Electron-Acceptor-Dependent Light Absorption and Charge-Transfer

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Electron-Acceptor Dependent Light Absorption and Charge Transfer Dynamics in N-Annulated Perylene Dye-Sensitized Solar Cells Lin Yang, Yameng Ren, Zhaoyang Yao, Cancan Yan, Wentao Ma, and Peng Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511687e • Publication Date (Web): 19 Dec 2014 Downloaded from http://pubs.acs.org on December 22, 2014

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The Journal of Physical Chemistry

Electron-Acceptor Dependent Light Absorption and Charge Transfer Dynamics in N-Annulated Perylene Dye-Sensitized Solar Cells

Lin Yang,†,‡ Yameng Ren,†,‡ Zhaoyang Yao,†,‡ Cancan Yan,†,‡ Wentao Ma,† and Peng Wang*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese

Academy of Sciences, Changchun, 130022, China ‡

University of Chinese Academy of Sciences, Beijing, 100049, China

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ABSTRACT: In this paper we report two metal-free perylene dyes (C269 and C270) featuring the respective benzothiadiazole-benzoic acid and ethynylbenzothiadiazole-benzoic acid electron-acceptors, in combination with a bis(4-(hexyl)phenyl)amino capped N-annulated perylene (NP) electron-donor. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations have disclosed that the use of ethynylbenzothiadiazole-benzoic acid can lower the lowest unoccupied molecular orbital (LUMO) level, reduce the energy-gap, and attenuate the Stokes shift of a NP dye, which are in good accord with electrochemical and photophysical measurements. When used in sensitized titania solar cells, the C270 dye exhibits reasonable good power conversion efficiency close to 9% at an irradiance of 100 mW cm-2, simulated AM1.5 sunlight. It has also been found that with respect to C269, the C270 dye forms a thinner and looser self-assembled dye-layer on the surface of titania, accounting for shorter electron lifetime and lower open-circuit photovoltage for cells made with it. Our femtosecond transient absorption (fs-TA) measurements have confirmed a positive relationship between driving energy and electron injection rate in spite of the close to unity electron injection yield for both dyes. In addition, target analysis of fs-TA data has unraveled that with respect to C269, more electrons are injected from the relaxed excited states for the C270 dye with a low LUMO level. KEYWORDS: Solar cell, organic dye, perylene, light absorption, charge transfer

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1. INTRODUCTION In the era of seeking for eco-friendly and low-cost technologies for solar-to-electricity conversion, a dye-sensitized solar cell (DSC) made with an interconnected network of titania nanocrystals has aroused tremendous research passions.1 In this nanostructured photovoltaic device, a sensitizing dye chemically grafted on the surface of semiconducting nanocrystals significantly gets command of light absorption and multi-channel charge transfer dynamics, impacting the cell efficiency to a large extent.2 Some ruthenium polypyridine3,4 and zinc porphyrin5,6 complexes have been demonstrated as the best performed sensitizers so far from the viewpoints of stability and efficiency, respectively. Meanwhile, considerable efforts have also been devoted to the design and synthesis of a large number of metal-free organic dyes, due mainly to the abundance of raw materials, the flexibility of molecular design, and the bright color.7−23 Organic conjugated compounds characteristic of a coplanar perylene skeleton have been extensively used as fluorescent lipid probes, light-emitting materials, automotive paints as well as pigments for synthetic fibers and engineering resins, because of their high luminescence yields and large molar absorption coefficients.24,25 Since the earlier work by Ferrere et al.,26 perylene has also been employed as a building block of metal-free photosensitizers for DSCs, however only moderate efficiencies lower than 9% has been attained so far.27−42 By use of a bis(4-(hexyloxy)phenyl)amino capped N-annulated perylene (NP) as an electron-donor, we previously synthesized a donor−acceptor (D−A) dye (C262, Figure 1)43 in combination with the benzothiadiazole-benzoic acid (BTBA)22 electron-acceptor. Our DFT calculation has however illustrated that the highest occupied molecular orbital (HOMO) energy level of −4.84 eV vs vacuum for C262 is too much close to the Fermi-level of −4.80 eV for a tris(2,2′-bipyridine)cobalt (Co-bpy) electrolyte.41 Thereby, in this paper we will remove the two oxygen atoms between hexyl and phenyl units to make its analogue (C269, Figure 1). This new dye shows an improved power conversion efficiency of 8% in comparison with that of 7.3% for C262. Furthermore, we will

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select the bis(4-(hexyl)phenyl)amino functionalized N-annulated perylene (D) as an electron-donor to assess the influence of electron-acceptors (A1, benzothiadiazole-benzoic acid22; A2, ethynylbenzothiadiazole-benzoic acid6) on energy level, light absorption, charge transfer dynamics, and photovoltaic performance, based upon photophysical and electrical measurements as well as theoretical calculations.

2. EXPERIMENTAL SECTION 2.1. Materials. Tetrahydrofuran (THF), acetonitrile (ACN), tert-butanol (TBA), and chloroform (CHCl3) were distilled before use. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide

(EMITFSI),

decamethylferrocene

(DMFC),

ferrocene

(FC),

and

4-tert-butylpyridine (TBP) were purchased from Sigma-Aldrich. The syntheses of C269 and C270 are detailed in the Supporting Information. 2.2. Theoretical Calculations. The Gaussian 09 software package was used for all quantum chemical calculations with a selection of the 6-311G(d, p) basis set. The conductor-like polarized continuum model (C-PCM) was picked for the simulation of solvent effects.44 The ground-state geometries were optimized by virtue of the popular B3LYP exchange-correlation functional.45 The TD-MPW1K hybrid functional,46 which includes 42% of Hartree-Fock exchange, was employed for the vertical electron transition calculations and excited-state geometry optimizations.47 2.3. Device Fabrication and Measurements. A fluorine doped tin oxide (FTO) conducting glass (NSG, Solar) partly screen-printed with a titania bilayer film was used as the negative electrode of DSCs. The bilayer semiconducting film is composed of a 4.2-µm-thick translucent layer of small particles (25 nm) and a 5.0-µm-thick light-scattering layer of large particles (350−450 nm). Its preparation details can be found from a previous paper.48 Dye-loading was performed by immersing a titania film into a solution of 150 µM dye in the

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solvent mixture of ACN/TBA/CHCl3 (v/v/v, 1/1/1) overnight. A 25-µm-thick Surlyn ring was heated at 130 oC to adhere a dyed titania electrode and a gold coated FTO electrode. The internal space of a partly sealed cell was infiltrated with a Co-bpy electrolyte. The electrolyte is composed of 0.25 M Co(bpy)3(TFSI)2, 0.05 M Co(bpy)3(TFSI)3, 0.5 M TBP, and 0.1 M LiTFSI in ACN. Herein bpy and TFSI denote 2,2'-bipyridine and bis(trifluoromethanesulfonyl)imide,

respectively.

The

external

quantum

efficiency

(EQE),

current

density−voltage (J−V), charge extraction (CE),49 and transient photovoltage decay (TPD)50 measurements have been detailed in our previous papers.51,52 2.4. Voltammetric, Electronic Absorption, Light Harvesting Yields ( φlh ), Photoluminescence (PL), and X-Ray Reflectivity (XRR) Measurements. Cyclic voltammogram (CV) of a THF dye solution was recorded on a CHI660C electrochemical workstation in combination with a three-electrode electrolytic cell, which consisted of a glassy carbon working electrode, a platinum gauze counter electrode, and a silver wire quasi-reference electrode. The iR drop was compensated and the potential scan rate was 5 mV s−1. Steady electronic absorption measurements were carried out on an Agilent G1103A spectrometer. We measured the wavelength dependent optical densities (OD) of 8-µm-thick dye-grafted translucent titania films in contact with a Co-bpy electrolyte and then derived φlh with the function: φlh = 1 − 10− OD . Steady PL spectra were recorded with an ICCD camera detector and a cw laser excitation at 490 nm. For spectroscopic measurements of dye molecules grafted onto titania, a dyed mesoporous oxide film deposited on FTO was stuck to a bare FTO, and the internal space was filled with the Co-bpy electrolyte employing the same procedure for DSC fabrication. XRR measurements were performed by using a Bruker D8 discover high-resolution diffractometer with Cu Kα X-ray radiation (λ=1.542 Å). The details on data collection and analysis were described in our pervious paper.53 2.5. Femtosecond Transient Absorption (fs-TA) Measurements. The same setup as outlined in our previous paper53 was employed for the fs-TA experiments. In brief, a mode-locked Ti:sapphire laser (Tsunami, Spectra

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Physics) in conjunction with a regenerative amplifier (RGA, Spitfire, Spectra Physics) was used to produce 3.2 mJ, 130 fs pulses at 800 nm. The main part passed through a 9/1 beam splitter was delivered to an optical parametric amplifier (TOPAS-C, Light Conversion) for the generation of pump pulses. The pump light was focused on a rotating sample after passing through a series of optical components and a phase-locked chopper. The minor portion of the RGA output was passed through an optical delay line and then focused on a sapphire to afford a super-continuum white light. The white light was split into two almost equal beams as the probe and reference lights. The polarization between pump and probe beams on a rotating sample was set at the magic angle. The probe and reference lights transmitted through a sample were detected with multi-channel optical sensors (MS 2022i, CDP Corp.). The ExciPRO software (CDP Corp.) was used for instrument control and data collection. All spectra were first corrected to remove the group velocity dispersion of the white light continuum with the Surface Xplorer software (version 2.3) and further modeled with the Glotaran software.54 3. RESULTS AND DISCUSSION The synthetic routes for the two new NP dyes (C269 and C270) are depicted in Scheme 1. We first prepared an aminated NP (3) in a good yield via a palladium catalyzed Buchwald-Hartwig cross-coupling of NP bromide 1 and bis(4-hexylphenyl)amine (2). Further bromation of 3 with NBS produced key intermediate 4. For the synthesis of C269, 4 underwent a Miyaura boration to form perylene borate ester 5 in a good yield. Suzuki−Miyaura cross-coupling of 5 with 6 yielded dye precursor 7, which was hydrolyzed in the presence of a strong base catalyst and then acidified with a diluted phosphoric acid aqueous solution to afford C269. To construct its analogue C270 with a triple bond inserted between the NP and BTBA units, we cross-coupled 6 with (triisopropylsilyl)acetylene by use of the Sonogashira coupling to prepare intermediate 8, which was then desilicated with tetra-n-butylammonium fluoride to quantitatively afford compound 9. Subsequently, intermediates 4 and 9 were cross-coupled via the Sonogashira coupling to produce dye precursor 10, which was

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converted to C270 by using the same protocol for C269.

CVs (Figure 2a) of C269 and C270 dissolved in anhydrous tetrahydrofuran (THF) were recorded under the atmosphere of nitrogen, to examine the influence of changing the electron acceptor from A1 to A2 upon the HOMO and LUMO energy levels. The derived parameters are compiled in Table 1. It is found that the herein incorporated triple-bond has slightly stabilized the HOMO energy level ( E CV H ) from −4.99 to −5.00 eV, but remarkably stabilized the LUMO energy level ( E CV L ) from −3.17 to −3.29 eV, resulting in a contracted LUMO/HOMO energy gap ( ∆E CV L/H ) of 1.71 eV for C270 in contrast to that of 1.82 eV for C269. Therefore, it is MEAS within expectation to observe a red-shifted maximum absorption wavelength ( λABS,MAX ) of 526 nm for C270 in

comparison with that of 471 nm for C269, as shown in Figure 2b. Moreover, the maximum molar absorption MEAS ) is augmented from 28.5 to 41.4×103 M−1 cm−1 upon the triple-bond incorporation. PL coefficient ( ε ABS,MAX

spectra (Figure 2b) of these two dye solutions were also measured by using a continuous-wave laser excitation at MEAS 490 nm. The maximum PL wavelengths ( λPL,MAX ) are 723 nm for C269 and 765 nm for C270, respectively. The

large Stokes shifts ( ∆v MEAS ) of 7400 cm−1 for C269 and 5900 cm−1 for C270 could be roughly ascribed to the vibrational, solvent, and conformational relaxations of their excited states.55−58

We also resorted to the density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations to gain insight into the influence of electron-acceptor alteration on energy levels, absorption spectra, and PL spectra of dye molecules. As listed in Table 1, theoretical calculations can nicely reproduce the relative alignments of the experimental LUMO and HOMO energy levels of these two dyes, and the red-shifted maximum absorption wavelength of C270 compared to C269. In general, the S0→S1 vertical electronic transitions to LUMO for both dyes are mainly originated from HOMO and HOMO−1, displaying a clear characteristic of intramolecular charge-transfer as perceived by looking at the topologies of molecular orbitals in Figure 3a. Also, the wavefunction overlap of bonding and anti-bonding orbitals for C270 is larger 7



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than that for C269, resulting in an augmented molar absorption coefficient as observed in electronic absorption measurements. Moreover, the HOMO and LUMO energy levels of electron-donor and electron-acceptor segments were also computed to disclose their intrinsic correlations with those of the D−A dyes. It can be interestingly perceived from Figure 3b that the HOMOs of both dyes are in general similar to that of their common perylene electron-donor (D), while the LUMOs of C269 and C270 are inherited from those of their respective electron-acceptors A1 and A2. As listed in Table 1, the experimentally observed relative larger ∆v meas could also be well reproduced by TDDFT calculations.

To understand the impact of electron-acceptor on electron injection dynamics and electron injection yield, we measured the femtosecond transient absorption (fs-TA) spectra (Figures 4a and 4b) of the dye-grafted mesoporous titania films, which were also permeated with a Co-bpy electrolyte. The very dissimilar kinetic traces presented in Figures S22, S23, S25, and S26 in the Supporting Information have suggested that there could be a severe spectroscopic superposition of the ground-state, excited-state, and charge-separated state in the recorded fs-TA spectra. Obviously, we cannot derive electron injection kinetics by monitoring the signal at a selected wavelength in the whole spectral region. At the photochemical functional interface of DSCs with organic D−A dyes, the light excitation promotes the dye molecule via intramolecular charge transfer to a non-equilibrium excited-state, i.e. the Franck-Condon (FC) excited-state. In the subsequent discussion we will call this “hot” state “CT1” owing to its charge transfer character. CT1 may undergo complicate vibrational, solvent, and torsional relaxations to produce an equilibrium excited state.55−58 During the relaxation, it is very much possible that electron injection to titania can occur from multiple excited-states including the unrelaxed and relaxed excited-states. Thereby, we employed a dynamic model of multiple-state electron injections as depicted Figure 4c to carry out target analysis59−61 and identified four key species, including CT1, two relaxed charge transfer excited-states (CT2 and CT3), and the charge-separated state (CS). The time constants of species 8


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evolution derived from target analysis were compiled in Table 2, which can be used to nicely reproduce the difference spectra at a suit of time delays (Figures S21 and S24), and the kinetic traces at a set of wavelengths (Figures S22, S23, S25 and S26). The species-associated difference spectra (SADS) of the key components of CT1, CT2, CT3, and CS are presented in Figures 5a and 5b and their kinetic traces are shown in Figures 5c and 5d. Note that we have also observed multiple-step excited-state relaxations for dye-grafted mesoporous alumina films in the presence or absence of the Co-bpy redox couple. These results will be detailed in our future paper. The C269 dye displays a time constant ( τ 1 ) of 1.1 ps for the evolution from the “hot” CT1 to the “partly relaxed” CT2, very similar to that of 1.2 ps for C270. The time constant ( τ 2 ) for electron injection from the “hot” CT1 is 3.1 ps for C269, smaller than that of 5.8 ps for C270. This could be easily understood in terms of a lowered LUMO energy level for C270. Overall, the averaged time constants ( τ avei ) for the three-exponential generation of CS from CT1, CT2, and CT3 are 30.6 ps for C269 and 41.9 ps for C270. It can be further derived that the percentage of electron injection via the “hot” CT1 state with respect to all excited states is 25% for C269, higher than that of 17% for C270. Moreover, the electron injection percentages for C270 are 33% via CT2 and 46% via CT3, higher than the corresponding values of 29% and 44% for C269. Obviously, more electrons are injected from the relaxed excited state for C270 albeit electron injection yields for both dyes are close to unity.

As presented in Figure 6a, EQEs of cells made with a bilayer dye-grafted titania film were plotted against the wavelengths of incident monochromatic lights. The cell fabrication details can be found in the Experimental Section. The EQE curves of both C269 and C270 show a maximum of ~88%. With respect to C269, there is a ~35 nm red-shifting of the onset wavelength of photocurrent response for C270, which is in rough accord with

φlh shown in Figure 6b although we used a bilayer titania film for DSCs and a transparent layer titania film for φlh measurement. The J−V curves of cells made with these two dyes were also tested at an irradiance of 100 9



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mW cm−2, simulated AM1.5 sunlight (Figure 6c). The extracted cell parameters are tabulated in Table 3. The C269 dye has a short-circuit photocurrent density ( J SC ) of 13.40 mA cm−2, an open-circuit photovoltage ( VOC )

of 813 mV, and a fill factor (FF) of 0.738, affording a power conversion efficiency (PCE) of 8.0%. In good accord with the prediction from photocurrent action spectra, the C270 dye with a smaller energy-gap has a higher J SC of 15.53 mA cm−2, generating an improved PCE of 8.8% albeit a 16 mV decreased VOC of 797 mV. We also recorded J−V curves at a set of light irradiances and plotted VOC versus J SC in Figure 6d. It can be derived from the fitting curves that at a certain JSC, there is an about 25 mV higher VOC for C269 in comparison with C270. Apparently there is also a good correlation between dark current shown in Figure 6c and VOC . To examine its interfacial energetic and dynamic origins of VOC variation, we further carried out CE and TPD measurements. As illustrated in Figure 7a, the dye alternation from C269 to C270 does not affect the conduction-band edge of titania with respect the redox electrolyte. It can be actually perceived from Figure 7b that at a given density of photo-injected electrons in titania, the half lifetime ( t1/2TPD ) for the C270 cell is shorter than the C269 counterpart, accounting for its reduced VOC . The self-assembled dye layer lying between the electron-transporting titania and hole-transporting electrolyte can be considered as a blocking layer to attenuate the electron-hole recombination by impacting the electron coupling matrix element. Thereby, it could be highly necessary to obtain a basic understanding on microstructure of dye layer on the surface of titania. As a well-established technique, XRR has been widely applied for a quantitative evaluation of the thickness, electron density, and roughness of an ultrathin layer.62 We therefore employed it to assess the microstructure parameters of self-assembled dye layers on the surface of planer titania, which was made via atomic layer deposition (ALD) on single-crystalline silicon (110). The ALD titania substrate used here has been verified to possess an atomically smooth surface by recording atom force microscopy images. In addition, it is known that its X-ray scattering length density ( SLD titania ) is significantly higher than that of an organic dye-layer. 10


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By fitting the initial reflectivity profiles (Figure 7c) of dye-grafted planar titania with a bilayer model to afford some key structural parameter (Table4), we have noticed that the dye alteration from C269 to C270 brings forth a decreased dye-layer thicknesses ( d D ) from 2.0 to 1.8 nm, a attenuated dye-layer X-ray scattering length density ( SLDD ) from 4.8×10−6 to 4.1×10−6 Å−1, and a reduced dye-layer mass density ( ρ D ) from 0.52 to 0.45 g cm−3. The surface concentration (cp) of dye molecules on the ALD titania is 7.3×10−11 mol cm−2 for C270, smaller than that of 9.8×10−11 mol cm−2 for C269. The dye loading amounts (cm) on the mesoporous titania film for DSCs were also examined with visible spectrometry, being 2.4×10−8 mol cm−2 µm−1 for C269 and 2.1×10−8 mol cm−2 µm−1 for C270. Its ratio is in general similar to that of cp. Our comparison presented here has implied a similar self-assembling of dye molecules on the planar and mesoporous titania substrates. Overall, the formation of a thinner and looser self-assembled dye-layer on the surface of titania by C270, albeit its larger molecular length, could have played a vital role on a shortened t1/2TPD and thereby a lower VOC .

4. CONCLUSIONS

To sum up, we have employed benzothiadiazole-benzoic acid and ethynylbenzothiadiazole-benzoic acid as the electron-acceptors to tune the energy-gaps of bisarylamino functionalized N-annulated perylene dyes, which present reasonable good power conversion efficiencies of 8−9% in dye-sensitized solar cells. Both theoretical calculations and experimental measurements have proved that the replacement of benzothiadiazole with ethynylbenzothiadiazole-benzoic acid can lower the LUMO energy level of a donor−acceptor dye to reduce the energy-gap for an enhanced light absorption, possibly without a decrease of the maximum of EQEs. Our femtosecond transient absorption measurements have however shown that the reduction of driving energy does result in a slower electron injection dynamics, and that more electrons are injected from the relaxed excited states. Thereby in order to further lower the LUMO level for a 11



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significantly improved light absorption, it will be important to design dye molecules with a long lifetime of the thermally relaxed excited state. In addition, it is still a big challenge to control and probe the microstructure of self-assembled dye layers on the surface of titania, which is believed to have a close relationship with charge recombination dynamics and thereby the final photovoltage output.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; Telephone: 0086-431-85262952 Notes

The authors declare no competing financial interest. ACKNOWLEDGMENTS

Acknowledgements are made to the National 973 Program (2011CBA00702 and 2015CB932204) and the National Science Foundation of China (No. 51103146 and No. 91233206) for financial support. SUPPORTING INFORMATION AVAILABLE

Synthetic details, NMR data, optimized ground-state and excited-state geometries, geometry parameters, Mulliken atomic charges, and additional spectroscopic data. This information is available free of charge via the Internet at http://pubs.acs.org.

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Berlin, 2009.

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Page 21 of 33

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


C262

O

C269

C270

N

N

O N

N

N

N

N N

S S

N

N

N

S N COOH

C OOH C OOH

Figure 1. Molecular structures of N-annulated perylene dyes C269 and C270, characteristic of the benzothiadiazole-benzoic acid and

ethynylbenzothiadiazole-benzoic acid electron-acceptors. The previously reported C262 dye with two more oxygen atoms compared to C269

is also included.

21



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Page 22 of 33

Scheme 1. Synthetic Routes for the N-Annulated Perylene Dyes C269 and C270 a N

S

N

Br

COOC 4H9 C 6H 13

C 6H13

6 iv C6H13

C6H13

C6H13

C6H13 C 8H17 S N N

C8H17 S N N

N

v

N

N

COOC4H9

COOH

C8H17 N

O B O

N

C6H13

C6H13

N

C6H13

C 6H13

C269

7

5

C8H17

iii

N Br

C6H 13

C6H13

C8H17 N

i

1

C8H17 N

ii

N

C6H13

C6H13

N

Br

C 6H 13 C6H13

C6H13

3

4

C6H13

C6H13

NH

C8H17 N

viii

S

N

N

C 6H 13

C 6H13

C6H 13 N

COOC 4H 9

C8H17 N

N

ix

S

N

N

COOH

2 C6H 13

C6H13

10 N

S

N

vi

Br

COOC4H9

6

a

N

S

N

Si

vii

N

S

COOC4H9

COOC4H9

8

C270

N

9

Reagents and conditions: (i) NaOt-Bu, Pd2(dba)3, XantPhos, toluene, reflux, 5 h; (ii) NBS, THF, 0

4,4,4′,4′,5,5,5′,5′-octamethyl-2,2′-bi(1,3,2-dioxaborolane), Pd(dppf)Cl2, KOAc, toluene, 80

o

C, 30 min; (iii)

o

C, 2 h; (iv) Na2CO3, Pd(PPh3)4,

ethanol/benzene/H2O (v/v/v, 1/4/2), reflux, 12 h; (v) KOH, THF/H2O (v/v, 3/1), reflux, 8 h; (vi) (triisopropylsilyl)acetylene, Pd2(dba)3,

P(t-Bu)3, Cs2CO3, dioxane, reflux, 5 h; (vii) tetra-n-butylammonium fluoride, CH2Cl2, RT, 3 h; (viii) Pd2(dba)3, P(t-Bu)3, Cs2CO3, dioxane,

reflux, 5 h; (ix) KOH, THF, reflux, 24 h. Note that herein C4H9, C6H13 and C8H17 denote n-butyl, n-hexyl, and n-octyl, respectively.

22


Page 23 of 33

(a)

C269 C270

i [a.u.]

DMFC

-2.0

-1.5

-1.0

-0.5

E [V] vs Fc/Fc+

0.0

4

0.5

−1

Wavenumber [10 cm ] 2.5

2.0

1.5

(b)

C270

450

600

750

PL [nm]

C269

ABS [nm]

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


900

Wavelength [nm]

Figure 2. (a) CVs of C269 and C270 in THF. EMITFSI was used as the supporting electrolyte and DMFC was added as the internal

reference. All potentials are further calibrated with Fc as the reference. (b) Electronic absorption and PL spectra of dye solutions in THF.

23



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Page 24 of 33

Table 1. Measured and Calculated Energy Levels, Electronic Absorption, and Photoluminescence Properties

E CV L

E B3LYP L

E CV H

E B3LYP H

MEAS λABS,MAX

TD-MPW1K λABS,MAX

MEAS ε ABS,MAX

MEAS λPL,MAX

TD-MPW1K λPL,MAX

[eV] a) [eV] b)

[eV] a)

[eV] b)

[nm] c)

[nm]

[×103 M−1 cm−1] c)

[nm] c)

[nm] d)

C269 −3.17 −2.76

−4.99

−4.98

471

494

28.5

723

663

7.4

5.1

C270 −3.29 −2.96

−5.00

−5.00

526

538

41.4

765

678

5.9

3.9

∆ν MEAS

∆ν TD-MPW1K

Dye

a)

[103 cm−1] c) [103 cm−1] d)

Measured molecular orbital energies ( E CV and E CV L H ) vs vacuum are calculated via Ε = −4.88 − eEonset , where Eonset is the onset

potential (Figure 2) of oxidation and reduction of the dye ground state measured with CV for a dye in THF. H and L represent HOMO and

LUMO, respectively.

b)

Theoretical molecular orbital energies ( E B3LYP and E B3LYP ) vs vacuum are calculated at the B3LYP/6-311G(d,p) L H

level for a dye in THF.;

c)

MEAS MEAS Measured maximum absorption wavelength ( λABS,MAX ), maximum molar absorption coefficient ( ε ABS,MAX ),

MEAS maximum PL wavelength ( λPL,MAX ), and Stokes shift ( ∆ν MEAS ) are derived from electronic absorption and PL spectra (Figure 2) for a dye in

TD-MPW1K TD-MPW1K THF. d) Theoretical maximum absorption wavelength ( λABS,MAX ), maximum PL wavelength ( λPL,MAX ), and Stokes shift ( ∆ν TD-MPW1K ) are

calculated at the TD-MPW1K/6-311G(d,p) level for a dye in THF.

24


Page 25 of 33

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


Figure 3. (a) The optimized ground-state geometry of a dye molecule in THF and the topologies of its molecular orbitals associated with the

S0→S1 transition. The isodensity surface values are fixed at 0.03. (b) The LUMO energy level (value above a color bar), and HOMO energy

level (value under a color bar), and energy gap (Eg) of a dye molecule and its electron donor and acceptor segments. To improve the

computational efficiency, the large aliphatic substituents were replaced with ethyl.

25



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

Figure 4. (a, b) fs-TA spectra of 2.1-µm-thick, mesoporous titania films grafted with C269 (panel a) and C270 (panel b), which are also in

contact with a Co-bpy electrolyte for DSCs. The pulse fluence of pump light at 490 nm is 40 uJ cm−2. (c) A dynamic model used in target

analysis of fs-TA spectra of dye-grafted titania films. The “hot” CT1 state is optically generated and can relax via torsional motions to form

the CT2 intermediate state and the CT3 equilibrium state.

26


Page 26 of 33

SADS

(a) 20

CT1 CT2 CT3 CS

15 10 5 0

700

800

900

1000

1100

λ [nm]

SADS

(b)

30

CT1 CT2 CT3 CS

20 10 0 700

800

900

1000

1100

1200

λ [nm] (c) 1.0

Population

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


0.5 0.0 CT1 CT2 CT3

-0.5 -1.0 0

1

2

CS GS IRF

10

100

t [ps] (d) 1.0

Population

Page 27 of 33

0.5 0.0 CT1 CT2 CT3

-0.5 -1.0 0

2

CS GS IRF

4

10

100

t [ps]

Figure 5. (a, b) Species-associated difference spectra (SADS) of CT1, CT2, CT3, and CS for the C269 (panel a) and C270 (panel b) samples,

which are generated via target analysis of the spectra in panels a and b by using the model in panel Figure 4c. (c, d) Kinetic traces generated

by target analysis, for GS (black), CT1 (blue), CT2 (green), CT3 (red), and CS (magenta) of the C269 (panel c) and C270 (panel d) samples.

The grey lines in panels c and d represent the instrument response functions (IRF).

27



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Page 28 of 33

Table 2. Time Constants Derived from Target Analysis of fs-TA Spectra of Dye-Grafted Titania Films Permeated with a Co-bpy

Electrolyte

Dye

τ1 [ps]

τ 2 [ps]

τ 3 [ps]

τ 4 [ps]

τ 5 [ps]

τ 6 [ps]

τ avei [ps]

C269

1.1

3.1

11.0

17.5

1043.7

58.7

30.6

C270

1.2

5.8

15.9

24.5

803.4

74.8

41.9

28


(a) 100 EQE [%]

80 60

C269 C270

40 20 0 400

500

600

700

λ [nm]

(b) 100 Φ [%] lh

80 60

C269

40

C270

20 0 400

500

600

700

λ [nm]

(c) −2

J [mA cm ]

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


15 10

C269/light C269/dark

5

C270/light C270/dark

0 -5 0.0

0.2

0.4

0.6

0.8

V [V]

(d) C269

0.8

VOC [V]

Page 29 of 33

C270

0.7 1

JSC [mA cm

−2

]

10

Figure 6. (a) External quantum efficiency (EQE) plotted against λ for cells made with a dye-grafted bilayer (4.2+5.0-µm thick) titania films

in combination with a Co-bpy electrolyte. (b) Light-harvesting yields ( φlh ) plotted versus wavelength (λ ) for the 4.2-µm-thick, dye-grafted

mesoporous titania films immersed in a Co-bpy electrolyte for DSC fabrication. (c) Current−voltage (J−V) characteristics of cells measured

under an irradiance of 100 mW cm−2, simulated AM1.5 sunlight and in the dark. (d) Plots of open-circuit photovoltage against short-circuit

photocurrent density. The linear fitting lines are also included. An antireflection film is adhered to a testing cell during measurements. The

aperture area of the employed metal mask is 0.160 cm−2.

29



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Page 30 of 33

Table 3. Averaged Photovoltaic Parameters of Four DSCs Measured at an Irradiance of 100 mW cm-2, Simulated AM1.5 Sunlighta

a

Dye

EQE [mA cm−2] J SC

J SC [mA cm−2]

VOC [mV]

FF

PCE [%]

C269

13.46±0.03

13.40±0.04

813±0.004

0.738±0.003

8.0±0.2

C270

15.77±0.03

15.53±0.03

797±0.003

0.715±0.002

8.8±0.2

J scEQE is derived via wavelength integration of the product of the standard AM1.5 emission spectrum (ASTM G173-03) and the EQEs

measured at the short-circuit. The validness of measured photovoltaic parameters is evaluated by comparing the calculated J scEQE with the

experimentally measured J sc .

30


Page 31 of 33

VOC [V]

(a) 1.0 0.8 C269

0.6

C270

10

20 30 CE Q [µC]

40 50 60

C269 10

1

10

0

C270

TPD

t 1/2 [ms]

(b)

10

20 30 CE Q [µC]

40 50 60

(c) 10-3 -5

10

R

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


bare

-7

10

C269

-9

10 10

C270

-11

0.1

0.2

0.3

0.4

0.5

−1

QZ/Å

Figure 7. (a) Plots of charge stored in a dye-grafted titania film ( Q CE ) measured with charge extraction (CE) method as a function of

TPD open-circuit photovoltage ( VOC ). (b) Comparison of electron half-lifetime ( t1/2 ) measured with the small-pulse transient photovoltage decay

(TPD) method against Q CE . (c) Measured (symbols) and simulated (solid lines) XRR curves for the bare and dye-grafted titania films. R is

the reflectivity and Qz is the perpendicular momentum transfer. Each curve is offset by 10−2 with respect to the previous one for clarity.

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Page 32 of 33

Table 4. Refined Structural Data of Self-Assembled Dye-Layers on the Surface of ALD Titania and the Amounts of Dye Molecules

Loaded onto a Mesoporous Titania Filma

a

Dye

d D [nm]

SLD D [10−6 Å−2]

ρD [g cm−3]

cp [10−11 mol cm−2]

cm [10−8 mol cm−2 µm−1]

C269

2.0

4.8

0.52

9.8

2.4

C270

1.8

4.1

0.45

7.3

2.1

The thickness ( d D ) and the X-ray scattering length density ( SLD D ) of a dye-layer on the surface of ALD titania are derived via fitting the

XRR data. The dye-layer mass density ( ρ D ) is calculated by the relation, ρ D = (SLD D M R ) / re N A Z , where re is the Bohr electron radius

(2.818×10−5 Å), N A is the Avogadro’s number, Z is the sum of atomic number (i.e. total number of electrons) for the dye molecule, and

M R is the relative molecular mass for the dye molecule. cp is the surface concentration of dye molecules on the planar ALD titania film

and can be calculated by the relation, cp = (SLD D d D ) / re N A Z . cm is the loading amount of dye molecules on the mesoporous titania film.

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TOC

33


Electron-Acceptor-Dependent Light Absorption and Charge-Transfer

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