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Cooperative effect of adsorbed cations on electron transport and recombination behavior in dye-sensitized solar cells
Accepted Manuscript Title: Cooperative Effect of Adsorbed Cations on Electron Transport and Recombination Behavior in Dye-Sensitized Solar Cells Author: Dongxing Kou Weiqing Liu Linhua Hu Songyuan Dai PII: DOI: Reference:
S0013-4686(13)00517-3 http://dx.doi.org/doi:10.1016/j.electacta.2013.03.105 EA 20213
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
1-11-2012 14-3-2013 14-3-2013
Please cite this article as: D. Kou, W. Liu, L. Hu, S. Dai, Cooperative Effect of Adsorbed Cations on Electron Transport and Recombination Behavior in Dye-Sensitized Solar Cells, Electrochimica Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.03.105 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Cooperative Effect of Adsorbed Cations on Electron Transport and Recombination Behavior in Dye-Sensitized Solar Cells Dongxing Kou, Weiqing Liu, Linhua Hu and Songyuan Dai*
ip t
Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, P. R. China
cr
Abstract. Lithium ion (Li+) and imidazolium cations (Im+s) had been reported to have
us
competitive effects on the photoinduced electrons in TiO2-electrolyte systems. Herein, a further investigation about their cooperative effect in dye-sensitized solar cells (DSCs) using organic liquid electrolyte is developed by altering alkyl chain length. Imidazolium iodides (Im+I-s) with
an
different alkyl chain length (3, 6, 12) were synthesized and used as iodide sources. The adsorption amount of Im+s onto TiO2, band edge shifts, trap states distribution, electron
M
recombination/transport processes and ion transport within the electrolyte for DSCs were detected. It is found that the multilayered adsorption of Im+s can induce a lower photoinduced
d
electron density. In-depth characterizations indicate that this negative effect can be reduced as the adsorption amount decreased with increasing alkyl chain length and the effect of Li+ is
te
consequently strengthened in varying degrees. The decisive role of Li+ in cation-controlled
Ac ce p
interfacial charge injection process finally contributes an ordinal increase of short-circuit photocurrent density Jsc for DSCs with increasing alkyl chain length because of the increasing charge injection efficiency ηinj. Additionally, a large power dissipation in ions transport process is induced by the long alkyl chain of Im+s. Overall, the cell efficiencies are relatively dependent of the trade-off between Jsc and FF, which is essentially related to the cooperative effect of adsorbed cations.
Keywords: imidazolium cations; lithium ion; dye-sensitized solar cells; charge transport and recombination
1. Introduction Over the past 20 years, the pioneering work of Michael Grätzel and coworkers has led to the development of high efficiency dye-sensitized solar cells (DSCs)[1-4]. Up to now, an landmark photoelectric conversion efficiency over 12.3% has been obtained[5], which gives a
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promising application foreground in renewable energy. The working principle of this device is based on the injection of electrons from photo-excited states of dye to the conduction band of the semiconductor followed by the reduction of the oxidized dye with a charge mediator[3,6,7].
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During these processes, electrolyte plays an important role in ions transport and charges transfer at TiO2/electrolyte or Pt/electrolyte interfaces[6,8,9]. It has been known that several species of cations existing in the electrolyte solution as the counteraction of I- and I3-, such as
cr
Li+, Im+ and so on. The adsorption or the intercalation of cations on TiO2 surface can influence
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the number or distribution of surface states, the strength of sensitizer surface attachment, the energetics of the semiconductor and sensitizer, the charge transport rate, the dynamics of
an
interfacial electron transfer as well as the rate of iodide oxidation[10-12]. Yanagida[6] had shown that the influence of cations on electron diffusion coefficient of TiO2 films in accordance with DMHI+ < TBA+ < Na+ < Li+. Furthermore, Fitzmaurice[11,13] reported that for a given
M
cation activity, the magnitude of the positive shift of Efb decreased in the order Mg2+ > Li+ > Na+. Their work strongly suggests that the magnitude of the effect of adsorbed cations mostly
d
depends on their nature, concentration and charge-to-radius ratio[14-16].
te
At the TiO2/electrolyte interface, Im+s multilayer adsorb onto TiO2 surfaces due to mutual electrostatic effect. Researches demonstrates that this adsorption can lead to a lower
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photoinduced electron density in comparison with other species[6]. On the other hand, the intercalation of Li+ results in the conduction band of TiO2 films shifts positively and enhances the electron transfer from the sensitized dye to TiO2 films, subsequently increases the quantum yield and promotes the photogeneration of charges[10]. Overall, the competition between effects of Im+s and Li+ on the photoinduced electrons truly exists and an optimal cell efficiency requires alternative species to harmony with each other. Many results had been published recently regarding the photovoltaic properties of DSCs
containing Im+s and Li+. But most of them were carried out in terms of ionic liquid electrolyte, where the Jsc was limited by the ions transport process[17,18]. Kyung[18] used Im+I-s with different substituents as electrolytes and the short-circuit photocurrent (Jsc) was found to increase with decrease in size of the substituent for the Li+ absented DSCs. After the addition of Li+, Kubo[17] proposed that the Jsc of DSCs could increase with increasing alkyl chain up to
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C7 of 1-alkyl-3-methylimidazolium. Their findings suggest us a promising conclusion that Li+ plays a significant role in the cations coexistence system. So far, there is no detailed study disclosing the cooperative effects of adsorbed cations on electron transport and recombination
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behavior in DSCs using organic liquid electrolyte. Herein, we develop an experiment aiming to verify the difference of dynamic response in both Im+s and Li+ containing DSCs by altering
cr
alkyl chain length of Im+s. Intensity-modulated photocurrent spectroscopy (IMPS)/intensitymodulated photovoltage spectroscopy (IMVS) measurements[19,20] and equivalent circuit
us
analysis[21-23] were carried out for kinetics characterization. This work will provide significant scientific contributions to the further understanding the detailed working mechanism
an
of adsorbed cations and high efficiency research for DSCs.
2. Experimental section
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2.1. Chemicals and materials
Anhydrous lithium iodide (LiI), Iodine (I2) and 3-Methoxypropionitrile (MePN) were purchased from Aldrich. Imidazolium salts with bromide as counterion (Im+Br-s) were
d
purchased from sinoreagent and used for detecting the absorbance of Im+s because Br- had no
te
absorption under UV spectra range from 200 to 400 nm. All the chemical reagents were used as received. 1,3-dialkylimidazolium iodides were synthesized as reported previously[24,25] and
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their purity had been confirmed by 1H-NMR. The Im+s are composed of two different alkyl subsistents in position R1 and R2, where R1 has been fixed by a methyl group and R2 is an alkyl substituent with different alkyl chain lengths (n=3, 6, 12). Molecular structures of 1, 3-dialkylimidazolium iodides are shown in fig. 1. Cells with electrolytes containing different Im+s and Li+ (0.1 M LiI, 0.08 M I2, 0.6 M MPII in MePN; 0.1 M LiI, 0.08 M I2, 0.6 M HMII in MePN; 0.1 M LiI, 0.08 M I2, 0.6 M C12MII in MePN; 0.1 M LiI, 0.08 M I2, 0.6 M TBAI in MePN) were assembled and denoted as C3, C6, C12 (Im+s cells) and TBA, respectively. The TBA cell was used as reference for investigating photoinduced electrons in comparaion with Im+s cells. Aiming to clarify the working mechanism of cations, TBP was excluded because of its mutual effect with Li+ and/or the influence on the adsorption of Im+s onto TiO2 films. Fig. 1
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2.2. The cell assembly The
colloidal
TiO2
nanoparticles
were
prepared
by
hydrolysis
of
titanium
tetraisopropoxide as described elsewhere[26]. Then the obtained nanoparticles were mixed with
ip t
polyethylene glycol (PEG, Aldrich) to form the viscous paste. The colloidal paste was coated on transparent conductive oxide (TCO) glass substrate by screen-printing. The films (5 mm×5
cr
mm) were dried at room temperature for 5 min and then sintered at 510 °C for 30 min in air. The films thickness was about 11 µm determined by surface profilometer (XP-2, AMBIOS
us
Technology Inc., U.S.A.). The photoelectrodes were immersed in a dye solution containing N719 at room temperature for 12 h. The platinized counter electrodes were obtained by
an
spraying H2PtCl6 solution on TCO glass, followed by heating at 410 °C for 20 min. The obtained counter electrode was placed directly on top of the dyed TiO2 films sealed with a 60 µm thermal adhesive film (Surlyn, Dupont). The electrolyte was filled from a hole on the
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counter electrode and then sealed with a surlyn adhesive film under a cover glass by heating. 2.3. Measurements
d
The photovoltaic performances of the DSCs were measured by a Keithley 2420 3A source
te
meter controlled by Testpoint software under solar simulator (solar AAA simulator, oriel USA, calibrated with a standard crystalline silicon solar).
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All the following electrochemical measurements were carried out on an IM6ex
electrochemical workstation (Germany, Zahner) at 25 °C. IMPS/IMVS measurements were performed on IM6ex workstation using light-emitting diodes (λ=610nm) driven by Expot (Germany, Zahner). The LED provided both dc and ac illumination components. The amplitude of the modulated component was 10% or less of dc component. The frequency range was 3 kHz to 0.1 Hz. Impedance measurements were performed by the IM6ex at reverse biases in the frequency range of 0.08 Hz-1000 kHz. The amplitude of the alternative signal was 5 mV. Steady-state voltammetry of the electrolytes were adopted in a conventional electrochemical cell equipped with a 1.0 µm platinum ultramicroelectrode (CHI107) as working electrode, and with a 1 mm radius platinum disk electrode (CHI102) as counter electrode and reference electrode. A slow scan rate of 5 mV/s was used in order to obtain steady-state current–voltage
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curves. The nitrogen (N) atom of Im+s can interact with the TiO2 films due to mutual electrostatic effect [6,18]. To investigate the differences of adsorption ability of Im+s onto TiO2 electrode,
ip t
the UV-Vis spectra of Im+Br-s in acetonitrile were performed by an ultraviolet-visible spectrophotometer (TU-1901, Persee Inc. China). Photometric scanning at 270 nm were carried
cr
out to detect the absorbance of solutions before and after an addition of small amount of dehydrated TiO2 (20 nm) as reported previously. According to Beer–Lambert law A = εcl[27],
us
where A is the absorbance, ε is the molar absorbtivity or extinction coefficient, l is the path length of the sample and c is the concentration of the compound in solution, absorbance is
an
proportional to the concentration of absorbing species in solution. Fig. 2 shows the absorbance of Im+s as a function of solution concentration (closed symbols), from which we can calculate the concentration of Im+s after the addition of TiO2 (open symbols) and deduce the adsorption
M
amount on TiO2 surface. The initial concentration is 0.1 M for Im+s and 1.0 M for TBA+, after the addition of TiO2 and the centrifugation, the adsorption amount decreased in the order of
d
MPI+ < HMI+ < C12MI+ < TBA+, indicating the absorption amount of Im+s decreased with
te
increasing alkyl chain length. One can conclude that the TBA+ is a weak adsorption species in accordance with the variation of absorbance. Additionally, the multilayered adsorption of Im+s
Ac ce p
on TiO2 surface is calculated by ca.85 molecules/nm2, which is consistent with previous report[6].
Fig. 2
3. Results and discussion
3.1. Photoinduced electron density and surface states distribution Fig. 3
The photoinduced electron density (Q, the product of Jsc and τn) in TiO2 films was detected by IMPS/IMVS measurements (Jsc was obtained under the same light intensity as well as τn)[28]. The dependence of Voc on Q for Im+s cells is shown in Fig. 3, where the TBA+ is used to be a weak adsorption species in comparison with Im+s. The TBA cell has the highest photoinduced electron density at the same energy level, which suggests that the absolute
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adsorption of Li+ in the mesoporous TiO2 films can promote the photogeneration of charges because of its enhancement to the quantum injection yield. When the cations of iodide source also adsorb onto TiO2 films, Im+s cells exhibit lower photoinduced electron density due to their
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multilayered adsorption and steric hindrance effects[6]. In both Im+s and Li+ containing system, their reverse effects on photoinduced electron density should be weakened by each other. That is to say, the falling of one will always along with the rising of the other. As the decreasing
cr
adsorption amount of Im+s with the increase of alkyl chain length is observed by UV-Vis
us
spectra, we can consider that the negative effect of Im+s on photoinduced electron density is reduced either. Simultaneously, the effect of Li+ is reasonably enhanced in varying degrees for
an
Im+s cells with the increase of alkyl chain length. Since the magnitude of the effect of adsorbed cations mostly depends on their charge-to-radius ratio[14-16], the decisive role of Li+ in cation-controlled interfacial charge injection process finally induces an ordinal increase of
M
photoinduced electron density at the same energy level with the increase of alkyl chain length as seen in Fig. 3. Overall, Li+ plays a determination role in the cations coexistence DSCs and
d
this role will be orderly enhanced with increasing alkyl chain length of Im+s. In-depth
te
characterizations about the cooperative effect of adsorbed cations on dynamic processes in DSCs will be discussed in detail.
Ac ce p
The accumulated charge in the conduction band determines the difference between the
conduction band and electron quasi-Fermi level. We can analysis band edge shifts by comparing the change of Voc at the constant Q. The relationship between Voc and ln(Q) can be expressed with the following linear equation[20,29]
Voc Vc mc ln(Q)
(1)
where Vc is the vertical intercept and mc is the slope rate. If mc = 26 mV, electrons recombination occurs principally via the conduction band. Otherwise, if mc > 26 mV, electrons recombination occurs principally via surface states. Data in Fig. 3 shows that the values of mc ranged from 62 to 95 mV for all samples, which suggests electrons recombination mainly via surface states. For Im+s cells, ∆Vc (the difference of conduction band edge between the comparison cells) indicates that the band edge of C6 and C12 cell shifts positively about 63 mV
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and 89 mV in comparison with C3 cell. The enhancement of the effect of Li+ on energy level of TiO2 films would lead to a more favorable driving force for electron injection. Fig. 4
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For impedance measurements in the dark, the charge stored in TiO2 films at any value of the potentials is given by the equation[30]
(2)
cr
Qdark VV00 Vbias VdU
where V0 is the initial potential and V is the potential dependent capacitance. So the calculated
us
capacitance of the TiO2 electrodes as a function of the applied potential offers us a useful description to the surface states energy distribution, which can be characterized as the
an
following
C Ca exp[ eV /(k BT )] Cb
(3)
M
where kB is the Boltzmann constant, T is the temperature, e is the elementary charge, V is the applied potential, Ca is the prefactor of the exponential, Cb is the quasi constant capacitance at
d
low potentials, and β is a coefficient describing an exponential distribution of surface states
te
below the conduction band edge (β < 1). A smaller β implies that the distribution is deeper on average with respect to the conduction band and that the relative proportion of deep traps is
Ac ce p
larger[30-32]. Fig. 4 presents the characterization of surface states over the energy range applied in Im+s cells by the potential dependent capacitance. Data is obtained by fitting impedance spectrum. In accordance with the variation of photoinduced electron density, the reduced adsorption amount of Im+s is accompanied with the enhancement of the effect of Li+. Their cooperative effect should influence the distribution of surface states in varying degrees. It can be seen in Fig. 4 that the fitted β is 0.19, 0.17 and 0.16 for Im+s cells with the increasing alkyl chain length as expected. Subsequently, the deeper surface states distribution has a profound influence on the transport and recombination dynamic processes of electrons in TiO2 films. 3.3. The electron transport and recombination behavior Nelson[33] proposed a random walk mechanism for electron transport in TiO2 films, in which each electron could move after a waiting time determined by the activation energy of the
Page 7 of 26
trap site. It is widely accepted that electron transport in TiO2 films is limited by trapping and detrapping processes[34,35]. Generally, the adsorption of cations can create the extra trap states on the surface of TiO2, in this case, the electron diffusion should be slowed. Fig. 5 shows
ip t
electron transit and recombination times for Im+s cells as a function of the light intensity I0, where the electron transit time τd is associated with the electron transport process from the injection sites to the TCO substrate and the recombination time τn is mostly determined by the
cr
rate of reaction between the electrons in the TiO2 films and I3-. Both the constants can be
us
estimated from the expression τd = 1/2πfImps and τn = 1/2πfImvs[23], where fImps and fImvs is the characteristic frequency minimum of the IMPS/IMVS imaginary component, respectively. Data
an
in Fig. 5a exhibits a power-law dependence on the light intensity of τd following the expression τd (I0)α-1[36]. With increasing light intensity, the quasi-Fermi energy of the electrons shifts closer to the conduction band edge, such that the trap depth involved in the electron transport
M
process decreases. This results in a more fast transmission process, as seen in Fig. 5a, τd is decreased with the increase of I0. Besides, it is observed that τd is increased with increasing
d
alkyl chain length at the same I0 for Im+s cells due to the increased trap depth mentioned above.
te
Additionally, steric hindrance of the long alkyl chain may also give a contribution to the delay of τd, which needs to be further researched.
Ac ce p
Generally, the transport-limited recombination of photocarriers[36] can induce a
proportionately increasing to τn along with the increasing of τd. Fig. 5b shows the dependence of τn on I0. It can be found that τn of C6 cell and C12 cell is much longer than C3 cell but there is no obvious difference between themselves. Though Im+s could restrain the recombination between I3- and photoinduced electrons[18], this steric hindrance effect should be weakened with increasing alkyl chain length due to the decreasing of absorption amount. The compromise as mentioned finally results in a comparable τn between C6 cell and C12 cell themselves. Fig. 5 In DSCs, photocurrents and photovoltages arise from a subtle balance of transport of photoinduced electrons toward the external circuit and recombination of these electrons with acceptors present in the system. This competition is a good description to the charge collection ability of photoelectrode. It can be characterized with the value of electron diffusion length (Ln),
Page 8 of 26
as expressed by the relation Ln Dn n , where Dn is electron diffusion coefficient determined from the expression Dn = d2/2.35τd and d is the film thickness[37]. So the Ln can be further expressed as (4)
n 2.35 d
ip t
Ln d
cr
An excellent photoelectrode for the charge collecting corresponds a much longer Ln than d. Fig. 6 shows that the C6 cell had the longest Ln due to the ratio τn/τd, indicating that there exists an
Fig. 6
an
Fig. 7
us
optimal alkyl chain length of Im+s for charge collection in photoelectrode.
In general, Jsc can be approximated by the following expression[38]: (5)
M
J sc qlhinj cl I 0
where q is the elementary charge, ηlh is the light-harvesting efficiency, ηinj is the charge
d
injection efficiency, and ηcl is the charge collecting efficiency. Among the three parameters, ηlh is mainly determined by the amount and extinction coefficient of adsorbed dye and the
te
light-scattering properties of the films; ηcl is totally determined by the competition between
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recombination and transport as discussed above; ηinj is largely related to the location of energy level between excited dye and the conduction band of semiconductor films. Fig. 7 shows the Jsc dependences on I0 for Im+s cells, where the Jsc is linearly changed along with I0 and the slopes increased with increasing alkyl chain length. Assuming ηlh for Im+s cells are the same, one can estimate the difference in slopes is determined by ηinj and ηcl. Since ηcl achieved the highest value by C6 cell, the increasing of slopes is mostly attributed to the enhancement of ηinj[39]. That is to say, the ordinal increasing of Jsc is essentially related to the location of conduction band determined by the cooperative effect of Im+s and Li+. This result is quite different from that of DSCs using ionic liquid electrolyte, where the enhanced Jsc is explained by the increased reductive activity of I- arising from increased alkyl chain length of Im+s[18]. 3.3. The effect of series resistance on cell performances Resistive effects in solar cells reduce the efficiency of the solar cells by dissipating power
Page 9 of 26
in internal resistance[22]. The internal resistance has a direct impact to FF and a low total series resistance contributes to a good FF value. In general, the series resistance roughly corresponds to the sum of system resistance, charge-transfer process occurring at the
ip t
Pt/electrolyte interface and ions transport process within the electrolyte[21,22]. Fig. 8 illustrates the equivalent circuit analysis to the influence of Im+s on internal processes of Im+s cells. The
cr
fitted data in Table 1 shows that Im+s with increasing alkyl chain length induces an ordinal increase in system resistance (Rh) and charge-transfer resistance at Pt/electrolyte interface (R1)
Fig. 8
transport process within the electrolyte is investigated by steady-state
an
Ions
us
due to the increased steric hindrance and viscosity.
voltammograms of solutions as seen in Fig.9. The apparent diffusion coefficient (D) of I3- and Iin MePN are calculated from the anodic and cathodic steady-state current (is) using the
M
equation is = 4nFDroc[40]. As we know, at the same temperature the viscosity of Im+s increases with increasing alkyl chain length because of van der Waals forces[17,18]. It is found that the D
d
values of I3- and I- (Table 1) both decreased with the increase of alkyl chain length. As a result,
te
the ions transport process within electrolyte becomes more slowly with increasing alkyl chain
Ac ce p
length, subsequently leading to large power dissipation. Fig. 9 Table 1
Table 2 lists the photocurrent–voltage properties of Im+s cells under AM 1.5, 100
mW·cm-2 irradiation. As discussed above, the cooperative effect of Im+s and Li+ contributes an ordinal increase of Jsc with increasing alkyl chain length. However, the ordinal increase of series resistance leads to large power dissipation and a lower FF. It can be seen in Table 2 that the cell efficiencies are relatively dependent of the trade-off between Jsc and FF. As a result, the C6 cell gives an optimal efficiency rather than C3 and C12 cell. Table 2
4. Conclusion In conclusion, the investigation of the cooperative effect of Im+s and Li+ in DSCs are
Page 10 of 26
developed in this work by altering alkyl chain length. It is found that the multilayered adsorption of Im+s can induce much less photoinduced electrons. In-depth research indicates that this negative effect is reduced with increasing alkyl chain length and the effect of Li+ is
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subsequently enhanced in varying degrees. Overall, the decisive role of Li+ in cation-controlled interfacial charge injection process induces a higher Q and a more positive band edge shift. The cooperative effect of Im+s and Li+ finally leads to an ordinal increase of Jsc for DSCs with
cr
increasing alkyl chain length. Equivalent circuit analysis suggests that Im+s have remarkable
us
influences on series resistance and can lead to a lower FF with increasing alkyl chain length. Consequently, we propose that the efficiencies of Im+s cells are relatively dependent of the
an
trade-off between Jsc and FF, which is essentially related to the cooperative effect of adsorbed cations. Further studies need to be developed to understand the detailed working mechanism of cations coexistence system.
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Acknowledgment
This work was financially supported by the National Basic Research Program of China under
d
Grant No. 2011CBA00700, the National High Technology Research and Development Program
te
of China under Grant No. 2011AA050527, the National Natural Science Foundation of China under Grant No. 21003130, the National Natural Science Foundation of China under Grant No.
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21173228, and the China Postdoctoral Science Foundation under Grant No. 20110490835.
References
[1] B. O'regan, M. Gratzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature 353 (1991) 737.
[2] H. G. Jung, Y. S. Kang, Y. K. Sun, Anatase TiO2 spheres with high surface area and mesoporous structure via a hydrothermal process for dye-sensitized solar cells, Electrochim. Acta 55 (2010) 4637. [3] L. M. Peter, Dye-sensitized nanocrystalline solar cells, Phys. Chem. Chem. Phys. 9 (2007) 2630. [4] S. J. Lim, Y. S. Kang, D. W. Kim, Dye-sensitized solar cells with quasi-solid-state cross-linked polymer electrolytes containing aluminum oxide, Electrochim. Acta 56 (2011) 2031. [5] A. Yella, H. W. Lee, H. N. Tsao, C. Y. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W. G. Diau, C. Y. Yeh, S. M. Zakeeruddin, M. Gratzel, Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency, Science 334 (2011) 629. [6] S. Kambe, S. Nakade, T. Kitamura, Y. Wada, S. Yanagida, Influence of the electrolytes on electron transport in mesoporous TiO2-electrolyte systems, J. Phys. Chem. B 106 (2002) 2967.
Page 11 of 26
[7] G. Boschloo, A. Hagfeldt, Characteristics of the iodide/triiodide redox mediator in dye-sensitized solar cells, Acc. Chem. Res. 42 (2009) 1819. [8] F. Fabregat-Santiago, J. Bisquert, G. Garcia-Belmonte, G. Boschloo, A. Hagfeldt, Influence of electrolyte in transport and recombination in dye-sensitized solar cells studied by impedance spectroscopy, Sol. Energy Mater. Sol .Cells 87 (2005) 117.
ip t
[9] S. Nakade, Y. Makimoto, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida, Roles of electrolytes on charge couples, J. Phys. Chem. B 109 (2005) 3488.
cr
recombination in dye-sensitized TiO2 solar cells (2): The case of solar cells using cobalt complex redox [10] C. A. Kelly, F. Farzad, D. W. Thompson, J. M. Stipkala, G. J. Meyer, Cation-controlled interfacial charge injection in sensitized nanocrystalline TiO2, Langmuir 15 (1999) 7047.
us
[11] D. F. Watson, G. J. Meyer, Cation effects in nanocrystalline solar cells, Coord. Chem. Rev. 248 (2004) 1391.
[12] B. C. O'Regan, J. R. Durrant, Calculation of activation energies for transport and recombination in
an
mesoporous TiO2/dye/electrolyte films-Taking into account surface charge shifts with temperature, J. Phys. Chem. B 110 (2006) 8544.
[13] M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphrybaker, E. Muller, P. Liska, N. Vlachopoulos, M.
M
Gratzel, Conversion of light to electricity by cis-X2bis(2,2'-bipyridyl-4,4'-dicarboxylate) ruthenium (II) charge-transfer sensitizers (X=Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline TiO2 electrodes, J. Am. Chem. Soc. 115 (1993) 6382.
te
Chem. B 103 (1999) 7860.
d
[14]G. Boschloo, D. Fitzmaurice, Electron accumulation in nanostructured TiO2 (anatase) electrodes, J. Phys. [15] G. Redmond, D. Fitzmaurice, Spectroscopic determination of flat-band potentials for polycrystalline
Ac ce p
TiO2 electrodes in nonaqueous solvents, J. Phys. Chem. 97 (1993) 1426. [16] B. I. Lemon, J. T. Hupp, Electrochemical quartz crystal microbalance studies of electron addition at nanocrystalline tin oxide/water and zinc oxide/water interfaces: Evidence for band-edge-determining proton uptake, J. Phys. Chem. B 101 (1997) 2426. [17] W. Kubo, T. Kitamura, K. Hanabusa, Y. Wada, S. Yanagida, Quasi-solid-state dye-sensitized solar cells using room temperature molten salts and a low molecular weight gelator, Chem. Commun. (2002) 374. [18] K. M. Son, M. G. Kang, R. Vittal, J. Lee, K. J. Kim, Effects of substituents of imidazolium cations on the performance of dye-sensitized TiO2 solar cells, J. Appl. Electrochem. 38 (2008) 1647. [19] L. M. Peter, Characterization and modeling of dye-sensitized solar cells, J. Phys. Chem. C 111 (2007) 6601. [20] G. Schlichthorl, S. Y. Huang, J. Sprague, A. J. Frank, Band edge movement and recombination kinetics in dye-sensitized nanocrystalline TiO2 solar cells: A study by intensity modulated photovoltage spectroscopy, J. Phys. Chem. B 101 (1997) 8141. [21] L. Y. Han, N. Koide, Y. Chiba, A. Islam, T. Mitate, Modeling of an equivalent circuit for dye-sensitized solar cells: improvement of efficiency of dye-sensitized solar cells by reducing internal resistance, C.R. Chim.
Page 12 of 26
9 (2006) 645. [22] M. Murayama, T. Mori, Evaluation of treatment effects for high-performance dye-sensitized solar cells using equivalent circuit analysis, Thin Solid Films 509 (2006) 123. [23] W. Q. Liu, L. H. Hu, S. Y. Dai, L. Guo, N. Q. Jiang, D. Kou, The effect of the series resistance in modulation techniques, Electrochim. Acta 55 (2010) 2338.
ip t
dye-sensitized solar cells explored by electron transport and back reaction using electrical and optical [24] P. Bonhote, A. P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Gratzel, Hydrophobic, highly
cr
conductive ambient-temperature molten salts, Inorg. Chem. 35 (1996) 1168.
[25] A. B. Pereiro, E. Tojo, A. Rodriguez, J. Canosa, J. Tojo, HMImPF(6) ionic liquid that separates the azeotropic mixture ethanol plus heptane, Green Chem. 8 (2006) 307.
us
[26]L. H. Hu, S. Y. Dai, J. Weng, S. F. Xiao, Y. F. Sui, Y. Huang, S. H. Chen, F. T. Kong, X. Pan, L. Y. Liang, K. J. Wang, Microstructure design of nanoporous TiO2 photoelectrodes for dye-sensitized solar cell modules, J. Phys. Chem. B 111 (2007) 358.
an
[27] P. M. Wiegand, S. R. Crouch, Evaluation of a programmable, hardware-driven, isolated-droplet generator, Appl. Spectrosc. 42 (1988) 567.
[28] K. Zhu, N. Kopidakis, N. R. Neale, J. van de Lagemaat, A. J. Frank, Influence of surface area on charge
M
transport and recombination in dye-sensitized TiO2 solar cells, J. Phys. Chem. B 110 (2006) 25174. [29] X. M. Ren, Q. Y. Feng, G. Zhou, C. H. Huang, Z. S. Wang, Effect of cations in coadsorbate on charge (2010) 7190.
d
recombination and conduction band edge movement in dye-sensitized solar cells, J. Phys. Chem. C 114
te
[30] F. Fabregat-Santiago, I. Mora-Sero, G. Garcia-Belmonte, J. Bisquert, Cyclic voltammetry studies of nanoporous semiconductors. Capacitive and reactive properties of nanocrystalline TiO2 electrodes in aqueous
Ac ce p
electrolyte, J. Phys. Chem. B 107 (2003) 758. [31] C. N. Zhang, Z. P. Huo, Y. Huang, L. Guo, Y. F. Sui, L. H. Hu, F. T. Kong, X. Pan, S. Y. Dai, K. J. Wang, Studies of interfacial recombination in the dyed TiO2 electrode using Raman spectra and electrochemical techniques, J. Electroanal. Chem. 632 (2009) 133. [32] J. A. Anta, F. Casanueva, G. Oskam, A numerical model for charge transport and recombination in dye-sensitized solar cells, J. Phys. Chem. B 110 (2006) 5372. [33] J. Nelson, R. E. Chandler, Random walk models of charge transfer and transport in dye sensitized systems, Coord. Chem. Rev. 248 (2004) 1181. [34] J. Nelson, S. A. Haque, D. R. Klug, J. R. Durrant, Trap-limited recombination in dye-sensitized nanocrystalline metal oxide electrodes, Phys. Rev. B 63 (2001). [35] J. van de Lagemaat, N. G. Park, A. J. Frank, Influence of electrical potential distribution, charge transport, and recombination on the photopotential and photocurrent conversion efficiency of dye-sensitized nanocrystalline TiO2 solar cells: A study by electrical impedance and optical modulation techniques, J. Phys. Chem. B 104 (2000) 2044. [36] N. Kopidakis, K. D. Benkstein, J. van de Lagemaat, A. J. Frank, Transport-limited recombination of
Page 13 of 26
photocarriers in dye-sensitized nanocrystalline TiO2 solar cells, J. Phys. Chem. B 107 (2003) 11307. [37] G. Schlichthorl, N. G. Park, A. J. Frank, Evaluation of the charge-collection efficiency of dye-sensitized nanocrystalline TiO2 solar cells, J. Phys. Chem. B 103 (1999) 782. [38] K. Zhu, N. R. Neale, A. Miedaner, A. J. Frank, Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays, Nano Lett. 7 (2007) 69.
ip t
[39] C. N. Zhang, Y. Huang, Z. P. Huo, S. H. Chen, S. Y. Dai, Photoelectrochemical effects of guanidinium thiocyanate on dye-sensitized solar cell performance and stability, J. Phys. Chem. C 113 (2009) 21779. [40] C. W. Shi, S. Y. Dai, K. J. Wang, X. Pan, L. Guo, L. Y. Zeng, L. H. Hu, F. T. Kong, Influence of
Ac ce p
te
d
M
an
us
dye-sensitized solar cells, Sol. Energy Mater. Sol. Cells 86 (2005) 527.
cr
1-methyl-3-propylimidazolium iodide on I3-/I- redox behavior and photovoltaic performance of
Page 14 of 26
ip t cr us
Table
Table 1 Influence of Im+s on internal resistances and carrier transport by I-/I3- within the
an
electrolyte
DI - /10-6 cm2·s-1
-6 2 -1 DI - /10 cm ·s
7.02
4.21
15.95
5.27
2.53
28.20
4.64
2.23
Rh/
R1/
C3
34.47
10.48
C6
40.99
C12
66.69
3
d
M
Cell
te
Table 2 Photovoltaic performances of DSCs with Im+I-s and LiI Voc/V
Jsc/mA·cm-2
FF
η/%
0.57
12.45
0.56
3.98
C6
0.60
14.79
0.52
4.61
C12
0.57
15.57
0.46
4.08
Cell
Ac ce p
C3
Page 15 of 26
ip t cr
Figure Capations
Chemical structures of 1-alkyl-3-methylimidazolium iodides
Fig. 2.
Absorbance of Im+s as a function of solution concentration (solid symbols) under 270
us
Fig. 1.
nm UV irradiation. Open symbols are the centrifuged solutions after the addition of TiO2 (10
an
mg for 5 mL solution).
Voc as a function of lnQ for DSCs with different cations of iodide source and Li+.
Fig. 4.
Results from the impedance data for capacitance of DSCs with Im+s and Li+.
Fig. 5.
Comparison of (a) transport and (b) recombination time constants as a function of light
M
Fig. 3.
intensities for DSCs with Im+s and Li+ under 610 nm LED irradiation.
te
Im+s and Li+.
d
Fig. 6. Dependence of effective electron diffusion length on light intensities for DSCs with Fig. 7. Dependence of short-circuit photocurrent densities on light intensities for DSCs with
Ac ce p
Im+s and Li+. Fig. 8.
Electrochemical impedance spectroscopy (Nyquist plots) for DSCs with Im+s and Li+
measured at -0.6 V. The lines show the fit results and the scatter dots are experiment data. The inset is the equivalent circuit, where Rh is the system resistance, R1 is resistance related to charge-transfer processes occurring at the Pt/electrolyte interface and R2 is charge transfer resistance occurring at TiO2/electrolyte interface. Fig. 9.
Steady-state voltammograms of solutions with Im+I-s and LiI in MePN at 25 °C.
Page 16 of 26
Chemical structures of 1-alkyl-3-methylimidazolium iodides
Ac ce p
Fig. 1.
te
d
M
an
us
cr
ip t
Figures
Page 17 of 26
ip t cr us an M d te
Absorbance of Im+s as a function of solution concentration (solid symbols) under 270
Ac ce p
Fig. 2.
nm UV irradiation. Open symbols are the centrifuged solutions after the addition of TiO2 (10 mg for 5 mL solution).
Page 18 of 26
ip t cr us an M d te Ac ce p Fig. 3.
Voc as a function of lnQ for DSCs with different cations of iodide source and Li+.
Page 19 of 26
ip t cr us an M d te Ac ce p
Fig. 4.
Results from the impedance data for capacitance of DSCs with Im+s and Li+.
Page 20 of 26
ip t cr us an M d te
Fig. 5.
Comparison of (a) transport and (b) recombination time constants as a function of light
Ac ce p
intensities for DSCs with Im+s and Li+ under 610 nm LED irradiation.
Page 21 of 26
ip t cr us an M d te
Ac ce p
Fig. 6. Dependence of effective electron diffusion length on light intensities for DSCs with Im+s and Li+.
Page 22 of 26
ip t cr us an M d te Ac ce p
Fig. 7.
Dependence of short-circuit photocurrent densities on light intensities for DSCs with
Im+s and Li+.
Page 23 of 26
ip t cr us an M d te
Fig. 8.
Electrochemical impedance spectroscopy (Nyquist plots) for DSCs with Im+s and Li+
Ac ce p
measured at -0.6 V. The lines show the fit results and the scatter dots are experiment data. The inset is the equivalent circuit, where Rh is the system resistance, R1 is resistance related to charge-transfer processes occurring at the Pt/electrolyte interface and R2 is charge transfer resistance occurring at TiO2/electrolyte interface.
Page 24 of 26
ip t cr us an M d te Ac ce p Fig. 9.
Steady-state voltammograms of solutions with Im+I-s and LiI in MePN at 25 °C.
Page 25 of 26
Disclose the mechanism of cooperative effects of adsorbed cations in DSCs. Characterize the influence of adsorption of Im+s on photoinduced electron density.
ip t
The effect of Li+ is orderly enhanced in DSCs with increasing alkyl chain length.
The DSCs efficiencies are relatively depended on the trade-off between Jsc and
Ac ce p
te
d
M
an
us
cr
FF.
Page 26 of 26
S0013-4686(13)00517-3 http://dx.doi.org/doi:10.1016/j.electacta.2013.03.105 EA 20213
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
1-11-2012 14-3-2013 14-3-2013
Please cite this article as: D. Kou, W. Liu, L. Hu, S. Dai, Cooperative Effect of Adsorbed Cations on Electron Transport and Recombination Behavior in Dye-Sensitized Solar Cells, Electrochimica Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.03.105 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Cooperative Effect of Adsorbed Cations on Electron Transport and Recombination Behavior in Dye-Sensitized Solar Cells Dongxing Kou, Weiqing Liu, Linhua Hu and Songyuan Dai*
ip t
Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, 230031, P. R. China
cr
Abstract. Lithium ion (Li+) and imidazolium cations (Im+s) had been reported to have
us
competitive effects on the photoinduced electrons in TiO2-electrolyte systems. Herein, a further investigation about their cooperative effect in dye-sensitized solar cells (DSCs) using organic liquid electrolyte is developed by altering alkyl chain length. Imidazolium iodides (Im+I-s) with
an
different alkyl chain length (3, 6, 12) were synthesized and used as iodide sources. The adsorption amount of Im+s onto TiO2, band edge shifts, trap states distribution, electron
M
recombination/transport processes and ion transport within the electrolyte for DSCs were detected. It is found that the multilayered adsorption of Im+s can induce a lower photoinduced
d
electron density. In-depth characterizations indicate that this negative effect can be reduced as the adsorption amount decreased with increasing alkyl chain length and the effect of Li+ is
te
consequently strengthened in varying degrees. The decisive role of Li+ in cation-controlled
Ac ce p
interfacial charge injection process finally contributes an ordinal increase of short-circuit photocurrent density Jsc for DSCs with increasing alkyl chain length because of the increasing charge injection efficiency ηinj. Additionally, a large power dissipation in ions transport process is induced by the long alkyl chain of Im+s. Overall, the cell efficiencies are relatively dependent of the trade-off between Jsc and FF, which is essentially related to the cooperative effect of adsorbed cations.
Keywords: imidazolium cations; lithium ion; dye-sensitized solar cells; charge transport and recombination
1. Introduction Over the past 20 years, the pioneering work of Michael Grätzel and coworkers has led to the development of high efficiency dye-sensitized solar cells (DSCs)[1-4]. Up to now, an landmark photoelectric conversion efficiency over 12.3% has been obtained[5], which gives a
Page 1 of 26
promising application foreground in renewable energy. The working principle of this device is based on the injection of electrons from photo-excited states of dye to the conduction band of the semiconductor followed by the reduction of the oxidized dye with a charge mediator[3,6,7].
ip t
During these processes, electrolyte plays an important role in ions transport and charges transfer at TiO2/electrolyte or Pt/electrolyte interfaces[6,8,9]. It has been known that several species of cations existing in the electrolyte solution as the counteraction of I- and I3-, such as
cr
Li+, Im+ and so on. The adsorption or the intercalation of cations on TiO2 surface can influence
us
the number or distribution of surface states, the strength of sensitizer surface attachment, the energetics of the semiconductor and sensitizer, the charge transport rate, the dynamics of
an
interfacial electron transfer as well as the rate of iodide oxidation[10-12]. Yanagida[6] had shown that the influence of cations on electron diffusion coefficient of TiO2 films in accordance with DMHI+ < TBA+ < Na+ < Li+. Furthermore, Fitzmaurice[11,13] reported that for a given
M
cation activity, the magnitude of the positive shift of Efb decreased in the order Mg2+ > Li+ > Na+. Their work strongly suggests that the magnitude of the effect of adsorbed cations mostly
d
depends on their nature, concentration and charge-to-radius ratio[14-16].
te
At the TiO2/electrolyte interface, Im+s multilayer adsorb onto TiO2 surfaces due to mutual electrostatic effect. Researches demonstrates that this adsorption can lead to a lower
Ac ce p
photoinduced electron density in comparison with other species[6]. On the other hand, the intercalation of Li+ results in the conduction band of TiO2 films shifts positively and enhances the electron transfer from the sensitized dye to TiO2 films, subsequently increases the quantum yield and promotes the photogeneration of charges[10]. Overall, the competition between effects of Im+s and Li+ on the photoinduced electrons truly exists and an optimal cell efficiency requires alternative species to harmony with each other. Many results had been published recently regarding the photovoltaic properties of DSCs
containing Im+s and Li+. But most of them were carried out in terms of ionic liquid electrolyte, where the Jsc was limited by the ions transport process[17,18]. Kyung[18] used Im+I-s with different substituents as electrolytes and the short-circuit photocurrent (Jsc) was found to increase with decrease in size of the substituent for the Li+ absented DSCs. After the addition of Li+, Kubo[17] proposed that the Jsc of DSCs could increase with increasing alkyl chain up to
Page 2 of 26
C7 of 1-alkyl-3-methylimidazolium. Their findings suggest us a promising conclusion that Li+ plays a significant role in the cations coexistence system. So far, there is no detailed study disclosing the cooperative effects of adsorbed cations on electron transport and recombination
ip t
behavior in DSCs using organic liquid electrolyte. Herein, we develop an experiment aiming to verify the difference of dynamic response in both Im+s and Li+ containing DSCs by altering
cr
alkyl chain length of Im+s. Intensity-modulated photocurrent spectroscopy (IMPS)/intensitymodulated photovoltage spectroscopy (IMVS) measurements[19,20] and equivalent circuit
us
analysis[21-23] were carried out for kinetics characterization. This work will provide significant scientific contributions to the further understanding the detailed working mechanism
an
of adsorbed cations and high efficiency research for DSCs.
2. Experimental section
M
2.1. Chemicals and materials
Anhydrous lithium iodide (LiI), Iodine (I2) and 3-Methoxypropionitrile (MePN) were purchased from Aldrich. Imidazolium salts with bromide as counterion (Im+Br-s) were
d
purchased from sinoreagent and used for detecting the absorbance of Im+s because Br- had no
te
absorption under UV spectra range from 200 to 400 nm. All the chemical reagents were used as received. 1,3-dialkylimidazolium iodides were synthesized as reported previously[24,25] and
Ac ce p
their purity had been confirmed by 1H-NMR. The Im+s are composed of two different alkyl subsistents in position R1 and R2, where R1 has been fixed by a methyl group and R2 is an alkyl substituent with different alkyl chain lengths (n=3, 6, 12). Molecular structures of 1, 3-dialkylimidazolium iodides are shown in fig. 1. Cells with electrolytes containing different Im+s and Li+ (0.1 M LiI, 0.08 M I2, 0.6 M MPII in MePN; 0.1 M LiI, 0.08 M I2, 0.6 M HMII in MePN; 0.1 M LiI, 0.08 M I2, 0.6 M C12MII in MePN; 0.1 M LiI, 0.08 M I2, 0.6 M TBAI in MePN) were assembled and denoted as C3, C6, C12 (Im+s cells) and TBA, respectively. The TBA cell was used as reference for investigating photoinduced electrons in comparaion with Im+s cells. Aiming to clarify the working mechanism of cations, TBP was excluded because of its mutual effect with Li+ and/or the influence on the adsorption of Im+s onto TiO2 films. Fig. 1
Page 3 of 26
2.2. The cell assembly The
colloidal
TiO2
nanoparticles
were
prepared
by
hydrolysis
of
titanium
tetraisopropoxide as described elsewhere[26]. Then the obtained nanoparticles were mixed with
ip t
polyethylene glycol (PEG, Aldrich) to form the viscous paste. The colloidal paste was coated on transparent conductive oxide (TCO) glass substrate by screen-printing. The films (5 mm×5
cr
mm) were dried at room temperature for 5 min and then sintered at 510 °C for 30 min in air. The films thickness was about 11 µm determined by surface profilometer (XP-2, AMBIOS
us
Technology Inc., U.S.A.). The photoelectrodes were immersed in a dye solution containing N719 at room temperature for 12 h. The platinized counter electrodes were obtained by
an
spraying H2PtCl6 solution on TCO glass, followed by heating at 410 °C for 20 min. The obtained counter electrode was placed directly on top of the dyed TiO2 films sealed with a 60 µm thermal adhesive film (Surlyn, Dupont). The electrolyte was filled from a hole on the
M
counter electrode and then sealed with a surlyn adhesive film under a cover glass by heating. 2.3. Measurements
d
The photovoltaic performances of the DSCs were measured by a Keithley 2420 3A source
te
meter controlled by Testpoint software under solar simulator (solar AAA simulator, oriel USA, calibrated with a standard crystalline silicon solar).
Ac ce p
All the following electrochemical measurements were carried out on an IM6ex
electrochemical workstation (Germany, Zahner) at 25 °C. IMPS/IMVS measurements were performed on IM6ex workstation using light-emitting diodes (λ=610nm) driven by Expot (Germany, Zahner). The LED provided both dc and ac illumination components. The amplitude of the modulated component was 10% or less of dc component. The frequency range was 3 kHz to 0.1 Hz. Impedance measurements were performed by the IM6ex at reverse biases in the frequency range of 0.08 Hz-1000 kHz. The amplitude of the alternative signal was 5 mV. Steady-state voltammetry of the electrolytes were adopted in a conventional electrochemical cell equipped with a 1.0 µm platinum ultramicroelectrode (CHI107) as working electrode, and with a 1 mm radius platinum disk electrode (CHI102) as counter electrode and reference electrode. A slow scan rate of 5 mV/s was used in order to obtain steady-state current–voltage
Page 4 of 26
curves. The nitrogen (N) atom of Im+s can interact with the TiO2 films due to mutual electrostatic effect [6,18]. To investigate the differences of adsorption ability of Im+s onto TiO2 electrode,
ip t
the UV-Vis spectra of Im+Br-s in acetonitrile were performed by an ultraviolet-visible spectrophotometer (TU-1901, Persee Inc. China). Photometric scanning at 270 nm were carried
cr
out to detect the absorbance of solutions before and after an addition of small amount of dehydrated TiO2 (20 nm) as reported previously. According to Beer–Lambert law A = εcl[27],
us
where A is the absorbance, ε is the molar absorbtivity or extinction coefficient, l is the path length of the sample and c is the concentration of the compound in solution, absorbance is
an
proportional to the concentration of absorbing species in solution. Fig. 2 shows the absorbance of Im+s as a function of solution concentration (closed symbols), from which we can calculate the concentration of Im+s after the addition of TiO2 (open symbols) and deduce the adsorption
M
amount on TiO2 surface. The initial concentration is 0.1 M for Im+s and 1.0 M for TBA+, after the addition of TiO2 and the centrifugation, the adsorption amount decreased in the order of
d
MPI+ < HMI+ < C12MI+ < TBA+, indicating the absorption amount of Im+s decreased with
te
increasing alkyl chain length. One can conclude that the TBA+ is a weak adsorption species in accordance with the variation of absorbance. Additionally, the multilayered adsorption of Im+s
Ac ce p
on TiO2 surface is calculated by ca.85 molecules/nm2, which is consistent with previous report[6].
Fig. 2
3. Results and discussion
3.1. Photoinduced electron density and surface states distribution Fig. 3
The photoinduced electron density (Q, the product of Jsc and τn) in TiO2 films was detected by IMPS/IMVS measurements (Jsc was obtained under the same light intensity as well as τn)[28]. The dependence of Voc on Q for Im+s cells is shown in Fig. 3, where the TBA+ is used to be a weak adsorption species in comparison with Im+s. The TBA cell has the highest photoinduced electron density at the same energy level, which suggests that the absolute
Page 5 of 26
adsorption of Li+ in the mesoporous TiO2 films can promote the photogeneration of charges because of its enhancement to the quantum injection yield. When the cations of iodide source also adsorb onto TiO2 films, Im+s cells exhibit lower photoinduced electron density due to their
ip t
multilayered adsorption and steric hindrance effects[6]. In both Im+s and Li+ containing system, their reverse effects on photoinduced electron density should be weakened by each other. That is to say, the falling of one will always along with the rising of the other. As the decreasing
cr
adsorption amount of Im+s with the increase of alkyl chain length is observed by UV-Vis
us
spectra, we can consider that the negative effect of Im+s on photoinduced electron density is reduced either. Simultaneously, the effect of Li+ is reasonably enhanced in varying degrees for
an
Im+s cells with the increase of alkyl chain length. Since the magnitude of the effect of adsorbed cations mostly depends on their charge-to-radius ratio[14-16], the decisive role of Li+ in cation-controlled interfacial charge injection process finally induces an ordinal increase of
M
photoinduced electron density at the same energy level with the increase of alkyl chain length as seen in Fig. 3. Overall, Li+ plays a determination role in the cations coexistence DSCs and
d
this role will be orderly enhanced with increasing alkyl chain length of Im+s. In-depth
te
characterizations about the cooperative effect of adsorbed cations on dynamic processes in DSCs will be discussed in detail.
Ac ce p
The accumulated charge in the conduction band determines the difference between the
conduction band and electron quasi-Fermi level. We can analysis band edge shifts by comparing the change of Voc at the constant Q. The relationship between Voc and ln(Q) can be expressed with the following linear equation[20,29]
Voc Vc mc ln(Q)
(1)
where Vc is the vertical intercept and mc is the slope rate. If mc = 26 mV, electrons recombination occurs principally via the conduction band. Otherwise, if mc > 26 mV, electrons recombination occurs principally via surface states. Data in Fig. 3 shows that the values of mc ranged from 62 to 95 mV for all samples, which suggests electrons recombination mainly via surface states. For Im+s cells, ∆Vc (the difference of conduction band edge between the comparison cells) indicates that the band edge of C6 and C12 cell shifts positively about 63 mV
Page 6 of 26
and 89 mV in comparison with C3 cell. The enhancement of the effect of Li+ on energy level of TiO2 films would lead to a more favorable driving force for electron injection. Fig. 4
ip t
For impedance measurements in the dark, the charge stored in TiO2 films at any value of the potentials is given by the equation[30]
(2)
cr
Qdark VV00 Vbias VdU
where V0 is the initial potential and V is the potential dependent capacitance. So the calculated
us
capacitance of the TiO2 electrodes as a function of the applied potential offers us a useful description to the surface states energy distribution, which can be characterized as the
an
following
C Ca exp[ eV /(k BT )] Cb
(3)
M
where kB is the Boltzmann constant, T is the temperature, e is the elementary charge, V is the applied potential, Ca is the prefactor of the exponential, Cb is the quasi constant capacitance at
d
low potentials, and β is a coefficient describing an exponential distribution of surface states
te
below the conduction band edge (β < 1). A smaller β implies that the distribution is deeper on average with respect to the conduction band and that the relative proportion of deep traps is
Ac ce p
larger[30-32]. Fig. 4 presents the characterization of surface states over the energy range applied in Im+s cells by the potential dependent capacitance. Data is obtained by fitting impedance spectrum. In accordance with the variation of photoinduced electron density, the reduced adsorption amount of Im+s is accompanied with the enhancement of the effect of Li+. Their cooperative effect should influence the distribution of surface states in varying degrees. It can be seen in Fig. 4 that the fitted β is 0.19, 0.17 and 0.16 for Im+s cells with the increasing alkyl chain length as expected. Subsequently, the deeper surface states distribution has a profound influence on the transport and recombination dynamic processes of electrons in TiO2 films. 3.3. The electron transport and recombination behavior Nelson[33] proposed a random walk mechanism for electron transport in TiO2 films, in which each electron could move after a waiting time determined by the activation energy of the
Page 7 of 26
trap site. It is widely accepted that electron transport in TiO2 films is limited by trapping and detrapping processes[34,35]. Generally, the adsorption of cations can create the extra trap states on the surface of TiO2, in this case, the electron diffusion should be slowed. Fig. 5 shows
ip t
electron transit and recombination times for Im+s cells as a function of the light intensity I0, where the electron transit time τd is associated with the electron transport process from the injection sites to the TCO substrate and the recombination time τn is mostly determined by the
cr
rate of reaction between the electrons in the TiO2 films and I3-. Both the constants can be
us
estimated from the expression τd = 1/2πfImps and τn = 1/2πfImvs[23], where fImps and fImvs is the characteristic frequency minimum of the IMPS/IMVS imaginary component, respectively. Data
an
in Fig. 5a exhibits a power-law dependence on the light intensity of τd following the expression τd (I0)α-1[36]. With increasing light intensity, the quasi-Fermi energy of the electrons shifts closer to the conduction band edge, such that the trap depth involved in the electron transport
M
process decreases. This results in a more fast transmission process, as seen in Fig. 5a, τd is decreased with the increase of I0. Besides, it is observed that τd is increased with increasing
d
alkyl chain length at the same I0 for Im+s cells due to the increased trap depth mentioned above.
te
Additionally, steric hindrance of the long alkyl chain may also give a contribution to the delay of τd, which needs to be further researched.
Ac ce p
Generally, the transport-limited recombination of photocarriers[36] can induce a
proportionately increasing to τn along with the increasing of τd. Fig. 5b shows the dependence of τn on I0. It can be found that τn of C6 cell and C12 cell is much longer than C3 cell but there is no obvious difference between themselves. Though Im+s could restrain the recombination between I3- and photoinduced electrons[18], this steric hindrance effect should be weakened with increasing alkyl chain length due to the decreasing of absorption amount. The compromise as mentioned finally results in a comparable τn between C6 cell and C12 cell themselves. Fig. 5 In DSCs, photocurrents and photovoltages arise from a subtle balance of transport of photoinduced electrons toward the external circuit and recombination of these electrons with acceptors present in the system. This competition is a good description to the charge collection ability of photoelectrode. It can be characterized with the value of electron diffusion length (Ln),
Page 8 of 26
as expressed by the relation Ln Dn n , where Dn is electron diffusion coefficient determined from the expression Dn = d2/2.35τd and d is the film thickness[37]. So the Ln can be further expressed as (4)
n 2.35 d
ip t
Ln d
cr
An excellent photoelectrode for the charge collecting corresponds a much longer Ln than d. Fig. 6 shows that the C6 cell had the longest Ln due to the ratio τn/τd, indicating that there exists an
Fig. 6
an
Fig. 7
us
optimal alkyl chain length of Im+s for charge collection in photoelectrode.
In general, Jsc can be approximated by the following expression[38]: (5)
M
J sc qlhinj cl I 0
where q is the elementary charge, ηlh is the light-harvesting efficiency, ηinj is the charge
d
injection efficiency, and ηcl is the charge collecting efficiency. Among the three parameters, ηlh is mainly determined by the amount and extinction coefficient of adsorbed dye and the
te
light-scattering properties of the films; ηcl is totally determined by the competition between
Ac ce p
recombination and transport as discussed above; ηinj is largely related to the location of energy level between excited dye and the conduction band of semiconductor films. Fig. 7 shows the Jsc dependences on I0 for Im+s cells, where the Jsc is linearly changed along with I0 and the slopes increased with increasing alkyl chain length. Assuming ηlh for Im+s cells are the same, one can estimate the difference in slopes is determined by ηinj and ηcl. Since ηcl achieved the highest value by C6 cell, the increasing of slopes is mostly attributed to the enhancement of ηinj[39]. That is to say, the ordinal increasing of Jsc is essentially related to the location of conduction band determined by the cooperative effect of Im+s and Li+. This result is quite different from that of DSCs using ionic liquid electrolyte, where the enhanced Jsc is explained by the increased reductive activity of I- arising from increased alkyl chain length of Im+s[18]. 3.3. The effect of series resistance on cell performances Resistive effects in solar cells reduce the efficiency of the solar cells by dissipating power
Page 9 of 26
in internal resistance[22]. The internal resistance has a direct impact to FF and a low total series resistance contributes to a good FF value. In general, the series resistance roughly corresponds to the sum of system resistance, charge-transfer process occurring at the
ip t
Pt/electrolyte interface and ions transport process within the electrolyte[21,22]. Fig. 8 illustrates the equivalent circuit analysis to the influence of Im+s on internal processes of Im+s cells. The
cr
fitted data in Table 1 shows that Im+s with increasing alkyl chain length induces an ordinal increase in system resistance (Rh) and charge-transfer resistance at Pt/electrolyte interface (R1)
Fig. 8
transport process within the electrolyte is investigated by steady-state
an
Ions
us
due to the increased steric hindrance and viscosity.
voltammograms of solutions as seen in Fig.9. The apparent diffusion coefficient (D) of I3- and Iin MePN are calculated from the anodic and cathodic steady-state current (is) using the
M
equation is = 4nFDroc[40]. As we know, at the same temperature the viscosity of Im+s increases with increasing alkyl chain length because of van der Waals forces[17,18]. It is found that the D
d
values of I3- and I- (Table 1) both decreased with the increase of alkyl chain length. As a result,
te
the ions transport process within electrolyte becomes more slowly with increasing alkyl chain
Ac ce p
length, subsequently leading to large power dissipation. Fig. 9 Table 1
Table 2 lists the photocurrent–voltage properties of Im+s cells under AM 1.5, 100
mW·cm-2 irradiation. As discussed above, the cooperative effect of Im+s and Li+ contributes an ordinal increase of Jsc with increasing alkyl chain length. However, the ordinal increase of series resistance leads to large power dissipation and a lower FF. It can be seen in Table 2 that the cell efficiencies are relatively dependent of the trade-off between Jsc and FF. As a result, the C6 cell gives an optimal efficiency rather than C3 and C12 cell. Table 2
4. Conclusion In conclusion, the investigation of the cooperative effect of Im+s and Li+ in DSCs are
Page 10 of 26
developed in this work by altering alkyl chain length. It is found that the multilayered adsorption of Im+s can induce much less photoinduced electrons. In-depth research indicates that this negative effect is reduced with increasing alkyl chain length and the effect of Li+ is
ip t
subsequently enhanced in varying degrees. Overall, the decisive role of Li+ in cation-controlled interfacial charge injection process induces a higher Q and a more positive band edge shift. The cooperative effect of Im+s and Li+ finally leads to an ordinal increase of Jsc for DSCs with
cr
increasing alkyl chain length. Equivalent circuit analysis suggests that Im+s have remarkable
us
influences on series resistance and can lead to a lower FF with increasing alkyl chain length. Consequently, we propose that the efficiencies of Im+s cells are relatively dependent of the
an
trade-off between Jsc and FF, which is essentially related to the cooperative effect of adsorbed cations. Further studies need to be developed to understand the detailed working mechanism of cations coexistence system.
M
Acknowledgment
This work was financially supported by the National Basic Research Program of China under
d
Grant No. 2011CBA00700, the National High Technology Research and Development Program
te
of China under Grant No. 2011AA050527, the National Natural Science Foundation of China under Grant No. 21003130, the National Natural Science Foundation of China under Grant No.
Ac ce p
21173228, and the China Postdoctoral Science Foundation under Grant No. 20110490835.
References
[1] B. O'regan, M. Gratzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature 353 (1991) 737.
[2] H. G. Jung, Y. S. Kang, Y. K. Sun, Anatase TiO2 spheres with high surface area and mesoporous structure via a hydrothermal process for dye-sensitized solar cells, Electrochim. Acta 55 (2010) 4637. [3] L. M. Peter, Dye-sensitized nanocrystalline solar cells, Phys. Chem. Chem. Phys. 9 (2007) 2630. [4] S. J. Lim, Y. S. Kang, D. W. Kim, Dye-sensitized solar cells with quasi-solid-state cross-linked polymer electrolytes containing aluminum oxide, Electrochim. Acta 56 (2011) 2031. [5] A. Yella, H. W. Lee, H. N. Tsao, C. Y. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W. G. Diau, C. Y. Yeh, S. M. Zakeeruddin, M. Gratzel, Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency, Science 334 (2011) 629. [6] S. Kambe, S. Nakade, T. Kitamura, Y. Wada, S. Yanagida, Influence of the electrolytes on electron transport in mesoporous TiO2-electrolyte systems, J. Phys. Chem. B 106 (2002) 2967.
Page 11 of 26
[7] G. Boschloo, A. Hagfeldt, Characteristics of the iodide/triiodide redox mediator in dye-sensitized solar cells, Acc. Chem. Res. 42 (2009) 1819. [8] F. Fabregat-Santiago, J. Bisquert, G. Garcia-Belmonte, G. Boschloo, A. Hagfeldt, Influence of electrolyte in transport and recombination in dye-sensitized solar cells studied by impedance spectroscopy, Sol. Energy Mater. Sol .Cells 87 (2005) 117.
ip t
[9] S. Nakade, Y. Makimoto, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida, Roles of electrolytes on charge couples, J. Phys. Chem. B 109 (2005) 3488.
cr
recombination in dye-sensitized TiO2 solar cells (2): The case of solar cells using cobalt complex redox [10] C. A. Kelly, F. Farzad, D. W. Thompson, J. M. Stipkala, G. J. Meyer, Cation-controlled interfacial charge injection in sensitized nanocrystalline TiO2, Langmuir 15 (1999) 7047.
us
[11] D. F. Watson, G. J. Meyer, Cation effects in nanocrystalline solar cells, Coord. Chem. Rev. 248 (2004) 1391.
[12] B. C. O'Regan, J. R. Durrant, Calculation of activation energies for transport and recombination in
an
mesoporous TiO2/dye/electrolyte films-Taking into account surface charge shifts with temperature, J. Phys. Chem. B 110 (2006) 8544.
[13] M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphrybaker, E. Muller, P. Liska, N. Vlachopoulos, M.
M
Gratzel, Conversion of light to electricity by cis-X2bis(2,2'-bipyridyl-4,4'-dicarboxylate) ruthenium (II) charge-transfer sensitizers (X=Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline TiO2 electrodes, J. Am. Chem. Soc. 115 (1993) 6382.
te
Chem. B 103 (1999) 7860.
d
[14]G. Boschloo, D. Fitzmaurice, Electron accumulation in nanostructured TiO2 (anatase) electrodes, J. Phys. [15] G. Redmond, D. Fitzmaurice, Spectroscopic determination of flat-band potentials for polycrystalline
Ac ce p
TiO2 electrodes in nonaqueous solvents, J. Phys. Chem. 97 (1993) 1426. [16] B. I. Lemon, J. T. Hupp, Electrochemical quartz crystal microbalance studies of electron addition at nanocrystalline tin oxide/water and zinc oxide/water interfaces: Evidence for band-edge-determining proton uptake, J. Phys. Chem. B 101 (1997) 2426. [17] W. Kubo, T. Kitamura, K. Hanabusa, Y. Wada, S. Yanagida, Quasi-solid-state dye-sensitized solar cells using room temperature molten salts and a low molecular weight gelator, Chem. Commun. (2002) 374. [18] K. M. Son, M. G. Kang, R. Vittal, J. Lee, K. J. Kim, Effects of substituents of imidazolium cations on the performance of dye-sensitized TiO2 solar cells, J. Appl. Electrochem. 38 (2008) 1647. [19] L. M. Peter, Characterization and modeling of dye-sensitized solar cells, J. Phys. Chem. C 111 (2007) 6601. [20] G. Schlichthorl, S. Y. Huang, J. Sprague, A. J. Frank, Band edge movement and recombination kinetics in dye-sensitized nanocrystalline TiO2 solar cells: A study by intensity modulated photovoltage spectroscopy, J. Phys. Chem. B 101 (1997) 8141. [21] L. Y. Han, N. Koide, Y. Chiba, A. Islam, T. Mitate, Modeling of an equivalent circuit for dye-sensitized solar cells: improvement of efficiency of dye-sensitized solar cells by reducing internal resistance, C.R. Chim.
Page 12 of 26
9 (2006) 645. [22] M. Murayama, T. Mori, Evaluation of treatment effects for high-performance dye-sensitized solar cells using equivalent circuit analysis, Thin Solid Films 509 (2006) 123. [23] W. Q. Liu, L. H. Hu, S. Y. Dai, L. Guo, N. Q. Jiang, D. Kou, The effect of the series resistance in modulation techniques, Electrochim. Acta 55 (2010) 2338.
ip t
dye-sensitized solar cells explored by electron transport and back reaction using electrical and optical [24] P. Bonhote, A. P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Gratzel, Hydrophobic, highly
cr
conductive ambient-temperature molten salts, Inorg. Chem. 35 (1996) 1168.
[25] A. B. Pereiro, E. Tojo, A. Rodriguez, J. Canosa, J. Tojo, HMImPF(6) ionic liquid that separates the azeotropic mixture ethanol plus heptane, Green Chem. 8 (2006) 307.
us
[26]L. H. Hu, S. Y. Dai, J. Weng, S. F. Xiao, Y. F. Sui, Y. Huang, S. H. Chen, F. T. Kong, X. Pan, L. Y. Liang, K. J. Wang, Microstructure design of nanoporous TiO2 photoelectrodes for dye-sensitized solar cell modules, J. Phys. Chem. B 111 (2007) 358.
an
[27] P. M. Wiegand, S. R. Crouch, Evaluation of a programmable, hardware-driven, isolated-droplet generator, Appl. Spectrosc. 42 (1988) 567.
[28] K. Zhu, N. Kopidakis, N. R. Neale, J. van de Lagemaat, A. J. Frank, Influence of surface area on charge
M
transport and recombination in dye-sensitized TiO2 solar cells, J. Phys. Chem. B 110 (2006) 25174. [29] X. M. Ren, Q. Y. Feng, G. Zhou, C. H. Huang, Z. S. Wang, Effect of cations in coadsorbate on charge (2010) 7190.
d
recombination and conduction band edge movement in dye-sensitized solar cells, J. Phys. Chem. C 114
te
[30] F. Fabregat-Santiago, I. Mora-Sero, G. Garcia-Belmonte, J. Bisquert, Cyclic voltammetry studies of nanoporous semiconductors. Capacitive and reactive properties of nanocrystalline TiO2 electrodes in aqueous
Ac ce p
electrolyte, J. Phys. Chem. B 107 (2003) 758. [31] C. N. Zhang, Z. P. Huo, Y. Huang, L. Guo, Y. F. Sui, L. H. Hu, F. T. Kong, X. Pan, S. Y. Dai, K. J. Wang, Studies of interfacial recombination in the dyed TiO2 electrode using Raman spectra and electrochemical techniques, J. Electroanal. Chem. 632 (2009) 133. [32] J. A. Anta, F. Casanueva, G. Oskam, A numerical model for charge transport and recombination in dye-sensitized solar cells, J. Phys. Chem. B 110 (2006) 5372. [33] J. Nelson, R. E. Chandler, Random walk models of charge transfer and transport in dye sensitized systems, Coord. Chem. Rev. 248 (2004) 1181. [34] J. Nelson, S. A. Haque, D. R. Klug, J. R. Durrant, Trap-limited recombination in dye-sensitized nanocrystalline metal oxide electrodes, Phys. Rev. B 63 (2001). [35] J. van de Lagemaat, N. G. Park, A. J. Frank, Influence of electrical potential distribution, charge transport, and recombination on the photopotential and photocurrent conversion efficiency of dye-sensitized nanocrystalline TiO2 solar cells: A study by electrical impedance and optical modulation techniques, J. Phys. Chem. B 104 (2000) 2044. [36] N. Kopidakis, K. D. Benkstein, J. van de Lagemaat, A. J. Frank, Transport-limited recombination of
Page 13 of 26
photocarriers in dye-sensitized nanocrystalline TiO2 solar cells, J. Phys. Chem. B 107 (2003) 11307. [37] G. Schlichthorl, N. G. Park, A. J. Frank, Evaluation of the charge-collection efficiency of dye-sensitized nanocrystalline TiO2 solar cells, J. Phys. Chem. B 103 (1999) 782. [38] K. Zhu, N. R. Neale, A. Miedaner, A. J. Frank, Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays, Nano Lett. 7 (2007) 69.
ip t
[39] C. N. Zhang, Y. Huang, Z. P. Huo, S. H. Chen, S. Y. Dai, Photoelectrochemical effects of guanidinium thiocyanate on dye-sensitized solar cell performance and stability, J. Phys. Chem. C 113 (2009) 21779. [40] C. W. Shi, S. Y. Dai, K. J. Wang, X. Pan, L. Guo, L. Y. Zeng, L. H. Hu, F. T. Kong, Influence of
Ac ce p
te
d
M
an
us
dye-sensitized solar cells, Sol. Energy Mater. Sol. Cells 86 (2005) 527.
cr
1-methyl-3-propylimidazolium iodide on I3-/I- redox behavior and photovoltaic performance of
Page 14 of 26
ip t cr us
Table
Table 1 Influence of Im+s on internal resistances and carrier transport by I-/I3- within the
an
electrolyte
DI - /10-6 cm2·s-1
-6 2 -1 DI - /10 cm ·s
7.02
4.21
15.95
5.27
2.53
28.20
4.64
2.23
Rh/
R1/
C3
34.47
10.48
C6
40.99
C12
66.69
3
d
M
Cell
te
Table 2 Photovoltaic performances of DSCs with Im+I-s and LiI Voc/V
Jsc/mA·cm-2
FF
η/%
0.57
12.45
0.56
3.98
C6
0.60
14.79
0.52
4.61
C12
0.57
15.57
0.46
4.08
Cell
Ac ce p
C3
Page 15 of 26
ip t cr
Figure Capations
Chemical structures of 1-alkyl-3-methylimidazolium iodides
Fig. 2.
Absorbance of Im+s as a function of solution concentration (solid symbols) under 270
us
Fig. 1.
nm UV irradiation. Open symbols are the centrifuged solutions after the addition of TiO2 (10
an
mg for 5 mL solution).
Voc as a function of lnQ for DSCs with different cations of iodide source and Li+.
Fig. 4.
Results from the impedance data for capacitance of DSCs with Im+s and Li+.
Fig. 5.
Comparison of (a) transport and (b) recombination time constants as a function of light
M
Fig. 3.
intensities for DSCs with Im+s and Li+ under 610 nm LED irradiation.
te
Im+s and Li+.
d
Fig. 6. Dependence of effective electron diffusion length on light intensities for DSCs with Fig. 7. Dependence of short-circuit photocurrent densities on light intensities for DSCs with
Ac ce p
Im+s and Li+. Fig. 8.
Electrochemical impedance spectroscopy (Nyquist plots) for DSCs with Im+s and Li+
measured at -0.6 V. The lines show the fit results and the scatter dots are experiment data. The inset is the equivalent circuit, where Rh is the system resistance, R1 is resistance related to charge-transfer processes occurring at the Pt/electrolyte interface and R2 is charge transfer resistance occurring at TiO2/electrolyte interface. Fig. 9.
Steady-state voltammograms of solutions with Im+I-s and LiI in MePN at 25 °C.
Page 16 of 26
Chemical structures of 1-alkyl-3-methylimidazolium iodides
Ac ce p
Fig. 1.
te
d
M
an
us
cr
ip t
Figures
Page 17 of 26
ip t cr us an M d te
Absorbance of Im+s as a function of solution concentration (solid symbols) under 270
Ac ce p
Fig. 2.
nm UV irradiation. Open symbols are the centrifuged solutions after the addition of TiO2 (10 mg for 5 mL solution).
Page 18 of 26
ip t cr us an M d te Ac ce p Fig. 3.
Voc as a function of lnQ for DSCs with different cations of iodide source and Li+.
Page 19 of 26
ip t cr us an M d te Ac ce p
Fig. 4.
Results from the impedance data for capacitance of DSCs with Im+s and Li+.
Page 20 of 26
ip t cr us an M d te
Fig. 5.
Comparison of (a) transport and (b) recombination time constants as a function of light
Ac ce p
intensities for DSCs with Im+s and Li+ under 610 nm LED irradiation.
Page 21 of 26
ip t cr us an M d te
Ac ce p
Fig. 6. Dependence of effective electron diffusion length on light intensities for DSCs with Im+s and Li+.
Page 22 of 26
ip t cr us an M d te Ac ce p
Fig. 7.
Dependence of short-circuit photocurrent densities on light intensities for DSCs with
Im+s and Li+.
Page 23 of 26
ip t cr us an M d te
Fig. 8.
Electrochemical impedance spectroscopy (Nyquist plots) for DSCs with Im+s and Li+
Ac ce p
measured at -0.6 V. The lines show the fit results and the scatter dots are experiment data. The inset is the equivalent circuit, where Rh is the system resistance, R1 is resistance related to charge-transfer processes occurring at the Pt/electrolyte interface and R2 is charge transfer resistance occurring at TiO2/electrolyte interface.
Page 24 of 26
ip t cr us an M d te Ac ce p Fig. 9.
Steady-state voltammograms of solutions with Im+I-s and LiI in MePN at 25 °C.
Page 25 of 26
Disclose the mechanism of cooperative effects of adsorbed cations in DSCs. Characterize the influence of adsorption of Im+s on photoinduced electron density.
ip t
The effect of Li+ is orderly enhanced in DSCs with increasing alkyl chain length.
The DSCs efficiencies are relatively depended on the trade-off between Jsc and
Ac ce p
te
d
M
an
us
cr
FF.
Page 26 of 26