Enabling Efficient Creation of Long-Lived Charge-Separation on Dye

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Enabling Efficient Creation of Long-Lived ChargeSeparation on Dye-Sensitized NiO Photocathodes Robert J. Dillon, Leila Alibabaei, Thomas J. Meyer, and John M. Papanikolas ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05856 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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ACS Applied Materials & Interfaces

Enabling Efficient Creation of Long-Lived Charge-Separation on Dye-Sensitized NiO Photocathodes Robert J. Dillon,* Leila Alibabaei, Thomas J. Meyer, and John M. Papanikolas* Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514, United States

Abstract The hole-injection and recombination photophysics for NiO sensitized with RuP ([RuII(bpy)2(4,4'-(PO3H2)2-bpy)]2+) are explored. Ultrafast transient absorption (TA) measurements performed with an external electrochemical bias reveal the efficiency for productive hole-injection, i.e. quenching of the dye excited state that results in a detectable charge-separated electron-hole pair, is linearly dependent on the electronic occupation of intragap states in the NiO film. Population of these states via a negative applied potential increases the efficiency from 0% to 100%. The results indicate the primary loss mechanism for dye-sensitized NiO is rapid non-geminate recombination enabled by the presence of latent holes in the NiO film. Our findings suggest a new design paradigm for NiO photocathodes and devices centered on the avoidance of this recombination pathway.

Keywords Hole-injection, recombination, dye-sensitized nickel oxide NiO, transient absorption, applied bias, solar fuels, photocathode

Introduction

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The dye-sensitized photoelectrosynthesis cell (DSPEC) is a promising technology in the pursuit of clean, renewable, and storable energy. Devices consisting of dye-loaded metal oxide photoanodes and photocathodes working in tandem target reactions such as lightdriven water-splitting and CO2 reduction.1–3 Historically, TiO2 based photoanodic systems made rapid initial gains, and have now matured considerably.4–6 On the other hand, more recent work on the development of analogous photocathodic systems has been met with limited success. Despite years of expertise gleaned in the development and characterization of photoanodes, devices using photocathodes composed of the most viable p-type metal oxide, NiO, perform poorly compared to its n-type analog, TiO2.7–9

Ultrafast transient absorption (TA), which has been used to great success in characterizing electron-injection on dye-sensitized TiO2, depicts fundamentally different behavior for hole-injection on NiO compared to the TiO2 analog. The transient spectra for RuII polypyridyl dyes on TiO2, which initially reflect the excited state of the dye, evolve to that of the oxidized dye. From these measurements has come the basic understanding that, for TiO2, electron-injection proceeds rapidly after dye excitation, frequently with injection efficiencies close to 100%, to produce a charge-separated state that persists for microseconds and longer.10–13

Literature results for hole-injection on dye-sensitized NiO do not seem to exhibit the same trend of excitation, rapid efficient injection, and then slow recombination that is observed for electron-injection on TiO2. Picosecond TA measurements on common sensitizers on NiO frequently display rapid quenching of the dye excited state without the


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appearance of any reduced dye. Or, if transient reduced dye is observed at all, the magnitude of the signal is usually miniscule compared to the initial excited state.14–26 Quenching of the excited state is often interpreted as arising from hole-injection, and the lack of any buildup of reduced dye is taken to imply a rapid recombination pathway. This apparent rapid recombination is oftentimes attributed to the low hole-mobility in NiO, which is perceived as preventing the injected hole at the NiO:dye interface from migrating away from the reduced dye, thus recombination prevails. This interpretation of the photophysics has spurred considerable synthetic effort to produce dyes that combat recombination by shuttling electron density away from the NiO:dye interface where, presumably, the injected hole is stuck.20–34 Small, simple chromophores have all but been abandoned in favor of complex ones with charge-transfer transitions and/or electronaccepting/withdrawing functional groups. The success attained by devices utilizing these new dyes has been limited. Furthermore, spectroscopic investigation of these newer dyes is hindered by their increasingly complicated excited states, which often feature intramolecular charge or electron transfer events, even when isolated in solution. The net result is that NiO device performance remains inexplicably inferior to its TiO2 analog despite all of these measures employed to combat its low hole-mobility.

In this work, we demonstrate that the commonly held beliefs associated with the electrontransfer dynamics on NiO are incorrect, and that the inefficient production of reduced dye observed in TA measurements is not due to the low hole-mobility in NiO, but rather the presence of vacant electronic states that extend well above the valence band, i.e. holes present in NiO films from the time of film manufacture. When these states are filled, for



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example by an external applied bias, quantitative hole-injection to produce a chargeseparated state that persists for microseconds is achieved. There have been a few other bias dependent TA studies of dye-loaded NiO conducted to date, all of which focus on nanoseconds (and slower) recombination kinetics at increasingly positive biases, where the hole-injection event is not resolved.35–37 In this work, we focus on the picosecond photophysics, where we observe the initial dye excited state prior to injection, and then precisely track its conversion into reduced dye to establish a quantitative description of the hole-injection process, at positive and negative biases.

Another significant departure from other applied bias TA studies is our choice of dye. Our approach utilizes a dye that is ill-suited for p-type applications: the prosaic phosphonated RuII polypyridyl complex "RuP" ([RuII(bpy)2(4,4'-(PO3H2)2-bpy)]2+, inset Figure 1A). This dye is the antithesis of the latest dyes for p-type devices: the excited/reduced state photophysics of RuP promote electron-hole recombination by pushing electron density towards the NiO surface and keeping it there.38 In the lowest energy triplet metal to ligand charge-transfer (3MLCT) excited state, an electron is promoted from the metal center to the phosphonated bpy that anchors it to NiO (Figure S1). Ideal for electron-injection, this arrangement is doubly disastrous for p-type applications. Hole-injection, i.e. transfer of an electron from NiO to the Ru3+ core, is hindered by the electron density on the reduced anchoring ligand, and, after holeinjection, recombination is promoted by the close proximity of the injected hole with the electron on the phosphonated bpy ligand, which happens to be the ligand closest to the NiO surface. The use of this dye, with its hole-scavenging characteristics, permits us to


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probe whether photoinjected holes are truly trapped at the surface of NiO. Experimental NiO films were prepared using commercial NiO nanoparticle paste, Ni-Nanoxide N/SP by Solaronix. A suspension of paste was prepared by adding 0.5 g of paste to 5 mL of ethanol, followed by sonication for 20 min, and then ultrasonication for 5 min. To this, 5 mL of a 10 wt% suspension of hydroxypropyl cellulose (1.3 g in 15 mL ethanol) was added, and the mixture was stirred overnight. Films were deposited on FTO glass via doctor blading; the thickness was controlled via scotch tape (1 layer). Films were annealed at 400 C for 1 hour, in air. The average thickness was 600 nm, determined by profilometry. The dye, RuP, and high surface area indium-tin-oxide (ITO) and ZrO2 substrates were made according to previously reported methods.39–41 Films were sensitized overnight in a pH1 HClO4 dye solution. Loaded films were briefly rinsed with neat acetonitrile, and then stored in 0.1 M TBAPF6 acetonitrile solution for at least 24 hours prior to experiments.

Ultrafast transient absorption was performed using a 1 kHz regeneratively amplified Ti:Sapphire laser system, Clark-MXR CPA2210. The pump wavelength, 471 nm, was generated by optical parametric amplification (OPA) of a portion of the 775 nm laser fundamental to 1200 nm, followed by sum frequency generation of the output 1200 nm beam with residual 775 nm in a BBO crystal. The white light continuum probe was typically generated by pumping a translating CaF2 plate with another portion of the 775 nm laser fundamental (for Figure 1A only, the white light was generated using a portion of the 1200 nm OPA output instead). Timing of the pump and probe pulses was achieved



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with a computer controlled delay stage. Shortpass filters were employed on the pump and probe beams to eliminate residual 775 nm fundamental. The per-pulse fluence was 480 µJ/cm2 or less. Long-time measurements were conducted using a similar laser setup but with a pulsed photonic crystal fiber laser as the white light probe, Ultrafast Systems EOS. Collection and analysis of the data was performed with custom made LabView software. Optical chirp was corrected according to the frequency resolved optical gating of a CCl4 solution.

Unless otherwise indicated, samples for TA were submerged in fresh 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) acetonitrile in a custom built spectroelectrochemistry cell. The sample was used in a three electrode setup as the working electrode. A Pt wire was used as the counter electrode, and a mini Ag/AgCl electrode served as the reference electrode. All three electrodes were fully encased within the cell, which was sparged with Ar for 1 hour and sealed. A Pine Wavenow potentiostat was used to control the potential during TA experiments. After changing the bias, the sample was given 3-5 minutes to equilibrate before beginning a TA measurement. During experiments, the entire cell was laterally translated in the focal plane of the TA setup to sample a large area of the film.

Cyclic Voltammetry was performed with the above cell and potentiostat, using a scan rate of 2 mV/s. Spectroelectrochemistry on dye-sensitized ITO was also performed with the apparatus described above in conjunction with an HP 8453 UV-Vis spectrophotometer.


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Transient Absorption

Figure 1: Spectral evolution of RuP-sensitized NiO freshly prepared with no applied bias (A), and with an applied bias of +0.4 V (B). In each case, the normally long-lived excited state (τ = 820 ns) is more or less quenched within a nanosecond, yet no reduced dye or any other species is observed. The gap in signal at 470 nm corresponds to where pump-scatter has been deleted. Experiments here were performed on different samples using different collection geometries and apparatus, and are thus qualitatively slightly different. Results in later figures were obtained on the sample used in (B), using the same experimental geometry, and were all collected on the same day.

Figure 1A shows the TA spectra for RuP in 0.1 M TBAPF6 acetonitrile on a freshly prepared NiO film (i.e. "out of the oven"). The spectrum contains all of the hallmarks of an excited state Ru-bpy complex: the induced absorption at 380 nm comes from the π→π* transition on the bpy⋅⋅− radical anion, negative signal at 455 nm is due to the loss of the ground state MLCT absorption, and the broad induced absorption beginning at 500 nm and extending across the visible to the red is attributed to a combination of π→π* transitions on the bpy⋅⋅− radical anion and ligand to metal charge-transfer.38 When RuPsensitized NiO is placed as the working electrode in a three-electrode setup and given a +0.4 V bias (all potentials in this study are versus Ag/AgCl unless otherwise noted; a Pt wire was used as the counter electrode), similar TA spectra are observed, Figure 1B. Figures 1A and 1B differ slightly owing to their being done on different samples with different experimental collection geometries. In Figure 1B, the experimental



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configuration has been improved such that excited state relaxation (subtle blue-shift of the 380 and 455 nm peaks) can be observed. In both Figures 1A and 1B, features pertaining to the reduced dye are not observed. The spectra are essentially identical to what is observed on ZrO2, where neither hole-injection nor electron-injection occurs (Figure S2). But, compared to ZrO2, where the excited state lifetime is 820 ns (from photoluminescence, Figure S3), on NiO the excited state relaxation is nearly complete within a nanosecond.

Quenching of the excited state with negligible or no formation of reduced dye is the general trend for picosecond TA of dye-sensitized NiO. The phenomenon has caused much confusion. Chromophores used in functioning p-type dye-sensitized solar cell (DSSC) devices have been reported as not undergoing hole-injection. For example, devices fabricated using RuP's carboxylate acid analog: "RuC," [RuII(bpy)2(4,4'(COOH)2-bpy)]2+, as the sensitizer show cell efficiencies of η = 0.019 %.42 But, based on the lack of transient formation of the reduced dye in picosecond TA experiments, along with unfavorable ligand energetics (similar to those of RuP), other workers concluded RuC does not inject holes into NiO.15 Simple RuII polypyridyl complexes like RuP and RuC, in solution or on inert glass, have excited state lifetimes of several hundred nanoseconds.38,43–45 In the mentioned TA study, the photoluminescence lifetime for RuC on NiO was found to be only 18 ns, suggesting the manifestation of hole-injection was detected after all.15


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In steady-state photocurrent vs. potential measurements for RuP and other dyes anchored to NiO, cathodic photocurrent emerges when the applied bias is negative to the valence band, +0.3 V, Figure S4.46–48 We have found when the TA experiment is performed with a negative applied bias, the rapid recombination mechanism is diminished, and the formation of transient reduced RuP is observed. Figure 2A shows the evolution over time of the transient spectra for the same sample used in Figure 1B, but now at a -0.2 V bias.



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Figure 2: Spectral evolution of RuP-sensitized NiO at -0.2 V (A), and comparison of normalized (to the 520 nm induced abs) spectra from TA at long and early time vs. steady-state spectroelectrochemistry on ITO (B). TA spectra at other applied biases in this study are provided in Figure S5.

The spectrum of reduced RuP retains the π→π* bpy⋅⋅− radical anion transitions, peak at 380 nm & broad abs in the red, present in the excited state spectrum, but the ground state MLCT absorption is no longer bleached but redshifted instead. This results in a new induced absorption around 520 nm and slight dip at 430 nm for the one electron reduced complex.49

The conversion of excited state dye to reduced dye occurs on the tens of picoseconds timescale, and can be qualitatively followed visually by the emergence of the 520 nm peak coupled with the loss of the ground state bleach at 450 nm. Once formed, reduced


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RuP persists for microseconds - a remarkable feat for a chromophore that is conducive to recombination. The spectra from TA, at early and long-time compare well with the steady-state difference spectrum obtained from electrochemically reducing the complex, Figure 2B. The spectral evolution at other applied biases may be found in the SI, Figure S5.

The magnitudes of the ultimate reduced dye signals shown in Figures 2A and S5 are large and on the same scale as the early time excited state signal. The molar absorptivities of the MLCT transition for the initial and the one electron-reduced complex are nearly 1:1; the comparatively large signal for reduced dye in the TA difference spectra is indicative of efficient formation of a long-lived charge-separated state.49 The magnitude of reduced dye formation is dependent on the applied bias, which we quantify in the next section.

Also worth noting is that the early time spectra consist of only the excited state spectrum. On TiO2, femtosecond electron-injection occurs from RuP's 1MLCT state and the earliest observed spectrum (in the picosecond TA spectra) is a combination of excited state and oxidized dye spectra.13 For RuP on NiO, we do not observe sub-picosecond holeinjection, regardless of bias.

Population Dynamics To evaluate the transition of excited state dye to reduced dye we employ a similar methodology used in electron-injection studies.50 The chirp-corrected TA spectra of RuP on NiO can be described by a linear combination of excited state and reduced state



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spectral components: ∆A = RuP* × ∆ARuP* + RuP × ∆A

(1)

where RuP* and RuP are the time-dependent populations of the excited state and reduced dye complex, respectively. The difference spectrum of excited state, ∆ARuP* , was determined by TA on ZrO2. The difference spectrum for reduced dye, ∆A , was determined by spectroelectrochemistry on ITO (Figure S6). The magnitudes of ∆ARuP* on ZrO2 and ∆A at t=0, at which time it is still purely excited state dye, were normalized to the ground state bleach at 450 nm. This bleach is directly proportional to the amount of dye excited. In the spectroelectrochemistry experiment, prior to reduction, the baselinesubtracted ground state absorption at 450 nm was normalized; then, after complete electrochemical reduction, the final spectrum was scaled by the same factor before computing the difference spectrum, ∆A . As the component spectra have well-defined and minimally overlapping spectral features, the transient spectra, ∆A, at each pumpprobe delay can be fit to Eq. 1 to extract the time-dependent populations for excited state and reduced state. The experimental time-dependent populations for the excited state and reduced state for each applied bias are presented as the scatter plots in Figures 3A and 3B, respectively.


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Figure 3: Population dynamics vs. applied bias for the excited state (A) and reduced dye (B). Solid lines in (A) are from the four-exponential global fit to the data, parameters for which are given in Table 1. Solid lines in (B) are calculated from the kinetic model discussed later using the ratios in Table 2 with an assumed 1 ps hole-transfer time.

Comparing Figures 3A and 3B, the most striking observation is that the formation of reduced dye does not match the decay the excited state. The disconnect between these two observables highlights the fast intermediary recombination mechanism governing whether or not an injected hole lives long enough for reduced dye to be observed. The mechanism at work is more complicated than the rate of injection simply becoming faster than recombination. This is most evident in the population dynamics at +0.4 and +0.2 V. These two data sets share nearly identical excited state decays, yet the latter potential has a reduced dye population that rapidly grows in with a 20% yield, whereas no reduced dye signal is observed for +0.4 V.

As the bias is shifted negative, the rapid recombination mechanism is diminished and the yield of long-lived reduced dye increases. By -0.4 V, the reduced dye yield is 91%. More importantly, at -0.4 V, the decay of the excited state and the growth of reduced dye nearly mirror each other. For almost every excited state chromophore that decays, a reduced one



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appears. In the previous section, we posited the quenching of an otherwise long-lived excited state is indicative of hole-injection for the lack of another decay mechanism. The observation here of a quantitative relationship in the population dynamics further reinforces this interpretation. Hereafter we will refer to excited state decay and holeinjection interchangeably.

A few additional observations emerge by examining Figures 3A and 3B separately, mainly in regard to their shape vs. bias. For hole-injection, Figure 3A, at +0.4 V the injection kinetics are highly multiexponential. The average injection time is about 2 ns, yet half of all injection is complete within 100 ps. As the applied bias is shifted progressively negative, the average injection time decreases and the shape becomes less multiexponential. At -0.4 V, the average injection time is only 39 ps and the kinetics are mostly biexponential. To capture the change in the shape of the decays from multiexponential (≥4 components) to biexponential, as well as to quantify the lifetimes, we chose to globally fit the excited state decay kinetics to a four-exponential decay function: RuP* = RuPA * /τinjA + RuPB * /τinjB + RuPC * /τinjC + RuPD * /τinjD

(2)

where each component, labeled RuPA * to RuPD * , has its own respective time constant, τinjA to τinjD . In the global fit, the decay times of each component were shared bias to bias, while the fractions of excited state dye for a given decay time, were unconstrained. The fits are displayed as the solid traces in Figure 3A, and Table 1 contains the fit parameters. A four-exponential function might be seen as overparameterization. However, its use as a global fit permits the entirety of all the bias-


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dependent decays to be fit with just four time-constants. As shown in Table 1, all four components are really only necessary for the two most positive potentials. By globally fitting the data in this manner, the decreasing complexity of the decays as the bias is shifted negative emerges quantitatively.

Table 1: Amplitudes and Lifetimes bias (V) +0.4 +0.2 0 -0.2 -0.4

initial excited state dye populations for each injection component [RuPA*]0 [RuPB*]0 [RuPC*]0 [RuPD*]0 0.11 0.30 0.40 0.19 0.12 0.24 0.46 0.18 0.08 0.51 0.36 0.05 0.22 0.60 0.19 0 0.43 0.51 0.06 0

average lifetimes‡ τavg (ps) 2040 1908 592 76.0 38.8

τinjA τinjB τinjC τinjD† lifetime (ps) 2.8 38.5 281 10,000 † Fixed for lifetimes persisting beyond the time range of the ultrafast TA instrument (1.6 ns). ‡ For the normalized data, τavg, was calculated by multiplying each time constant by its coefficient: τavg = RuPA * × τinjA + RuPB * × τinjB + RuPC * × τinjC + RuPD * × τinjD

The reduced dye population dynamics, Figure 3B, follow a different trend. The total magnitude increases with each 200 mV shift in applied bias. At +0.4 V, no reduced dye is observed. When the applied bias is shifted 200 mV negative, the signal for reduced dye seemingly "turns on," and the yield jumps to 20%. The largest increase in long-lived reduced dye yield, +35%, occurs from +0.2 to 0 V; subsequent 200 mV shifts grant successively smaller increases. While magnitude changes, the rate and overall shape of the growth remain the same bias to bias. Once formed, transient reduced dye decays with the same lifetime of 6 ns; bias dependence in the recombination is not observed on the timescale of the ultrafast TA experiment, but is observed on the ns-us timescale and will be discussed later.



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The growth of transiently formed reduced dye seemingly only corresponds to the fast components of the excited state decay. For +0.2 V for example, the excited state decay kinetics are predominantly, 64%, described by the two slow injection components, τC and τD; yet these slow injection components are not reflected in the reduced dye dynamics by a slow growth component. Only the fast injection components, τA and τB, seem to ultimately create long-lived electron-hole pairs. At -0.4 V, where the excited state decay is mostly, 94%, described by τA and τB, the yield is 91%. This observed relationship will be exploited later on in the development of a mathematical model for hole-injection & recombination, from which the solid lines in Figure 3B are calculated.

Reduced Dye Yield vs. the NiO Density of States The range of applied potentials used in the TA study is well within the window where the dye is neither electrochemically reduced nor oxidized, and hole-injection is always energetically favorable. From cyclic voltammetry on ITO, the redox potentials of RuP are −1.49 V and +1.45 V for the Ru2+/+ and Ru3+/2+ couples, (Figure S7). The E00 energy for RuP (Ru2+)* is 2.10 eV, (from photoluminescence, Figure S8), thus RuP's excited state reduction potential, (Ru2+/+)*, is +0.61 V.


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Figure 4: Normalized Density of States for the NiO films used in this study. The area from +0.3 V to -0.5 V was normalized. The percentages given correspond to the total percent area of intragap states filled in at each potential used in the TA experiments. Plotting the percent intragap states filled vs. the maximal yields in the population dynamics (see Figure 3B) reveals a linear relationship, inset.

The density of states (DOS) for the NiO films used in this study was obtained via cyclic voltammetry is shown in Figure 4.51 The result compares well what has been observed by others.52–55 The valence band (VB) position for NiO is accepted at +0.3 V; however, assigning a single value for the valence band position is an oversimplification. The tail of the band should be non-exponential due to splitting of the 3d orbitals.56 In this study and the others mentioned, the DOS includes a large distribution of trap states and extends more than a volt beyond the +0.3 V assigned value. Where the valence band ends and the trap states begin is a grey area. Work on TiO2 has encountered a similar conundrum with regard to trap states just below the conduction band.11 For comparison with the TA experiments, in Figure 4 we integrated and normalized the total area of the DOS pertaining to intragap states, with the assumption they begin sharply at +0.3 V. The direct effect of applied bias is the adjustment of the Fermi level in the NiO film. In the figure, the percentage of intragap states filled at each potential is reported and is color coded to match with its corresponding TA experiment. The filling of intragap states correlates well



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with the percent yields for productive hole-injection (i.e. the maximum value of observed transiently formed reduced dye) observed by TA. We found this relationship is linear and has a slope of nearly 1, Figure 4 inset. The fit also goes through the origin, suggesting the +0.3 V as the approximate beginning of the trap states is reasonable.

We are still exploring wider the ramifications of this relationship, but the correlation does fit well with our observations and those of other workers who performed picosecond TA directly on unwired dye-sensitized NiO films only to see little or no long-lived reduced dye. The Fermi level of unbiased NiO films similar to those used in this study has been reported by others as being close to the VB at +0.25 V, hence the yield of transiently formed reduced dye is expected to be low.36

Mechanistic Interpretation Intragap states have been long been implicated as sources for detrimental recombination in TiO2.3 In NiO, it has been suggested that intragap states serve as traps for photoinjected holes, leading to faster geminate recombination.36,57 This interpretation is a likely carryover from nanosecond TA measurements on TiO2 to explain slow recombination in systems where rapid recombination, of any type, is not observed. For newly prepared NiO however, in the absence of an applied bias, the Fermi level of a freshly prepared film sits below the valence band maximum. Intragap states in freshly prepared NiO are essentially holes already in the material. Given the magnitude of their effect on hole-injection and recombination, both surface properties, the latent holes in NiO are likely accumulated at the surface. Due to the small particle sizes in NiO film, the


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extent to which band-bending plays a role is unclear, but upward band-bending due to the Fermi level being below the valence band maximum is also expected to exacerbate the accumulation of holes at the surface.58 Following hole-injection, the primary loss mechanism in NiO is fast (picoseconds/sub-picosecond) recombination of the electron on the reduced dye with one of many nearby non-geminate holes already in the NiO. The actual ability of the injected hole to migrate away from the reduced dye is masked by this recombination mechanism.

An alternative hypothesis for the debilitating role of empty intragap states in NiO is that they serve as traps to which electron-injection occurs.54 Electron-injection to these intragap states is energetically feasible for RuP. However, we do not observe any hint of oxidized dye in the TA spectra. If oxidative quenching were a substantial loss mechanism, we would also expect the rate of excited state decay to increase with the creation of additional, deeper, traps at increasingly positive bias, which it does not.

Latent holes actually extend the lifetime of RuP's excited state. At positive bias, photoexcited chromophores experience slow hole-injection possibly due to the relative scarcity of electrons at the NiO surface, changes in the surface orientation of the dye, or simply because the reducing electron from the NiO bulk must transfer through a surface full of holes to reach the dye. Driving force might also be a factor; however, this would not explain the minimal difference of the +0.4 V and +0.2 V excited state decay kinetics. At the same time, the abundance of latent holes overwhelmingly results in rapid nongeminate recombination, limiting the buildup of reduced dye in the positive bias scenario.



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As the Fermi level is shifted negative, NiO surface sites become increasingly equivalent as they are reduced; the excited state decay quickens and becomes less multiexponential. At -0.4 V, the majority of intragap states are filled, the hole-injection rate is very fast and biexponential, and non-geminate recombination is significantly eliminated. The contrasting phenomena for NiO at high and low (negative) bias are summarized in Figures 5A and 5B.

Figure 5: Illustrations for the photophysics occurring at high (A) and low (B) negative applied bias. For (A), the low concentration of holes in the NiO surface is coupled with rapid hole-injection and minimal nongeminate recombination. For (B), use of low bias results in only partial filling of the VB resulting in a large concentration of surface holes. In this scenario, hole-injection is slowed resulting in a longer excited state (compared to A). At the same time, the latent surface holes are available to instantly quench the chargeseparated state as soon as it is formed.

To better understand the bias dependence on the photophysics of the NiO:RuP system, we developed several deterministic kinetic models and evaluated their ability to describe the experimental time-dependent populations in Figure 3. The mismatch of the excited state decay and the appearance of reduced dye necessitates an intermediate surface site in the model. However, the inclusion of a surface site adds considerable complexity to the interpretation of the results. The observables in the TA experiment only relate to the state of the dye; the experiment is insensitive to whether an injected hole is in the surface site


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or the bulk, both have the same observable. To further complicate matters, two types of surface site were required to faithfully simulate the experimental data.

The simplest model capable of describing the experimental data is shown in Scheme 1. In this scheme we exploit the observed correlations found in the experimental data. Both hole-injection and immediate recombination are related to the Fermi level in the NiO film, and are thus related to each other. We rationalized slow hole-injection coupled with immediate (non-geminate) surface recombination were indicative of a NiO surface depleted of electrons. Another way of looking at it is that the rate of hole-injection not only informs us on the likelihood for immediate recombination, but also provides insight into the electronic occupancy of the NiO surface to which the injection occurs. In the kinetic model, we utilize two types NiO surface site: one that is predominantly electron filled, sNiOF, and one that is predominantly electron depleted, sNiOD. The site to which a given photexcited dye injects into is then assigned by the experimentally observed injection rate. The four injection rates and the initial populations in Scheme 1 are obtained from the global fit of the experimentally observed excited state population decays, and used as fixed parameters. The two fast components are assigned to injection into the predominantly electron filled surface, and the two slow components are assigned to the primarily depleted surface. In this way, the distinction of which surface the dye injects into is much less arbitrary. Following hole-injection, with the hole at a surface site, the charge-separated state can either be quenched by non-geminate recombination, or prolonged by transfer of the hole to the bulk. The observed "slow" recombination, i.e. the



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6 ns experimentally observed decay of the reduced dye signal, is also treated as a fixed parameter for the eventual recombination of injected holes that reach the bulk.

Scheme 1: The simplest scheme capable of reproducing the experimental picosecond population dynamics. Surface site processes, kh-trans, kF-rec, and kD-rec, were evaluated deterministically, all other rates and initial excited state populations were obtained experimentally.

Events occurring in the first hundred picoseconds, after hole-injection but before the slow recombination, dictate the ultimate survival of injected holes. In the model, the three unknown variables, kh-trans, kF-rec, and kD-rec, describe hole-transfer from the surface to the bulk, recombination at the electron-filled surface site, and recombination at the electron depleted surface site, respectively. These three rates cannot be directly ascertained by TA. We evaluated them mathematically and compared the simulated kinetics with the experimentally observed data. For each potential, all three rates were programmatically varied from (1 fs) -1 to (10 ns) -1 and used to deterministically simulate the growth of the reduced dye (the rate constant matrix, concentration vector, and other parameters may be found in the supporting information).59 The simulated kinetics were then compared to the experimental data, and scored by the least squares method. While certain trends in the


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data were apparent with just a few permutations, a computationally excessive set of over four million unique combinations of kh-trans, kF-rec, and kD-rec was evaluated for each potential.

Figure 6 shows the output topology map for the best fit to the 0 V experimental population dynamics for a given set of τD-rec:τF-rec:τh-trans. As the surface processes are interrelated and their rates are indirectly inferred, the results of our evaluation are essentially limited to their relative values. For each data set, we observe the best fits occur over a range of values where there exists a linear relationship among τD-rec:τF-rec:τhtrans.

For the 0 V data set, a linear valley spans much of the plot, where the ratio, τD-rec:τF-

rec:τh-trans,

is 0.30:0.44:1 ps. Observed ratios for the other potentials are provided in Table

2, and topology plots are provided in the SI, Figure S9.



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Figure 6: Topology plot for the simulation of the photophysics at 0 V showing the goodness of fit, χ2 versus the recombination times τD-rec and τF-rec. Additional coarse contours showing the change in holetransfer time, τh-trans, used to achieve the χ2 minimum are overlaid (dashed lines). For each data set, a linear valley for the χ2 minimum stretching across a range of recombination times was observed.

Table 2: Kinetic Model Results bias (V)

τD-rec:ττF-rec:ττh-trans

+0.4 +0.2 0 -0.2 -0.4

0.020:0.023:1 0.30:0.44:1 0.30:6.2:1 0.125:308:1 1.28:16,200:1


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At +0.4 V, recombination is very fast and even the fast injecting components produce no long-lived reduced dye. As the bias is increased, the general trend in the dynamics is the ratio of the two surface recombination times, τD-rec:τF-rec, increases substantially, from 0.020:0.023, at +0.4 V, to 1.28:16,200, at -0.4 V. The difference in recombination for electron-filled and electron-depleted surfaces validates the use of two surface sites in the kinetic model, and makes physical sense; a surface with a higher concentration of latent holes will experience a faster quenching of the charge-separated state.

With regard to hole-transfer and recombination, the exact nature of the bias dependence on the surface dynamics is an open question; the change in relative rates can be construed as surface recombination slowing down, hole-transfer being accelerated, or some combination of both. As such small changes in applied bias are unlikely to significantly alter hole-transport in NiO, we venture its main effect is to slow surface recombination by the elimination of latent holes. The best evidence for this is observed when the sample's electrical connection is interrupted. When the circuit is broken, the electronic occupancy of the film becomes fixed. We observe the photophysics of the unplugged, unbiased, sample remain those of the most recently applied bias, even after a significant period of time (45 days), Figure S10. Similar behavior has been observed previously for electron-injection in RuII polypyridyl sensitized TiO2.11 Using TA with an applied bias to explore electrolyte effects, the authors observed the kinetics became constant and insensitive to changes in electrolyte in the absence of an electrical connection.



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Finally, the most significant result from the kinetic modeling is the observation of an upper limit on the time taken for photoinjected holes to migrate from the surface, τh-trans. In Figure 6, the range of values for τF-rec and τD-rec able to provide a reasonable fit to the data is ultimately limited; above a hole-transfer time of ~10 ps, the quality of the fit is inescapably worse. In the +0.2 V data set, which we found to be the least forgiving to simulate, the best possible fit for a given hole-transfer time becomes systematically worse when it takes longer than 1 ps (Figure S11). If it were to change at all, we do not expect hole-transfer from the surface to the bulk to slow down at negative bias; we interpret the observed 1 ps limit at low bias as the upper limit for the system, regardless of bias.

Extreme Negative Bias In the previous sections we describe the bias dependent photophysics where a fast nongeminate recombination mechanism is active. In that range of potentials, +0.4 to -0.4 V, as high energy sites are filled, fast recombination is minimized. For negative potentials beyond the tapered end of the valence band, -0.5 V, the photophysics take on a new characteristic response which we now reveal.


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Figure 7: Population dynamics at extreme negative bias (A), nanosecond reduced RuP population dynamics vs. applied bias (B), and illustration of the extreme negative bias photophysics (C). At -0.4 V, the excited state decay and reduced dye growth roughly mirror each other in Figure 3; here in (A) at -0.6 V and -0.8 V, the mirror image relationship between the excited state decay (solid lines & filled symbols) and the reduced dye growth (dashed lines & empty symbols) is nearly perfect, signaling the non-geminate recombination is % now minimal. Solid lines in (B) are stretched exponential fits of the form " = # /$ , with a fixed β of 0.5 and lifetimes of 432, 566, 750, 3400 ns for applied potentials -0.2 V to -0.8 V (200 mV steps).



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According to our mechanistic interpretation, at potentials beyond -0.5 V, excited state decay and reduced dye formation should mirror each other exactly because there is no longer the possibility for non-geminate recombination. Figure 7A shows the early time population dynamics for -0.6 and -0.8 V where we do indeed observe a perfect mirror image relationship. We also expected the hole-injection rate to have plateaued, but the rate unexpectedly continues to change with bias; now becoming seemingly slower. A small fraction of the excited state population, ~20%, exhibits delayed hole-injection, while remaining dyes have the same, unchanging, fast injection. On the microsecond timescale, reduced dye lives just slightly longer with each -200 mV shift; but from -0.6 to -0.8 V, the lifetime jumps, more than quadrupling, Figure 7B. In steady-state photocurrent measurements, the photoresponse is also enhanced at high negative bias (Figure S4).

Our electronic occupation model does not predict continued bias dependence above the valence band maximum, -0.5 V. Increased bias where the NiO DOS is zero, and the Fermi level is pinned, rules out driving force explanations for both slower injection and recombination. In regard to the new slow injection component, Marcus inverted electrontransfer is not expected for electron transfer in dye-loaded metal oxides.50 We attribute the behavior at high negative bias to the partial desorption of the dye by the electrolyte. At high negative potential, bulky tetrabutylammonium (TBA+) cations in the electrolyte crowd the NiO surface and a subset of dyes experiences slower injection and recombination as a result being displaced, Figure 7C. RuP's inherent radiative excited state decay is sufficiently slow, τ = 820 ns, that it does not compete with delayed


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injection, otherwise this would constitute an additional loss mechanism. The buildup of positive cations at the NiO surface probably stabilizes the reduced dye resulting in the increased recombination time; the subtle blue-shift of the reduced dye at very long time, Figure 2B, might be attributable to this stabilization. When the bulky TBA+ is replaced with tetramethylammonium (TMA+), the slow injection/growth components are eliminated, and the photophysics become uniformly fast, Figure 7A.

Comparison with the Reported Bias Dependence of TiO2 Our results for RuP on NiO can be viewed in a broader context by comparison with the electron-injection photophysics of dye-sensitized TiO2. Application of a negative bias on RuP sensitized NiO results in rapid quantitative hole-injection along with slow recombination. With a sufficiently negative bias, the overall photophysics become simplified and even analogous to those reported for electron-injection on TiO2. RuP has traditionally found use as a chromophore for n-type systems like TiO2, where, without any applied bias, its efficiency for electron-injection is 100%.12,13 The same is true for many other RuII polypyridyl complexes on TiO2. TA studies on TiO2 are routinely conducted on dye-sensitized slides that are simply placed into solution. Rapid quantitative injection and the absence of a fast recombination mechanism, without the need for an applied bias, are likely the inherent blessings of doing electron-injection into a TiO2 surface having few electrons. It is likely with this mindset that dye-sensitized NiO substrates have similarly been studied as isolated slides in solution. For NiO however, the surface of the film is unfortunately not empty of holes by default, thus results on isolated slides so far have been largely uninformative and misleading.



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While an applied bias on NiO can make the hole-injection photophysics resemble the bias-free electron-injection kinetics on TiO2, a few reports of the bias dependence of RuII polypyridyl-sensitized TiO2 seem to show that the electron-injection can be made to resemble the hole-injection kinetics of unbiased NiO. As this is undesirable, only few reports of the TiO2 photophysics at negative bias have been reported. Furthermore, we have shown for NiO it is vital to track both the reagent, excited state dye, and the product, reduced dye, to fully grasp its photophysics. As the efficiency of electron-injection on TiO2 is often very high, most studies tend to track either the loss of the excited state or the growth of the oxidized dye (or reduced TiO2 signal), and not both. Nevertheless, piecing together observations made in separate studies on the bias dependence of RuII polypyridyl-sensitized TiO2 shows the photophysics of electron-injection on TiO2 begin to resemble those of hole-injection on NiO when a negative bias is applied to TiO2. In TiO2, shifting the Fermi level negative fills intragap states below the conduction band, creating a situation that is analogous to electron vacancies above the NiO valence band. At sufficiently negative potentials, electrons accumulate at the surface of TiO2.11,58 Just like NiO, telltale signs of slow injection coupled with a fast recombination mechanism emerge with the unfavorable adjustment of the Fermi level. In terms of the excited state, work by the Grätzel group found electron-injection slows down with negative bias to the extent short-lived photoluminescence is observable.60 Meanwhile for the product, the Durrant group observed kinetics similar to RuP's reduced dye dynamics on NiO, Figure 3B, for the signal of the injected electron in TiO2.61 In the plot, with an increasingly negative bias on TiO2, the amplitude of the reduced TiO2 product decreases, while the rate at which it grows in is unaffected. In the terms of our model for NiO, we would


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interpret the increased photoluminescence as due to electron-injection being impeded by electrons already at the surface, lengthening the excited state; while the variation in the magnitude of the charge-separated state is due to it being quenched by a rapid process, probably non-geminate recombination from electrons already in the film.

Conclusions We have identified non-geminate recombination due to electron vacancies already in the NiO as the main loss mechanism of the charge-separated state, and have demonstrated RuP, a dye unsuited for p-type function, can achieve quantitative, 100%, long-lived holeinjection when these vacancies are filled. From the high yields of transient reduced dye in the TA experiments, and kinetic modeling of the results, we posit the low hole-mobility of NiO does not trap photoinjected holes at the NiO:dye interface; from our calculations, the time required for a photoinjected hole to transport into the bulk is less than 1 ps.

In a broader scope, we have shown how the electronic occupation of NiO is linked with its observable TA dynamics. Where, by adjustment of the Fermi level, hole-injection on NiO can be optimized and even made to resemble electron-injection on TiO2. The results suggest the electronic condition of the NiO film plays a vital role in the early-time creation of long-lived charge-separated states on NiO. The similarities in the bias dependent NiO hole-injection and TiO2 electron-injection photophysics suggest the mechanistic insight obtained in this study is likely applicable in the understanding and optimization of other dye-sensitized solar energy conversion systems.



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Impact on Devices Finally, in the context of devices, a tapered partially oxidized distribution of intragap states above the valence band for "out of the oven" substrates might be an inherent flaw of NiO as a sintered metal oxide film (though a blessing for TiO2). For optimal device function, the condition of these states must be considered. Our findings suggest a chromophore-catalyst assembly on NiO may experience a profound boost in a tandem configuration with a photoanode that is capable of filling the intragap states. In this case, the reduction potential of the catalyst should ideally be above of the maximum of the intragap states. Similarly, for NiO DSSC devices a new sufficiently negative redox shuttle is required. The I3-/I- shuttle, another carryover from TiO2 devices, is simply too positive. We note the success of the most efficient NiO DSSC device to date, η = 2.51%, falls mostly its use of a more negative redox shuttle, tris(acetylacetonato)-FeIII/II.37 The authors report when the I3-/I- shuttle is used instead, the efficiency dramatically drops to η = 0.60%. While there is some concern a negative redox shuttle might promote recombination of the reduced shuttle with transient photoinjected holes, we posit the larger issue at hand is the avoidance of the redox mediator leeching electrons from the NiO intragap states and opening up the rapid non-geminate recombination pathway.

Associated Content Supporting Information Molecular orbitals from density functional theory (Figure S1), transient absorption spectra for RuP on ZrO2 (Figure S2), time-resolved fluorescence kinetics of RuP on ZrO2 (Figure S3), photocurrent vs. applied bias for RuP on NiO (Figure S4), transient


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absorption spectra for RuP on NiO at applied potentials from +0.4 V to -0.8 V (Figure S5), spectroelectrochemical reduction of RuP on ITO (Figure S6), cyclic voltammetry of RuP on ITO (Figure S7), steady-state photoluminescence spectrum of RuP on ZrO2 (Figure S8), simulation details and results (Figure S9), comparison of transient absorption spectra after disconnecting the applied bias (Figure S10), χ2 vs. hole-transfer time and simulation results for the +0.2 V data set (Figure S11).

Author Information Corresponding Authors E-mail: [email protected], [email protected]

Acknowledgements This material is based upon work solely supported as part of the UNC EFRC: Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001011. RJD acknowledges Mark Wicker in the UNC Scientific Glassblowing Department for his assistance in constructing the sample cell used in this study.

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Enabling Efficient Creation of Long-Lived Charge-Separation on Dye

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