Application of Diaminium Iodides in Binary Ionic Liquid Electrolytes for Dye-Sensitized Solar Cells

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, 0.15 mol·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).


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][2][3].Various functionalized imidazolium iodide salts [4][5][6][7][8][9] and the additives containing basic N atoms, such as pyridine [10,11], benzimidazole [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 I 3 − /I − redox couples and contain basic N atoms.However, their application in the electrolytes of DSCs 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 I 3 − /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.

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][16][17][18].Briefly, The CV measurement was achieved by using an electrochemical workstation (CHI660B).The working electrode was a 5.0 μm radius Pt 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 5 mV•s −1 was used to obtain steady-state current-voltage curves.The EIS measurement was carried out with a two-platinum-blackelectrode 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 ±10 mV over the bias 0 V at frequencies from 1 to 10 kHz.The TiO 2 paste was printed on transparent conducting glass sheets (TEC-8, LOF)   N-(2-hydroxyethyl)ethylenediamine (45.00 g, 0.43 mol, HEEDA) was dissolved in the equal weight of deionized water.Then hydriodic acid (122.81g, 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 6 h.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%). 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 pK b1 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.Component of solutions A∼D was the same as that in Figure 5. R s : serial resistance (resistance of solutions); R ct : charge-transfer resistance; CPE: constant phase element; Y 0 : constant phase angle element; n: a constant ranging from 0 ≤ n ≤ 1.

Thermal Analysis of HEEDAI and HEPI. Figures
Table 3: Photovoltaic performance of DSCs with electrolytes A∼G.
Electrolyte 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.

Influence of HEEDAI and HEPI on I 3
− /I − Redox Behavior in Binary Ionic Liquid.Figure 4 showed steady-state voltammograms of the solutions A∼D at 25 • C. Due to the large excess of I − relative to I 3 − , only the cathodic steady-state diffusion current of I 3 − was measured in solutions A∼D and the apparent diffusion coefficient (D app ) of I 3 − was listed in Table 1.Compared with solution A and B, D app value of I 3 − 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 I 3 − 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 I 3 − 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 × 10 −7 cm 2 •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][16][17][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 R s and R ct increased.This result was consistent with the diffusion coefficient of I 3 − in binary ionic liquid of HEEDAI and MPII.Moreover, the values of R s and R ct in binary ionic liquid of HEPI and MPII were higher than that in binary ionic liquid of HEEDAI with MPII. 3 and Figure 6 showed the photovoltaic performance of DSCs.The electrolyte C gave the short-circuit photocurrent density (J sc ) of 9.36 mA•cm −2 , open-circuit photovoltage (V oc ) of 0.67 V, fill factor (FF) of 0.52, and yielded an overall photoelectric conversion efficiency (η) 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 TiO 2 surface to suppress the recombination between I 3 − and the injected electrons in TiO 2 conduction band.Therefore, HEEDAI can be used as the alternative of TBP in the electrolyte of DSCs and has lower volatility than TBP.

Photovoltaic Performances of DSCs. Table
Compared with electrolytes C and A, the short-circuit photocurrent density (J 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 I 3 − and the injected electrons in TiO 2 conduction band.Compared with electrolytes E and C, the short-circuit photocurrent density (J sc ) and fill factor (FF) of electrolyte E were lower than those of electrolyte C.This result were because the diffusion coefficient of I 3 − in electrolyte E is lower those electrolyte C.Moreover, there were no obvious differences of photoelectric conversion efficiency between electrolytes A and G.

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 I 3 − /I − 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 I 3 − and the injected electrons in TiO 2 conduction band and be used as the alternative of TBP in the electrolyte of DSCs.Moreover, The electrolyte C, 0.15 mol•L −1 I 2 , HEEDAI and MPII with mass ratio of 1 : 4, gave the shortcircuit 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%.

Table 2 :
Parameters obtained by fitting the experimental spectra with the equivalent circuit.