Thiolate Based Redox Shuttle for Dye-Sensitized Solar Cells

Article pubs.acs.org/JPCC

Disulfide/Thiolate Based Redox Shuttle for Dye-Sensitized Solar Cells: An Impedance Spectroscopy Study Xiaobao Xu,† Kun Cao,† Dekang Huang,‡ Yan Shen,*,‡ and Mingkui Wang*,† †

Michael Grätzel Center for Mesoscopic Solar Cells, ‡Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, 1037 Luoyu Road, 430074 Wuhan, P. R. China S Supporting Information *

ABSTRACT: Redox electrolyte has been proven to be extremely important in determining the optoelectronic properties in dye-sensitized solar cells (DSCs). Herein, we report on the application of various disulfide/thiolate redox shuttles within organic or aqueous electrolyte in conjunction with a dye-sensitized heterojunction. Electrochemical impedance spectroscopy is used to explore the difference in DSCs’ performance using disulfide/thiolate-based redox shuttles system in term of solvent effect and cations effect. A long interfacial charge recombination lifetime is found in disulfide/thiolate-based cells compared with that in iodide/triiodide cells, which can be attributed to different recombination states in these devices. semiconductor film or in the electrolyte.12 The most common redox couple is iodide/triiodide (I−/I3−) acting as hole transporting material.8 The counter electrode is often a platinum coated transparent conductive oxidize (TCO) glass where the reduction of the redox mediator occurs to complete the circuit.13 Up to now the I−/I3− redox couple has been demonstrated to be one of the best cases. However the interest in finding a suitable alternative is still strong due to a number of reasons,14,15 such as the adsorption of I−/I3− redox electrolytes in the visible part of solar spectrum, corrosion of I3− to several materials, and a mismatch between the redox potential of sensitizer cation D+ and that of I−/I3−. To date, only a handful of cationic redox couples (Co(II/III) complexes for instance) and p-type semiconductors have functioned as effective redox mediators.16,17 Yella et al. boosted the power conversion efficiency (PCE) of a DSC to 12.3% by using a donor−π-bridge−acceptor zinc porphyrin along with a cosensitizer and Co(II/III)(bipyridine)3-based redox electrolyte.18 Recently, organic redox electrolytes have attracted the most intensive investigation.19,20 For example, we reported the utilization of a disulfide/thiolate-based redox couple for DSCs.21 Electrolyte based on this redox couple has almost no adsorption in the visible part of the spectrum and without mass transport limitations even at full sunlight intensity, as well as showing fast dye regeneration kinetics. Using this novel, iodidefree redox electrolyte in conjunction with a dye-sensitized heterojunction, an efficiency of 6.4% was achieved under standard illumination test conditions.21 Recently, Liu et al. reported new colorless redox couples consisting of tetramethylthiourea and its oxidized dimer tetramethylformaminium

1. INTRODUCTION The end of the fossil fuel era is fast approaching. We must find renewable, sustainable energy options. Extensive study has concluded that there is no one single solution. Multiple solutions will be needed. Among these suggested energy sources, solar power has attracted a great deal of notice. Solar cells are considered as a major technology for obtaining energy from the sun, since they can efficiently convert sunlight to electricity. The current photovoltaic industry is exclusively dominated by silicon-based p-n junction cells and others based on inorganic materials.1,2 Fabrication of semiconductor devices requires a large amount of energy. It is highly desirable to provide solar cells with different shapes and explore different materials. Research has now focused on the third generation of photovoltaics, the most interesting version being the dyesensitized solar cell (DSC) as a low cost alternative for high efficiency solar cell production.3−5 Since the initial report by Grätzel, the efficiency of DSCs has been increased largely due to systematical study on sensitizers, electrolytes, cathodes, and device architecture.5−8 For instance, recent improvements in high efficiency DSCs have been evidenced through development of new ruthenium complexes capable of absorbing in a broad visible range as well as in the near-IR region.9 In addition to the well-documented sensitizers, significant improvements have been made due to the development of electrolytes. The redox couple in the electrolyte is of crucial importance for the stable operation of DSCs,10,11 which ensures an intimate contact with the surface of the nanostructured electrode, sufficient ionic conductivity, and a rapid reduction of the oxidized dye at the electrolyte/ TiO 2 interface. The redox couple influences the dye regeneration as well as other processes in a DSC, including electron-transfer kinetics at the counter electrode, dark current reactions, ion-pairing with the dye, and charge transport in the © XXXX American Chemical Society

Received: August 14, 2012 Revised: November 3, 2012

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dx.doi.org/10.1021/jp308109t | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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disulfidedication for DSCs. In combination with carbon based counter electrodes, the devices showed up to 3.1% power conversion efficiency under AM 1.5 light illumination and 4.5% under weaker light intensities.22 Kloo et al. designed a series of organic redox couples based on 2-mercapto-5-methyl-1,3,4thiadiazole and its disulfide dimer and studied them in an organic dye based DSC, showing a promising efficiency of 6.0% under AM 1.5 G light illumination.23,24 These characteristics make organic redox shuttles a promising candidate for DSCs. Stability of solar cells based on organic redox shuttles is not satisfied as yet but a process can be noticed. For example, the utilization of conductive polymer (PEDOT) or CoS counter electrodes25 or by introducing an ionic liquid thiolate26 can significantly improve the thiol-based DSC device stability. In this study, we investigated two types of disulfide/thiolate redox shuttles within organic or aqueous electrolyte in conjunction with a dye-sensitized heterojunction. Electrochemical impedance spectroscopy (EIS) was used to study the interfacial recombination process in the disulfide/thiolate-based and iodide/triiodide-based shuttles. Although electrons of most dye-sensitized heterojunctions tend to be very short-lived, there are notable exceptions and few technologies by which to predict their lifetimes. Even less is understood about how the internal recombination dynamics depend on the molecular structure and the local environment. Our main interest lies in understanding the effect of molecular structure of redox shuttles and solvents on the fast interfacial recombination dynamics between electrons in nanocrystal films and the oxidized redox in electrolytes.

Figure 1. Redox shuttles and dye used in this study: (a) 1-methyltetrazole-5-thiolate (coded T2/T−), (b) 2-mercapto-5-methyl-1,3,4thiadiaole-2-thiolate (coded M2/M−), (c) triiodide/iodide redox shuttle, and (d) Z907 Na sensitizer.

2. EXPERIMENTAL SECTION Two thiolate redox molecules have been synthesized, including the above-mentioned 1-methy-1-H-tetrazole-5-thiolate (T−) and its dimer (T2) redox shuttle (coded as T2/T−), and 2mercapto-5-methyl-1,3,4-thiadiaole-2-thiolate (M−) and its dimer (M2) redox shuttle (coded as M2/M−) through neutralization of the corresponding thiol in methanol with tetramethylammonium hydroxide.21 The molecular structures of various redox molecules and dye sensitizer (NaRu(4carboxylic acid-4′-carboxylate) (4,4′-dinonyl-2,2′-bipyridyl) (NCS)2 (coded as Z907Na))27,28 are presented in Figure 1. The NMR characterization of the prepared redox molecules (T2/T− and M2/M−) and sensitizer (Z907Na) are presented in the Supporting Information (Figure S1−3). A 7.5-μm thick transparent layer of 20 nm TiO2 particles was first printed on the FTO conducting glass electrode and then coated with a 5-μm thick second layer of 400 nm light scattering anatase particles (WER2-O, Dyesol). The details of the preparation of TiO2 films have been described elsewhere.29 The TiO2 film was first sintered at 500 °C for 30 min and then cooled to about 80 °C in air. Then the TiO2 film electrodes were dipped into a 300 μM Z907Na solution in a mixture of acetonitrile and tert-butyl alcohol (volume ratio, 1:1) at room temperature for 16 h. After being washed with acetonitrile and dried by air flow, the sensitized titania electrodes were assembled with thermally platinized conductive glass electrodes. The working and counter electrodes were separated by a 25 μm thick hot melt ring (Surlyn, DuPont) and sealed by heating. The internal space was filled with liquid electrolytes using a vacuum backfilling system. The electrolyte for device A was 0.4 M T2 and 0.4 M T− with the tetramethylammonium cation in the acetonitrile, device B being 0.4 M T2 and 0.4 M T− with the tetramethylammonium cation in the acetonitrile with 0.5 M

tert-butylpyridine (tBP) and 0.05 M LiClO4, device C being 0.4 M M2 and 0.4 M M− with the potassium cation in the acetonitrile, device D being 0.4 M T2 and 0.4 M T− with the tetramethylammonium ion in the mixture of water and acetonitrile (volume ratio, 1:19) with 0.5 M tBP and 0.05 M LiClO4, and device E being 0.8 M 1,3-dimethylimidazolium iodide and 0.4 M I2, respectively. The compositions of various electrolytes are listed in Table 1. A 450 W xenon light source solar simulator (Oriel, model 9119) with AM 1.5G filter (Oriel, model 91192) was used to Table 1. Detailed Composition of Electrolytes Used in This Study electrolyte I II III IV

V

composition 0.4 M T2 and 0.4 M T− with the tetramethylammonium cation in the acetonitrilea 0.4 M T2 and 0.4 M T− with the tetramethylammonium cation in the acetonitrile with 0.5 M tBP and 0.05 M LiClO4a 0.4 M M2 and 0.4 M M− with potassium cation in the acetonitrileb 0.4 M T2 and 0.4 M T− with the tetramethylammonium ion in the mixture of water and acetonitrile (volume ratio, 1:19) with 0.5 M tBP and 0.05 M LiClO4a 0.8 M 1,3-dimethylimidazolium iodide and 0.4 I2 in the acetonitrile

redox potentialc 0.485 0.485 0.473 0.435

0.536

a −

T is 1-methyl-1-H-tetrazole-5-thiolate anion and T2 is its dimer; M− is 2-mercapto-5-methyl-1,3,4-thiadiaole-2-thiolate and M2 is its dimer; cV vs NHE, obtained from the cyclic voltammetry of redox shuttle in the electrolyte with glossary carbon electrode as the work electrode, Pt wire as the counter electrode, and Ag/AgCl (3 M KCl in ethanol). b

B



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give an irradiance of 100 mW cm−2 at the surface of the solar cell. The current−voltage characteristics of the cell under these conditions were obtained by applying external potential bias to the cell and measuring the generated photocurrent with a Keithley model 2400 digital source meter (Keithley, USA). A similar data acquisition system was used to control the incident photon-to-current conversion efficiency (IPCE) measurement. A white light bias (1% sunlight intensity) was applied onto the sample during the IPCE measurements with ac model (10 Hz). The devices with the photoanode area of 0.2826 cm2 were tested with a metal mask: 0.159 cm2. Electrochemical impedance spectroscopy (EIS) measurements were measured using the PGSTAT302N frequency analyzer from Autolab (Eco Chemie B.V, Utrecht, The Netherlands) together with the Frequency Response Analyzer module providing voltage modulation in the desired frequency range. The Z-view software (v2.8b, Scribner Associates Inc.) was used to analyze the impedance data. The EIS experiments were performed at a constant temperature of 20 °C in the dark. The impedance spectra of the DSC devices were recorded at potentials varying from −0.9 V to −0.35 V at frequencies ranging from 0.02 Hz to 200 kHz, the oscillation potential amplitudes being adjusted to 10 mV. The photoanode (TiO2) was used as the working electrode and the Pt counter electrode (CE) was used as both the auxiliary electrode and the reference electrode. These obtained spectra were fitted (error α > 0 indicate an exponential distribution of traps below the conduction band edge. Comparatively larger values of α correspond to a steeper drop of the DOS below the conduction band edge.44 The value for α corresponding to devices A−E was found to be 0.36, 0.33, 0.31, 0.26, and 0.38 (see Table 1), respectively. Figure 6 shows the calculated charge density distributions (nc, nL, and ntotal) for devices A−E as a function of the potential expressed on the vacuum scale based on impedance measurements. A reasonable level for the TiO2 conduction band edge of −4.05 to ∼−4.1 eV vs vacuum is assumed and the redox potentials for various electrolytes are also included. As illustrated in Figure 6, because the density of trapped electrons (nL) in the TiO2 nanoparticles is much larger than that of free electrons (nc) present in the conduction band when Ec − EF ≫ kBT, the total electron concentration ntotal (ntotal = nc + nL) is essentially equal to nL. If we set the difference between the Fermi level of the TiO2 and the redox potential of electrolyte at 600 mV, a 35-fold increase in the nL could be found for device D comparing with device E (see Figure 6 and Table 3). This explains the larger Cμ obtained in device D comparing with that in device E (Figure 5c). From the energy diagrams of stained titania films shown in Figure 6, we may conclude that density of trapped electrons has made a major contribution to the interfacial recombination process in the thiols-based devices. Figure 7 compares that the apparent recombination lifetime (τn, τn = RctCμ) and electron diffusion coefficient (Dn, Dn = (RtCμ)−1) as a function of charge density. Note that the charge densities were obtained from the integration charge calculation based on eq 2d. Device E with I3−/I− has the shortest apparent electron lifetime (τn) at a given electron density among the tested samples. The devices A (T2/T−) and C (M2/M−) have

The chemical capacitance (Cμ) of electrons in the DSC can be expressed by Cμ = e 2

∂(ntotal) ∂(n + nL) = e2 c = Cμ(cb) + Cμ(trap) ∂μn ∂μn

(3)

The first term in eq 3, due to the free electrons (nc) in the conduction band can be obtained by n Cμcb = e 2 c kBT (3a) Ccb μ,

The second component,Ctrap μ , is related to the contribution of localized electron trapping states as expressed by Cμtrap = e 2

∂nL = e2 ∂E

∫E

E1

f ( E , E F ) g ( E ) d E ≈ e 2g ( E )

2

(3b)

where the density of energy states (DOS) can be described by the g(E) of eq 3c g (E ) =

⎛ E − Ec ⎞ exp⎜α ⎟ kBT ⎝ kBT ⎠

αNt,0

(3c)

Thus, the localized electron trapping states can be obtained by using numerical integration as showing eq 3d nL =

∫E

E F,redox + qUphoto F,reodx

g (E ) f (E , E F ) d E

(3d)

where E is the quasi Fermi level (QFL) position, f(E,EF) is the Fermi−Dirac distribution at room temperature. nL stands for localized trap states, which is usually got by using numerical integration of the product g(E) f(E) up to Ec. Thus, the chemical capacitance Cμ obtained from impedance study contains the all the necessary information ascribed to free electrons from the conduction band and the electrons from the surface trap states in the band gap. Clearly, the chemical capacitance in the titania nanoparticles with an exponential F



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kinetic dynamics.48,49 In order to better understand the charge recombination dynamics of various redox shuttles at the dyesensitized heterojunction interface, we have modeled the electron−hole recombination procedure by considering the continuous-time random walk of electron transport in a trapdominated material as presented in Figure S5.31,42,50 The dark current reaction contributes to the loss of photovoltaic performance in DSCs. The kinetic of electron transfer to the oxidized state of redox shuttle can be described in terms of the electron lifetime (∼k−1, k being the recombination rate constant).51 According to the quasi-static treatment developed by Bisquert and Vikhrenko,34 the apparent electron lifetime, τn, is related to the conduction band electron lifetime, τ0, by the expression

Table 3. Parameters for the Total Density of the Localized States, Electron Lifetime, and Rate Constants for Charge Transfer in Dye-Sensitized Solar Cells device τ0 (ms) ntrap (×1018 cm−3) nc (×1016 cm−3) kcbCox (×102 s−1) kcb (10−18 cm3 s−1) kstCox (s−1) kst (×10−20 cm3 s−1) kcb/kst

A

B

C

D

E

9.00 1.47 1.86 17.1 1.1 86 35.6 31

11 1.95 6.14 5.5 2.26 20 8.24 28

8 3.28 13.5 1922 7.98 78 32.4 25

40 13.1 44 3.3 1.38 96 40.1 3

0.8 0.37 0.3 194 80.5 844 351 23

⎛ ∂n ⎞ τn = τ0⎜1 + L ⎟ ∂nc ⎠ ⎝

(4)

Therefore, from eqs 1, 3d, and 4, one can find the following relationship: ⎡ ⎛ Ecb − E Fn ⎞1 − α ⎤ αNt,0 ⎢ exp⎜ τn = τ0 1 + ⎟ ⎥ ⎢⎣ NcT ⎝ kBT ⎠ ⎥⎦

(5)

where Nt,0 and Nc are the total density of the localized states and the accessible density of states in the conduction band; others have the normal physical meaning. The measured apparent charge recombination lifetime τn differs substantially from the free electron lifetime (τ0) and is dependent on the trap occupational level. The fitting data of electron lifetime with eq 5 is presented in Figure S5, giving conduction band electron lifetimes (τ0) of 9, 11, 8, 40, and 0.8 ms for devices A−E in the dark, respectively. In the fitting procedure, the rate of reduction of the oxidized species was assumed to be first order in electron density,52 though it is still under debate.51,53 The rate of charge transfer can also be expressed by the product of the concentration of electrons present in a particular electronic state of the semiconductor and a transition frequency.34 The rate of transfer from surface traps at the energy level E is

Figure 7. Plots of (a) apparent electron lifetime (τn) and (b) electron diffusion coefficients (Dn) obtained by EIS measurements in dark as a function of the electron charge density.

similar behavior of recombination lifetime. The T2/T− based electrolyte with water (device D) shows a longer recombination lifetime than that of electrolytes using organic solvents (devices A−C), which could be attributed to the solvent effect. Note that the organic rodox based electrolyte for device A−D have smaller effective electron diffusion coefficients (Dn) than device E with I3−/I− redox couple at the same charge densities. This is consistent with the observation that the density of the trapped electrons in devices A−D is nearly 1 order of magnitude higher than in device E at a given bias (see Table 3). Peter et al. suggested that the electron trap distributions may not be intrinsic properties of the TiO2 nanoparticles but may be associated with electron-ion interactions, and the electrolyte composition could alter the electron trap distribution significantly.45,46 An important issue for understanding on the interfacial charge recombination is to figure out the effect from traps in TiO2 gap band on the kinetic dynamics,47 which includes the conduction band states (or transport states or extended states), and the localized trap states. There are several methods available (especially time-dependent techniques) to measure the redox shuttle related interfacial charge recombination

−

dn(st) (st) (E) = gs(E)fs (E , E F)eox (E ) dt

(6)

For conduction band states −

dn(cb) (cb) (E) = nceox (Ec) dt

(7) (st)

eox(cb)

The transition probabilities, eox and (st: surface traps and cb: conduction band states), correspond to the product of the rate constant for isoenergetic electron transfer, k(st) and t k(cb) t , and the probability densities of the fluctuating energy levels in the electrolyte, given by Marcus−Gerscher model for electron transfer (i) eox (E) = 2kBTk t(i)

⎛ (E − E )2 ⎞ ox ⎟ exp⎜ − 4πλkBT ⎠ 4πλkBT ⎝ Cox

(8)

where the superscript i stands for st or cb. The overall transition frequency corresponds to the sum of the transition frequency for the electron transfer from the surface traps and the conduction band state. Then the rate constants for isoenergetic electron transfer, k(st) and k(cb) t t , and the resulted charge transfer G



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conduction band and an exponential distribution of surface states. It was found that cations, presumably through the association with the surface adsorption of the compound onto the TiO2 surface, play the key influence on the conduction band edge and, thus, on the interfacial charge recombination. The aqueous electrolyte shows an increased charge transfer rate constant at the surface state in comparison to the pure organic redox electrolyte, which might be related to the solvent dynamics on the interfacial charge transfer reaction. It is worth noting that surface state recombination dominates the interfacial recombination in the case of the organic redox shuttle, whereas the one being the conduction band electron for the case of I3−/I−. This study helps us improve the organic redox shuttle based DSCs by controlling the interfacial recombination process.

parameter through conduction band states and surface traps, (cb) k(st) t Cox and kt Cox, can be obtained by the known τ0 and τn. The results from fitting electron lifetime (τ0), recombination rate constants (k) and transition probabilities in the dark are summarized in Table 3. An example for E − EF, redox = 600 mV as the applied bias is given in Table 3. In the dark, though there is a lower value of the localized electron density (nL) in the TiO2, device E has the fastest rate constants (kst and kcb) for isoenergetic electron transfer from surface trap states and conduction band states of 3.51 × 10−18 and 8.05 × 10−17 cm3 s−1 among the tested samples. This is consistent with the result from conduction band electron lifetime, showing that device E has the shortest free electron lifetime (0.8 ms). The additives (Li+ and tBP) have obvious influence on the interfacial recombination from the surface trap states (kst being 35.6 × 10−20 and 8.24 × 10−20 cm3 s−1 for device A and B, respectively). The addition of water into the electrolyte (device D) increases the charge transfer parameter kst (40.1 × 10−20 cm3 s−1) in comparison with device B, which might be caused by the influence of solvent dynamics on the rate of the interfacial charge transfer reaction. The fitting results of devices A and C indicate that the charge transfer parameters kst and kstCox are similar; however the kcb and kcbCox are much larger for device C. This result could be caused by the surface modification by the cation adsorption (K+) as discussed above. Comparing with thiols-based redox shuttle (devices A and C), the iodide-based redox shuttle (device E) shows much faster charge transfer parameters (k(cb) and k(st) t t ). The dark current is the flow of current around the device, resulting from the interfacial charge recombination process. There are a number of factors to control the dark current, including the two most common: the rate of electron transfer between the TiO2 and the oxidized species in electrolyte, and the transport of redox shuttles to and from the TiO2 surface. The thoils-based redox couple electrolytes have an apparent diffusion coefficient of ∼2 × 10−5 cm2 s−1 at 20 °C, which is close to the iodide system.21 A low electrolyte diffusion coefficient of redox shuttle could potentially limit the photovoltaic performance of DSC, such as the photocurrent collection efficiency. Since the same concentration of oxidized species for different shuttles (0.4 M) and the organic solvent (acetonitrile) were used in this study, we suspect that the redox shuttle properties, including the molecular structure and size, may influence the interfacial charge recombination kinetics. An evolution of molecular dimension of various redox shuttles was present in Figure S6, showing that a bigger oxidized species volume for the T2 and M2 than that of I3−. The value of ratio between kcb and kst (kcb/ kst) at the E − EF,redox = 600 mV was calculate to be 31, 28, 25, 3, and 23 for device A-E, respectively (see Table 3). This indicates that the interfacial recombination mostly takes place between those electrons trapped by surface states with the oxidized state of the organic redox shuttle, which are electrons in conduction band for I3−/I−.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Synthesis of T2/T− and M2/M− redox shuttles, Z907Na sensitizer, NMR characterization of the redox shuttles (Figure S1 and S2) and the sensitizer (Figure S3), the charge transfer resistance (RCE) at the counter electrode/electrolyte interface (Figure S4), apparent electron lifetime versus electron energy levels from impedance measurements in dark (Figure S5), and the molecular size of redox shuttles with Chemdraw 3D (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected] (Y.S.); mingkui.wang@ mail.hust.edu.cn (M.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Director Fund of the WNLO, the 973 Program of China (2011CBA00703 and 2013CB922104), the NSFC (21103578, 21161160445, 20903030, and 201173091), the “Talents Recruitment Program” of the HUST, the Fundamental Research Funds for the Central Universities (HUST: 2011TS021 and 2011QN040), and the CME with the Program of New Century Excellent Talents in University (NCET-10-0416) is gratefully acknowledged. The authors thank the Analytical and Testing Center of the HUST for support.

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REFERENCES

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5. CONCLUSION Recent studies on new organic redox couples, such as the disulfide/thioalte based shuttle, have shown promising potential for application in DSCs. The interfacial charge recombination lifetime in disulfide/thiolate-based cells was found to be longer than that in triiodide/iodide based cells. EIS measurements were performed on this organic redox based DSC to rescue the interfacial charge recombination from different sources of electrons in the TiO2 nanocrystal: the H



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Thiolate Based Redox Shuttle for Dye-Sensitized Solar Cells

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