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Dye-sensitized solar cell based on Rose Bengal dye and nanocrystalline TiO2
ARTICLE IN PRESS Solar Energy Materials & Solar Cells 92 (2008) 909– 913
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Dye-sensitized solar cell based on Rose Bengal dye and nanocrystalline TiO2 M.S. Roy a,, P. Balraju b, Manish Kumar a, G.D. Sharma b a b
Defence Laboratory, Air Force, Ratanada, Jodhpur-342011, Rajasthan, India Department of Physics, Jai Narain Vyas University, Jodhpur-342005, Rajasthan, India
a r t i c l e in fo
abstract
Article history: Received 8 August 2007 Received in revised form 29 November 2007 Accepted 17 February 2008 Available online 14 April 2008
Dye-sensitized solar cell is fabricated using Rose Bengal dye (RB) for sensitization of nanocrystalline TiO2 and that imparts extension in spectral response towards visible region by modifying the semiconductor surface. Further, the photoresponse of the cell was evaluated by analyzing its J–V and impedance characteristics under illumination with metal halide light source of 400 W with an incident light of 73 mW/cm2. Various photovoltaic parameters like Jsc, Voc, FF were evaluated and found to be 3.22 mA, 890 mV, 0.53, respectively, resulting conversion efficiency (Z) of 2.09%. Impedance analysis of the cell was carried out to investigate the internal resistance of the cell by recording Cole–Cole plots in between real and imaginary impedance in dark and with illumination under variable biasing, i.e. from 0 to 3 V. & 2008 Published by Elsevier B.V.
Keywords: Dye-sensitized solar cell Nanocrystalline TiO2 Electrolyte Conversion efficiency Photoresponse Impedance
1. Introduction Dye-sensitized solar cells have attracted much attention for the last more than a decade since reported first time by Gratzel and coworkers [1]. Attempts are continuously being made to promote the adsorption of dye to harvest more solar light, smooth the progress of transport of photoexcited electrons and facilitate the diffusion of an electrolyte ion. The process involves the injection of electrons from the photoexcited dye into the conduction band of the semiconductor oxide, from where they pass through the nanoparticles to the transparent conducting oxide fluorine-doped tin oxide (FTO) current collector and finally into the external circuit. Further, the sensitizer is regenerated by electron transfer from a donor, typically iodide ions, which are dissolved in the electrolyte that is present in the porous semiconductor. The triiodide ions formed during the reaction diffuse to the counter electrode, where they are reduced back to iodide by the conduction band electrons that have passed though the external circuit that performs the electrical work (Fig. 1). In the present research paper, a dye-sensitized solar cells (DSSC) is fabricated by employing Rose Bengal dye (RB) dye adsorbed into the nc-TiO2 film and further sandwiched with another counter electrode made with 100 nm PEDOT-PSS film grown over graphite-coated FTO substrate. Photoresponse of the cell was investigated by recording its J–V characteristics and photoaction spectrum. Further, impedance analysis was carried Corresponding author. Tel.: +91 291 2567318; fax: +91 291 2511191.
E-mail address: [email protected] (G.D. Sharma). 0927-0248/$ - see front matter & 2008 Published by Elsevier B.V. doi:10.1016/j.solmat.2008.02.022
out to monitor the internal resistance of the cell under irradiation, which in turn influences the charge-transportation process and conversion efficiency of the cell.
2. Experimental 2.1. Materials Titanium dioxide (Degussa P-25) and RB dye were procured from Orion Chem. Pvt. Ltd, India and Hi-Media Laboratories Pvt. Ltd. Mumbai, respectively. For electrolyte preparation, polyethylene glycol (PEG) (MW 400) from Central Drug House Pvt. Ltd., New Delhi, potassium iodide and iodine from s.d. fiNE-cHEM, Ltd., Mumbai were procured and used as such. 2.2. Fabrication of DSSC The process involves firstly growing of nc-TiO2 layer over fluoride doped tin oxide-coated glass substrate having resistance of 10 O/cm2. The doctor blade approach is used for making 10 mm TiO2 layer and further the film was kept for annealing at 450 1C for 30 min. RB dye in 0.3 m mol proportion was dissolved in acetonitrile solvent and diffused into the TiO2 layer by dipping the film within the solvent for overnight. Then the film is taken out, washed with distilled acetonitrile and kept for drying at 60 1C for another 30 min. Meanwhile, counter electrode was made by developing a 100 nm thin film of PEDOT-PSS over graphite-coated FTO glass as reported [2]. In the process, firstly FTO is coated with
ARTICLE IN PRESS 910
M.S. Roy et al. / Solar Energy Materials & Solar Cells 92 (2008) 909–913
e-
e-
LOAD LOAD
Counter electrode e-
e-
e-
eLUMO
3I
_ e-
e-
hν _ I3
RB
HOMO Electrolyte
FTO TiO2
FTO
Modified Graphite coating PEDOT:PSS
graphite and then PEDOT-PSS was grown over the top of the film by spin-coater maintaining spin speed initially at low rpm to let the material spread over the desired rectangular area and further accelerated to 6000 rpm till it becomes completely dried. The film was dried at 80 1C for 30 min. Finally, both the electrodes were clamped together sandwiching the layered structure of DSSC. Electrolyte solution was prepared by taking the proportionate quantity of 0.5 mol KI and 0.05 mol of iodine and PEG (MW 400) (0.14 mol) in 50 ml acetonitrile solvent. The conductivity of the electrolyte solution was recorded with conductivity bridge meter and it was found to be 13.6 mS cm1. One drop of this electrolyte solution was poured from the edge of the sandwiched structure and sealed properly.
Absorbance (a.u)
Fig. 1. Schematic diagram of a dye-sensitized solar cell.
2.3. Photoresponse measurement Photoresponse of the DSSC was evaluated by recording J–V characteristics with Semiconductor Parameter Analyzer (model HP 4145 B) in dark as well as under illumination. For illumination, metal halide light source of 400 W was used with and incident light of 73 mW/cm2. Incident photon to electron conversion efficiency (IPCE) was evaluated by Photodiode array spectrometer (Analytic jena, Germany) having provision for selecting excitation wavelength under irradiation with 0.16 mW/cm2 in built light source. For the measurement of respective photocurrent, PG Stat/ECIS (model autolab 30, Echo Chemie, Netherlands) was used. Further, impedance analysis was carried out with an objective to investigate the overall internal resistance of the cell which in turn influences the cell performance. Cole–Cole plot in between Zreal and Zimag impedance was recorded with the same PG Stat/ ECIS (model autolab 30) in the frequency range from 100 Hz to 1 MHz under the biasing from 0 to 3 V.
3. Results and discussion The optical spectrum of the nc-TiO2, RB and dye-sensitized TiO2 is recorded in the form of film and shown in Fig. 2. It is seen in the figure that before sensitization with dye, the nc-TiO2 shows an
300
400
500 600 700 Wavelength (nm)
800
900
Fig. 2. Absorption spectra of TiO2 (. . . . .), Rose Bengal (- - - -) and RB-sensitized TiO2 (—).
absorption band extending from 320 to 400 nm and extrapolation of the absorption peak edge down to zero x-axis makes a cut at around 420 nm that yields optical band gap of 3.0 eV as reported elsewhere [3]. Further, sensitization with RB imparts widening of spectral response covering complete visible region and extrapolation of the peak absorption edge results a substantial decrease in band gap down to 1.7 eV. It does mean that sensitization with RB imparts an enhancement in photoconductivity and broadening of photoaction spectrum towards low photon energy. In other way, this can be well understood by drawing a plot in between incident photon energy (hu) and absorbed energy (ahu)m (Fig. 3(a and b)). It is observed that dye sensitization results an accelerated improvement in absorbed photon energy (ahu)m through visible region of the spectrum extending from 1.6 to 3.3 eV following direct allowed transitions (m ¼ 12).
ARTICLE IN PRESS M.S. Roy et al. / Solar Energy Materials & Solar Cells 92 (2008) 909–913
911
4 102
Direct Allowed (m=1/2) x Indirect Allowed (m=2) x 1013 Direct Forbidden (m=3/2) x 1010 Indirect Forbidden (m=3) x 1021
300
200
100
3.5 Current density (mA cm-2)
(αhυ)m (eV-cm-1)
400
3 2.5 2 1.5 1 0.5
0 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 hυ (eV)
400
Direct Allowed (m=1/2) x 102 Indirect Allowed (m=2) x 1013 Direct Forbidden (m=3/2) x 1010 Indirect Forbidden (m=3) x 1021
0 0
0.1
0.2
0.3
0.4 0.5 0.6 Voltage (V)
0.7
0.8
0.9
1
Fig. 4. Photocurrent–voltage characteristic of a solar cell-based on TiO2 sensitized by RB dye.
30 25 200
IPCE (%)
(αhυ)m (eV-cm-1)
35 300
100
20 15 10 5
0 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 hυ (eV) Fig. 3. The transitions of the (a) TiO2 thin film and (b) RB-TiO2 thin film.
0 360
460
560 660 Wavelength (eV)
760
860
Fig. 5. Photoaction spectra of RB-sensitized TiO2-based DSSC.
Photovoltaic response of the cell was investigated by recoding the J–V characteristics under illumination as shown in Fig. 4. It is found that short circuit current was 3.22 mA, with a Voc of 890 mV and FF of 0.53 leading to conversion efficiency Z of 2.09%.
3.1. Photoelectrochemical analysis of cell The incident photon to current conversion efficiency IPCE was plotted as a function of excitation wavelength under irradiation with 0.16 mW/cm2 in built light source of PDA spectrometer (analytik jena, Germany) as shown in Fig. 5 and measurement of photocurrent was carried out with the help of Keithley electrometer 6517 A. The observation of a peak at 550 nm with a maximum IPCE of 20.3% is attributed electron injection into the conduction band of TiO2 which is the characteristics of dye absorption spectrum [4] whereas the peak at 400 nm with 17.2% IPCE can be assigned to direct excitation of TiO2 band gap. The impedance analysis of the cell was carried out with an objective to investigate internal resistance of the cell attributable to charge transfer process. For improving the conversion efficiency of the cell it is necessary to comprehend the charge transfer process and internal resistances of the cell. The components of
DSSC that contributes to impedance are porous TiO2 electrode, counter electrode, and electrolyte [5]. The analysis over wide frequency range enables to measure internal resistances of each electrochemical step at the same time. In the present cell, on analyzing the Cole–Cole plot in between real and imaginary impedance in dark and with illumination under the frequency range from 100 Hz to 1 MHz and biasing from 0 to 3 V, it is found that at high frequency region one small semicircle appears which is followed by almost a straight line towards low frequency region as shown in Fig. 6(a–e). The semicircle region is characteristics of charge transfer process whereas the straight-line region is the resultant of diffusioncontrolled step as reported elsewhere for other cases [6]. At high frequency region the series resistance (Rs) is seems to be effective which is Ohmic or uncompensated resistance induced by the sheet resistance of the FTO and electrolyte. Additional effective impedance is considered to be contributed by (i) charge transfer resistance (Rct) originated by the electron transfer process in between electrode and electrolyte. The diameter of the semicircle signifies the extent of charge transportation and it is reciprocal with respect to Rct. In dark, Rct was 10 O and on illumination with light it reduces to 4 O. (ii) Double layer capacitance (Cdl) caused by
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M.S. Roy et al. / Solar Energy Materials & Solar Cells 92 (2008) 909–913
60
20 15
40 Z'' (Ohm)
Z'' (Ohm)
50
30 20
5
10 0 100
110
120 130 Z' (Ohm)
140
0
150
100
40 35 30 25
Z'' (Ohm)
Z'' (Ohm)
10
20 15 10 5 0 110
115
120 125 Z' (Ohm)
130
135
105 Z' (Ohm)
110
100 105 Z' (Ohm)
110
50 45 40 35 30 25 20 15 10 5 0 95
10
Z''(Ohm)
8 6 4 2 0 90
95
100
105
Z' (Ohm) Fig. 6. Cole–Cole plots of DSSC (a) in dark and under illumination at different biasing (b) 0 V, (c) 1 V, (d) 2 V and (e) 3 V.
accelerated conversion efficiency. The complex equivalent circuit model is shown in Fig. 7 that represents a cell where polarization is due to a combination of kinetics and diffusion processes [7–9].
Cdl Rs
Rct
Zwar
Fig. 7. Equivalent circuit of the DSSC.
the formation of an electrical double layer on the interface in between electrode and surrounding electrolyte. (iii) Warburg impedance (Zw) induced by diffusion of ionic species, i.e. I3 and appears as a diagonal line towards low frequency region and decreases with illumination. Biasing imparts decrease in overall impedance of the cell leading to effective charge transport process responsible for
4. Conclusion Dye-sensitized solar cell is fabricated by adopting Doctor Blade technique using RB dye for sensitization. The cell shows photoresponse in terms of Jsc 3.22 mA; Voc 890 mV; FF 0.53 and the conversion efficiency Z ¼ 2.09%. The incident photon to current conversion efficiency IPCE was found to be 20.3% at 550 nm contributed by electron injection into the conduction band of TiO2 and 17.2% at 400 nm which is assigned to direct excitation of TiO2 band gap. Impedance analysis was carried out by plotting Cole–Cole plot in between real and imaginary impedance with illumination under the frequency range from 100 Hz to 1 MHz and biasing from 0 to 3 V. It is found that at high frequency region one
ARTICLE IN PRESS M.S. Roy et al. / Solar Energy Materials & Solar Cells 92 (2008) 909–913
small semicircle appears which is followed by a diagonal line towards low-frequency region. The semicircle reveals characteristics charge transfer process whereas the subsequent straight line reveals the resultant of diffusion controlled. The equivalent circuit of the cell was made with the help of Cole–Cole plot and further used to describe the over all internal resistance of the cell. References [1] B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737. [2] J.G. Chen, H.Y. Wei, K.C. Ho, Sol. Energy Mater. Sol. Cells 91 (2007) 1472.
913
[3] The-Vinh Nguyen, HyunCheol Lee, O-Bong Yang, Sol. Energy Mater. Sol. Cells 90 (2006) 967. [4] A.F. Nogueira, M.A. De Paoli, Sol. Energy Mater. Sol. Cells 61 (2000) 135. [5] Toyohisa Hoshikawa, Ryuji Kikuchi, Koichi Eguchi, J. Electrochem. Chem. 589 (2006) 59. [6] Jiaquing Li, Luoping Li, Lei Zheng, Yuezhong Xian, Litong Jin, Electrochim. Acta 51 (2006) 4942. [7] C. Gabrielle, Identification of Electrochemical Processes by Frequency Response Analysis, Solartron Instrumentation Group, 1980. [8] J.R. Scully, D.C. Silverman, M.W. Kendig, Electrochemical Impedance: Analysis and Interpretation, ASTM, 1993. [9] Electrochemical Impedance Spectroscopy (EIS): 4 Equivalent Circuit Models Autolab Application Note, 2007.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Dye-sensitized solar cell based on Rose Bengal dye and nanocrystalline TiO2 M.S. Roy a,, P. Balraju b, Manish Kumar a, G.D. Sharma b a b
Defence Laboratory, Air Force, Ratanada, Jodhpur-342011, Rajasthan, India Department of Physics, Jai Narain Vyas University, Jodhpur-342005, Rajasthan, India
a r t i c l e in fo
abstract
Article history: Received 8 August 2007 Received in revised form 29 November 2007 Accepted 17 February 2008 Available online 14 April 2008
Dye-sensitized solar cell is fabricated using Rose Bengal dye (RB) for sensitization of nanocrystalline TiO2 and that imparts extension in spectral response towards visible region by modifying the semiconductor surface. Further, the photoresponse of the cell was evaluated by analyzing its J–V and impedance characteristics under illumination with metal halide light source of 400 W with an incident light of 73 mW/cm2. Various photovoltaic parameters like Jsc, Voc, FF were evaluated and found to be 3.22 mA, 890 mV, 0.53, respectively, resulting conversion efficiency (Z) of 2.09%. Impedance analysis of the cell was carried out to investigate the internal resistance of the cell by recording Cole–Cole plots in between real and imaginary impedance in dark and with illumination under variable biasing, i.e. from 0 to 3 V. & 2008 Published by Elsevier B.V.
Keywords: Dye-sensitized solar cell Nanocrystalline TiO2 Electrolyte Conversion efficiency Photoresponse Impedance
1. Introduction Dye-sensitized solar cells have attracted much attention for the last more than a decade since reported first time by Gratzel and coworkers [1]. Attempts are continuously being made to promote the adsorption of dye to harvest more solar light, smooth the progress of transport of photoexcited electrons and facilitate the diffusion of an electrolyte ion. The process involves the injection of electrons from the photoexcited dye into the conduction band of the semiconductor oxide, from where they pass through the nanoparticles to the transparent conducting oxide fluorine-doped tin oxide (FTO) current collector and finally into the external circuit. Further, the sensitizer is regenerated by electron transfer from a donor, typically iodide ions, which are dissolved in the electrolyte that is present in the porous semiconductor. The triiodide ions formed during the reaction diffuse to the counter electrode, where they are reduced back to iodide by the conduction band electrons that have passed though the external circuit that performs the electrical work (Fig. 1). In the present research paper, a dye-sensitized solar cells (DSSC) is fabricated by employing Rose Bengal dye (RB) dye adsorbed into the nc-TiO2 film and further sandwiched with another counter electrode made with 100 nm PEDOT-PSS film grown over graphite-coated FTO substrate. Photoresponse of the cell was investigated by recording its J–V characteristics and photoaction spectrum. Further, impedance analysis was carried Corresponding author. Tel.: +91 291 2567318; fax: +91 291 2511191.
E-mail address: [email protected] (G.D. Sharma). 0927-0248/$ - see front matter & 2008 Published by Elsevier B.V. doi:10.1016/j.solmat.2008.02.022
out to monitor the internal resistance of the cell under irradiation, which in turn influences the charge-transportation process and conversion efficiency of the cell.
2. Experimental 2.1. Materials Titanium dioxide (Degussa P-25) and RB dye were procured from Orion Chem. Pvt. Ltd, India and Hi-Media Laboratories Pvt. Ltd. Mumbai, respectively. For electrolyte preparation, polyethylene glycol (PEG) (MW 400) from Central Drug House Pvt. Ltd., New Delhi, potassium iodide and iodine from s.d. fiNE-cHEM, Ltd., Mumbai were procured and used as such. 2.2. Fabrication of DSSC The process involves firstly growing of nc-TiO2 layer over fluoride doped tin oxide-coated glass substrate having resistance of 10 O/cm2. The doctor blade approach is used for making 10 mm TiO2 layer and further the film was kept for annealing at 450 1C for 30 min. RB dye in 0.3 m mol proportion was dissolved in acetonitrile solvent and diffused into the TiO2 layer by dipping the film within the solvent for overnight. Then the film is taken out, washed with distilled acetonitrile and kept for drying at 60 1C for another 30 min. Meanwhile, counter electrode was made by developing a 100 nm thin film of PEDOT-PSS over graphite-coated FTO glass as reported [2]. In the process, firstly FTO is coated with
ARTICLE IN PRESS 910
M.S. Roy et al. / Solar Energy Materials & Solar Cells 92 (2008) 909–913
e-
e-
LOAD LOAD
Counter electrode e-
e-
e-
eLUMO
3I
_ e-
e-
hν _ I3
RB
HOMO Electrolyte
FTO TiO2
FTO
Modified Graphite coating PEDOT:PSS
graphite and then PEDOT-PSS was grown over the top of the film by spin-coater maintaining spin speed initially at low rpm to let the material spread over the desired rectangular area and further accelerated to 6000 rpm till it becomes completely dried. The film was dried at 80 1C for 30 min. Finally, both the electrodes were clamped together sandwiching the layered structure of DSSC. Electrolyte solution was prepared by taking the proportionate quantity of 0.5 mol KI and 0.05 mol of iodine and PEG (MW 400) (0.14 mol) in 50 ml acetonitrile solvent. The conductivity of the electrolyte solution was recorded with conductivity bridge meter and it was found to be 13.6 mS cm1. One drop of this electrolyte solution was poured from the edge of the sandwiched structure and sealed properly.
Absorbance (a.u)
Fig. 1. Schematic diagram of a dye-sensitized solar cell.
2.3. Photoresponse measurement Photoresponse of the DSSC was evaluated by recording J–V characteristics with Semiconductor Parameter Analyzer (model HP 4145 B) in dark as well as under illumination. For illumination, metal halide light source of 400 W was used with and incident light of 73 mW/cm2. Incident photon to electron conversion efficiency (IPCE) was evaluated by Photodiode array spectrometer (Analytic jena, Germany) having provision for selecting excitation wavelength under irradiation with 0.16 mW/cm2 in built light source. For the measurement of respective photocurrent, PG Stat/ECIS (model autolab 30, Echo Chemie, Netherlands) was used. Further, impedance analysis was carried out with an objective to investigate the overall internal resistance of the cell which in turn influences the cell performance. Cole–Cole plot in between Zreal and Zimag impedance was recorded with the same PG Stat/ ECIS (model autolab 30) in the frequency range from 100 Hz to 1 MHz under the biasing from 0 to 3 V.
3. Results and discussion The optical spectrum of the nc-TiO2, RB and dye-sensitized TiO2 is recorded in the form of film and shown in Fig. 2. It is seen in the figure that before sensitization with dye, the nc-TiO2 shows an
300
400
500 600 700 Wavelength (nm)
800
900
Fig. 2. Absorption spectra of TiO2 (. . . . .), Rose Bengal (- - - -) and RB-sensitized TiO2 (—).
absorption band extending from 320 to 400 nm and extrapolation of the absorption peak edge down to zero x-axis makes a cut at around 420 nm that yields optical band gap of 3.0 eV as reported elsewhere [3]. Further, sensitization with RB imparts widening of spectral response covering complete visible region and extrapolation of the peak absorption edge results a substantial decrease in band gap down to 1.7 eV. It does mean that sensitization with RB imparts an enhancement in photoconductivity and broadening of photoaction spectrum towards low photon energy. In other way, this can be well understood by drawing a plot in between incident photon energy (hu) and absorbed energy (ahu)m (Fig. 3(a and b)). It is observed that dye sensitization results an accelerated improvement in absorbed photon energy (ahu)m through visible region of the spectrum extending from 1.6 to 3.3 eV following direct allowed transitions (m ¼ 12).
ARTICLE IN PRESS M.S. Roy et al. / Solar Energy Materials & Solar Cells 92 (2008) 909–913
911
4 102
Direct Allowed (m=1/2) x Indirect Allowed (m=2) x 1013 Direct Forbidden (m=3/2) x 1010 Indirect Forbidden (m=3) x 1021
300
200
100
3.5 Current density (mA cm-2)
(αhυ)m (eV-cm-1)
400
3 2.5 2 1.5 1 0.5
0 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 hυ (eV)
400
Direct Allowed (m=1/2) x 102 Indirect Allowed (m=2) x 1013 Direct Forbidden (m=3/2) x 1010 Indirect Forbidden (m=3) x 1021
0 0
0.1
0.2
0.3
0.4 0.5 0.6 Voltage (V)
0.7
0.8
0.9
1
Fig. 4. Photocurrent–voltage characteristic of a solar cell-based on TiO2 sensitized by RB dye.
30 25 200
IPCE (%)
(αhυ)m (eV-cm-1)
35 300
100
20 15 10 5
0 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 hυ (eV) Fig. 3. The transitions of the (a) TiO2 thin film and (b) RB-TiO2 thin film.
0 360
460
560 660 Wavelength (eV)
760
860
Fig. 5. Photoaction spectra of RB-sensitized TiO2-based DSSC.
Photovoltaic response of the cell was investigated by recoding the J–V characteristics under illumination as shown in Fig. 4. It is found that short circuit current was 3.22 mA, with a Voc of 890 mV and FF of 0.53 leading to conversion efficiency Z of 2.09%.
3.1. Photoelectrochemical analysis of cell The incident photon to current conversion efficiency IPCE was plotted as a function of excitation wavelength under irradiation with 0.16 mW/cm2 in built light source of PDA spectrometer (analytik jena, Germany) as shown in Fig. 5 and measurement of photocurrent was carried out with the help of Keithley electrometer 6517 A. The observation of a peak at 550 nm with a maximum IPCE of 20.3% is attributed electron injection into the conduction band of TiO2 which is the characteristics of dye absorption spectrum [4] whereas the peak at 400 nm with 17.2% IPCE can be assigned to direct excitation of TiO2 band gap. The impedance analysis of the cell was carried out with an objective to investigate internal resistance of the cell attributable to charge transfer process. For improving the conversion efficiency of the cell it is necessary to comprehend the charge transfer process and internal resistances of the cell. The components of
DSSC that contributes to impedance are porous TiO2 electrode, counter electrode, and electrolyte [5]. The analysis over wide frequency range enables to measure internal resistances of each electrochemical step at the same time. In the present cell, on analyzing the Cole–Cole plot in between real and imaginary impedance in dark and with illumination under the frequency range from 100 Hz to 1 MHz and biasing from 0 to 3 V, it is found that at high frequency region one small semicircle appears which is followed by almost a straight line towards low frequency region as shown in Fig. 6(a–e). The semicircle region is characteristics of charge transfer process whereas the straight-line region is the resultant of diffusioncontrolled step as reported elsewhere for other cases [6]. At high frequency region the series resistance (Rs) is seems to be effective which is Ohmic or uncompensated resistance induced by the sheet resistance of the FTO and electrolyte. Additional effective impedance is considered to be contributed by (i) charge transfer resistance (Rct) originated by the electron transfer process in between electrode and electrolyte. The diameter of the semicircle signifies the extent of charge transportation and it is reciprocal with respect to Rct. In dark, Rct was 10 O and on illumination with light it reduces to 4 O. (ii) Double layer capacitance (Cdl) caused by
ARTICLE IN PRESS 912
M.S. Roy et al. / Solar Energy Materials & Solar Cells 92 (2008) 909–913
60
20 15
40 Z'' (Ohm)
Z'' (Ohm)
50
30 20
5
10 0 100
110
120 130 Z' (Ohm)
140
0
150
100
40 35 30 25
Z'' (Ohm)
Z'' (Ohm)
10
20 15 10 5 0 110
115
120 125 Z' (Ohm)
130
135
105 Z' (Ohm)
110
100 105 Z' (Ohm)
110
50 45 40 35 30 25 20 15 10 5 0 95
10
Z''(Ohm)
8 6 4 2 0 90
95
100
105
Z' (Ohm) Fig. 6. Cole–Cole plots of DSSC (a) in dark and under illumination at different biasing (b) 0 V, (c) 1 V, (d) 2 V and (e) 3 V.
accelerated conversion efficiency. The complex equivalent circuit model is shown in Fig. 7 that represents a cell where polarization is due to a combination of kinetics and diffusion processes [7–9].
Cdl Rs
Rct
Zwar
Fig. 7. Equivalent circuit of the DSSC.
the formation of an electrical double layer on the interface in between electrode and surrounding electrolyte. (iii) Warburg impedance (Zw) induced by diffusion of ionic species, i.e. I3 and appears as a diagonal line towards low frequency region and decreases with illumination. Biasing imparts decrease in overall impedance of the cell leading to effective charge transport process responsible for
4. Conclusion Dye-sensitized solar cell is fabricated by adopting Doctor Blade technique using RB dye for sensitization. The cell shows photoresponse in terms of Jsc 3.22 mA; Voc 890 mV; FF 0.53 and the conversion efficiency Z ¼ 2.09%. The incident photon to current conversion efficiency IPCE was found to be 20.3% at 550 nm contributed by electron injection into the conduction band of TiO2 and 17.2% at 400 nm which is assigned to direct excitation of TiO2 band gap. Impedance analysis was carried out by plotting Cole–Cole plot in between real and imaginary impedance with illumination under the frequency range from 100 Hz to 1 MHz and biasing from 0 to 3 V. It is found that at high frequency region one
ARTICLE IN PRESS M.S. Roy et al. / Solar Energy Materials & Solar Cells 92 (2008) 909–913
small semicircle appears which is followed by a diagonal line towards low-frequency region. The semicircle reveals characteristics charge transfer process whereas the subsequent straight line reveals the resultant of diffusion controlled. The equivalent circuit of the cell was made with the help of Cole–Cole plot and further used to describe the over all internal resistance of the cell. References [1] B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737. [2] J.G. Chen, H.Y. Wei, K.C. Ho, Sol. Energy Mater. Sol. Cells 91 (2007) 1472.
913
[3] The-Vinh Nguyen, HyunCheol Lee, O-Bong Yang, Sol. Energy Mater. Sol. Cells 90 (2006) 967. [4] A.F. Nogueira, M.A. De Paoli, Sol. Energy Mater. Sol. Cells 61 (2000) 135. [5] Toyohisa Hoshikawa, Ryuji Kikuchi, Koichi Eguchi, J. Electrochem. Chem. 589 (2006) 59. [6] Jiaquing Li, Luoping Li, Lei Zheng, Yuezhong Xian, Litong Jin, Electrochim. Acta 51 (2006) 4942. [7] C. Gabrielle, Identification of Electrochemical Processes by Frequency Response Analysis, Solartron Instrumentation Group, 1980. [8] J.R. Scully, D.C. Silverman, M.W. Kendig, Electrochemical Impedance: Analysis and Interpretation, ASTM, 1993. [9] Electrochemical Impedance Spectroscopy (EIS): 4 Equivalent Circuit Models Autolab Application Note, 2007.