Correlating Dye Adsorption Behavior with the Open-Circuit Voltage of

27 May 2010 - dye had been attributed to its lying orientation after comparing ...... Moon, S. J.; Baker, R. H.; Ko, J.; Nazeeruddin, M. K.; Grätzel,...
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10992

J. Phys. Chem. C 2010, 114, 10992–10998

Correlating Dye Adsorption Behavior with the Open-Circuit Voltage of Triphenylamine-Based Dye-Sensitized Solar Cells Yanliang Liang, Bo Peng, and Jun Chen* Institute of New Energy Material Chemistry and Key Laboratory of AdVanced Micro/Nanomaterials and Batteries/Cells (Ministry of Education), Chemistry College, Nankai UniVersity, Tianjin 300071, People’s Republic of China ReceiVed: March 16, 2010; ReVised Manuscript ReceiVed: May 3, 2010

To study the relationship between dye adsorption behavior and open-circuit voltage (VOC) of dye-sensitized solar cells (DSCs), four triphenylamine-based organic sensitizers with closely related molecular structures but with two different adsorption orientations were engineered and compared. The origin of VOC was investigated in terms of band-edge movement of the TiO2 conduction band (CB) and interfacial charge recombination, with the latter found to be the governing factor. The two dyes with cyanoacetic acid as an anchoring group (TC dyes) adopt a standing adsorption mode and exert a larger surface dipole potential on TiO2 than their counterparts bearing rhodanine-3-acetic acid (TR dyes), which lie along the surface. TR dyes exhibit a greater extent of charge recombination than TC dyes because of the low surface-blocking efficiency of the dye layer and the intimacy between the I3--bound dyes and the TiO2. The differences in both CB movement and charge recombination between TR and TC dyes amplify with the expansion of the π-conjugated system. The present result shows that molecules able to stand on the TiO2 surface upon adsorption would be structures of interest in the design of organic sensitizers for DSCs. Introduction Since the milestone work from Gra¨tzel’s group, dye-sensitized solar cells (DSCs) have received extensive development efforts over the past two decades and now stand out as one of the most promising alternatives for solar energy conversion.1,2 High power conversion efficiencies (η) of over 11% have been reported for DSCs based on Ru complexes, volatile electrolytes, the I-/I3redox couple, and mesoporous TiO2 films.3 To further improve efficiencies, organic sensitizers with high molar extinction coefficients (ε) have been developed. Unfortunately, although comparable or even higher short-circuit photocurrent densities (JSC) were achieved with these sensitizers, efficiencies were not proportionally improved because of the somehow reduced opencircuit voltages (VOC).4,5 Low VOC values have become the major problem limiting the efficiency of DSCs based on organic dyes. The relationship between the VOC and the molecular structures of organic dyes has been actively studied only recently. Generally, the low VOC values of organic dyes are attributed to short electron lifetimes, mainly due to fast interfacial charge recombination.6,7 The extent of charge recombination was reported to be related to the molecular size of the dyes. For example, small-sized organic dyes were expected to adopt close packing upon adsorption, and the resulting dye layer was thought to protect the TiO2 surface and prevent charge recombination, while elongation of the π-conjugated linker leads to decreased VOC values, which were attributed to the unfavorable packing of the large-sized molecules.7 However, comparison of a series of coumarin dyes and a Ru dye (N719) suggested that it was not the size of conjugated system but the location of the negatively charged atoms that influences VOC.6 In some cases, the tendency of dyes with expanded conjugated systems to bind the electron acceptor species such as I3- and/or I2 was considered * To whom correspondence should be addressed. Fax: +86 22 23502604. E-mail: [email protected].

to facilitate the recombination process in analogy to systems based on Ru complexes.7–9 The effects of additional functions in dye backbone such as long alkyl chains were also examined but resulted in diverse conclusions.10–12 On the whole, factors influencing VOC are still far from well-understood. In our development of triphenylamine-based organic sensitizers, we noticed an interesting trend in which sensitizers incorporating cyanoacrylic acid (CA) as the acceptor/anchoring group always gave higher VOC values than the corresponding dyes bearing identical donors and π-bridges but a different acceptor of rhodanine-3-acetic acid (RA).13–15 A search of the literature found that this trend applies in liquid-state DSCs based on not only triphenylamine dyes16–18 but also virtually all types of organic dyes including coumarin,19 indoline,20 and phenothiazine dyes.21 Although this phenomenon is related to charge recombination, there is only one report focused on the origin of such a difference, which attributed it to the different electronic structures of CA and RA dyes.18 Alternatively, we think that the dye adsorption behavior is worth concern. Howie et al. proposed that indoline dyes bearing CA and RA exhibit standing and lying orientations on the TiO2 surface, respectively.22 In another study, the serious electron recombination of a squaraine dye had been attributed to its lying orientation after comparing several adsorption modes.23 Thus, it seems reasonable to expect some correlation between the possible lying mode and the fast charge recombination of RA dyes. In fact, the effect of the adsorption mode has been successfully used to explain the lower VOC values of heteroleptic Ru complexes than N719.24 However, to the best of our knowledge, the contribution of dye adsorption behavior to VOC has not yet been systematically investigated in organic dye-based DSCs. Herein, we report on a systematic comparison of DSCs based on four triphenylamine-based organic sensitizers (two of them are new) with closely related structures but with two different adsorption behaviors. They are divided into two classes, namely,

10.1021/jp1023873  2010 American Chemical Society Published on Web 05/27/2010

Triphenylamine-Based Dye-Sensitized Solar Cells

Figure 1. Molecular structures of the four triphenylamine-based organic sensitizers investigated in this study.

TR dyes (TR1425 and TR15) and TC dyes (TC14 and TC1526) bearing RA and CA as acceptor/anchoring groups, respectively (Figure 1). Combined Fourier transform infrared (FT-IR) analysis and theoretical molecular modeling indicates that TR dyes lie along while TC dyes stand aslant on the TiO2 surface; hence, these dyes provide appropriate models to study the effect of dye adsorption behavior on the VOC. Furthermore, one more double bond (-CdC-) is inserted into the conjugated system of the 14 series (TR14 and TC14) to form the 15 series (TR15 and TC15). This design gives a clear view of the influence coming from molecular size/conjugation length without making significant changes to the whole molecule. Experimental Section Materials and Methods. N,N-Dimethylformamide (DMF) and dichloromethane (DCM) were distilled from CaH2. Tetrahydrofuran (THF) was dried over anhydrous MgSO4. Potassium tert-butoxide, RA, and cyanoacetic acid were purchased from Acros and used as received. Acetonitrile (Guangfu, China) was chromatographic grade. All other reagents (analytical grade from China) were used without further purification. TiO2 nanoparticles (P25, a mixture of 30% rutile and 70% anatase) were purchased from Degussa AG (Germany). The triphenylamine dyes were synthesized under Wittig reaction, Vilsmeier reaction, and Knoevenagel condensation with the synthetic routes and detailed procedures (synthesis and characterization, Supporting Information). Melting points of the samples were taken on an RY-1 melting point apparatus (Tianfen, China). 1H and 13C NMR spectra were carried out on a Varian Mercury Vx300 MHz spectrometer with the chemical shifts reported relative to tetramethylsilane (for CDCl3) or the residual solvent peak (DMSO-d6 signal δ 2.50). High-resolution mass spectral analysis (HR-MS) was performed on a high-resolution ESI-FTICR mass spectrometer (Varian 7.0 T). Optical and Electrochemical Measurements. The absorption spectra of the dyes in DCM solutions and on sensitized TiO2 films (Figure S1 in the Supporting Information) were measured with a Jasco V-550 UV/vis spectrophotometer. The steady-state fluorescence spectra (Figure S2 in the Supporting Information) of the dyes in DCM solutions were recorded on a Cary Eclipse fluorescence spectrophotometer. The FT-IR spectra of the neat dyes and their potassium salts were recorded using the KBr disk technique, and those of the dye-loaded TiO2 films were recorded using a reflection method on a FTIR-650 spectrometer (Gangdong, China) at a resolution of 2 cm-1.

J. Phys. Chem. C, Vol. 114, No. 24, 2010 10993 Cyclic voltammetry (Figure S3 in the Supporting Information) was performed with a PARSTAT 2273 potentiostat/galvanostat/ FRA at room temperature at a scan rate of 100 mV s-1. A glassy carbon freshly polished with alumina pastes was used as working electrode, along with a Pt-wire counterelectrode and an Ag/ AgCl reference electrode to form a three electrode system. The potential of the Ag/AgCl electrode was internally calibrated with ferrocene (0.63 V vs NHE). The supporting electrolyte was 0.1 M tetrabutylammonium perchlorate in DCM, with the concentration of dyes fixed at 1 × 10-3 M. All of the first oxidation waves of the dyes studied in this work were reversible; therefore, the half-wave oxidation potentials (Eox) were reported as the average of anodic and cathodic peak potentials. The excitedstate oxidation potentials (Eox*) of the dyes were calculated as the difference between oxidation potential and optical transition energy (E0-0), while E0-0 was estimated from the intersection of the normalized absorption and emission spectra in DCM solution (5 × 10-5 M). Electrochemical impedance spectroscopy (EIS) in the frequency range of 100 mHz to 100 kHz was performed with a PARSTAT 2273 potentiostat/galvanostat/FRA in the dark with the alternate current amplitude set at 10 mV. Forward biases of 500-750 mV were applied to the dyesensitized TiO2 electrode during the measurement. The resulting curves were fitted to the appropriate equivalent circuit (Figure S4 in the Supporting Information). Theoretical Calculation Methods. The geometrical structures of the four dyes were optimized by employing the density functional theory at the B3LYP/6-31+G(d) level with the Gaussian 03W program package.27 Several possible rotamers and geometrical isomers were considered with a similar procedure as reported.22 Electronic properties were calculated on the basis of the optimized geometry at the same theory level. Solvent accessible surfaces were generated with the ChemBio3D ultra 11.0 program. Fabrication and Photovoltaic Measurement of DSCs. Solar cells studied in this work were fabricated with a procedure similar to that described previously.13 In brief, a transparent conducting substrate (F-doped SnO2, 20 Ω/sq, >80% transparency in the visible region, Yaohua, China) was treated with TiCl4 (aqueous, 40 mM) at 70 °C for 30 min, followed by doctorblading a paste consisted of TiO2 (16%), ethyl cellulose (20-30 cP, 5%), and terpinol (79%). The film was successively fired at 450 °C under air for 30 min, treated with TiCl4 solution (aqueous, 40 mM), and fired again to give a ∼10 µm thick mesoscopic TiO2 film. After it was cooled to room temperature, the TiO2 electrode was immersed in a dye solution (5 × 10-4 M in DCM) for 12 h and then rinsed with DCM and dried under an Ar flow. The counterelectrode was prepared by spin-coating H2PtCl4 (50 mM in isopropanol) on an FTO substrate and sintering at 390 °C under air for 30 min. A redox electrolyte (0.1 M LiI, 0.05 M I2, and 0.6 M DMPImI in acetonitrile) was introduced between the dye-sensitized photoanode and the Pt electrode spaced by adhesive tapes (45 µm thick), and the cell was finalized by firmly clamping the two electrodes together. For photovoltaic measurements of the DSCs, a 500 W xenon lamp served as the light source in combination with a bandpass filter (400-800 nm) to remove the ultraviolet and most of the infrared radiation. The light intensity was held at 100 mW cm-2 at the surface of the cells with the aid of a radiant power energy meter (model 70260 with 70268 Probe, Oriel). Current density-voltage (J-V) characteristics were recorded with a digital source meter (Keithley 2400) controlled by a computer. The incident photon-to-current conversion efficiency (IPCE) spectra for the cells were measured with a home-built system

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Figure 2. FT-IR spectra of neat dyes (a and d), potassium salts (b and e), and anchored dyes (c and f) of TR14 (left) and TC15 (right).

equipped with a bromine tungsten lamp and a grating spectrometer (SBP300, Zolix). The IPCE system was calibrated using a silicon reference cell (the 18th Research Institute of Electronics Industry Ministry, China). Results and Discussion Determination of Dye Adsorption Behavior on the TiO2 Surface. Dye molecules with carboxyl as an anchoring group attach to TiO2 particles by chemisorptions through either monodentate or bidentate binding.16 The adsorption behaviors of three indoline dyes bearing RA and CA have been studied by Howie et al. by comparing the dye coverages.22 The packing densities for both the monodentate and the bidentate binding modes were theoretically estimated, and the bidentate mode was assigned to both of the two types of dyes because it showed better agreement with experimental values. This method, although convenient, could be adventurous because the results would be obscured if dye multilayers form, which has been reported for organic dyes.11,28 Photoelectron spectroscopy has also been employed to study the surface molecular structure but was only able to give a general description of the orientation of the adsorbed dyes.29 On the other hand, the FT-IR technique provides a powerful tool in determining the type of carboxylate coordination and has been applied to the determination of the adsorption mode of both organic and organometallic sensitizers.16,30,31 On the basis of a wide range of IR data of carboxylate complexes, Deacon et al. concluded that monodentate coordination increases the separation (∆) of νasym(O-C-O) and νsym(O-C-O) relative to that of free carboxylate ion (taken as the values of sodium or potassium salts), while bidentate chelation/bridging decreases it.32 By comparing the ∆ value for potassium salt and an anchored form of a certain dye, the adsorption mode can be clarified. TR14 (left) and TC15 (right) were chosen as representatives of dyes bearing RA and CA, respectively, and the FT-IR spectra are shown in Figure 2. The peak at 1712 cm-1 for neat TR14 (Figure 2a) is characteristic of the stretching band ν(CdO) of the carboxyl group. Upon adsorption to TiO2 (Figure 2c), the ν(CdO) disappeared due to deprotonation, and a new absorption band at 1421 cm-1 was observed, which is attributed to νsym of carboxylate group. The corresponding νasym is likely to be overlapped by the stretching band of the phenyl ring at 1578 cm-1, resulting in a broader peak than that in neat TR14. A ∆ () νasym - νsym) value of 157 cm-1 was thus derived, which is smaller than that of the potassium salt of TR14 (1587 - 1412 ) 175 cm-1, Figure 2b), indicating a bidentate binding mode

Figure 3. Photocurrent density-voltage curves (top) and IPCE spectra (bottom) of DSCs based on the four dyes shown in Figure 1 under 100 mW cm-2 irradiation. The transmittance of the FTO is also shown.

for TR14. Similarly, attachment of TC15 to TiO2 brings about the disappearance of ν(CdO) at 1678 cm-1 and the appearance of νasym and νsym at 1578 and 1399 cm-1, respectively. The ∆ value (179 cm-1) was again smaller than that of the ionic form of TC15 (247 cm-1); therefore, TC15 also binds to TiO2 in a bidentate manner. The bidentate binding mode is preferred over the monodentate mode for several types of organic sensitizers bearing CA and benzoic acid as anchoring groups,16,31,33,34 and we now provide reliable evidence that it also holds true for those bearing RA. Combining the bidentate binding mode and the optimized molecular structures (as will be detailed in the following sections concerning surface dipole moments), TC dyes are established to be standing aslant on TiO2. As for TR dyes, the methylene group connecting the rhodanine ring and the carboxyl group induces a bend in the conjugation plane, causing them to lie along the surface. These two different adsorption behaviors lead to differences in interfacial properties and photovoltaic performances, which will be discussed in detail in the following sections. Photovoltaic Performances. For fair comparison, working electrodes were sensitized with DCM dye baths, because all four dyes freely dissolve. To get a clear view of how the dye adsorption behavior influences VOC, the additive tert-butylpyridine commonly used to increase VOC was not added to the electrolyte. The J-V curves of DSCs based on the four dyes measured under 100 mW cm-1 simulated sunlight (equivalent to AM1.5, 1 sun) are displayed in Figure 3a and Table S2 in the Supporting Information. TC dyes show significantly higher VOC values than TR dyes. Expansion of the π-conjugated bridge results in increased JSC values but reduced VOC values. As mentioned above, lower VOC values for organic dyes bearing RA as an anchoring group have been repeatedly reported.15–21 The value of VOC is determined by the potential difference between the Fermi level of TiO2 (EFn) and the chemical potential of the redox species (Ered) in the electrolyte, and EFn is determined by the potential of the conduction band edge (ECB) and the electron density (n) in TiO2. VOC could be described as35

VOC ) Ered - ECB -

( )

γkBT Ne ln e n

where γ is a characteristic constant of TiO2 tailing states, kB is the Boltzmann constant, T is temperature (293 K in this work), e is

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elementary charge, and Ne is the effective density of states at the CB. Considering that Ered, γ, and Ne would not change strongly in DSCs fabricated under similar conditions, the parameters influencing VOC are ECB and n, with the latter closely related to the charge recombination. The photoresponse ranges of TR dyes are broader than the corresponding TC dyes with the same conjugation lengths, but this does not lead to proportionally higher JSC values because of a ca. 16% reduction in the IPCE plateaus for TR dyes (Figure 3b). IPCE at a specific wavelength is given by

IPCE(λ) ) LHE(λ)ηinjηcol where LHE stands for light-harvesting efficiency, ηinj is the electron injection efficiency, and ηcol is the electron collection efficiency. At the wavelengths where absorption and IPCE reach their maxima, complete absorption of incident light is expected. Further correction of the IPCE maxima with the transmittance of the FTO gives internal quantum efficiencies (IQE)

IQE(λ) ) IPCE(λ)/LHE(λ) ) ηinjηcol which are only limited by ηinj and ηcol. IQEs of 0.98 and 0.95 are found for TC14 and TC15, respectively, which are very close to unity. Low IQEs for TR dyes (0.77 for TR14 and 0.75 for TR15) are derived, indicating inefficient electron injection and/or collection. Many research groups, including ours, have attributed the low IPCE values for RA-based dyes to poor electron injections into TiO2, because the π-conjugation in the RA dyes backbone is disrupted by the methylene group in the RA, and the lowest unoccupied molecular orbital (LUMO) electron density (Figure S5 in the Supporting Information) fails to extend to the anchoring carboxylate group.15,17,21 However, a recent study pointed out that electron injections for RA and CA dyes are both ultrafast and indistinguishable.18 An insufficient driving force for electron injection is also possible to induce poor ηinj. Eox* of TR14, TC14, TR15, and TC15 are estimated to be -1.19, -1.73, -1.12, and -1.22 V vs NHE, respectively (Table S1 in the Supporting Information). We find that the driving forces for electron injection (Eox* - ECB) are very similar for TR14 and TC15, but the two dyes show quite different IPCE maxima; besides, TR15 managed to achieve a comparable IPCE with TR14 in spite of its 70 mV smaller driving force. These results suggest that it is not ηinj but ηcol that limits the IPCE of TR dyes. If we assume that the loss of injected electrons is proportional to n and the rate constants for charge collection and charge recombination are independent of each other, then ηcol could be expressed by36

ηcol ) 1 -

τSC τOC

where τSC is the time constant of the combined processes of charge injection and recombination at short circuit, and τOC is the time constant of charge recombination at open circuit. Given comparable charge injection, ηcol is clearly limited by charge recombination. Theoretical Calculation of Surface Dipole Moments. The position of the TiO2 CB has been reported to shift as a function of the dipole moment of adsorbed dipolar molecules.37,38 Molecules having a dipole moment pointing toward the TiO2

Figure 4. Optimized molecular structures and calculated total dipole moments (µtot) together with the vector components (µy and µz) of the four dyes shown in Figure 1. The x-axis extends out of the plane of the page, and µx ) 0. The dyes are positioned in such a way that their orientations after anchoring onto the TiO2 surface are simulated. The TiO2 surface plane is parallel to both the x- and the y-axis.

surface plane (here the direction of the dipole moment is defined as pointing from negative to positive charge) shift the CB to more positive levels, while molecules with their dipole moment pointing out of the plane lead to negative shifts of CB and thus an increase in the VOC. For systems where the directions of the dipole moments of the modifying molecules are the same, the ones with larger values of the vertical dipole components (µvert) cause greater movement of the CB. This phenomenon has recently been used to explain the origin of VOC values of DSCs based on both organic and organometallic sensitizers.24,39 The geometries of the adsorbed dyes derived from combined theoretical calculation and FT-IR analysis are shown in Figure 4. The TiO2 surface plane is parallel to the x- and y-axis. For calculation simplicity and appropriate accuracy, the protonated forms of the dyes are employed for molecular modeling to represent the dye molecules chemisorbed to the TiO2.22 Considering their bidentate binding mode, the dye molecules are positioned in such a way that the C2 axis of the carboxylate is parallel to the z-axis; therefore, their orientations after anchoring onto the TiO2 surface are simulated. The components of dipoles parallel to the x-axis are fixed at zero, facilitating a straightforward observation of the effect of adsorption angles (θ) on the vertical and horizontal components of the dipole moments in a two-dimensional manner. All four dyes possess comparable total dipole moments (µtot) in the range of 6.040-8.645 D (Table S3 in the Supporting Information). The vertical component of dipoles is defined as

µvert ) µtot sin θ It is sensitive to the angle between µtot and the TiO2 plane. For TR dyes that lie along the surface, the values of θ are small (23.14-24.65°) and the µvert values are only ∼40% of the corresponding µtot values. In contrast, the large θ values for TC dyes (43.02-45.64°) ensure ∼70% of the µtot values contribute to the vertical components. As a result, µvert values for TC dyes are nearly twice as high as those for TR dyes. To our experience, the µtot of Ti4+-bound dyes could vary (10% with respect to the protonated form.40 Such a change, however, is small when compared to the ∼100% difference

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Figure 5. Typical impedance spectra of DSCs based on the four dyes shown in Figure 1. The spectra were measured in the dark and displayed in the form of Nyquist plot. A forward bias of -0.6 V was applied to the working electrode.

Liang et al.

Figure 6. Dark-current density-voltage curves of DSCs based on the four dyes.

in µvert values between the two types of dyes. The change in surface potential can be estimated by37

∆V )

Nµvert εε0

where N is the coverage of dipoles (Table S1 in the Supporting Information), ε0 is the permittivity of the dipole layer, and ε is the vacuum permittivity. If we use a value of ε ) 5.3,22 ∆V values can be approximated as TC15 (401 mV) > TC14 (428 mV) > TR15 (244 mV) > TR14 (188 mV), implying that sensitizers adopting standing modes on the TiO2 could induce a higher VOC via surface modification. It is readily expected that dipole moments increase with the expansion of π-conjugation, as is confirmed by comparing the µtot values of the 15 series with the 14 series. Interestingly, we find that the length of the π-bridge affects µvert in yet another aspect, that is, the adsorption angle θ. For TC dyes that stand aslant on the TiO2 surface, the insertion of a -CdC- into the dye backbone results in an increase in θ from 43.02° to 45.64°. This trend has been further established by calculation of the θ value of an organic sensitizer, JK-2,39 with a longer π-conjugated bridge, for which an even larger θ (48°) is obtained. It seems that long linkers are preferred structures for requiring a large upward shift of the CB of the sensitized TiO2. Relationship between Dye Adsorption Geometry and Interfacial Charge Recombination. The charge recombination at the TiO2/electrolyte interface is investigated by EIS, which is a versatile steady-state method to elucidate the electronic and ionic processes occurring in DSCs. Typical EIS Nyquist plots for DSCs based on the four dyes measured in the dark under a forward bias of -0.6 V are shown in Figure 5. The fitted interfacial charge transfer resistances (RCT) vary dramatically from one dye to another in the sequence TC14 (409 Ω) > TC15 (130 Ω) > TR14 (71 Ω) > TR15 (46 Ω), indicating that recombination of conduction band electrons to the electrolyte occurs more easily for DSCs based on TR dyes than TC dyes. For dyes with the same anchoring group, recombination is faster for those with more expanded π-bridge (the 15 series). This observation can be cross-checked by measuring the J-V characteristics of the DSCs in the dark. By extrapolating the high potential region of the J-V curves to J ) 0 (Figure 6), the onset potentials of dark currents for the four dyes are estimated as TC14 (647 mV) > TC15 (625 mV) > TR14 (543 mV) > TR15 (488 mV), the sequence of which is in agreement with that of RCT values. The faster charge recombination at the interface for RA dyes than CA dyes has been reported by several research groups including ours,15–18 but understanding of the origins of such a difference is still limited. Tian et al. tentatively proposed that

Figure 7. Graphical illustration of the geometries of the adsorbed dyes and the dye-I3- interaction.

the different spatial array between RA dyes and CA dyes on TiO2 surface could affect dark current differently.16 From analysis of the transient absorption traces in the near-infrared region, Wiberg et al. concluded that RA dyes facilitate recombination by injecting electrons to short-lived surface trap states, suffering from their specific electronic structures.18 This finding is reasonable but still not the whole picture because it would lead to an expectation of shorter electron lifetimes for RA dyes than CA dyes in any DSCs; however, the opposite was observed in solid-state DSCs.22 Besides the electronic structures, the most apparent difference of TR dyes from TC dyes is their adsorption geometry on TiO2. We closely compare the geometries and possible interaction with I3- of the four dyes as illustrated in Figure 7. Bulky aryl groups in organic dyes are reported to enhance electron lifetimes by blocking the redox species from approaching TiO2.41–43 For the four dyes in this work, the diphenylvinyl and the two outer phenyl groups in the triphenylamine core (marked with blue ellipse) are expected to exert similar blocking effects. In the case of TC dyes, these “blocking moieties” locate well above the dye molecule. In contrast, TR dyes, because of their lying orientation, have their blocking moieties placed aside, leaving the π-conjugated system open to the electrolyte. If the TiO2 surface is completely coated with a monolayer of dye, the electron acceptor species, I3-, would be difficult to penetrate a TC dye layer considering the large ionic size of I3- (cf. the ionic diameter of the monatomic anion I- is already 4.4 Å) and the small “unprotected” voids (the widths are approximated from their solvent accessible surfaces to be 2.2 Å for TC14 and 3.1 Å for TC15). The corresponding void of TR14 is, on the other hand, considerably wider (8.8 Å), and I3- can readily approach the vicinity of TiO2. The situation becomes worse for TR15 with an unprotected void as wide as 11.5 Å, reflecting the poorest blocking efficiency. An additional disadvantage of TR dyes’ lying geometry can be found when the complex-forming mechanism of dye molecules with I3- is taken into account. Take TR14 as an example, the bound I3- would be brought very close to TiO2 by the low-lying binding site in TR14 (Figure 7, left). Although this qualitative interpretation is based upon an RA dye, we see no reason why it should not extend to other organic dyes adopting a lying adsorption mode such as some squaraine dye.23

Triphenylamine-Based Dye-Sensitized Solar Cells

Figure 8. Chemical capacitance Cµ (a), interfacial charge transfer resistance RCT (b), and electron lifetime τr (c) fitted from impedance spectra under a series of applied potentials.

Note that the above conclusion specifically applies to DSCs where I-/I3- (and possibly small-sized species like Br-/Br3-) is used as a redox couple. For instance, large molecules such as spiro-MeOTAD used in solid-state DSCs would have difficulty in infiltrating a dense dye layer. Besides, if electron acceptor species do not form complexes with dyes, the conjugation system’s accessibility and distance from TiO2 are not important to charge recombination. In these occasions, achievement of high VOC values shall not be so restricted by dye adsorption geometry.22 Dependence of Photovoltage on the Conduction Band Movement and Charge Recombination. To investigate the influences of the shift in the TiO2 CB and charge recombination on VOC, EIS spectra were measured under a range of applied potentials near the VOC values of the four DSCs, and the fitted chemical capacitances (Cµ) and RCT values together with electron lifetimes τr () Cµ × RCT) are plotted in Figure 8. Cµ follows an exponential rise with the increase of forward bias as is given by44

Cµ )

[

]

R e2 exp (E - ECB) kBT kBT Fn

where R is a constant related to the distribution of electronic states below the conduction band and

EFn ) Ered + eVappl where Vappl is the potential at the electrode. At a given value of Cµ, the cell potentials are in the order TR14 < TC15 < TC14 < TR15, indicating a sequential upward shift of the CB edge. We notice that this result does not coincide with that which we acquired from the calculation of surface dipole potential (TR14 < TR15 < TC14 < TC15). Such a deviation is understandable, remembering that many other ionic species like H+ released

J. Phys. Chem. C, Vol. 114, No. 24, 2010 10997 from dyes during adsorption and Li+ in the electrolyte shift CB positively,45,46 diminishing the upward-shifting effect exerted by the sensitizing dyes. ECB is therefore an interplay of several counteracting factors, and precise prediction of it requires more qualitative analysis. In the range of potentials studied, τr increases in the order TR15 < TR14 < TC15 < TC14. This trend is in good accordance with that for VOC, implying that it is charge recombination, rather than the position of CB, that governs VOC. In other words, there is a close association among lying adsorption mode, fast charge recombination, and low photovoltages. On the basis of the above interpretations, we propose that molecules able to stand on the TiO2 surface upon adsorption would be structures of interest in the design of organic sensitizers. They possess a 3-fold advantage on achieving high VOC values over their counterparts bearing bent building blocks (such as RA), which make the molecules tend to lie along the binding surface, due to the reasons that follow. First, they could exert large vertical components of surface dipole potential on TiO2 because of a large adsorption angle. Second, they ensure a close packing of dye molecules and therefore the bulky blocking moieties, effectively passivating TiO2 against I3- and leading to reduced interfacial charge recombination. Third, unlike the lying dyes, their complexes with I3- are relatively far from TiO2, thus avoiding intimate contact with electrons in the TiO2 CB. It is noted that controlled charge recombination contributes to not only VOC but also JSC because of the improved charge collection efficiency at the photoanode. Consequently, sensitizers adopting a standing orientation (TC dyes in this study) stand out as more promising candidates for DSCs with TC15 showing an overall efficiency as high as 5.70%, >40% higher than structurally similar TR dyes, say, TR15, in spite of its narrower photoresponse range than the later. Conclusions Four triphenylamine-based organic sensitizers with RA and CA as acceptor/anchoring groups are engineered and compared in DSCs. Combined FT-IR analysis and theoretical modeling show that TR and TC dyes adopt different adsorption behaviors of lying flat and standing aslant, respectively. TR dyes exhibit a broader photoresponse range but lower VOC values as compared to TC dyes. The origin of VOC was investigated regarding movement of the TiO2 CB and charge recombination at the TiO2/electrolyte interface. TC dyes, because of their larger adsorption angles, show larger dipole moments in the direction normal to the TiO2 surface plane than TR dyes, which is likely to shift the CB to more negative position and leads to higher VOC values. Such a difference amplifies with the expansion of the π-conjugated system. A significantly larger extent of charge recombination was found for TR dyes than TC dyes, which was correlated to dye adsorption behavior in terms of the surface blocking effect of the dye layer and the spatial distance between the I3--bound dye and the TiO2. From our findings, we propose that dyes with standing adsorption mode should be preferred in the future development of organic sensitizers. Acknowledgment. This work was supported by the Research Programs from National MOST (2005CB623607 and 2009AA05Z421) and Tianjin High-Tech (07ZCGHHZ00700). We acknowledge the computational support by NKSTARS provided by the Center of Theoretical and Computational Chemistry at Nankai University. Supporting Information Available: Synthesis and characterization of TC14, TR15, and TC15; IR spectra of TR14 and

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