.4270 Å, c = 10.8480 Å. The product obtained after annealing exhibited four strong peaks at 2u = 28.5 , 47.3 , and 56.2 and four weak peaks at 2u = 32.36 , 58.02 , 69.44 and 76.79 , which were indexed to diffraction of the (11 2), (2 0 0), (2 2 0), (3 1 2), (0 0 8) and (3 3 2) planes, respectively, of the tetragonal CZTS phase with cell constants a = 5.4270 Å and c = 10.8480 Å (JCPDS 26-0575) [24].
Fig. 4 illustrates the typical Raman spectrum of the CZTS nanoflakes annealed at 450 C. From the both spectrum, one can evidently observe a single peak at about 333.5 cm1, which corresponds to quaternary CZTS. This result is in good agreement with the Raman spectra of single phase Cu2ZnSnS4 [21]. In addition, there are no additional peaks related to the presence of other compounds, which means that the single phase CZTS film was obtained. Fig. 5 shows the current density–voltage (J–V) curves of Pt/FTO and CZTS/FTO counter electrode based DSSC devices under dark and illumination. The DSSC based on Pt counter electrode produced conversion efficiency (h%) of 3.82% with short circuit current density (JSC) = 9.35 mA cm2, open circuit voltage (VOC) 0.649 V and fill factor (FF) 0.61. The DSSCs based on CZTS counter electrode shows 4.84% power conversion efficiency (PCE) (JSC = 13.35 mA cm2, VOC = 0.595 V, FF = 0.63). The TiO2 morphology is useful for the effective dye loading and can be increase the light harvesting within the electrode and thus enhance the photoncapture probability. Also it works as a ‘pathway’ for electrolyte diffusion throughout a TiO2films (Fig. 5). The solar cell parameters based of CZTS and Pt based counter electrodes are summarized in Table 1. In ordinary DSSCs the VOC is determined by the Fermi level around the CB of nanostructured TiO2 and oxidation/reduction
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S.S. Mali et al. / Materials Research Bulletin 59 (2014) 249–253 Table 1 Photovoltaic performance of DSSCs with different counter electrodes. Counter electrode
JSCa (mA/cm2)
VOC (V)
RS (V/cm2)
RSh (V/cm2)
FF
Pt CZTS
9.35 13.35
0.649 0.595
114 98
8727 5623
0.61 0.63
a
Fig. 3. XRD pattern of deposited CZTS thin film onto glass substrate by using SILAR technique.
potential of redox couple I/I3. While in case of CZTS counter electrode DSSCs the VOC is determined by the quasi-Fermi levels above the valence band (VB) of CZTS and below the CB of TiO2 [14]. Therefore, the VOC of the CZTS counter electrode DSSCs solar cell is lower than that of the traditional DSSCs solar cell, which is due to the decrease of VOC from 0.649 V to 0.595 V. In case of CZTS/FTO based counter electrode the JSC increased significantly from 9.35 mA/cm2 to 13.35 mA/cm2. From the table it is clear that CZTS/FTO based DSSC device shows lowest series resistance (RS)
Fig. 4. Micro-Raman Spectrum of CZTS nanoflakes.
h (%) 3.82 4.84
Devices were tested at room temperature with 0.25 cm2 active area.
98 V/cm2 compared to both Pt/FTO (114 V/cm2) counter electrodes. It is well known that the JSC of the DSSC depends on the RS of the devices. A smaller RS is favourable to a higher JSC and can be calculated using inverse slop of the J–V curves. Electrochemical impedance spectroscopy (EIS) has been used to study the interfaces, carrier transport, electronic and ionic processes and recombination in Pt and CZTS counter electrode based DSSCs devices. In order to confirm the effect of counter electrodes on the conversion efficiency of TiO2 thin films, EIS measurement was conducted to the Pt/FTO and CZTS/FTO based counter electrodes. Fig. 6(a) shows the Impedance spectrum of Pt/ FTO counter electrode based DSSCs. The measured impedance spectra data for TiO2–dye/electrolyte/Pt was fitted with the equivalent circuit given in inset of Fig. 6(a). The (R3, Q2) represents the electrode/electrolyte interface and R1 is the shunt resistance in parallel with the TiO2–dye structure represented by W1, R2, C1. R3 is the charge transfer resistance and Q2 is the double layer capacitance. While W1 and R2 represent the diffusion resistance and series resistance of the TiO2–dye structure, C2 represents the TiO2–dye junction capacitance [25]. Fit values indicated that R2 is lesser (and R1 higher) for CZTS/FTO counter electrode compared to Pt/FTO counter electrode. The product of the series resistance R2 and the capacitance C2 (at the TiO2/dye interface corresponds to the electron lifetime in the device [26]. There are differences in the series resistance (R2) values of both devices, where R2 of the Pt/FTO device is more than that CZTS/FTO (R2 = 238 and 195 V cm2 for devices with Pt/FTO and CZTS/FTO measured at 0.7 V, respectively). Lower R2 for CZTS/FTO electrode results in higher fill factor, thereby increasing efficiency when compared to Pt/FTO based device.
Fig. 5. J–V curves for (a) the Pt/FTO counter electrode (b) CZTS/FTO counter electrode.
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References
Fig. 6. Impedance spectra of DSSCs samples (a) Pt/FTO counter electrode and (b) CZTS/FTO counter electrode, measured in the dark under 0.70 V applied bias. Inset shows its respective equivalent circuits.
4. Conclusions In summary, we have successfully demonstrated SILAR deposited p-type CZTS quaternary materials with novel nanoflakes like morphology beneficial for efficient DSSCs. We have also demonstrated CZTS/FTO counter electrodes for TiO2 DSSCs shows good enhancement as compared to Pt/FTO. The highest solar energy conversion efficiency of 4.84% has been achieved for CZTS/ FTO based device. Such type of counter electrodes will be best alternative counter electrodes for Gratzel solar cells. Acknowledgment This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2009-0094055).
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