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Enhanced Internal Quantum Efficiency in Dye-sensitized Solar Cells: Effect of Long-lived Charge-separated State of Sensitizers Haiya Sun, Dongzhi Liu, Tianyang Wang, Ting Lu, Wei Li, Siyao Ren, Wenping Hu, Lichang Wang, and Xueqin Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14993 • Publication Date (Web): 03 Mar 2017 Downloaded from http://pubs.acs.org on March 5, 2017

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ACS Applied Materials & Interfaces

Enhanced Internal Quantum Efficiency in Dyesensitized Solar Cells: Effect of Long-lived Chargeseparated State of Sensitizers Haiya Sun,†, ‡Dongzhi Liu,†, ‡, ǁTianyang Wang,†, ‡, §Ting Lu,†, ‡Wei Li,†, ‡, ǁSiyao Ren,†, ‡Wenping Hu,‡, ┴Lichang Wang,†, ‡, §and Xueqin Zhou†, ‡, ǁ,* †School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China; ‡Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072, China; §Department of Chemistry and Biochemistry and the Materials Technology Center, Southern Illinois University, Carbondale, IL 62901, United States; ǁTianjin Engineering Research Center of Functional Fine Chemicals, Tianjin, 300072, China; ┴School of Science, Tianjin University, Tianjin 300072, China

ABSTRACT：：Effective charge separation is one of the key determinants for the photovoltaic performance of the dye-sensitized solar cells (DSSCs). Herein, two charge-separated (CS) sensitizers, MTPA-Pyc and YD-Pyc, have been synthesized and applied in DSSCs to investigate the effect of the CS states of the sensitizers on the device’s efficiency. The CS states with lifetimes of 64 ns and 177 ns for MTPA-Pyc and YD-Pyc, respectively, are formed via the photo-induced electron transfer (PET) from the 4-styryltriphenylamine (MTPA) or 4-

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styrylindoline (YD) donor to the pyrimidine cyanoacrylic acid (Pyc) acceptor. DSSCs based on MTPA-Pyc and YD-Pyc exhibit high internal quantum efficiency (IQE) values of over 80% from 400 nm to 600 nm. In comparison, the IQEs of the charge transfer (CT) sensitizer cells are 10-30% lower in the same wavelength range. The enhanced IQE values in the devices based on the CS sensitizers are ascribed to the higher electron injection efficiencies and slower charge recombination. The results demonstrate that taking advantage of the CS states in the sensitizers can be a promising strategy to improve the IQEs and further enhance the overall efficiencies of the DSSCs.

KEYWORDS: charge-separated (CS) sensitizers, photo-induced electron transfer (PET), internal quantum efficiency (IQE), electron injection, charge recombination

Introduction Natural photosynthesis, which is the most efficient process of converting sunlight into electrical or chemical energy, is attractive to researchers focused on energy conversion and utilization. In a natural photosynthetic system, long-lived charge-separated (CS) states are formed via multistep electron transfers upon light absorption and are believed to be the key for the subsequent chemical reactions.1-3 Inspired by this, a great deal of research effort has been dedicated to designing electron donor-acceptor (D-A) systems in which long-lived CS states were attained through photo-induced single or multistep electron transfer.4-21 In some of the artificial photosynthetic complexes, CS states have reached the lifetimes from 380 ms to 2 h that are comparable or even longer than those in natural photosynthetic systems.5,7,12,21 However, when the long-lived charge separation organic compounds were applied into solar cells, the overall power conversion efficiencies (PCEs) were found to be far from satisfactory.16-20,22-24 As


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such, it remains unclear how the CS states of the organic molecules affect the photovoltaic properties of their solar cells and how to improve the device performances by taking advantage of the long-lived CS states. The major types of the organic solar cells in which charge separation molecules are applied are the single-layer solar cells (SLSC) and heterojunction solar cells. Several molecular dyads or triads in which fullerenes were covalently linked to the conjugated electron donor architectures have been assembled into SLSCs.18,25-27 Though the CS state lifetimes of these molecules were found to be around 1 µs, the cell efficiencies of the devices were modest, ranging from 0.02% to 0.37%. Recently, relatively good PCEs of 9.22% and 3.5% were reported with inverted bulk heterojunction (BHJ) solar cells in which CS thiophene polymers28 or fullerene dyads29 were employed as the donor or acceptor component, respectively. However, the PCEs of these BHJ solar cells closely depended on the structures and manufacturing processes of the devices which varied dramatically from each other, thus making it difficult to analyze the superiority of the CS states in the devices. Dye-sensitized solar cell (DSSC) is one of the most promising varieties of organic solar cells as many great strides have been achieved since Gratzel’s pioneering work in 1991.30-39 The charge separation of the photo-induced excitons at the dye/metal oxide interface is one of the key factors that determines the PCEs of the DSSCs.31,40,41 Dyes with D-A structure have been revealed to be beneficial in improving the interfacial charge separation via intramolecular charge transfer (ICT) effects.42 ICT effects are also found to be able to reduce the energy band-gap and extend the light absorption range to longer wavelengths. As such, DSSCs based on various D-A sensitizers have been extensively developed among which the highest efficiency of 14% was reported.36 Taking into account the importance of charge separation, Fukuzumi and co-workers


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introduced dye molecules or supramolecules with long-lived CS states into DSSCs.17,19,22,23,43 Though the performance of the DSSCs was revealed to be higher than those fabricated with dyes showing no detection of CS states, the best efficiency among these DSSCs was found to be only 2.1%. This was attributed to the inefficient electron injection from the dye molecules or supramolecules to the metal oxides due to absence of effective bonding to the SnO2 thin film of the cathodes. Therefore, the effect of the CS states was negligible in evaluating the overall performance of the DSSCs. The discussions above suggest that using dye molecules with long-lived CS states as well as anchoring groups as the sensitizers are expected to promote charge separation and electron injection in the DSSCs, thus leading to better photovoltaic performances. We have previously designed a series of D-A systems in which multistep electron transfer resulted in long-lived CS states.44-46 Improved photovoltaic performances have been realized in SLSCs when the lifetimes of the CS states were elongated.44 Unfortunately, the efficiencies were still quite moderate due to the limitation of device structures. We herein report two dyes with long-lived CS states and an anchoring group which allows them to bond to theTiO2 nanoparticles. DSSCs were fabricated employing these dyes as sensitizers and their photovoltaic performances were investigated in detail and correlated to the CS states. To distinguish these sensitizers having long-lived CS states and active groups from other sensitizers, we call them CS sensitizers. Structures of the two CS sensitizers,

(E)-2-Cyano-3-(2-((E)-4-(2-(4-N,N-bis(4-

methylphenyl)aminophenyl)vinyl)phenylamino)pyrimidin-5-yl)-acrylic acid (MTPA-Pyc) and (E)-2-cyano-3-(2-((E)-4-(2-(4-(4-methylphenyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7yl)vinyl)phenylamino)pyrimidin-5-yl)acrylic acid (YD-Pyc) were presented in Figure 1. Four reference dyes: (E)-1-phenyl-2-(4-N,N-bis(4-methylphenyl)aminophenyl)ethene (MTPA)47,48,


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(E)-1-phenyl-2-(4-(4-methylphenyl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indole-7-yl)ethene (YD)49, (E)-2-Cyano-3-((E)-4-(2-(4-N,N-bis(4-methylphenyl)aminophenyl)vinyl)phenyl)acrylic acid

(MTPAcc)50

and

(E)-2-Cyano-3-((E)-4-(2-(4-(4-methylphenyl)-1,2,3,3a,4,8b-

hexahydrocyclopenta[b]indol-7-yl)vinyl)phenyl)acrylic acid (YDcc) were also listed in Figure 1. MTPA and YD with strong electron-donating triphenylamine44,48 or indoline51-53 play the role of the donor in the two CS sensitizers. MTPAcc and YDcc containing 2-cyanoacrylic acid as the anchoring group showed charge transfer (CT) character but no long-lived CS state was detected upon photo-excitation in our nanosecond transient absorption measurement. They are referred to as CT sensitizers in this paper corresponding to CS sensitizers. The synthetic procedures of these dyes were given in Scheme S1 of the Supporting Information. The CS sensitizers were revealed to be capable of promoting the electron injection efficiency while retarding the charge recombination. Further analysis of the photovoltaic properties revealed that the internal quantum efficiencies (IQEs) of the DSSC devices based on MTPA-Pyc and MIND-Pyc were enhanced in comparison to those of the CT sensitizers. Finally, the effects and advantages of CS sensitizers for the DSSCs were concluded.


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Figure 1. Chemical structures of the CS sensitizers and the reference CT sensitizers. Results and Discussion The synthetic routes of the CS and CT sensitizers were listed in Scheme S1 (Supporting Information). The synthetic and characterization details of the sensitizers were given in the Supporting Information. The CS sensitizers were acquired via a four-step synthetic procedure including the Wittig reaction, reduction, nucleophilic substitution and Knoevenagel condensation reaction. Wittig reaction, Heck reaction and Knoevenagel condensation were conducted to obtain the reference CT sensitizers. Spectral and Energy Level Characters: CS Sensitizers vs CT Sensitizers Figure 2 gives the UV-vis absorption spectra of six dyes in toluene. The absorption maxima (λabsmax) of the main absorption band are slightly red-shifted from 370 nm for MTPA to 380 nm for MTPA-Pyc and from 374 nm for YD to 388 nm for YD-Pyc. However, significant red-shifts of the maximum absorption band were found in the two CT sensitizers: the λabsmax of MTPAcc and YDcc are 463 nm and 465 nm, respectively. Moreover, changes of the light absorption


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intensities are also different between the CS sensitizers and CT sensitizers with regard to their corresponding donors. For the CS sensitizers, increased molar extinct coefficients (ε) were observed: from 30700 M -1 cm-1 for MTPA to 33200 M -1 cm-1 for MTPA-Pyc and from 28400 M -1

cm-1 for YD to 31200 M -1 cm-1 for YD-Pyc, respectively. On the contrary, MTPAcc and YDcc

show slightly reduced ε of 29200 M -1 cm-1 and 27200 M -1 cm-1, respectively. Similar results concerning the differences between the absorption properties of the CS sensitizers and CT sensitizers were also observed in dichloromethane (Figure S1, Supporting Information).

Figure 2. Uv-vis absorption spectra of the triphenylamine sensitizers (a) and indoline sensitizers (b) in toluene (5 × 10-6 M).


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The results above reveal apparent differences between the effects of photo-induced electron transfer (PET) in CS sensitizers and photo-induced charge transfer (PCT) in CT sensitizers on the absorption spectra. The absorption maxima of MTPA and YD are derived from the transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) which has been discovered with PCT character from the triphenylamine or indole to the styrene module.54 The minor red-shift of the λabsmax of MTPA-Pyc and YD-Pyc suggests that the corresponding transitions of these CS sensitizers are similar to those of the donor modules MTPA and YD.44,46 This is further confirmed by the calculation results shown in Figure 3 in which the transition energy of the CS sensitizers corresponding to the major absorption band of the calculated absorption spectra is in good accordance with the HOMO-LUMO+1 energy gap. However, both experimental and computational results reveal that the longest absorption band of MTPAcc and YDcc are due to the HOMO-LUMO transition which shows an enhanced PCT character with the electron distribution on the LUMO extended to the 2-cyanoacrylic acid group. Therefore, it is concluded that the strong conjugation and electron withdrawing effect of the 2cyanoacrylic acid group lead to a huge red-shift in the absorption spectra of MTPAcc and YDcc, while the introduction of the pyrimidine acceptor induces a weak conjugation and thus only contributes a limited red-shift in the absorption spectra of the CS sensitizers. The computational data are listed in Table S1 (Supporting Information).


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Figure 3. Optimized geometrical structures, calculated molecular orbitals and energies in dichloromethane at the B3LYP/6-31+G(d,p) level (a), calculated absorption spectra of triphenylamine (b) and indoline (c) sensitizers in dichloromethane at the ωB97XD/6-311G (d, P) level. Corresponding data are given in Table S1. The carbon, hydrogen, oxygen and nitrogen atoms are represented by the gray, white, red and blue balls, respectively. The curve and the vertical line in the calculated absorption spectra represent the molar extinct coefficient (ε) and the oscillator strength, respectively.


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Figure 4 plots the fluorescence emission spectra of six dyes excited at their λabsmax. According to the emission maxima (λemmax), the Stokes shifts of MTPA-Pyc (75 nm) and YD-Pyc (34 nm) were found to be close to those of their corresponding donors MTPA (51 nm) and YD (48 nm). In contrast, much larger Stokes shifts were observed in MTPAcc (127 nm) and YDcc (111 nm) which should be attributed to the extended conjugation and stronger PCT feature.55-57 On the other hand, drastic differences between the fluorescence intensities of the CS and CT sensitizers were also observed. The fluorescence quantum yields (Φf) of the dyes were calculated using MTPA (Φf = 0.750 in dichloromethane) as the reference.47 The fluorescence intensities of the CS sensitizers are substantially quenched with the estimated quenching ratio being 99.4% for MTPA-Pyc (Φf = 0.004) and 99.5% for YD-Pyc (Φf = 0.003), respectively. This dramatic fluorescence quenching in the CS sensitizers is resulted from the PET which is further confirmed in the detection of the CS states. In comparison, the fluorescence intensities of the CT sensitizers are not intensely reduced, with Φf of 0.458 for MTPAcc and 0.477 for YDcc, respectively.


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Figure 4. Fluorescence emission spectra of the triphenylamine (a) and indoline sensitizers (b) in toluene (5 × 10-6 M). To explore the difference between the energy levels of CS and CT sensitizers, cyclic voltammetry (CV) studies were conducted in 2×10-4 M dichloromethane solution. The CV curves are shown in Figure S2 and the electrochemical data are listed in Table S2 in Supporting Information. MTPA-Pyc, YD-Pyc, MTPAcc and YDcc all possess a quasi-reversible oxidation wave and an irreversible reduction wave. The oxidation waves are attributed to the donor module (MTPA or YD) because their potentials agree well with the oxidation potential of MTPA (0.60 V vs Ag/Ag+) or YD (0.24 V vs Ag/Ag+). For MTPA-Pyc and YD-Pyc, the reduction waves are


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attributed to the Pyc moduledue to their well-matched reduction potentials (Figure S2, Supporting Information). For MTPAcc and YDcc, the reduction waves are attributed to the 3phenyl-2-cyanoacrylic acid module. The energy levels of the frontier molecular orbitals of the dyes were determined from the oxidation potentials (Eox) and reduction potentials (Ered). The HOMO energy levels of MTPA-Pyc and YD-Pyc are -5.56 eV and -5.21 eV, which are similar to those of their corresponding CT sensitizers MTPAcc (-5.53 eV) and YDcc (-5.25 eV). The LUMO levels are nearly identical for MTPA-Pyc (-3.69 eV) and YD-Pyc (-3.66 eV), respectively, due to their same acceptor structure. Meanwhile, the LUMO energy levels of the CT sensitizers were found to be -3.61 eV for MTPAcc and -3.54 eV for YDcc, respectively. The results suggest that the electrochemically determined energy levels of the four dyes remain similar in spite of the difference between the electron transfer and charge transfer process. However, the CV curves of MTPA-Pyc and YD-Pyc at 1-10 CV circles exhibit fewer changes than those of MTPAcc and YDcc (Figure S3), indicative of higher electrochemical stability the CS sensitizers possess in comparison with the CT sensitizers. Notably for the CS sensitizers, the optical energy band-gaps (Egop) (2.47 eV for MTPA-Pyc and 2.28 eV for YD-Pyc) obtained from the onset of the absorption spectra are apparently larger than the electrochemical energy band-gaps (EgCV) (1.87 eV for MTPA-Pyc and 1.55 eV for YDPyc) which corresponds to the difference between the first oxidation potential and the first reduction potential in the CV curves. Moreover, the major absorption peak of MTPA-Pyc and YD-Pyc is at a similar wavelength to that of MTPA and YD, respectively. This further confirms our calculated results that the Egop of MTPA-Pyc and YD-Pyc is derived from the HOMOLUMO+1 transition. Hence, EgCV which reveals the band gap between HOMO and LUMO differs from Egop for MTPA-Pyc and YD-Pyc. As illustrated in Figure 5, the difference between


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the Egop and EgCV in the CS sensitizers also reflects the PET from LUMO+1 to LUMO.44 In comparison, the Egop are in accordance with the EgCV in the CT sensitizers, indicative of their typical PCT feature.58

Figure 5. Energy level schematics comparing the Egop and EgCV of MTPA-Pyc and YD-Pyc. The green and red arrows indicate the electron transition and the electron transfers process, respectively. Detection of Long-lived Charge-separated (CS) States To explore the formation dynamics of the CS states, a time-correlated single photon counting (TCSPC) measurement was carried out in toluene and the fluorescence decay pathways of the dyes were analyzed. The emission decay curves are shown in Figure 6 and the fluorescence lifetimes (τf), fluorescence quantum yields (Φf), radiative (kf) and non-radiative (knr) decay rate constants are listed in Table 1. Bi-exponential decays of the fluorescence were observed for MTPA-Pyc and YD-Pyc. According to the determined energy levels, the fast decay is attributed to the electron transfer from the LUMO+1 on the donor to the LUMO on the acceptor. Therefore,


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it is estimated that 97.1% of the excited state in MTPA-Pyc is converted into the CS state with the rate of 4.76 ×109 s-1. Meanwhile, the formation rate of the CS state in YD-Pyc is 5.26 ×109 s1

from 96.3% of the excited state. In addition, the calculated non-radiative decay rate constant

(knr) is two magnitudes larger than the radiative decay rate constant (kf) of these CS sensitizers (Table 2). Thus, it is reasonable to conclude that the fast intramolecular electron transfer is also responsible for the fluorescence quenching in the CS sensitizers. The slow fluorescence decay of these two dyes, which is ascribed to the solvation relaxation of the excited states, is only of an insignificant proportion within the overall decay.


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Figure 6. Emission decay curves of the triphenylamine (a) and indoline (b) sensitizers in toluene (5 × 10-6 M).


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Table 1. Fluorescence lifetimes, fluorescence quantum yields and the rate constants of the sensitizers and the donor molecules in toluene (5 × 10-6 M).

Dyes MTPA

τfa (ns) 1.80

Φf

kfb

knrb

(s-1)

(s-1)

0.690 3.83×108 1.72×108

MTPA-Pyc 0.21 (97.1%), 1.68 (2.9%) 0.004 1.58×107 3.94×109 MTPAcc

2.60

0.458 1.76×108 2.08×108

YD

1.26

0.713 5.66×108 2.28×108

YD-Pyc YDcc

0.19 (96.3%), 2.15 (3.7%) 0.003 1.14×107 3.80×109 2.25

0.477 2.16×108 2.28×108

a

The fluorescence lifetimes were measured at the emission maxima upon excited at 457 nm for MTPAcc and YDcc, and 366 nm for the rest of the dyes; bThe radiative (kf) and nonradiative (knr) decay rate constants for the excited state of the dyes are estimated using the observed Φf and τf values and applying the following relations: kf = Φf/τf; knr = 1/τf - kf in which τf of the CS sensitizers are the average values of the two decay lifetimes.59,60 The lifetimes of the CS states of the dyes in toluene were detected using the nanosecond transient absorption measurement. Nanosecond transient absorption spectra and the kinetics at 600 nm of MTPA-Pyc and YD-Pyc are demonstrated in Figure 7. A peak around 330 nm and a broad absorption band from 500 nm to 700 nm were observed upon excited at 410 nm. The 330 nm signal are attributed to the radical anion absorption of pyrimidine unit61,62 and the broad peaks at longer wavelengths belong to the absorption of the triphenylamine44 or indoline46 radical cations. These absorption signals confirm the formation of the CS state in MTPA-Pyc and YD-Pyc. Since the signal at 330 nm is affected by the ground state bleach, the decay kinetics of the radical cations at 600 nm was employed to reveal the lifetimes of the CS states. The decays of the signals follow the first-order fitting and the lifetimes of the CS states are estimated as 64 ns for MTPA·+-Pyc·- and 177 ns for YD·+-Pyc·-, respectively. However, transient absorption signals with the lifetimes of longer than 10 ns (the response limit of the instrument)


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were not detected in MTPAcc and YDcc, agreeing well with their mono-exponential fluorescence decay (Figure 6 and Table 1).

Figure 7. Nanosecond transient absorption spectra of MTPA-Pyc (a) and YD-Pyc (b) when excited at 410 nm. The bottom figures are the decay kinetics of the radical cations in MTPA-Pyc (c) and YD-Pyc (d) probed at 600 nm. The red solid lines are the fitting curves by the singleorder exponential decay equation. Effect of CS Sensitizers on Internal Quantum Efficiency (IQE) Solar cells were fabricated by adsorbing dyes on the surface of the TiO2 thin films and assembling the devices with volatile iodine electrolyte. The internal quantum efficiency (IQE) of the devices was investigated and compared between the CS sensitizers and CT sensitizers. The


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IQE value is determined by the quantum yield of electron injection into the TiO2 conduction band (φinj) and the charge collection efficiency (ηcoll) as expressed in Equation (1):63

IQE = φinj ηcoll (1) Therefore, IQE is an important property to examine the influence of the CS states on the interfacial charge transfer efficiency. As established in previous reports,37,64,65 the actual IQE values of these dyes were calculated through dividing the incident photon-to-current conversion efficiencies (IPCE) by the light-harvesting efficiencies (LHE) according to Equation (2). =

(2)

The IPCE spectra of the DSSCs based on the four sensitizers are presented in Figure 8. The highest IPCE value of MTPA-Pyc reaches 79% at 405 nm and exceeds 70% from 390 nm to 470 nm. In comparison, the IPCE spectrum of MTPAcc stays beneath that of MTPA-Pyc before 470 nm and shows a narrow peak of 79% around 490 nm. Similar results were also observed on the films sensitized by the indoline dyes with the IPCE maxima found at 435 nm for YD-Pyc (83%) and 505 nm for YDcc (82%), respectively.


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Figure 8. Incident photon-to-current conversion efficiency (IPCE) curves for DSSCs based on the triphenylamine (a) and indoline sensitizers (b). The LHE of the 11-µm double layer TiO2 films (8-µm transparent layer and 3-µm scattering layer) sensitized by the CS and CT sensitizers were obtained and shown in Figure 9. The LHE values for MTPAcc and YDcc remain close to 100% in the wavelength range of 350 nm to 550 nm due to their red-shifted absorption band. For MTPA-Pyc and YD-Pyc, however, the peak values of LHE begin to drop from 450 nm and stay lower than those of their reference dyes in the longer wavelength range. This suggests that MTPA-Pyc and YD-Pyc sensitized photoanodes are comparatively less effective in capturing the visible light.


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Figure 9. Light harvesting efficiency (LHE) as a function of wavelength for the triphenylamine (a) and indoline sensitizers (b) adsorbed on 11-µm double layer TiO2 films. The IQE values as a function of the wavelength are shown in Figure 10. For the DSSCs based on MTPA-Pyc and YD-Pyc, the IQEs stay between 80% and 90% from 400 nm to 600 nm. In the same wavelength range, however, the IQE values of MTPAcc and YDcc are 10% to 30% lower than those of the CS sensitizers. The enhanced IQEs of the MTPA-Pyc and YD-Pyc based devices indicate that the long-lived charge separation has led to superior electron injection or charge collection efficiencies in the DSSCs.


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Figure 10. Internal quantum efficiency (IQE) as a function of wavelength for DSSCs based on the triphenylamine (a) and indoline sensitizers (b). Electron Injection and Charge Recombination The electron injection efficiencies (φinj) and charge collection efficiencies (ηcoll) of the DSSCs are discussed to further unravel the effect of the charge separation on the enhanced IQE values in MTPA-Pyc and YD-Pyc. The φinj was estimated from the fluorescence lifetimes of the dyes adsorbed on the TiO2 film surface (τTiO2) and the lifetimes of the excited states (MTPAcc and


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YDcc) or the CS states (MTPA-Pyc and YD-Pyc) in solution (τsolution) according to Equation (3):66

φ = 1-

τTiO2 solution

(3)

The τsolution of the dyes have been obtained from the transient fluorescence or transient absorption measurements discussed above. The τTiO2 were measured by the TCSPC technique and the results are summarized in Table 2. The τTiO2 of the four sensitizers are close to each other, ranging from 0.21 ns to 0.38 ns. The calculated φinj for the CT sensitizers MTPAcc and YDcc is 90.0% and 90.7%, respectively. As for the CS sensitizers, the estimated φinj reach almost unity due to their long-lived CS states in comparison to the much shorter lived excitons in the CT sensitizers. The improvement of the φinj for the MTPA-Pyc and YD-Pyc is in good accordance with the computational results, which demonstrated higher electron densities on the cyanoacrylic acid part of the CS sensitizers than those of the CT sensitizers due to the good spatial separation of the HOMO and LUMO orbitals. Even taking into consideration the energy losses during the formation of the CS states (2.9% for MTPA-Pyc and 3.7% for YD-Pyc, according to the proportion of the solvation relaxation among the overall decay), the actual φinj from the excited states to the TiO2 conduction band still reaches 96.5% and 96.1% for MTPA-Pyc and YD-Pyc, respectively.


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Table 2. Fluorescence lifetimes of the sensitizers on 3-µm TiO2 film, lifetimes of the excited states or the CS states in toluene (5×10-6 M) and the calculated electron injection efficiencies. τTiO2

τsolution

φinjb

(ns)

(ns)

(%)

MTPA-Pyc

0.31

64a

99.5 (96.5)c

MTPAcc

0.26

2.60

90.0

YD-Pyc

0.38

177a

99.7 (96.1)c

YDcc

0.21

2.25

90.7

Dyes

a

The τsolution of MTPA-Pyc and YD-Pyc are obtained from the transient absorption measurement; bThe electron injection efficiencies are calculated according to the equation: φinj = 1 - τsolution/τTiO2; cThe data in the parentheses are the estimated electron injection efficiencies of MTPA-Pyc and YD-Pyc when the energy losses during the formation of the CS states are taken into consideration. Nanosecond laser photolysis experiments for the sensitized TiO2 films were also conducted without the electrolyte and the decay kinetics at 620 nm are shown in Figure 11. All the dyes display an absorption band of the radical cation from 550 nm to 700 nm (Figure S4, Supporting Information). It should be noted that the radical cations of the sensitizers could only be reduced by the injected electrons from the semiconductor in the absence of the electrolyte. Therefore, the lifetimes of the cations reflect the rate of charge recombination. This process is expected to be slow so that efficient charge collection as well as dye regeneration can be ensured. As shown in Figure 11, the signal of the MTPA·+ in MTPA-Pyc lasts for 23.1 µs after the electron injection. In comparison, the recombination of the radical cation in MTPAcc with the injected electron is more than twice as fast with a lifetime of 9.5 µs. This unfavorable fast recombination may compete with the electron transport through the TiO2 thin layer to the back electrode and therefore should be one of the causes for the lower IQE of MTPAcc. A similar situation was observed in the YD-Pyc and YDcc with the lifetimes of the YD·+ being 27.7 µs in the former and


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12.7 µs in the latter. The longer lifetimes of the YD·+ compared to those of the MTPA·+ is ascribed to the stronger electron donating ability of the indoline moiety than the triphenylamine unit.

Figure 11. Decay kinetics of the radical cations in MTPA-Pyc (a), YD-Pyc (b) and their corresponding CT sensitizers MTPAcc (c) and YDcc (d) adsorbed at the surface of the 3-µm TiO2 films probed at 620 nm. The red solid lines are the fitting curves by the single-order exponential decay equation. In summary, the improved φinj and the elongated interfacial charge separation lifetimes of the devices based on MTPA-Pyc and YD-Pyc meet our expectation that the CS sensitizers are beneficial for optimizing the conversion of the absorbed photons into electrons, thus providing the premises for realizing good cell performances. Overall Efficiencies of DSSC


24

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The photocurrent-voltage (J-V) characteristics of the cells were measured under the condition of AM 1.5 illumination (100 mW cm-2) and the J-V curves of all four dyes are shown in Figure 12. The open-circuit photovoltage (VOC), short-circuit photocurrent density (JSC), fill factor (FF) and overall energy conversion efficiencies (η) are summarized in Table 3. The DSSCs based on MTPA-Pyc and YD-Pyc exhibit overall efficiencies of 5.04% and 5.78%, respectively, which are comparable to or better than the performance of the MTPAcc and YDcc based devices. Since all of the cells were assembled with the same TiO2 photo-anode and electrolyte, the VOC and FF showed no significant differences among the devices. Therefore, the JSC is the determinant of the overall efficiency. JSC is derived by integrating the IPCE spectra and can be calculated by the Equation 4:37,63,67 = λ λ dλ = !λ "#$% &'()) λ dλ

(4)

in which Is is the incident photon flux, e is the element charge. The lower LHE after 450 nm of the MTPA-Pyc and YD-Pyc is due to the insufficient absorptions in the longer wavelengths of the CS sensitizers in comparison to the CT sensitizers. However, the φinj and ηcoll of the CS sensitizers were higher than those of their corresponding CT sensitizers. The combined effect of these factors resulted in higher (YD-Pyc) or comparable (MTPA-Pyc) JSC values and enhanced overall efficiencies of the CS sensitizers. This result also reveals that IQE plays a more significant role than the absorption ability in DSSCs. The higher JSC and η of MTPA-Pyc than (E)-2-cyano-3-(4-(di-p-tolylamino)phenyl)acrylic acid of which the absorption maxima (415 nm) is obviously longer than that of MTPA-Pyc also confirm this point.68 In addition, the two CS sensitizers have similar IQEs but the JSC of YD-Pyc is far higher than that of MTPA-Pyc, indicating that the JSC is closely dependent on their absorption range. The 10 nm red-shift of the absorption maxima from 388 nm for MTPA-Pyc to 398 nm for YD-Pyc on the TiO2 photo-


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anode (Figure S5, Supporting Information) brings about a noticeably improved JSC in the DSSC based on YD-Pyc than that of MTPA-Pyc.

Figure 12. Photocurrent density-voltage (J-V) curves for DSSCs based on the triphenylamine (a) and indoline sensitizers (b) under AM 1.5 illumination.


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Table 3. Photovoltaic performances of DSSCs based on the CS and CT sensitizersa. JSC

VOC

(mA cm-2)

(mV)

MTPA-Pyc

9.72± 0.11

721± 3

0.72± 0.01

5.04± 0.07

MTPAcc

9.88± 0.14

687±5

0.73± 0.01

4.97± 0.09

YD-Pyc

10.81± 0.09

723± 2

0.74± 0.01

5.78± 0.06

YDcc

10.46± 0.16

708±4

0.71± 0.01

5.26±0.11

Dyes

a

FF

η (%)

Based on five measurements

3. Conclusion In summary, two CS sensitizers, MTPA-Pyc and YD-Pyc, were synthesized and characterized to reveal the effects of the CS states on the photovoltaic performances of the DSSCs. Steady state spectra show insignificant red-shift of the absorption peak and drastic quenching of the fluorescence intensity in MTPA-Pyc and YD-Pyc in regard to the donor modules MTPA and YD, which is indicative of the PET process in the CS sensitizers. On the other hand, the CT sensitizers show large red-shift of the absorption band while the fluorescence emission remains intense. Electrochemical results reveal the difference between the EgCV and Egop in the CS sensitizers which is favorable for intramolecular PET and charge separation. CS states in MTPA-Pyc and YD-Pyc are formed with the time of around 200 ps and the lifetimes of the CS states are 64 ns for MTPA·+-Pyc·- and 177 ns for YD·+-Pyc·-, respectively. In the CT sensitizers MTPAcc and YDcc, however, CS states in nanosecond timescale are absent. DSSCs based on MTPA-Pyc and YD-Pyc exhibit high IQE values of over 80% in a wide wavelength range from 400 nm to 600 nm, which are 10%-30% higher than those of the MTPAcc and YDcc devices. The enhanced IQE values in the DSSCs based on the CS sensitizers are ascribed to the higher electron injection efficiencies and slower charge recombination, which are largely derived from


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the CS states of the sensitizers. This work reveals that the CS sensitizers are capable of enhancing the IQEs of the DSSCs, thus benefiting the overall PCEs. However, only a slightly broadened absorption range is found in CS sensitizers due to the limited contribution of the acceptor to the conjugation of the donor. DSSCs of more competitive overall efficiencies can be expected for the CS sensitizers with extended absorption range in the visible light. This work is currently in process in our laboratory. 4. Experimental Section Synthesis. The detailed synthetic procedures and characterization data of the sensitizers and their corresponding donor and acceptor modules were given in the Supporting Information. Commercially available starting materials were used as received. All solvents were in reagent grade and were purified by standard methods. Structural and Optical Characterization: 1H NMR spectra (500 MHz) and

13

C NMR (125.7

MHz) spectra were obtained on a VARIAN INOVA spectrometer and the testing temperature was 25°C. High resolution ESI mass spectra (HRMS-ESI) were obtained on a Bruker mior OTOF-Q II Mass Spectrometer. Elemental analysis was performed on a Vario MICRO CHNOS elemental analyzer (Elementar Analysen systeme, Hanau, Germany). The absorption spectra were taken on a Thermo Spectronic, Helios Gamma spectrometer. For dye solutions, quartz cells with a path length of 1 cm were utilized for the UV-vis measurement. Uv-vis absorption spectra of the solid thin films were obtained by soaking the 11 µm TiO2 layer into the 3×10-4 M dye solution of dried THF. The fluorescence spectra were recorded on a Varian CARY ECLIPSE Fluorospectrophotometer. Electrochemistry Measurements. The electrochemical properties were measured by a CHI 600E Electrochemical Workstation utilizing a three-electrode configuration with a glassy carbon


28

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electrode as the working electrode, Ag/AgNO3 electrode as the reference electrode and platinum as

the

auxiliary

electrode.

The

three-electrode

system

was

calibrated

using

a

ferrocene/ferrocenium (Fc/Fc+) redox couple as the external standard prior to the measurements. The scan rate was 50 mV s-1. The dichloromethane containing 0.1 M tetra-butyl ammonium hexafluorophosphate (TBAPF6) was employed as the medium for the cyclic voltammetric determination. The concentration of the dyes in dichloromethane was 2×10-4 M. Computational Details. The calculations were performed using the Gaussian 09 package.69 The geometry optimization and frequency calculations were performed using the density functional theory (DFT) method with Becke’s three-parameter hybrid functional and Lee–Yang– Parr’s gradient-corrected correlation functional (B3LYP) at 6-31+G(d,p) level. All geometry optimizations were done with the SCF, gradient, and energy convergence of 10-8, 10-6, and 10-6 a. u., respectively. While B3LYP is a good choice for geometry optimizations, it failed to predict the absorption spectra of organic molecules with CS and CT characters, in which long range correction to the exchange functional is needed and is presented in ωB97XD functional. As such, we chose to use ωB97XD/6-311G(d,p) to calculate absorption spectra. All calculations were carried out in dichloromethane and the polarizable continuum model (PCM) was used for evaluating solvent effects. Time-correlated Single Photon Counting (TC-SPC). Excitation of the samples was done with picosecond diode lasers (Horiba Jobin Yvon Instruments) at 366 nm (1.2 ns pulses) or 457nm (1.2 ns pulses) and the time resolution was ~ 150 ps. The laser's pulse energy was ca. 15 pJ and attenuated (often more than an order of magnitude) to the desired count rate of ca. 1% or less of the excitation frequency. A cooled (ca. -40°C) Hamamatsu MCP photomultiplier R3809U 51 was used for detection of single photons, and the signal passed through a discriminator (Ortec


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9307) and into a TAC (Ortec 566, 100 ns range used). The electrical trigger signal from the laser was also passed through a discriminator (Tennelec TC454) and on to the TAC (Ortec 566). The TAC output was read by a DAQ-1 MCA computer card using 1024 channels and collected with Horiba Jobin Yvon Data Station 2.5. Measurements were done in reverse mode at 5 MHz and under magic angle polarisation. A cut-off filter, GG400 (Excitation at 366nm), was used to block stray excitation light. A dilute solution of Ludox was used to record the instrument response function without any filter for solution measurements. No monochromator was used, i.e. all wavelengths transmitted by the cut-off filter were collected. The dye concentrations were 5×10-6 M and the solutions were bubbled with nitrogen for 30 min before the measurements.

Nanosecond Transient Absorption Spectroscopy. Nanosecond transient absorption measurements were performed on a LP-920 Laser Flash Photolysis Setup (Edinburgh). The dye solvent samples in toluene with the concentration of 1×10-5 M were excited at 410 nm by a computer-controlled Nd:YAG laser/OPO system from Opotek (Vibrant 355 II) (operating at 10 Hz was directed to the sample at the excitation wavelength). The laser and analyzing light beam passed perpendicularly through a 1 cm quartz cell. The solvent samples were bubbled with nitrogen for 30 min before the flash photolysis experiments. Nanosecond transient absorption measurements for the photo-anode were applied to dyesensitized transparent TiO2 mesoporous films (3-µm thick, 20 nm particle diameter) deposited on normal glass. The samples, which were kept at a 45° angle to the laser beam, were excited at 410 nm. The laser fluence was kept around 30 µJ cm-2 per pulse to ensure that less than one electron was injected per TiO2 nanoparticle on pulsed irradiation on average.


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The complete transient absorption spectra were obtained using a gated CCD camera (AndoriSTAR); the kinetic traces were detected by a Tektronix TDS 3012B oscilloscope and a R928P photomultiplier and analyzed by Edinburgh analytical software (LP920). DSSC Device Fabrication. The double layer nanocrystalline TiO2 photo-anode used in this study is fabricated by screen printing. Indium Tin Oxide (ITO, 1.1 mm thick, 10 ohms sq-1, Nippon Sheet Glass, Japan) conducting glass electrode was washed with soapy water and ultrasonicated in acetone and isopropanol for 30 min, respectively. After a drying period, the electrodes were submerged in a 40 mM aqueous solution of TiCl4 for 30 min at 75°C, and then washed by water and ethanol. On the electrodes, an 8-µm thick nanocrystalline TiO2 layer (20 nm particle diameter) and 3-µm thick TiO2 light scattering layer (400 nm particle diameter, PST400C) were deposited onto the glass by screen-printing method. The TiO2 electrodes were heated at 500°C for 30 min, followed by treating with a 40 mM aqueous solution of TiCl4 for 30 min at 75°C and subsequent sintering at 500°C for 30 min. The thickness of TiO2 films was measured by a profiler, Sloan, Dektak 3. The TiO2 photo-anodes were immersed in the 2 × 10-4 M dye solutions in anhydrous THF for 16 h at room temperature. The dyed electrodes were then rinsed with ethanol to remove excess dye molecules. This sensitized photo-anode and a platinum counter electrode was sealed together after the electrolyte solution was then introduced. The electrolyte contains 0.68 M dimethyl imidiazolium iodide, 0.05 M iodine, 0.10 M LiI, 0.05 M guanidinium thiocyanate, and 0.40 M tert-butyl pyridine in the mixture of acetonitrile and valeronitrile (85:15, v/v). Photovoltaic Characterization. The photocurrent-voltage (J-V) characteristics were recorded at room temperature using a computer-controlled Keithley 2400 source meter under air mass (AM) 1.5 simulated illumination (100 mW cm-2, Oriel, 67005). The action spectra of


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monochromatic incident photon-to-current conversion efficiency (IPCE) for solar cells were performed using a commercial setup (PV-25 DYE, JASCO). A 300 W Xenon lamp was employed as light source for generation of a monochromatic beam. Calibrations were performed with a standard silicon photodiode.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Detailed synthetic methods and characterizations of the sensitizers and the corresponding intermediates, UV-vis absorption in dichloromethane and on TiO2 thin films, cyclic voltammograms, transient absorption spectra of the sensitizers adsorbed on TiO2 thin films, calculated absorption and transition and electrochemical data of the sensitizers were found there. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Funding Sources National Nature Science Foundation of China (Grant No. 21576195 and 21506151)and Tianjin Science and Technology Support Program (No. 14TXGCCX001). ACKNOWLEDGMENT This work was supported by the National Nature Science Foundation of China (Grant No. 21576195 and 21506151) and Tianjin Science and Technology Support Program (No.


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14TXGCCX001). Lichang Wang acknowledges the support by the Tianjin 1000 talent program for her stay at Tianjin University. REFERENCES (1) McConnell, I.; Li, G.; Brudvig, G. W. Energy Conversion in Natural and Artificial Photosynthesis. Chem. Biol. 2010, 17, 434-447. (2) Cowan, A. J.; Durrant, J. R. Long-lived Charge Separated States in Nanostructured Semiconductor Photoelectrodes for the Production of Solar Fuels. Chem. Soc. Rev. 2013, 42, 2281-2293. (3) Kodis, G.; Liddell, P. A.; de la Garza, L.; Clausen, P. C.; Lindsey, J. S.; Moore, A. L.; Moore, T. A.; Gust, D. Efficient Energy Transfer and Electron Transfer in an Artificial Photosynthetic Antenna-reaction Center Complex. J. Phys. Chem. A 2002, 106, 2036-2048. (4) Kashiwagi, Y.; Ohkubo, K.; McDonald, J. A.; Blake, I. M.; Crossley, M. J.; Araki, Y.; Ito, O.; Imahori, H.; Fukuzumi, S. Long-lived Charge-separated State Produced by Photoinduced Electron Transfer in a Zinc Imidazoporphyrin-C60 Dyad. Org. Lett. 2003, 5, 27192721. (5) Guldi, D. M.; Imahori, H.; Tamaki, K.; Kashiwagi, Y.; Yamada, H.; Sakata, Y.; Fukuzumi, S. A Molecular Tetrad Allowing Efficient Energy Storage for 1.6 s at 163 K. J. Phys. Chem. A 2004, 108, 541-548. (6) Fukuzumi, S.; Saito, K.; Ohkubo, K.; Khoury, T.; Kashiwagi, Y.; Absalom, M. A.; Gadde, S.; D'Souza, F.; Araki, Y.; Ito, O. Multiple Photosynthetic Reaction Centres Composed of Supramolecular Assemblies of Zinc Porphyrin Dendrimers with a Fullerene Acceptor. Chem. Commun. 2011, 47, 7980-7982. (7) Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Y.; Luo, C.; Sakata, Y.; Fukuzumi, S. Charge Separation in a Novel Artificial Photosynthetic Reaction Center Lives 380 ms. J. Am. Chem. Soc. 2001, 123, 6617-6628. (8) Kawashima, Y.; Ohkubo, K.; Kentaro, M.; Fukuzumi, S. Electron Transfer in a Supramolecular Complex of Zinc Chlorin Carboxylate Anion with Li+@C60 Affording the Long-Lived Charge-Separated State. J. Phys. Chem. C 2013, 117, 21166-21177. (9) Kelber, J. B.; Panjwani, N. A.; Wu, D.; Gomez-Bombarelli, R.; Lovett, B. W.; Morton, J. J. L.; Anderson, H. L. Synthesis and Investigation of Donor-porphyrin-acceptor Triads with Long-lived Photo-induced Charge-separate States. Chem. Sci. 2015, 6, 6468-6481. (10) Kamimura, T.; Ohkubo, K.; Kawashima, Y.; Nobukuni, H.; Naruta, Y.; Tani, F.; Fukuzumi, S. Submillisecond-lived Photoinduced Charge Separation in Inclusion Complexes Composed of Li+@C60 and Cyclic Porphyrin Dimers. Chem. Sci. 2013, 4, 1451-1461. (11) Bill, N. L.; Ishida, M.; Kawashima, Y.; Ohkubo, K.; Sung, Y. M.; Lynch, V. M.; Lim, J. M.; Kim, D.; Sessler, J. L.; Fukuzumi, S. Long-lived Charge-separated States Produced in Supramolecular Complexes Between Anionic and Cationic Porphyrins. Chem. Sci. 2014, 5, 3888-3896. (12) Fukuzumi, S.; Kotani, H.; Ohkubo, K.; Ogo, S.; Tkachenko, N. V.; Lemmetyinen, H. Electron-Transfer State of 9-Mesityl-10-methylacridinium Ion with a Much Longer Lifetime and Higher Energy Than That of the Natural Photosynthetic Reaction Center. J. Am. Chem. Soc. 2004, 126, 1600-1601.


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Graphic Abstract:


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Enhanced Internal Quantum Efficiency in Dye ... - ACS Publications

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