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Spectroscopy and Photochemistry; General Theory

Role of the Electronic Relaxation in the Injection Process of Organic Push-Pull Dyes in Complete Dye Sensitized Solar Cells Valentin Maffeis, Hakan Dogan, Elsa Cassette, Bruno Jousselme, and Thomas Gustavsson J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01947 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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The Journal of Physical Chemistry Letters

Role of the Electronic Relaxation in the Injection Process of Organic Push-Pull Dyes in Complete Dye Sensitized Solar Cells Valentin Maffeis,a,b1 Hakan Dogan,a Elsa Cassette,a Bruno Jousselmeb and Thomas Gustavsson*,a a

b

LIDYL, CEA, CNRS, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France

LICSEN, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay 91191 Gif-sur-Yvette Cedex, France

Corresponding author *Thomas Gustavsson Tel: +33 169 089 309, Email: [email protected]

1

Current address: ICIQ, Institute of Chemical Research of Catalonia ICIQ, Avda. Països Catalans,

16, 43007 Tarragona, Spain

1 ACS Paragon Plus Environment

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ABSTRACT There is a growing consensus that the charge separation taking place in dye-sensitized solar cells is a multiscale process occurring on a times scale from a few to hundreds of picoseconds. We studied the excited state dynamics of the robust and efficient push-pull dye RK1 in solution, on mesoporous films and in complete photovoltaics cells by femtosecond fluorescence upconversion. On polar environment and cells, the dynamics at early times are dominated by an intramolecular electronic relaxation while electron injection is predominant on thin films only. In cells, the electron injection process becomes visible on a later stage, from tens to hundreds of picoseconds. Our study shows that it is crucial to record and analyze full time-resolved fluorescence spectra in order to obtain wavelength independent dynamics and get a correct description of the nature and the population of the excited state.

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Organic dyes characterized by a push-pull structure are extensively used for the sensitization of mesoporous metal-semiconductor providing high absorption, stability and efficiency in dye sensitized solar cells (DSSCs).1–3 The process of electron transfer from the photoexcited dye to the mesoporous semiconductor, the so-called injection process, has been reported to occur on a picosecond timescale.4 Such a fast process is obtained if the injection driving force is superior to 0.2 eV,5 which leads to high injection efficiency as the fluorescence lifetime for a typical excited dye is an order of magnitude longer.6 Ultrafast fluorescence spectroscopy has been used to compare the excited state lifetime of the dye when grafted on a semiconductor, such as TiO27 or ZnO,8 where injection takes place, with that on an insulator such as Al2O39 or ZrO210,11 where it does not. The difference in quenching rate is then taken as a measure of the electron injection. Still, an ultrafast (~10 ps) partial fluorescence intensity quenching is frequently observed also on sensitized insulator films. This quenching has been attributed to electron injection into trap states12 or dye-dye self-quenching13,14 but other studies evoked the involvement of an intramolecular electronic relaxation.15,16 It should be stressed that in the preceding studies, most measurements were done at single wavelengths. This can lead to misinterpretations since an observed quenching of the fluorescence intensity may not only be due to a decrease of the emitting state population but also, for example, a time-dependent spectral shift of the fluorescence spectrum. We have, in a previous work on another push-pull dye,17 shown the need to record full time-resolved fluorescence spectra (TRFS) in order to decorrelate the total, spectrally integrated, fluorescence intensity (𝐼(𝑡)) and the spectral relaxation (represented by the mean position of the emission band 𝜎(𝑡)). In this work, we use the femtosecond fluorescence upconversion (FU) technique in order to record the TRFS of the push-pull dye RK1 in solution, on mesoporous Al2O3 and TiO2 films in air



and in closed cells with an electrolyte and a counter-electrode. The latter comparison is important since different dynamics have been reported in the literature for films and cells.18–20 RK1 is a commercially available triphenylamine-based dye with a D-π-A-π-A structure. It is highly photostable and a power conversion efficiency of 10.2 % has been reported for RK1 sensitized TiO2 based DSSCs.21,22 The results obtained indicate the presence of an electronic relaxation from the directly excited Franck-Condon (FC) state to a less bright state, and not the injection, as the main reason for the ultrafast fluorescence quenching. The charge injection emerges at a later stage (tens of picoseconds) as a multi-scale non-exponential process extending to the end of our measurements (hundreds of picoseconds) and probably beyond. Our observations are in clear contradiction with the general assumption that a picosecond fast charge injection is needed for high photoconversion efficiency. The TRFS of RK1 were measured in toluene, tetrahydrofuran (THF) and acetonitrile (ACN), three solvents of increasing polarity. Chosen contour plots are shown in the Supporting Information (Figure S9). The excitation wavelength used was 450 nm, corresponding to the blue side of the first absorption band. The TRFS were fitted using a log-normal function in order to extract the integrated intensity and the position. Like other push-pull dyes in solution,23 a strong red-shift of the fluorescence band of RK1 ensues during the first tens of picoseconds after photoexcitation. This is expected as the S0→S1 transition in these dyes has a distinct CT character.24 Another noticeable feature is that this picosecond spectral relaxation is occurring alongside a partial quenching of the spectrally integrated fluorescence on the same timescale (see Figure S10). It is well known that the instantaneous fluorescence intensity depends on the excited state population 𝑁 ′ (𝑡), the emission transition dipole moment, 𝜇𝑒𝑚 , and on the cube of the emission


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wavenumber, 𝜈 3 .25,26 In order to get a better picture of 𝜇𝑒𝑚 , the TRFS were accordingly divided by 𝜈 3 before integration. The resulting integrated values, denoted 𝑁 ′ (𝑡) × 𝜇𝑒𝑚 are shown in Figure 1.

Figure 1. Corrected total fluorescence intensities, divided by 𝜈 3 (see text), of RK1 in toluene (black), tetrahydrofuran (red) and acetonitrile (blue). Solid lines indicate fitted curves. The dashed blue line indicates the normalized and scaled traces of the excited state absorption in acetonitrile (at 650 nm) obtained by transient absorption spectroscopy (see Supporting information). In the less polar solvents, 𝑁 ′ (𝑡) × 𝜇𝑒𝑚 is practically flat during the first hundreds of picoseconds, in accordance with a long-lived excited state population and constant emission transition dipole moment. On a longer time-scale, the fluorescence decays mono-exponentially with a time constant dropping from 4.1 ns (toluene) to 1.7 ns (THF) as determined by timecorrelated single photon counting (see Figure S8). The red-shift of the emission also increases with the solvent polarity. This can be explained as due to a reduced energy gap relative to the ground state, favoring an internal conversion. In the polar solvent ACN, the situation is radically different. As shown in Figure 1, the value of 𝑁 ′ (𝑡) × 𝜇𝑒𝑚 decreases rapidly in a bi-exponential manner with



time constants of 1.1 ps and 18 ps. This may be due to a fast population decay and/or a timedependent change of the emission transition dipole moment 𝜇𝑒𝑚 (𝑡). Femtosecond transient absorption measurements of RK1 in ACN allow us to assign the 18 ps time constant to the excited state population decay while the faster time constant of 1.1 ps rather corresponds to an ultrafast change in the emission transition dipole moment. Besides, the radiative lifetime of RK1 in ACN (57 ns) is much higher than what is observed in the less polar solvents (∼ 15 − 25 ns, see Table S2 in the Supporting Information) and that predicted by the Strickler-Berg equation (8 ns, details in the Supporting Information).27 Therefore, in toluene and THF, the fluorescence dynamics of RK1 can be described as emission from a single excited state undergoing a solvent controlled stabilization while in ACN, the rapid bi-phasic drop of the fluorescence indicates a very different process. Drawing on our previous study of a similar push-pull dye, we attribute this behavior to an electronic relaxation from the initially excited Franck-Condon (FC) state towards a new excited state with an enhanced CT character. This state is more polar and less conjugated resulting in a decrease of the fluorescence intensity during the electronic relaxation. Therefore, the electronic relaxation could go along with a structural relaxation towards a less planar geometry. The CT state is then quenched in 18 ps, probably via an internal conversion to the ground state as proposed for similar molecules.28 The results obtained in solution show clearly that the polarity of the solvent, i.e. of the local environment, strongly governs the excited state dynamics of RK1. The fluorescence decays of RK1 sensitized Al2O3 and TiO2 films in air and in complete cells extend over several hundreds of picoseconds and show a strong non-exponential behavior. However, the fluorescence dynamics can be separated in two distinct phases, one covering the first few tens of picoseconds and a second phase extending to several hundreds of picoseconds.


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The time evolutions of the total fluorescence intensity and of the spectral relaxation of the four samples are shown in Figure 2 for the first 30 or 40 picoseconds. The scales were chosen in order to overlay the spectral relaxation and the fluorescence intensity decay at time zero and at 30 or 40 picoseconds.

Figure 2. (top) Time-resolved fluorescence spectra and (bottom) corresponding extracted mean position of the emission band (red) and normalized integrated fluorescence intensity (black) of RK1 grafted a) on Al2O3 under air, b) on TiO2 under air, c) on Al2O3 in a complete cell and d) on TiO2 in a complete cell. In all cases, an important spectral relaxation occurs during the first phase. On the same timescale, the fluorescence is partially quenched both on TiO2 and Al2O3. Both processes are faster for films under air. In complete cells, the amplitude associated with this first quenching is roughly 60 % of the initial intensity after 40 picoseconds whatever the nature of the substrate. The fitting of the time-evolution of the mean position of the emission band during this first phase using a multiexponential model yield characteristic times of 0.9 and 12.5 ps for RK1 grafted on Al2O3 under air and 1.7 and 21.5 in a complete cell; 0.7 ps on TiO2 under air and 0.7 and 5.8 ps in a complete 7 ACS Paragon Plus Environment


cell. Similar time constants are obtained for the spectrally integrated fluorescence decays (see Table S4). As discussed below, to describe the spectral relaxation observed for the TiO2-based films and cells during the second phase a third, longer component has to be added. It is noteworthy that the spectral relaxation and the fluorescence quenching follow the same time-evolution for the four samples during the first phase of the relaxation. Only in the case of TiO2 under air can a slight mismatch be seen. Finally, 𝑁 ′ (𝑡) × 𝜇𝑒𝑚 decays non-exponentially on every substrate (see Figure S17). These observations suggest that the two processes have a common origin. We recall that a similar behavior was found for RK1 in ACN. Besides, we stress that the quenching has similar amplitude and dynamics in complete cells on both substrates. Therefore, we believe that there is only one single process explaining the spectral relaxation and the fluorescence quenching of RK1 in both TiO2 and Al2O3 complete cells during the first 40 picoseconds after photoexcitation. We refute the possibility of electron injection as a common cause due to the similar amplitudes of the quenching in TiO2 and Al2O3. Although the possible injection from the excited dye to trap states of Al2O3 have been evoked, it cannot be as efficient as electron injection in a semi-conductor. Excitonic relaxation is another hypothesis for this quenching but it does not explain the observed spectral relaxation dynamics. Such a relaxation is also highly improbable since (i) RK1 possess an alkyl sidechain to reduce the formation of aggregates and (ii) all our films were prepared with CDCA as a co-sensitizer. Therefore, we assign the concomitant initial fluorescence quenching and spectral relaxation to an intramolecular electronic relaxation from the Franck-Condon state towards a less radiative and more polar CT state and not to the electron injection from the excited state of the dye to the substrate.


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For RK1 on TiO2 film under air, the fluorescence quenching is faster than the spectral relaxation. It is possible that the charge injection is faster in this case and competes with the electronic relaxation on the 0-10 picosecond timescale (𝑘𝑖𝑛𝑗,𝐹𝐶 ∼ 𝑘𝑒𝑙 ). Indeed, it has already been shown for TiO2 films sensitized with similar push-pull dyes and studied under air that the injection process occurs in a time range of 1-10 ps29–34. For RK1|TiO2 in complete cells, on the other hand, we stress that the injection process cannot be detected during the first 40 picoseconds but occurs only on a much longer time range. Figure 3 shows the time evolutions of the spectrally integrated fluorescence intensity decay and the mean position of the emission band of RK1 in complete cells on a longer timescale to emphasize the presence of two phases in the fluorescence quenching.



Figure 3. Mean position of the emission band (red) and normalized fluorescence intensity (black) on a longer time scale of RK1 grafted on (a) Al2O3 and (b) TiO2 in complete cells. Insert: the temporal dependence of the full width at the half maximum of the emission band (fwhm). In complete cells, differences between dynamics on Al2O3 and TiO2 only arise after several tens of picoseconds. During this second phase, the fluorescence intensity decreases very slowly (〈𝜏〉 = (232  37 ps) on Al2O3 whereas the band does not shift any longer (Figure 3a). On TiO2, the quenching of the fluorescence is faster (〈𝜏〉 = (171  12 ps) and goes together with a further spectral relaxation of the emission band that lasts until the end of our measurements (Figure 3b). Once again, the integrated fluorescence decay and the spectral relaxation have similar dynamics and the two curves overlap nicely. We attribute the differences of behavior in the fluorescence of RK1 grafted on TiO2 and Al2O3 in the second phase to the electron injection process. This explains not only the faster quenching on TiO2 but also the slower "never-ending" spectral relaxation process occurring after the initial electronic relaxation in complete RK1|TiO2 cells. Due to the inhomogeneous environment on the mesoporous film, the energy distribution (𝑊(𝐸)) of the electronically excited dye will be quite large. On the other hand, the corresponding density of acceptor states (𝐷(𝐸)) in the semiconductor conduction band increases with the energy meaning that the injection process is faster for dyes with higher excited energies. Therefore, an inhomogeneous relaxation occurs. As a consequence, the fluorescence emitted by the dye molecule at shorter wavelengths will be quenched faster than at longer ones35 causing a macroscopic spectral relaxation. This also explains the continuous drop in fwhm in the second phase (Inserts in Figure 3). The effects of the electronic and the inhomogeneous relaxations on the TRFS are shown schematically respectively in Figure 4a and 4b. 10 ACS Paragon Plus Environment

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Figure 4. Schematic model describing the effects of the electron injection process on the timeresolved fluorescence spectra and on the dynamic Stokes shift (Δ𝜎𝑑 ) of a) an electronic FC-CT relaxation and b) a subsequent non-homogeneous injection process from the CT state of RK1 into the conduction band of TiO2. Thin transparent films, often used for time-resolved spectroscopic studies of injection processes, show completely different dynamics than real DSSCs. Indeed, with the addition of an electrolyte, the conduction band of TiO2 is shifted upwards resulting in slower injection.36,37 Furthermore, this energy shift has a more complex consequence on the interfacial electron transfer for a sensitizer with a strong charge-transfer character such as RK1. In the presence of an electrolyte, the electron injection from the FC state becomes slower than the solvation process or than the intramolecular electronic FC→CT relaxation resulting in an injection process only involving the relaxed CT state. This electronic relaxation causes the fluorescence quenching occurring during the first tens of picoseconds after photoexcitation while it is partially responsible for the associated redshift. 11 ACS Paragon Plus Environment


Consequently, the injection process for RK1 in a complete cell is much slower than what has been reported for similar push-pull dyes in the literature. An important evidence is our observation that the spectral relaxation process in RK1|TiO2 complete cells is a highly non-exponential process, ranging from the sub-picosecond time range to, at least, several hundreds of picoseconds. Still, having characterized the fluorescence dynamics over this wide range, we do not exclude that some electron injection may occur at very early times (< 100 fs) or that it continues beyond the timelimit of our setup (a couple of hundreds of picoseconds, see Figure S18). The implications of the present work are multiple. We have shown that it is crucial to record and analyze full TRFS instead of fluorescence decays at single wavelengths in order to decorrelate the spectrally integrated fluorescence intensity from the spectral relaxation. Moreover, we demonstrate that the electron injection from RK1 into TiO2 in complete cells is a strongly multiscale process, having its real effect on a timescale from tens to hundreds of picoseconds due to faster solvation and electronic relaxation processes. We do not believe this finding to be specific to the RK1 dye but that this is a very general phenomenon of push-pull dye sensitized semiconductor films. In this context, we recall that RK1|TiO2 cells display conversion efficiencies up to 10 % and therefore the injection efficiency of RK1 should be on par with or better than similar dyes. Experimental Methods. To prepare mesoporous Al2O3 and TiO2 films, RK1 was co-sensitized with CDCA in order to avoid aggregation. The films were studied directly under air or in a complete dye sensitized solar cells comprising the aforementioned films, 25 µm of AN-50 electrolyte (composed of ACN and 𝐼3− /𝐼 − redox electrolyte and other additives) and a transparent Pt counter electrode. For the time-resolved fluorescence measurements, we used a “conventional” transmission-mode FU setup38 to measure in solution and a front-face reflexion-mode setup to


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study films and opaque, scattering and highly absorbing DSSCs.39,40 Both setups allow the direct recording of time-resolved fluorescence spectra, corrected for the "chirp" induced by the wavelength-dependent difference in group velocity. Acknowledgment. This work was supported by the CEA program DSM Energie, contract PHLUMVIR – E113-9. E.C. thanks C. Cornaggia for his help with the laser and the French Agence Nationale de la Recherche for funding (grant ANR-16-ACHN-0022-01). Supporting information. Details of the experimental methods including information about the chemicals used, preparation of the samples, the spectrometers and setups used as well as the analytical methods, study of RK1 in solution, absorption and fluorescence spectra of films and cells, time evolution of 𝑁′(𝑡) × 𝜇𝑒𝑚 for films and cells and nanosecond-fluorescence decays for complete cells.



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