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Improved performance of dye-sensitized solar cells with surface-treated TiO2 as a photoelectrode
Materials Research Bulletin 47 (2012) 2722–2725
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Improved performance of dye-sensitized solar cells with surface-treated TiO2 as a photoelectrode Su Kyung Park a, Chinkap Chung a, Dae-Hwan Kim b, Cham Kim b, Sang-Ju Lee b, Yoon Soo Han c,* a
Department of Chemistry, Keimyung University, Daegu 704-701, Republic of Korea Green Energy Research Division, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 711-873, Republic of Korea c Department of Advanced Energy Material Science and Engineering, Catholic University of Daegu, Gyeongbuk 712-702, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 24 April 2012
We report on the effects of surface-modified TiO2 on the performance of dye-sensitized solar cells (DSSCs). TiO2 surface was modified with Na2CO3 via a simple dip coating process and the modified TiO2 was applied to photoelectrodes of DSSCs. By dipping of TiO2 layer into aqueous Na2CO3 solution, the DSSC showed a power conversion efficiency of 9.98%, compared to that (7.75%) of the reference device without surface treatment. The UV–vis absorption spectra, the impedance spectra and the dark current studies revealed that the increase of all parameters was attributed to the enhanced dye adsorption, the prolonged electron lifetime and the reduced interfacial resistance. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Semiconductors A. Surfaces C. Impedance spectroscopy D. Surface properties
1. Introduction Dye-sensitized solar cells (DSSCs) are of great interest as an alternative to the conventional Si-based device because of their low-cost production and high performance [1,2]. After a decade of first report, the overall conversion efficiency has already enhanced to more than 11% using ruthenium dyes [3]. However, an unusual feature of these porous film-based solar cells is the lack of depletion layer at the electrode/dye/electrolyte interface [3]; therefore, the cells still suffer a series of energy losses. The back electron transfer (i.e., charge recombination) that occurred at the interfaces still remains one of the major limiting factors for enhancing the cell performance [4–8]. By modification of TiO2 surface, the interfacial charge recombination could be reduced by formation of energy barrier layer on TiO2 [5–8]. For example, SrO [5], Al2O3 [6], Nb2O5 [7], and MgO [8] were applied to modify TiO2 electrodes, and the resulting DSSCs showed an enhancement in power conversion efficiency (PCE). It has been shown that the improved efficiency of DSSCs is attributed to the following two factors. First, the wide band gap overlayer as a barrier layer retards the back electron transfer at the TiO2/dye/ electrolyte interfaces, and minimizes the charge recombination. Second, the overlayer enhances the dye adsorption, leading to the improved cell performance. Meanwhile, several research groups have reported that metal carbonates such as CaCO3 [9] and BaCO3 [10] employed onto TiO2 surface effectively formed an energy
* Corresponding author. Tel.: +82 53 850 3491; fax: +82 53 850 3397. E-mail address: [email protected] (Y.S. Han). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.04.049
barrier or a surface dipole, resulting in improvement in Voc (open circuit voltage) and Jsc (short circuit current). In this study, a dilute aqueous solution of Na2CO3 was applied to modify TiO2 surface via a very simple dip coating process. We could successfully introduce Na2CO3 on the TiO2 surface without calcinations process, and applied the resulting TiO2 films to photoanodes of DSSCs. The overall performance of DSSCs was investigated, and the effects of modified TiO2 layer on the performance were discussed.
2. Experimental 2.1. Materials A commercial TiO2 paste (T20/SP, Ti-nanoxide 300; Solaronix) and Na2CO3 (Sigma–Aldrich) were selected as the photoelectrode and the coating materials, respectively. Commercial N719 dye (Solaronix) was employed as the sensitizer. An iodide-based commercial electrolyte (AN-50; Solaronix) and a Pt source (H2PtCl655H2O; Kojima Chemicals) were selected. All of the chemicals were used without any further purification. 2.2. Fabrication of DSSCs Except for modification process of the TiO2 surface, the same procedures presented in our earlier work [11] were applied to prepare DSSCs. The modification process is as follows. The Na2CO3modified TiO2 electrode (Na2CO3-TiO2) was fabricated by dipping the TiO2/FTO electrode into the aqueous Na2CO3 solution (0.05 M) for 300 s, followed by rinsing with water and drying at 100 8C for
S.K. Park et al. / Materials Research Bulletin 47 (2012) 2722–2725
2723
30 min. To keep similar conditions between the standard and modified electrodes, the bare-TiO2/FTO electrodes were also rinsed, and then dried at 100 8C for 30 min. The resulting DSSCs with 25 mm2 active layer were used to obtain photovoltaic data, impedance spectra and dark currents. 2.3. Measurements X-ray photoelectron spectroscopy (XPS) was performed using VG Multilab ESCA 2000 (ThermoVG scientific) with Al Ka radiation. The attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were measured with a Nicolet 380 FT-IR spectrometer (Thermo Electron Corporation). The photovoltaic current–voltage characteristics were measured under AM1.5 verified by an AIST-calibrated Si-solar cell. The UV–vis absorption spectra were obtained from a Lambda 750 UV-Vis spectrophotometer (PerkinElmer). Nyquist and Bode plots were acquired using an electrochemical impedance analyzer (Iviumstat Tec.) under 1 Sun at open-circuit potential. 3. Results and discussion When a TiO2/FTO film is dipped into 0.05 M Na2CO3 aqueous solution, the Na+ and CO32 ions penetrate the porous TiO2 film, and then they are adsorbed onto the surface of TiO2. After dry at 100 8C for 30 min, the Na2CO3 was formed on the TiO2 surface. The incorporation of Na2CO3 onto the TiO2 surface was verified by the XPS and ATR-FTIR measurement. XPS spectrum (not shown here) of the Na2CO3-TiO2 film showed a peak detected at 1072 eV corresponding to the binding energy of 1 s in Na, indicating the existence of Na. To further confirm the presence of Na2CO3, ATRFTIR spectra (not shown here) of Na2CO3-TiO2/FTO and bare-TiO2/ FTO films were recorded. The IR spectrum of the bare-TiO2/FTO film was featureless except the peak (1640 cm1) for adsorbed water. In contrast to bare-TiO2/FTO film, the broad peak at 1360 cm1, assignable to the O–C–O stretching vibration, appeared in the spectrum of the Na2CO3-TiO2 film, strongly supporting the presence of O–C–O moieties which are from carbonate [11]. DSSCs with bare-TiO2/FTO and Na2CO3-TiO2/FTO electrodes were fabricated, and their photovoltaic properties were characterized. The DSSC with Na2CO3-TiO2/FTO showed an increase in Jsc, Voc and fill factor (FF), resulting in a PCE of 9.98% compared to that (7.75%) of reference device with bare-TiO2/FTO. Fig. 1 shows the current density and the voltage curves of DSSCs with bare-TiO2/ FTO and Na2CO3-TiO2/FTO electrodes. Important physical parameters governing the efficiency of the DSSCs were determined from the photocurrent-voltage curve, and the results are presented in Table 1. The enhanced PCE in DSSC with Na2CO3-TiO2/FTO was caused by an increase in all parameters. The significant increase in Jsc is particularly notable. In order to investigate the physical origins of the improved Jsc value, the influence of the Na2CO3-TiO2 on the dye adsorption was first investigated. Fig. 2 shows the UV–vis absorption spectra of the desorbed dye molecules. The absorbance intensity was increased by Na2CO3 coating on the TiO2 surface. On the basis of the observed optical absorption spectra, the amount of adsorbed dye molecules were calculated [12,13]. As presented in Table 1, the amount of adsorbed
Fig. 1. Photocurrent–voltage curves of the DSSCs employing bare TiO2/FTO and Na2CO3-TiO2/FTO electrodes.
dye molecules on Na2CO3-TiO2/FTO electrode was increased by 37.5% compared with that of bare-TiO2/FTO electrode (from 0.88 107 to 1.21 107 mol/cm2), suggesting that the presence of Na2CO3 enhances the dye attachment to the TiO2 surface, probably due to the more basic property than the bare TiO2 surface [9]. In addition to the enhanced dye adsorption, the collection efficiency of the photoinjected electrons can contribute to the increase in Jsc value. Electrochemical impedance spectroscopy (EIS) has been widely used for investigating the kinetics and energetic of transport and recombination in DSSCs [14,15]. Fig. 3 shows the Bode and Nyquist phase plots of EIS spectra for the bare-TiO2/FTO and Na2CO3-TiO2/ FTO electrodes. In the Bode phase plots, the frequency peak related to the Na2CO3-TiO2/FTO electrode slightly shifted to a relatively low frequency. The electron lifetime (tn) has been estimated from equation, tn = 1/2pfmax, where fmax is peak frequency. The tn was found to be 15.9 and 19.0 ms for DSSCs with bare-TiO2 and Na2CO3-TiO2 film, respectively. This enhancement in electron lifetime can contribute to an increase of the electron collection efficiency, leading to the improved Jsc value. We believe that the increased Jsc value in DSSC with Na2CO3-TiO2/FTO electrode is attributed to the enhancement of the dye adsorption and electron lifetime. The Voc value of DSSC with Na2CO3-TiO2/FTO electrode was also increased. The Voc is given by the following equation [4]: Iin j kT ln V oc ¼ (1) e ncb ket ½I3 where k and T are Boltzmann constant and the absolute temperature, respectively. Iinj is the flux of charge resulting from the sensitized injection, and ncb is the concentration of electrons at the TiO2 surface. ket and [I3] are the rate constant for the reduction of I3 by the conduction band electrons and the concentration of I3 as shown in chemical equation (2), respectively. ket
I3 þ 2ecb ðTiO2 Þ!3I
(2)
As stated above, the lifetime of the electrons injected from dyes was increased by employing the modified TiO2 film. This fact means that the ket is decreased by the modification of TiO2 film, suggesting that the surface modification by Na2CO3 induces
Table 1 Photovoltaic properties of the DSSCs employing bare-TiO2/FTO and Na2CO3-TiO2/FTO as photoanodes. Applied electrodes
Voc (mV)
Jsc (mA/cm2)
FF (%)
h (%)
Adsorbed dye (107 mol/cm2)
R1 (V)
R2 (V)
R3 (V)
tn (ms)
Bare-TiO2/FTO Na2CO3-TiO2/FTO
727 757
15.23 18.00
69.98 73.26
7.75 9.98
0.88 1.21
27.25 27.98
3.021 3.061
11.69 10.76
15.9 19.0
2724
S.K. Park et al. / Materials Research Bulletin 47 (2012) 2722–2725
Fig. 2. UV–vis absorption spectra of the DSSCs employing bare-TiO2/FTO and Na2CO3-TiO2/FTO electrodes.
formation of energy barrier on TiO2, which can suppress charge recombination [9,12]. From Eq. (1), we can understand that the decreased ket leads to an increment in Voc. The dark current is also a good method to indirectly verify the charge recombination of DSSCs. Fig. 4 shows the dark current of DSSCs with bare-TiO2/FTO and Na2CO3-TiO2/FTO electrode as a function of the applied potential. It can be seen from Fig. 4 that, throughout the measured potential range, the dark current of the device with Na2CO3-TiO2/ FTO is lower than that of the reference device, indicating that charge recombination between injected electrons and I3 ions is retarded by the surface modification. The observation of retarded charge recombination is well consistent with the results of the enhanced electron lifetime.
Fig. 4. Dark current of the DSSCs employing bare TiO2/FTO and Na2CO3-TiO2/FTO.
Thus, it is believed that the increased Voc value of the device with Na2CO3-TiO2/FTO electrode is due to the prolonged lifetime (the retarded recombination), i.e., the decreased ket. FF enhancement in the device with Na2CO3-TiO2/FTO electrode is another contribution to the improved PCE, compared to the reference device with pristine TiO2/FTO. In general, FF is influenced by internal resistance in cells. From the Nyquist plots, the interfacial charge transfer resistances (Table 1) of the DSSCs can be determined. R1 includes the RFTO (resistance of the FTO substrate) and electrolyte resistance [16]. R2 is the electrochemical reaction resistance at the Pt counter electrode and R3 is the charge transfer resistance at the TiO2/dye/electrolyte interface. As we can expect, R1 and R2 were not changed. However, R3 was slightly reduced by the surface modification. Thus, the lowering of internal resistance at the interface of the TiO2/electrolyte is responsible for the increase in FF of the device with Na2CO3-TiO2/FTO electrode from 69.98 to 73.26%. 4. Conclusion In summary, we incorporated Na2CO3 onto the surface of TiO2/ FTO electrodes and applied the resulting electrodes (Na2CO3-TiO2/ FTO) to the photoanode of DSSCs. When Na2CO3-TiO2/FTO was used as the photoanode, all parameters (Jsc, Voc, and FF) were increased. It was confirmed that the Jsc improvement was attributed to the increase in the dye adsorption and electron lifetime; the enhancement in Voc was caused by the retarded charge recombination between injected electrons and I3 ions; FF was increased by the reduced interfacial resistance. Consequently, we obtained ca. 28.8% improvement in PCE by using Na2CO3-TiO2/ FTO as the photoanode rather than bare TiO2/FTO. Acknowledgment This work was supported by the DGIST R&D Program of the Ministry of Education, Science and Technology of Korea (12-BD-03). References B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737–739. Q. Wang, J.E. Moser, M. Gra¨tzel, J. Phys. Chem. B 109 (2005) 14945–14953. M. Gra¨tzel, J. Photochem. Photobiol. A 164 (2004) 3–14. M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Muller, P. Liska, N. Valchopoulos, M. Gra¨tzel, J. Am. Chem. Soc. 115 (1993) 6382–6390. [5] S. Yang, C. Huang, X. Zhao, Chem. Mater. 14 (2002) 1500–1504. [6] X.T. Zhang, H.W. Liu, T. Taguchia, Q.B. Meng, O. Satoa, A. Fujishima, Sol. Energy Mater. Sol. Cells 81 (2004) 197–203. [7] A. Zaban, S.G. Chen, S. Chappel, B.A. Gregg, Chem. Commun. (2000) 2231–2232.
[1] [2] [3] [4]
Fig. 3. EIS spectra of (up) Bode and (below) Nyquist plots of the DSSCs with bare TiO2/FTO and Na2CO3-TiO2/FTO.
S.K. Park et al. / Materials Research Bulletin 47 (2012) 2722–2725 [8] H.S. Jung, J.-K. Lee, J.-Y. Kim, J.-S. Park, K.S. Hong, H. Shin, Langmuir 21 (2005) 10332–10335. [9] Z.-S. Wang, M. Yanagida, K. Sayama, H. Sugihara, Chem. Mater. 18 (2006) 2912– 2916. [10] G.D. Sharma, P. Suresh, M.S. Roy, J.A. Mikroyannidis, J. Power Sources 195 (2010) 3011–3016. [11] S.K. Park, C. Kim, J.H. Kim, J.Y. Bae, Y.S. Han, Curr. Appl. Phys. 11 (2011) S131–S135.
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[12] H. Alarcon, M. Hedlund, E.M.J. Johansson, H. Rensmo, A. Hagfeldt, G. Boschloo, J. Phys. Chem. C 111 (2007) 13267–13274. [13] M. Gra¨tzel, Inorg. Chem. 44 (2005) 6841–6851. [14] K. Kern, R. Sastrawan, J. Feber, R. Stangi, J. Luther, Electrochim. Acta 47 (2002) 4213–4225. [15] L. Han, N. Koide, T. Chiba, T. Mitate, Appl. Phys. Lett. 84 (2004) 2433–2435. [16] T. Peng, K. Fan, D. Zhao, J. Chen, J. Phys. Chem. C 114 (2010) 22346–22351.
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Improved performance of dye-sensitized solar cells with surface-treated TiO2 as a photoelectrode Su Kyung Park a, Chinkap Chung a, Dae-Hwan Kim b, Cham Kim b, Sang-Ju Lee b, Yoon Soo Han c,* a
Department of Chemistry, Keimyung University, Daegu 704-701, Republic of Korea Green Energy Research Division, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 711-873, Republic of Korea c Department of Advanced Energy Material Science and Engineering, Catholic University of Daegu, Gyeongbuk 712-702, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 24 April 2012
We report on the effects of surface-modified TiO2 on the performance of dye-sensitized solar cells (DSSCs). TiO2 surface was modified with Na2CO3 via a simple dip coating process and the modified TiO2 was applied to photoelectrodes of DSSCs. By dipping of TiO2 layer into aqueous Na2CO3 solution, the DSSC showed a power conversion efficiency of 9.98%, compared to that (7.75%) of the reference device without surface treatment. The UV–vis absorption spectra, the impedance spectra and the dark current studies revealed that the increase of all parameters was attributed to the enhanced dye adsorption, the prolonged electron lifetime and the reduced interfacial resistance. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Semiconductors A. Surfaces C. Impedance spectroscopy D. Surface properties
1. Introduction Dye-sensitized solar cells (DSSCs) are of great interest as an alternative to the conventional Si-based device because of their low-cost production and high performance [1,2]. After a decade of first report, the overall conversion efficiency has already enhanced to more than 11% using ruthenium dyes [3]. However, an unusual feature of these porous film-based solar cells is the lack of depletion layer at the electrode/dye/electrolyte interface [3]; therefore, the cells still suffer a series of energy losses. The back electron transfer (i.e., charge recombination) that occurred at the interfaces still remains one of the major limiting factors for enhancing the cell performance [4–8]. By modification of TiO2 surface, the interfacial charge recombination could be reduced by formation of energy barrier layer on TiO2 [5–8]. For example, SrO [5], Al2O3 [6], Nb2O5 [7], and MgO [8] were applied to modify TiO2 electrodes, and the resulting DSSCs showed an enhancement in power conversion efficiency (PCE). It has been shown that the improved efficiency of DSSCs is attributed to the following two factors. First, the wide band gap overlayer as a barrier layer retards the back electron transfer at the TiO2/dye/ electrolyte interfaces, and minimizes the charge recombination. Second, the overlayer enhances the dye adsorption, leading to the improved cell performance. Meanwhile, several research groups have reported that metal carbonates such as CaCO3 [9] and BaCO3 [10] employed onto TiO2 surface effectively formed an energy
* Corresponding author. Tel.: +82 53 850 3491; fax: +82 53 850 3397. E-mail address: [email protected] (Y.S. Han). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.04.049
barrier or a surface dipole, resulting in improvement in Voc (open circuit voltage) and Jsc (short circuit current). In this study, a dilute aqueous solution of Na2CO3 was applied to modify TiO2 surface via a very simple dip coating process. We could successfully introduce Na2CO3 on the TiO2 surface without calcinations process, and applied the resulting TiO2 films to photoanodes of DSSCs. The overall performance of DSSCs was investigated, and the effects of modified TiO2 layer on the performance were discussed.
2. Experimental 2.1. Materials A commercial TiO2 paste (T20/SP, Ti-nanoxide 300; Solaronix) and Na2CO3 (Sigma–Aldrich) were selected as the photoelectrode and the coating materials, respectively. Commercial N719 dye (Solaronix) was employed as the sensitizer. An iodide-based commercial electrolyte (AN-50; Solaronix) and a Pt source (H2PtCl655H2O; Kojima Chemicals) were selected. All of the chemicals were used without any further purification. 2.2. Fabrication of DSSCs Except for modification process of the TiO2 surface, the same procedures presented in our earlier work [11] were applied to prepare DSSCs. The modification process is as follows. The Na2CO3modified TiO2 electrode (Na2CO3-TiO2) was fabricated by dipping the TiO2/FTO electrode into the aqueous Na2CO3 solution (0.05 M) for 300 s, followed by rinsing with water and drying at 100 8C for
S.K. Park et al. / Materials Research Bulletin 47 (2012) 2722–2725
2723
30 min. To keep similar conditions between the standard and modified electrodes, the bare-TiO2/FTO electrodes were also rinsed, and then dried at 100 8C for 30 min. The resulting DSSCs with 25 mm2 active layer were used to obtain photovoltaic data, impedance spectra and dark currents. 2.3. Measurements X-ray photoelectron spectroscopy (XPS) was performed using VG Multilab ESCA 2000 (ThermoVG scientific) with Al Ka radiation. The attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were measured with a Nicolet 380 FT-IR spectrometer (Thermo Electron Corporation). The photovoltaic current–voltage characteristics were measured under AM1.5 verified by an AIST-calibrated Si-solar cell. The UV–vis absorption spectra were obtained from a Lambda 750 UV-Vis spectrophotometer (PerkinElmer). Nyquist and Bode plots were acquired using an electrochemical impedance analyzer (Iviumstat Tec.) under 1 Sun at open-circuit potential. 3. Results and discussion When a TiO2/FTO film is dipped into 0.05 M Na2CO3 aqueous solution, the Na+ and CO32 ions penetrate the porous TiO2 film, and then they are adsorbed onto the surface of TiO2. After dry at 100 8C for 30 min, the Na2CO3 was formed on the TiO2 surface. The incorporation of Na2CO3 onto the TiO2 surface was verified by the XPS and ATR-FTIR measurement. XPS spectrum (not shown here) of the Na2CO3-TiO2 film showed a peak detected at 1072 eV corresponding to the binding energy of 1 s in Na, indicating the existence of Na. To further confirm the presence of Na2CO3, ATRFTIR spectra (not shown here) of Na2CO3-TiO2/FTO and bare-TiO2/ FTO films were recorded. The IR spectrum of the bare-TiO2/FTO film was featureless except the peak (1640 cm1) for adsorbed water. In contrast to bare-TiO2/FTO film, the broad peak at 1360 cm1, assignable to the O–C–O stretching vibration, appeared in the spectrum of the Na2CO3-TiO2 film, strongly supporting the presence of O–C–O moieties which are from carbonate [11]. DSSCs with bare-TiO2/FTO and Na2CO3-TiO2/FTO electrodes were fabricated, and their photovoltaic properties were characterized. The DSSC with Na2CO3-TiO2/FTO showed an increase in Jsc, Voc and fill factor (FF), resulting in a PCE of 9.98% compared to that (7.75%) of reference device with bare-TiO2/FTO. Fig. 1 shows the current density and the voltage curves of DSSCs with bare-TiO2/ FTO and Na2CO3-TiO2/FTO electrodes. Important physical parameters governing the efficiency of the DSSCs were determined from the photocurrent-voltage curve, and the results are presented in Table 1. The enhanced PCE in DSSC with Na2CO3-TiO2/FTO was caused by an increase in all parameters. The significant increase in Jsc is particularly notable. In order to investigate the physical origins of the improved Jsc value, the influence of the Na2CO3-TiO2 on the dye adsorption was first investigated. Fig. 2 shows the UV–vis absorption spectra of the desorbed dye molecules. The absorbance intensity was increased by Na2CO3 coating on the TiO2 surface. On the basis of the observed optical absorption spectra, the amount of adsorbed dye molecules were calculated [12,13]. As presented in Table 1, the amount of adsorbed
Fig. 1. Photocurrent–voltage curves of the DSSCs employing bare TiO2/FTO and Na2CO3-TiO2/FTO electrodes.
dye molecules on Na2CO3-TiO2/FTO electrode was increased by 37.5% compared with that of bare-TiO2/FTO electrode (from 0.88 107 to 1.21 107 mol/cm2), suggesting that the presence of Na2CO3 enhances the dye attachment to the TiO2 surface, probably due to the more basic property than the bare TiO2 surface [9]. In addition to the enhanced dye adsorption, the collection efficiency of the photoinjected electrons can contribute to the increase in Jsc value. Electrochemical impedance spectroscopy (EIS) has been widely used for investigating the kinetics and energetic of transport and recombination in DSSCs [14,15]. Fig. 3 shows the Bode and Nyquist phase plots of EIS spectra for the bare-TiO2/FTO and Na2CO3-TiO2/ FTO electrodes. In the Bode phase plots, the frequency peak related to the Na2CO3-TiO2/FTO electrode slightly shifted to a relatively low frequency. The electron lifetime (tn) has been estimated from equation, tn = 1/2pfmax, where fmax is peak frequency. The tn was found to be 15.9 and 19.0 ms for DSSCs with bare-TiO2 and Na2CO3-TiO2 film, respectively. This enhancement in electron lifetime can contribute to an increase of the electron collection efficiency, leading to the improved Jsc value. We believe that the increased Jsc value in DSSC with Na2CO3-TiO2/FTO electrode is attributed to the enhancement of the dye adsorption and electron lifetime. The Voc value of DSSC with Na2CO3-TiO2/FTO electrode was also increased. The Voc is given by the following equation [4]: Iin j kT ln V oc ¼ (1) e ncb ket ½I3 where k and T are Boltzmann constant and the absolute temperature, respectively. Iinj is the flux of charge resulting from the sensitized injection, and ncb is the concentration of electrons at the TiO2 surface. ket and [I3] are the rate constant for the reduction of I3 by the conduction band electrons and the concentration of I3 as shown in chemical equation (2), respectively. ket
I3 þ 2ecb ðTiO2 Þ!3I
(2)
As stated above, the lifetime of the electrons injected from dyes was increased by employing the modified TiO2 film. This fact means that the ket is decreased by the modification of TiO2 film, suggesting that the surface modification by Na2CO3 induces
Table 1 Photovoltaic properties of the DSSCs employing bare-TiO2/FTO and Na2CO3-TiO2/FTO as photoanodes. Applied electrodes
Voc (mV)
Jsc (mA/cm2)
FF (%)
h (%)
Adsorbed dye (107 mol/cm2)
R1 (V)
R2 (V)
R3 (V)
tn (ms)
Bare-TiO2/FTO Na2CO3-TiO2/FTO
727 757
15.23 18.00
69.98 73.26
7.75 9.98
0.88 1.21
27.25 27.98
3.021 3.061
11.69 10.76
15.9 19.0
2724
S.K. Park et al. / Materials Research Bulletin 47 (2012) 2722–2725
Fig. 2. UV–vis absorption spectra of the DSSCs employing bare-TiO2/FTO and Na2CO3-TiO2/FTO electrodes.
formation of energy barrier on TiO2, which can suppress charge recombination [9,12]. From Eq. (1), we can understand that the decreased ket leads to an increment in Voc. The dark current is also a good method to indirectly verify the charge recombination of DSSCs. Fig. 4 shows the dark current of DSSCs with bare-TiO2/FTO and Na2CO3-TiO2/FTO electrode as a function of the applied potential. It can be seen from Fig. 4 that, throughout the measured potential range, the dark current of the device with Na2CO3-TiO2/ FTO is lower than that of the reference device, indicating that charge recombination between injected electrons and I3 ions is retarded by the surface modification. The observation of retarded charge recombination is well consistent with the results of the enhanced electron lifetime.
Fig. 4. Dark current of the DSSCs employing bare TiO2/FTO and Na2CO3-TiO2/FTO.
Thus, it is believed that the increased Voc value of the device with Na2CO3-TiO2/FTO electrode is due to the prolonged lifetime (the retarded recombination), i.e., the decreased ket. FF enhancement in the device with Na2CO3-TiO2/FTO electrode is another contribution to the improved PCE, compared to the reference device with pristine TiO2/FTO. In general, FF is influenced by internal resistance in cells. From the Nyquist plots, the interfacial charge transfer resistances (Table 1) of the DSSCs can be determined. R1 includes the RFTO (resistance of the FTO substrate) and electrolyte resistance [16]. R2 is the electrochemical reaction resistance at the Pt counter electrode and R3 is the charge transfer resistance at the TiO2/dye/electrolyte interface. As we can expect, R1 and R2 were not changed. However, R3 was slightly reduced by the surface modification. Thus, the lowering of internal resistance at the interface of the TiO2/electrolyte is responsible for the increase in FF of the device with Na2CO3-TiO2/FTO electrode from 69.98 to 73.26%. 4. Conclusion In summary, we incorporated Na2CO3 onto the surface of TiO2/ FTO electrodes and applied the resulting electrodes (Na2CO3-TiO2/ FTO) to the photoanode of DSSCs. When Na2CO3-TiO2/FTO was used as the photoanode, all parameters (Jsc, Voc, and FF) were increased. It was confirmed that the Jsc improvement was attributed to the increase in the dye adsorption and electron lifetime; the enhancement in Voc was caused by the retarded charge recombination between injected electrons and I3 ions; FF was increased by the reduced interfacial resistance. Consequently, we obtained ca. 28.8% improvement in PCE by using Na2CO3-TiO2/ FTO as the photoanode rather than bare TiO2/FTO. Acknowledgment This work was supported by the DGIST R&D Program of the Ministry of Education, Science and Technology of Korea (12-BD-03). References B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737–739. Q. Wang, J.E. Moser, M. Gra¨tzel, J. Phys. Chem. B 109 (2005) 14945–14953. M. Gra¨tzel, J. Photochem. Photobiol. A 164 (2004) 3–14. M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Muller, P. Liska, N. Valchopoulos, M. Gra¨tzel, J. Am. Chem. Soc. 115 (1993) 6382–6390. [5] S. Yang, C. Huang, X. Zhao, Chem. Mater. 14 (2002) 1500–1504. [6] X.T. Zhang, H.W. Liu, T. Taguchia, Q.B. Meng, O. Satoa, A. Fujishima, Sol. Energy Mater. Sol. Cells 81 (2004) 197–203. [7] A. Zaban, S.G. Chen, S. Chappel, B.A. Gregg, Chem. Commun. (2000) 2231–2232.
[1] [2] [3] [4]
Fig. 3. EIS spectra of (up) Bode and (below) Nyquist plots of the DSSCs with bare TiO2/FTO and Na2CO3-TiO2/FTO.
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