Correlating Multichannel Charge Transfer Dynamics with Tilt Angles of

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Correlating Multichannel Charge Transfer Dynamics with Tilt Angles of Organic Donor−Acceptor Dyes Anchored on Titania Yinglin Wang, Lin Yang, Mingfei Xu, Min Zhang, Yanchun Cai, Renzhi Li, and Peng Wang* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China S Supporting Information *

ABSTRACT: Attaching the side chains onto the electronic backbone of photosensitizing dye molecules has been a widely employed method to enhance the performance of dye-sensitized solar cells (DSCs). However, at the present time the interfacial microstructure as well as its influence on the charge transfer dynamics have not been fully comprehended, both of which underpin the phenomenological photovoltaic characteristics. In this work, we derived the tilt angles of two organic donor− acceptor dyes anchored on the surface of titania by X-ray reflectivity measurements and probed the broad time scale dynamics of charge transfer reactions in DSCs, unveiling the intrinsic roles of bulky or branched side chains attached to the conjugated segment in modulating photovoltaic behaviors.

1. INTRODUCTION Apart from light-harvesting, the kinetic competitions from several pairs of charge transfer reactions occurring at the titania/dye/electrolyte heterojunctions to a large extent control the solar to electricity conversion of dye-sensitized solar cells (DSCs).1−3 Comprehensive analysis of these photoinduced charge transfer processes on a broad time scale ranging from femtoseconds to milliseconds is highly instructive for the target optimization of smart materials and functional interfaces to further boost the cell performance. In terms of the Marcus theory of electron transfer,4−7 the rate of a heterogeneous electron transfer reaction is determined by the change of free energy, the electronic coupling between the donor and acceptor states, and the reorganization energy. Thereby, aside from the energy levels of the donor and acceptor states, the interfacial microstructure albeit so far rarely addressed in DSCs is also of paramount importance in modulating the rate constants of the photoinduced charge transfer reactions at the buried interfaces.8−14 In this paper, we will tune the tilt angles of two organic donor−acceptor (D−A) dyes anchored on the surface of titania by varying the side substituents attached to the conjugated dithienopyrrole π-linker. Figure 1a shows the molecular structures of the herein studied D−A dyes, C241 and C260, characteristic of the linear n-hexyl and branched 2-hexyldecyl side moieties, respectively. It is valuable to point out that both density functional theory (DFT) (not shown) and experimental measurements (Figure S1 in Supporting Information) on electronic absorption, fluorescence, and electrochemistry have proved a negligible influence of the side chain transformation on the electronic backbone of these two dyes. Furthermore, we © 2014 American Chemical Society

will unveil the crucial impacts of altering the tilt angles of D−A dyes on multichannel charge transfer dynamics as illustrated in Figure 1b.

2. EXPERIMENTAL SECTION Materials. Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), 4-tert-butylpyridine (TBP), acetonitrile, and 4-tertbutanol were purchased from Aldrich and used without further purification. The preparation of C241 was reported in our previous paper15 and the synthetic details of C260 are presented in Supporting Information. X-ray Reflectivity (XRR) Measurements. XRR measurements were carried out on a Bruker D8 discover high-resolution diffractometer using the Cu Kα X-ray radiation (λ = 1.542 Å). The X-ray beam was collimated using a Göbel mirror with a 0.2 mm slit. Reflectivity spectra were recorded over the angular range 0.20° ≤ 2θ ≤ 8.00° with a step size of 0.005° and a counting time of 1 s per step. Collected reflectivity data were plotted as a function of perpendicular momentum transfer (Qz), Qz = 4π(sin θ)/λ, and were refined by the MOTOFIT package16 to obtain structural parameters associated with layers of the bare and dye-grafted titania films prepared by atomic layer deposition (ALD). The ALD titania films were prepared according to the literature method.17 Initial structural models were prepared using estimated values of the X-ray scattering length density (SLD) of 31.2 × 10−6 Å−2 for the ALD titania Special Issue: Michael Grätzel Festschrift Received: November 6, 2013 Revised: December 22, 2013 Published: January 29, 2014 16441

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excitation light. The minor portion of 800 nm laser went through a delay line (2 ns, CDP Corp.) and was focused on the sapphire to generate white light continuum. The white light was then split into two equal beams to form probe and reference light. Polarization between pump and probe beams was set as a magic-angle. The probe and reference beams were then collected by two optical fibers connected to a monochromator and detected with a multichannel optical sensor (1024 elements, MS 2022i, CDP Corp.). The obtained signal was processed by the ExciPRO software (CDP Corp.). We further implemented pump-power-dependent measurements to get rid of pump intensity dependent dynamics in our measurements. All the experiments were carried out at room temperature. The raw data were analyzed on Glotaran software by target analysis. The experimental details on nanosecond transient absorption kinetic measurements were described in our previous paper.18 Cell Fabrication and Characterization. A 4.2 + 5.0 μm bilayer titania films was deposited by screen-printing a 4.2 μm thick transparent layer and a 5.0 μm thick scattering layer on fluorine-doped tin oxide (FTO) conducting glass (Nippon Sheet Glass, Solar, 4 mm thick), which was dye-loaded by immersing it into a 150 μM dye solution in the mixed solvent of acetonitrile and 4-tert-butanol (v/v, 1/1). The film thickness was measured by a benchtop Ambios XP-1 stylus profilometer. Then the dye-grafted titania electrode was assembled with a gold coated FTO electrode by use of a 25 μm thick Surlyn ring to produce a thin-layer electrochemical cell. The cobalt electrolyte is composed of 0.25 M tris(1,10-phenanthroline)cobalt(II) di[bis(trifluoromethanesulfonyl)imide], 0.05 M tris(1,10-phenanthroline)cobalt(III) tris[bis(trifluoromethanesulfonyl)imide], 0.5 M TBP, and 0.1 M LiTFSI in acetonitrile. Details on cell fabrication and measurements of photocurrent action spectrum, photocurrent−voltage, transient photovoltage decay, and charge extraction can be found in our previous publications.18,19

Figure 1. (a) Chemical structures of two D−A dyes investigated in this work. R1 represents the n-hexyl moiety for C241 and 2-hexyldecyl for C260. (b) Schematic for the state energy diagram and associated multichannel charge transfer reactions characteristic of various rate constants in DSCs with a cobalt electrolyte, following light absorption. The forward processes including photoexcitation (kex), electron injection (kei), and hole injection (khi) are indicated by blue arrows. The competing loss pathways of radiative (kr1) and nonradiative (knr1) deactivations, and electron recombination of photoinjected electrons in titania either with dye cations (kr2) or with cobalt(III) ions (kr3) are marked by red arrows.

film and 10.0 × 10−6 Å−2 for the dye layer. A native silicon oxide layer was not considered in the structural model, as little contrast was present between the SLD of the Si wafer (20.1 × 10−6 Å−2) and that of the native oxide (18.9 × 10−6 Å−2). In the structural model, the thickness and the SLD of each layer were first estimated by the genetic optimization method and further refined by the Levenberg−Marquardt method until minimal χ2 values were obtained. Transient Fluorescence and Absorption Measurements. For the femtosecond fluorescence up-conversion measurements, a mode-locked Ti:sapphire laser (Tsunami, Spectra Physics) pumped by a Nd:YVO4 laser (Millennia Pro sSeries, Spectra Physics) was employed to seed a regenerative amplifier (RGA, Spitfire, Spectra Physics). The output of 3.7 mJ femtosecond pulse (130 fs pulse width) at 800 nm from the amplifier was divided into two parts with a 9/1 beam splitter. The large proportion was sent to an optical parametric amplifier (TOPAS-C, Light Conversion) to generate a 480 nm pump pulse (pulse fluence: 19 μJ cm−2). Both the 720 nm photons emitted from the rotating sample and the minor one of the RGA output employed as the optical gate were focused onto a 0.3 mm thick BBO crystal to generate a sum frequency light in a femtosecond fluorescence up-conversion spectrometer (FOG 100-DA, CDP Corp.). The fwhm of our instrument response function was approximately 180 fs. Femtosecond transient absorption experiments used the same laser source and TOPAS for our fluorescence up-conversion tests. A pump pulse at 560 nm from TOPAS went through a special phaselocked chopper (1 kHz) and was focused on the sample as

3. RESULTS AND DISCUSSION With the aid of XRR measurements, the thickness (dD) of a self-assembled monolayer of dye molecules anchored on a planar titania substrate made via atomic layer deposition (ALD) can be reliably determined owing to the high contrast of X-ray scattering length densities (SLD) for the titania substrate and dye layer.17,20−22 A bilayer model (Figure 2a) was resorted to fit the initial reflectivity profiles of dye-grafted titania films (Figure 2b), giving the dD values of 13.6 and 20.7 Å for C241 and C260, respectively. We noted from our DFT calculations that these two dyes feature the same molecular length (lD) of 24.9 Å along the D−A direction and thus derived the average tilt angles (θD) of C241 and C260 with respect to the substrate normal to be 57 and 34°, respectively. The proposed molecular orientation on the titania surface can be visualized from Figure 2c. The more erective geometry of C260 on titania could arise from the interaction of the branched or bulky 2-hexyldecyl side chain. Moreover, the SLD of a reflective layer is given by the relation, SLD = (reNAρZ)/MR, where re is the Bohr electron radius (2.818 × 10−5 Å), NA is the Avogadro’s number, ρ is the mass density of each layer, Z is the sum of atomic number (i.e., total number of electrons) for the molecule in each layer, and MR is the relative molecular mass for the molecule in each layer.22 Thereby, the surface concentrations (cp) of dyes on the planar ALD titania can be estimated via the relation of cp = (SLDdD)/reNAZ, being 2.6 × 10−10 mol cm−2 for C241 and 1.8 16442

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Figure 3. PL decay traces upon femtosecond laser excitation of (a) alumina and (b) titania films dyed with C241 (blue) and C260 (red). Excitation wavelength, 480 nm; pulse fluence, 19 μJ cm−2; probe wavelength, 720 nm. The solid gray lines are fittings of normalized PL intensity (IPL) via an equation comprising an exponential rise and three-exponential decays convoluted with a Gaussian instrument response function I(t), IPL = A0 − Ar exp(−t/τr) ⊕ I(t) + ∑i 3= 1Ai exp(−t/τi) ⊕ I(t) where A0 is the baseline signal, Ar and Ai denote the amplitudes, and τr and τi are the time constants.

Figure 2. (a) The bilayer model used for XRR to characterize a selfassembled monolayer of dye molecules on titania. dD is the average thickness of a dye layer. (b) Measured (symbols) and simulated (solid lines) XRR curves for the bare and dye-grafted ALD titania films. Each curve is offset by 10−2 with respect to the previous one for clarity. (c) Pictorial representation of the average tilt angle variation induced by the side chain alternation.

Table 1. Rate Constants of Various Charge Transfer Reactions dye

kr1 + knr1 [109 s−1]

C241 C260

32.3 9.0

kei,aa kei,bb [1011 s−1] [1011 s−1] 8.8 3.3

7.9 3.9

kr2 [102 s−1]

khi [103 s−1]

kr3c [s−1]

5.5 2.2

5.0 35.7

574.7 5.0

a kei,a was calculated via the equation of kei,a = 1/⟨τex,tatania⟩ − 1/ ⟨τex,alumina⟩. bkei,b was determined from the target analysis of TA data. c Values at an extracted charge of 2.6 × 10−5 C.

× 10−10 mol cm−2 for C260. In addition, the mass densities in dye layers derived from XRR are 1.52 g cm−3 for C241 and 0.83 g cm−3 for C260. The dye loading amounts (cm) of C241 and C260 on our mesoporous titania films were measured to be 2.7 × 10−8 and 1.5 × 10−8 mol cm−2 μm−1, respectively, the ratio of which is in good agreement with that of cp values, suggesting the grafting of dye molecules on the planar and mesoporous titania substrates should be very similar. The side-chain induced variation of tilt angle as well as dye packing density motivated us to scrutinize their relationships with broad time scale charge transfer dynamics. We first employed the femtosecond fluorescence upconversion technique to compare photoluminescence (PL) transients of these two dyes grafted on mesoporous alumina and titania films, which were permeated with an inert electrolyte of 0.1 M LiTFSI and 0.5 M TBP in acetonitrile. Owing to the absence of energy-offset for electron injection from the electronically excited states of dye molecules to alumina, the PL decays (Figure 3a) could be ascribed to the radiative and nonradiative deactivation of excited states, featuring the excited-state amplitude-averaged lifetimes on alumina (⟨τex,alumina⟩) of 30.9 ps for C241 and 111.6 ps for C260 (Supporting Information Table S1). The rate constants (kr1 + knr1) of excited-state deactivations were thereby obtained from the reciprocals of ⟨τex,alumina⟩, being 32.3 × 109 s−1 for C241 and 9.0 × 109 s−1 for C260 (Table 1). Note that these two dyes in chloroform (10 μM) present the same PL lifetime of 270 ps at the probe wavelength of 720 nm. The almost four times longer lifetime of the C260 excited-state on alumina has suggested a weaker intermolecular interaction for C260 on alumina, which can be ascribed to the branched and insulating

side chains attached to the conjugated backbone and the increased average distance resulting from the lower dye loading amount. The weaker intermolecular interaction between C260 is further supported by a smaller apparent charge diffusion coefficient in the C260 layer than that of C241 (see Supporting Information Figure S2).23−25 Our observation on prolongation of excited state lifetime and reduction of dye loading amount by tethering a branched or bulky side chains to dye molecules can be easily understood by the previous notion on the reduction of dye aggregation.26,27 Moreover, as displayed in Figure 3b, the energy-offset at the dye/titania interface switched on the electron injection channel, and significantly shortened the excited-state amplitude-averaged lifetimes on titania ⟨τex,titania⟩ to 1.1 ps for C241 and to 3.0 ps for C260, generating the respective rate constants (kei,a) of electron injection of 8.8 × 10−11 and 3.3 × 10−11 s−1 (Table 1). The electron injection yields (ϕei) of C241 and C260 on titania were estimated by referring to the relationship ϕei = 1−⟨τex,titania⟩/⟨τex,alumina⟩ to be 96 and 97%, respectively. Note that for the estimated rate and efficiency of electron injection the alumina samples were used as references to simulate the PL dynamics of dyes on titania in case that electron injection could not occur. To directly examine the electron injection dynamics, we further carried out femtosecond transient absorption (TA) measurements (Figure 4a,b) of the preceding dye-coated titania films. Our preliminary analysis has shown that in the visible and near-IR (not shown here) regions, there is a severe absorption 16443

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Figure 4. Temporal absorption spectra of 2.1 μm thick titania films grafted with C241 (panel a) and C260 (panel b) and immersed in an inert electrolyte. Species-associated difference spectra derived from target analysis of raw data for the C241 (panel c) and C260 (panel d) samples. Kinetics traces of different components (S1,h, red; S1, blue; D+, green) for the C241 (panel e) and C260 (panel f) samples. The gray lines in panel e and f represent the instrument response functions. Pump wavelength, 560 nm; pulse fluence, 4.4 μJ cm−2.

dynamics of the electron transfer reaction involving the photoinjected electrons in titania and D+. From the transient absorption traces (Figure 5a) of the aforementioned inert cells, the average time constants (⟨τr2⟩) can be derived through stretched exponential fitting. It is noted that C241 exhibits a ⟨τr2⟩ of 1.8 ms, much shorter than that of 4.5 ms for C260. Thus, the rate constants (kr2) for this recombination process are calculated to be 5.5 × 102 and 2.2 × 102 s−1 for C241 and

overlap of the ground states, excited states, and oxidized states of these two dyes and the injected electrons in titania. This has nearly ruled out the possibility to select a single wavelength in this region to monitor the electron injection dynamics, thereby we have performed the target analysis of the time-resolved data using the kinetic model depicted in Supporting Information Scheme S2.28−31 The evolution of TA spectra could be contributed by three kinetic species represented in the speciesassociated difference spectra of Figure 4c,d, including the hot excited singlet state (S1,h), the thermally equilibrated excited singlet state (S1), and the oxidized state (D+). Their key kinetic parameters are listed in Supporting Information Table S2 and can be employed to nicely reproduce dynamic traces recorded at various wavelengths (see Supporting Information Figures S3 and S4 for details). The kinetic traces of these components are revealed in Figure 4e,f, for C241 and C260, respectively. The time constants of the transition from the ground state (S0) to S1,h are within several femtoseconds and cannot be estimated accurately due to the limitation of our instrument response function. The time constants of S1 formation (τs1) were fitted to be 1.27 ps for C241 and 1.73 ps for C260. The time constant of D+ generation (τD+) was obtained from a two-exponential rise function to be 1.3 ps for C241, twice shorter than that of 2.6 ps for C260. The rate constants of electron injection for C241 and C260 were therefore obtained to be 7.7 × 1011 and 3.8 × 1011 s−1 (Table 1), respectively, from the reciprocals of τD+. Note that the Gibbs free energies are comparable due to the identical redox potentials (Supporting Information Figure S1) of these two dye molecules. In addition, the outer-sphere reorganization energy accounts for the main contribution to the total reorganization energy and is only weakly dependent on the dye structure.32 It is apparent that the faster electron injection from the C241 excited state with respect to C260 is consistent with the spatial orientation between the dye molecules and titania accepting states, as illustrated in Figure 2c, if one does not consider the possible influence of reorganization energy. Nanosecond-to-millisecond transient absorption measurements were further executed to compare the tilt angle related

Figure 5. Absorption transients upon nanosecond pulsed laser excitation of 4.2 μm thick titania films grafted with C241 and C260 and immersed in the inert (panel a) and Co-phen (panel b) electrolyte. Pulse fluence: 21.5 μJ cm−2. Excitation wavelength, 640 nm, C241/inert; 629 nm, C260/inert; 645 nm, C241/Co-phen; 632 nm, C260/Co-phen; probe wavelength, 785 nm. The excitation wavelengths were carefully selected according to a 0.2 optical density of dye-grafted titania films to ascertain a similar excited state distribution in the testing films. The solid gray lines are fittings of normalized absorption (ΔA) via the stretched exponential decay function of ΔA ∝ A0 exp[−(t/τ)α], where A0 is the pre-exponential factor, α is the stretching parameter, and τ is the characteristic time. By use of the gamma function Γ(x), the averaged-times of these charge transfer reactions were derived through ⟨τ⟩ = (τ/α)Γ(1/α). 16444

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electron density in titania. These impacts can be perceived by comparing photocurrent action spectra in Figure 6b and photocurrent density−voltage (j−V) curves in Figure 6c. Lower monochromatic incident photon-to-collected electron conversion efficiencies (IPCEs) for the green and red photons (Figure 6b) were noted for the C241 cell in spite of its larger light-harvesting ability in the visible light region (Supporting Information Figure S5). Because our preceding dynamic spectroscopic measurements have shown that these two dyes have a comparable carrier photogeneration yield, the lower IPCEs measured for C241 must be ascribed to a reduced electron collection yield. The significantly attenuated rates of charge-recombination of photoinjected electrons in titania with cobalt(III) ions by using a branched side chain also gave birth to an 137 mV increase in Voc at an irradiance of 100 mW cm−2 simulated AM 1.5 sunlight from 825 mV for C241 to 962 mV for C260. Taking the Voc and short-circuit photocurrent density (jsc) improvements together, the C260 dye generated an enhanced efficiency from 7.5 to 9.6%.

C260 (Table 1), respectively. It is apparent that the larger tilt angle is responsible for the observed faster backward electron transfer of C241. Moreover, the replacement of the inert electrolyte with an electroactive Co-phen (tris(1,10-phenanthroline)cobalt) electrolyte brought forth a charge transfer pathway of hole injection from D+ to divalent Co-phen and accelerated apparently the transient absorption decays (as depicted in Figure 5b). The mean reaction times of the hole injection (⟨τhi⟩) are 201 μs for C241 and 28 μs for C260, and the corresponding rate constants (khi) of hole injection reaction are 5.0 × 103 for C241 and 35.7 × 103 s−1 for C260 (Table 1), respectively. The much slower hole injection for the C241 sample can be associated with the weaker electronic coupling between the dye oxidized state and the cobalt(II) ion in view that the comparable change of free-energy and reorganization energy. As shown in Figure 6a at a certain extracted charge a DSC made from the C260-grafted titania film has a nearly 2 orders of

4. CONCLUSIONS In summary, we have systematically investigated the impacts of augmenting the side chain of organic D−A dyes on multichannel charge transfer dynamics on a broad time scale from femtosecond to millisecond. The side chain associated tilt angle of dye molecules anchored on the surface of titania was directly measured via X-ray reflectometry, which is identified to be well correlated with carrier photogeneration and recombination dynamics. Our finding on the crucial role of tilt angle related thickness of a self-assembled dye layer rather than the intuition of its packing density in controlling the dynamics of determinant charge recombination channel (i.e., photoinjected electrons in titania with the electron-accepting species in electrolytes) for photovoltage should shed a new light on the further dye design and interface engineering of dye-sensitized solar cells.

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ASSOCIATED CONTENT

* Supporting Information S

Synthetic details and additional data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

Figure 6. (a) Comparison of electron lifetimes at a certain charge extracted from the C241 and C260 grafted titania films in combination with a Co-phen electrolyte. (b) Photocurrent action spectra. (c) j−V curves of the C241 and C260 cells measured under an irradiance of 100 mW cm−2, simulated AM 1.5 sunlight. An antireflection film was adhered to a testing cell during IPCE and j−V measurements. Aperture area of the employed metal mask: 0.160 cm−2.

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Acknowledgements are made to the National 973 Program (2011CBA00702), the National Science Foundation of China (No. 51103146, No. 51203150, No. 21203175, No. 51125015, and No. 91233206), and the National 863 Program (No. 2011AA050521) for financial support.

magnitude enlarged electron lifetime (Table 1) in comparison with C241. It can be inferred that compared to the compact but thin C241 dye layer, the looser but thicker C260 dye layer can more efficiently isolate the photoinjected electrons in titania against the cobalt(III) ions. In addition, a faster recombination between the photoinjected electrons in titania with the oxidized C241 dye could also make a minor contribution to the shorter electron lifetimes in transient photovoltage decay measurements. The attenuated charge transfer dynamics (kr3) conferred by a smaller tilt angle is advantageous for both collection of photoinjected electrons by the titania film and augmentation of

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REFERENCES

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