Twisted Fused-Ring Thiophene Organic Dye-Sensitized Solar Cells

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Twisted Fused-Ring Thiophene Organic Dyes Sensitized Solar Cells Huanhuan Dong, Mao Liang, Chunyao Zhang, Yungen Wu, Zhe Sun, and Song Xue J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06604 • Publication Date (Web): 23 Sep 2016 Downloaded from http://pubs.acs.org on September 25, 2016

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The Journal of Physical Chemistry

Twisted Fused-Ring Thiophene Organic Dyes Sensitized Solar Cells Huanhuan Dong, Mao Liang,* Chunyao Zhang, Yungen Wu, Zhe Sun, Song Xue* Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, Department of Applied Chemistry, Tianjin University of Technology, Tianjin 300384, P.R.China

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ABSTRACT:

Fused-ring thiophene compounds emerged as an important type of building blocks for organic dyes toward the dyes sensitized solar cells (DSSCs) because of their good charge transfer and light harvesting properties. Nevertheless, some fused-ring thiophenes have a lack of desired alkyl chains or side alkyl chains, which may induce a severe charge recombination in devices. In this work, the hex-1-en-1-ylbenzene (HEYB) unit was introduced in two new fused-ring thiophene organic dyes (M60 and M59), resulting in a modest dihedral angle (around 36o) between the donor and spacer in dyes. The effect of the HEYB unit on optical, electrochemical, and photovoltaic properties has been investigated by comparing with their congeners (M42 and M58) without the HEYB unit. It is found that introduction of the HEYB unit in arylamine donor enhanced the driving force for dye regeneration and beneficial for suppressing dye aggregation as well as reducing the charge recombination. Device performance characteristics demonstrate that introduction of the HEYB unit in the arylamine donor is a feasible strategy towards enhancing the performance of fused-ring thiophene organic dyes. Benefiting from this strategy, a dye-sensitized solar cell employing the M60 photosensitizer and a cobalt electrolyte exhibits a good power conversion efficiency of 9.75% measured under the 100 mW cm-2, simulated AM1.5 sunlight.


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INTRODUCTION Research that focuses on energy conversion and storage technologies receive great attentions owing to the growing global energy demands.1 Dye-sensitized solar cells (DSSCs) have been under focus as one of the most promising technologies because their low cost, stability, high efficiency and ease of fabrication. The photosensitizer that functions to light harvesting and electricity conversion is a key component in the DSSCs. Consequently, photosensitizers such as ruthenium(II) complexes,2 porphyrin-based dyes3-6 and metal-free organic dyes7-46 have been developed. Among them, metal-free organic dyes featuring a donor-π-spacer-acceptor (D−π−A) framework show promising advantages such as high efficiency, chemical versatility and low cost. To achieve high power conversion efficiencies (PCEs), molecular engineering of the electron donor, spacer and acceptor for broader and higher light response is necessary. Meanwhile, molecular design has to take into account the suppressing charge recombination and dye aggregation. This recognition has translated into massive efforts in establishing new strategy to reduce aggregation and suppress charge recombination in devices. Scientists proposed several strategies for diminishing the dye aggregation and the electron recombination. Firstly, alkyl chains introduced into donor (e.g. dihexyloxy-substituted triphenylamine) and spacer (e.g. dihexylcyclopentadithiophene) for construction of organic sensitizers.7 Secondly, the incorporation of bulky rigid groups such as dipropylfluorene8 and hexapropyltruxene7 unit can notably retard the charge recombination at the titania/electrolyte interface. Finally, it is valuable to note that a combination of alkyl side chains and a twisted linked backbone in a dye molecule is an effective way for the suppression of dye aggregation and charge recombination.7 This design can be found in some outstanding systems. For instances,


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ADEKA-1,9 SM3155 and TPA-TTAR-T-A10 (Scheme S1 in the supporting information) based DSSCs employing the cobalt electrolyte exhibited PCEs of 12.5, 13 and 10.1%, respectively. No matter there were different explanations on the above exciting results, to our opinion, alkyl side chains and twisted structures significantly contribute to the high performance of these dyes. As presented in Scheme S1, ADEKA-1 contains two 4,3ʹ -dihexylbithiophene units with a twisted structure, which increases electron lifetimes of cells.9 On the other hand, SM315 incorporates two different twisted structures.5 In this regard, design and synthesis new twisted structures in organic dyes are one of the basic motivations for photosensitizers studies in DSSCs field. Recently, fused-ring thiophenes and multifused thiophenes have been widely exploited as crucial building blocks for organic dyes. Meanwhile, our group have synthesized a series of 4Hdithieno[3,2-b:2ʹ ,3ʹ -d]pyrrole (DTP) dyes to fabricate high performance DSSCs, through attachment of different substituents to DTP.11-13 As demonstrated in these works, better πconjugation can be readily achieved by increasing the coplanarity of bridged π-systems. Although highly coplanarization of the rigidified π-spacer is beneficial to extend the light absorption of the sensitizer, lack of twisted structure may result in undesirable dye aggregation and charge recombination. For example, X76 (Scheme S1) displays a low PCE of 4.1% owing to severe charge recombination.13 In contrast, TPA-TTAR-T-A (Scheme S1) displays a high PCE of 10.1% partially because of the twisted linked backbone between the triphenylamine and multifused thiophene.10 Therefore, we think that a twisted structure is indispensable for organic dyes based on fused-ring thiophene or multifused thiophene. However, sometimes, it is difficult to introduce desired alkyl chains to the framework of them due to a big challenge of synthesis. A search of the literature found that most of the fused-ring thiophene or multifused thiophene


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derivatives have alkyl chains at the central part of the unit, and they have a lack of alkyl chains at the edge side. This defect undermines the performance of organic dyes. To avoid this dilemma for fused-ring thiophene based organic dyes, we extended our work by employing the hex-1-en-1-ylbenzene (HEYB) unit to construct arylamine donor (Figure 1). In this article, we synthesized M59 and M60 with a HEYB unit between arylamine donor and DTP spacer.

To understand the effect of the HEYB

unit, M58 and M42 without this unit were

introduced as reference dyes. Hexyl-substituted DTP, a representative fused-ring thiophene with strong electron-donating ability,47 was employed as π-spacer for these dyes because its alkyl chain is not perfect and there is no alkyl chain at the edge of fused-ring (Figure 1). Density functional theory (DFT) calculations revealed that there is a modest dihedral angle (around 36o) between the donor and spacer in M59 and M60. With this small modification, driving force for dye regeneration enhancing, suppressing dye aggregation as well as reducing the charge recombination can be realized for M59 and M60, but without a big loss in light harvesting. Device performance characteristics demonstrate that introduction of HEYB unit in arylamine donor is a feasible strategy towards enhancing the performance of fused-ring thiophene organic dyes. Importantly, these results also suggested that, no matter what arylamine donor employed (bulky or small), this twisted structure has a positive effect on the photovoltaic performance of dyes.


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Figure 1. Chemical structures of the M58-60 and M42. EXPERIMENTAL SECTION

Scheme 1. Schematic routes for the M58-60: (a) t-BuONa, Pd2(dba)3, Xphos, toluene, reflux 8 hours; (b) Piperidine, Cyanoacetic acid, CHCl3/CH3CN (v/v, 1/2), reflux, 8 hours.


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Synthesis of dyes The synthetic route of M58-60 is presented in Scheme S1 of the Supporting Information. Synthesis of compound 2a The compound 2a was synthesized based on the method reported in the literature.33 Orange solid (150 mg, yield 69%). 1 H NMR (400 MHz, CDCl3): δ 9.87 (s, 1H), 7.63 (s, 1H), 7.47 (d, J = 8.76 Hz, 2H), 7.12 (d, J = 8.92 Hz, 4H), 7.10 (s, 1H), 6.96 (d, J = 8.76 Hz, 2H ), 6.88 (d, J = 8.96 Hz, 4H ), 4.25-4.21 (t, J =7.04 Hz, 2H), 3.84 (s, 6H), 1.95-1.88 (m, 2H), 1.34-1.30 (m, 6H), 0.91-0.88 (t, J = 6.72 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ 182.62, 156.28, 149.86, 148.94, 148.04, 143.68, 140.34, 130.91, 130.89, 128.85, 128.81, 126.91, 126.42, 123.77, 120.06, 114.84, 104.97, 68.17, 55.51, 31.39, 30.24, 26.68, 22.50, 14.01. HRMS (ESI) calcd for C35H35N2O3S2 (M+H+): 595.2089, found: 595.2070. Synthesis of compound M58 M58 was synthesized with the compound 2a and cyanoacetic acid. The method was reported in our previous studies.11,12 M58: red power (122 mg, yield 87%). 1H NMR (400 MHz, CDCl3): δ 8.24 (s, 1H), 7.48 (s, 1H), 7.35-7.28 (m, 2H), 7.01 (d, J = 8.2 Hz, 4H), 6.84 (d, J = 8.12 Hz, 3H), 6.81 (d, J = 8.64 Hz, 4H), 4.02 (m, 2H), 3.77 (s, 6H), 1.79-1.67 (m, 2H), 1.31-1.24 (m,6H), 0.890.83 (t, J = 6.80 Hz, 3H).

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C NMR (100 MHz, CDCl3): δ 156.15, 150.01, 148.74, 144.40,

140.32, 132.69, 132.40, 130.90, 128.79, 126.79, 126.34, 120.05, 114.79, 114.02, 113.19, 104.84, 68.27, 55.47, 38.73, 31.32, 26.58, 22.45, 13.94. HRMS (ESI) calcd for C38H36N3O4S2 (M+H+): 662.2192, found: 662.2147. Characterizations of 4a, 4b, M59 and M60 are shown in the supporting information.


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RESULTS AND DISCUSSION Photophysical Properties The electronic absorption spectra of diluted dichloromethane solutions of these dyes were recorded to evaluate the photophysical properties of dyes, and the corresponding properties are summarized in Table 1. As depicted in Figure 2, the major absorption peaks (λmax) for M58, M59, M42 and M60 are at 502, 492, 538 and 518 nm, respectively. A red-shifted maximum absorption wavelength along with enhanced maximum molar absorption coefficient (ε) 4.56 ×104 M−1 cm−1 at 502 nm was measured for the M58 dye, compared to that of 3.92 × 104 M−1 cm−1 at 492 nm for M59. Similar trend can be found for M42 vs M60. Clearly, the introduction of a HEYB

unit

in arylamine donor reduce the conjugation between the donor and spacer in M59 and M60. In view of the light harvesting capability, traditional triarylamine is superior to the HEYB based triarylamine. Nevertheless, this difference is not significant. Note that, the light harvesting capability of M42 and M60 are better than that of M58 and M59, respectively, because of the larger conjugation of truxene unit as well as the strong electron-donating

ability

of

carbazole,

which enhances the donor strength of the amine segment and result in a pronounced donoracceptor interaction.48


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Figure 2. Absorption spectra of M58-60 and M42 in dichloromethane. Apart from the light harvesting capability of dyes, the light harvesting function of sensitized electrode depends on the adsorption of dyes on TiO2 surface. The absorption spectra of 3 µmthick TiO2 sensitized electrodes are shown in Figure 3a. It can be found that the absorption of M58/59 sensitized electrodes are much higher, in comparison with their analogues with truxenecarbazole based donor. This sharply contrasts the trend observed in Figure 2, indicating a higher adsorption amount of M58/59. The surface coverage (Γ) of M58, M59, M42 and M60 anchored on TiO2 film are determined to be 9.82 × 10−8 and 9.53 × 10−8, 4.55 × 10−8 and 4.35 × 10−8 mol cm-2, respectively. Apparently, bulky rigidity groups (i.e. truxene-carbazole based arylamine) noticeably reduced the surface coverage of M42 and M60 on electrodes. Fortunately, the introduction of the HEYB unit near the DTP spacer has small impact on the surface coverage of dyes. There are no significant difference in surface coverage for dyes with and without the HEYB unit.


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Figure 3. Absorption spectra of the studied dyes sensitized electrodes with (a) and without (b) electrolytes. Moreover, the M58/59 sensitized electrodes show superior light harvesting performance to M42/60 sensitized electrodes when immersed in the Co-phen electrolyte (Figure 3b). Benefiting from the augmentation of dye load amount, the dye transformation from M42/60 to M58/59 brings forth a slight red-shifts of the absorption onset. On the other hand, the light harvesting capability of cells based on M59/60 with the HEYB unit between the donor and spacer are inferior to those of their analogues without it.


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Figure 4. Absorption spectra of the M58 (a), M59 (b), M42 (c) and M60 (d) in DCM, DCM+TEA and on TiO2. Compared with the absorption spectrum in dichloromethane solution, the ICT band for these sensitized electrodes are blue shifted by 53, 34, 40 and 35 nm for M58, M59, M42 and M60, respectively. To understand the influence of the HEYB

unit on dyes aggregation, the absorption

spectrum of dyes in dichloromethane solution with the addition of an excess amount of triethylamine (TEA) have also been recorded. The λmax for M58, M59, M42 and M60 in DCM +TEA are at 477, 463, 483 and 467 nm, respectively. As shown in Figure 4a, a blue-shift of absorption (28 nm) has been observed for M58 sensitized electrode when compared to that of M58 in DCM+TEA. This result indicates that the blue shift of M58 sensitized electrode is due to a combination of the deprotonation effect and H-aggregation. In contrast, the absorption peak of M59 in DCM+TEA is close to that of M59 sensitized electrode (Figure 4b). Apparently, the introduction of

the

HEYB

unit in M59 suppress the intermolecular aggregation effectively.

Nevertheless, similar trend has not been observed for truxene dyes. Both the absorption peaks of M42 and M60 in DCM+TEA are close to those of M42 and M60 sensitized electrodes, respectively. This suggests that the blue shift in absorption peak for truxene dyes should be predominated by deprotonation effect. Of course, the bulky structure of hexapropyltruxene

unit

control the aggregation of dye molecules on TiO2 film.


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Table 1. Optical properties and electrochemical properties of the M58, M59, M42 and M60. Dye

λmax/nm a

λmax/nm b

ε/103M–1cm–1

E0–0/eV

HOMO/V vs NHE

LUMO/V vs NHE

M58

502

449

45.6

2.19

0.84

-1.35

M59

492

458

39.2

2.21

0.94

-1.27

M42

538

498

65.6

2.10

0.87

-1.23

M60

518

483

63.3

2.13

0.91

-1.22

The absorption spectra of dyes in DCM a and sensitized electrodes b.

Electrochemical Properties. Electrochemical properties of the four dyes sensitized electrodes were investigated by cyclic voltammetry (CV) (Figure 5a). The HOMO, LUMO and energy gap can be found in Table 1. The first oxidation potential (HOMO) of M58, M59, M42 and M60 sensitized electrodes are determined to be 0.84, 0.94, 0.87 and 0.91 V (vs NHE), respectively. Apparently, the HOMO of dyes was down-shifted when the conjugation was partially broken, because the charge transfer interactions from amine to acceptor become difficult. However, the shift degree also depends on the electron donor. As presented in Figure 5b, a significantly positive shift of the HOMO (0.1 V) can be observed for M59 vs M58, while that for M60 vs M42 is only 0.04 V. The

donor-acceptor

interaction in carbazole based dyes is so strong that the twisted structure effect on the HOMO level of M60 diminish. As the electron-donating capability of donor becomes weak, the dye M59 exhibits a pronounced twisted structure effect. Some studies have shown that the photocurrent of DSSCs based on CoII/III polypyridyl electrolytes is dependent on the driving force for regeneration (ΔGreg).46 Therefore, the introduction of the HEYB

unit between the donor and

spacer has a positive effect on dye regeneration in cobalt cells. Thus, M59 possessed a larger driving force for regeneration as compared to that of other dyes.


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Figure 5. (a) Cyclic voltammograms of M58-60 and M42 sensitized electrodes. (b) HOMO, LUMO and energy gap of M58-60 and M42. Theoretical Approach In order to gain better understanding of the molecular geometries and molecular orbitals, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were conducted using the Gaussian 09 program package at the B3LYP/6-31G (d) level. To make this paper more concise, in the main text we focus our analyses solely on M58 and M59 to investigate the influence of the HEYB unit on conjugation. As shown in Figure 6a, the dihedral angle between the triphenylamine donor and DTP spacer is 20.2o for M58, ensuring a good co-planarity of the main π system to make a better light response with sunlight. Just as expected, the replacement of phenyl group by HEYB unit in M59 did not affect the dihedral angle significantly, resulting in a modest dihedral angle of 36.2° between the triphenylamine donor and DTP spacer as calculated. A similar dihedral angle found for M60 (Figure S2 in SI). Note that, a large dihedral angle in spacer may lead to poor light response owing to the broken of conjugation. For example, limited by di(3-hexylthiophene) spacer (the dihedral angle is around 59°), C239 shows the poor light


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harvesting capacity and low incident photon-to-collected electron conversion efficiencies.17 Therefore, we think the dihedral angle in M59 and M60 is suitable for organic dyes. Figure 6b shows a representation of the HOMO and LUMO topologies for M58 and M59 (those for M60 can be find in Figure S3 in SI). The HOMOs of them are delocalized over the molecular with more localized at the 4-methoxy-N-(4-methoxyphenyl)-N-phenylaniline, while the LUMOs are primarily localized on the DTP and cyanoacrylic acid unit. Nevertheless, by close comparison we can find the HEYB unit in M59 has some impact on the delocalization of the HOMOs. The electron distribution on DTP unit for M59 is slightly smaller than that of M58 (Figure 6b), which contributes to the HOMO/TiO2 spatial separation. Thus, a slower recombination rate in cells can be expected when M59 was used as photosensitizer.

Figure 6. The optimized geometries (a) as well as the isodensity surface plots for the HOMO and LUMO of M58 and M59 (b). Photovoltaic Performance We further carried out the incident photon-to-collected electron conversion efficiency (IPCE) measurement of DSSCs employing the Co-phen electrolyte. As depicted in Figure 7a, the


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photocurrent action spectra of all these sensitizers exhibit a high plateau of over 80%. The dye transformation from M60 to M42 brings forth a slightly red-shifts of the IPCE onset, in good agreement

with

the

absorption spectra of the dyes sensitized electrodes as shown in Figure 3.

Moreover, we observe a similar trend when compare the IPCE of M58 and M59. However, M59 and M60 with the HEYB unit show higher IPCE values than those of M58 and M42, respectively, indicating that the introduction of HEYB unit in triphenylamine donor could trigger a favorable influence on the IPCE summit. Despite the better absorption spectra of the sensitized electrode, M58 displays a lower IPCE plateau than other dyes, leading to a relative lower photocurrent density.

Figure 7. IPCE (a) and J–V characteristic (b) curves for DSSCs employing the Co-phen electrolyte. As shown in Figure 7b, the short–circuit photocurrent density (JSC), open–circuit photovoltage (VOC) and fill factor (FF) of the M58 cell measured under AM 1.5 irradiation (100 mW cm–2) are 14.2 mA cm–2, 843 mV and 0.66, respectively, affording an overall power conversion efficiency (PCE) of 7.90%. By contrast, an improved JSC of 15.5 mA cm–2 is concomitant with an increased VOC of 866 mV for M59 based cell, contributing to a high PCE of 9.13%. The cumulative effect


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of the reduced dye aggregation and attenuated rates of charge-recombination contribute the higher JSC for M59 based cell. This result suggests that the introduction of the HEYB unit in triphenylamine donor could induce a large PCE improvement in small organic dyes sensitized cobalt cells. Moreover, we also observe this positive effect of the HEYB unit in truxene dyes. With respect to that of M42, M60 also achieves an improvement in PCE, resulting in a promising efficiency as high as 9.75%. Nevertheless, the photocurrent density of M60 based cell is equal to that of M42. Therefore, we ascribed this PCE improvement to an increase in VOC, rather than the JSC. Note that, the VOC of truxene dyes are much higher than those of triphenylamine dyes, primarily owing to the bulky insulating truxene donor, which forms a more favorite organic layer to inhibit charge recombination at the interface between the redox shuttle and titania. Overall, no matter what arylamine donor employed (bulky or small) by organic dyes, the HEYB unit is beneficial to the photovoltaic performance of cobalt cells. To compare these dyes with other reported thiophene based dyes, it is better to avoid D-A-π-A organic dyes, since auxiliary acceptor favors the light harvesting. Based on our experiences, the efficiency of M60 is higher than that of C218, its efficiency measured in our lab is around 8.5%. In our opinion, the PCE of over 9% is high for D-π-A organic dyes. Table 3. Photovoltaic parameters for the cobalt and iodine cells based on M58-60 and M62. Dye

JSC/ mA cm–2

VOC/mV

M58

14.2±0.4

843±9

0.66±0.01

7.90±0.3

Cobalt

M59

15.5±0.3

866±8

0.68±0.01

9.13±0.3

Cobalt

M42

15.4±0.3

920±8

0.67±0.01

9.49±0.3

Cobalt

M60

15.4±0.3

945±8

0.67±0.01

9.75±0.3

Cobalt

M58

12.5±0.2

600±5

0.68±0.01

5.10±0.1

Iodine

FF

PCE / %


Electrolyte

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M59

13.0±0.2

663±6

0.66±0.01

5.68±0.3

Iodine

M42

14.8±0.2

682±8

0.66±0.01

6.66±0.2

Iodine

M60

15.5±0.3

742±9

0.67±0.01

7.70±0.3

Iodine

Figure 8. Charge density as a function of open circuit (a) and electron lifetime as a function of pseudo-Fermi level (b) for the cobalt cells. Figure 8a shows the relation between the VOC and extracted charge density (dn) at open circuit. The curves for the DSSCs employing Co-phen electrolyte are roughly parallel to each other. For example, at a fixed dn, the VOC of M58 and M59 are very similar, indicative of a small shift of CB (< 5 mV). Therefore, CB shift has a small effect on the VOC variations of congener dyes. Figure 8b presents the relation between the electron lifetime (τ) and pseudo-Fermi level (EF), which shows an increase in the order of M60 > M42 > M59 > M58, indicating an attenuated rates of charge recombination between electrons in TiO2 and electrolyte acceptor species and oxidized dye molecules at the interface. The observed enhancement of VOC from M58/M42 to M59/M60 dye can be attributed to the retarded charge recombination caused by the HEYB unit. It is well known that the performance of dyes also depends on the electrolytes employed. To better understanding the effect of the HEYB unit on the performance of the dyes, the iodine electrolyte was introduced to prepare DSSCs. This electrolyte displays a sluggish electron


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transfer kinetics in dye regeneration and interfacial recombination compared with cobalt electrolyte.40 IPCE curves for DSSCs employing the iodine electrolyte are shown in Figure S4 in the SI. The electrolyte composition is listed in the experimental section of the SI. The J–V characteristics of iodine cells under an irradiance of 100 mW cm-2, simulated AM1.5 sunlight shown in Figure 9a, and the detailed parameters listed in Table 2. M60 exhibits a notably improved VOC of 742 mV concomitant with an increased JSC of 15.5 mA cm–2 in comparison with its congener (M42), resulting in an improvement from 6.66% to 7.70%. Moreover, we can find that the M59 (PCE = 5.68%) is superior to M58 (PCE = 5.10%) in terms of photovoltaic performance. This trend agree with the results from measurements of cobalt cells. Clearly, regardless of the electrolyte selection, triphenylamine or truxene based triarylamine dyes with the HEYB unit between the donor and the fused-ring thiophene spacer are outperforms their congeners without it. Note that, the effect of the twisted structure on the photovoltage of iodine cells seems more significant than that of cobalt cells. For example, compared with M42, the introduction of the HEYB unit in M60 successfully realized a significant increase in VOC around 60 mV in iodine cells, while that for cobalt cell is only 25 mV.

Figure 9. J–V characteristic curves (a) and electron lifetime as a function of pseudo-Fermi level (b) for the iodine cells.


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IMVS measurements (Figure 9b) revealed that the lifetime of the iodine cell based on M60 is 2.38-fold higher than that of M42. By contrast, the lifetime of the cobalt cell based on M60 is 1.39-fold higher than that of M42 (Figure 8b). In addition, M59 also shows a higher lifetime when compared to that of M58 in iodine cells. In other words, the HEYB unit in triarylamine donor slows down the recombination of injected electrons more effectively in the iodine system. Since cobalt redox couples display a much faster electron transfer kinetics in interfacial charge recombination than that of the iodide/triiodide system, we suspect that fast interfacial charge recombination in cobalt cells lessened the impact of the HEYB unit somewhat. CONCLUSIONS In summary, we have synthesized a new twisted structure for fused-ring thiophene organic dyes. The introduction of the HEYB unit in arylamine donor leads to a modest dihedral angle (around 36o) between the donor and spacer. With this rational design, enhancing of driving force for dye regeneration, suppressing dye aggregation as well as reducing the charge recombination have been achieved for M59 and M60, but without a big loss in light harvesting. It is found that, regardless of the electrolyte selection, the dyes M59 and M60 with the HEYB unit have achieved better photovoltaic properties when compared to those dyes (M58 and M42) without it. Importantly, no matter what arylamine donor employed (bulky or small), the use of the HEYB unit in arylamine donor has a positive effect on the photovoltaic performance of dyes. The strategy presented in this work on the construction of fused-ring thiophene organic dyes could extend to the design of other more comprehensive sensitizers such as multifused thiophenes organic dyes.


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ASSOCIATED CONTENT Supporting Information. Electrochemical impedance spectroscopy (EIS) analysis of Cobalt cells (Figure S5) and iodine cells (Figure S6). A preliminary experiment for stability measurement (Figure S7). 1H,

C NMR and HRMS of sensitizers and important intermediates

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(Figure S8-13). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author * E-mail: [email protected] (Mao Liang); [email protected] (Song Xue) Phone: +86 60214259 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We gratefully acknowledge the financial support from the National Science Foundation of China (No. 21373007, 21376179), and the Tianjin Natural Science Foundation (13JCZDJC32400, 14JCYBJC21400). REFERENCES 1.

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Twisted Fused-Ring Thiophene Organic Dye-Sensitized Solar Cells

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