Metal-Free Tetrathienoacene Sensitizers for High-Performance Dye-Sensitized Solar Cells

Article abstract of DOI:10.1021/ja513254z

A new series of metal-free organic chromophores (TPA-TTAR-A (1), TPA-T-TTAR-A (2), TPA-TTAR-T-A (3), and TPA-T-TTAR-T-A (4)) are synthesized for application in dye-sensitized solar cells (DSSC) based on a donor-π-bridge-acceptor (D-π-A) design. Here a simple triphenylamine (TPA) moiety serves as the electron donor, a cyanoacrylic acid as the electron acceptor and anchoring group, and a novel tetrathienoacene (TTA) as the π-bridge unit. Because of the extensively conjugated TTA π-bridge, these dyes exhibit high extinction coefficients (4.5-5.2 × 10⁴ M^-1 cm^-1). By strategically inserting a thiophene spacer on the donor or acceptor side of the molecules, the electronic structures of these TTA-based dyes can be readily tuned. Furthermore, addition of a thiophene spacer has a significant influence on the dye orientation and self-assembly modality on TiO₂ surfaces. The insertion of a thiophene between the π-bridge and the cyanoacrylic acid anchoring group in TPA-TTAR-T-A (dye 3) promotes more vertical dye orientation and denser packing on TiO₂ (molecular footprint = 79 ?²), thus enabling optimal dye loading. Using dye 3, a DSSC power conversion efficiency (PCE) of 10.1% with V_oc = 0.833 V, J_sc = 16.5 mA/cm², and FF = 70.0% is achieved, among the highest reported to date for metal-free organic DSSC sensitizers using an I^-/I₃^- redox shuttle. Photophysical measurements on dye-grafted TiO₂ films reveal that the additional thiophene unit in dye 3 enhances the electron injection efficiency, in agreement with the high quantum efficiency. (Graph Presented)

Full text of DOI:10.1021/ja513254z

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Article
Metal-Free Tetrathienoacene (TTA) Sensitizers
for High-Performance Dye-Sensitized Solar Cells
Nanjia Zhou, Kumaresan Prabakaran, Byunghong Lee, Sheng Hsiung Chang, Boris Harutyunyan, Peijun
Guo, Melanie R. Butler, Amod Timalsina, Michael J. Bedzyk, Mark A. Ratner, Sureshraju Vegiraju, Shuehlin
Yau, Chun-Guey Wu, Robert P. H. Chang, Antonio Facchetti, Ming-Chou Chen, and Tobin J. Marks
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja513254z • Publication Date (Web): 13 Mar 2015
Downloaded from http://pubs.acs.org on March 18, 2015
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MetalꢀFree Tetrathienoacene (TTA) Sensitizers for Highꢀ
Performance DyeꢀSensitized Solar Cells
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a‡
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Nanjia Zhou, Kumaresan Prabakaran, Byunghong Lee, Sheng Hsiung Chang, Boris Harutyunyan,
a
d
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a*
d*
Peijun Guo, Melanie R. Butler, Amod Timalsina, Michael J. Bedzyk, Mark A. Ratner, Sureshraju
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Vegiraju, Shuehlin Yau, ChunꢀGuey Wu, Robert P. H. Chang, * Antonio Facchetti, * MingꢀChou
b
d
Chen, * Tobin J. Marks *
a
Department of Materials Science and Engineering and the Materials Research Center, the Argonneꢀ
Northwestern Solar Energy Research Center, Northwestern University, 2145 Sheridan Road, Evanston,
Illinois, 60208, USA.
b
Department of Chemistry, National Central University, ChungꢀLi, Taiwan 32054, ROC.
c
Research Center for New Generation Photovoltaics, National Central University, ChungꢀLi, Taiwan
3
2054, ROC.
d
Department of Chemistry and the Materials Research Center, the ArgonneꢀNorthwestern Solar Energy
Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, USA.
e
Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077, USA.
‡
These authors contributed equally.
Abstract: A new series of metalꢀfree organic chromophores (TPAꢀTTARꢀA (1), TPAꢀTꢀTTARꢀA (2),
TPAꢀTTARꢀTꢀA (3), and TPAꢀTꢀTTARꢀTꢀA (4)) are synthesized for application in dye sensitized soꢀ
lar cells (DSSC) based on a donorꢀπꢀbridgeꢀacceptor (DꢀπꢀA) design. Here a simple triphenylamine
(TPA) moiety serves as the electron donor, a cyanoacrylic acid as the electron acceptor and anchoring
group, and a novel tetrathienoacene (TTA) as the πꢀbridge unit. Due to the extensively conjugated TTA
4
ꢀ1
ꢀ1
πꢀbridge, these dyes exhibit high extinction coefficients (4.5ꢀ5.2 × 10 M cm ). By strategically insertꢀ
ing a thiophene spacer on the donor or acceptor side of the molecules, the electronic structures of these
TTAꢀbased dyes can be readily tuned. Furthermore, addition of a thiophene spacer has a significant inꢀ
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fluence on the dye orientation and selfꢀassembly modality on TiO surfaces. The insertion of a thiophene
2
between the πꢀbridge and the cyanoacrylic acid anchoring group in TPAꢀTTARꢀTꢀA (dye 3) promotes
2
more vertical dye orientation and denser packing on TiO (molecular footprint = 79 Å ), thus enabling
2
optimal dye loading. Using dye 3, a DSSC power conversion efficiency (PCE) of 10.1% with V =
oc
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0.833 V, J = 16.5 mA/cm , and FF = 70.0 % is achieved, among the highest reported to date for metalꢀ
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free organic DSSC sensitizers using an I¯/I ¯ redox shuttle. Photophysical measurements on dyeꢀgrafted
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TiO films reveal that the additional thiophene unit in dye 3 enhances the electron injection efficiency,
2
in agreement with the high quantum efficiency.
Introduction
Since the first successful demonstration of dye sensitized solar cells (DSSCs) based on a
1
polypyridyl ruthenium (II) dye, the past decade has witnessed a steady increase of power conversion
2
efficiencies (PCEs) for metal complexꢀsensitized DSSCs. While each device component optimization
has impacted the progress of this technology, the understandingꢀbased development of more effective
sensitizers is undoubtedly one of the most promising components to investigate for further advances in
3ꢀ5
PCE. Among these sensitizers, the most prominent examples are the zinc porphyrin sensitizers:
SM371 and SM315, (A. Scheme 1) which provide PCEs of 12.5ꢀ13.0% when employing a Co(II/III)
2
,6,7
redox shuttle as the electrolyte system.
With the goal of broadening our understanding of DSSC
function and performance limitations, metalꢀfree organic photosensitizers have attracted great recent
attention owing to their: a) chemical versatility and facile synthetic approaches to diverse molecular
structures; b) energetic and structural tunability to realize broad and high levels of solar spectral absorpꢀ
tion (sizable molar extinction coefficients) within the visible region, and in some cases, the nearꢀIR reꢀ
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ꢀ11
gion as well; c) potential cost reductions vs. DSSCs employing noble metal containing dyes.
Neverꢀ
theless, despite these attractions, to date the performance of DSSCs based on metalꢀfree organic dyes
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has lagged behind those using traditional metalꢀorganic dyes, largely as a result of their high recombinaꢀ
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tion losses and lower open circuit voltages (V s).
However, note that a few recent reports have
oc
shown PCEs = 10.0ꢀ10.65% employing single metalꢀfree organic dyes (e.g. YA442, B. Scheme 1) in
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combination with a Co(II/III) redox shuttle, demonstrating the potential of this approach.
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Scheme 1. Examples of A. zinc porphyrinꢀbased DSSC dyes, B. The highest PCE metalꢀfree organic dye reported
to date, and C. metalꢀfree fused thiophene based DꢀπꢀA dyes. Power conversion efficiencies and redox shuttle
employed are given in blue.
The design of molecular DSSC dyes requires careful consideration of multiple optoꢀelectronic
properties, such as band alignment and optical absorption coefficient, as well as solidꢀstate properties
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1ꢀ23
such as dye aggregation, morphology, and mode of assembly on the TiO photoanode.
A promising
2
strategy for organic sensitizer design employs a donorꢀπꢀbridgeꢀacceptor (DꢀπꢀA) framework which can
24ꢀ28
satisfy these requirements.
Among them, most organic dyes typically feature a hydrophobic elecꢀ
tronꢀdonating triphenylamine (TPA) moiety connected via a πꢀconjugated bridge unit to an hydrophilic
electronꢀaccepting cyanoacrylic acid group which acts as the anchoring site on the TiO photoanode
2
2
7,29ꢀ31
(Scheme 1 and Figure 1).
For this class of dyes, the traditional focus has been to design πꢀ
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conjugated spacer units which facilitate charge transfer from the ground to excited state and increase
light absorption. Representative bridging units often employ wellꢀknown πꢀconjugated thiophene sysꢀ
tems, such as oligothiophenes, thienyleneꢀ vinylenes, in addition to fused thiophenes, including
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thienothiophenes (TT) and dithienothiophene (DTT).
These systems afford excellent efficiencies,
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up to 9.8% (I; Scheme 1). Note that there are several potential advantages to further extending the
bridge extent such as a smaller band gap for extended light absorption and a better match with the enerꢀ
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getics of the I /I redox shuttle, thereby enhancing DSSC charge transfer characteristics. For example,
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organic dyes based on the simplest fused thiophene, TT enable PCEs up to 7.8% (II; Scheme 1), while
34
with a more conjugated DTT, the maximum PCE is increased to 8.0% (III; Scheme 1). Towards creatꢀ
ing even more conjugated structures, an attractive organic dye design would be to introduce a highly
3
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planar tetrathienoacene (TTA) core as the πꢀbridge unit. We previously disclosed that this unit features
strong intramolecular SꢁꢁꢁS interactions enabling long range order in the solid state and excellent charge
transport in organic thin film transistors (OFETs) with hole/electron mobilities as high as 0.42 and 0.30
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ꢀ1 ꢀ1
37,38
cm V s , respectively.
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Figure 1. Schematic representation of the donorꢀπꢀbridgeꢀacceptor molecular dye design concept and the
chemical structures of TTA dyes 1ꢀ4.
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With the premise that additional core extension should enhance DSSC performance even further,
here we introduce a new TTAꢀbased sensitizer series. To maintain adequate dye processability, we have
introduced a long alkyl substituent (R = nꢀC H ) on the TTA core in all sensitizers, which also supꢀ
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presses dye aggregation and charge recombination on the TiO surface. In addition, we systematically
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investigate the effects of thiophene introduction between the bridge and the donor and/or acceptor moieꢀ
ties. Starting from the simplest TTAR structure, TPAꢀTTARꢀA (dye 1), a thiophene spacer is either inꢀ
serted on the donor side between TTAR and TPA to yield TPAꢀTꢀTTARꢀA (dye 2), or on the acceptor
side between TTAR and cyanoacrylic acid to yield TPAꢀTTARꢀTꢀA (dye 3), and finally on both sides
to yield TPAꢀTꢀTTARꢀTꢀA (dye 4) (Figure 1). These πꢀextended systems should improve panchromatic
lightꢀharvesting, as the result of the uplifted TTA highest occupied molecular orbital (HOMO) and lowꢀ
37,39
er lowest unoccupied molecular orbital (LUMO) compared to TT and DTT based molecules.
In this
work, a simple, baseline iodineꢀbased electrolyte is employed to investigate the DSSC characteristics. It
will be seen that the additional thiophene spacer provides a simple strategy to modify the optoelectronic,
chemical and physical properties of these organic sensitizers, and thereby enhance DSSC PCEs.
Results and Discussion
In the following section we discuss the design rationale and the synthesis of new metalꢀfree DSSC
dyes 1ꢀ4. Next, these systems are characterized at the molecular level using optical spectroscopy, moꢀ
lecular orbital computations, and cyclic voltammetry (CV). Subsequently, DSSC devices are fabricated
using these dyes to evaluate performance metrics. To better understand the device response, electroꢀ
chemical impedance (EIS) analysis is performed on the DSSCs, and dye molecular orientation and covꢀ
erage are characterized by Xꢀray reflectivity (XRR) and optical spectroscopic measurements. Finally,
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femtosecond timeꢀresolved photoluminescence (FTRꢀPL) measurements are performed to investigate in
detail the photophysical processes operative in these organic dyes.
Synthetic Strategy
The synthetic approaches to dyes 1ꢀ4 are presented in Scheme 2. Dyes 1 and 2 are obtained by reꢀ
acting bromoformyltetrathienoacene (6) with the corresponding aminoꢀdonor moieties 8 and 9 via Suꢀ
zuki and Stille coupling protocols to give formylated tetrathienoacenes 10 and 11, respectively. The reꢀ
sulting TTAs are then reacted with cyanoacetic acid in a Knoevenagel condensation to yield dyes 1 and
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in good yields. Dyes 3 and 4 were synthesized from dibromotetrathienoacene (7) in good yields via
three steps. First, dibromotetrathienoacene 7 is monoꢀcoupled with the corresponding aminoꢀdonor moiꢀ
eties 8 and 9 to afford the mono brominated tetrathienoacenes, 12 and 13, respectively. Secondly, the
resulting TTAs are then linked with 5ꢀ(1,3ꢀdioxolanꢀ2ꢀyl)thiophenꢀ2ꢀyl)tributylstannane via Stille couꢀ
pling to yield the corresponding aldehydes 14 and 15. Finally, both aldehydes are then treated with cyꢀ
anoacetic acid in the presence of piperidine to afford dyes 3 and 4. Note, the preparation of dyes 1 and 2
from the intermediate 7 was not successful. As shown in Scheme 2, 6 was prepared by monoꢀ
formylation of TTA (5) via a VilsmeierꢀHaack protocol, followed by bromination. Detailed synthetic
procedures are described in the SI.
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Scheme 2. Synthetic routes to metalꢀfree DSSC sensitizers 1ꢀ4.
Characterization of Dyes 1ꢀ4
The optical absorption spectra of the four new dyes in 1,2ꢀdichlorobenzene (oꢀDCB) solution are
shown in Figure 2, indicating the presence of two strong absorptions assignable to a chargeꢀtransfer
(
CT) band [~498 nm (1), ~482 nm (2), ~490 nm (3), ~513 nm (4)] and a higher energy πꢀπ* transition
~340 nm (1), ~389 nm (2), ~369 nm (3), ~402 nm (4)]. Importantly, the insertion of one or two thioꢀ
[
phenes in dyes 2ꢀ4 noticeably redꢀshifts the absorption onset compared to dye 1 as a result of their exꢀ
tended conjugation lengths. All four dyes 1ꢀ4 exhibit large molar extinction coefficients at their absorpꢀ
tion maxima, with values ~3ꢀ4x larger than those of typical Ru(II) polypyridyl complexes (N719; 1.40 ×
4
−1
−1 40
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0 M cm ). Therefore, these dyes are expected to efficiently harvest light and enable large photoꢀ
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ꢀ
currents in a DSSC device. To estimate the energy level alignment of dyes 1ꢀ4 versus TiO and the I
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ꢀ
/I redox mediator, CV measurements were performed in solution with addition of ferrocene as the inꢀ
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ternal standard (Figure 3a). All four dyes exhibit two distinct and reversible oxidations without any reꢀ
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ductive features. HOMO energies of the four dyes were estimated from the onset potentials (E ) of
ox
4
2
the first oxidation peaks; EHOMOꢀCV= ꢀ(4.8+ E ); and found to be ꢀ5.29, ꢀ5.16, ꢀ5.22, and ꢀ5.09 eV for
ox
dyes 1ꢀ4, respectively. Evidently, the addition of the thiophene spacer(s) into dye 1 upshifts the HOMO
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energies. This result, combined with the HOMO and LUMO energy gap contraction on going from 1
(2.18 eV) to 2/3 (2.08 eV) to 4 (2.04 eV) as indicated by the optical spectroscopy data (Figure S3),
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yields a dye independent LUMO energy of ~ 3.1 eV for all dyes.
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Figure 2. UVꢀvis absorption spectra of dyes 1ꢀ4 and their corresponding molar absorption coefficients measured
ꢀ5
in oꢀDCB in concentration of 10 M.
Electronic structure calculations were performed at the B3LYP/6ꢀ31G** level of density functional
theory (DFT; Figures 3c and S1). Figure 3c shows a representation of the HOMO and LUMO topoloꢀ
gies for dyes 1ꢀ4, indicating that the HOMOs are clearly more localized at the TPA donor moieties
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3
while the LUMOs are primarily localized on the cyanoacrylic acid unit. Such a directional electronic
distribution is ideal for electron injection into TiO from anchoring sites and reduction of oxidized dyes
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ꢀ
ꢀ 29
by I /I . Importantly, the DFT computed HOMO energies are in excellent agreement with the experiꢀ
3
mental HOMO energies estimated from CV measurements (Figure 2a). Regarding the DFTꢀcalculated
dye geometries, the insertion of a thiophene spacer in between the TPA and the TTA units (dyes 2 and
4), noticeably reduces core planarity (Figure S1), possibly suggesting nonꢀideal dye assembly and hamꢀ
30
pered charge injection characteristics (See more below).
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. Electronic structure characterization of dyes 1ꢀ4. (a) Solution cyclic voltammograms with the midpoint
+
potential of the ferrocene/ferrocenium (Fc/Fc ) internal standard referenced to 0.0 V; (b) MO energy diagram
with respect to the conduction band of TiO , where the HOMO levels are determined from the onset of first
2
oxidation peak, and LUMO levels are derived from HOMOꢀLUMO energy gap from optical measurements on
films; (c) Isodensity surface plots for the HOMO and the LUMO of dyes 1ꢀ4.
Table 1. Summary of optical and electrochemical properties of dyes 1ꢀ4.
a
b
d
e
c
Dye
UVꢀVis λmax
nm)
EHOMOꢀCV
eV)
E
HOMOꢀ
ELUMO
(eV)
E
LUMOꢀ
ꢁEg
ꢁEg
ꢀ
c
c
(
DFT (eV)
DFT (eV) (eV)
(eV)
(
(Debye)
1
2
3
4
498
482
490
513
ꢀ5.29
ꢀ5.16
ꢀ5.22
ꢀ5.09
ꢀ5.12
ꢀ5.01
ꢀ5.06
ꢀ4.95
ꢀ3.11
ꢀ3.08
ꢀ3.14
ꢀ3.05
ꢀ2.64
ꢀ2.69
ꢀ2.78
ꢀ2.80
2.18
2.08
2.08
2.04
2.48
2.32
2.28
2.15
11.3
11.5
7.65
7.79
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a
ꢀ5
ꢀ1 b
Measured in oꢀDCB at a concentration of 10 M . E
= ꢀ(4.8+ E ) where E = Onset potential of the first
ox ox
HOMOꢀCV
+
c
d
oxidation peak when the Fc/Fc internal standard is referenced to 0.0 V; by DFT calculation. Estimated from
e
EHOMO+ꢂE . ꢂE calculated from absorption onsets.
g
g
DyeꢀSensitized Solar Cells
DSSCs were fabricated using THF/EtOH (1:1 v/v) solutions of dyes 1ꢀ4. The TiO photoelectrode
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
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5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
4
4
was prepared by electroꢀspray according to our previous report. The dye adsorption was performed by
immersing the TiO electrode into the dye solution for 10 h at room temperature. After preparation of
2
the Pt counter electrode from H PtCl solution, the two electrodes were sealed together using a thermal
2
6
melt polymer film. The liquid electrolyte was composed of 0.6 M 1ꢀbutylꢀ3ꢀmethylimidazolium iodide
BMII), 0.03 M of I , 0.1 M of guanidinium thiocyanate (GSCN), and 0.5 M of 4ꢀtertꢀbutylpyridine
(
2
(tBP) in acetonitrile and valeronitrile (85:15 v/v), and was injected between the two electrodes (see
more details in Experimental). Figure 4a shows representative DSSC device JꢀV characteristics, while
all device parameters are summarized in Table 2. The DSSC based on the smallest dye 1 shows a modꢀ
2
est PCE of 4.76% with a relatively low J of 7.66 mA/cm and a FF of 65.8%. The addition of a thioꢀ
sc
phene spacer between TPA and TTA (dye 2) substantially increases the PCE to 6.15%, mainly due to
2
the extended absorption enhancing J (10.1 mA/cm ). Remarkably, by inserting the thiophene spacer
sc
between the cyanoacrylic acid and TTA moieties (dye 3), the highest PCE reaches 10.1%, with a V =
oc
2
0
.833 V, J = 16.5 mA/cm , and FF = 70.0 %. Note that these are the highest DSSC performance metꢀ
sc
rics for any fusedꢀthiophene based dye sensitizer reported to date, and approach the record PCE of
0.65% for metalꢀfree organic sensitizers recently reported by Yang et al using a Co(II/III) redox shutꢀ
1
2
0
tle. However, when the thiophene spacers are placed on both donor/acceptorꢀbridge sides (dye 4), a
modest PCE of 6.91% is obtained despite having the lowest HOMOꢀLUMO energy gap in this series.
Interestingly, all four dyes in this study yield very high V s of 0.832ꢀ0.946 V. This experimental data
oc
can be explained by the TiO conduction band energy shift as a result of different dye coverages, molecꢀ
2
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45ꢀ49
ular dipole moments, and/or molecule tilt angles.
The dark JꢀV characteristics also reveal that the
2
3
4
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6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
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2
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5
6
dark currents fall in the order of 4>3>2>1, in accord with the trend in the V s. The spectra of monoꢀ
oc
chromatic incident photonꢀtoꢀcurrent conversion efficiencies (IPCEs) for the four DSSCs are shown in
Figure 4b. These data clearly show that the addition of thiophene units contributes to the extended phoꢀ
ton absorption of dyes 2ꢀ4 versus the original dye 1, while the highest IPCE for dye 3 is found to exceed
0
1
2
3
4
5
6
7
8
9
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0
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2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
8
0% from ~385 to ~540 nm.
Figure 4. Device characterization for DSSCs based on organic dyes 1ꢀ4. (a) Illuminated JꢀV characteristics; (b)
IPCE spectra; (c) Normalized open circuit voltage decay (OCVD).
Table 2. Summary of DSSC performance parameters for dyes 1ꢀ4.
2
Dye
1
VOC (V)
JSC (mA/cm )
FF (%)
65.8
PCE (%)
4.76
0
0
.946
.893
7.66
2
10.1
68.1
6.15
3
0.833
16.5
73.7
10.1
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4
0
.832
11.8
70.3
6.91
Openꢀcircuit voltage decay (OCVD) dynamics are presented in Figure 4c to evaluate the lifetimes of
5
0,51
photogenerated electrons, τ , according to eq 1.
where k is the Boltzmann constant and T is the
B
n
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
−
1
τ = −(k T/e)(dV /dt)
(1)
n
B
oc
temperature. Analysis of the OCVD dynamics reveals that of the present four organic DSSCs, the slowꢀ
est decay dynamics are observed for the dye 3ꢀbased cell, followed by dye 2 and dye 4, and the most
rapid decay for dye 1, indicating the lowest recombination velocity for the dye 3ꢀ based DSSCs.
The charge transport dynamics of the present DSSCs were investigated via electrochemical imꢀ
pedance spectroscopy (EIS) measurements performed under one sun conditions (Figure S2). This analyꢀ
52ꢀ54
sis is based on the kinetic models of Adachi, Kern, and Bisquert, et al
(see detailed analysis in SI).
Under open circuit conditions, three semicircles are observed in the frequency range 0.05−150 kHz.
Among them, the semicircles in the middle region are attributed to the resistance due to the photoinjecꢀ
tion of electrons into the TiO films. It is found that under one sun conditions (with different V s), dye 3
2
oc
exhibits the smallest recombination and electron diffusion resistance, suggesting a higher J , consistent
sc
with the experimental observations. Although this model can compare the cell transport mechanisms at
V and explains the different cell characteristics (primarily in J ), it cannot be used to rationalize the
oc
sc
voltage dependent recombination rates of the different cells. Nevertheless, these data suggest that chemꢀ
isorption of dye 3 contributes to the relatively small cell internal resistance, possibly resulting from faꢀ
vorable electron density delocalization and enhanced selfꢀassembly on the TiO surface (see more beꢀ
2
low). This may reduce the contact area between the TiO and the electrolyte, and in principle, decrease
2
ꢀ
the interception rate of the electrons in the TiO conduction band by I .
2
3
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Chemisorbed Dye Characterization on TiO2
2
3
4
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1
1
1
1
1
1
1
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1
1
2
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5
5
6
To obtain further insight into the dye selfꢀorganization, molecular orientation, coverage, and aggreꢀ
gation on the TiO surface, XRR measurements were performed on monolayerꢀcoated TiO substrates
2
2
grown by atomic layer deposition (ALD) onto Si/SiO substrates. The selfꢀassembled monolayer (SAM)
2
0
1
2
3
4
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0
chemisorption process was achieved by immersing TiO ꢀcoated substrates in dye 1ꢀ4 solutions in
2
THF/EtOH for ~24 h (see Experimental). Similar sample preparation procedures and XRR characterizaꢀ
5
5
56
tion data were reported by Wagner et al. and Griffith et al. The XRR data were fit according to the
5
7
procedures reported by Nelson et al. Interestingly, the XRR data indicate the formation of very thin
Figure 5. XRR measurements of monolayer dye on TiO surfaces. (a) XRR data and model fits. The top three
2
curves are vertically offset for purposes of clarity. Scattering vector q = 4πsin(2θ/2) / λ; (b) Electron density vs.
depth along the sample surface normal.
monolayers (7.3 and 6.6 Å) for the dyes without the thiophene spacer proximate to the cyanoacrylic acid
anchoring group (dyes 1 and 2, respectively). This result suggests that these molecules are not aligned
with their long axes perpendicular to the substrate plane (tilt angle = 0°) on the TiO substrates, but raꢀ
2
ther exhibit significant tilt angles from the surface normal of ~70° (dye 1) and ~74° (dye 2), estimated
5
6
from the DFTꢀcomputed molecular lengths of 20.9 Å (dye 1) and 24.0 Å (dye 2). The molecular footꢀ
2
prints for these two dye molecules are estimated to be 152 and 183 Å , respectively. Importantly, the
addition of a thiophene unit close to the cyanoacrylic acid group in dye 3 greatly enhances film thickꢀ
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ness to 16.8 Å, thereby drastically changing the molecular orientation, with a smaller estimated binding
tilt angle from the surface normal of only ~48° (estimated molecular length = 24.9 Å), resulting in a sigꢀ
2
nificantly smaller molecular footprint of 79 Å . Thus 3 can form more densely packed monolayers on
TiO , which not only enhances dye loading for more efficient light harvesting, but also suppresses direct
2
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contact between the TiO electrode and the electrolyte, reducing recombination losses. This result is
2
consistent with the highest FF of 70% for DSSCs employing dye 3 (Table 3). On the other hand, despite
having the longest backbone (molecular length = 28.9 Å), dye 4 exhibits a slightly reduced monolayer
2
thickness (15.4 Å), thus a larger tilt angle of 58°, and a larger molecular footprint of 104 Å
.
Table 3. Summary of XRRꢀderived SAM thickness and molecular footprint data for dyes 1ꢀ4 on ALDꢀ
grown TiO substrates.
2
Dye
Thickꢀ
ness
SLD
(Å )
Air/dye interꢀ TiO /dye interꢀ TiO2
layer TiO /SiO2 interꢀ Molecular
footꢀ
2
2
ꢀ
2
2
face roughꢀ face
roughꢀ thickness (Å)
face
(Å)
roughness print (Å )
(Å)
ness(Å)
4.5
ness(Å)
1
2
3
4
7.3
11.5
11.5
11.1
9.0
3.0
2.7
3.5
3.6
16.4
16.5
16.6
16.2
5.0
5.9
3.6
4.3
152
183
79
6.6
3.5
16.8
15.4
5.0
4.7
104
To quantify the dye loading in the present DSSCs, the TiO anodes were immersed in 0.30 mM
2
THF/EtOH (1:1) dye solutions, and the dye concentration difference before and after anode soaking was
5
8
quantified by optical spectrophotometry. (Figure S3) These experiments indicate that TiO samples
2
ꢀ8
ꢀ8
coated with dyes 1 and 2 achieve a relatively low dye loading of 2.1× 10 mol/mg and 1.3× 10
ꢀ8
ꢀ8
mol/mg, respectively, versus those of dyes 3 (7.4 × 10 mol/mg) and 4 (6.6 × 10 mol/mg). This result
is consistent with the smaller molecular footprints and larger tilt angles for these dye molecules as idenꢀ
tified in the XRR data. Clearly, introducing a thiophene unit close to the cyanoacrylic anchoring group
plays a crucial role in enhancing both vertical dye orientation and dye coverage for dyes 3 and 4 versus
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1
and 2. Additionally, dyes 1 and 3 also show higher dye loadings than dyes 2 and 4, respectively. This
2
3
4
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8
9
1
1
1
1
1
1
1
1
1
1
2
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2
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2
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2
2
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6
result can be rationalized at the molecular level from the DFT computations which indicate that thioꢀ
phene insertion at the donor position undesirably twists molecular backbone (Figure S1), which in other
SAM systems is known to suppress molecular selfꢀorganization and densification on the substrate surꢀ
0
1
2
3
4
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9ꢀ61
face.
Photophysical Measurements
FTRꢀPL is a powerful tool for investigating photophysical processes associated with intraꢀ and inꢀ
terꢀmolecular charge transfer and charge injection into TiO . Here, the hot and cold electron lifetimes of
2
62,63
dyes 1ꢀ4 were measured in solution using broadband FTRꢀPL.
The prominent peaks of dyes 1ꢀ4 PL
spectra are all redꢀshifted from ~570 nm to ~610 nm over the time range of 2 ps ꢀ 100 ps (SI, Figure
6
4
S4). Compared to the time for electron injection from a dye to the TiO nanoparticles (~1 ps), the timeꢀ
2
65
dependent PL spectrum evolution for the present systems is relatively slow. The competition between
the radiative decay of the excited electron in the dye and the electron injection into TiO nanoparticle
2
6
6
mainly occurs at the highest metastable state (λ~570 nm). Therefore, hot and cold electron properties
must be observed at the emission wavelength of ~570 nm. The electron lifetimes of the present dyes are
obtained by fitting the rising (for hot electron) and falling (for cold electron) curves of the TRPL with a
67
twoꢀparameter biꢀexponential decay function (Figures S4 and 6a). As summarized in Table S1, dye 1
exhibits the longest hot electron lifetime (1.01 ps), matching well with the highest V of 0.943 V for the
oc
corresponding cells, suggesting efficient hot electron injection into the TiO nanoparticles. In contrast,
2
the hot electron lifetimes of dyes 2ꢀ4 are smaller than the instrumental response function (IRF; ~150 fs).
Comparing the cold electron decay dynamics, dye 3 exhibits the strongest emission intensity and the
longest cold electron lifetime among dyes 2ꢀ4 (Figures 6a, at the excitation wavelength of 420 nm) in
agreement with its highest IPCE in the series of dyes in this study. Note that the cold electron lifetime of
dye 1 is the longest among the four dyes in this study.
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Figure 6. FTRꢀPL spectra of dyes 1ꢀ4. a. dyes 1ꢀ4 measured in solution. (1:1 v/v in THF + EtOH solvent). b.
dyeꢀgrafted TiO nanoparticle film. The pulse duration, pulse repetition rate, and central wavelength of excitation
2
are 100 fs, 80 MHz, and 420 nm, respectively.
FTRꢀPL quenching experiments for dye/TiO nanoparticle films were carried out to assess the effiꢀ
2
ciency of cold electron injection from the dye into the TiO nanoparticles to the DSSC IPCE (Figure
2
6
b). Figure S6 shows the FTRꢀPL spectra of the dye/TiO nanoparticle films at 0.5 ps, which indicate
2
that the photoꢀexcited electrons in dyes 1ꢀ4 relax to the lower metastable state through rapid electronꢀ
phonon interactions. In the metastable state, the cold electron injection efficiency can be evaluated at the
emission peak wavelength for the dye/TiO nanoparticle films. Figure 6b presents the FTRꢀPL characꢀ
2
teristics of dyes 1ꢀ4 adsorbed on the TiO nanoparticle films. To obtain the relaxation times of the cold
2
electrons in dyes 1ꢀ4, the experimental results are fitted according to a triexponential decay function.
The three time constants are assigned as the cold electron injection (exciton dissociation) time, the exciꢀ
tonꢀexciton annihilation time, and the cold electron lifetime. For dyes 1ꢀ4, the trend of cold electron lifeꢀ
times in dyes adsorbed on the TiO nanoparticle films is the same as in the solutionꢀbased FTRꢀPL reꢀ
2
sults described above, showing the longest lifetimes for dyes 1 and 3 (23.6 and 19.3 ps, respectively),
followed by dyes 4 and 2 (15.1 and 13.6 ps, respectively). Such fast decay dynamics suggest highly efꢀ
ficient electron transfer from the excited states of the dyes to TiO . The efficiency of cold electron injecꢀ
2
68
tion from the dye to TiO nanoparticles can be calculated from the equation η = (1/τ )/(1/τ + 1/τ ),
2
in
e
in
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where τ is the cold electron injection time and τ is the cold electron lifetime. The fitting results of the
in
e
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
PL dynamics and the efficiency of cold electron injection are summarized in Table 4. It is evident that
the electron injection efficiencies for all four dyes (1 > 3 > 4 > 2) are very high (>90% at 420 nm). The
relatively high electron injection efficiency for dye 3 agrees very well with its high IPCE of ~85% at
420 nm.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 4. Temporal characteristics of photoꢀexcited electron dynamics for dyes 1ꢀ4 adsorbed on TiO naꢀ
2
noparticles and their corresponding DSSC IPCE metrics.
Dye/TiO2
Cold electron injecꢀ Excitonꢀexciton
Cold
electron Electron injection IPCE at 420
tion time (exciton disꢀ annihilation time lifetime (ps)
efficiency at 420 nm nm
sociation time) (ps)
(ps)
1
2
3
4
0.68
1.46
0.74
0.84
4.35
1.47
3.08
2.77
23.56
13.63
19.30
15.07
97.2
90.3
96.3
94.7
~69%
~58%
~85%
~61%
Conclusions
A novel series of four DꢀπꢀbridgeꢀA metalꢀfree organic DSSC dyes based on a highly planar and
extensively conjugated TTA bridge were synthesized and characterized. All four dyes exhibit large exꢀ
tinction coefficients for efficient photon absorption. For structural optimization, the role of the thioꢀ
phene spacer(s) on the molecular donor or acceptor side was the principal focus. Energetically, the addiꢀ
tion of a thiophene increases the conjugation length, extends absorption wavelength, and gradually eleꢀ
vates the HOMO energies. The insertion of the thiophene spacer between the TTA bridge unit and the
cyanoacrylic acid anchoring sites improves both electron delocalization as well as selfꢀassembly and
dye loading on the TiO surface. In contrast, the insertion of the thiophene between the TPA unit and the
2
TTA bridges slightly disrupts molecular backbone planarity, leading to suboptimal dye assembly on
TiO and increased recombination losses. Due to its densely packed structure on TiO , we demonstrate a
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highest PCE of 10.1% for dye 3 – one of the highest performing organic dyes reported to date, and the
highest for fusedꢀthiophene based sensitizers. We anticipate that further PCE improvements can be obꢀ
tained by replacing the iodide/triiodide redox couple with a cobaltꢀbased electrolyte. This result suggests
that the linear conjugated TTA core is a very promising bridging unit for designing metalꢀfree, organic
DSSC dyes. Our investigation also establishes important rules correlating organic dye molecular design
versus selfꢀassembly for further optimizations.
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Experimental
Materials and Synthesis. All chemicals and solvents were of reagent grade and were obtained from Alꢀ
drich, Arco, or TCI Chemical Co. Solvents for reactions (toluene, benzene, ether, and THF) were disꢀ
tilled under nitrogen from sodium/benzophenone ketyl, and halogenated solvents were distilled from
CaH . NMR spectra were measured using a Bruker 300 or 500 MHz spectrometer with chloroformꢀd
2
and/or tetrahydrofuranꢀd as solvents. UV–Vis spectra were recorded using a Shimadzu UV3600 UV–
8
Vis–NIR spectrophotometer, with EtOH/THF as solvents. Electrospray ionization mass spectrometry
was performed using a JMSꢀ700 HRMS spectrometer.
Electrode Preparation and DSSC assembly: To prepare mesoporous TiO spheres, the aqueous solvent
2
6
9,70
in hydrothermally treated TiO nanoparticle solutions was replaced by ethanol.
This solution was
2
then loaded into a syringe equipped with a 27ꢀgauge stainless steel needle. The spaying rate (25 ꢃL/min)
for nanoparticle growth was controlled using a syringe pump (KD Scientific model 220). An electric
field (12ꢀ15 kV) was applied between a metal orifice and the aluminum foil substrate at a distance of 10
cm using a BERTAN Series 230 power supply, and the nanoparticle suspension was electrosprayed onto
the aluminum foil. After electrospraying, the asꢀprepared TiO spheres were made into a paste by stirꢀ
2
ring with a mixture 0.5 g of the TiO spheres, 100 ꢃl of Triton Xꢀ100, 0.2 g of polyethylene oxide (PEO,
2
Adrich, Mw = 100,000) into 3 mL acetic acid (0.1 M). The resulting TiO paste was then spread onto
2
SnO :F coated glass substrates (Pilkington, TEC 8 glass, 8ꢄ/□, 2.3 mm thick ) by doctorꢀblading to give
2
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a flat, smooth surface. Finally, the TiO coated electrode was calcined to remove the polymer under an
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o
o
o
air flow at 150 C for 15 min, then at 320 C for 10 min, and at 500 C for 30 min, leaving a pure anaꢀ
tase TiO nanosphere film. TiO nanosphere electrode was then immersed in a THF/ethanol solution of
2
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the dye molecule (0.30 mM) for 10 h at 25°C. The dyeꢀadsorbed TiO electrode was then rinsed with
2
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ethanol and dried under an N flow. No chenodeoxycholic acid (CDCA) additive was added. The counꢀ
2
terꢀelectrode was produced by coating F:SnO glass with a thin layer of a 5 mM solution of H PtCl in
2
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isopropanol and was then heated at 400°C for 20 min. The two electrodes were sealed together with
thermal melt polymer film (24 ꢃm thick, DuPont). The liquid electrolyte composed of 0.6 M of 1ꢀ butylꢀ
3
0
ꢀmethylimidazolium iodide (BMII), 0.03 M of iodine, 0.1M of guanidinium thiocyanate (GSCN) and
.5 M of 4ꢀtertꢀbutylpyridine (tBP) in acetonitrile and valeronitrile (85:15 v/v) was injected between
two electrodes.
Solar Cell Measurements: A NewportꢀOriel ® IQEꢀ200 ACDC was used to measure incident photon to
2
charge carrier efficiency (IPCE). The DSSC devices were evaluated under 100 mW/cm AM1.5G simuꢀ
lated sunlight with a class A solar cell analyzer (Spectra Nova Tech.). A silicon solar cell fitted with a
KG3 filter tested and certified by the National Renewable Energy Laboratory (NREL) was used for calꢀ
ibration. The KG3 filter accounts for the different light absorption between the dye sensitized solar cell
and the silicon solar cell, and it ensures that the spectral mismatch correction factor approaches unity.
Electrochemical measurements: Electrochemistry was performed on a C3 Cell Stand electrochemical
station equipped with BAS Epsilon software (Bioanalytical Systems, Inc., Lafayette, IN), a glassy carꢀ
bon working electrode, a Ptꢀwire counter electrode and a Agꢀwire reference electrode. Dyes 1ꢀ4 were
dissolved in anhydrous dichloromethane (~1 mg/mL) along with Bu N(PF ) (0.1 M) electrolyte. All
4
6
scans were performed at 100 mV/s scan rate under Nitrogen atmosphere. A ferrocene/ferrocenium redox
couple was used as an internal standard.
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XRR Measurements: XRR curves were obtained on Rigaku Smartlab with standard slits for a 300mm
goniometer. Curves of Rq4 vs q were fitted by Motofit with error bars which included counting and maꢀ
chine measurement errors, and edge effects for small q regions. TiO films of ~16ꢀ17 Å were prepared
2
via atomic layer deposition (ALD) on bare Si substrates, and subsequently sintered at 500º C for 2 h to
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obtain the TiO anatase phase. XRR measurements of these TiO films on Si shows crystalline randomly
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oriented TiO in the anatase phase. XRR samples were prepared by immersing the TiO coated subꢀ
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strates in THF/EtOH solvent (1:1) for dye 1ꢀ4, and allowing selfꢀassembly monolayer (SAM) growth
over 24 h. The substrates were subsequently washed with pure solvents.
EIS Measurements and Analysis: The electrochemical impedance results were measured under the
same light illumination with a Solartron 1260 impedance analyzer and a Solartron 1287 potentiostat
with the device at Voc.
Femtosecond timeꢁresolved photoluminescence (FTRꢁPL): To understand the photoexcited electron
dynamics in dyes 1ꢀ4, the FTRꢀPL was measured by the sumꢀfrequency technique (FluoMax, IB Phoꢀ
tonics Ltd.). The dyes dissolved in THF/EtOH solutions were pumped at the wavelength of 420 nm. To
observe the ultraꢀshort lifetime of hot electrons, the pumping laser was focused onto the sample with a
short Rayleigh length which gives an instrument response function of ~ 150 fs.
Supporting Information Available
Synthetic procedures and characterization data of all new compounds. Details for all device characteriꢀ
zations. This information is available free of charge via the internet at http://pubs.acs.org.
Corresponding Authors
rꢀchang@northwestern.edu;
aꢀfacchetti@northwestern.edu;
mcchen@cc.ncu.edu.tw;
tꢀ
marks@northwestern.edu; ratner@northwesern.edu; bedzyk@northwestern.edu
Contributions
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The manuscript was written through contributions of all authors. All authors have given approval to the
final version of the manuscript. ‡These authors contributed equally.
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Acknowledgements
This research is supported as part of the ANSER Center, an Energy Frontier Research Center funded by
the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award
Number DEꢀSC0001059). We thank the NSFꢀMRSEC program through the Northwestern University
Materials Research Science and Engineering Center for characterization facilities (DMRꢀ1121262) and
Institute for Sustainability and Energy at Northwestern (ISEN) for partial equipment funding. Financial
assistance for this research was also provided by the National Science Council, Taiwan, Republic of
China (Grant Numbers NSC102ꢀ2113ꢀMꢀ008ꢀ004 and NSC102ꢀ2923ꢀMꢀ008ꢀ004ꢀMY2).
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