Correlation Between Charge Recombination and Lateral Hole

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Correlation Between Charge Recombination and Lateral HoleHopping Kinetics in a Series of cis-Ru(phen′)(dcb)(NCS)2 DyeSensitized Solar Cells Renato N. Sampaio,† Andressa V. Müller,‡ André S. Polo,*,‡ and Gerald J. Meyer*,† †

Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States Centro de Ciências Naturais e Humanas, Universidade Federal do ABC−UFABC, Av dos Estados, 5001, 09210-580 Santo André, São Paulo, Brazil

‡

ABSTRACT: Four complexes of the general form cis-Ru(phen′)(dcb)(NCS)2, where dcb is 4,4′-(CO2H)2-2,2′-bipyridine and phen′ is 1,10-phenanthroline (phen), 4,7-(C6H5)2-phen (Ph2phen), 4,7-(CH3)2-phen (Me2-phen), or 3,4,7,8-(CH3)4-phen (Me4-phen), were anchored to mesoporous TiO2 thin films for applications as sensitizers in dye-sensitized solar cells (DSSCs). The compounds displayed metal based reductions Eo(RuIII/II) = 1.01 ± 0.05 V vs NHE and were potent reductants competent of excited-state electron transfer to TiO2 with yields ϕinj ≥ 0.75 in acetonitrile electrolytes. Average charge recombination rate constants, kcr, abstracted from nanosecond transient absorption measurements, and the apparent diffusion coefficients for lateral hole-hopping, abstracted from chronabsorptometry measurements, showed the same sensitizer dependency: Ru(Me4-phen) > Ru(Ph2-phen) > Ru(Me2-phen) ≈ Ru(phen). When used in operational solar cells, Ru(Ph2-phen) was most optimal with an efficiency of (6.6 ± 0.5)% in ionic liquids under 1 sun illumination. The superior performance of Ru(Ph2-phen) was traced to a higher injection yield and more efficient regeneration due to an unusually small sensitivity of kcr to the number of injected electrons. KEYWORDS: DSSC, ruthenium, hole-hopping, charge recombination, regeneration, efficiency

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INTRODUCTION Ruthenium diimine compounds have been widely utilized in dye-sensitized solar cells (DSSCs) because of their exceptional light harvesting ability and stability under solar illumination.1−4 These compounds have also been of tremendous utility for understanding the fundamental electron transfer reactions that drive the energy conversion process. Compounds with a cisRu(NCS)2 core were identified by Nazeeruddin as gold standard sensitizers for DSSCs that invoked iodide electrolytes.5 Herein we report systematic studies of a series of sensitizers with the structure cis-Ru(phen′)(dcb)(NCS)2, where dcb is 4,4′-(CO2H)2-2,2′-bipyridine and phen′ is 1,1-phenanthroline (phen) or a substituted phen ligand, Figure 1. The motivation for this choice of sensitizers was that the substitution of a single phenanthroline ligand was expected to induce only a small change in the ground- and excited-state reduction potentials, thereby allowing systematic study of a relatively subtle structural change.6−9 Interestingly, the electron transfer kinetics and the global energy conversion efficiencies were sensitive to the nature of the substituents on the phenanthroline ligand. An unexpected correlation between the rates of lateral intermolecular RuIII/II “hole-hopping” and the rate constants for charge recombination between the injected electrons and the oxidized sensitizers was discovered © 2017 American Chemical Society

that provides further evidence that the two processes are linked. Indeed, this correlation provides credence to the notion that charge recombination is influenced by translation of the oxidized dye away from the injection site.10 Studies in functional DSSCs were performed in the absence of potential determining cations, such as Li+, so as to optimize open circuit photovoltages. Of the four sensitizers studied, Ru(Ph2-phen) was found to be the most optimal in both ionic liquids and acetonitrile electrolytes. The superior performance of this sensitizer was traced to a higher injection yield and an unusually weak sensitivity of the charge recombination rate constants to the number of electrons injected into TiO2. The implications of these findings for solar energy conversion are discussed.

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EXPERIMENTAL SECTION

Materials. The following materials and reagents were used as received from the indicated commercial suppliers: acetonitrile (CH3CN; Burdick & Jackson, spectrophotometric grade; or SigmaAldrich, HPLC grade); ethanol (EtOH; Sigma-Aldrich, ≥ 99.5%); Special Issue: Hupp 60th Birthday Forum Received: January 31, 2017 Accepted: April 17, 2017 Published: May 4, 2017 33446

DOI: 10.1021/acsami.7b01542 ACS Appl. Mater. Interfaces 2017, 9, 33446−33454

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Figure 1. Molecular structures and abbreviations of the cis-Ru(phen′)(dcb)(NCS)2 sensitizers utilized in this manuscript. Dye-Sensitized Solar Cells (DSSCs). Preparation. Mesoporous thin films of 0.196 cm2 area and 10.3 ± 0.2 μm height were prepared by screen-printing TiO2 paste onto FTO glasses (8 Ω/□), followed by equilibration under an ethanol saturated atmosphere for 6 min. The thin films were then dried at 125 °C for 6 min. The deposition process was repeated three times. After TiO2 depositions, the films were sintered for 5 min at 325 °C, 5 min at 375 °C, 15 min at 450 °C, and 15 min at 500 °C.19 Films thicknesses were measured by using a Tencor P-7 profiler. After sintering, the films were sensitized by immersing TiO2 electrodes into mM ethanolic sensitizer solutions for 24 h. The Pt counter-electrodes were fabricated by the deposition of hexachloroplatinic acid on FTO and heating to 450 °C for 30 min. The two electrodes were sealed together with a Surlyn film heated to 110 °C in a custom-built sealing apparatus.20 For photoelectrochemical studies, 38 mg of iodine, 59 mg of guanidinium thiocyanate, 0.38 mL of 4-tert-butylpyridine, and 0.80 mg of 1-butyl-3-methylimidazolium iodide were dissolved in 85:15 v:v acetonitrile:valeronitrile. Alternatively, 32 mg of iodine and 0.46 g of TBAI were dissolved in acetonitrile. Both mediators were prepared to result in 5 mL solution. The mediator was placed between the photoanode and counterelectrode through a hole drilled on the counter-electrode. Characterization. Current−potential data for the DSSCs were determined under simulated solar irradiation with a source-meter (Keithley model 2410). The simulated solar light was provided by a solar simulator (Newport, model 96000), composed of a 150 W xenon lamp (Newport, model 6255) in a lamp housing (Newport, model 67005) equipped with a water filter and A.M. 1.5G filter. The intensity was measured with a thermopile detector (Newport, model 818P-00112), connected to a power-meter (Newport, model 842-PE). The photoelectrochemical data reported were for the average of at least 5 distinct experiments. Photocurrent action spectra were determined using a 0.25 m Czerny−Turner monochromator (Newport, model Cornerstone 260), equipped with a diffraction grating of 1200 lines mm−1 and band-pass filters to avoid second-order effects, which decomposes the light provided by a 300 W xenon lamp (Newport, model 6258) placed in a lamp housing (Newport, model 66902). The quasi-monochromatic light beam is divided (50:50) by a beam splitter to the sample and a silicon detector (Newport, model 818-UV), which was connected to a virtual power-meter (Newport, model 841-p-USB). The photocurrent was measured using a source-meter (Keithley− model 2410).

isopropanol (Synth); tetra-n-butylammonium perchlorate (TBAClO4; Sigma-Aldrich, ≥ 99.0%); tetra-n-butylammonium iodide (TBAI; Sigma-Aldrich, ≥ 99.0%); argon gas (Airgas, > 99.998%); titanium(IV) isopropoxide (Sigma-Aldrich, 97%); hexachloroplatinic acid (H2PtCl6; Acros, 99.9%); TiO2 anatase nanoparticles paste (18NR-T, Dyesol); fluorine-doped SnO2-coated glass (FTO; Hartford Glass Co., Inc., 2.3 mm thick, 15 Ω/□ or Aldrich, 3.2 mm thick, 8 Ω/□); Surlyn (30 μm, Dyesol); glass microscope slides (Fisher Scientific, 1 mm thick); iodine (Sigma-Aldrich, ≥ 99.8%); guanidine thiocyanate (SigmaAldrich, ≥ 97%); 4-tert-butylpyridine (Aldrich, 96%) and 1-butyl-3methylimidazolium iodide (Aldrich, 98%). All sensitizers investigated were available from previous studies.6,7 Preparation. Transparent mesoporous nanocrystalline TiO2 thin films were prepared as previously described in the literature.11 Glass microscope slides were used as substrates for spectroscopic measurements while transparent FTO conductive substrates were chosen for electrochemical measurements. The as-prepared mesoporous TiO2 thin films were immersed in mM ethanolic sensitizer solutions to yield ∼1 × 10−7 mol/cm2 surface coverages that were determined using a modified Beer−Lambert law.12 The films were then washed with neat acetonitrile prior to use. Sensitized TiO2 thin films were positioned at 45° angle in cuvettes filled with the desired acetonitrile solutions and purged with argon gas for a minimum of 30 min prior to transient absorption or electrochemical studies. Mesoporous thin films of tin(IV) indium-doped oxides nanoparticles (nanoITO) were prepared as previously described.13 Spectroscopic. UV−Visible Absorption. The UV−visible absorption spectra were measured with an Agilent Cary 60 spectrophotometer and 1.0 cm square cuvettes. Transient Absorption. The apparatus for nanosecond transient absorption has been previously described.14 Relative excited-state electron injection yields were measured by comparative actinometry15,16 on the nanosecond time scale with Ru(Ph2-phen) as the reference actinometer. Electrochemistry. Electrochemical measurements were made with a Bioanalytical Scientific Instrument, Inc. (BAS) model CV-50W potentiostat in a standard three-electrode arrangement with a nanoITO or TiO2 thin film working electrode, a Pt counter electrode (BAS), and a nonaqueous Ag/AgCl pseudoreference electrode (BAS). The ferrocenium/ferrocence (Fc+/0) half-wave potential was used as an external standard for calibration of the pseudoreference electrode before and after experiments in a 0.1 M TBAClO4/acetonitrile electrolyte. All potentials were converted to the normal hydrogen electrode (NHE) through the use of the Fc+/Fc half-wave potential, + 630 mV vs NHE.17 Quantification of the apparent diffusion coefficient, Dapp, was obtained through chronoabsorptometry (CA) experiments performed in argon purged CH3CN containing 0.1 M TBAClO4 as the supporting electrolyte. Spectroelectrochemistry. Spectroelectrochemistry of the sensitized thin films was achieved with the BAS potentiostat and the Cary spectrophotometer. The applied potential was typically held for 2−3 min before UV−visible absorption spectrum were recorded. The TiO2 density of states were obtained from a previously described analysis of the single-wavelength absorption features plotted as a function of the applied potential.18

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RESULTS The UV−vis absorption spectra of all sensitizers exhibited an intense absorption band between 400 and 700 nm attributed to metal-to-ligand charge-transfer (MLCT) transitions.20,21 The extinction coefficients were ∼1.25 × 104 M−1 cm−1 for the Ru(phen), Ru(Me2-phen), Ru(Me4-phen) sensitizers whereas Ru(Ph2-phen) provided slightly higher extinction coefficients, 1.5 × 104 M−1 cm−1 when measured in acetonitrile.6,7 Visiblelight excitation into the MLCT transitions led to weak room temperature photoluminescence in acetonitrile with peak maxima that were red-shifted with increased electron-donating 33447


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ACS Applied Materials & Interfaces character of the phenanthroline ligand substituents, Ru(Ph2phen) < Ru(phen) < Ru(Me2-phen) ≈ Ru(Me4-phen).6,7 The spectra of the compounds were essentially preserved when anchored to the TiO2, with surface coverages of ∼10−7 mol/cm2, and immersed in acetonitrile electrolyte, Figure 2.

Abs = εl[N]TiO2(e−) = 1040(M−1 cm−1)2.828 × 10−4 (cm)50% TiO2 (e−) 4/3π (10 × 10−9(m))3 NA × 103(L m−3)

(1)

III/II

Self-exchange intermolecular Ru electron transfer, commonly referred to as “hole-hopping”, was quantified by chronoabsorptometry experiments.25,26 The oxidation of the sensitized TiO2 thin films were spectroscopically monitored as a function of time after a potential step 0.5 V more positive then E0(RuIII/II). Figure 3 shows comparative data for all the

Figure 2. Normalized visible absorption spectra of the indicated sensitizers anchored to nanocrystalline TiO2 thin films.

Cyclic voltammetry of the sensitized thin films on mesoporous nanoITO in 0.1 TBAPF6/CH3CN electrolyte displayed quasi-reversible RuIII/II redox waves at 1.06, 0.99, 0.96, and 1.03 V vs NHE for Ru(phen), Ru(Me2-phen), Ru(Me4-phen), and Ru(Ph2-phen), respectively. The excitedstate reduction potential was estimated through a free energy cycle, E0(RuIII/2+*) = E0(RuIII/II) − ΔGES, where ΔGES is the Gibbs free energy stored in the MLCT excited-state determined from the photoluminescence onset, Table 1.

Figure 3. Mole fraction after a potential step sufficient to oxidize the indicated sensitizers anchored to TiO2 thin films plotted against the square root of time. The absorption changes were monitored at 800 nm. Overlaid are fits to eq 2.

sensitized thin films. A linear fit to the Cottrell equation, eq 2, over the first 60% of the oxidation provided the apparent diffusion coefficient, Dapp, where ΔAf is the final change in absorbance and d is the TiO2 film thickness. The Dapp followed the order Ru(phen) > Ru(Me2-phen) > Ru(Ph2-phen) ≫ Ru(Me4-phen). These values were also used to estimate the effective rate constants for intermolecular “hole-hopping”,26 Table 2.

Table 1. Thermodynamic Properties of the Sensitized TiO2 Thin Films in CH3CN compd

E0(RuIII/II)a (V)

ΔGES (eV)

E0(RuIII/2+*)a (V)

Ru(phen) Ru(Me2-phen) Ru(Me4-phen) Ru(Ph2-phen)

1.06 0.99 0.96 1.03

1.92 1.84 1.81 2.05

−0.86 −0.85 −0.85 −1.03

ΔGa,b (V) 0.17 0.24 0.27 0.20

(−0.13) (−0.06) (−0.03) (−0.10)

E (RuIII/II), E0(RuIII/2+*), and ΔG given in V vs NHE. bCalculated free energy change for outer-sphere reaction based on E0(I•/I−) = 1.23 24 − V and E0(I•− 2 /2I ) = 0.93 V vs NHE shown in parentheses. Standard deviations are ±0.01 V (or eV). a 0

ΔA =

1/2 1/2 2ΔA f Dapp t

dπ 1/2

(2)

Pulsed 532 nm light excitation of the sensitized films immersed in neat acetonitrile at the open circuit condition resulted in absorption changes consistent with the formation of an interfacial charge separated state, i.e., an oxidized sensitizer and TiO2(e−), Figure 4. The long-wavelength absorption bands have previously been assigned to NCS− → RuIII ligand-to-metal charge-transfer transitions.27,28 Some minor changes in the transient spectra were observed as a function of time and were assigned to a Stark effect at the TiO 2 interface. 22,23 Comparative actinometry measurements performed 20 ns after pulsed laser excitation (∼0.1 mJ/cm2) indicated that the relative excited-state electron injection yield was sensitive to the different phenantholine ligands, Table 2. Single-wavelength absorption kinetics monitored at 700 nm after pulsed 532 nm light excitation of sensitized TiO2 thin films immersed in neat acetonitrile are shown in Figure 5. The changes in absorption were attributed to charge recombination of the injected electron, TiO2(e−), with the oxidized sensitizer that required micro- to milli-seconds to complete. The transient

Application of forward (negative) bias to the sensitized thin films immersed in 0.1 M TBAPF6/CH3CN electrolyte resulted in reduction of TiO2 that was monitored by a long wavelength absorption, > 700 nm, characteristic of TiO2(e−). Concomitant with the appearance of TiO2(e−), small changes in the MLCT absorption band were observed and attributed to a Stark effect.22,23 The measured absorbance at 900 nm was converted to the number of TiO2(e−) per 20 nm diameter spherical particle through Beer’s law using the extinction coefficient ε = 1040 M−1 cm−1 at 900 nm, and an effective 2 μm optical path length for a film of 50% porosity, positioned 45° to the probe light. For example, the absorption due to 20 electrons per particle was calculated with eq 1. 33448


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ACS Applied Materials & Interfaces Table 2. Efficiencies and Kinetic Quantities of the Ru(phen′) Sensitized TiO2 Thin Films ϕinj

compd Ru(phen) Ru(Me2-phen) Ru(Me4-phen) Ru(Ph2-phen) std dev

0.85 0.90 0.75 1.00 ±0.05

Dapp (cm2 s−1) −8

2.5 × 10 2.0 × 10−8 0.3 × 10−8 1.1 × 10−8 ±10%

khh (s−1)a

kcr (s−1)

kcr‑PP (s−1)b

kreg (M−1 s−1)

ϕregc

3.8 × 10 2.9 × 106 0.4 × 106 1.6 × 106 ±10%

2.7 × 10 2.6 × 103 0.8 × 103 1.4 × 103 ±10%

3.9 × 10 4.0 × 105 1.6 × 105 0.4 × 105 ±10%

1.6 × 10 0.7 × 107 0.2 × 107 1.4 × 107 ±10%

0.92 0.85 0.71 0.99 ±0.1

6

3

5

7

a

The effective intermolecular hole-hopping rate constants were calculated using khh = 4Dapp/R2,26 where R is the intermolecular distance. bRate constants determined at power point (20 e− per TiO2 particle). cRegeneration yields calculated using eq 5 for [I−] = 0.3 M and the charge recombination rate constants at the power-point condition ([TiO2(e−)] ≈ 20).

Figure 4. Transient absorption difference spectra after pulsed 532 nm excitation of (a) Ru(phen), (b) Ru(Me2-phen), (c) Ru(Me4-phen), and (d) Ru(Ph2-phen). All experiments were performed with the sensitized thin films immersed in neat acetonitrile.

electrons reside in the TiO2 nanoparticles. The average charge recombination rate constants were acutely sensitive to the initial concentration of TiO2(e−) generated after reduction of TiO2 with forward bias. The charge recombination rate constants, kcr, were found to increase exponentially with the applied potential from +0.2 to −0.7 V vs NHE, Figure 6.

Figure 5. Absorption changes monitored at 700 nm after pulsed 532 nm excitation of the Ru(phen′) sensitized thin films in neat acetonitrile. Overlaid in yellow is the best fit to the KWW kinetic model, β = 0.2.

data were modeled using the Kohlrausch−William−Watts (KWW) kinetic function.29,30

A(t ) = A 0 exp( −k t)β

Figure 6. Logarithmic plot of the representative charge recombination rate constants, kcr, as a function of the number of electrons per TiO2 nanocrystallite, TiO2(e−). Here, the applied potentials were converted to TiO2(e−)/particle using eq 1.

(3)

where β is inversely related to the width of the underlying Lévy distribution of the rate constants, 0 < β < 1, A0 is the initial absorbance, and k is the characteristic observed rate constant. A representative charge recombination rate constant, kcr, was taken as ⎡ 1 ⎛ 1 ⎞⎤−1 kcr = ⎢ Γ⎜ ⎟⎥ ⎣ kβ ⎝ β ⎠⎦

Regeneration of the oxidized sensitizers were investigated in 0.1 M TBAI/CH3CN electrolyte solution. Single wavelength kinetics were monitored at 700 nm where the oxidized sensitizer predominantly absorbed light. The experiments were performed under low laser irradiances conditions to help ensure pseudo-first-order conditions in iodide concentration [I−], where about 1 injected electron was estimated to be present in each TiO2 nanocrystallite.31−34 The kinetic data could not be fit to a single exponential function, but were adequately described by the KWW model, from which an average rate constant was calculated. The obtained observed

(4)

where β = 0.2 for all measurements. Charge recombination rate constants are summarized in Table 2. Charge recombination experiments were also performed with an applied bias to simulate operational conditions of dyesensitized solar cells, where a steady-state concentration of 33449


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ACS Applied Materials & Interfaces rate constants, kobs, were linear in the I− concentration, Figure 7, and the slopes were taken as the second-order rate constants

photon-to-current efficiency (IPCE) values as a function of the excitation wavelength provided photocurrent action spectra that were in good agreement with the absorptance spectra, Figure 8a. The maximum IPCE values followed the trend, Ru(Ph2-phen) > Ru(Me2-phen) > Ru(phen) > Ru(Me4-phen). Current−voltage curves are shown in Figure 8b. Short-circuit current densities (JSC) and open-circuit voltages (VOC) were sensitive to the identity of the molecular sensitizer. Power conversion efficiencies (η) were calculated with eq 6 η(%) =

100

(6)

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DISCUSSION The four sensitizers investigated in this study differ only by the substituents on a single phenanthroline ligand and are based on a cis-Ru(NCS)2 core that has long been a gold standard in dyesensitized solar cells (DSSCs).5,36,37 All sensitizer thin films showed small, yet measurable, changes in their ground state redox potentials, Eo(RuIII/II) = 1.01 ± 0.05 V vs NHE. However, the MLCT excited-state of Ru(Ph2-phen) stored considerably more free energy and hence was a more potent photoreductant. Photoelectrochemical measurements showed that the global conversion efficiencies were sensitive to the identity of the sensitizer, with Ru(Me4-phen) displaying the poorest efficiency and Ru(Ph2-phen) being most optimal. Detailed mechanistic studies led to a clearer understanding of the origin(s) of these disparate efficiencies and also revealed an unexpected correlation between the kinetics for charge recombination of the injected electron with the oxidized sensitizer and lateral intermolecular “hole-hopping” selfexchange electron transfer. Below we discuss these kinetics followed by a discussion of the solar energy conversion efficiencies. Charge Recombination and Hole-Hopping. It has previously been shown that all the redox active molecules anchored to mesoporous TiO2 thin films can be reversibly oxidized and reduced in standard electrochemical cells provided that the molecular surface coverage exceeds a percolation threshold.25,26 A mechanism for this has been proposed. The reactions are initiated at the transparent conductive oxide that

for sensitizer regeneration, kreg. Regeneration efficiencies, ϕreg, as a function of the initial concentration of TiO2(e−) were calculated from eq 5 using the measured rate constants for charge recombination and regeneration, Table 2. k reg[I −] k reg[I −] + kcr[TiO2 (e−)]

Pirr

where ff is the fill factor and Pirr is the incident light irradiance of 100 mW/cm2. The results are summarized in Table 3 and the efficiency values followed the trend Ru(Ph2-phen) > Ru(Me2-phen) > Ru(phen) > Ru(Me4-phen) in both TBAI and ionic liquid electrolyte solution.

Figure 7. Plot of the calculated rate constants (kobs) as a function of the [I−] concentration for Ru(phen) (black), Ru(Me2-phen) (red), Ru(Me4-phen) (blue), and Ru(Ph2-phen) (green). The linear fit provides an estimate of the average regeneration rate constant kreg.

ϕreg =

JSC VOC ff

(5)

Transient absorption spectra in the presence of the I− were also recorded. At early time delays, < 1 μs, the difference spectra resembled that of the oxidized sensitizer followed by an increasing positive absorption near 375 nm associated with the formation of I3−.24,33,34 At time delays longer than 1 μs, sensitizer regeneration by the redox mediator was complete and the spectra resembled a first derivative feature of the groundstate absorption spectra attributed to an electric field generated by the injected electron (Stark effect).22,23,35 Recombination of the injected electrons with I3− was monitored at 375 nm and was found to be insensitive to the identity of the sensitizer and was well modeled by KWW function with a β value of 0.5, yielding averaged rate constants of 2 × 102 s−1. Photoelectrochemical experiments were performed under similar conditions to those from kinetic analysis with TBAI and I2 acetonitrile electrolytes. In addition, cell performances were also quantified with an ionic liquid based mediator. Incident

Figure 8. (a) Incident-photon-to-current efficiency (IPCE) and (b) current−voltage curves under 1 sun AM 1.5 solar illumination for the indicated sensitizer in DSSCs fabricated with TBAI, I2/CH3CN electrolyte. 33450


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Table 3. Measured and Calculated Short Circuit Current Density (JSC), Open Circuit Potential (VOC), Fill Factor (ff), overall Efficiency (η), under 1 Sun Irradiation (100 mW/cm2) at the Indicated Electrolyte Conditions Ru(phen) ionic liquid-based mediator

TBAI-based mediator

JSC (mA cm−2) VOC (V) ff η (%) JSC (mA cm−2) VOC (V) ff η (%)

12.3 0.71 0.62 5.1 9.2 0.67 0.62 3.8

± ± ± ± ± ± ± ±

0.4 0.01 0.01 0.3 0.3 0.01 0.01 0.3

supports the mesoporous thin film and proceeds by lateral intermolecular self-exchange “hole-hopping” across the sensitized TiO2 nanocrystallites. Electrochemical experiments where a potential step is used to initiate hole transfer and the current (chronoamperometry) or absorption (chronoabsorptometry) are quantified and analyzed with the Cottrell equation provide apparent diffusion coefficients, Dapp, that with some assumptions have been directly linked to the self-exchange rate constants.25,26 The Dapp values reported here spanned about a factor of 7 and followed the trend: Ru(Me4-phen) ≪ Ru(Ph2phen) < Ru(Me2-phen) ≈ Ru(R-phen). Density functional theory indicates that oxidation of cisRu(dcb)2(NCS)2 removes significant electron density from both the Ru metal center and the S atoms in the isothiocyanate ligands. Hence theory indicates that the highest occupied molecular orbital of cis-Ru(NCS)2 diimine compounds has both Ru and NCS character.38−41 Cyclic voltammetry studies provide experimental evidence for this with two closely spaced waves at positive potentials, the first assigned to RuIII/II and the second to NCS−/•. Interestingly, Dapp values were reported to be significantly smaller for cis-Ru(dcb)2(NCS)2 than for the sensitizer called Z907 where one of the dcb ligands was replaced by 4,4′-(nonyl)2-bpy (dnb), i.e., cis-Ru(dcb)(dnb)(NCS)2.42 This observation was rationalized by different orientations of the two sensitizers on the anatase TiO2 nanocrystallite surface. One carboxylate from each dcb ligand was anchored to TiO2 in cis-Ru(dcb)2(NCS)2, while both carboxylates from the single dcb ligand of cis-Ru(dcb)(dnb)(NCS)2 were surface anchored resulting in better intermolecular electronic coupling for hole-hopping. Comparative timeresolved absorption anisotropy studies on much shorter nanoto milli-second time scales were also consistent with more rapid hole-hopping for cis-Ru(dcb)(dnb)(NCS)2 relative to cisRu(dcb)2(NCS)2.43 The four sensitizers utilized in this study have only a single dcb ligand and the relatively subtle change induced by the substituents on the phenanthroline ligand were not expected to dramatically influence the sensitizer-TiO2 surface orientation. The trend is instead tentatively assigned to the intermolecular distance the hole has to hop. The unsubstituted Ru(phen) sensitizers are packed the most tightly and hence are in closer contact to one another than are those sensitizers with substituents in the 4,7-phenanthroline positions, Ru(Ph2phen) < Ru(Me2-phen). The electron donating character of the methyl groups relative to the phenyl groups may also provide greater electron density at the metal center and to the isothiocyanate ligands. The inclusion of methyl groups in the 3 and 8 positions in Ru(Me4-phen) was found to result in by far the smallest Dapp value, presumably because these substituents project along the surface normal and prevent close sensitizer

Ru(Me2-phen) 12.4 0.72 0.65 5.7 9.3 0.68 0.62 4.0

± ± ± ± ± ± ± ±

0.6 0.01 0.01 0.2 0.3 0.01 0.01 0.2

Ru(Me4-phen) 10.6 0.70 0.63 4.7 8.3 0.66 0.61 3.4

± ± ± ± ± ± ± ±

0.2 0.01 0.01 0.1 0.3 0.02 0.01 0.3

Ru(Ph2-phen) 14.2 0.73 0.65 6.6 11.7 0.69 0.63 4.8

± ± ± ± ± ± ± ±

0.9 0.01 0.01 0.5 0.2 0.01 0.01 0.5

contact. A recent study showed that inclusion of bulky tertiary butyl groups in the 4 and 4′ positions of bipyridine ligands resulted in a significant decrease in hole-hopping relative to methyl groups in Ru(dcb)(LL)22+ sensitizers; behavior that was also explained by the extent of electronic coupling.26 Charge Recombination. Charge recombination of the injected electrons with the oxidized sensitizers was quantified by nanosecond transient absorption spectroscopy. Consistent with many previous results, the kinetics were complex and could not be fit to a first-order kinetic model. Instead the Kohrlausch−Williams−Watts (KWW) function modeled the data very well. This function was proposed empirically by Kohlrausch in the 1800s and the much later derivation by Scherr and Montroll has made it a paradigm for quantifying transport in disordered media.29,30 Indeed, early application of the KWW function to sensitized TiO2 interfaces was based on the assumption that transport of the injected electron within the metal oxide nanoparticles represented the rate limiting step in a composite charge recombination mechanism.44,45 Very recently, the notion that charge recombination is rate limited only by transport of the injected electron has been questioned.10 Of particular relevance to this study, the work of Moia et al. has shown that charge recombination is highly affected by the sensitizer surface coverage and tracks the holehopping kinetics.10 The authors concluded that transport of the injected electron and the oxidized sensitizer (by intermolecular hole-hopping) influence charge recombination rate constants. The results from the Ru(phen′) sensitizers investigated herein support this conclusion. The average rate constants, kcr, abstracted from the temporal data followed the same trend as did the Dapp values, Ru(Me4-phen) > Ru(Ph2-phen) > Ru(Me2phen) ≈ Ru(phen). The correlation was not unity as the kcr values spanned less than a factor of 4, whereas Dapp values displayed a 7-fold change. The results indicate that models for charge recombination need take into account transport of both the injected electron and the oxidized sensitizer. Solar Conversion Efficiencies. Global solar energy conversion efficiencies of the sensitized thin films were quantified under 1 sun of air mass 1.5 (100 mW/cm2) illumination with I−/I3− redox mediators in either acetonitrile or an imidazolium ionic liquid. The conversion efficiencies were significantly higher in the ionic liquids, yet the relative trends were the same for both solvents with Ru(Ph2-phen) > Ru(phen) ≈ Ru(Me2-phen) > Ru(Me4-phen). This trend cannot be explained trivially by differences in the number of solar photons absorbed which were in fact near parity for all four sensitizers. The discussion below is focused on understanding the molecular origin(s) of the energy conversion efficiency with emphasis on the solar cell data acquired in acetonitrile that is most relevant to the kinetic studies. 33451


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Figure 9. (a) Interfacial energetics of Ru(phen) (black), Ru(Me2-phen) (red), Ru(Me4-phen) (blue), and Ru(Ph2-phen) (green) in 0.1 M TBAClO4/acetonitrile solution. (b) Calculated regeneration efficiency as a function of the initial number of electrons per TiO2 nanoparticle.

point, where about 20 extra injected electrons are expected to be present in each TiO2 nanocrystallite.48 These excess electrons are known to increase charge recombination and hence could lower the regeneration efficiency and decrease Voc values. One method to probe this is to complete transient studies on the operational DSSC; however this is difficult experimentally due to poor light transmission and iodide redox chemistry at the counter electrode.49 Another method is to inject electrons into the TiO2 thin film with an applied potential.34 In doing this, the kcr values were found to increase exponentially with the number of TiO2(e−) from +100 to −600 mV vs NHE. At the simulated power point, kcr values were about 2 orders of magnitude larger than when no excess electrons were present and consequently the regeneration efficiency decreases to below unity. The data indicate that regeneration is not quantitative in DSSCs, Figure 9, and that some injected electrons do indeed recombine with the oxidized sensitizers. Interestingly, charge recombination was least affected by the presence of excess electrons for Ru(Ph2-phen), whereas the other three sensitizers were within experimental error the same. This behavior likely explains the previous report by Sun et al., who found that the open circuit photovoltages obtained from DSSCs base on Ru(Ph2-phen) were on average superior to that of the well-known cis-Ru(dcb)2(NCS)2 (or N3) sensitizer.9 These authors utilized Li+ as a potential determining cation in the electrolyte solution. Hence unlike the data reported here that can all be reconciled based on the excited-state injection yields, the larger open circuit photovoltages reported previously likely result from more efficient regeneration that is nearly unaffected by the number of electron in TiO2 as shown in Figure 9.

A significant difference between the conditions utilized here and those of most literature DSSC studies is that “potential determining” cations, such as Li+, were not present in the acetonitrile electrolyte.1−4 Instead, the tetrabutyl ammonium cation, TBA+, was used. Lewis acidic cations like Li+ are termed potential determining as they are known to have a significant influence on the energetic position of the acceptor states in TiO2. Large concentrations of such cations shift the TiO2 acceptor states positive on an electrochemical scale, i.e., away from the vacuum level, resulting in more favorable energetics for excited-state injection. For example, heteroleptic Ru sensitizers of the type Ru(dcb)(bpy′)22+ have very low injection yields ϕinj < 0.2 in TBA+ electrolytes that are increased to near unity when Li+ cations are present.46 The Ru(phen′) sensitizers studied here are much more potent photoreductants than Ru(dcb)(bpy′)22+*; however, the injection yields measured 10 ns after pulsed excitation was unity for only Ru(Ph2-phen) and dropped to 0.75 for Ru(Me4-phen). As expected from Gerischer theory, the injection yields scaled with the overlap of the excited-state reduction potential with the TiO2 acceptor states, Figure 9.47 Hence the superior conversion efficiency and higher photocurrents for Ru(Ph2-phen) are understood by more optimal excited-state injection yield. The quantitative injection yield for Ru(Ph2-phen) is expected to enhance the open circuit photovoltage, Voc. The Voc value represents the maximum Gibbs free energy that can be abstracted from a regenerative solar cell. Excited-state injection shifts the quasi-Fermi level of TiO2 away from the Fermi level of the Pt counter electrode that is fixed at the I−/I3− electrolyte redox potential. Hence provided that the recombination does not change, a higher injection yield will result in a larger Voc value. To test whether other factors might influence Voc values, we performed additional studies to test how rapidly and efficiently the oxidized sensitizers were regenerated by iodide after excited-state injection. The second-order kreg rate constants varied by about a factor of 8 and tracked the ground state reduction potentials. The presence of electron donating methyl substituents induced a small yet measurable decrease in the Eo(RuIII/II) potentials that resulted in slower regeneration. Nevertheless, at 0.3 M I− the regeneration was much faster than recombination and the quantum yield for regeneration was unity. The regeneration kinetics were measured under conditions where the number of injected electrons and oxidized sensitizers was equal. This differs from a DSSC operating at the power

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CONCLUSION Four cis-Ru(phen′)(dcb)(NCS)2 sensitizers were utilized in DSSCs based on mesoporous TiO2 thin films. The sensitizer where phen’ was 4,7-(C6H5)2-phenanthroline, i.e. Ru(Ph2phen), was the most optimal with an efficiency of 6.6% in ionic liquids under 1 sun illumination. The superior performance of Ru(Ph2-phen) was traced to a higher injection yield and an unusually small sensitivity of the average charge recombination rate constant kcr to the number of injected electrons. A correlation between kcr and the apparent diffusion coefficients for lateral hole-hopping was discovered that supports the view that transport of both the injected electron and the oxidized 33452


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sensitizer yields an encounter complex before charge recombination electron transfer occurs. The data support previous conclusions that Ru(Ph2-phen) is a viable alternative sensitizer for practical application in DSSCs.9

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Andressa V. Müller: 0000-0002-3999-7798 Gerald J. Meyer: 0000-0002-4227-6393 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.V.M. and A.S.P. are grateful to Fundaçaõ de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for financial support (2013/25173-5 and 2015/00605-5). The UNC-Chapel Hill research was supported by the National Science Foundation (NSF) under Award CHE-1213357

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REFERENCES

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Correlation Between Charge Recombination and Lateral Hole

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