Understanding the Role of Electron Donor in ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Understanding the Role of Electron Donor in Truxene Dye Sensitized Solar Cells with Cobalt Electrolytes Panpan Dai,† Huanhuan Dong,† Mao Liang,*,†,‡ Hua Cheng,† Zhe Sun,† and Song Xue† †

Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, Department of Applied Chemistry, Tianjin University of Technology, No. 391 Binshui Xidao, Xiqing District Tianjin 300384, P. R. China ‡ Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Collaborative Innovation Center of Chemical Science and Engineering, College of Chemistry, Nankai University, No. 94 Weijin Road, Nankai District Tianjin 300071, P. R. China S Supporting Information *

ABSTRACT: To understand the role of electron donor in organic dye sensitized solar cells employing cobalt electrolytes, two new organic dyes (M41 and M42) and a reference dye (M29) have been synthesized. We investigated the effects of the electron-withdrawing/electron-rich group in the electron donor on the photophysical and electrochemical properties of these dyes. In addition, the electron transfer kinetics of devices were compared by using two cobalt electrolytes (Co(II/III)phen and Co(II/III)-Clphen) with different oxidation potentials. Regardless of the electrolyte selection, M42 shows a higher efficiency in comparison with those of M41 and M29. M42 in combination with the Co(II/III)-phen redox couples yielded an efficiency of 9.69% under the standard light illumination (AM 1.5G, 100 mW cm−2). We also evaluate the influence of light harvesting capacity, regeneration force of oxidized dyes, and charge recombination on the performance of studied DSSCs. This study suggests that, given the good capacity of retarding charge recombination, light harvesting capacity is the critical factor behind the photocurrent as well as the efficiency for organic dyes, but not the regeneration force of oxidized dyes. KEYWORDS: Organic dyes, Electron donor, Light harvesting capacity, Photovoltaic performance

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toward cobalt electrolytes.5−12 In view of electron transfer kinetics characteristics, the cobalt electrolyte is different from the traditional iodine electrolyte. So, the design of organic dyes also depends on the electrolyte used.46,47 Carefully tuning the molecular energy levels of organic dyes is beneficial to maximizing the advantages of cobalt cells. Ensuring an efficient regeneration of the oxidized sensitizers in DSSCs is one of the aims of energy tuning. The reduction rate of the sensitizer cations must be at least 2 orders of magnitude faster than the recombination rate, ensuring a sufficient driving force.48 However, there is a trade-off between dye regeneration and absorption. How can the dye regeneration and sunlight harvesting capacity of dyes be balanced, and which is the key factor for cobalt cells? Such an interesting question deserves better understanding. Besides the energy matching and light harvesting improvement, charge recombination between TiO2 electrons and redox oxidized dyes/electrolytes must be controlled in cobalt cells. Overall, understanding the balance

INTRODUCTION Energy conversion and storage are central to harnessing the limitless power of the sun.1 Because of high electronic power conversion efficiencies (PCEs) and more cost-efficiency in production, dye sensitized solar cells (DSSCs) are attractive functional devices for the use of solar energy. During the past two decades, PCEs in such cells have been substantially increased, mainly due to improvements of exploring new photosensitizers.2 Photosensitizers such as ruthenium(II) complexes,2,3 porphyrin-based dyes,4 and metal-free organic dyes5−44 have been investigated systematically. For a long time, the PCE of DSSCs sensitized with metal-free organic dyes were behind those of ruthenium(II) complexes and porphyrin-based dyes, partly due to the lack of absorption at long wavelength. Nevertheless, Kakiage et al. demonstrated an impressive PCE of 14.3% based on organic dyes sensitized DSSCs very recently.45 This report indicates the great potential of metal-free organic dyes in DSSCs application. Donor, spacer, and acceptor are three basic parts of organic dyes. In the past decade, considerable efforts have been made on the exploration of various donors. Representative arylamine donors include triphenylamine, triarylamine, indoline, and carbazole; they are widely utilized to construct new sensitizers © 2016 American Chemical Society

Received: April 6, 2016 Revised: September 28, 2016 Published: October 24, 2016 97

DOI: 10.1021/acssuschemeng.6b00706 ACS Sustainable Chem. Eng. 2017, 5, 97−104

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ACS Sustainable Chemistry & Engineering

Figure 1. Chemical structures of M41, M42, and M29. SI. A combination of Buchwald−Hartwig coupling and Knoevenagel condensation has been employed in the synthesis of M41 and M42. M29 was prepared according to our previous publication.33

between the dye regeneration, the light absorption, and controlling of the charge recombination is necessary. For this purpose, in this contribution, M41 and M42 (Figure 1) were synthesized. For comparison, M29 was introduced as the reference dye. In addition, two cobalt electrolytes (Co(II/ III)-phen and Co(II/III)-Clphen) with different oxidation potentials have been applied here to change the regeneration force of oxidized dyes in cobalt cells, which will enable us to better understand the topic of this work.

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RESULTS AND DISCUSSION Optical Properties and DFT Calculations. The optical properties of M41, M42, and M29 were studied in dichloromethane solution (Figure 2) with data summarized in Table 1.

EXPERIMENTAL SECTION

Materials and Instruments. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), t-BuONa, XPhos, Pd2(dba)3, and 4-tert-butylpyridine (TBP) were purchased from JKChemical (China). Some commonly used solvents, such as dichloromethane (DCM) and N,N-dimethylformamide (DMF), was dried and distilled. TiO2 colloid was prepared according to the literature method.49 Two types of electrolytes were used for devices fabricating, i.e. Co(II/III)-phen and Co(II/III)-Clphen. The composition of Co(II/ III)-phen electrolyte: 0.25 M [Co(II)(phen)3](PF6)2, 0.05 M [Co(III)(phen)3](PF6)3, 0.5 M TBP, and 0.1 M LiTFSI in acetonitrile. The composition of Co(II/III)-Clphen electrolyte: 0.25 M [Co(II)(Cl-phen)3](PF6)2, 0.05 M [Co(III)(Cl-phen)3](PF6)3, 0.5 M TBP, and 0.1 M LiTFSI in acetonitrile. [Co(II)(phen)3](PF6)2, [Co(III)(phen)3](PF6)3, [Co(II)(Cl-phen)3](PF6)2, and [Co(III)(Cl-phen)3](PF6)3 were synthesized according to the literature method.50 Also, the redox properties of the two redox couples are obtained from the same literature protocol. A Bruker AM-300 or AM-400 spectrometer was used to record the 1 H NMR and 13C NMR spectra of new compounds. The Micromass GCT-TOF mass spectrometer gives the high resolution mass spectra. Finally, we used the RY-1 melting point apparatus (Tianfen, China) to measure the melting points of the samples. The details of optical/electrochemical measurements, cell fabrication, and photovoltaic characterization are included in the SI. Synthesis of Dyes. The synthetic routes (Scheme S1) and procedures to the M41 and M42 dyes are presented in the SI. The synthetic routes of compounds 2 and 4 are shown in Scheme S2 in the

Figure 2. Normalized absorption and emission spectra of M41, M42, and M29 in DCM and the solution of M41 and M42 (insert).

We observed a broad and strong absorption in the region 400− 650 nm for M41, M42, and M29, which corresponds with an intramolecular charge transfer (ICT) between the truxenebased donor and the cyanoacrylic acid group. M42 exhibits a maximum absorption peak (λmax) at 536 nm. The λmax of M41 was blue-shifted by approximately 43 nm compared with M42 because of the insertion of the methyl acetate group (electronwithdrawing group). By comparison, the electron-rich donor 98


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ACS Sustainable Chemistry & Engineering Table 1. Absorbance and Electrochemical Properties of the Dyes

a

Dye

λmax/ nma

ε/ 103 M−1cm−1

E0−0 /eV

HOMO/V vs NHE

LUMO/V vs NHE

M41 M42 M29

493 536 530

48900 65600 66700

2.21 2.10 2.15

0.96 0.87 0.97

−1.25 −1.23 −1.18

The absorption peaks in solution.

alternation from 9-hexyl-9H-carbazole (M42) to hexyloxybenzene (M29) has caused a λmax slightly blue-shifted of 6 nm. We observed that the color of M41 is obviously lighter than that of M42 (Figure 2). To clarify the origin of the absorption blue-shift of M41, we have computed equilibrium structures for M41 and M42 dyes using density functional theory (DFT) at the B3LYP level. The calculation results indicate that insertion of the methyl acetate group into the triarylamine donor leads to a large (64.1°, Figure S1 in SI) dihedral angle between the phenyl and dithieno[3,2b:2′,3′-d]pyrrole (DTP) groups in M41. In contrast, the dihedral angle in the same location for M42 is only 23.0°. Thus, the observed blue-shifted absorption peak of M41 is essentially due to the broken planar conjugating system. Upon adsorption onto TiO2 films (Figure S2 in the SI), the absorption spectra of M41, M42, and M29 were generally blueshifted as compared to the dyes dissolved in DCM, implying the deprotonation of the dyes at the surface of the TiO2 films and H-aggregates. Like most organic dyes, there is a severely blue-shifted absorption peak for M42 (40 nm) and M29 (49 nm) sensitized films. In contrast, the ICT absorption peak is slightly blue-shifted by only 17 nm for M41. It can be concluded that the strong electron withdrawing capacity of the methyl acetate group in M41 weakens the deprotonation effect and results in a small blue-shift in the absorption peak as the M41 becomes anchored on the TiO2. This effect is similar to that of the electron-withdrawing groups, such as benzothiadiazole in the spacer.51 In spite of that, the maximum absorption wavelength of the M42 sensitized film (496 nm) is still 20 nm longer than that of M41 (476 nm). Therefore, among the three dyes sensitized films, M42 displays a better light harvesting capacity. Electrochemical Properties. To estimate the HOMO level of the dyes, cyclic voltammetry (CV) was employed using a typical three-electrode electrochemical cell (Figure 3, details of this experiment can be found in the Supporting Information). As presented in Table 1, the HOMO levels of sensitizers of M41, M42, and M29 are 0.96, 0.87, and 0.97 V vs NHE, respectively. Both the HOMO levels of M41 and M29 are positive shifted related to that of M42, thus providing higher regeneration force of oxidized dyes. Nevertheless, it should be note that M41 and M29 achieved the higher driving force in different ways. M29 slightly reduced the donating capacity of the triarylamine donor by replacing the 9-hexyl-9Hcarbazole with hexyloxybenzene; on the other hand, M41 keeps the carbazole unit but introduces an electron-withdrawing group (methyl acetate) into the triarylamine donor. Unfortunately, incorporation of methyl acetate significantly increases the dihedral angle between the phenyl and DTP groups, enhancing the energy gap (2.21 V, Figure S3 in SI) between the HOMO and LUMO levels. In addition, M41 (−1.25 V) exhibits a slightly higher HOMO level than that of M42 (−1.23 V). Clearly, without the enhancement of the dihedral angle, the

Figure 3. Cyclic voltammograms of the dyes sensitized TiO2 films.

LUMO of M41 could be positively shifted. Therefore, we suggest that incorporation of an electron-withdrawing group into the triarylamine donor may be an effective way to lower both the HOMO and LUMO levels. Recently, Xie, Zhu, and co-workers proposed that reducing the energy “waste” of the LUMO level is important for efficiency improvement.6 In view of this, the LUMOs of M41 and M42 are too high. It seemed that M29 gives an optimizing of the HOMO and LUMO levels. However, this is not a whole picture of energy level modulating, because the light absorption is also vital for efficiency improvement, which relies on the energy gap between the HOMO and LUMO. In addition, charge recombination also affects the key interfacial processes in DSSCs. In fact, light absorption and controlling the charge recombination may outweigh the modulating of HOMO/ LUMO energy levels. We will discuss this issue in the following. Photovoltaic Performance. As shown in Figure 4a, the incident photon-to-collected electron conversion efficiencies (IPCEs) of cells based on the cobalt electrolyte are plotted as a function of wavelength. Although all dyes show a similar IPCE plateau around 80%, the response ranges of the IPCE for these devices are different. It is impressive that M42 bestows a very broad IPCE response. The IPCE onset of M41 is smaller than those of M29 and M42, in good agreement with the absorption measurement. The integrated current values obtained from the IPCE curve are shown in Figure 4b, and the trend is consistent with the experimental results. Though the onset of IPCE for M42 sensitized DSSCs (Co(II/III)-phen electrolyte) is longer than those of M41 and M29, we could not say the light response area is the sole factor behind the photocurrent. In fact, the onset of the light response is not the whole picture. A search of the literature revealed that sometimes a broader light absorption or IPCE response may not result in a better photocurrent.35,46 So, other factors also play major roles in photoelectric conversion. In the following section, we present some primary discussions on this issue. It is well-known that the IPCE response of DSSCs depends on the electron injection efficiency (Φinj), the regeneration 99


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Figure 4. (a) IPCE and (b) integrated current for M41, M42, and M29 sensitized DSSCs with the Co(II/III)-phen electrolyte.

Table 2. Photovoltaic Parameters of DSSCs Dye M41 M42 M29 M41 M42 M29

JSC/ mA cm−2

VOC/mV

± ± ± ± ± ±

900 ± 8 920 ± 8 900 ± 8 978 ± 6 980 ± 10 975 ± 7

12.0 15.5 13.5 10.5 11.3 10.8

0.4 0.2 0.3 0.3 0.3 0.2

FF 0.69 0.68 0.68 0.67 0.68 0.68

± ± ± ± ± ±

0.01 0.01 0.01 0.01 0.01 0.01

PCE/%

Electrolyte

± ± ± ± ± ±

Co(II/III)-phen Co(II/III)-phen Co(II/III)-phen Co(II/III)-Clphen Co(II/III)-Clphen Co(II/III)-Clphen

7.20 9.69 8.26 6.91 7.53 7.16

0.3 0.2 0.2 0.3 0.3 0.2

Figure 5. J−V characteristics for cells based on M41, M42, and M29 (Co(II/III)-phen (a); Co(II/III)-Clphen (b)).

our group indicate that the regeneration efficiency of truxene dye could reach more than 90% even though the regeneration force of oxidized dyes is only 0.3 V.11 Therefore, we can rule out ηreg as the main reason for heavily constraining the IPCE response of M41. In addition, the charge collection yield could not induce a significant difference in IPCE, because we do not observe an obvious height difference in IPCEs among these cells.55 Thereby, we conclude that the differences in the electron injection efficiency, the regeneration efficiency, and the collection efficiency do not make a great contribution to the observed IPCE height variation. In our opinion, LHE is the key factor that affects the IPCE response of M41, M42, and M29 sensitized devices. We recorded the photocurrent density voltage characteristics (J−V) of our devices with the Co(II/III)-phen electrolyte under AM 1.5 irradiation (100 mW cm−2). The detailed short− circuit photocurrent density (JSC), open−circuit photovoltage (VOC), fill factor (FF), and PCE parameters are collected in Table 2. As shown in Figure 5a, DSSCs made with M41 yielded JSC = 12.0 mA cm−2, VOC = 900 mV, and FF = 0.69, affording a

efficiency (ηreg), the collection efficiency (ηcc), and light harvesting efficiency (LHE); see eq 1: IPCE = LHE(λ) ·Φinj ·ηreg ·ηcc

(1)

First, the driving forces for electron injection are 0.75, 0.73, and 0.68 V for M41, M42, and M29, respectively. These values are much higher than 0.2 V,52,53 indicating the Φinj is not the main handicap in these DSSCs process. On the other hand, the IPCE value of DSSCs with cobalt electrolyte are very sensitive to the efficient regeneration of the oxidized dyes.50 Sufficient driving force is an indispensable condition in high efficiency DSSCs employing cobalt electrolytes. However, high driving force for regeneration gives no guarantee of high photocurrent, since the recombination could reduce the produce of photocurrent. In fact, attenuation of charge recombination rate could make a contribution to the high photocurrent of DSSCs.54 The regeneration driving forces (ΔGreg) for M41, M42, and M29 are 0.34, 0.25, and 0.35 V, respectively. Clearly, the driving force for M42 is significantly lower than those of M41 and M29. However, the IPCE response of M42 is better than those of M41 and M29. Furthermore, recent works from 100


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Figure 6. Charge density as a function of open circuit (a) and electron lifetime (τ) as a function of pseudo-Fermi level (EF, b) for the studied DSSCs.

figure provides some information about the shift of the conduction band (CB); that is, a higher VOC value at a fixed Q means a negative-shifted CB, while a lower VOC value suggests a contrary shift of CB. For these devices, the Q−VOC plots revealed a small range of CB. Compared with that of M29, the TiO2 bands of M42 based cells upward shifted by about 10 mV. In addition, EF-τ plots revealed that the electron lifetime of M42 is longer than that of M29. Thus, M42 exhibits a slightly higher VOC (920 mV) as compared to that of M29 (900 mV). Interestingly, the twist conformation in M41 does not cause an enhancement of electron lifetimes, a trend that is different from other organic dyes. For instance, Chai et al. found that the efficiency of the dye with a larger twist angle is higher than that of the dye without it, because of increased electron lifetimes.19 This result indicates that the twist conformation in organic dyes may not be necessary for organic dyes with a bulky electron donor. EF−τ plots also revealed that the electron lifetimes of Co(II/ III)-Clphen cells are lower than those of the corresponding Co(II/III)-phen cells regardless of the dye section, which could be correlated with the reduced regeneration force of oxidized dyes in the Co(II/III)-Clphen system. For M42 sensitized DSSCs, the electron lifetime of the Co(II/III)-Clphen system is lower than those of the Co(II/III)-phen system by around 2.77-fold (Figure 6b). By contrast, the electron lifetime of the former is closer to that of the latter in M41 sensitized DSSCs, despite the same value of reduced ΔGreg due to the change of electrolyte. With the Co(II/III)-Clphen electrolyte, the ΔGreg values of M41 and M42 are 0.24 and 0.15 V, respectively. The results suggest that the decrease of the dye regeneration rate accelerated as the regeneration force of oxidized dyes became lower than 0.2 V, because of the competition between charge recombination and dye regeneration. The more oxide dyes DSSCs have, the higher the opportunity of charge recombination. A fall in both dye regeneration rate and electron lifetimes contributed to the significant decrease in current intensity for M42. In spite of that, the photocurrent of M42 sensitized DSSCs containing the Co(II/III)-Clphen electrolyte still outperforms the devices based on M41. Therefore, we arrive at this conclusion: that is, given good capacity of retarding charge recombination, light harvesting capacity but not the regeneration force of oxidized dyes is the critical factor behind the photocurrent as well as the efficiency for organic dyes. Electrochemical impedance spectroscopy (EIS) was used to complete the analysis of interfacial charge transfer in devices.

PCE of 7.20%. In contrast, both JSC and VOC of M42 are significantly greater than those of M41, leading to 34.5% enhancement of the overall power conversion efficiency for the device based on M42 (PCE = 9.69%). Note that this high efficiency was achieved without use of electron-deficient groups such as benzothiadiazole (BTD), which is a popular auxiliary group used for construction of organic dyes. The PCE of M29 (8.26%) is also lower than that of M42, but higher than that of M41 under the same conditions. Clearly, the PCE of these devices highly depends on the JSC but not the VOC, highlighting the role of electron donor in determining the photocurrent. Despite the low driving force (0.25 V), M42 still exhibited a high JSC, owing to an evidently suppressed interfacial recombination (see next section). In other words, the joint advantages of better light harvesting and controlling the charge recombination outweigh the limitation of dye regeneration. If this is true, we will observe a similar trend of photovoltaic performance for devices with different cobalt electrolytes. So, we employed the Co(II/III)-Clphen electrolyte, which has a higher oxidation potential (0.72 V) than the Co(II/III)-phen, to fabricate the devices. As presented in Figure 5b, with the Co(II/III)-Clphen electrolyte, M41, M42, and M29 sensitized DSSCs show a dramatically high VOC but a relative low JSC, leading to a PCE of 6.91%, 7.53%, and 7.16%, respectively. The main reason for the lower JSC of DSSCs based on Co(II/III)− Clphen electrolyte could be attributed to the reduced driving force for regeneration.50 In addition, the bulkier size of Co(ClPhen)3 could lead to mass transport problems.8 Evidently, the trend of PCE for the Co(II/III)-Clphen system is the same as that of the Co(II/III)-phen system. Therefore, we think that good light harvesting of organic dyes and reduced charge recombination in devices are of great importance to photoelectric conversion of organic dyes-based DSSCs. Nevertheless, this does not mean that the regeneration force of oxidized dyes has no influence on photocurrent. In fact, we found the photocurrent of M42 and M29 to decrease by 37% and 20%, respectively (Figure S4 in SI) as the electrolyte changed to the Co(II/III)-Clphen system. In contrast, the corresponding photocurrent variation for M41 is small (decreased by 12.5%). This indicates that charge recombination rate and dye regeneration efficiency are bound to be changes when the new electrolyte system is introduced. Intensity Modulated Photovoltage Spectroscopy (IMVS) Measurements. Figure 6a shows the relation between the VOC and extracted charge density (Q) at open circuit. This 101


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Figure 7. Nyquist plots (a) and Bode phase plots (b) for DSSCs based on the studied dyes.

Nyquist and Bode phase plots for DSSCs based on the M41, M42, and M29 are presented in Figure 7a and Figure 7b, respectively. The larger semicircle at lower frequencies of Nyquist plots is correlated to the interfacial charge transfer resistances (RCT) at the TiO2/dye/electrolyte interface. The smaller the RCT, the faster the charge recombination is. For Co(II/III)-phen cells, the RCT increases in the order M42 > M41 > M29. By comparison, the electrolyte alternation from Co(II/III)-phen to Co(II/III)-Clphen has caused an increase of RCT at the same forward bias for M41 and M29 sensitized cells. A result can be attributed to the increased bulk of the Clphen group. However, this electrolyte alternation leads to an attenuation of RCT for M42 sensitized cells, sharply contrasting an enhancement in M41 and M29 sensitized cells. As a matter of fact, the RCT for M42 is much lower than those of M41 and M29 in the Co(II/III)-Clphen electrolyte system. The results indicate that, for M42 cells with the Co(II/III)-Clphen electrolyte, the charge recombination rate between the TiO2 electrons and redox electrolyte accelerated as the regeneration force of oxidized dyes is lower than 0.2 V, in agreement with observations from Figure 6b. Nevertheless, we should note that IMVS measurements are something different from that of EIS measurements. For example, Figure 6b provides information on the charge recombination rate between the TiO2 electrons and redox electrolyte/oxidized dye molecules, while that for Figure 7b is between the TiO2 electrons and redox electrolyte. Based on EIS measurements, we obtained the electron lifetime of devices using the equation τ = 1/ωmin,56 where ωmin is the angular frequency at the midfrequency peak. Clearly, the lower frequency peak of the cell based on M42/Co(II/III)Clphen is shifted to much higher frequency when compared to that of the cell based on M41/Co(II/III)-phen, indicating a significant decrease in the electron lifetime in the dark, which is in agreement with the result from IMVS measurements. On the other hand, for the Co(II/III)-phen system, the electron lifetime increases in the order M29 < M41 < M42.

ability, resulting in decreased energy gap between the HOMO and LUMO levels. Though the regeneration force of M42 is lower than those of M41 and M29, the former displays a better efficiency regardless of the electrolyte selected. M42-sensitized DSSCs employing the Co(II/III)-phen electrolyte yielded a PCE of 9.69% under standard AM 1.5 sunlight. This study suggests that, given the good capacity of retarding charge recombination, light harvesting capacity is the critical factor behind the photocurrent as well as the efficiency for organic dyes, but not the regeneration force of oxidized dyes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00706. Optical and electrochemical measurements, fabrication and characterization of DSSCs, Figure S1−5, and synthetic details of M41, M42, and their intermediates (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Author Contributions

P.P.D. and H.H.D. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Science Foundation of China (No. 21373007, 21376179), and the Tianjin Natural Science Foundation (13JCZDJC32400, 14JCYBJC21400).

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CONCLUSIONS In conclusion, we have investigated the effect of electron donor for organic dyes toward cobalt cells. The introduction of the methyl acetate group in the triarylamine donor results in an obvious blue-shift of the absorption peak for M41, owing to a twist conformation and downshift of the HOMO level. Compared with the hexyloxybenzene group (M29), 9-hexyl9H-carbazole in M42 displays a stronger electron-donating

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