Highly Twisted Dianchoring DâÏâA Sensitizers for Efficient Dye

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Highly Twisted Dianchoring D-#-A Sensitizers For Efficient Dye-Sensitized Solar Cells Chun-Yuan Lo, Dhirendra Kumar, Shu-Hua Chou, Chih-Han Chen, Chih-Hung Tsai, Shih-Hung Liu, Pi-Tai Chou, and Ken-Tsung Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10162 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on September 28, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

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

ACS Applied Materials & Interfaces

Highly Twisted Dianchoring D-π-A Sensitizers For Efficient Dye-Sensitized Solar Cells Chun-Yuan Lo,† Dhirendra Kumar,† Shu-Hua Chou†, Chih-Han Chen,‡ Chih-Hung Tsai,*,‡ Shih-Hung Liu,† Pi-Tai Chou†*, Ken-Tsung Wong*,†,§ † ‡

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan Department of Opto-Electronic Engineering, National Dong Hwa University, Hualien

97401, Taiwan §

Institute of Atomic and Molecular Science, Academia Sinica, Taipei 10617, Taiwan

ABSTRACT Two new organic dyes BPDTA and BTTA possessing dual D-π-A units have been synthesized, characterized, and employed as efficient sensitizers for dye-sensitized solar cells.

The

two

individual

D-π-A,

which are

based

on

(E)-3-(5'-(4-(bis(4-

(hexyloxy)phenyl)amino)phenyl)-[2,2'-bithiophen]-5-yl)-2-cyanoacrylic acid unit (D21L6), are connected directly between phenylene or thiophene within linear π-conjugated backbone to constitute a highly twisted architecture for supressing the dye aggregation. The new dianchoring dyes exhibited pronounced absorption profile with higher molar extinction coefficient, which is consistent with the results obtained from DFT calculations. The theoretical analysis also indicated that the charge transfer transition is mainly constituted of HOMO/HOMO-1 to LUMO/LUMO+1 that were found to be located on donor and acceptor segments respectively. Theoretical calculations give the distance between two binding sites of 19.50 Å for BPDTA and 12.04 Å for BTTA. The proximity between two anchoring units of BTTA results in superior dye loading and hence higher cell efficiency. The BTTA-based device yielded an optimized efficiency of 6.86% as compared to 6.61% of BPDTA-based device, whereas the model sensitizer D21L6 only delivered an inferior performance of 5.33% under similar conditions. Our molecular design strategy thus opens up a new horizon to establish efficient dianchoring dyes. KEYWORDS: dye-sensitized solar cell, organic sensitizer, donor-π-acceptor, dianchoring dye, dye–TiO2 interaction, dye aggregation

1 ACS Paragon Plus Environment


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

1. INTRODUCTION Dye-sensitized solar cells (DSSCs) have received great research attentions because of their potentially low cost and easy fabrication, comparable device efficiency, and readily available sensitizers as compared to other photovoltaic technologies.1-4 In DSSCs, the sensitizer has been renowned as one of the significant components, which governs the device efficiency besides the electrolyte and semiconductor.5,6 Although, metal-containing sensitizers including ruthenium dyes,7,8 Zn-porphyrins,9,10 perovskites11,12 etc. achieved remarkable power conversion efficiency (PCE) so far, but their practical uses are still limited as considering the cost, scarcity and environmental aberration. In contrast, metal-free organic sensitizers are fascinating because of their easier accessibility via manageable synthetic protocols and versatile structure tailoring along with superior light harvesting efficiency, in particular, of higher molar extinction coefficient.13-15 The structure of the organic dyes plays an important role for governing the optical and electronic properties, and hence the photovoltaic performance. To induce effective intramolecular charge transfer (ICT) for charge separation, the donor-(π-bridge)-acceptor (Dπ-A) has been a widely exploited architecture, offering molecules capable of absorbing a wide range of sun light.16,17 In particular, molecular design having arylamine donors, different π-conjugated linkers including aromatics and/or heteroaromatics and cyanoacrylic acid as acceptor/anchor is promising to attain outstanding device performance.18-21 However, in spite of the linear/rod shaped D-π-A skeleton affording competent charge transfer from donor to acceptor and therefore remarkable absorption, it is also prone to aggregate due to the planar structure/conformation that eventually deteriorates the DSSC efficiency.22,23 Tethering of either bulky groups or alkyl chains onto donor/linker may resolve the aggregation problems, in part, if the co-planarity of the molecule is not disrupted.24-27 Unfortunately, such modification may also lead to the reduction of dye loading on semiconductor surface.28 Recently, DSSC dyes with two anchoring groups (dianchoring) were found to resolve the aforesaid aggregation issues and therefore to impede the charge recombination, leading to the hike in open circuit voltage (VOC) and short-circuit current (JSC).29-32 Some of the dianchoring dyes render remarkable light harvesting than mono-congener due to better spectral responses to sunlight induced by extended molecular π-conjugation. In addition, the dianchoring dye can be regarded as a bidentate ligand that can bind to TiO2 with better affinity and impart compact packing to block electrolyte interactions and thus hamper the charge recombination; such binding also ensues the better electron injection through the multiple anchoring units,


Page 2 of 27

Page 3 of 27

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


which may boost the device efficiency and stability. It is worthy to note that the structure and molecular size of dianchoring dye strongly govern its loading on semiconductor surface and generally it would be higher than mono-anchors. Even in some case, although dianchor dye would have lower dye-loading than mono-anchor counterpart but the former have double DA units thus might still have higher light harvesting.33 There are several crucial aspects regarding the different architectures of dianchoring dyes and their effects on the photophysical properties and photovoltaic performances. For example, the distance between two anchoring units is one of the crucial factors. The large distances between two anchors, such as the linear/ladder-shaped or fused coplanar molecules having the anchoring units on the opposite periphery, generally do not perform well in spite of the higher absorption profile.34-37 This can be rationally attributed to their relative position of anchors that is far-separated each other such that they can neither bind simultaneously nor acquire extensive area to bind TiO2, rendering the reduced dye loading. Conversely, if the distance between two anchor units is rather close, for example, dye configured as a (D-π-A)2 dyad, then there may exhibit serious intramolecular steric interactions between two constituting D-π-A units. As a result, the two anchors are also difficult to bind TiO2 simultaneously. Hence, an optimal distance between two anchoring units seems to be a prerequisite for boosting the cell efficiency. The monoachoring congener (D-π-A) can be inter-connected by either donor or spacer segments to produce dianchoring dyes with various architectures that ultimately modulate the photovoltaic performance. Typically, π-bridge or σ-bridge was employed to combine the two D-π-A units, yielding the dianchoring dyes with different cell performance, depending on the structural features employed.38-43 Among various structures, dianchor dyes with directly linked (D-π-A)2 structures are scarce. In addition to the synthetic challenges, directly connect two D-π-A units without employing any bridge unit may impart twisted conformation and thus induce non-planar structure. This may cause spectral blue shift and hence is disadvantageous to the cell performance. However, it is worth of note that the sensitizer should be adequately loaded and packed to act as a blocking layer between semiconductor surface and electrolyte for reducing undesired dark current.44,45 Accordingly, the packing of planar dye occasionally encounters severe aggregation. In this regard, tailor-made (D-π-A)2type molecules with highly twisted configuration may offer the advantage to overcome dye aggregation and hence facilitate the charge-recombination.46 Hence, (D-π-A)2-type molecules



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

with two anchoring sites appropriately optimized to bind TiO2 may efficiently improve the solar cell performance. In this study, we chose dye D21L6 (see Figure 1) as the core D-π-A model unit for designing dual chromophore (D-π-A)2-type dianchoring dye because of its high incident monochromatic photon to current conversion efficiency and remarkable photostability.47 Moreover, the presence of terminal hexyloxy group on the diphenylamino donor moiety assures to improve the electron-donating ability as well as to retard the dye aggregation. Noteworthily, based on D21L6, a few of dianchor-type examples have been reported where the phenylamine units are bridged through different π-conjugates (DC1-5, Figure 1).48-50 Unfortunately, these dianchor dyes suffered inferior efficiency as compared to that of parent D21L6 due to severe aggregation and poor dye loading. In this work, we report two new dianchoring dyes where two D-π-A (D21L6) units were directly connected either through phenylene or thiophene designated as BPDTA and BTTA, respectively (Figure 2). The direct connection of two D21L6 molecules without a spacer imposes strong propensity on these new molecules, giving highly twisted conformation that is beneficial for reducing dye aggregation. Moreover, the two weakly coupled individual D-π-A chromophores can modulate the binding distance on TiO2 surface depending on the distance between the connection point (phenylene or thiophene) and the anchor group (cyanoacrylic acid). The results from FT-IR spectroscopy indicated that two anchors of dyes BPDTA and BTTA are bound to TiO2 simultaneously with favourable position, the relative position of which can be interpreted by DFT calculations. The corresponding DSSCs performance based on these new dyes is superior to that of D21L6. The BTTA-based cell yielded an optimized efficiency of 6.86% (~92% of N-719 model dye under similar condition) with JSC, VOC and FF of 14.71 mA/cm2, 0.74 V and 0.63, respectively, which is among the best efficiency reported for dianchor DSSC dyes.51-54 Details of results and discussion are elaborated as follows.


Page 4 of 27

Page 5 of 27

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


Figure 1. Molecular structure of the D21L6 and its extended dianchoring dyes.

Figure 2. Molecular structures of two new sensitizers BPDTA and BTTA. 2. RESULT AND DISCUSSIONS 2.1. Synthesis Different strategies were employed for the synthesis of two dyes BPDTA and BTTA. For the synthesis of BPDTA (Scheme 1), the reaction initially proceeded by introducing diarylamine onto dibromo 1 under Buckwald-Hartwig C-N bond coupling55,56 condition to give 2, which was then either iodinated or brominated by NIS or NBS respectively to give the intermediate 3 with excellent yield. The iodo or bromo derivative underwent Stille coupling57 with tin reagent derived from bithiophene acetal to afford compound 4. It was obvious to have higher product yield with the iodo derivative than that of bromo counterpart. Finally, 4



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

Page 6 of 27

was hydrolyzed to furnish dialdehyde 5, which was converted to BPDTA by Knoevenagel reaction.58

Scheme 1. The synthetic route of BPDTA. The synthesis of BTTA (Scheme 2) was initiated by the introduction of thiophene onto the trimethylsilyl (TMS) group protected dibromo compound 6 by Stille coupling reaction. The product was then lithiated with LDA and subsequently reacted with DMF to furnish dialdehyde 8. The TMS group of 8 was converted to the bromo group with NBS. Finally, the Stille coupling with tin derivative 10 yielded 11, followed by the Knoevenagel reaction to give BTTA.


Page 7 of 27

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


Scheme 2. The synthetic route of BTTA.

2.2. Photophysical Properties The absorption spectra of the dyes BPDTA and BTTA recorded in dichloromethane are displayed in Figure 3. As shown, BPDTA and BTTA in CH2Cl2 exhibited similar absorption profile, having three distinct bands that are comparable to those of the parent dye D21L6. The higher lying absorption bands are mainly attributed to the π-π* transition, while the band appeared in the visible region is ascribed to the charge transfer transitions between aminedonor and cyanoacrylic acid acceptor mixed, to a certain extent, with delocalized π-π* transitions. It is obvious that the extinction coefficient of BPDTA and BTTA is higher than that of D21L6 because of the presence of two D-π-A chromophores, which in turn is beneficial for better light harvesting capability. Interestingly, compared with D21L6 slightly blue shift of the lowest lying absorption peak wavelength was observed for both BPDTA and BTTA. The result can be rationalized by the loss of co-planarity for BPDTA and BTTA in



the individual π-backbone due to their highly twisted conformation stemming from the steric interactions between two D-π-A components. The absorption spectra (Figure 4) of the dyes anchored on TiO2 displayed broad, featureless and blue-shifted spectral feature, which is in stark contrast to those observed in dichloromethane solution (Figure 3). This hypsochromic shift is typically attributed to the deprotonation of carboxylic acid units after chelation with TiO2 surface. As shown in Figure 4, under the same TiO2 thickness (7 µm) the absorbance of BTTA and BPDTA is higher than that of the mono anchor D21L6, manifesting their higher capability of light harvesting. In addition, the absorbance of BTTA is higher than that of BPDTA, which can be rationally ascribed to higher BTTA dye loading because of the shorter binding distance between two anchors. This viewpoint will be verified by theoretical analysis elaborated in the later section.

60

-‐1 c m -‐1 ) ε ( 10 3 M

ε (10 3 M -‐1 c m -‐1 )

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

Page 8 of 27

D 21L 6 B P D T A B T T A

40

20

0 300

400

500

600

W av elen g th (n m ) Figure 3. Absorption spectra of the dyes recorded for CH2Cl2 solutions.


700

Page 9 of 27

4

A b s o rb a n c e (a .u .)

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



3

2

1

0 400

500

600

700

Wav elen g th (n m ) Figure 4. Absorption spectra of various dyes anchoring on the porous TiO2 nanoparticle films (7 µm in thickness).

Table 1. Photophysical and electrochemical characteristics of the dyes Dye

λabs, nm (ε, M-1 cm-1)a

λabs on TiO2, nmb

E0-0, eVc

Eox, Vd

Eox*, Ve

D21L6

516 (2.87 × 104)

443

2.02

0.82

-1.20

BPDTA

503 (4.81 × 104)

450

2.03

0.88

-1.15

BTTA

483 (3.70 × 104)

467

2.05

0.87

-1.18

a

Absorption spectra recorded in CH2Cl2 (10-5 M). b Absorption spectra of the dyes anchored

on 7 µm porous TiO2 nanoparticle films. absorption spectra in CH2Cl2.

d

c

E0-0 was determined from the onset of the

Oxidation potentials were measured by CV in CH2Cl2;

potential was measured versus FC/FC+ were converted to normal hydrogen electrode (NHE) by addition of 630 mV. e Excited state oxidation potential was calculated by Eox - E0-0.

The FT-IR analyses on the dyes anchored on TiO2 were then performed to probe the surface interactions between dye and semiconductor. The results shown in Figure 5 indicate that the CN band remains intact whereas the -COOH band disappeared as compared to those of free neat dyes. In particular, the appearance of characteristic stretching absorption band of



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

Page 10 of 27

COO– along with complete disappearance of the COOH band confirmed the involvement of both carboxylic acids anchoring to TiO2. Such bidentate chelation of dye is beneficial to the higher electron injection, leading to higher cell performance (vide infra).

Figure 5. FT-IR spectra of pristine dye (top) and dye adsorbed on the TiO2 film (bottom).

2.3. Electrochemical Properties The cyclic voltammetry experiments were performed to investigate the electrochemical characteristics of BPDTA and BTTA for interpreting the thermodynamic driving forces accessible for the electron injection and dye regeneration. Both BPDTA and BTTA showed two oxidation potential waves together with one irreversible reduction wave (Figure 6). The higher oxidation wave was irreversible and other was quasi-reversible for dianchoring dyes (BPDTA and BTTA). In comparison, D21L6 showed both quasi-reversible oxidation peaks. The lower oxidation potential can be assigned to the oxidation of the chromophore having triphenylamine associated π-backbone. The higher oxidation peak with a large potential difference against the first one indicated that the second oxidation potential occurred on the same chromophore with delocalized radical cation characteristics. The reduction wave is attributed to the cyanoacrylic acid coupled with π-backbone. The oxidation potential of BPDTA and BTTA was anodically shifted as compared to that of monoanchoring D21L6. The highly twisted conformation, to a certain degree, reduces the electron delocalization from donor to acceptor along the linear π-conjugation backbone, lowering down both the oxidation and reduction potentials for BPDTA and BTTA.


Page 11 of 27

D 21L 6

B TTA

C u rre n t

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


B P DTA

1.5

1.0

0.5

0.0

-‐0.5

P o ten tial ( V v s . A g /A g C l )

-‐1.0

-‐1.5

Figure 6. Cyclic voltammograms of the dyes recorded in CH2Cl2. The excited state oxidation potential of BPDTA and BTTA were calculated and found remarkably more negative than the TiO2 conduction band (-0.5 V versus NHE)59 which is favorable for electron injection (Table 1, Figure 7). On the other hand, the oxidation potential is more positive than the I-/I3- redox couple (0.4 V versus NHE),60 which ascertains the efficient dye regeneration reaction of the dye cation radical by recapture of the injected electrons from electrolyte. The Eox of BPDTA and BTTA was lower than that of D21L6, which should be in favor of dye regeneration. Moreover, compared with BPDTA, the photoexcited BTTA is expected to undergo more favorable electron injection into TiO2 owing to its marginally higher Eox* value.



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

Page 12 of 27

Figure 7. Energy level diagram showing the ground and excited state oxidation potentials for the studied dyes.

2.4. Computational Approach To gain in-depth insight into the electronic features of the dyes, the geometries of BPDTA and BTTA were optimized with Gaussian 09 program,61 in which density functional theory62 was employed using B3LYP with 6-31G(d,p) basis sets in a polarizable continuum model (PCM)63 using CH2Cl2 as media. The computed lowest energy excitation along with their oscillator strengths and their assignments are collected in the Table 2. Note that the computed lowest electronic transitions and oscillator strength are in agreement with the experimentally observed onset of the lowest lying absorption and high peak molar extinction coefficients, respectively. The frontier molecular orbitals (HOMO-1, HOMO, LUMO, LUMO+1) for the dyes BPDTA and BTTA are shown in Figure 8. For both BPDTA and BTTA the HOMO is well localized on the tripheylamine-donor and marginally spread to the thiophene while the LUMO was contributed mainly from cyanoacrylic acid associated, in part, with thiophene. The relative HOMO-LUMO overlap was found to be more distinctive in BTTA, which may be responsible for its broader absorption. It was noteworthy that the HOMO /HOMO-1 as well as LUMO/ LUMO+1 distribution was quite similar, manifesting that the lowest electronic transitions comprise mainly charge transfer and thus the electron migration from donor to acceptor can be predicted upon photoexcitation from HOMO/ HOMO-1 to LUMO /LUMO+1. The favourable photoinduced electron transfer may facilitate the charge separation in the molecule and the subsequent electron injection into the conduction band of TiO2. Table 2. Computed vertical excitation wavelengths, oscillator strengths (f) and their orbital contribution for the dyes using B3LYP/6-31G(d,p) for CH2Cl2 with PCM Model. Dye

λabs, nm (calc.)

BPDTA

670.4

f

Assignment

0.679 HOMO-1→LUMO+1(43%) HOMO-1→LUMO(23%) HOMO→LUMO(22%) HOMO→LUMO+1(12%)

BTTA

685.2

0.579 HOMO-1→LUMO+1(63%) HOMO→LUMO(37%)


Page 13 of 27

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


Figure 8. Electronic distribution in the frontier molecular orbitals of the BPDTA and BTTA. The intermolecular interactions between dye and TiO2 nanocluster ((TiO2)70) were also analyzed to shed light on the binding nature of dye on the semiconductor surface.64 The results of calculation shown in Figure 9 revealed firm bidentate binding of the dyes onto (TiO2)70, in which the distance between two carboxylic units was calculated to be 19.50 Å for BPDTA and 12.04 Å for BTTA (Figure 9), supporting the conclusion made in the FT-IR measurement (vide supra). The well-defined distance between two anchoring units is necessary for the provocative photovoltaic parameters. The closer vicinity of anchoring units in BTTA ascertains its more confined binding on TiO2 and thus better loading than BPDTA.



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

Page 14 of 27

Figure 9. Contour plot for the lowest energy transition of dye/(TiO2)70. Pink and green meshes represent HOMO and LUMO, respectively. 2.5. Photovoltaic Properties The photovoltaic characteristics of D21L6, BPDTA, and BTTA as the sensitizers in DSSCs were evaluated with a sandwich DSSC cell comprising 0.6 M 1-butyl-3methylimidazolium iodide (BMII), 0.05 M LiI, 0.03 M I2, 0.5 M 4-tert-butylpyridine (4TBP), and 0.1 M guanidinium thiocyanate (GuSCN) in a mixture of acetonitrile-valeronitrile (85: 15, v/v) as the redox electrolytes (details of the device preparation and characterization are described in the Experimental Section). The incident monochromatic photon-to-current conversion efficiency (IPCE) spectra of the DSSCs are shown in Figure 10. The IPCE of the device using the D21L6 sensitizer exceeds 70% in the spectral region ranging from 420 to 530 nm and reaches a maximum of 76% at 450 nm. In comparison, the IPCE spectra of BPDTA and BTTA exhibit significantly higher values than that of D21L6, showing IPCEs >70% from 340 to 590 nm maximized at 88-90% (380-540 nm), which correlates well with the absorption behaviours.


Page 15 of 27

100 D 21L 6 B P D T A

80

IP C E (% )

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


B T T A

60 40 20 0 300

400

500

600

700

Wa v elen g th (n m ) Figure 10. IPCE spectra of DSSCs based on D21L6, BPDTA, and BTTA. Figure 11 shows the current density-voltage (J-V) characteristics of the DSSCs under standard global AM 1.5G 100 mW/cm2 solar irradiation. The photovoltaic parameters including short-circuit photocurrent density (JSC), open-circuit voltage (VOC), fill factor (FF), and the overall conversion efficiency (η) are listed in Table 3. The standard deviations of five devices made under identical conditions are also listed in Table 3. The JSC, VOC, and FF of the BPDTA cell and the BTTA cell are (14.42 mA/cm2, 0.74 V, 0.62) and (14.71 mA/cm2, 0.74 V, 0.63), respectively, yielding rather high overall power conversion efficiencies (PCE) of 6.61% and 6.86%, respectively. Under the same conditions, DSSC based on D21L6 shows inferior conversion efficiencies of 5.33% with relatively lower JSC (12.11 mA/cm2) and VOC (0.71 V). Moreover, the dyes have much higher efficiency than previously reported D21L6based dual-anchoring dyes DC1-DC5.48-50



Table 3. DSSC performance parameters of the dyes.a Dye

JSC, mA cm-2

VOC, V

FF

D21L6

12.11±0.06

0.71±0.01

0.62

5.33±0.10

BPDTA

14.42±0.05

0.74±0.01

0.62

6.61±0.11

BTTA

14.71±0.06

0.74±0.01

0.63

6.86±0.11

N719

15.08±0.07

0.74±0.01

0.67

7.48±0.13

DC1

2.50

0.52

0.78

1.00

DC2

3.72

0.58

0.66

1.43

DC3

9.25

0.63

0.61

3.56

DC4

7.92

0.62

0.68

3.34

DC5

1.34

0.45

0.76

0.46

a

Efficiency, %

Performances of DSSCs based on D21L6, BPDTA, BTTA, and N719 were measured

with a 0.125 cm2 working area. Irradiating light was AM 1.5G light (100 mW/cm2). All the parameters are in the absence of co-adsorbent including for DC1-DC5.

16 -‐2

C u rre n t D e n s ity ( m A c m )

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

Page 16 of 27

12

8

4


0 0.0

0.2

0.4

0.6

0.8

V o ltag e (V ) Figure 11. Current density-voltage curves for DSSCs based on D21L6, BPDTA, and BTTA organic dyes under AM 1.5G simulated solar light (100 mW/cm2).


Page 17 of 27

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


The higher VOC of the dianchors, BPDTA and BTTA can be attributed to the lower lying HOMO than the monoanchor D21L6;2 while their higher absorptivity rendered higher JSC. For a fair comparison, the N719-sensitized DSSC was also fabricated and tested under similar conditions (Table 3). The efficiency of the BTTA cell reaches ~92% of the N719 cell efficiency. It is worthy to observe the same VOC for BPDTA, BTTA, and N719. The higher JSC and efficiency of BTTA (cf. BPDTA) can be attributed to its higher LUMO as well as close vicinity of two anchoring units on semiconductor surface, resulting better dye-loading and thus more charge collection on TiO2 despite of its inferior absorption properties. The device properties were further characterized by electrochemical impedance spectroscopy (EIS). EIS is a useful tool for characterizing important interfacial chargetransfer processes in DSSCs, such as the electron transfer/charge recombination at the TiO2/dye/electrolyte interface, electron transport in the TiO2 electrode, electron transfer at the counter electrode and I3- transport in the electrolytes etc.65 In this study, the impedance spectroscopy was carried out by subjecting the cell to the constant AM 1.5G 100 mW/cm2 illumination and to the bias at the open-circuit voltage VOC of the cell, namely, under the conditions of no dc electric current, for better manifesting the process at the TiO2/dye/electrolyte interface of a cell under operation. Figure 12 shows the EIS Nyquist plots (i.e., minus imaginary part of the impedance, -Z", vs. the real part of the impedance, Z', when sweeping the frequency) for DSSCs based on organic dyes. For the frequency range investigated (0.1 Hz to 1 MHz), three regimes are generally distinguished: a small semicircle in the lowest frequency range (∼0.1 to 1 Hz), a larger semicircle in the middle frequency range (~1 to 100 Hz) and another smaller semicircle in the highest frequency range (> 100 Hz). With the bias illumination and voltage are applied, the small semicircle at lowest frequencies is associated with ion diffusion in the electrolyte, the larger semicircle at middle frequencies corresponds to the charge-transfer processes at the TiO2/dye/electrolyte interface, while the smaller semicircle at highest frequencies corresponds to the charge-transfer processes at the Pt/electrolyte interface. In Figure 12, one sees that DSSCs based on BPDTA and BTTA show more similar impedance characteristics in their Nyquist plots. The D21L6 cell, on the other hand, shows significantly larger middle-frequency semicircle. The larger width of the middle-frequency semicircles of D21L6 cell indicates a larger charge-transfer resistance at the TiO2/dye/electrolyte interface. The smaller middle-frequency semicircle widths together with higher DSSC efficiencies of BPDTA and BTTA cells strongly suggest more efficient electron generation and thus larger electron population at their



TiO2/dye/electrolyte interfaces. Thus, EIS results are in good agreement with the results of JSC, IPCE, and overall power conversion efficiencies of the DSSCs.

40 D 21L 6 B P D T A B T T A

30

20

-‐ Z '' (o h m )

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

Page 18 of 27

10

0 0

40

80

120

160

Z ' (o h m ) Figure 12. EIS Nyquist plots for DSSCs based on D21L6, BPDTA, and BTTA organic dyes. Figure 13 shows the EIS Bode plots (i.e., the phase of the impedance vs. the frequency) of the DSSCs based on D21L6, BPDTA, and BTTA dyes. The characteristic frequency of the middle frequency peak in the Bode plot is related to the charge recombination rate, and its reciprocal is associated with the electron lifetime. The electron lifetime values of the DSSCs based on D21L6, BPDTA, and BTTA organic dyes are 7.96, 13.14, and 13.09 ms, respectively. The electron lifetime values of the DSSCs based on BPDTA and BTTA are larger than those of DSSCs based on D21L6. These factors may in turn lead to the higher VOC (30 mV higher) of the BPDTA and BTTA cells than that of the D21L6 cell.


Page 19 of 27

-‐30 D 21L 6

-‐25

θ (D e g )

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


B P D T A B T T A

-‐20 -‐15 -‐10 -‐5 0 -‐1 10

10

0

10

1

10

2

10

3

10

4

10

5

F req u en c y (H z ) Figure 13. The EIS Bode plots of the DSSCs based on D21L6, BPDTA, and BTTA dyes.

3. CONCLUSION In summary, two dianchoring dyes BPDTA and BTTA, featuring two D21L6 D-π-A units connected directly in a symmetrical fashion at different positions, i.e., phenylene for BPDTA and thiophene for BTTA, of the π-conjugated backbone to investigate the impact on physical and photovoltaic properties. The steric interactions between two directly linked D-π-A chromophores lead to highly twisted molecular structure, which is beneficial for suppressing dye aggregations upon loaded on TiO2. These dianchoring dyes are bound to TiO2 efficiently with desired bidentate fashion as evidenced by the absence of COO- band observed by FT-IR analysis. The distance between two binding sites was calculated to be 19.50 Å for BPDTA and 12.04 Å for BTTA, the latter of which has closer proximity between two anchoring units, resulting in superior dye loading and thus higher efficiency. The device fabricated using these dyes yielded efficiency of 6.61% and 6.86% for BPDTA and BTTA, respectively, while an inferior performance of 5.33% was observed for D21L6. The devices employing these new dyes performed much better than previously reported D21L6-based dianchoring dyes (DC1DC5) without the aid of co-adsorbents, signifying the advantage of current molecular design strategy.



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

Page 20 of 27

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI: Experimental details, characterization data of the compounds, copies of 13

1

H, and

C NMR spectra for new compounds.

AUTHOR INFORMATION Corresponding Authors *

E-mail: [email protected] (C.-H.T.); [email protected] (P.-T.C); [email protected]

(K.-T.W.) ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from Ministry of Science and Technology (MOST) of Taiwan (Grant Nos. 104-2113-M-002-006-MY3 and 104-2221-E259-020). REFERENCES (1) O'Regan, B.; Grätzel M. A Low-Cost, High-Efficiency Solar Cell Based on DyeSensitized Colloidal TiO2 Films. Nature 1991, 353, 737-740. (2) Ning, Z.; Fu, Y.; Tian, H. Improvement of Dye-Sensitized Solar Cells: What We Know and What We Need to Know. Energy Environ. Sci. 2010, 3, 1170-1181. (3) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-Sensitized Solar Cells. Chem. Rev. 2010, 110, 6595-6663. (4) Jung, H. S.; Lee, J.-K. Dye Sensitized Solar Cells for Economically Viable Photovoltaic Systems. J. Phys. Chem. Lett. 2013, 4, 1682-1693. (5) Robertson, N. Optimizing Dyes for Dye-Sensitized Solar Cells. Angew. Chem. Int. Ed. 2006, 45, 2338-2345. (6) Clifford, J. N.; Ferrero, M. E.; Viterisi, A.; Palomares, E. Sensitizer Molecular StructureDdevice Efficiency Relationship in Dye Sensitized Solar Cells. Chem. Soc. Rev. 2011, 40, 1635-1646.


Page 21 of 27

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


(7) Chen, C.-Y.; Wang, M.; Li, J.-Y.; Pootrakulchote, N.; Alibabaei, L.; Ngoc-le, C.-H.; Decoppet J.-D.; Tsai, J.-H.; Grätzel, C.; Wu, C.-G.; Zakeeruddin, S. M.; Grätzel, M. Highly Efficient Light-Harvesting Ruthenium Sensitizer for Thin-Film Dye-Sensitized Solar Cells. ACS Nano 2009, 3, 3103-3109. (8) Aghazada, S.; Gao, P.; Yella, A.; Marotta, G.; Moehl, T.; Teuscher, J.; Moser, J.-E.; Angelis, F. D., Grätzel, M.; Nazeeruddin, M. K. Ligand Engineering for the Efficient DyeSensitized Solar Cells with Ruthenium Sensitizers and Cobalt Electrolytes. Inorg. Chem. 2016, 55, 6653-6659. (9) Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Grätzel, M. Porphyrin-Sensitized Solar Cells with Cobalt (II/III)-Based Redox Electrolyte Exceed 12 Percent Efficiency. Science 2011, 334, 629-634. (10) Mathew, S.; Yella, A.; Gao, P.; Baker, R. H.; Curchod, B. F. E.; Astani, N. A.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Dye-Sensitized Solar Cells with 13% Efficiency Achieved Through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6, 242-247. (11) Choi, H.; Paek, S.; Lim, N.; Lee, Y. H.; Nazeeruddin, M. K.; Ko, J. Efficient Perovskite Solar Cells with 13.63% Efficiency Based on Planar Triphenylamine Hole Conductors. Chem. Eur. J. 2014, 20, 10894-10899. (12) Meng, T.; Liu, C.; Wang, K.; He, T.; Zhu, Y.; Al-Enizi, A.; Elzatahry, A.; Gong, X. High Performance Perovskite Hybrid Solar Cells with E-Beam-Processed TiOx Electron Extraction Layer. ACS Appl. Mater. Interfaces 2016, 8, 1876-1883. (13) Mishra, A.; Fischer M. K. R.; Bäuerle, P. Metal-Free Organic Dyes for Dye-Sensitized Solar Cells: From Structure: Property Relationships to Design Rule. Angew. Chem. Int. Ed. 2009, 48, 2474-2499. (14) Ooyama, Y.; Harima, Y. Molecular Structures of Organic Dyes for Dye-Sensitized Solar Cells. ChemPhysChem 2012, 13, 4032-4080. (15) Ahmad, S.; Guillén, E.; Kavan, L.; Grätzel, M.; Nazeeruddin, M. K. Metal Free Sensitizer and Catalyst for Dye-Sensitized Solar Cells. Energy Environ. Sci. 2013, 6, 34393466.



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

Page 22 of 27

(16) Kim, B.-G.; Chung, K.; Kim, J. Molecular Design Principle of All-Organic Dyes for Dye-Sensitized Solar Cells. Chem. Eur. J. 2013, 19, 5220-5230. (17) Kloo, L. On the Early Development of Organic Dyes for Dye-Sensitized Solar Cells. Chem. Commun. 2013, 49, 6580-6583. (18) Liang, M.; Chen, J. Arylamine Organic Dyes for Dye-sensitized Solar cells. Chem. Soc. Rev. 2013, 42, 3453-3488. (19) Yang, Z.; Shao, C.; Cao, D. Screening Donor Groups of Organic Dyes for DyeSensitized Solar Cells. RSC Adv. 2015, 5, 22892-22898. (20) Chou, S.-H.; Tsai, C.-H.; Wu, C.-C.; Kumar, D.; Wong, K.-T. Regioisomeric Effects on the Electronic Features of Indenothiophene-Bridged D-π-A'-A DSSC Sensitizers. Chem. Eur. J. 2014, 20, 16574-16582. (21) Ooyama, Y.; Harima, Y. Molecular Designs and Syntheses of Organic Dyes for DyeSensitized Solar Cells. Eur. J. Org. Chem. 2009, 2903-2934. (22) Pastore, M.; Angelis, F. D. Aggregation of Organic Dyes on TiO2 in Dye-Sensitized Solar Cells Models: An ab initio Investigation. ACS Nano 2010, 4, 556-562. (23) Feng, S.; Li, Q.-S.; Sun, P.-P.; Niehaus, T. A.; Li, Z.-S. Dynamic Characteristics of Aggregation Effects of Organic Dyes in Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 22504-22514. (24) Lu, M.; Liang, M.; Han, H.-Y.; Sun, Z.; Xue, S. Organic Dyes Incorporating BisHexapropyltruxeneamino Moiety for Efficient Dye-Sensitized Solar Cells. J. Phys. Chem. C 2011, 115, 274-281. (25) Feng, Q.; Zhang, Q.; Lu, X.; Wang, H.; Zhou, G.; Wang, Z.-S. Facile and Selective Synthesis of Oligothiophene-Based Sensitizer Isomers: An Approach Toward Efficient DyeSensitized Solar Cells. ACS Appl. Mater. Interfaces 2013, 5, 8982-8990. (26) Demeter, D.; Roncali, J.; Jungsuttiwong, S.; Melchiorre, F.; Biagini, P.; Po, R. Linearly π-Conjugated Oligothiophenes as Simple Metal-Free Sensitizers for Dye-Sensitized Solar Cells. J. Mater. Chem. C 2015, 3, 7756-7761. (27) Shen, Z.; Chen, J.; Li, X.; Li, X.; Zhou, Y.; Yu, Y.; Ding, H.; Li, J.; Zhu, L.; Hua, J. Synthesis and Photovoltaic Properties of Powerful Electron-Donating Indeno[1,2-


Page 23 of 27

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


b]thiophene-Based Green D-A-π-A Sensitizers for Dye-Sensitized Solar Cells. ACS Sustainable Chem. Eng. 2016, 4, 3518-3525. (28) Murakami, T. N.; Koumura, N.; Yoshida, E.; Funaki, T.; Takano, S.; Kimura, M.; Mori, S. An Alkyloxyphenyl Group as Sterically Hindered Substituent on a Triphenylamine Donor Dye for Effective Recombination Inhibition in Dye-Sensitized Solar Cells. Langmuir 2016, 32, 1178-1183. (29) Hong, Y.; Liao, J.-Y.; Cao, D.; Zang, X.; Kuang, D.-B.; Wang, L.; Meier, H.; Su, C.-Y. Organic Dye Bearing Asymmetric Double Donor-π- Acceptor Chains for Dye-Sensitized Solar Cells. J. Org. Chem. 2011, 76, 8015-8021. (30) Ren, X.; Jiang, S.; Cha, M.; Zhou, G.; Wang, Z.-S. Thiophene-Bridged Double D-π-A Dye for Efficient Dye-Sensitized Solar Cell. Chem. Mater. 2012, 24, 3493-3499. (31) Li, H.; Hou, Y.; Yang, Y.; Tang, R.; Chen, J.; Wang, H.; Han, H.; Peng, T.; Li, Q.; Li, Z. Attempt to Improve the Performance of Pyrrole-Containing Dyes in Dye-Sensitized Solar Cells by Adjusting Isolation Groups. ACS Appl. Mater. Interfaces 2013, 5, 12469-12477. (32) Manfredi, N.; Cecconi, B.; Abbotto, A. Multi-Branched Multi Anchoring Metal-Free Dyes for Dye-Sensitized Solar Cells. Eur. J. Org. Chem. 2014, 7069-7086. (33) Cao, D.; Peng, J.; Hong, Y.; Fang, X.; Wang, L.; Meier, H. Enhanced Performance of the Dye-Sensitized Solar Cells with Phenothiazine-Based Dyes Containing Double D-A Branches. Org. Lett. 2011, 13, 1610-1613. (34) Ambre, R.; Chen, K.-B.; Yao, C.-F.; Luo, L.; Diau, E. E.-G. Effect of Porphyrinic mesoSubstituents on the Photovoltaic Performance of Dye-Sensitized Solar Cells: Number and Position of p-Carboxyphenyl and Thienyl Groups on Zinc Porphyrins. J. Phys. Chem. C 2012, 116, 11907-11916. (35) Yu, L.; Xi, J.; Chan, H. T.; Su, T.; Antrobus, L. J.; Tong, B.; Dong, Y.; Chan, W. K.; Phillips, D. L. Novel Organic D-π-2A Sensitizer for Dye Sensitized Solar Cells and its Electron Transfer Kinetics on TiO2 Surface. J. Phys. Chem. C 2013, 117, 2041-2052. (36) Lee, T.-H.; Hsu, C.-Y.; Liao, Y.-Y.; Chou, H.-H.; Hughes, H.; Lin, J. T. Dye-Sensitized Solar Cells Based on (Donor-π-Acceptor)2 Dyes with Dithiafulvalene as the Donor. Chem. Asian J. 2014, 9, 1933-1942.



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

Page 24 of 27

(37) Dai, X.-X.; Feng, H.-L.; Huang, Z.-S.; Wang, M.-J.; Wang, L.; Kuang, D.-B.; Meier, H.; Cao, D. Synthesis of Phenothiazine-Based Di-Anchoring Dyes Containing Fluorene Linker and Their Photovoltaic Performance. Dyes Pigm. 2015, 114, 47-54. (38) Abbotto, A.; Leandri, V.; Manfredi, N.; Angelis, F. D.; Pastore, M.; Yum, J.-H.; Nazeeruddin, M. K.; Grätzel, M. Bis-Donor-Bis-Acceptor Tribranched Organic Sensitizers for Dye-Sensitized Solar Cells. Eur. J. Org. Chem. 2011, 6195-6205. (39) Li, Q.; Shi, J.; Li, H.; Li, S.; Zhong, C.; Guo, F.; Peng, M.; Hua, J.; Qin, J.; Li, Z. Novel Pyrrole-Based Dyes for Dye-Sensitized Solar Cells: From Rod-Shape to "H" Type. J. Mater. Chem. 2012, 22, 6689-6696. (40) Lu, X.; Jia, X.; Wang, Z.-S.; Zhou, G. X-Shaped Organic Dyes with Quinoxaline Bridge for Use in Dye-Sensitized Solar Cells. J. Mater. Chem. A 2013, 1, 9697-9706. (41) Leandri, V.; Ruffo, R.; Trifiletti, V.; Abbotto, A. Asymmetric Tribranched Dyes: An Intramolecular Cosensitization Approach for Dye-Sensitized Solar Cells. Eur. J. Org. Chem 2013, 6793-6801. (42) Hong, Y.; Iqbal, Z.; Yin, X.; Cao, D. Synthesis of Double D-A Branched Organic Dyes Employing Indole and Phenoxazine as Donors for Efficient DSSCs. Tetrahedron 2014, 70, 6296-6302. (43) Huang, Z.-S.; Cai, C.; Zang, X.-F.; Iqbal, Z.; Zeng, H.; Kuang, D.-B.; Wang, L.; Meier, H.; Cao, D. Effect of the Linkage Location in Double Branched Organic Dyes on the Photovoltaic Performance of DSSCs. J. Mater. Chem A 2015, 3, 1333-1344. (44) Griffith, M. J.; James, M.; Triani, G.; Wagner, P.; Wallace, G. G.; Officer, D. L. Determining the Orientation and Molecular Packing of Organic Dyes on a TiO2 Surface Using X-ray Reflectometry. Langmuir 2011, 27, 12944-12950. (45) Grey, J. M.; Cole, J. M.; Evans, P. J.; Preferred Molecular Orientation of Coumarin 343 on TiO2 Surfaces: Application to Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 16404-16409. (46) Zhang, F.; Fan, J.; Yu, H.; Ke, Z.; Nie, C.; Kuang, D.; Shao, G.; Su, C. Nonplanar Organic Sensitizers Featuring a Tetraphenylethene Structure and Double ElectronWithdrawing Anchoring Groups. J. Org. Chem. 2015, 80, 9034-9040.


Page 25 of 27

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


(47) Yum, J.-H.; Hagberg, D. P.; Moon, S.-J.; Karlsson, K. M.; Marinado, T.; Sun, L.; Hagfeldt, A.; Nazeeruddin, M. K.; Grätzel, M. A Light-Resistant Organic Sensitizer for Solar-Cell Applications. Angew. Chem. Int. Ed. 2009, 48, 1576-1580. (48) Seo, K. D.; You, B. S.; Choi, I. T.; Ju, M. J.; You, M.; Kang, H. S.; Kim, H. K. DualChannel Anchorable Organic Dyes with Well-Defined Structures for Highly Efficient DyeSensitized Solar Cells. J. Mater. Chem. A 2013, 1, 9947-9953. (49) Seo, K. D.; You, B. S.; Choi, I. T.; Ju, M. J.; You, M.; Kang, H. S.; Kim, H. K. DyeSensitized Solar Cells Based on Organic Dual-Channel Anchorable Dyes with Well-Defined Core Bridge Structures. ChemSusChem 2013, 6, 2069-2073. (50) Seo, K. D.; You, B. S.; Choi, I. T.; Ju, M. J.; You, M.; Kang, H. S.; Kim, H. K. DualChannel Anchorable Organic Dye with Triphenylamine-Based Core Bridge Unit for DyeSensitized Solar Cells. Dyes Pigm. 2013, 99, 599-606. (51) Liu, J.; Zhang, J.; Xu, M.; Zhou, D.; Jing, X.; Wang, P. Mesoscopic Titania Solar Cells with the Tris(1,10-phenanthroline)cobalt Redox Shuttle: Uniped versus Biped Organic Dyes. Energy Environ. Sci. 2011, 4, 3021-3029. (52) Lin, R. Y.-Y.; Wu, F.-L.; Chang, C.-H.; Chou, H.-H.; Chuang, T.-M.; Chu, T.-C.; Hsu, C.-Y.; Chen, P.-W.; Ho, K.-C.; Lo, Y.-H.; Lin, J. T. Y-Shaped Metal-Free D-π-(A)2 Sensitizers for High-Performance Dye-Sensitized Solar Cells. J. Mater. Chem. A 2014, 2, 3092-3101. (53) Jiang, S.; Fan, S.; Lu, X.; Zhou, G.; Wang, Z.-S. Double D-π-A Branched Organic Dye Isomers for Dye-Sensitized Solar Cells. J. Mater. Chem. A 2014, 2, 17153-17164. (54) Hung, W.-I.; Liao, Y.-Y.; Lee, T.-H.; Ting, Y.-C.; Ni, J.-S.; Kao, W.-S.; Lin, J. T.; Wei, T.-C.; Yen, Y.-S. Eugenic Metal-Free Sensitizers with Double Anchors for High Performance Dye-Sensitized Solar Cells. Chem. Commun. 2015, 51, 2152-2155. (55) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buckwald, S. L. Rational Development of Practical Catalysts for Aromatic Carbon-Nitrogen Bond Formation. Acc. Chem. Res. 1998, 31, 805-818. (56) Hartwig, J. F.; Transition Metal Catalysed Synthesis of Arylamines and Aryl Ethers from Aryl Halides and Triflates: Scope and Mechanism. Angew. Chem. Int. Ed. 1998, 37, 2046-2067.



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

Page 26 of 27

(57) Stille, J. K. The Palladium-Catalyzed Cross-Coupling Reactions of Organotin Reagents with Organic Electrophiles. Angew. Chem. Int. Ed. Engl. 1986, 25, 508-524. (58) Knoevenagel, E. Ueber eine Darstellungsweise des Benzylidenacetessigesters. Ber. Dtsch. Chem. Ges. 1896, 29, 172-174. (59) Hagfeldt, A.; Grätzel, M. Light-Induced Redox Reactions in Nanocrystalline Systems. Chem. Rev. 1995, 95, 49-68. (60) Grätzel, M. Photoelectrochemical Ccells. Nature 2001, 414, 338-344. (61) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A. Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N; Staroverov, V. N; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.; Gaussian 09, revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (62) Adamo, C.; Jacquemin, D. The Calculations of Excited State Properties with TimeDependent Density Functional Theory. Chem. Soc. Rev. 2013, 42, 845-856. (63) Balanay, M.; Kim, D. H. DFT/TD-DFT Molecular Design of Porphyrin Analogues for Use in Dye-Sensitized Solar Cells. Phys. Chem. Chem. Phys. 2008, 10, 5121-5127. (64) Liu, S.-H.; Fu, H.; Cheng, Y.-M.; Wu, K.-L.; Ho, S.-T.; Chi, Y.; Chou, P.-T. Theoretical Study of N749 Dyes Anchoring on the (TiO2)28 Surface in DSSCs and Their Electronic Absorption Properties. J. Phys. Chem. C 2012, 116, 16338-16345. (65) Wang, Q.; Moser, J.-E.; Grätzel, M. Electrochemical Impedance Spectroscopic Analysis of Dye-Sensitized Solar Cells. J. Phys. Chem. B 2005, 109, 14945-14953.


Page 27 of 27

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


Graphic Abstract:


Highly Twisted Dianchoring DâÏâA Sensitizers for Efficient Dye

Recommend Documents

Highly Twisted Dianchoring DâÏâA Sensitizers for Efficient Dye