Effect of Inorganic Iodides on Performance of Dye-Sensitized Solar Cells

20 Sep 2007 - Brisbane, 4001, Australia, and Dyesol Limited Company, Queanbeyan, NSW 2620, Australia. ReceiVed: July 8, 2007; In Final Form: August 6,...
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J. Phys. Chem. C 2007, 111, 15125-15131

15125

Effect of Inorganic Iodides on Performance of Dye-Sensitized Solar Cells Hongxia Wang,*,† John Bell,† Johann Desilvestro,‡ Michael Bertoz,‡ and Graeme Evans‡ Faculty of Built EnVironment and Engineering, Queensland UniVersity of Technology, Brisbane, 4001, Australia, and Dyesol Limited Company, Queanbeyan, NSW 2620, Australia ReceiVed: July 8, 2007; In Final Form: August 6, 2007

In this work, we investigate the effect of iodides based on a variety of cations (Mn+) in an electrolyte system composed of 1-propyl-3-methylimidazolium iodide (0.6 M) and methoxypropionitrile on the photocurrent density-voltage (J-V) characteristics and the kinetics process of electron transfer/transport of corresponding dye-sensitized solar cells (DSCs). It is observed that the ionic conductivity of the electrolyte is reduced slightly after introduction of Mn+-based iodides. The investigation of the performance of DSCs as a function of Mn+ shows that the short-circuit photocurrent density, Jsc, linearly depends on the charge/radius ratio of Mn+ in the electrolyte up to 1.5 Å-1. Beyond this value, Jsc asymptotically reaches saturation. The highest Jsc is obtained with an AlI3-based electrolyte. Open circuit voltage, Voc, is reduced after addition of monocations or trications into the electrolyte, whereas for dication-based iodides such as CaI2, Voc is increased. Electrochemical impedance spectra for the DSCs show that the charge-transfer resistance, Rct, related to the reaction between I3- and electrons at the Pt electrode/electrolyte interface, decreases with the cations displaying higher charge/radius ratio and with increasing cation concentrations in the electrolyte. An inverse dependence of Rct on logarithmic NaI concentrations is observed. Meanwhile, the resistance related to the electron backtransfer reactions from the dyed TiO2 to I3- in electrolyte, Rbr, is significantly influenced by the nature as well as the concentration of different cations. A maximum Rbr is observed with CaI2-based electrolyte, whereas AlI3-based electrolyte displays the lowest Rbr values, consistent with the variation of Voc as a function of Mn+. We explain that electrostatic interactions between cations in the electrolyte system and the TiO2 surface, and also with conduction band electrons, govern the electron injection efficiency and electron mobility in the nanostructured TiO2 film as well as the performance of the DSCs.

Introduction Due to their technical and economical promise as an alternative to present silicon-based p-n junction photovoltaic devices, dye-sensitized solar cells (DSCs) have attracted much attention from the academic and commercial community since a light-to-electricity conversion efficiency of above 7% was first reported in 1991.1-4 A cascade of physicochemical processes involving electron-transfer reactions and transport governs the operation of DSCs. The reaction where conduction band electrons are transferred from the photoelectrode to I3- in the electrolyte and the oxidized state of the dye (dye+) results in an effective loss of cell voltage and output power.5,6 Strategies to improve the performance of DSCs include suppressing the electron back-transfer reaction more effectively and enhancing the effective electron injection efficiency. Tailoring the molecular structure and composition at the nanoparticulate TiO2/dye/ electrolyte interface is expected to control the electron-transfer and transport processes, thus improving the performance of the device. It has been well documented that cations in the electrolyte exert profound effects on the energetics of TiO2 films and thus the performance of a DSC.1,7-13 Most results in the literature are based on high-volatility electrolyte system such as acetonitrile. From the practical point of view for DSCs applications, however, low-volatility or quasi-solid/solid-state * To whom correspondence should be addressed. Tel.: +61 02 6299 1592. Fax: +61 02 6299 1698. E-mail: [email protected]. † Queensland University of Technology. ‡ Dyesol Limited Company.

electrolyte systems are desired. In this work, the effects of inorganic iodides based on different cations on the short-circuit photocurrent density, Jsc, and open circuit voltage, Voc, of DSCs, were investigated using a low-volatility electrolyte system consisting of methoxypropionitrile (MPN, boiling point 166 °C) and high concentrations of an ionic liquid iodide, 1-propyl-3methylimidazolium iodide (0.6 M, PMII). The influence of various cations and their concentrations on the ionic conductivity of the electrolyte and on the performance of corresponding DSCs is presented. Evolution of the kinetics processes in the cell with different cations was investigated through electrochemical impedance spectroscopy (EIS). Experimental Methods Preparation of Electrolyte. All chemicals used in this work were stored in a dry room (relative humidity 99%, Merck), 0.1 M iodine (I2, 99.8%, Aldrich), 0.45 M 1-methylbenzimidazole (99%, Aldrich) in methoxypropionitrile (MPN, 99+%, Fluka). The used inorganic iodides including LiI (99.9%), NaI (99%), KI (99%), CsI (99.9%), MgI2 (98%), CaI2 (99.95%), BaI2 (99.995%), AlI3 (99%), LaI3 (99.9%) were anhydrous unless otherwise stated and were purchased from Aldrich. Tetrabutylammonium iodide (TBAI, 99+%) was supplied by Fluka, and Mg(SCN)2 was from City Chemicals. The above iodides with variable concentrations were added to the stock solution and then dissolved using magnetic stirring followed by sonication for 30 min. KI, CaI2, BaI2, AlI3,

10.1021/jp075305f CCC: $37.00 © 2007 American Chemical Society Published on Web 09/20/2007

15126 J. Phys. Chem. C, Vol. 111, No. 41, 2007 and LaI3 did not completely dissolve in the stock solution due to the high concentration of I- from PMII. The solubility of the inorganic iodides is thus limited. Fabrication of DSCs. TiO2 paste (titania paste-transparent, Dyesol) was deposited onto fluorine-doped tin oxide (FTO) conducting glass (TEC15) by screen printing to form a transparent TiO2 film. The TiO2 film was dried at room temperature for 20 min and then fired at 525 °C for 30 min to remove any organics and to form a porous film structure. The thickness of the TiO2 film was 12 µm. The film was immersed into an ethanol solution of 3 × 10-4 M cis-bis(isothiocyanato) bis(2,2′bipyridyl-4,4′-dicarboxylate) ruthenium(II) bis-tetrabutylammonium (N-719, Dyesol) for 16 h. The dye-coated TiO2 film was then rinsed with absolute ethanol to remove any excess dye before assembling a cell. The platinized counter electrode was prepared by screen-printable platinum paste (PT1, Dyesol) onto FTO conducting glass (TEC15), followed by firing at 500 °C for 30 min. DSCs were fabricated by sealing the counter electrode and the dye-coated photoanode with a thermal plastic spacer (50 µm Surlyn, Dupont) in a test cell assembly machine (TCAM, Dyesol). The electrolyte was introduced into the cell through a 2 mm hole predrilled in the counter electrode. The hole was subsequently sealed with an aluminum foil covered with a layer of thermoplastic (Bynel, U.S.A.) using a heat press. The active area of a DSCs is 0.88 cm2. Four cells of each type of electrolyte were assembled to assess reproducibility, and an average of the four cells is reported for Jsc and Voc. Characterizations. A Gamry Instruments model PCI4-300 potentiostat/galvanostat/zero-resistance ammeter was employed to characterize the transport properties of charge carriers in the electrolyte and the kinetic properties of electron transport and transfer in DSCs through EIS. The frequency range for EIS measurement was from 300 kHz to 0.1 Hz with an ac modulation signal of 5 mV. A calibrated platinum conductance probe with a cell constant of 0.998 cm-1 (Shanghai Rousull Technology Co., China) was employed to measure ionic conductivities of the electrolytes through EIS for temperatures of 20-80 °C. Conductivity values of the electrolytes were determined from the high-frequency intercept with the real axis in the Nyquist plot. EIS characterization of DSCs was carried out in the dark under ambient temperature conditions (20 °C) by applying a forward bias dc voltage of 0.68 V. EIS spectra were analyzed with the software Echem Analyst from Gamry Instruments Inc. Photocurrent density-photovoltage (J-V) characteristics of DSCs were recorded with a computer-controlled digital source meter (Keithley 2420) by applying an external potential bias to the cell and measuring the photocurrent generated under a solar light simulator (1000 W high-pressure sodium lamp) which was calibrated by a silicon reference cell and corrected for any spectral difference on a clear day under AM1.5 outdoor illumination. The incident light intensities were adjusted with neutral wire mesh attenuators. All cell voltages are reported as the potential difference between counter electrode and photoanode (Ecounter electrode - Ephotoanode). Results and Discussion Ionic Conductivity Characteristics of Electrolyte. It is known that, for an electrolyte to be used in DSCs, a sufficiently fast diffusion of ionic species such as I-/I3- is essential for a satisfactory performance of solar cells. Therefore, the variation of the ionic conductivity of the base electrolyte after addition of variable cations with different concentrations was first investigated. Figure 1 shows the ionic conductivity at 20 °C as

Wang et al.

Figure 1. Ionic conductivity of the electrolyte composed of 0.1 M I2, 0.45 M 1-methylbenzimidazole, and iodides in methoxypropionitrile (MPN) as a function of the iodide salt concentration (curve 1, 0.6 M PMII + x M LiI; curve 2, 0.6 M PMII + x M NaI; curve 3, x M PMII) at 20 °C.

a function of iodide salt concentrations for the electrolyte solutions containing various iodides. The conductivity of the base electrolyte (containing only 0.6 M PMII without other iodides) is 8.46 × 10-3 S/cm, and the conductivity increases with the increase of the PMII concentrations and reaches 1.0 × 10-2 S/cm at 1.0 M PMII. However, addition of inorganic iodides into the base electrolyte results generally in a slight decrease of the ionic conductivity probably due to the formation of ionic clusters, leading to the decrease of the effective concentrations of free charge carriers. Figure 2a shows that Arrhenius plots for ionic conductivities of the base electrolyte, to which either 0.2 M PMII (total PMII content is 0.8 M), 0.1 M MgI2, or 0.1 M CaI2 was added, cannot be adequately fitted by the Arrhenius equation (eq (1)). A better fit of the data is obtained by applying the Vogel-TammannFulcher (VTF) equation (eq (2)) (Figure 2b).

σ(T) ) A exp[-Ea/RT]

(1)

σ(T) ) AT(-1/2) exp[-B/(T - T0)]

(2)

where A and B are constants, T is the absolute temperature, R is the air constant, Ea is the activation energy, and T0 is the thermodynamic Kauzmann temperature.14 Good fits were obtained for T0 ) 155 K, which is the same T0 value determined by Wang et al. for a similar electrolyte system.15 It is observed that the conductivity of 0.1 M MgI2 in the base electrolyte solution is similar to the electrolyte containing 0.1 M CaI2 and the conductivities of both are slightly lower than with the 0.8 M PMII based system. A similar trend is observed with LiI and NaI added to the base electrolyte solution as shown in Figure 1. Moreover, it is noticed that the slope of the three plots is close, indicating that the addition of inorganic iodides does not change the activation energy for the conduction of the electrolyte significantly. Photovoltaic Properties of DSCs. Figure 3a shows the J-V curves of DSCs with the base electrolyte containing different iodides (0.1 M MIn, 1e n e 3) under an illumination level of 0.1 suns. Jsc of these cells show the sequence AlI3 > MgI2 > CaI2 > LiI > NaI > base electrolyte > TBAI. The linear relationship of Jsc with illumination intensity (Figure 3b) indicates that diffusion of I3- in the electrolyte is not the primary factor limiting Jsc of the cells though the ionic conductivity is decreased for the electrolyte containing different inorganic iodides. Additionally, it is worthwhile to note that when a salt

Effect of Iodides on Dye-Sensitized Solar Cells

Figure 2. Ionic conductivity as a function of electrolyte temperature for different iodides in the base electrolyte composed of 0.1 M I2, 0.45 M 1-methylbenzimidazole, and 0.6 M PMII in MPN (A, 0.2 M PMII; B, 0.1 M CaI2; C, 0.1 M MgI2) in (a) Arrhenius and (b) VTF coordinates, with symbols for experimental values and solid lines for fitted curves.

of the same cation but with a different anion was used to replace the corresponding iodide (such as Mg(SCN)2 instead of MgI2), a similar photovoltaic performance was achieved (not shown). This indicates that the cation of the added salt significantly influences the performance of the DSCs, whereas the influence from the anion, here it is I-, on DSCs performance is negligible. It is observed from Figure 3, parts a and b, that, compared to the base electrolyte solution where only 0.6 M PMII was used, addition of TBAI reduces Jsc of DSCs. This differs from Nakade et al.’s previous report where DSCs with an electrolyte system containing TBAI provided much higher efficiencies than DSCs based on the same concentration of 1,2-dimethyl-3-propylimidazolium iodide (DMPII), an ionic liquid similar to PMII.16 However, because of the difference in the compositions of the used electrolytes, the results are not directly comparable. The electrolyte investigated in Nakade et al.’s work contained cations of either DMPI+ or TBA+, whereas in our case, two different kinds of cations including PMI+ from PMII and TBA+ are contained in the electrolyte. In order to confirm whether the coexistence of TBA+ with PMI+ in the electrolyte is responsible for the decreased Jsc and power conversion efficiency, DSCs based on the electrolyte containing 0.6 M PMII or 0.6 M TBAI were assembled, respectively. It is found that, in contrast to the low Jsc and power conversion efficiency of the DSCs based on the electrolyte containing 0.1 M TBAI in the base electrolyte investigated in this work, a higher energy conversion efficiency was indeed obtained with the 0.6 M TBAI based cell (Supporting Information Figure 1S), consistent with Nakade et al.’s result. This indicates that the added TBA+ into the base electrolyte is responsible for the decreased Jsc as well as the reduced power conversion efficiency for the DSCs. It is known

J. Phys. Chem. C, Vol. 111, No. 41, 2007 15127 that the ionic radius of both TBA+ and PMI+ are rather large, but the latter has a relatively strong adsorption ability on the TiO2 surface and may exert more electrostatic interaction with the TiO2 surface than the former. It is generally assumed that TBA+ adsorbs only outside the Helmholtz layer and does not interfere with the dye/TiO2 interface.16-18 In comparison with the base electrolyte where only PMII is present, electrostatic shielding of conduction band electrons by adsorbed cations is expected to be reduced in the electrolyte containing TBA+ due to the larger distance of the positive charge from the TiO2 surface. Consequently, electron mobility and lifetime (which will be discussed in the following) is decreased and thus Jsc lowered. Figure 4a shows the dependence of Jsc for the DSCs on the charge/radius ratio for the cations of the iodides in the base electrolyte under illumination of 98 mW/cm2. The dashed line shows Jsc for DSCs based on the base electrolyte. It is observed that Jsc increases with increasing the cationic charge/radius ratio up to 1.5 Å-1. Beyond this value, Jsc starts to saturate and only increases slightly with further increase of the charge/radius ratio of the cations. TBA+ with the lowest charge/radius ratio of 0.242 Å-1 generates the smallest Jsc for the cell (Jsc ) 4.3 mA/cm2), whereas the AlI3-based cell (charge/radius ratio ) 5.88 Å-1) results in the highest Jsc (Jsc ) 13.1 mA/cm2). The variation of Jsc with the cationic charge/radius ratio is in good agreement with a previous report where the relative quantum yield for electron injection, Φinj, from the dye excited state to the conduction band of TiO2 increased with increasing charge/radius ratio of cations present in the electrolyte.10 This indicates that the variation of Jsc with different cations in the electrolyte is governed, at least to some extent, by the difference in Φinj. It is assumed that Φinj of a DSC increases with increasing degree of overlap between the dye excited-state distribution function and the density of semiconductor acceptor states (in the conduction band).19 Cations in the electrolyte can lead to a positive shift of the TiO2 conduction band through intercalation into the crystal lattice or adsorption to the surface of the TiO2 electrode, leading to an increased integral overlap of states, Φinj, and thus Jsc. As we will show below, higher electron injection efficiency is probably not the sole reason for higher Jsc with certain cations. Another important characteristic parameter for a DSC is Voc. Theoretically, Voc is the difference between the Fermi energy level of TiO2 and the potential at the counter electrode which is governed by the redox potential of the electrolyte. According to eq (3),20

Voc )

Iinj kT ln e ncbkbr[I3-]

(3)

Voc of a DSC is determined by several factors including the flux Iinj of injected electrons, the electron concentration ncb at the conduction band edge of TiO2, the rate constant kbr for the electron back-transfer reaction of conduction band electrons with I3- in the electrolyte, and the I3- concentration [I3-] in the electrolyte. Since Iinj and [I3-] were kept identical for the DSCs investigated, Voc should be largely controlled by ncb and kbr in this work. Figure 4b shows the dependence of Voc on the charge/radius ratios of cations present in the base electrolyte. Voc of the DSCs with the base electrolyte is 0.73 V and is shown by the horizontal dashed line. Addition of cations to the base electrolyte mostly decreases Voc. For monocations, no obvious trend is observed for Voc as a function of cationic charge/radius ratio. Voc values

15128 J. Phys. Chem. C, Vol. 111, No. 41, 2007

Wang et al.

Figure 3. Photocurrent density-voltage (J-V) curves of DSCs based on electrolyte solutions containing different 0.1 M iodide additives in the base electrolyte composed of 0.1 M I2, 0.45 M 1-methylbenzimidazole, and 0.6 M PMII in MPN (1, TBAI; 2, none; 3, NaI; 4, LiI; 5, CaI2; 6, MgI2; 7, AlI3) under 0.1 sun level illumination (a) and Jsc as a function of illumination intensity of DSCs (b).

Figure 4. Jsc (a) and Voc (b) of DSCs based on electrolyte solutions containing different iodides (0.1 M MIn) as a function of the charge/radius ratio of Mn+. The horizontal dashed lines show Jsc and Voc for the base electrolyte. Illumination level: 98 mW/cm2.

for Na+-, K+-, Cs+-, and TBA+-based cells are all very similar (0.68-0.69 V) though the size of these cations varies widely with the ionic radii varying from 0.97 to 4.13 Å. Voc of Li+based DSCs is even slightly higher (Voc ) 0.71 V) than for other alkali cations. This phenomenon differs from Liu et al.’s results where Voc linearly increased with the ionic radius of the alkali cations.9 In order to relate the results from literature9 with ours, we have to consider the high concentration of supporting electrolyte of 0.6 M PMII in our work. It appears that PMII exerts, in our case, a more pronounced effect on Voc than the other cations present at significantly lower concentration. To confirm this hypothesis, the performance of DSCs based on electrolytes consisting of high concentration of iodides (0.5 M LiI or NaI) in MPN with or without PMII added was investigated (Supporting Information Table 1S). As can be seen from Table S1, in both cases, 0.5 M NaI based DSCs generate higher Voc and lower Jsc than LiI-based cells, in accordance with literature observations. Moreover, Voc of the DSCs containing 0.5 M LiI in the base electrolyte decreases significantly in comparison with the DSCs containing 0.1 M LiI. Apparently, the effect of cations on Voc is masked by the high concentration of PMII in this work. In contrast to the monocations, the dication- and the tricationbased iodides have a more pronounced effect on Voc. The addition of CaI2 into the base electrolyte slightly enhances Voc to 0.74 V, whereas AlI3 reduces Voc significantly to 0.59 V. As discussed above, Voc should be controlled by the electron concentration ncb at the conduction band edge of TiO2 and the electron back-transfer reaction kbr. The Coulombic force exerted by cations adsorbed to the TiO2 surface is expected to positively

shift the band energies, the Fermi level, and thus decrease Voc. In fact, Al3+, the ion with the largest surface electron density, shows the lowest Voc among the electrolyte systems investigated. Following this reasoning, one would expect lower Voc for Mg2+and Ca2+-based salts in comparison with Li+ or Na+ salts. This has indeed been observed by Redmond and Fitzmaurice with polycrystalline TiO2 electrodes in acetonitrile solutions containing Mg2+, Li+, or Na+, respectively, where the positive shift of the TiO2 Fermi level was reported to follow the trend Mg2+ > Li+ > Na+.7 In this work, it is found, however, that cations used in the electrolyte do not exhibit the same clear-cut trends according to ncb. We observed that lower relative charge/radius ratios do not necessarily lead to higher Voc values (see Figure 4b). This suggests that the difference in the kbr should be responsible for at least some of the variation of Voc. To clarify this, EIS was employed to investigate the electron-transfer processes in the DSCs employing iodides based on various cations. Electrochemical Impedance Spectra Characterization. EIS has shown its usefulness in exploring the kinetics processes in DSCs.21-25 On the basis of a simplified equivalent circuit (Figure 5), three distinct features are generally observed in ac impedance spectrum for a DSC under high enough forward bias (see Supporting Information Figure 2S, Nyquist plot, and Bode plot (inset)). The semicircle in the high-frequency region (>100 Hz) is due to the counter electrode and can be described by the charge-transfer resistance Rct and its double layer capacitance Cdl (expressed as a constant phase element). The quasisemicircle at intermediate frequencies (∼100-1 Hz) is ascribed to the electron back-transfer resistance, Rbr, and the chemical

Effect of Iodides on Dye-Sensitized Solar Cells

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Figure 5. Equivalent circuit used for modeling ac impedance spectra of DSCs.

TABLE 1: Characteristic Parameters of the ac Impedance Spectra of DSC with Variable Iodides in Base Electrolytea iodides (0.1 M)

Rbr (Ω)

Rct (Ω)

τ (ms)

AlI3 LaI3 BaI2 LiI TBAI Basic electrolytea MgI2 CaI2

8.2 15.4 19.1 23.8 25.8 33.7 35 39.6

2.3 8.51 5.3 15.2 20.7 25.6 7.1 2.3

68 105 130 172 21 34 170 213

a Compositions of the base electrolyte: 0.6 M 1-propyl-3-methylimidazolium iodide (PMII), 0.1 M I2, 0.45 M 1-methylbenzimidazole (MBI) in methoxypropionitrile (MPN).

capacitance Ccc (expressed as a constant phase element) of the TiO2 nanoparticles. The feature observed at the lowest frequencies (