Increasing the Open-Circuit Voltage of Dye-Sensitized Solar Cells via

Sep 5, 2017 - Metal-ion coordination is shown to increase the open-circuit voltage ... Considerable efforts are dedicated to increasing the open-circu...
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Increasing the Open-Circuit Voltage of Dye-Sensitized Solar Cells via Metal-Ion Coordination Omotola O. Ogunsolu,† Jamie C. Wang,‡ and Kenneth Hanson*,†,‡ †

Materials Science and Engineering and ‡Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States S Supporting Information *

ABSTRACT: Considerable efforts are dedicated to increasing the open-circuit voltage (Voc) of dye-sensitized solar cells (DSSCs) by slowing charge recombination dynamics using atomic layer deposition, alkyl-substituted dyes, coadsorbents, and other strategies. In this report, we introduce metal-ion coordination to a metal oxide bound dye as an alternative means of increasing Voc. Metal-ion coordination has minimal influence on the photophysical and electrochemical properties of the N3 dye, but presumably because of increased steric hindrance at the interface, it slows charge recombination kinetics and increases Voc by upwards of 130 mV relative to the parent N3 DSSC. With respect to the nature of the metal ion, the trend in decreasing short-circuit current (Jsc) and increasing Voc correlates with the charge of the coordinated metal ion (MIV → MIII → MII). We attribute this trend to electrostatic interactions between the metal cation and I− or I3−, with the more highly charged cations maintaining a higher concentration of mediator anions in proximity to the surface and, as a result, increasing the regeneration and recombination rates.

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INTRODUCTION Dye-sensitized solar cells (DSSC) are a promising alternative to conventional silicon solar cells because of their ease of fabrication, flexibility, and low-light-harvesting abilities.1 However, the current record efficiency for DSSCs stands at 14.1%, leaving much room for improvement.2 In terms of increasing power conversion efficiencies (PCEs) in these devices, a majority of the research focuses on increasing the short-circuit current (Jsc) and/or open-circuit voltage (Voc). Efforts aimed at improving the Jsc include modifying the dye structure,3 codepositing complementary dyes,2,3 increasing the dye extinction coefficients,4,5 and improving the dye regeneration rates.6−8 Efforts to increase Voc have focused on shifting the conduction band potential of the metal oxide,9 lowering the potential of the redox mediators,10,11 and slowing recombination between injected electrons in metal oxide and the dye/ mediator11,12 by using atomic layer deposition,13,14 alkylsubstituted dyes,15 coadsorbents,16 or self-assembled bilayers.17,18 Recently, in an effort to increase Jsc, we introduced selfassembled bilayers 19 composed of two complementary chromophores on nanocrystalline TiO2 as a means of increasing the light absorption and solar spectrum overlap.12 Bilayer films were prepared using a stepwise procedure20−22 by first soaking TiO2 in a solution of pN3 dye, followed by ZrOCl2 linking ion, and finally p1M dye. The bilayer DSSCs had increased Jsc, Voc, and PCE values relative to the monolayer DSSCs. One unexpected but notable observation from this study was that simply coordinating the ZrIV metal ion to pN3, without the second dye, decreased Jsc but, remarkably, increased Voc by ∼30 © 2017 American Chemical Society

mV relative to the pN3 dye alone. Given that pN3 is the phosphonated equivalent of N3, one of the bench mark dyes for DSSCs, we were particularly intrigued by the promise and simplicity of using metal “linking” ions to influence the DSSC performance. However, the role of the metal ion in dictating the device performance was unclear. In this report, we coordinate several different metal cations (CdII, CuII, FeII, LaIII, MnII, SnIV, ZrIV, and ZnII) to TiO2−N3 (Figure 1a), with the goal of understanding how the metal ion influences the electrochemical/photophysical properties of N3, as well as the dynamic events (Figure 1b) and performance metrics of DSSCs.

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EXPERIMENTAL SECTION

Materials. TiO2 paste (18 NR-T) and ruthenium(II) cisbis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ditetrabutylammonium (N719) were purchased and used as received from Dyesol, while ruthenium(II) cis-bis(isothiocyanato)bis(2,2′bipyridyl-4,4′-dicarboxylic acid) (N3) was purchased from Solaronix. Fluorine-doped tin oxide (FTO) glass substrates were purchased from Hartford Glass Co. (sheet resistance 15 Ω □−1). 1-Butyl-3methylimidazolium iodide (BMII; Aldrich), iodine (I2; Fisher Scientific), lithium iodide (LiI; Aldrich), 4-tert-butylpyridine (TBP; Aldrich), guanidine thiocyanate (GSCN; Aldrich), dihydrogen hexachloroplatinate(IV) hexahydrate (H2PtCl6·6H2O; Alfa Aesar), tetrabutylammonium hexafluorophosphate(V) [(TBA)PF6; Aldrich], lanthanum(III) acetate sesquihydrate [La(OOCCH3)3·1.5H2O; Alfa Aesar], cadmium(II) acetate dihydrate [Cd(OOCCH3)2·2H2O; Alfa Aesar], tin(IV) acetate [Sn(OOCCH3)4; Aldrich], copper(II) acetate Received: June 16, 2017 Published: September 5, 2017 11168

DOI: 10.1021/acs.inorgchem.7b01531 Inorg. Chem. 2017, 56, 11168−11175

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Inorganic Chemistry

using a 93.9 pass energy and 0.8 eV step−1 on an analysis area of 200 μm diameter. DSSC Fabrication. The functionalized TiO2 films (TiO2−N3 and TiO2−N3−Mn) on FTO glass (1.5 × 2.0 cm) served as the photoanode of the DSSCs. The cathode was prepared by first drilling a small hole (d = 1.1 mm) in the corner of the 2.0 × 2.2 cm glass slide. The drilled slides were thereafter sonicated with EtOH for 10 min. This was followed by the deposition of platinum by drop-casting 75 μL of a ∼4 mM solution of H2PtCl6 in EtOH on FTO glass followed by heat treatment (400 °C) using a Leister hot air blower (model 142.636). DSSC sandwich cells were prepared using a home-built assembly apparatus.17 A 2-mm-wide 1.2 × 1.2 cm Surlyn thermoplastic resin (25-μm-thick Meltonix 1170-25 from Solaronix) was placed between the anode and cathode and the entire ensemble heated to ∼150 °C for 7 s. The electrolyte (0.6 M BMII, 0.1 M LiI, 0.05 M I2, 0.1 M GSCN, and 0.5 M TBP in a 85:15 mixture of MeCN and BuCN) was introduced into the cell through a hole on the counter electrode using vacuum backfilling. The cell was then sealed by heating a Meltonix film and a 4 × 4 mm portion of a microcover glass slide (18 × 18 mm VWR). Film Characterization. Differential pulse voltammetry, attenuated-total-reflectance infrared (ATR-IR) absorption, steady-state emission, and nanosecond transient absorption measurements were performed as previously reported.12 Details of these procedures are reported in the Supporting Information. Device Characterization. Photocurrent density−voltage (J−V) curves, incident photon-to-current efficiency, electrochemical impedance spectroscopy (EIS), and open-circuit voltage decay (OCVD) measurements were performed as previously reported.12 Details of these procedures are reported in the Supporting Information.

Figure 1. (a) Schematic representation of the N3 dye and metal ion (Mn = CdII, CuII, FeII, LaIII, MnII, SnIV, ZnII, and ZrIV) coordination. (b) Energy-level diagram and dynamic events in a DSSC (kex = light absorption rate, kr = radiative decay rate, knr = nonradiative decay rate, kinj = electron injection rate, kbet = back-electron-transfer rate, kregen = regeneration rate, krecomb = recombination rate, and kred = mediator reduction rate).

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RESULTS AND DISCUSSION Film Formation. MO2−N3−Mn films were composed of nanocrystalline metal oxide, TiO2 or ZrO2, N3, and a metal ion Mn (where Mn = CdII, CuII, FeII, LaIII, MnII, SnIV, ZnII, and ZrIV). N3 dye was selected for this study because of its wellunderstood properties and behavior in DSSCs,23,24 and the metal cations were selected because they are known to readily coordinate to CO2H functional groups, as described in the selfassembled bilayer17,25,26 and metal−organic framework literature.27,28 For consistency, the same salt (acetate), solvent (MeOH), and concentration (0.5 mM) were used for the metal-ion-coordination step. The MO2−N3−Mn films were prepared using a stepwise soaking procedure, with the MO2 films first immersed in an EtOH solution of N3 dye. The MO2−N3 films were then soaked in a MeOH solution of the metal ion, rinsed with MeCN, and dried under a stream of air. Each of these steps was monitored by UV−vis and ATR-IR spectroscopy. The ATR-IR spectra of the TiO2−N3 and TiO2−N3−Mn films are shown in Figure 2. In accordance with previously published assignments for TiO2−N3, peaks at 1537 and 1438 cm−1 are characteristic of the bipyridyl ligand, 2110 cm−1 corresponds to the NCS group, ∼1720 cm−1 is characteristic of the CO group of COOH, 1262 cm−1 is a C−O stretching vibration, and the peaks at 1608 and 1379 cm−1 correspond to the symmetric and asymmetric vibrations of COO, respectively.29,30 Similar spectral features for TiO2−N3 have been reported by Shomali et al.31 and Ma et al.32 For TiO2−N3−Mn, the disappearance of the non-surface-bound CO stretch at ∼1720 cm−1,30 as well as the change in intensity of the bipyridyl and COO peaks between ∼1300 and 1700 cm−1, is consistent with metal-ion coordination (Figure 2). The decrease in the peak separation between asymmetric and symmetric bands of COO upon metal-ion coordination is suggestive of a bidentate (one metal per COO) or a bridging bidentate (two metals per COO)

monohydrate [Cu(OOCCH3)2·1H2O; Aldrich], zinc(II) acetate dihydrate [Zn(OOCCH3)2·2H2O; Alfa Aesar], manganese(II) acetate tetrahydrate [Mn(OOCCH3)2·4H2O; Aldrich], iron(II) acetate [Fe(OOCCH 3 ) 2 ; Aldrich], zirconium(IV) acetate hydroxide [(CH3CO2)xZr(OH)y, where x + y ≈ 4; Aldrich], tetraisopropyl orthotitanate (C12H28O4Ti; TCI), zirconium(IV) propoxide (Zr[O(CH2)2CH3]4; Alfa Aesar), and valeronitrile (BuCN; Aldrich) were all used as purchased without further purification. Acetonitrile (MeCN; Aldrich), methanol (MeOH; Aldrich), and ethanol (EtOH; Koptec) solvents were all used without further purification. Film Preparation. FTO glass substrates were sonicated in EtOH and then HCl/EtOH (15/85% mix) for 20 min. Nanocrystalline TiO2 films of ∼5 μm thickness, coating an area of 7 × 7 mm on top of TiCl4-treated FTO glass, were prepared by screen-printing Dyesol 18 NR-T TiO2 paste with an ATMA screen printer and subsequent sintering at 500 °C for 30 min. A TiO2−N3 film was prepared by soaking the TiO2 thin film in a 0.4 mM EtOH solution of N3 dye. The slides were then removed after 24 h, rinsed with MeCN, and dried under a stream of air. TiO2−N3−Mn films (where Mn = CdII, CuII, FeII, LaIII, MnII, SnIV, ZnII, and ZrIV) were prepared by immersing the TiO2−N3 films in a 0.5 mM solution of the respective metal acetate salt in MeOH. After complete coordination, as determined from a time-dependent loading study, the slides were rinsed with MeCN and dried under a stream of air. A similar procedure was followed for the ZrO2 thin film but with an active area of 10 mm × 15 mm. TiO2(Mn)− N3 films were prepared by soaking the TiO2 films in a 0.5 mM solution of the respective metal acetate salt in MeOH for the same period as that for TiO2−N3−Mn films. The films were thereafter rinsed with MeCN and immersed in a 0.4 mM EtOH solution of N3 dye. After complete loading, the slides were rinsed with MeCN and dried under a stream of air. X-ray Photoelectron Spectroscopy (XPS). XPS was performed using a PHI 5000 VersaProbe II (ULVAC PHI, Inc.) and used to determine the relative ratio of dye to metals in each sample. The survey XPS spectra were recorded with a monochromatic Al Kα source 11169

DOI: 10.1021/acs.inorgchem.7b01531 Inorg. Chem. 2017, 56, 11168−11175

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individually and not as metal oxide or metallic particles. If we assume that two of the carboxylate groups of N3 are bound to the surface29,30 and two carboxylates are free to coordinate a single metal ion each, then we would anticipate a N3−M ratio of 1:2. The larger than 1:2 ratio in conjunction with the ATRIR data indicates that at least one metal ion is coordinated to each free carboxylate group. The larger number of metal ions beyond the ideal ratio could be attributed to (1) more than one ion coordinating to the carboxylate groups via bridging bidentate modes, (2) some metal ions coordinating to the NCS group (vida supra), (3) at least some of the metal ions adsorbing directly to the metal oxide surface, and/or (4) the limited escape depth of the photoelectron preventing quantitative determination of the N(1s) peak. Without direct structural characterization of the interface, it is difficult to differentiate between these possibilities. However, the relatively small N3−M ratio for the films, except for ZrIV, suggests that these contributions are limited. Photophysical and Electrochemical Characterization. The room temperature absorption spectra of TiO2−N3 and select examples of TiO2−N3−Mn can be seen in Figure 3, with

Figure 2. ATR-IR absorption spectra for TiO2−N3 and TiO2−N3− Mn (Mn = CdII, CuII, FeII, LaIII, MnII, SnIV, ZnII, and ZrIV).

coordination mode.29 The minimal change in the intensity and peak positions of the ATR-IR spectrum of TiO2−N3 after 2 h of soaking in neutral (neat), basic (0.5 mM NaAc), and acidic (0.5 mM HAc) MeOH solutions (Figure S1) suggests that the solvent treatment or acid−base chemistry of the counterion is not solely responsible for the spectral changes during M(OAc)x treatment. It is worth noting that, with at least some of the metal ions, especially MnII and CdII, there may be a decrease in the NCS peak intensity, suggesting partial metal-ion coordination with the NCS group.33 Metal-ion coordination to thiocyanate groups is an effective means of generating NCSbridged bimetallic complexes34,35 and has even been suggested as a means of sensing mercury ions.36 The metal-ion loading time, the time when no more changes were observed in the ATR-IR spectrum (Figures S2 and S3), decreases in the order of CdII ≈ CuII ≈ LaIII (3 h) > SnIV ≈ ZrIV ≈ ZnII ≈ FeII (2 h) > MnII (1 h). The observed trend in the loading time roughly correlates with the metal-ion size [LaIII (117 pm) > CdII (109 pm) > CuII (91 pm) ≈ ZnII (88 pm) ≈ ZrIV (86 pm) > SnIV (83 pm) > MnII (81 pm) > FeII (75 pm)], where large metal ions such as LaIII and CdII have the longest loading times and MnII and FeII have the shortest loading times. It is worth noting that there is little to no correlation between the metal-ion loading time and the charge, charge/size ratio,37 or hard−soft acidity−basicity of the metal ion,38,39 indicating that the loading process may be diffusion-limited and not metal-ion-coordination-limited. XPS was used to determine the N3-to-metal-ion ratio in TiO2−N3−Mn, and the results are summarized in Table 1 with

Figure 3. UV−vis absorption spectra for TiO2−N3, TiO2−N719, and TiO2−N3−Mn (Mn = ZnII and ZrIV).

the remaining spectra in Figure S12. N3 dye exhibits a metal-toligand charge-transfer (MLCT) transition from ∼430 to 560 nm with a peak maximum at ∼500 nm.24 For a majority of the metal-ion-coordinated films, except for CuII, FeII, and ZrIV, there is a notable red shift in the MLCT absorption feature relative to the TiO2−N3 film. Interestingly, the red-shifted spectra for TiO2−N3−Mn resemble that of TiO2−N719 (Figure 3),40 where N719 is the doubly deprotonated derivative of N3. Given the similarity between the oxidation potentials of the TiO2−N3−Mn and TiO2−N3 (vida infra), we attribute the red-shifted absorption to stabilization of the 2,2′-bipyridyl-4,4′dicarboxylic acid ligand through deprotonation of the carboxylate groups and metal-ion coordination, effectively lowering the energy of the MLCT transition. To investigate the influence of metal-ion coordination on the ground- and excited-state potentials of the dye, differential pulse voltammetry and steady-state emission were performed, and the results are summarized in Table 2 and Figures S13 and S14. Because of the irreversible oxidative wave of N3,41 differential pulse voltammetry, rather than cyclic voltammetry, was used to determine the oxidation potential [E1/2(RuIII/II)]. With the exception of TiO2−N3−CuII, which had a ∼180 mV shift, there is less than a 60 mV shift in E1/2(RuIII/II) of TiO2− N3−Mn relative to the TiO2−N3 film, indicating that metal-ion coordination to the ligand has minimal influence on the metal-

Table 1. N3-to-Metal-Ion Ratio from XPS in the TiO2−N3− M Films Mn II

Cd CuII FeII LaIII

N3−Mn 1:4 1:6 1:3 1:3

Mn II

Mn SnIV ZnII ZrIV

N3−Mn 1:3 1:4 1:9 1:30

all spectra included in Figures S4−S11. The N3-to-M ratio was calculated by comparing 1/6 of the N(1s) peak area to the peak area of M. The N(1s) peak was chosen to represent N3 (containing six nitrogen atoms) as opposed to ruthenium because the Ru(3d) doublet peak overlaps significantly with the C(1s) peak. The relatively small N3−M ratio, SnIV > ZrIV ≈ LaIII > MnII > CdII ≈ ZnII ≫ CuII ≈ FeII. The TiO2−N3 and TiO2−N3−Mn devices exhibit similar spectral shapes in the incident photon-to-current efficiency

9.89 ± 5.28 ± 0.75b 0.29b 8.23 ± 7.33 ± 8.97 ± 5.24 ± 8.49 ±

0.33 0.44

0.34 0.18 0.39 0.27 0.57

Voc (mV) 609 ± 11 736 ± 19 690b 382b 657 ± 5 613 ± 12 563 ± 16 740 ± 4 591 ± 17

FF (%) 63.79 ± 72.84 ± 73.72b 58.784b 65.61 ± 67.49 ± 64.22 ± 73.82 ± 64.58 ±

0.15 2.50

2.34 1.84 1.40 1.54 1.42

η (%) 3.81 ± 2.80 ± 0.38b 0.07b 3.53 ± 3.02 ± 3.21 ± 2.85 ± 3.22 ±

0.18 0.09

0.26 0.14 0.24 0.19 0.25

a

All data, except for with CuII and FeII, are the average value from the measurement of three different DSSCs, and the error bars are the standard deviation of those measurements. bBecause of their exceptionally low performance, only one DSSC was measured.

curves (Figure S16), so the large variation in Jsc cannot be attributed to differences in the absorption overlap with the solar spectrum. The trend in Jsc also does not correlate with the excited- or ground-state potential of the TiO2−N3−Mn, suggesting that the driving force for electron injection and/or regeneration by the electrolyte is not the primary variable dictating Jsc. Interestingly, Jsc does trend with the charge on the metal ion, MIV > MIII > MII, as well as the charge/size ratio, SnIV ≈ ZrIV > LaIII > MnII > ZnII > CdII. Electrostatic interactions between the metal ion and the I− mediator may play a role in increasing the regeneration rate (kregen), as has been observed in systems with thiol−iodide interactions48,49 and halogen bonding.50 The significantly lower Jsc for the CuII and FeII films can be attributed to competitive excited-state quenching by the metal ion prior to electron injection into the metal oxide. When the exceptionally low performance of FeII is ignored, the trend in Voc is nearly the inverse of that for Jsc, increasing in the order SnIV < ZrIV < MnII < LaIII < CuII < CdII ∼ ZnII. The CuII bilayer is particularly notable in this regard in that, despite having a very low Jsc, it retains an Voc value of 690 mV.26 In a DSSC, the maximum Voc is dictated by the energy difference between the redox potential of the mediator, in this case I−/I3−, and the quasi Fermi level of the electrons within TiO2.51 Because all of the cells are composed of the same cathode and redox mediator, the increase in Voc can be attributed to an increased electron density within TiO2, which is dictated by an equilibrium between the electron injection rate and the TiO2(e−) recombination rates with N3+ or I3−.51 Given the 11171

DOI: 10.1021/acs.inorgchem.7b01531 Inorg. Chem. 2017, 56, 11168−11175

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counter electrode, the similarities in RPt are expected. In contrast, there is a large variation in Rk given by the diameter of the major arc, and a measure of the resistance to interfacial recombination (Rk) between injected electrons and oxidized species increases in the order SnIV < ZrIV ∼ N3 < MnII < LaIII < CdII ∼ ZnII, a trend previously seen for Voc. The recombination rate (keff) and electron lifetime (τeff) of the coordinated films, with the exception of LaIII, also trend with Voc. Similarly, with the exception of TiO2−N3−ZnII, OCVD traces also increase in the order SnIV < MnII ≤ ZrIV < N3 < LaIII ∼ CdII (Figure S19). These results further support that one of the primary contributors to Voc is resistance to the recombination process between injected electrons and oxidized electrolytes.54,55 Given the correlation between the increasing charge of the metal ion and the increasing recombination rate, presumably electrostatic interactions between the oxidized mediator (I3−) and the metal cation play an influential role in dictating the resistance to recombination. The influence of the conduction-band-edge shift induced by metal-ion coordination was investigated by performing EIS over a range of applied potentials (Figure S20).55 This method also enabled determination of the recombination resistance at each of these potentials. Figure S20a shows that the trend in Rk is consistent across a range of potentials. The chemical capacitance (Cμ) of the TiO2 electrode at these voltages were also calculated and plotted against the applied voltages in Figure S20b. Cμ is known to give quantitative information regarding the conduction-band position from eqs 1 and 2.54−56

similarities in the driving force for electron injection, we attribute the increased Voc to slowed recombination in the TiO2−N3−Mn films with lower charged metal ions. Alkaline-metal-ion treatments are known to influence the conduction band energies of TiO2 via surface loading or lattice intercalation.52,53 To differentiate any possible contribution of the noncoordinated metal ions, as opposed to N3−M coordination, control samples were prepared by first soaking the films in the metal-ion solution and then in N3 dye [TiO2(Mn)−N3]. With the exception of CdII and CuII, which have notably decreased Jsc values, all TiO2(Mn)−N3 samples are within 10% of the parent TiO2−N3 (Figure S17). In terms of Voc, there is less than a 60 mV difference between the TiO2(Mn)−N3 samples and TiO2−N3. Treatments of TiO2− N3 with neutral (neat), basic (0.5 mM NaAc), or acidic (0.5 mM HAc) MeOH also had relatively minimal influence on the devices and only served to decrease Voc relative to the untreated film (Figure S15). These observations strongly suggest that the influence of solvent treatments or the metal ions on TiO2 cannot solely account for, or is even a major contributor to, the large variation in the device performance of TiO2−N3−Mn DSSCs. Interestingly, the ZnII pretreatment of TiO2 significantly perturbs the diode behavior of the devices. While we are not entirely sure why this occurs, it does not appear to be an issue in the TiO2−N3−ZnII device. Regardless of the mechanism, it is notable that treatment with CdII and ZnII ions increased Voc by nearly 150 mV relative to that of the untreated N3. This is particularly remarkable in that, despite being one of the benchmark dyes for DSSCs, this is one of the first examples of improving Voc for an N3 device without cosurfactants, synthetic modification of the dye, or treatment/changes to the metal oxide. Because of their exceptionally low performance, CuII- and FeII-coordinated films will be omitted from the rest of the discussion. EIS. The influence of the metal ions on recombination events in TiO2−N3−Mn was investigated by EIS, and the results are shown as a Nyquist plot in Figure 5. The curves were

⎛ αqVF ⎞ Cμ = C0 exp⎜ ⎟ ⎝ kBT ⎠

C0 = L(1 − p)α

(1)

⎡ α(Eredox − Ec) ⎤ q2Nt exp⎢ ⎥ kBT kBT ⎦ ⎣

(2)

where C0 is an exponential prefactor, α is the exponential electron-trap distribution parameter, q is the elementary charge, VF is the applied voltage, kB is Boltzmann’s constant, T is the temperature, L is the film thickness, p is the porosity, Nt is the total number of trap states, and Eredox − Ec is the difference in the potential of the redox mediator and conduction band. Because the total number of traps in TiO2 (Nt) remains constant, capacitance shifts are often attributed to band-edge shifts because parameters such as VF, L, and p are the same across the films.55 For most of the cells, there is a change in the slope of the line at higher voltages. DSSCs with TiO2−N3−Mn (Mn = ZnII and CdII) as the photoanodes, which gave the highest Voc values, also show the largest conduction-band shifts. Electron-Transfer Kinetics. The electron-transfer dynamics was measured by nanosecond transient absorption spectroscopy with a 480 nm light excitation of TiO2−N3 and TiO2−N3−Mn films in MeCN and by monitoring of the ground-state recovery of N3 [TiO2(e−)−N3+ → TiO2−N3] at 535 nm, and the results are summarized in Table 4. Electron injection efficiencies (Φinj) were obtained by using thin-film actinometry with TiO2−N3 (Φinj = 100%) as the reference.57 The recovery kinetics for N3 with (w) and without (w/o) the iodide−triiodide electrolyte (device concentrations: 0.6 M BMII and 0.1 M LiI) were used to determine the dye regeneration rate (kregen), and the example kinetics for TiO2− N3−ZrIV are shown in Figure 6, with the remaining traces in Figure S21.

Figure 5. Nyquist plot for for DSSCs with photoanodes composed of TiO2−N3 and TiO2−N3−Mn (Mn = CdII, LaIII, MnII, SnIV, ZnII, and ZrIV).

fit by using the equivalent circuit previously published by Bisquert et al.54 (Figure S18), and the results are summarized in Table S1. All samples exhibit two distinct semicircles from 106 to 103 Hz and from 103 to 1 Hz, corresponding to the chargetransfer resistances at the platinum/electrolyte interface (RPt) and the TiO2/dye/electrolyte interface (Rk).17,54 Given that the samples are composed of the same electrolyte and platinum 11172

DOI: 10.1021/acs.inorgchem.7b01531 Inorg. Chem. 2017, 56, 11168−11175

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using eq 5. From Table 4, kregen decreases in the order of N3 ∼ ZrIV > SnIV ∼ MnII > CdII > LaIII > ZnII. This trend is similar to that of Jsc, with the exception of LaIII, and indicates that the regeneration process plays a significant role in dictating Jsc.

Table 4. Injection Yields, Weighted Average Lifetimes with (w) or without (w/o) an Iodide−Triiodide Electrolyte, and kbet and kregen from Transient Absorption Measurements of TiO2−N3 and TiO2−N3−Mn (Mn = CdII, LaIII, MnII, SnIV, ZnII, and ZrIV) in a MeCN Solution of 0.3 M LiClO4 (λex = 480 nm and λabs = 535 nm)

TiO2−N3 CdII LaIII MnII SnIV ZnII ZrIV

Φinja

τw/o (ns)

τw (ns)

kbet (×10 s−1)b

kregen (×10 s−1)c

100 60.0 57 90 73 63 81

219 223 207 1637 332 368 302

54 73 111 78 88 71 57

4.6 4.5 4.8 0.6 3.0 2.7 3.3

13.8 9.1 8.0 10.7 10.9 7.2 14.0

6

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CONCLUSIONS In this report, we demonstrate that metal-ion coordination can have a large influence on the performance of N3-based DSSCs. Metal-ion coordination to N3 was confirmed by UV−vis, ATRIR, and XPS but had minimal influence on the ground- and excited-state potentials of the dye. The metal ions decrease Jsc but increase Voc by upwards of 130 mV relative to the parent N3 DSSCs. Through a combination of EIS, OCVD, and transient absorption measurements, we attribute the decrease in Jsc and increase in Voc to the metal ion slowing dye regeneration and recombination kinetics, respectively, presumably because of the introduction of additional steric bulk hindering electrontransfer events. With respect to the type of metal ion, the trend in increasing Jsc and decreasing Voc correlates with the increasing charge and charge/size of the coordinated metal ion. This observation suggests that, because of electrostatic interactions between the metal cation and I− or I3−, the more highly charged cations maintain a higher concentration of mediator anions in proximity to the surface and, as a result, increase the regeneration and recombination rates. This report demonstrates that metal-ion coordination to a surface-bound dye offers an additional tool to tune the performance of DSSCs. The use of a strategically designed neutral/cationic solid state or cobalt-based mediators with metal-ion-coordinated dyes may offer a means of preferentially increasing the regeneration dynamics but slowing recombination, effectively increasing both Jsc and Voc and increasing the overall DSSC performance.

6

Acquired with the laser intensity fixed to 5 mJ pulse−1. bτw/o = 1/kbet. kregen = 1/τw − 1/τw/o.

a c

Figure 6. Transient absorption trace at 535 nm for TiO2−N3−ZrIV with and without an iodide−triiodide electrolyte (λex = 480 nm).

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As seen in Table 4, all films with metal-ion coordination have injection yields larger than ∼55% but still lower than the parent TiO2−N3 film. Interestingly, for TiO2−N3−Mn, there is no correlation between the injection yield and Jsc. Given that Jsc is dependent on the injection yield and dye regeneration rate, these results suggest that the latter has a stronger role in influencing Jsc. Likewise, there is no correlation between the back-electrontransfer rate (kbet) and Voc. As mentioned above, Voc is partially dictated by the TiO2(e−) recombination rates with N3+ or I3−. The lack of correlation of kbet with Voc, but a strong correlation to the resistance to interfacial recombination (Rk), suggests that recombination with I3− at the interface has a much larger influence on Voc than kbet. In the absence of the redox mediator, the rate of back electron transfer (kbet) is equal to 1 over the lifetime for N3+ (τw/o).18,49 (eq 3) k bet = 1/τw/o

(3)

k bet + k regen = 1/τw

(4)

k regen = 1/τw − 1/τw/o

(5)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01531.

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Nanoparticle paste preparation, film and device characterization, equivalent circuit, absorption and emission spectra, and tables of values (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kenneth Hanson: 0000-0001-7219-7808 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Army Research Office under Grant W911NF-14-1-0660 and the National Science Foundation Graduate Research Fellowship under Grant DGE1449440. Transient absorption measurements were performed on a spectrometer supported by the National Science Foundation under Grant CHE-1531629.

As expected, the recovery kinetics for TiO2−N3 and TiO2− N3−Mn are notably faster in the presence of I−/I3−. Assuming that, upon the addition of I−/I3−, the only additional recovery pathway for N3 is the reduction of N3+ by I− or some other I2 species (kregen),49 and then the lifetime with I−/I3− is equal to the sum of kbet and kregen (eq 4) and kregen can be calculated 11173

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Inorganic Chemistry

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