Electrolyte Interface

Article pubs.acs.org/JPCC

Photoelectrochemical Analysis of the Dyed TiO2/Electrolyte Interface in Long-Term Stability of Dye-Sensitized Solar Cells Changneng Zhang,† Yang Huang,† Shuanghong Chen,† Huajun Tian,† Li’e Mo,† Linhua Hu,† Zhipeng Huo,† Fantai Kong,† Yingwen Ma,‡ and Songyuan Dai*,† †

Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, Anhui 230031, People’s Republic of China ‡ School of Materials Science and Technology, Harbin Institute of Technology, Harbin, Heilongjiang 150001, People’s Republic of China S Supporting Information *

ABSTRACT: The interface structure induced by electrolyte cations was found to play a significant role in determining the performance and stability of dye-sensitized solar cells. The trap state density in the nanostructure TiO2 electrodes was affected by the adsorbed 1,2-dimethyl-3-propylimidazolium cation (DMPI+) or alkali cations, such as Li+, Na+, K+, and Cs+, on the dyed TiO2 electrode and was found to increase with the order of decreasing cation radius DMPI+ < Cs+ < K+ < Na+ < Li+. The change in interface structure resulted from the accumulation of the adsorbed cations to increase trap states in the nanostructure TiO2 electrodes during long-term accelerated aging tests. The size effect of electrolyte cations on the cell performance suggested that the reduced surface cations, when small cations penetrated into titania lattice, resulted in a negative shift of the TiO2 conduction band edge and a weaker interaction of Li+ with dyes to obtain the decline in photocurrent and efficiency. The overall efficiency of dye-sensitized solar cells with large DMPI+ in the electrolyte retained over 110% of its initial value after 2100 h. Also, no obvious differences in the efficiency for dye-sensitized solar cells with electrolyte cations, such as Li+, Cs+, and DMPI+, were observed after 1270 h under one sun light soaking in our experiment. The results suggested that large DMPI+ chemisorbed on TiO2 surface could not intercalate into the TiO2 lattice for the enhanced stability of dye-sensitized solar cells in practical application.

1. INTRODUCTION Dye-sensitized solar cells (DSCs) have been expected to be a very compelling device in the conversion of solar energy directly into electricity due to their low production cost and high performance.1−6 The typical DSC comprises a dyed TiO2 electrode, a platinized counter electrode, and an electrolyte containing an I−/I3− redox couple between the electrodes. It has been shown that the interfacial reaction between the dyed TiO2 electrode and electrolyte plays an important role in the photoelectric conversion efficiency of DSC.7−9 The interfacial processes, such as photogeneration, charge separation, and electron recombination at the dyed TiO2 electrode/electrolyte interface, were found to depend on the electrolyte component. A number of studies implied that the electrolyte component could induce a shift in the conduction band edge of TiO2 electrode and a change in surface recombination centers to control the short-circuit current density (Jsc), open circuit voltage (Voc), and fill factor.8−13 Recently, several groups have found that the dyed TiO2 electrode/electrolyte interface appears to be crucial for the long-term stability of DSCs in practical application.3,5,14−20 The stability analysis of DSC modules under outdoor working conduction indicated that the N719 dyed TiO2 electrode and counter electrode were almost stable after about 2.5 years.14 © 2012 American Chemical Society

Gregg et al. showed that UV irradiation inducing surface states in TiO2 film resulted in instability of the dyed TiO2 electrode/ electrolyte interface and their degradation efficiency of solar cells.15 Also, we observed that the adsorption of 1methylbenzimidazole (MBI) on the surface of TiO2 photoelectrode could improve the interface stability of the dyed TiO2 electrode/electrolyte and decrease the loss of iodine in the electrolyte under UV irradiation to enhance the stability of DSCs.17 Nguyen et al. investigated the different nitrogen additive effect on the thermal stability of DSCs and reported that the 2,6-lutidine additive prevented the N719 dye reaction with I3− in the electrolyte to keep the dyed TiO2 electrode/ electrolyte interface stable.20 With respect to a change in the interface performance of the dyed TiO2 electrode/electrolyte, Grätzel and co-workers investigated the influence of heat and light on the long-term stability of DSCs with ionic liquid electrolyte, and they believed that the decreased cell performance under accelerated aging conduction was attributed to the change of charge recombination and chemical capacitance in the dyed TiO2 electrode.3 Received: May 21, 2012 Revised: August 27, 2012 Published: August 27, 2012 19807

dx.doi.org/10.1021/jp304911u | J. Phys. Chem. C 2012, 116, 19807−19813

The Journal of Physical Chemistry C

Article

sandwich-type cell with a thermal adhesive film (Surlyn, Dupont). The electrolyte was filled from a hole made on the counter electrode, which was later sealed by a cover glass and thermal adhesive films. The total active electrode area of DSCs was 0.8 cm2. 2.3. Methods. Intensity-modulated photocurrent spectroscopy (IMPS) measurements were carried out on an IM6ex (Germany, Zahner Co.) using light-emitting diodes (λ = 610 nm) driven by Expot (Germany, Zahner Co.). The LED provided both dc and ac components of the illumination. A small ac component was 10% or less than that of the dc component. The frequency range is from 0.1 Hz to 3 kHz. Impedance measurements were done using an IM6ex electrochemical workstation (Zahner-Elektrick, Germany) in the frequency range of 100 mHz to 1000 kHz at room temperature. The amplitude of the alternative signal was 10 mV. The impedance measurements were carried out in the dark, and the obtained spectra were fitted with Z-View software in terms of appropriate equivalent circuit as shown in Figure 1. Action

Nakade et al. reported that the surface adsorption of cations, such as 1,2-dimethyl-3-propylimidazolium (DMPI+), could not intercalate into the TiO2 lattice to obtain the interface stable DSC with the stable cation concentration in the dyed TiO2 interfaces under dark conditions.16 It was presented that the surface adsorption of small cations, such as Li+ and Na+, can be intercalated into the TiO2 lattice by a negative applied potential.21 However, there is no detailed study about the influence of electrolyte cations on the long-term photostability of the dyed TiO2 interfaces. In our traditional electrolyte component of DSCs, small Li+ and large DMPI+ cation could be adsorbed on the TiO2 surface to form the Helmoltz layer at the interface. The adsorption or intercalation of electrolyte cations is expected to influence the recombination and cell performance. In this work, we first study the influence of electrolyte cation (Li+, Na+, K+, Cs+, and DMPI+) inducing interface properties on the long-term stability of DSCs. Charge-transfer resistance and chemical capacitance induced by electrolyte cations at the dyed TiO2 electrodes are also discussed for DSCs during the photostability test.

2. EXPERIMENTAL SECTION 2.1. Materials. 1-Methylbenzimidazole (MBI) and alkali iodides including LiI (99.9%), NaI (99%), KI (99%), and CsI (99.9%) were purchased from Aldrich. 3-Methoxypropionitrile (MePN) and iodine (I2) were purchased from Fluka. 1,2Dimethyl-3-propylimidazolium iodide (DMPII) was synthesized by the quaternization of 1-methylimidazole and 1iodopropane, and its purity has been confirmed by 1H NMR.9 Five different electrolytes were employed for DSCs as shown in Table 1. Table 1. Composition and Concentration of Various Electrolytes Used in This Study sample name DMPI+ Li+ Na+ K+ Cs+

Figure 1. Transmission line model of DSCs used to fit the impedance experimental data. rct, RPt, and RFTO: charge-transfer resistance at the dyed TiO2/electrolyte, the electrolyte/Pt interface, and the TiO2/ electrolyte interface, respectively. Cμ, CPt, and CFTO: the chemical capacitance at the dyed TiO2/electrolyte, the electrolyte/Pt interface, and the TiO2/electrolyte interface, respectively. rt is the transport resistance of the electrons in the TiO2 film. Zd(sol) is the Warburg element showing the Nernst diffusion of I3− in the electrolyte. Rs is the series resistance, including the sheet resistance of the FTO glass and the contact resistance of the cells.

electrolyte composition 0.6 0.1 0.1 0.1 0.1

M M M M M

DMPII, 0.1 M I2, 0.45 M MBI in MePNa LiI, 0.6 M DMPII, 0.1 M I2, 0.45 M MBI in MePN NaI, 0.6 M DMPII, 0.1 M I2, 0.45 M MBI in MePN KI, 0.6 M DMPII, 0.1 M I2, 0.45 M MBI in MePN CsI, 0.6 M DMPII, 0.1 M I2, 0.45 M MBI in MePN

a In the experimental section, the increase of [I−] from 0.6 to 0.7 M had little influence on the rate of dye regeneration and cell performance.

spectra of the monochromatic incident photon-to-current conversion efficiency (IPCE) of the DSCs were measured with a 300 W xenon lamp (Newport) and a monochromator (model, Newport 74125, U.S.) as a function of excitation wavelength over the 350−800 nm spectral range. The photocurrent−photovoltage (J−V) characteristics of DSC using the shading mask with the active area of 0.8 cm2 were measured under an illumination of AM 1.5 (100 mW cm−2), which was realized on a solar simulator (Oriel Sol3A, Newport Stratford Inc., U.S., calibrated with standard crystalline silicon solar cell; spectral mismatch was not considered) with a Keithley 2420 source meter. During successive one sun light soaking experiment, the solar cells covered with a 50 μm thick polymer film as a UV cutoff filter (up to 394 nm) were irradiated at open circuit under a 450 W xenon lamp (XQ3000, 100 mW/cm2, Shanghai B.R. science instrument Co., Ltd., China), and the air temperature was set to approximately 50 °C and was kept the same during the whole process.

2.2. DSC Assembly. The TiO2 colloidal nanoparticle was prepared by the thermal reaction of titanium tetraisopropoxide as described in our previous work.9 The morphology of TiO2 sample was investigated by HRTEM, and the observed spacing between the lattice planes of TiO2 sample was about 0.359 nm for (101) plane of the anatase crystal.22 The TiO2 paste was printed on the transparent conducting glass sheets (FTO, TEC8, LOF) by using the screen-printing technique, and sintered in air at 450 °C for 30 min to obtain a nano-TiO2 electrode. The film thickness was about 12 μm, which was determined with a profilometer (XP-1, AMBIOS Technology, Inc., U.S.). After being cooled to 80 °C, the films were immersed in an ethanol solution (0.5 mM) of dye N719 [NaRu(4,4′-bis(5-(hexylthio) thiophen-2-yl)-2,2′-bipyridine)(4-carboxylic acid-4′-carboxylate-2,2′-bipyridine) (NCS)2] overnight. The counter electrode was platinized by spraying H2PtCl6 solution to FTO glass and fired in air at 410 °C for 20 min. The dyed TiO2 electrodes and platinized counter electrodes were sealed together in a 19808



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3. RESULTS AND DISCUSSION Figure 2 shows the J−V curves of DSCs with various electrolyte cations under 100 mW cm−2 illumination. The fresh cells with

Figure 2. J−V characteristics of the Ru(dcbpy)2(SCN)2-sensitized solar cells with different electrolyte cations in 100 mW cm−2 simulated sunlight.

only DMPI+ (base electrolyte) in our experiments indicated a short-circuit current density (Jsc) of 9.4 mA cm−2, an opencircuit voltage (Voc) of 0.76 V, and fill factor (FF) of 0.64, leading to an energy conversion efficiency (η) of 4.84%. When alkali cations, such as Li+, Na+, K+, and Cs+, were introduced into the base electrolyte, respectively, Jsc of these cells displayed the sequence Li+ > Na+ > K+ > Cs+ > DMPI+ (base electrolyte), while Voc gradually decreased as shown in Figure 2. It is apparent that the addition of alkali cations such as Li+, Na+, K+, and Cs+ into the base electrolyte could obviously improve the cell photovoltaic performance. To elucidate the efficient cell behaviors, IMPS, EIS, and IPCE measurements were used to study the interfacial properties in the dyed TiO2 electrode. The calculated distribution of trap states in the TiO2 electrode was measured by IMPS at different applied light intensity for DSC with various electrolyte cations. In the traplimited electron transport model, depending on the short circuit current density Jsc, the electron transport time τd obtained from IMPS was found to be expressed as22−24 τd ∝ Jsc β− 1

Figure 3. Electron transport time τd as a function of short-circuit current density (Jsc) (a) and trap state distribution β as a function of ionic radius (b) for DSCs with various electrolyte cations. The lines represent power-law fits (eq 1) to the data, and the inset indicates the values of the logarithmic slope (β − 1) obtained by the best fit.

During electron diffusion in TiO2 electrode under opencircuit or working conductions, the recombination reaction occurring with the oxidized sensitizers and I3− ions on the surface is generally considered to be the predominant recombination in DSCs. The recombination reaction can be elucidated by EIS. Figure 4 shows the Nyquist plots of DSCs

(1)

where β was related to the steepness of the exponential trapstate distribution. The slopes in β − 1 indicated the specific adsorption of the electrolyte cations to affect the distribution of trap states at the TiO2 crystal lattice surface. The Jsc dependence of τd in DSCs with various electrolyte cations is shown in Figure 3. The τd for DSCs with alkali cations such as Li+, Na+, K+, and Cs+ (Figure 3a) was larger than that for DSCs with only DMPI+ in the electrolyte at the same short circuit current level. The τd depended on the radius of electrolyte cations, and it decreased from small Li+ cation to large Cs+ cation. Figure 3b presents the plot of the trap-state distribution β verse the electrolyte cation radius for DSCs. The observed β was reduced from small Li+ cation to large Cs+ cation, which suggested that the smaller cations could easily be adsorbed and efficiently binded on the dyed TiO2 electrode.25 Thus, we understand that the smaller adsorbed cation on the dyed TiO2 electrode could increase the distribution of trap states in the nanostructure TiO2 electrodes to decrease the electron collection efficiency in order of Li+ < Na+ < K+ < Cs+ < DMPI+ (base electrolyte).

Figure 4. Nyquist plots of DSCs with different electrolyte cations measured at −0.67 V in the dark. The lines show the fitted results.

with the electrolyte cations measured at forward bias of −0.67 V in the dark. The small semicircle in the higher frequencies corresponding to the charge-transfer reaction at Pt electrode is attributed to the redox charge-transfer resistance and double layer capacitance at the electrolyte/Pt interface. The large semicircle at lower frequencies is ascribed to the charge-transfer resistance and chemical capacitance at the dyed TiO2/ electrolyte. To obtain the chemical capacitance (Cμ) and charge transport resistance (Rct) at the dyed TiO2/electrolyte interface, transmission line model of DSCs has been used to fit above EIS data. It was indicated that the chemical capacitance Cμ at the dyed TiO2/electrolyte interface slightly decreased 19809



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TiO2 film, and ηcoll is the charge collection efficiency. In our experiment, due to the approximately same TiO2 film thickness, the LHE(λ) of the dyed TiO2 electrodes was considered to be the same. Thus, the electron injection yield of DSCs (eq 2) can be written as

from 1.86 to 1.07 mF, and the electron transfer resistance Rct increased from 12.65 to 58.55 Ω, yielding an increased electron lifetime τr in the order of Li+ < Na+ ≈ K+ < Cs+ < DMPI+(base electrolyte), which has also been investigated by Fredin et al.10 It is suggested that the increase of surface states led to an increase the chemical capacitance with decrease of cation radius, and enhance the recombination reaction of photoinjected electrons with I3− that resulted in the decrease of Voc due to its leakage of chemical capacitance. To gain insights on the electrolyte cation effect on the efficiency of electron injection (ηinj) from the excited sensitizers to TiO2, the incident photon-to-current efficiency (IPCE) of DSCs with various electrolyte cations is shown in Figure 5. The

ηinj ∝ IPCE/ηcoll

The charge collection efficiency ηcoll was affected by the electron transport time τd and electron lifetime τr in the nanoTiO2 film, which was described by the relation ηcoll = 1 − τd/τr. From Figures 3 and 4, it was noted that τd decreased from small Li+ cation to large DMPI+ cation and τr increased from small Li+ cation to large DMPI+ cation in the nanostructure TiO2 electrodes. This revealed that the ηcoll was DMPI+ (base electrolyte) > Cs+ > Li+ in order. However, IPCE decreased with increase of the cation radius in the order Li+ > Na+ > K+ > Cs+ > DMPI+. According to eq 3, it was found that the electron injection yield for DSCs was increased from large DMPI+ cation to small Li+ cation. This was mainly due to an increase in chemical binding between electrolyte cations and TiO2 particles with the decrease of electrolyte cation radius, which resulted in the more energetic overlap between the sensitizer excited-state distribution function and the density of semiconductor acceptor states.9 It can be concluded that the increased radius of cation from small Li+ to large DMPI+ to obtain the more negative shift in the band edge of the TiO2 electrode resulted in a decreased electron injection yield for DSCs and thus Jsc. To get more information of the electrolyte cation effect on the interface stability of the dyed TiO2 electrode, the DSCs using electrolyte cations with different size and adsorption properties were investigated for a long-term accelerated aging test. Figure 6 shows the photovoltaic performance of DSCs using electrolyte cations under one sun light soaking. After few weeks, an obvious decrease in Jsc led to the reduced efficiency η for DSCs with the small and adsorptive cation Li+. When LiI was introduced into DMPII electrolyte without additive such as

Figure 5. Spectra of monochromatic incident photon-to-current conversion efficiencies (IPCEs) for DSCs based on different electrolytes.

electron injection yield of DSCs with various electrolyte cations can be expressed by the relation:12 ηinj = IPCE/(ηcoll *LHE(λ))

(3)

(2)

where IPCE is the incident photon-to-current efficiency for DSCs, the LHE(λ) is the light absorbance of the dye-sensitized

Figure 6. The parameters evolution during long-term accelerated aging tests under one sun light soaking for DSCs with different electrolyte cations. 19810



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Figure 7. Results from the impedance data for DSC with various electrolyte cations measured at forward bias of −0.67 V in the dark. (a) Chargetransfer resistance at Pt electrode with various electrolyte cations with the aging time. (b) Capacitance at TiO2 electrode/electrolyte interface of DSCs with the aging time. (c) Charge-transfer resistance at TiO2 electrode/electrolyte interface of DSCs with the aging time. (d) The electron lifetime obtained from the product of Rct and Cμ of DSCs with the aging time.

MBI, an obvious decreased Jsc of DSCs was also obtained under one sun light soaking (as shown in Figure S1). However, employing DMPI+ or Cs+ cation showed better stability even after prolonged exposure to sunlight. A slight increase in Jsc and FF led to the moderately enhanced efficiency η for DSCs with Cs+ after 2100 h of aging test. The efficiency η for DSCs with only DMPI+ in the base electrolyte retained over 110% of its initial value after 2100 h, which was mainly due to the fact that the decrease of 40−50 mV in Voc was well compensated by the increase in Jsc and FF. It is also interesting to find that no obvious differences in the value of efficiency η for DSCs with different electrolyte cations, such as Li+, Cs+, and DMPI+, were observed after 1270 h under one sun light soaking in our experiment. To understand the electrolyte cation inducing interface properties, the photovoltaic parameter evolution with aging time for DSC was investigated by EIS measurements. Through the analysis of the impedance data of Nyquist plots at forward bias of −0.67 V in the dark (Figure S2, Supporting Information), Figure 7 shows the change in RPt, Rct, Cμ, and electron lifetime with different aging. The higher chargetransfer resistance RPt at the counter electrode was obtained for DSC with base electrolyte (in Figure 7a). RPt decreased from 7.42 to 3.03 Ω with the cation radius in the order DMPI+ > Cs+ > Li+, and RPt for DSCs with various electrolyte cations obviously decreased after heating at 60 °C in the light for 640 h. The decreased electron exchange reaction at the Pt/electrolyte interface of the DSC explained the increase of FF observed upon aging the cells over the time of 2100 h under one sun light soaking. The electron transport resistance could be other parameters for influencing FF of DSCs.26 In the experiment, the electron transport resistance for DSCs with DMPI+ (base electrolyte) was found to decrease from 7.5 to 0.5 Ω after 2100 h of aging test (Figure S2, Supporting Information). The reduction of electron transport resistance could be a cause for

the increase of FF for DSCs based on DMPII (base electrolyte) in the long-term photostability. The decreased charge-transfer resistance and the increased chemical capacitance Cμ at TiO2 electrode/electrolyte interface were obviously found, and the electron lifetime decreased for DSCs with Li+, Cs+, and DMPI+ (base electrolyte) after the aging test. More specifically, very rapid charge-transfer resistance decay to obtain the electron lifetime from 62.3 to 25.2 ms was observed in the case DMPI+ after 640 h in comparison with DSCs containing Li+. This result demonstrated that the smaller cations could be easily adsorbed and penetrated in the dyed TiO2 electrode. Also, large DMPI+ in the electrolyte needs more time to be adsorbed at saturation on the TiO2 surface. Little change of charge-transfer resistance and electron lifetime was found for the aging cells with various electrolyte cations between 1270 and 2100 h. This result indicated a close relationship between charge-transfer resistance and electron lifetime as well as Voc. Therefore, the increased chemical capacitance and charge-transfer resistance under the long-term accelerated aging test was attributed to the accumulation of the adsorbed cations in the TiO2 electrodes. The possible explanation to understand the accumulated cations in the vicinity of TiO2 is the trap states in the nanostructure TiO2 electrodes upon the aging cells. The chemical capacitance and recombination resistance were governed by the trap state density in the nanostructure TiO2 electrodes.27,28 It was likely due to an accumulation of the adsorbed cations to increase the trap states in the nanostructure TiO2 electrodes and alter the chemical capacitance and recombination resistance after aging test. If MBI or dyes departed from the TiO2 surface in the light at 60 °C, electrolyte cations can be adsorbed and further accumulate to form the better stable interface structure in the TiO2 electrode. From the energy band diagram of the nanostructured TiO2 electrode depicted in Figure 8 in our cell system, it can be seen that the charge recombination reaction with triiodide was increased at 19811



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Figure 9 were obtained from the IPCE values for the long-term stability of DSCs as shown in Figure S3 (Supporting Information). For the DSCs with large Cs+ cation or DMPI+ (base electrolyte), the ratio of IPCE/IPCE (fresh) markedly increased after heating at 60 °C in the light for 640 h. This observation indicated that DMPI+ and Cs+ could be adsorbed on the dyed TiO2 electrode/electrolyte interface and cause a positive shift of the TiO2 conduction band edge to improve the electron injection yield and Jsc in the light. Also, no degradation in the ratio of IPCE/IPCE (fresh) was observed after 640 h, which showed a saturated adsorption for DSCs with larger Cs+ or DMPI+ cation. For the DSCs with small Li+ cation, the ratios of IPCE/IPCE (fresh) slightly reduced after heating at 60 °C in the light for 640 h, but an obvious decrease in the ratio of IPCE/IPCE (fresh) was observed at longer wavelengths in our study after 1270 h. We expected that the increase in Jsc for DSCs with Li+ cations was calculated from the schematic diagram in Figure 8, but the decreased trend in Jsc was obtained in our experiment. Li+ adsorption on the TiO2 surface can cause the positive shift of conduction band edge, whereas Li+ intercalation can result in the negative shift of TiO2 conduction band edge to some extent.29,30 It was suggested that the reduced Li+ in the TiO2 surface, when small Li+ cations penetrated into TiO2 lattice for DSCs under one sun light soaking, led to the negative shift of band edge to obtain the decreased Jsc. Also, Li+ cations can interact with dye molecules to result in the red-shift of the photocurrent action spectrum of Ru dye.31,32 The weaker interaction of Li+ with dyes could reduce the photocurrent generation efficiency of DSCs, especially in the near-IR range of the photocurrent action spectrum, due to the reduced Li+ in the TiO2 surface for longterm application. In this work, heat stress was applied as well as light soaking, suggesting that heat stress-induced dye desorption may be a cause for the drop of Jsc for DSCs with aging time. Because of heat stress-induced recombination reaction, the dyes can desorb from the TiO2 surface and cannot readsorb on the TiO2 surface for long-term application,3 which resulted in the decline in the light absorbance of the dyesensitized TiO2 film to obtain the reduced Jsc. These results strongly suggested that large cations could be adsorbed in the vicinity of TiO2 and did not intercalate into the TiO2 lattice, which was considered for the balanced loss of the smaller cations adsorption to improve the photoinjection stability and long-term performance of DSCs.

Figure 8. Schematic diagram to explain the electron injection and charge recombination reaction at the dyed TiO2 electrode with the cation adsorption for DSCs with aging time under one sun light soaking.

the TiO2 photoelectrode/electrolyte to reduce the Voc due to the increased trap state density for DSCs with the accumulated adsorption of cation on the TiO2 suface. To study the change of IPCE during long-term aging test, Figure 9 shows the ratio of IPCE collected for DSC with various electrolyte cations. The ratios of IPCE/IPCE (fresh) in

4. CONCLUSIONS The stability of dyed TiO2 electrode/electrolyte interface has been studied for DSCs containing different cations in the electrolyte. The adsorbed electrolyte cation on the dyed TiO2 electrode could increase the trap state density in the nanostructure TiO2 electrodes with the decreased cation radius in the order DMPI+ < Cs+ < K+ < Na+ < Li+. EIS experiments suggested that the increased chemical capacitance and decreased charge-transfer resistance at TiO2 electrode/electrolyte interface resulted from the accumulation of the adsorbed cations to increase the trap states in the nanostructure TiO2 electrodes during long-term accelerated aging tests. The surface adsorption of electrolyte cations can desorb MBI in the vicinity of TiO2 as the recombination centers to decrease the Voc for DSCs with aging time. The smaller intercalated Li+ cations into titania lattice could reduce the ηinj and Jsc. Also, the surface adsorption of large DMPI+ could not intercalate into the TiO2 lattice to improve the photoinjection stability. The high

Figure 9. The radio of monochromatic incident photon-to-current conversion efficiency (IPCE) for DSCs based on various electrolyte cations with aging time under one sun light soaking. 19812



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photostability of DSCs using only DMPI+ cations can retain over 110% of its initial value after 2100 h, and no obvious differences in the efficiency for DSCs with Li+, Cs+, and DMPI+ were observed after 1270 h under one sun light soaking. The results indicated that the electrolyte component in the traditional electrolytes should be further optimized to improve the interface stability of the dyed TiO2 electrode/electrolyte.

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(16) Nakade, S.; Kanzaki, T.; Kambe, S.; Wada, Y.; Yanagida, S. Langmuir 2005, 21, 11414−11417. (17) Zhang, C.; Dai, J.; Huo, Z.; Pan, X.; Hu, L.; Kong, F.; Huang, Y.; Sui, Y.; Fang, X.; Wang, K.; et al. Electrochim. Acta 2008, 53, 5503− 5508. (18) Zhang, C.; Huang, Y.; Huo, Z.; Chen, S.; Dai, S. J. Phys. Chem. C 2009, 113, 21779−21783. (19) Kato, N.; Takeda, Y.; Higuchi, K.; Takeichi, A.; Sudo, E.; Tanaka, H.; Motohiro, T.; Sano, T.; Toyoda, T. Sol. Energy Mater. Sol. Cells 2009, 93, 893−897. (20) Nguyen, P. T.; Andersen, A. R.; Skou, E. M.; Lund, T. Sol. Energy Mater. Sol. Cells 2010, 94, 1582−1590. (21) Alarcon, H.; Hedlund, M.; Johansson, E. M. J.; Rensmo, H.; Hagfeldt, A.; Boschloo, G. J. Phys. Chem. C 2007, 111, 13276−13274. (22) Zhang, C.; Chen, S.; Mo, L.; Huang, Y.; Tian, H.; Hu, L.; Huo, Z.; Dai, S.; Kong, F.; Pan, X. J. Phys. Chem. C 2011, 115, 16418− 16424. (23) Liu, W.; Kou, D.; Cai, M.; Hu, L.; Sheng, J.; Tian, H.; Jiang, N.; Dai, S. J. Phys. Chem. C 2010, 114, 9965−9969. (24) Schlichthorl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J. Phys. Chem. B 1997, 101, 8141−8155. (25) Liu, Y.; Hagfeldt, A.; Xiao, X.; Lindquist, S. Sol. Energy Mater. Sol. Cells 1998, 55, 267−281. (26) Shi, Y.; Wang, Y.; Zhang, M.; Dong, X. Phys. Chem. Chem. Phys. 2011, 13, 14590−14597. (27) Bisquert, J.; Zaban, A.; Salvador, P. J. Phys. Chem. B 2002, 106, 8774−8782. (28) Bisquert, J.; Vikhrenko, V. S. J. Phys. Chem. B 2004, 108, 2313− 2322. (29) Harima, Y.; Kawabuchi, K.; Kajihara, S.; Ishii, A.; Ooyama, Y.; Takeda, K. Appl. Phys. Lett. 2007, 90, 103517. (30) Jennings, J. R.; Wang, Q. J. Phys. Chem. C 2010, 114, 1715− 1724. (31) Hara, K.; Nishikawa, T.; Kurashige, M.; Kawauchi, H.; Kashima, T.; Sayama, K.; Aika, K.; Arakawa, H. Sol. Energy Mater. Sol. Cells 2005, 85, 21−30. (32) Furube, A.; Katoh, R.; Hara, K.; Sato, T.; Murata, S.; Arakawa, H.; Tachiya, M. J. Phys. Chem. B 2005, 109, 16406−16414.

ASSOCIATED CONTENT

S Supporting Information *

(i) Time-dependent normalized Jsc of the DSC containing different Li+ cation concentrations without MBI under one sun conditions; (ii) EIS for DSCs with different electrolyte cations measured at forward bias of −0.67 V in the dark; and (iii) IPCEs for DSCs with different electrolyte cations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-551-5591377. Fax: +86-551-5591377. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (Grant No. 2011CBA00700), National High Technology Research and Development Program of China (Grant No. 2011AA050510), and National Natural Science Foundation of China (Grant Nos. 21173227, 21173228, 21103197, and 21003130).

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REFERENCES

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