Exploiting Conformational Dynamics of Structurally Tuned Aryl

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Exploiting Conformational Dynamics of Structurally Tuned Aryl-Substituted Terpyridyl Ruthenium(II) Complexes to Inhibit Charge Recombination in Dye-Sensitized Solar Cells Karen E Spettel, and Niels H. Damrauer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03302 • Publication Date (Web): 26 Apr 2016 Downloaded from http://pubs.acs.org on May 2, 2016

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The Journal of Physical Chemistry

Exploiting Conformational Dynamics of Structurally Tuned ArylSubstituted Terpyridyl Ruthenium(II) Complexes to Inhibit Charge Recombination in Dye-Sensitized Solar Cells Karen E. Spettel and Niels H. Damrauer* Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, CO, 80309 *[email protected] ABSTRACT To explore the impact of dye structure on photoinduced interfacial electron transfer (ET) processes, a series of systematically tuned 4′-aryl-substituted terpyridyl ruthenium(II) complexes have been studied in TiO2 film and dye-sensitized solar cell (DSSC) device settings. Structural tuning is achieved by the introduction of methyl substituents at the ortho positions of a ligand aryl moiety. Solar power conversion efficiencies are measured and these values are deconstructed to better understand the fundamental processes that control light-to-current conversion. Injection yields are identified as the primary factor limiting efficiencies, due in large part to significant non-radiative decay pathways in these bis-terpyridyl Ru(II) systems. Encouragingly, the addition of methyl steric bulk is found to inhibit charge recombination, with measured recombination lifetimes increasing by over 12-fold across the series of structurally-tuned complexes. If injection yields can be improved, the structural tuning of recombination rate constants may be an important design strategy for improving solar conversion efficiency in solar cells and water splitting devices.

INTRODUCTION Photoinduced electron transfer (ET) phenomena are key mechanistic components in many solar energy conversion strategies. In photosynthesis – both natural1,2 and artificial3-10 – stored chemical potential is derived from spatially separated oxidative and reductive equivalents whose generation is fundamentally tied to ET and proton-coupled ET events initiated by electronic excited states. In photovoltaics (PV), electrical rather than chemical potential is operative and naturally derives from charge separation. In the 1

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case of thin-film PV technologies, including layered and heterojunction organic solar cells11,12 and dye-sensitized solar cells (DSSCs; the subject of this work),13-16 charge separation originates in interfacial ET where driving forces are supplied by excited states. One can generalize that in all these strategies, optimizing rate constants for photoinduced ET from an excited-state donor (D*) to an acceptor (A) is an insufficient condition for success. To afford the needed time to productively process redox equivalents with additional thermal ET steps or with chemical reactions, it is critical to also minimize energy wasting recombination events wherein the transferred electron recombines with oxidized species within the system, including, but not limited to, D+. Ruthenium (II) polypyridyl complexes17 have underwritten the development of DSSCs and dye-sensitized strategies for artificial photosynthesis, serving as champion dyes as well as workhorses for mechanistic exploration.14,15,18-20 Within these chromophores, visible light absorption is enabled by spin and dipole-allowed transitions into singlet metal-to-ligand charge transfer (1MLCT) excited states, which can inject electrons on ultrafast time scales into the conduction band of TiO2 nanoparticles to which the dyes are covalently bound.21-23 A competing process is ultrafast intersystem crossing and thermalization within the 3MLCT manifold, from which electron injection into TiO2 can also occur but on slower time scales.24,25 The quantum yield for electron injection (φINJ) speaks to the competition of electron injection from both 1MLCT and 3MLCT manifolds (kinj) relative to mostly non-radiative relaxation pathways (krelax) available to the dye (see Scheme 1). For light-to-photocurrent generation in DSSCs, the injected electrons diffusively transfer through the TiO2 nanoparticle network of the high surface area thin film and are collected at the back contact of the working electrode (anode). In this context, large values for electron collection efficiency (ηCOLL) (approaching 1) are desirable. Concurrently, the ground state dye is regenerated from the reduced form of a redox shuttle. Here, large values for dye regeneration efficiency (ηREG) are desirable, again approaching 1.

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Scheme 1. Summary of pathways in DSSCs. Photoexcitation of dye (hν), electron injection from the excited state of the dye (not distinguishing between 1MLCT and 3MLCT) to the conduction band of TiO2 (kinj), electron transport within the TiO2 network to the FTO substrate (ktrans), and regeneration of ground state dye by iodide (kreg) are all productive processes and are shown in green. Energy wasting processes are shown in red for the decay of the excited state of the dye to the ground state (krelax), interception of TiO2 electrons by the triiodide redox mediator (kint), and recombination of TiO2 electrons with the oxidized dye (krec). Serving to limit both ηCOLL and ηREG are the same kinds of energy-wasting back ET events alluded to in the first paragraph that undermine productive use of conduction band electrons. Namely, charge interception (kint), wherein the oxidized form of the redox shuttle is reduced, and charge recombination (krec), wherein the oxidized dye is reduced. In this paper, we make the distinction between events whose control is intimately tied to the dye versus those dictated largely by the nanocrystalline TiO2 network and its relationship with the redox shuttle. In this sense, ηREG is dye-centric and speaks to a competition between the regeneration rate constant (kreg) that is productive versus krec that is unproductive. On the other hand ηCOLL is TiO2 and redox shuttle centric and speaks to a

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competition between the electron transport rate constant (ktrans, which is productive) and kint (unproductive). While ηCOLL is important, it plays a relatively minor role in the subject of this work. We utilize the I3-/I- redox shuttle and gladly accept the long interception time scales that result. The focus, rather, is on design strategies for dye structure that aim to maximize ηREG without compromising φINJ. Several approaches have been implemented in the literature to inhibit charge recombination (krec) in dye/semiconductor conjugates.26-30 For example, systems are known where the driving force for back ET is larger in magnitude than the reorganization energy such that the process is slowed by Marcus inverted regime reactivity.31-33 However, this is a challenging control knob when faced with also ensuring efficient light harvesting and driving force for electron injection.28 Other sensitizer studies describe inhibition of recombination rates using increased donor/acceptor separation for the back ET reaction from the TiO2 surface.28,34-36 In future designs, if distance is to be controlled by ligand structure affecting attachment between the dye and the TiO2 surface – rather than using, for example, the spatial position of the hole3, 11 – then it is imperative that one also factor the impact of increasing distance on φINJ.6 We have begun to consider a different approach, the design of which originates in work by our group involving solution-phase covalent donor-bridge-acceptor (DBA) systems for study of photoinduced ET and back ET phenomena.37-40 In those systems, an aryl bridge is exploited whose relative conformation is sensitive to both the electronic state of the neighboring donor (ground versus MLCT) and to its oxidation state. In a systematic study of several related DBA systems differing only in the degree of methylation of the bridge, we showed that while forward ET slows by a factor of two across the series, the energy wasting back ET (i.e., recombination) slows by a factor of eight.39 In other words, these systems are exhibiting properties of a rectifier or an ‘electron turnstile’.41 The effect orginates in modifications to the electronic coupling for both ET events (forward and backward) and is related to work where controlled torsions between aryl-subunits in an ET pathway influence ET and triplet energy tranfer rates.42-55 Scheme 2, while idealized, is presented to help summarize the application of these original DBA ideas in the context of DSSCs, using the kinds of dye systems explored herein (vide infra). Step 1 shows a vertical (Franck Condon) preparation of an MLCT

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state. There, a significant dihedral angle θ exists between the terpyridyl ligand moiety and the aryl substituent that bridges the metal-complex donor to the TiO2 film. Subsequently in step 2, aryl-ring rotation occurs that is marked by a decrease in θ (shown as -∆θ) and driven by excited-state intraligand electron delocalization.37,56-60 This is expected on an approximate 2 ps time scale53,61,62 within the 3MLCT manifold which is populated on a significantly shorter ~ 100 fs time scale.63-66 In step 3, forward ET occurs, in this case as electron injection into the conduction band of TiO2. Our expectation within the overall design is that the smaller dihedral angle θ (step 2) serves to enhance electronic coupling for ET through enhanced π-conjugation between the metal-complex donor and the TiO2 acceptor. Once the electron has been transferred to TiO2 (step 3), the intraligand electron delocalization that drove the smaller dihedral angle θ is inoperative and the angle should increase in step 4 (+∆θ), approaching a value comparable to that of the ground state. Here, our expectation within the overall design is that the larger dihedral angle θ will decrease electronic coupling for recombination (step 5) of the transferred electron with the Ru(III) hole, thereby making recombination timescales slower. Finally, it is noted that variations in substituents at positions R1 and R2 are used in order to tune starting, intermediate, and final values of θ. As described below, three new dye systems are explored with zero, one, and two methyl groups in R1 and R2 positions.

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Scheme 2. Design scheme for conformational trapping of electrons in DSSCs. Ligandbased ring rotation occurs at steps indicated by ∆θ. Both injection and recombination rate constants (kinj and krec) are expected to be modulated by this ligand-based motion. The break in energy is intentional to show the amount of energy expended is small compared to the fraction absorbed. At a fundamental level, slower recombination timescales designed into dye structure are expected to contribute to more efficient devices. These ideas have support in the review by Hupp and coworkers67 who have evaluated the complex function of DSSCs by summarizing data from the Durrant group68-72 using N719 dye at its maximum power point (i.e. under device conditions with the maximum product of current and voltage). Their review looks into alternative photoanodes with better charge transport characteristics while highlighting that the kinetic processes in DSSCs are complex and do not always conform to simple rate laws.67 Therefore, in their review, half-life times are reported instead of rate constants, to allow for a more meaningful comparison between different processes (we will also use half-life times herein). Based on the half-life times for charge recombination (3 µs) and regeneration (1 µs) reported,67 ηREG is calculated to be 0.75 (see eq 1). 6


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=

1// ≈ + 1// + 1//

(1)

If one could increase the recombination half-life time by merely a factor of two without affecting the regeneration half-life time, then ηREG would be enhanced by 14%. This in turn would enhance power conversion efficiency, provided that other factors, including ηCOLL, φINJ, and light harvesting efficiency (LHE), were held at comparable levels to what is achieved in the highly optimized N719 DSSCs. Similar implementations yielding slower charge recombination rates may find applications in settings beyond DSSCs. For example, solar water splitting photoelectrochemical cells can utilize frameworks similar to those of DSSCs, with added catalysts for water oxidation and reduction.3,73 Currently, the low quantum yield of oxidative water splitting photoanodes is attributed to the rapid back ET from TiO2 to the oxidized sensitizer, which is about an order of magnitude faster than the ET from a water oxidation catalyst (such as IrO2) to the oxidized sensitizer.3,6,73-75 Therefore, it is increasingly important to better understand systems that are capable of slowing charge recombination. In this paper, we explore the sensitization of nanocrystalline TiO2 films (in fully assembled DSSCs as well as the photoanode component on its own) using several chromophores

referred

to

as

the Steric Series.

These are [Ru(1)(tbtpy)]2+,

[Ru(2)(tbtpy)]2+, and [Ru(3)(tbtpy)]2+ (see Scheme 3), where steric bulk in the form of methyl substituents is sequentially added at positions R1 and R2 across the series. The series design aims to exploit the valuable properties discussed above for solution-phase DBA systems where structure (via manipulation of θ) allows us to tune and improve the rectifier-like properties for forward versus back ET. DSSCs and photoanodes of N719 are also investigated to provide points of comparison to a commonly studied champion ruthenium dye from literature (see Scheme 3).

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Scheme 3. Structures of dyes used in DSSCs and thin film studies. Counter ions are tetrabutylammonium (TBA+) and hexafluorophosphate (PF6-). In part A of Results and Discussions, we measure current-voltage characteristics of fully assembled DSSCs and obtain power conversion efficiencies. It is stated at the outset that the trends observed go against our expectations based on the design. With this in mind we turn to measurements in section B (framed in the context of φINJ, ηCOLL, and ηREG) that aim to uncover the underlying reasons for systematic efficiency lowering. Finally in section C we use transient absorption to measure charge recombination timescales. These results highlight the potential value of the design provided that other factors, including non-radiative pathways within the dyes, can be properly managed. EXPERIMENTAL All chemicals were purchased from Sigma Aldrich unless otherwise noted. [Ru(1)(tbtpy)]2+, [Ru(2)(tbtpy)]2+, and [Ru(3)(tbtpy)]2+ were prepared according to previously published procedures.76 N719 dye was purchased from Solaronix. 3methoxypropionitrile was purchased from TCI America. Chloroplatinic acid hexahydrate was purchased from Strem Chemicals. TiO2 Film Preparation. TiO2 electrodes were prepared on pre-cut 2.5 cm x 2.5 cm fluorine-doped tin oxide (FTO) glass slides (TEC 15, Hartford Glass). The FTO glass slides were first cleaned with acetone, water and methanol for 30 min each in an ultrasonic bath.77-80 After air-drying, approximately 5 mm of the edge of one side of the FTO glass slides were protected for lead connectivity with high-heat tape (polyimide Kapton tape, Marian Chicago) to prepare for TiCl4 treatment. Following standard procedures,81 taped FTO slides were placed in 40 mM TiCl4 solution at 70 °C for 30 min, removed from solution, washed with water and ethanol, and air-dried. 8


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According to known procedures in literature,15 a layer of nanocrystalline TiO2 paste (Ti-Nanoxide T/SP, 15 to 20 nm-sized particles, Solaronix) was coated on FTO glass slides by doctor-blading onto a masked area of Scotch tape with a razor blade (active area = 0.704 cm2). The paste was allowed to dry overnight and then slides were heated at 450 °C for 30 min in a furnace under air atmosphere (FB 1300, Barnstead/Thermolyne). After sintering, the slides were given an additional TiCl4 treatment using the same procedure as described above, followed by heating again to 450 °C before sensitization. Film thickness was measured using a profilometer (Dektak 3, Veeco) and was, on average, 8 ± 1 µm. For each efficiency measurement and each transient absorption measurement, four similar thickness TiO2 films (agreeing within 0.4 µm) were selected. TiO2 sensitization was carried out immediately after heating the TiO2 films to limit physisorption of water on the TiO2 surface.82 Slides were cooled to 85 °C and immersed in 0.5 mM dye solutions of 1:1 acetonitrile/tert-butanol for 24 hrs. Sensitizing solutions were sonicated prior to film immersion to avoid the presence of dye aggregates.24 After sensitization, the slides were thoroughly rinsed with acetonitrile to remove excess dye and air-dried. Sensitized TiO2 slides for transient absorption (TA) measurements were allowed to equilibrate additionally in pure acetonitrile (six hrs or more) until the time of measurement, while sensitized TiO2 slides for device studies were immediately processed into complete cells. This additional equilibration step was implemented for TA studies due to the observation of dye desorption in 5 mL of acetonitrile with freshly sensitized films. However, this observation is not expected to impact the device performance given the small amount of methoxypropionitrile electrolyte solution that is utilized. In fact, current-voltage characteristics were indistinguishable among a selection of four DSSCs prepared with or without an additional acetonitrile equilibration time. To test that observed transient absorption dynamics were independent of the number of injected electrons in our studies, we also considered sensitized TiO2 films at lower surface coverage. These were prepared using shorter soaking times (~ 4 hrs instead of 24 hrs) in lower concentration sensitizing solutions (0.15 mM of [Ru(1)(tbtpy)]2+ or N719 in 1:1 tert-butanol/acetonitrile). Lower absorbances were confirmed with UV-Vis. Counter Electrode Preparation. Platinized FTO counter electrodes were prepared by drilling a hole (~ 1 mm) into FTO glass slides with an easily removable waxed

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backing to prevent cracking. Prior to platinum deposition, the drilled slides were washed with 1:1 toluene/hexane mixture, 0.1 M HCl, and water. The slides were then cleaned with acetone, water and methanol for 30 min each in an ultrasonic bath, as was done with the FTO slides prior to the TiO2 film preparation. The platinum catalyst was deposited on top of the FTO by drop casting 5 mM chloroplatinic acid in isopropanol (5 μl/cm2).83,84 The coated counter electrodes were then slowly heated to 380 °C and held at 380 °C for 20 min to allow for thermal decomposition; forming platinum islands.85 DSSC Assembly. Solar cells were assembled in a conventional sandwich-type arrangement using one of the sensitized TiO2 slides and one of the platinized FTO slides with a 25-µm thick Surlyn thermoplastic gasket (Meltonix 1170-25, Solaronix) as the spacer and sealant. Electrolyte solution – comprised of 0.6 M 1-methyl-3propylimidazolium iodide (PMII), 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine (TBP) in 3-methoxypropionitrile – was injected into the predrilled hole on the platinum counter electrode side with a Vac’n’Fill Syringe (Solaronix). An additional square of Surlyn and a small cover slip was used to seal the drilled hole on the side, thus completing the cell assembly. Cells were stored for three to five days in the dark before photovoltaic measurements were made, in accordance with literature.86,87 Each DSSC that was properly sealed was found to be stable by UV-vis absorption after several months of storage in the dark. UV-Vis Absorption. Steady-state absorption spectra were recorded using a Hewlett-Packard diode array UV-vis spectrophotometer (HP8452A). Sensitized TiO2 films were rinsed with acetonitrile immediately prior to measurement, and held by a filter holder (Thor Labs) in an attempt to maintain the same angle and interrogation area for each UV-vis measurement. We note that an integrating sphere was not used. For all TA experiments, film absorbances were kept below an absorbance of one at the MLCT peak and between 0.35 - 0.6 at the excitation wavelength of 532 nm. Current-Voltage Characteristics. Photovoltaic measurements incorporated the use of a 100 W Xenon lamp (6257, Newport/Oriel) equipped with an AM1.5 global filter (AM1.5 G, Newport). The power of the simulated light was calibrated to 100 mWcm-2 using a thermopile (PowerMax 500D, Molectron/Coherent). Photovoltaic performance was tested using a black metal mask 1.5 mm wider on each side than the active area of

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TiO2 (mask area = 1.316 cm2). This mask, of greater aperture than that of the active area of TiO2, was used in order to capture both direct and diffuse light while eliminating multiple reflections due to light piping (see the S.I. for more details).87-89 The cells were kept at 25 ± 0.5 °C over the course of the measurement, as measured by a thermocouple kept in direct contact with the illuminated cell. Temperature stability was achieved using a home-built water-cooled metal cell holder (that also acted as the mask) that was connected to a refrigerated constant temperature circulator (VWR). Current-voltage characteristics were measured with linear-sweep voltammograms using an electrochemical analyzer (601C, CH Instruments). A two-electrode setup was employed with the sensitized TiO2/FTO slide acting as the working electrode and the Pt/FTO slide acting as both the counter electrode and the reference electrode. Care was taken to ensure the leads were connected in a similar fashion for each measurement as this was deemed significant in achieving reproducibility. Due to a known hysteresis with scan direction among DSSC J-V curves,90 all results are reported as an average of forward and reverse scan directions. Before measurement, samples were allowed to equilibrate to 25 °C for 30 min. J-V measurements were taken on DSSCs of each dye on the same day, with three trials after adjustments to lead connectivity. The trial with lead connectivity leading to the greatest photocurrent (and subsequent efficiency) was reported in each case. The edges of the slides were sanded for optimal lead connectivity. IPCE Setup. Incident photon-to-current conversion efficiency measurements were made with the same light source and potentiostat as discussed above. However, to increase total excitation intensity, the Xe lamp controller was set to 110 W and a rear reflector (60005, Oriel) was added to the source. The light was passed through a 125 mm focusing lens (F/3.7, Newport) into a monochromator (77250, Oriel) at 10 nm bandpass (entrance and exit slits 1.5 mm). A water-cooled metal cell holder held the complete DSSC sample with a metal circle mask of known aperture (area = 0.1257 cm2) in front of the illuminated TiO2 working electrode side. The cell temperature was set to 25 ± 0.5 °C, with no equilibration time needed. The excitation power was measured at the sample, directly after the mask, before and after each measurement. This was repeated with a FTO slide in place to account for power losses due to FTO absorption and reflection.89 Care was taken to ensure that the

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excitation between 400-700 nm remained in the linear power regime of at least 2.5 mW/cm2. This value was determined from literature based on known concerns of nonlinear photocurrents below a photon flux of 1017 cm-2s-1.91 Unfortunately, excitation dropped to between 0.7-2.5 mW/cm2 from 700-800 nm. Photocurrent action spectra were measured with 10 nm bandpass every 10 nm under short circuit conditions (Jsc(λ), potential = 0 V), with the room lights off. The experiment duration at each given wavelength was adjusted to ensure that steady-state values of Jsc(λ) were reached before recording an average photocurrent at a particular wavelength. For the measurement of all wavelengths greater than 630 nm, a 420 nm long-pass filter (Newport) was needed to filter out the apparent intensity arising from higher order diffraction. Jsc values determined by J-V characteristics are within error of those calculated from integrated IPCE measurements (see S.I. for more information). Transient Absorption. For all transient absorption (TA) experiments, an electrochemical sample setup was utilized to allow for an external bias to be applied to the sensitized TiO2 film. The degree of external bias was measured with a potentiostat (601C CH Instruments) under a controlled potential coulometry setup (also known as bulk electrolysis). The sensitized TiO2 film formed the working electrode in a threeelectrode configuration in a glass cell (4 × 4 × 1 cm, NSG Precision Cells, Inc.) shown schematically in Figure S2 of the S.I. The rest of the setup consisted of a coiled Pt wire counter electrode and a freshly prepared non-aqueous 0.01 M Ag/Ag+ reference electrode in 5 mL of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in anhydrous acetonitrile. All cells were internally referenced to ferrocene (Fc+/Fc) at the end of each experiment. In order to report electrochemical values vs. SCE, a conversion constant of +380 mV from Fc+/Fc was applied.92 Sensitized TiO2 films were positioned in the glass cell (with a Teflon stand) normal to the probe light (vide infra) and at a 25° angle to the pump light (see Figure S2 in the S.I.). Nanosecond to millisecond TA measurements utilized a 532 nm laser excitation (1 Hz; 3-5 ns pulse width; beam diameter ~3.2 mm) generated by a Q Switched Nd:YAG Laser (Continuum Minilite II). A low fluence (3.7 mJ/cm2) was established in each experiment by reducing the pulse energy to 0.3 mJ/pulse, as measured with a thermopile power meter (PowerMax 500D, Molectron/Coherent).

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A Xenon lamp (75 or 100 W, Oriel) served as a broadband probe source. A one-inch diameter beam of probe light was focused into the sample using a plano-convex lens (focal length = 100 mm) achieving an approximate 1.5 mm diameter spot size at the sample. Prior to reaching the sample, the probe light was passed through a long pass filter with an onset at 420 nm (Thorlabs) to prevent TiO2 band gap excitation. The samples were excited with pump light at a 25° angle to the normal of the sample (see Figure S2 in the S.I. for a diagram of the sample setup). Passing through normal to the sample, the probe beam was re-collimated and then focused via a plano-convex lens (focal length = 50 mm) into a monochromator (SLM instruments, entrance and exit slits 1 mm, 2 nm bandpass).93 A 530 nm notch filter (Edmund Optics) placed after the sample eliminated pump scatter. Probe intensity was monitored with a negatively-biased PMT (Hamamatsu R-928) operating at –1000 V DC. The signal from the PMT was passed into a digital oscilloscope (LeCroy 9384L) interfaced with a computer. For long time-window collection (>100 µs), signals were terminated using a 1000 ohm external resistor to amplify measured voltages. In all other cases, the signal was terminated at 50 ohm via the oscilloscope setting. The instrument response time was measured to be ~10 ns. Transient absorption kinetics at particular wavelengths were obtained by averaging 30 oscilloscope time-traces of probe intensity with the pump laser on, followed immediately by averaging 30 time-traces of probe intensity with the pump off. These pump-on and pump-off time traces were used to determine a ∆A signal as a function of time. The whole collection process was repeated at least three times and an average ∆A kinetic trace was determined. Single-wavelength kinetic traces were fit with a stretched exponential decay model given in eq 2.94 ∆() =

(2)

Here, Ao is the initial change in absorption, τ is a characteristic lifetime, and β is the degree of dispersion, which is inversely related to the width of the underlying Lévy distribution of rate constants, 0 < β < 1, with lower values of β being more dispersive.95 For ruthenium dyes on TiO2, β values in charge recombination studies have been reported to range from 0.25 to 0.5 depending on the electrolyte employed.96 To have a sense of recombination kinetics independent of the chosen fitting parameters, the time required to 13



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observe 50% of the initial change in absorption (half-life time, t1/2) was measured as well. For a single set of measurements, the sensitized TiO2 film was immersed in the 5 mL electrolyte solution in the glass cell. Stable lifetime measurements were observed under applied bias as confirmed by 1 hr of repeated trials (see S.I. for more details). The film was rinsed with acetonitrile, and a UV-Vis absorption spectrum was measured to ensure that no changes occurred. This entire process was repeated three times with the same electrolyte solution topped off to 5 mL with fresh acetonitrile to account for evaporation. RESULTS AND DISCUSSION A. Characterization of Photovoltaic Performance. To first evaluate the impact of structural changes within our Steric Series on DSSC function, current-voltage characteristics (J-V curves) were measured under AM1.5 G solar irradiation (100 mW/cm2) at 25°C. J-V curves are shown in Figure 1 for the three dyes [Ru(1)(tbtpy)]2+, [Ru(2)(tbtpy)]2+, and [Ru(3)(tbtpy)]2+. Comparable data for the champion dye, N719, lies considerably outside the range of the Steric Series and is shown separately in Figure S4 in the S.I. From the measured J-V curves, one can determine the total power conversion efficiency (η) of each DSSC according to eq 3,97,98 where η is a unitless efficiency standard characterized by the maximum electrical power output (Pmax) divided by the total solar power input (Pin). This is measured at 25 °C with the incident irradiance of sunlight (Po) in eq 3 fixed at 100 mW/cm2. =

!" %&' ()' ** = #$ )

14


(3)

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0.5 0.4 0.3 Current/ mAcm-2

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0.2 0.1 0.0 -0.1

% Jsc /mAcm -2 Voc /V FF Ru(1) 0.17 0.49 0.48 0.71 Ru(2) 0.10 0.32 0.42 0.72 Ru(3) 0.05 0.18 0.40 0.65

-0.2 -0.3 0.0

-0.1

-0.2 -0.3 Potential/ V

-0.4

-0.5

Figure 1. J-V curves for the Steric Series measured under AM1.5 G solar irradiance (100 mW/cm2) and in the dark (dashed lines) at 25 °C. An insert is provided for the respective solar energy parameters obtained from each J-V curve. Ru(1), Ru(2) and Ru(3) are shorthand for [Ru(1)(tbtpy)]2+, [Ru(2)(tbtpy)]2+, and 2+ [Ru(3)(tbtpy)] , respectively. Several noteworthy features and trends are observed in these data. Most drastically, one sees a large decrease in short-circuit (V = 0) photocurrent (Jsc) with each addition of a methyl substituent across the Steric Series. These changes are similar in magnitude to the large (over threefold) decrease in efficiencies (η) across the Steric Series, thus suggesting that Jsc plays an important role in the efficiency trend. There are more subtle trends as well that impact efficiency. For example, a slight decrease in both the fill factor (FF) and open-circuit photovoltage (Voc) are observed with each addition of a methyl substituent across the Steric Series. As both of these observables are influenced by the curvature of the J-V data, we looked to dark current measurements for more insight. Such measurements assess the onset of the flow of electrons out of the working electrode as they are intercepted by the triiodide electrolyte (i.e. the charge interception pathway).99,100 These data indicate a later onset (more negative potential) of cathodic current with the less stericly encumbered [Ru(1)(tbtpy)]2+ compared to [Ru(3)(tbtpy)]2+. We believe this is a result of a slightly lower surface coverage across the Steric Series101 (with additional steric bulk) leading to more open

15



surface sites for intercalation of electrolyte species.102-104 This trend may be explainable based on systematic increases in dye pKa across the series (we have taken care to ensure a constant concentration of dye in all sensitizing solutions). To quantify measurement uncertainties that might arise, for example, due to surface coverage issues discussed above, we evaluated all four dyes (including N719) with three independent DSSCs. In each case, separate measurements were made on days three and five following construction of the complete cell. No trends in data were observed between day three and day five indicating that equilibrium had been reached by the third day. Histograms are provided in Figure 2 for all photovoltaic parameters obtained with the Steric Series, with averaged data provided in Table 1. 3

3 3

4

2 2

4

2

1

3

1 1

3

6

0.05

1

4

5

2

6

0.10 0.15 Efficiency/ %

[Ru(1)(tbtpy)] 2+ [Ru(2)(tbtpy)] 2+ [Ru(3)(tbtpy)] 2+

2

# Complete Cells

# Complete Cells

5

5

2 2

0.20

1

0.25

5

1

4

4

6

3

2

4

5

1

3

6

1

6

7

0.1

3

0.2

5

0.3 0.4 0.5 Jsc/ mAcm-2

6

0.6

7 4

2

6

6 1

# Complete Cells

# Complete Cells

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5 3

6

4 5

2

3 4

1

2 3

4

4

5 3

4 2

3 1

6

6

1

1

6

2 2

3

6

5

3

4

5

5

1 1

2

0.40

3

5

0.44

5

4

0.48

1

0.52

0.55

2

0.60

Voc /V

0.65 FF

0.70

6

0.75

Figure 2. Histograms of solar cell parameters measured under AM1.5 G solar irradiance (100 mW/cm2) at 25 °C. Three solar cells were made for each dye with measurements taken on both day three and day five of equilibration with electrolyte after sealing (six total measurements of each dye). Cell numbers are shown for ease of comparing different sensitizers.

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Perhaps best seen in the histograms of Figure 2, there is a clear trend towards decreasing η, Jsc, and Voc as methyl substituents are added within the Steric Series. Compared to the complex with no methyl substituents, [Ru(1)(tbtpy)]2+, there is a fourfold reduction in η with the most stericly encumbered species, [Ru(3)(tbtpy)]2+. We conclude that this is primarily due to Jsc values (with a threefold reduction), but with a minor contribution from Voc values, which decrease modestly presumably due to the decreasing Jsc trends105107

(see Table 1). The quantity FF is the only parameter with no obvious correlation

across the series. This is not unreasonable as FF values are heavily influenced by cell components and inherent defects, which are common to all of these DSSCs. Table 1. Photovoltaic parameters measured under AM1.5 G solar irradiance at 25 °C.a %η Jsc/ mAcm-2 Voc / V FF Complex 0.19 ± 0.02 0.55 ± 0.07 0.49 ± 0.02 0.68 ± 0.03 [Ru(1)(tbtpy)]2+ 2+ 0.12 ± 0.04 0.40 ± 0.10 0.44 ± 0.03 0.70 ± 0.01 [Ru(2)(tbtpy)] 2+ 0.05 ± 0.02 0.18 ± 0.06 0.40 ± 0.01 0.64 ± 0.06 [Ru(3)(tbtpy)] 4.2 ± 0.7 13 ± 1 0.74 ± 0.02 0.42 ± 0.06 N719 a Errors are reported as the standard deviation of six measurements. Summarizing, it can be seen that decreases in η across the Steric Series are being driven largely by trends in Jsc. Further, DSSCs from all of the members of the Steric Series are significantly less efficient than when N719 is used, in agreement with reports from similar thiocyanate-free systems in the literature: DSSCs built from [Ru(1)2](PF6)2 have been previously reported ranging from 0.05%108 to 0.77%.109 Again, Jsc values are the most significant driver in these observations. At this point in the paper it is noted that our original design hypothesis of using structural dynamics controlled by sterics to increase efficiency is not reflected in device performance. This highlights a need to dig deeper into the details of what controls trends in the Steric Series and that is the goal of section B below. With those details in place we can then explore whether there is merit in the original design hypothesis. That is the goal of section C that follows. B. Understanding the Low PV Conversion in the Steric Series and Trends Therein. IPCE results. To better understand the overall low η values in the Steric Series, we have sought wavelength-specific information. The first step in this is through

17



measurement of incident photon-to-current conversion efficiency (IPCE). A set of %IPCE spectra collected immediately following η measurements (on days three and five) are shown in Figure 3b. Details about the measurement and equation used are found in the S.I. along with a %IPCE spectrum of N719 (Figure S5) showing strong agreement with literature under similar cell conditions.110

1.5

[Ru(1)(tbtpy)]2+ [Ru(2)(tbtpy)]2+ [Ru(3)(tbtpy)]2+

(b) 4 IPCE = 3 %IPCE

Absorbance/AU

(a) 2.0

1.0

500 600 700 Wavelength/nm

2

0 400

800

(c) 100


(d) 6 LHE = 1 10

A

APCE =

5 %APCE

80 60 40 20 0 400

1240 Jsc( ) P( )

1

0.5 0.0 400

%LHE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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800

IPCE LHE

4 3 2 1


800

0 400


800

Figure 3. (a) Sensitized TiO2 film absorption specta (b) %IPCE spectra (c) %LHE spectra and (d) %APCE spectra of the Steric Series. %LHE are converted from the absorption spectra of the sensitized TiO2 film that is subtracted for unsensitized TiO2 absorption. %IPCE and %APCE spectra are corrected for the loss of light intensity due to absorption and reflection by FTO. As expected, the %IPCE spectra mirror aspects of the absorption properties of the sensitized TiO2 films (Figure 3a and b) inasmuch as they both peak at wavelengths corresponding to the MLCT maxima of these systems. The peak %IPCE values are

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presented in Table 2. These data, in comparison to cell parameters shown in Table 1, confirm and highlight the role being played by Jsc in controlling efficiency η. Table 2. Photon conversion efficiencies of each DSSC at 25 °C. Peak Average Peak Complex %IPCE %APCE %APCEa 5±2 2.3 ± 0.2 [Ru(1)(tbtpy)]2+ 3.2 ± 0.5 2+ 2.1 ± 0.3 3.2 ± 0.4 1.8 ± 0.7 [Ru(2)(tbtpy)] 2+ 0.8 ± 0.3 1.4 ± 0.6 0.7 ± 0.3 [Ru(3)(tbtpy)] 84 ± 5 90 ± 7 70 ± 10 N719 a Average %APCE are analyzed between 400 nm and 700 nm. Of note, we were surprised by the low peak %IPCE values obtained for cells built from the Steric Series given that the peak %IPCE value for a cell with the homoleptic version of ligand 1 (i.e., [Ru(1)2]2+) was reported at 13.8%,111 compared to our studies with [Ru(1)(tbtpy)]2+ at 3.2% (see Table 2). Because our %IPCE spectrum for the N719 dye is well matched to literature, we rule out measurement error. As the following section will indicate, the low %IPCE values are partially dictated by dye absorption properties. But this is likely not the entire explanation, thereby highlighting the need to factor in interfacial ET kinetics and competing nonradiative relaxation pathways. Internal Efficiency Components. To better understand factors influencing efficiency on a molecular level, IPCE is further deconstructed into absorbed photon-to-current efficiency (APCE), a quantity that measures photocurrent independent of how well a sensitizer absorbs light (eq 4, where +,-(.) = 1 − 101(2) ). 3-(.) =

43-(.) = 5678 ')99 +,-(.)

(4)

The %LHE spectra of each dye as a function of wavelength are provided in Figure 3c. The %APCE spectra generated from the IPCE and LHE data are shown in Figure 3d. A %APCE spectrum of N719 is overlaid with its %IPCE spectrum in Figure S5 in the S.I. Given that both %IPCE and LHE spectra approach zero to the red of 700 nm, the %APCE values are only analyzed between 400 and 700 nm. Maximum %APCE values are reported in Table 2, along with an averaged %APCE. Examination of the trends in Table 2 reveals a total decrease in average %APCE of close to threefold across the Steric Series with the introduction of methyl substituents. Reiterating, this is independent of the

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

light absorption abilities of the dyes (e.g. %LHE) and highlights fundamental differences in their function in these cells. To understand the implications of this efficiency drop across the Steric Series, APCE measurements are deconstructed into the components of collection efficiency, ηCOLL, regeneration efficiency, ηREG, and injection quantum yield, φINJ, as shown in eq 4.15 Tackling the first of these, it can be stated at the outset that ηCOLL is largely dictated by the transport of electrons through TiO2 by diffusion, and in this sense the quantity is expected to be dye independent.112 However, a relevant photocurrent loss pathway involves interception of electrons in TiO2 with the oxidized form of the redox mediator in the electrolyte. In principle, the interfacial dye species may serve in blocking this process but because of the similarities of the dyes in this series, it is not possible to implicate structure-specific function in this context. On the other hand, it is noted that dye-specific surface coverage issues were implicated in our dark current measurements described earlier suggesting a possible role in ηCOLL. However, the small percentage differences in the dark current onset voltages across the Steric Series – with nearly identical onset voltages for [Ru(2)(tbtpy)]2+ compared to [Ru(3)(tbtpy)]2+ – suggest that only a minor role is being played in this context in affecting photocurrent. The second quantity, regeneration efficiency (ηREG) is a measure of net productive pathways for regeneration of the dye that is oxidized following photoinduced ET to TiO2 according to eq 1. In this expression, kreg is the rate constant for regeneration of the ground state dye by the redox mediator. This is a productive pathway. The rate constant krec also corresponds to regeneration of the ground state dye but here it is through recombination with an injected electron and is therefore unproductive. In the limit that krec goes to zero, the regeneration efficiency, ηREG is unity. The regeneration rate constant kreg that is central to ηREG is known to be dependent on the driving force for ET between the reduced form of the redox mediator and the oxidized form of the dye.70 To assess whether any variations in driving force might exist in this series, the Ru3+/2+ couples were measured and are shown graphically in Figure 4 along with other relevant redox data.

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TiO2

Potential/ V vs. SCE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1.0

N719

Ru(1)2

Ru(1)

Ru(2)

Ru(3)

∆GINJ = -0.34 eV

-0.05 eV

-0.07 eV

-0.13 eV

-0.14 eV

(-0.75 V)

(-0.77 V)

(-0.83 V)

(-0.84 V)

Ru3+/2+* ECB

∆GINJ

(-1.04 V)

(-0.7 V)

µF

0.0

I-/I3-

VOC

(0.27 V)

Ru3+/2+ 1.0

Electrolyte

(0.71 V) (1.23 V)

(1.21 V)

(1.21 V)

(1.22 V)

Figure 4. Reduction potentials of individual DSSC components. Potentials are provided from literature for TiO2,98 N719,67 Ru(1)2,113 and I-/I3-.114 Ru(1)-Ru(3) are shorthand for the Steric Series: [Ru(1)(tbtpy)]2+ through [Ru(3)(tbtpy)]2+. Values for Ru3+/2+ are determined from the one-electron electrochemical potential measured in solution.76 Values for Ru3+/2+* are calculated from :;

Exploiting Conformational Dynamics of Structurally Tuned Aryl

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