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0Hindawi Publishing Corporation International Journal of Photoenergy Volume 2011, Article ID 986869, 5 pages doi:10.1155/2011/986869
Research Article
Application of Diaminium Iodides in Binary Ionic Liquid Electrolytes for Dye-Sensitized Solar Cells
Yanzhen Yang,1 Renjie Sun,2,3 Chengwu Shi,2,3 Yucheng Wu,1 and MeiXia2,3
1 School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China
2 School of Chemical Engineering, Hefei University of Technology, Hefei 230009, China
3 Key Lab of Novel Thin Film Solar Cells, Chinese Academy of Sciences, Hefei 230031, China
Correspondence should be addressed to Chengwu Shi, shicw506@gmail.com and Yucheng Wu, ycwu@hfut.edu.cn Received 27 January 2011; Revised 24 April 2011; Accepted 2 May 2011 Academic Editor: Gion Calzaferri
Copyright © 2011 Yanzhen Yang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
N-(2-hydroxyethyl)ethylenediaminium iodides (HEEDAIs) and N-(2-hydroxyethyl)piperazinium iodides (HEPIs) were synthesized, and their thermal properties were analysed. The influence of HEEDAI and HEPI on I3-/I- redox behavior in binary ionic liquid was investigated. The result revealed that HEEDAI can suppress the recombination between I3- and the injected electrons in TiO2 conduction band and be used as the alternative of 4-tert-butylpyridine in the electrolyte of dye-sensitized solar cells. The electrolyte C, 0.15mol-L-1 I2, HEEDAI and MPII with mass ratio of 1:4, gave the short-circuit photocurrent density of 9.36 mA-cm-2, open-circuit photovoltage of 0.67 V, fill factor of 0.52, and the corresponding photoelectric conversion efficiency of 3.24% at the illumination (air mass 1.5, 100 mW-cm-2, active area 0.25 cm2).
1. Introduction
Dye-sensitized solar cells (DSCs) have been extensively studied due to their high light-to-electric energy conversion yield. The electrolyte is one of the critical components of DSCs and has the remarkable effect on the stability under long-term thermal and light-soaking dual stresses [1-3]. Various func-tionalized imidazolium iodide salts [4-9] and the additives containing basic N atoms, such as pyridine [10, 11], benzim-idazole [12, 13], aminotriazole [14], have been evaluated as electrolytes in DSCs with some success. Compared with the imidazolium iodides and pyridine derivatives, diaminium iodides serve as the iodide resource of I3-/I- redox couples and contain basic N atoms. However, their application in the electrolytes ofDSCs has not been reported.
In this paper, N-(2-hydroxyethyl)ethylenediaminium iodides (HEEDAI) and N-(2-hydroxyethyl)piperazinium iodides (HEPI) were synthesized, and their thermal properties were characterized by the thermogravimetric analysis and differential scanning calorimeter. The influence of HEEDAI and HEPI on I3-/I- redox behavior in binary ionic liquid was investigated by cyclic voltammetry using a Pt disk ultramicroelectrode and electrochemical impedance spec-
troscopy. And the photovoltaic performances of the corresponding DSCs were measured.
2. Experimental
2.1. Materials. All chemical reagents were commercially available and used without further purification. 1-methyl-3-propylimidazolium iodide (MPII) was synthesized according to our previous report [15].
2.2. Measurement. The melting and freezing points of HEEDAI and HEPI were characterized by differential scanning calorimeter using a Thermal Analysis DSC821 with heating rate at 10°C-min-1, under a nitrogen flow rate of about 30 mL- min-1. And their thermal stability was analyzed on a TG209F3 Tarsws NETZSCH thermogravimetric balance at a heating rate of 10°C-min-1 under nitrogen atmosphere.
The measurements of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for binary ionic liquid and the fabrication of DSCs were similar to our previous reports [15-18]. Briefly, The CV measurement was achieved by using an electrochemical workstation (CHI660B). The working electrode was a 5.0 ^m radius Pt
I- , H
HOCH2CH2-HNCH2CH2
H + HI
HOCH2CH2
HNCH2CH2 HEEDAI
NV ' ' 1 1 H H
HOCH2CH2 -N
H + HI
HOCH2CH2 -N
HEPI \
Scheme 1: The synthesis reaction of HEEDAI and HEPI.
100 80 60
50 100 150 200 250 300 350 400 450 500 Temperature (°C)
Figure 1: Thermogravimetric analysis curves of HEEDAI.
-15 s 20
50 100 150 200 250 300 350 400 450 500 Temperature (°C)
Figure 2: Thermogravimetric analysis curves of HEPI.
disk ultramicroelectrode (CHI107); the auxiliary electrode and the reference electrode were a 1.0 mm radius Pt disk electrode (CHI102). A slow scan rate at 5mV-s_1 was used to obtain steady-state current-voltage curves. The EIS measurement was carried out with a two-platinum-black-electrode conductance cell (DJS-1C, REX, cell constant is 0.974). After every EIS measurement, the conductance cell was washed with anhydrous ethanol and immersed in distilled water overnight. EIS spectra were obtained by applying sinusoidal perturbations of ±10mV over the bias 0 V at frequencies from 1 to 10 kHz. The TiO2 paste was printed on transparent conducting glass sheets (TEC-8, LOF)
30 60 90 120 150 180 Temperature (°C)
Figure 3: Differential scanning calorimeter curves of (A) HEEDAI and (B) HEPI.
6 -0-4
-0.4 -0.3 -0.2 -0.1
Potential (V)
...... C
Figure 4: Steady-state voltammograms of solutions A~D at 25°C. Solution component: (A) m(HEEDAI) : m(MPII) = 1:9; (B) m(HEPI) :m(MPII) =1:9; (C) m(HEEDAI) : m(MPII) = 1:4; (D) m(HEEDAI) :m(MPII) = 1: 1, and0.15mol-L~1 I2.
N -140
Table 1: Apparent diffusion coefficient (Dapp) of I3 A~D.
in solutions
800 1000 Z' (Ohm)
--A-fit
--B-fit
--C-fit
--D-fit
Figure 5: EIS and its equivalent circuit of solutions A~D at 25°C. Solution component: (A) m(HEEDAI): m(MPII) = 1:9; (B) m(HEPI) :m(MPII) =1:9; (C) m(HEEDAI): m(MPII) = 1:4; (D) m(HEEDAI): m(MPII) = 1: 1, and0.15 mol-L"1 I2.
0.2 0.3 0.4 0.5 Photovoltage (V)
............E
Figure 6: Photocurrent-photovoltage characteristics of DSCs with electrolytes A~G. Component of electrolytes A~G is the same as that in Table 3.
and sintered in air at 450°C for 30min. The film was ~10 ^m thick, which was determined by profilometer (XP-2, AMBIOS Technology Inc. USA). A 4pm thick light scattering layer was used. After cooling to 80°C, the TiO2 films were immersed in anhydrous ethanol solution with 5.0 X 10"4 mol-L"1 a's-dithiocyanate-N,N'-bis-(4-carbox-ylate-4-tetrabutylammonium carboxylate-2,2'-bipyridine) ruthenium (II) (N719) overnight. The excess of N719 dye in TiO2 films was rinsed off with anhydrous ethanol before assembly. The counterelectrode was platinized by spraying
Solution 10-7Dapp(I3-)/(cm2-s-1)
(A) m(HEEDAI): m(MPII) =1 : 9 5.0
(B) m(HEPI): m(MPII) = 1 : 9 4.4
(C) m(HEEDAI) :m(MPII) =1 : 4 2.5
(D) m(HEEDAI) : m(MPII) =1 : 1 1.8
The other component of solutions A~D was 0.15 mol-L 1 I2.
H2PtCl6 solution to transparent conducting glass and fired in air at 410°C for 20 min. Then, it was placed directly on the top of the dye-sensitized TiO2 films. The gap between the two electrodes was sealed by thermal adhesive (Surlyn, Dupont). The electrolyte was filled from a hole made on the counter electrode, and the hole was later sealed by a cover glass and thermal adhesive films. The active area of DSCs was 0.25 cm2. The photovoltaic performances of DSCs were measured with a solar simulator (Class AAA, Oriel, Newport, USA, Air Mass 1.5, 100mW-cm~2) and a Keithley 2420 source meter controlled by Testpoint software. The irradiation intensity was calibrated with standard crystalline silicon solar cell (Oriel, Newport, USA).
2.3. Synthesis Procedure. HEEDAI and HEPI were synthesized by the neutralization reaction of N-(2-hydroxyeth-yl)ethylenediamine/N-(2-hydroxyethyl)piperazine and HI using the stoichiometric amounts of reactants (Scheme 1).
N-(2-hydroxyethyl)ethylenediamine (45.00 g, 0.43 mol, HEEDA) was dissolved in the equal weight of deionized water. Then hydriodic acid (122.81 g, 0.43 mol HI, 45% mass fraction) was slowly dropped to the above solution with magnetic stirring at room temperature. Subsequently, the reaction mixture was stirred for 30 min, and the deionized water was removed by rotary evaporator at 90°C and dried in vacuum at 80°C for 6h. A red viscous liquid, HEEDAI, was obtained. Yield: 94.26 g (94%).
The synthesis of HEPI was similar to that of HEEDAI. A white solid, HEPI, was obtained with the reaction of N-(2-hydroxyethyl)piperazine (45.54 g, 0.35 mol, HEP) and hydroiodic acid (99.50 g, 0.35 mol HI, 45% mass fraction). Yield: 86.70 g (96%).
3. Results and Discussion
3.1. Thermal Analysis of HEEDAI and HEPI. Figures 1 and 2 showed thermogravimetric analysis curves of HEEDAI and HEPI. The decomposition temperatures of HEEDAI and HEPI were 345°C and 325°C. This result revealed that the thermal stability of HEEDAI was superior to that of HEPI. Because the pKb\ of HEEDA and HEP were 4.4 and 4.8, respectively [19], the basicity of HEEDA was more intense than that of HEP, and the N-H bond energy in the resulting iodide salt, HEEDAI, was higher than that in HEPI. The N-H bond break of HEEDAI required higher decomposition temperature than HEPI.
Table 2: Parameters obtained by fitting the experimental spectra with the equivalent circuit.
Solution _Rs/Ohm .Rtf/Ohm Yo/wFs"-1 n
(A) m(HEEDAI) :m(MPII) = 1:9 440 120 33 0.8
(B) m(HEPI) :m(MPII) =1:9 549 156 25 0.8
(C) m(HEEDAI) :m(MPII) = 1:4 769 340 32 0.8
(D) m(HEEDAI) : m(MPII) = 1:1 1209 303 46 0.8
Component of solutions A~D was the same as that in Figure 5.
Rs: serial resistance (resistance of solutions); Rct: charge-transfer resistance; CPE: constant phase element; Yo: constant phase angle element; n: a constant ranging from 0 < n < 1.
Table 3: Photovoltaic performance of DSCs with electrolytes A~G.
Electrolyte 7sc/(mA-cm-2) Voc/V FF n (%)
(A) m(HEEDAI) :m(MPII) =1 : 9 Without TBP 6.88 0.68 0.46 2.16
(B) m(HEEDAI) :m(MPII) = 1 : : 9, 0.5mol-L"1 TBP 6.64 0.69 0.45 2.06
(C) m(HEEDAI) :m(MPII) =1 : 4 Without TBP 9.36 0.67 0.52 3.24
(D) m(HEEDAI) :m(MPII) =1 :4, 0.5mol-L"1 TBP 8.96 0.66 0.52 3.06
(E) m(HEEDAI) :m(MPII) = 1 : 1 Without TBP 5.76 0.63 0.50 1.80
(F) m(HEEDAI) :m(MPII) = 1 : 1,0.5 mol-L"1 TBP 5.68 0.64 0.48 1.74
(G) m(HEPI) : m(MPII) =1 : 9 Without TBP 7.20 0.65 0.53 2.44
The electrolyte component of A-G: (A) m(HEEDAI): m(MPII) = 1:9; (B) m(HEEDAI): m(MPII) = 1:9, 0.5 mol-L"1 TBP, (C) m(HEEDAI): m(MPII) = 1:4; (D) m(HEEDAI): m(MPII) = 1:4, 0.5 mol-L"1 TBP; (E) m(HEEDAI): m(MPII) = 1:1; (F) m(HEEDAI): m(MPII) = 1:1, 0.5 mol-L"1 TBP; and 0.15 mol-L"1 I2; (G) m(HEPI): m(MPII) =1: 9 and 0.15 mol-L"1 I2, 0.1 mol-L"1 LiI. Illumination: AM 1.5, 100mV-cm"2, active areas of DSCs: 0.25 cm2.
Figure 3 showed differential scanning calorimeter curves of HEEDAI and HEPI. HEEDAI had a glass transition temperature at -28°C, a crystallization peak followed by a broad melting transition between — 10°C and 170°C. And the melting point of HEPI was 94° C, while the freezing points of HEEDAI and HEPI were not observed. It was indicated that the two iodide ionic liquids were transferred from equilibrium liquid state into the metastable supercooled liquid state when refrigerated from high temperature to low temperature.
3.2. Influence of HEEDAI and HEPI on I3"/I" Redox Behavior in Binary Ionic Liquid. Figure 4 showed steady-state voltam-mograms of the solutions A-D at 25°C. Due to the large excess of I" relative to I3", only the cathodic steady-state diffusion current of I3" was measured in solutions A-D and the apparent diffusion coefficient (Dapp) of I3" was listed in Table 1. Compared with solution A and B, Dapp value of I3" in binary ionic liquid of HEEDAI and MPII was higher than that in HEPI and MPII. This was because the molecular size of HEPI was larger than that of HEEDAI. From solutions A, C, and D, the diffusion coefficients of I3" increased with the increase of MPII. This was because MPII had lower macroscopic viscosity than that of HEEDAI. It was noteworthy that the diffusion coefficients of I3" in binary ionic liquid of HEEDAI and MPII, the mass ratio of 1: 4 and 1: 9, were higher than pure MPII solution (1.88 X 10"7cm2-s"1) [20].
Figure 5 showed the EIS spectra of the solutions A-D at 25°C. Experimental data were represented by symbols and the solid lines corresponded to the fit using the equivalent circuit [15-18]. The parameters obtained by fitting the experimental spectra with the equivalent circuit were listed in Table 2. With the increase of HEEDAI, the viscosity of
solution increased, and Rs and Rct increased. This result was consistent with the diffusion coefficient of I3" in binary ionic liquid of HEEDAI and MPII. Moreover, the values of Rs and Rct in binary ionic liquid of HEPI and MPII were higher than that in binary ionic liquid of HEEDAI with MPII.
3.3. Photovoltaic Performances of DSCs. Table 3 and Figure 6 showed the photovoltaic performance of DSCs. The electrolyte C gave the short-circuit photocurrent density (/sc) of 9.36 mA-cm"2, open-circuit photovoltage (Voc) of 0.67 V, fill factor (FF) of 0.52, and yielded an overall photoelectric conversion efficiency (n) of 3.24%. From Table 3, it was interesting that the addition of 0.50 mol-L"1 4-tert-butylpyridine (TBP) in binary ionic liquid of HEEDAI and MPII cannot improve the photoelectric conversion efficiency of DSCs. This was because HEEDAI, similar to TBP, contains the basic N atoms. These N atoms can be adsorbed on the TiO2 surface to suppress the recombination between I3" and the injected electrons in TiO2 conduction band. Therefore, HEEDAI can be used as the alternative of TBP in the electrolyte of DSCs and has lower volatility than TBP.
Compared with electrolytes C and A, the short-circuit photocurrent density (/sc) and fill factor (FF) of electrolyte C were higher than those of electrolytes A. This result was because the HEEDAI content in electrolyte C was higher than electrolyte A, which can suppress the recombination between I3" and the injected electrons in TiO2 conduction band. Compared with electrolytes E and C, the short-circuit photocurrent density (/sc) and fill factor (FF) of electrolyte E were lower than those of electrolyte C. This result were because the diffusion coefficient of I3" in electrolyte E is lower those electrolyte C. Moreover, there were no obvious differences of photoelectric conversion efficiency between electrolytes A and G.
4. Conclusion
HEEDAI and HEPI were synthesized, and their thermal properties were characterized by the thermogravimetric analysis and differential scanning calorimeter. The influence of HEEDAI and HEPI on I3VI- redox behavior in binary ionic liquid was investigated by cyclic voltammetry using a Pt disk ultramicroelectrode and electrochemical impedance spectroscopy. It was found that HEEDAI had a glass transition temperature at -28°C, the melting point of HEPI was 94° C, and the freezing points of HEEDAI and HEPI were not observed. And HEEDAI can suppress the recombination between I3- and the injected electrons in TiO2 conduction band and be used as the alternative of TBP in the electrolyte of DSCs. Moreover, The electrolyte C, 0.15mol-L-112, HEEDAI and MPII with mass ratio of 1: 4, gave the short-circuit photocurrent density of 9.36 mA-cm-2, open-circuit photovoltage of 0.67 V, fill factor of 0.52, and the corresponding photoelectric conversion efficiency of 3.24%.
Acknowledgment
This work is financially supported by the College Natural Science Foundation of Anhui Province (KJ2010A266), Anhui Province Science and Technology Plan Project of China (2010AKND0794), and the National High Technology Research and Development Program of China (2009AA 050603).
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