Effects of donors of bodipy dyes on the performance of dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2017.02.017

A new series of donor-acceptor-π-linker-acceptor (D–A–π–A) featured organic dyes containing Bodipy derivative as the auxiliary acceptor, furan unit as the π conjugated spacer, and 2-cyanoacrylic acid as the anchor group were synthesized and applied in dye-sensitized solar cells (DSSCs). The incorporated electron-withdrawing unit of Bodipy enhances light harvesting by decreasing the molecular energy gap and red-shifting absorption spectra. For comparison, three different arylamine chromophores namely 3-methoxy-9H-carbazole (CBZ-B), triphenylamine (TPA-B) and phenoxazine (POZ-B) were separately appended onto the 6-position of Bodipy unit to study the effect of the electron donating groups on device performance. The UV–vis absorption spectra, electrochemical properties, density functional theory calculation, photovoltaic properties and electrochemical impedance measurements of DSSCs with these three dyes were systematically investigated. The Bodipy dyes have broad absorption spectra covering the range of 250–700?nm with the highest molar extinction coefficient up to >55?000?M^?1?cm^?1. The molecular energy level tuning can be conveniently accomplished by alternating the donor moiety. The DSSCs based on TPA-B showed the best photovoltaic performance: a maximum monochromatic incident photon-to-current conversion efficiency (IPCE) of 70%, a short-circuit photocurrent density (J_sc) of 13.95?mA?cm^?2, an open-circuit photovoltage (V_oc) of 557?mV, and a fill factor (ff) of 0.61, corresponding to an overall conversion efficiency of 4.76% under standard global AM 1.5G solar light conditions. Our research demonstrates that Bodipy based D–A–π–A molecular architecture is a highly promising class for the improvement of the performance of DSSCs in the future.

Full text of DOI:10.1016/j.dyepig.2017.02.017

Accepted Manuscript
Effects of donors of bodipy dyes on the performance of dye-sensitized solar cells
Mao Mao, Qing-Song Li, Xiao-Lin Zhang, Guo-Hua Wu, Chun-Guang Dai, Yong Ding,
Song-Yuan Dai, Qin-Hua Song
PII:
S0143-7208(16)31228-1
10.1016/j.dyepig.2017.02.017
DYPI 5794
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 20 November 2016
Revised Date: 23 January 2017
Accepted Date: 11 February 2017
Please cite this article as: Mao M, Li Q-S, Zhang X-L, Wu G-H, Dai C-G, Ding Y, Dai S-Y, Song Q-H,
Effects of donors of bodipy dyes on the performance of dye-sensitized solar cells, Dyes and Pigments
(2017), doi: 10.1016/j.dyepig.2017.02.017.
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ACCEPTED MANUSCRIPT
Graphical abstract:

ACCEPTED MANUSCRIPT
Effects of donors of Bodipy dyes on the performance of
dye-sensitized solar cells
a,b,*
a
a, *
c
b
d
Mao Mao , Qing-Song Li , Xiao-Lin Zhang , Guo-Hua Wu , Chun-Guang Dai , Yong Ding ,
d
b,*
Song-Yuan Dai , Qin-Hua Song
a
School of Atmospheric Physics, Nanjing University of Information Science & Technology, Nanjing
10044, PR China
2
b
Department of Chemistry, University of Science and Technology of China, Hefei 230026, PR China
College of Science and Technology, Nihon University, 1-18-14 Kanda Surugadai, Chiyodaku, Tokyo
01-8308, Japan
c
1
d
Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of
Sciences, Hefei 230031, PR China
Abstract
A new series of donor-acceptor-π-linker-acceptor (D–A–π–A) featured organic dyes containing
Bodipy derivative as the auxiliary acceptor, furan unit as the π conjugated spacer, and 2-cyanoacrylic
acid as the anchor group were synthesized and applied in dye-sensitized solar cells (DSSCs). The
incorporated electron-withdrawing unit of Bodipy enhances light harvesting by decreasing the
molecular energy gap and red-shifting absorption spectra. For comparison, three different arylamine
chromophores namely 3-methoxy-9H-carbazole (CBZ-B), triphenylamine (TPA-B) and phenoxazine
*
Corresponding authors. Tel/Fax: +86-25 58699771.
E-mail address: mmao@nuist.edu.cn (M. Mao), xlnzhang@nuist.edu.cn (X. Zhang),
qhsong@ustc.edu.cn (Q. Song)
1

ACCEPTED MANUSCRIPT
(POZ-B) were separately appended onto the 6-position of Bodipy unit to study the effect of the
electron donating groups on device performance. The UV-vis absorption spectra, electrochemical
properties, density functional theory calculation, photovoltaic properties and electrochemical
impedance measurements of DSSCs with these three dyes were systematically investigated. The
Bodipy dyes have broad absorption spectra covering the range of 250 to 700 nm with the highest molar
–
1
–1
extinction coefficient up to > 55 000 M cm . The molecular energy level tuning can be conveniently
accomplished by alternating the donor moiety. The DSSCs based on TPA-B showed the best
photovoltaic performance: a maximum monochromatic incident photon-to-current conversion
–
2
efficiency (IPCE) of 70%, a short-circuit photocurrent density (J ) of 13.95 mA cm , an open-circuit
sc
photovoltage (V ) of 557 mV, and a fill factor (ff) of 0.61, corresponding to an overall conversion
oc
efficiency of 4.76% under standard global AM 1.5G solar light conditions. Our research demonstrates
that Bodipy based D–A–π–A molecular architecture is a highly promising class for the improvement of
the performance of DSSCs in the future.
Keywords: Bodipy; Carbazole; Triphenylamine; Phenoxazine; Photovoltaic performances; sensitizers
1
. Introduction
With increasing demand for accessible and sustainable energy, various approaches for solar energy
[
1]
conversion have been developed over the past years . Among them, dye-sensitized solar cells (DSSCs)
have been considered as a promising generation of photovoltaic for commercial applications because of
their decent light-to-electricity conversion efficiencies, low cost of production, ease of fabrication,
[
2,3]
light-weight and aesthetic features of vivid color
. In a typical DSSC, the dye molecules absorb the
sunlight as sensitizers and inject the electrons into the conduction band of nanostructured metal oxide
(
usually TiO ), to produce electrons for the external circuit. The oxidized dyes are subsequently
2
2

ACCEPTED MANUSCRIPT
[
4]
regenerated by a redox couple present in the electrolyte . The photo-electricity conversion efficiency
(PCE) of devices is significantly influenced by the visible light harvesting ability of the sensitizers.
Generally, organic sensitizers can be divided into two mainstreams: metal complexes (ruthenium
polypyridyl, zinc porphyrin) and metal free dyes. The disadvantages of limited sources for ruthenium
dyes and low synthetic yield for zinc porphyrin dyes drove research attention toward metal free organic
sensitizers. In particular, the flexibility of synthetic chemistry has allowed the exploration of various
design criteria to improve the performance of DSSCs with respect to photon absorption, electron
injection, chemical stability and other aspects. Numerous efforts on organic sensitizers emphasized that
an electron-donating (D) fragment with a π-conjugation linker coupled to a strong
electron-withdrawing group (A), i.e., D–π–A configuration, is a promising structural arrangement for
enhancing the photoinduced intramolecular charge transfer and thereby boost solar energy conversion
[
5,6]
.
Slight variations with these three sections may cause significant change in photovoltaic
performance. For further improving the photovoltaic efficiency, it is pivotal to broaden and extend the
absorption spectra in D–π–A organic sensitizers. Recently, the incorporation of an auxiliary
electron-withdrawing unit between the donor and conjugated bridge to form D–A–π–A configuration
has been intensively developed, because additional acceptor can decrease the HOMO–LUMO energy
gap, as well as bring a beneficial photo-induced electron transfer from the donor to anchor unit, thus
efficiently resulting in a great enhancement of both the dye stability and photovoltaic conversion
[
7-11]
efficiency
.
Among the various metal free organic dyes, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (hereafter
abbreviated to Bodipy) derivatives have been recognized as promising candidates for DSSCs due to the
4
-1
-1
following intrinsic advantages: i) sharp and strong absorption near 500 nm (4–10×10 M cm ),
3

ACCEPTED MANUSCRIPT
leading to efficient light-harvesting capability; ii) diverse modification on the core structure, at all
positions and with any desired functionality; iii) long excited state lifetimes, excellent photo-stability
[
12-35]
and good solubility in organic solvents
. Especially, the Bodipy is a moderately electron-deficient
chromophore due to its high electron affinity originating from the unsaturated nitrogen and fluorine
atoms. In the last decade, the molecular configuration for Bodipy dyes has undergone a development
[
14-35]
process
. Originally, Fukuzumi and co-workers designed the Bodipy-based DSSCs using 8-phenyl
[
14]
with electron-donating substituents as a donor and the Bodipy core itself as an acceptor . The dyes
gave the best overall efficiency of 0.16%. Obviously, there is much room in the optimization of both
short-circuit current density and open-circuit voltage. Subsequently, a series of panchromatic Bodipy
dyes with 8-acceptor and 3,5-distyryl linked by donors were designed and synthesized by Akkaya and
[15-17, 26]
Erten-Ela et al
. This class of Bodipy dyes yields dramatically improved efficiencies of DSSCs
[
17]
up to 2.46% . On the basis of the above work, our group began to design 2,6-modified Bodipy dyes
[
18-22, 28]
with a donor/acceptor
. This class of Bodipy dyes displays better conjugation between the
Bodipy core and donor/acceptor, and better electron flow under light irradiation, consequently giving
[
21]
better efficiencies of up to 5.31% . Quite interestingly, ethynyl group incorporated into the molecular
skeleton of 2,6-modified Bodipy is beneficial for extending the light absorption of a dye to the near-IR
[
28]
region, however, the photovoltaic value is unsatisfactory with only 1% in our study
and 0.22% in
[
29]
other group . Butterfly-shaped Bodipy dyes featured no strong arylamine donors, were prepared by
[
24]
Kubo el al., and its power-to-current conversion efficiency is as high as 6.06%
. Ziessel group
demonstrated that replacing fluorine ion on the boron center with an insert alkynyl-poly-ethylene
[
25]
glycol chain is an efficient way to solve the problem of aggregation
. Gibson et al. designed and
characterized a number of NiO photosensitizers based on triphenylamine-bodipy conjugates for p-type
4

ACCEPTED MANUSCRIPT
[
30]
solar cells
. Feng group believed that the Bodipy sensitizer with a greater fluorescence quantum
[
33]
yield and longer fluorescence lifetime would lead to the better photovoltaic performance . Recently,
Xue and co-workers developed various Bodipy dyes with different linkers between the Bodipy antenna
and the acceptor, and they found that accompanied with the increasing length of linker at the 8-position
of the Bodipy core, the light harvesting capability in longer wavelength is improved without obvious
[
34]
change of the highest occupied molecular orbital (HOMO) levels . However, the lab recorded
photovoltaic performance of Bodipy dyes is still lagging behind that of other organic dyes. Herein, it is
urgent to establish efficient strategy for designing Bodipy dyes
In fact, the electron donor is of paramount importance in tuning the molecular energy-level as well
as the physicochemical interactions of multiple components at the TiO /dye/electrolyte interface.
2
Questions may be raised on which is the best chromophore for 2,6-modified Bodipy dyes on the device
performance. As a typical sensitizer, the triphenylamine (TPA)-based dye B3 (Chart 1), employing the
Bodipy unit as both the auxiliary acceptor and conjugated bridge, shows relatively low efficiency of
[
19]
1
.83% . An intrinsic drawback for dye B3 is the extremely weak absorption in the range of 350−450
nm, which hampers further improvement of the DSSC efficiency. One of the promising approaches for
overcoming this problem is introduction additional electron-rich spacer on the acceptor side between
Bodipy and cyanoacrylic acid with filling up the Bodipy absorption valley for achieving highly
efficient light harvesting. According to our previous studies, the furan unit could provide a simple
strategy to modify the optoelectronic, chemical and physical properties of the sensitizers due to
elongated conjugation of the molecular, and thereby enhanced device power conversion efficiency.
With this encouragement, we incorporated furan unit into B3 to construct sensitizer TPA-B for
optimizing the light-harvesting capability. Meanwhile, we designed and synthesized other two new
5

ACCEPTED MANUSCRIPT
dyes coded as CBZ-B and POZ-B, along with the same additional acceptor, conjugated bridge and
anchoring group, but different arylamine groups including 3-methoxy-9H-carbazole (MeO-CBZ) and
phenoxazine (POZ) as electron donors, in order to systematically have a better understanding of the
molecular structure of the dye vs. DSSC performance (Chart 1).
Chart 1 here
2
. Experimental section
2
.1. Materials and reagents
Starting materials
-bromo-10-propyl-10H-phenoxazine (1c) , N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan
[
36]
3-bromo-6-methoxy-9-propyl-9H-carbazole
(1a)
,
[
37]
3
[
38]
[21]
-
2-yl)aniline (2b) , , compound 3 and 4
were synthesized according to literature methods. Dyes
[
18]
[19]
R2
and B3
were synthesized according to our earlier publications. All solvents and chemicals
were purchased from Sigma-Aldrich Company and used as received. All reactions and manipulations
were carried out under nitrogen atmosphere.
2.2. Analytical instruments and measurements
1
13
H NMR (400 MHz) and C NMR (100 MHz) spectra were recorded on a Bruker AV400
spectrometer by using chloroform-d (CDCl ) or dimethyl sulfoxide-d (DMSO [D ]) as the solvent.
1
3
6
6
FTIR spectra were acquired on a Bruker VECTOR22 infrared spectrophotometer. Mass spectra were
determined using a MALDI-TOF analyzer. Absorption spectra of the dyes in solution and on the TiO2
films (thickness 2.5 ꢀm) were measured with a Shimadzu 2450 UV-Vis spectrophotometer. Cyclic
voltammetric (CV) measurement was performed on a computer-controlled CHI620D electrochemical
workstation using a conventional three-electrode cell with a glass carbon working electrode, a Pt wire
counter electrode, and a saturated calomel electrode reference electrode. The supporting electrolyte was
0
.1 M tetrabutylammonium hexafluorophosphate (TBAPF ) in anhydrous CH Cl . The results were
6 2 2
calibrated using a ferrocene reference as an external standard at the end of each measurement (0.63 V
6

ACCEPTED MANUSCRIPT
vs NHE).
Density functional theory (DFT) calculations with Gaussian 09 program
[
39]
were performed on these
dyes to investigate their molecular structures and electron distribution. The structures were optimized
using B3LYP/6-31G(d) to obtain the molecular orbitals. The low-energy 20 electronic excited states of
these molecules were then obtained by using the time-dependent DFT (TD-DFT) calculations with the
same functional and basis set.
2
2
.3. Synthesis procedures and characterization data
.3.1. 3-Methoxy-9-propyl-6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (2a)
[
36]
A
solution of 3-bromo-6-methoxy-9-propyl-9H-carbazole
(318 mg, 1.0 mmol),
bis(pinacolato)diborane B Pin (559 mg, 2.2 mmol), KOAc (490 mg, 5.0 mmol), and Pd(PPh ) Cl (35
2
2
3 2
2
o
mg, 0.05 mmol) in dry toluene (40 mL) was refluxed at 110 C under a nitrogen atmosphere overnight.
When the reaction was complete, toluene was removed under high vacuum, and the mixture was
poured into deionized water (40 mL) and extracted with CH Cl (3 × 40 mL). The collected organic
2
2
phases were dried over MgSO , filtered, and evaporated under vacuum, and then the residue was
4
purified by column chromatography on silica gel with petroleum ether and ethyl acetate as eluent to
1
afford 2a (314 mg, 86%) as a white solid. H NMR (400 MHz, CDCl ): δ 8.56 (s, 1H, ArH), 7.89 (d, J
3
=
7.7 Hz, 1H, ArH), 7.64 (s, 1H, ArH), 7.37 (d, J = 8.1 Hz, 1H, ArH), 7.30 (d, J = 8.7 Hz, 1H, ArH),
7
.09 (d, J = 8.5 Hz, 1H, ArH), 4.23 (t, J = 6.6 Hz, 2H, CH ), 3.92 (s, 3H, OCH ), 1.92–1.87 (m, 2H,
2
3
1
3
CH ), 1.40 (s, 12H, CH ), 0.94 (t, J = 7.2 Hz, 3H, CH ). C NMR (100 MHz, CDCl ): δ 153.9, 143.2,
2
3
3
3
135.6, 132.0, 127.9, 123.4, 122.4, 115.0, 109.6, 108.3, 103.2, 83.6, 56.0, 44.7, 25.0, 22.4, 11.8.
+
TOFMS (EI) calcd for [M] C H BNO : 365.2162, found: 365.2155.
22
28
3
2.3.2. 10-Propyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-10H-phenoxazine (2c)
7

ACCEPTED MANUSCRIPT
The
synthetic
method
was
similar
to
that
of
2a
except
that
3
3
-bromo-6-methoxy-9-propyl-9H-carbazole (318 mg, 1.0 mmol) was replaced with
[
37]
-Bromo-10-propyl-10H-phenoxazine
(304 mg, 1.0 mmol). Compound 2c (316 mg, 90%) was
1
isolated as a white solid. H NMR (400 MHz, CDCl ): δ 7.28 (d, J = 7.9 Hz, 1H, ArH), 7.06 (s, 1H,
3
ArH), 6.80 (t, J = 7.5 Hz, 1H, ArH), 6.69–6.62 (m, 2H, ArH), 6.50–6.46 (m, 2H, ArH), 3.47 (t, J = 7.8
13
Hz, 2H, CH ), 1.74–1.69 (m, 2H, CH ), 1.35 (s, 12H, CH ), 1.05 (t, J = 7.8 Hz, 3H, CH ). C NMR
2
2
3
3
(
100 MHz, CDCl ): δ 145.3, 144.3, 136.2, 132.9, 131.1, 123.4, 121.2, 120.9, 115.4, 111.5, 110.6, 83.5,
3
+
4
2
6
5.5, 24.9, 18.3, 11.1. TOFMS (EI) calcd for [M] C H BNO :351.2006, found: 351.2007.
21
26
3
4
.3.3. 5-(5,5-Difluoro-8-(6-methoxy-9-propyl-9H-carbazol-3-yl)-1,3,7,9-tetramethyl-10-pentyl-5H-5λ ,
4
λ -dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinin-2-yl)furan-2-carbaldehyde (5a).
[
21]
Compound 2a (241 mg, 0.66 mmol), Bodipy 4
(178 mg, 0.33 mmol), K CO solution (2 M, 20
2 3
mmol), THF (13 mL), toluene (13 mL) and Pd(PPh ) (23 mg, 0.02 mmol) were added into a flask (50
3
4
mL). The mixture was refluxed overnight under N . After being cooled to room temperature, the
2
mixture was poured into 50 mL water and the organic layer was separated. The aqueous layer was
extracted with CH Cl (3 × 20 mL) and the combined organic layers were dried over anhydrous
2
2
Na SO . Solvent was evaporated and product 5a (186 mg, 87%) was obtained as a red solid via a silica
2
4
1
column chromatography using petroleum ether and CH Cl as the eluent. H NMR (400 MHz, CDCl ):
2
2
3
δ 9.65 (s, 1 H, CHO), 7.88 (s, 1 H, ArH), 7.58 (s, 1 H, ArH), 7.44 (d, J = 7.6 Hz, 1 H), 7.36–7.34 (m, 2
H, ArH), 7.26 (m, 1 H, ArH), 7.13 (d, J = 7.7 Hz, 1 H), 6.53 (s, 1 H, ArH), 4.28 (t, J = 6.3 Hz, 2 H,
CH ), 3.94 (s, 3 H, OCH ), 3.14 (m, 2 H, CH ), 2.69 (s, 3 H, CH ), 2.60 (s, 3 H, CH ), 2.55 (s, 3 H,
2
3
2
3
3
CH ), 2.42 (s, 3 H, CH ), 1.95–1.92 (m, 2 H, CH ), 1.74 (m, 2 H, CH ), 1.57–1.53 (m, 2 H, CH ),
3
3
2
2
2
1
3
1
.42–1.41 (m, 2 H, CH ), 1.00 (t, J = 6.4 Hz, 3 H, CH ), 0.95 (t, J = 6.7 Hz, 3 H, CH ). C NMR (100
2 3 3
8

ACCEPTED MANUSCRIPT
MHz, CDCl ): δ 177.0, 169.7, 157.2, 155.4, 153.7, 151.8, 150.7, 147.6, 140.3, 139.0, 137.9, 136.6,
3
1
3
35.9, 132.9, 130.6, 127.8, 122.8, 122.7, 121.9, 120.3, 115.3, 111.3, 109.7, 108.8, 103.2, 56.1, 44.8,
+
2.5, 31.6, 28.8, 26.9, 22.5, 22.4, 14.9, 14.5, 14.1, 13.8, 11.8. TOFMS (ESI) calcd for [M+H]
C H BF N O : 650.3366, found: 650.3370.
39
43
2
3
3
4
4
2
.3.4. 5-(8-(4-(Diphenylamino)phenyl)-5,5-difluoro-1,3,7,9-tetramethyl-10-pentyl-5H-5λ ,6λ -dipyrrolo
[
1,2-c:2',1'-f][1,3,2]diazaborinin-2-yl)furan-2-carbaldehyde (5b)
The synthetic method was similar to that of 5a except that compound 2a (241 mg, 0.66 mmol) was
[38]
replaced with compound 2b
(245 mg, 0.66 mmol). Compound 5b (180 mg, 83%) was isolated as a
1
red solid. H NMR (400 MHz, CDCl ): δ 9.65 (s, 1H, CHO), 7.35 (s, 1H, ArH), 7.31–7.26 (m, 4H,
3
ArH), 7.16–7.12 (m, 6H, ArH), 7.06–7.04 (m, 4H, ArH), 6.54 (s, 1H, ArH), 3.11 (m, 2H, CH ), 2.68 (s,
2
3
2
1
1
2
6
2
H, CH ), 2.59 (s, 3H, CH ), 2.54 (s, 3H, CH ), 2.40 (s, 3H, CH ), 1.70 (m, 2H, CH ), 1.53–1.49 (m,
3 3 3 3 2
1
3
H, CH ), 1.42–1.37 (m, 2H, CH ), 0.94 (t, J = 7.0 Hz, 3H, CH ). C NMR (100 MHz, CDCl ): δ
2
2
3
3
77.1, 156.5, 155,4, 151.8, 151.2, 148.2, 147.8, 147.6, 147.2, 138.7, 136.1, 135.2, 132.8, 131.0, 130.8,
29.4, 129.1, 127.6, 126.5, 124.7, 123.3, 122.9, 122.8, 121.8, 120.5, 116.3, 111.4, 32.5, 31.6, 29.7, 28.8,
+
2.5, 14.9, 14.5, 14.1, 13.7. TOFMS (ESI) calcd for [M+H] C H BF N O : 656.3260, found:
4
1
41
2
3
2
556.3261.
.3.5. 5-(5,5-Difluoro-1,3,7,9-tetramethyl-10-pentyl-8-(10-propyl-10H-phenoxazin-3-yl)-5H-5λ ,6λ -
4
4
dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinin-2-yl)furan-2-carbaldehyde (5c)
The synthetic method was similar to that of 5a except that compound 2a (241 mg, 0.66 mmol) was
replaced with compound 2c (231 mg, 0.66 mmol). Compound 5c (188 mg, 90%) was isolated as a red
1
solid. H NMR (400 MHz, CDCl ): δ 9.64 (s, 1H, CHO), 7.35 (d, J = 3.5 Hz, 1H, ArH), 6.80 (t, J = 7.6
3
Hz, 1H, ArH), 6.64–6.60 (m, 3H, ArH), 6.53–6.47 (m, 4H, ArH), 3.48 (t, J = 7.5 Hz, 2H, CH ), 3.10
2
9

ACCEPTED MANUSCRIPT
(
m, 2H, CH ), 2.67 (s, 3H, CH ), 2.58 (s, 3H, CH ), 2.52 (s, 3H, CH ), 2.38 (s, 3H, CH ), 1.76–1.69 (m,
2
3
3
3
3
4
6
1
1
H, CH ), 1.57–1.49 (m, 2H, CH ), 1.42–1.38 (m, 2H, CH ), 1.05 (t, J = 6.5 Hz, 3H, CH ), 0.93 (t, J =
2 2 2 3
1
3
.3 Hz, 3H, CH ). C NMR (100 MHz, CDCl ): δ 177.0, 156.4, 155.3, 151.8, 151.1, 147.7, 144.8,
3
3
44.7, 138.6, 136.1, 134.6, 133.0, 132.8, 132.7, 130.7, 125.5, 125.2, 123.8, 123.0, 120.9, 120.5, 117.0,
15.4, 111.4, 111.3, 111.1, 45.8, 32.5, 31.6, 28.7, 26.9, 22.5, 18.3, 14.7, 14.4, 14.0, 13.7, 13.6. TOFMS
+
(
ESI) calcd for [M+H] C H BF N O : 636.3209, found: 636.3208.
38
41
2
3
3
2.3.6. (E)-2-cyano-3-(5-(5,5-difluoro-8-(6-methoxy-9-propyl-9H-carbazol-3-yl)-1,3,7,9-tetramethyl-10-
4
4
pentyl-5H-5λ ,6λ -dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinin-2-yl)furan-2-yl)acrylic acid (CBZ-B)
A mixture of precursor 5a (97 mg, 0.15 mmol) with cyanoacetic acid (51 mg, 0.60 mmol) in CHCl3
(
15 mL) and CH CN (15 mL) solution was refluxed overnight in the presence of piperidine (10 ꢀL)
3
under a nitrogen atmosphere. Then, the mixture was poured to CHCl (50 mL) and washed with 2N
3
HCl (30 mL) and water (2 × 50 mL). Next, solvent was evaporated under vacuum and the residue was
purified by column chromatography on silica gel with CH Cl and MeOH as the eluant to afford
2
2
1
compound CBZ-B (77 mg, 72%) as a dark powder. H NMR (400 MHz, DMSO [D ]): δ 8.14 (s, 1H,
6
CH), 8.07 (s, 1H, ArH), 7.82 (s, 1H, ArH), 7.66 (d, J = 8.1 Hz, 1H, ArH), 7.60 (s, 1H, ArH), 7.57 (d, J
=
8.8 Hz, 1H, ArH), 7.35 (d, J = 8.4 Hz, 1H, ArH), 7.11 (d, J = 8.8 Hz, 1H, ArH), 6.99 (s, 1H, ArH),
4
.37 (m, 2H, CH ), 3.86 (s, 3H, OCH ), 3.17 (m, 2H, CH ), 2.66 (s, 3H, CH ), 2.61 (s, 3H, CH ), 2.51
2
3
2
3
3
(
s, 3H, CH ), 2.44 (s, 3H, CH ), 1.86–1.80 (m, 2H, CH ), 1.70 (m, 2H, CH ), 1.51 (m, 2H, CH ),
3
3
2
2
2
1
3
1
1
1
3
.42–1.39 (m, 2H, CH ), 0.91 (t, J = 7.0 Hz, 6H, CH ). C NMR (100 MHz, DMSO [D ]): δ 164.4,
2
3
6
56.6, 154.7, 153.7, 150.9, 148.3, 148.0, 140.2, 139.5, 137.6, 136.4, 136.3, 135.8, 132.6, 130.5, 130.0,
28.0, 126.2, 122.8, 122.7, 122.6, 122.3, 120.6, 116.9, 115.4, 113.7, 110.5, 109.7, 103.9, 56.1, 44.4,
-
1
2.3, 31.4, 28.5, 22.5, 22.2, 14.9, 14.5, 14.3, 14.2, 14.1, 11.9. IR (KBr, cm ): 2928 (m), 1614 (s), 1538
10

ACCEPTED MANUSCRIPT
–
(
s), 1198 (s), 1079 (m), 1014 (m), 805 (m). TOFMS (ESI) calcd for [M–H] C H BF N O : 715.3267,
42 42 2 4 4
found: 715.3281.
2
5
.3.7.(E)-2-cyano-3-(5-(8-(4-(diphenylamino)phenyl)-5,5-difluoro-1,3,7,9-tetramethyl-10-pentyl-5H-
4
4
λ , 6λ -dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinin-2-yl)furan-2-yl)acrylic acid (TPA-B)
The synthetic method was similar to that of CBZ-B except that compound 5a (97 mg, 0.15 mmol)
was replaced with compound 5b (98 mg, 0.15 mmol). Dye TPA-B (86 mg, 78%) was isolated as a dark
1
solid. H NMR (400 MHz, DMSO [D ]): δ 8.09 (s, 1H, CH), 7.62 (s, 1H, ArH), 7.38–7.35 (m, 4H,
6
ArH), 7.23 (d, J = 7.7 Hz, 2H, ArH), 7.12–7.10 (m, 6H, ArH), 7.04 (d, J = 8.0 Hz, 2H, ArH), 7.00 (s,
1
H, ArH), 3.15 (m, 2H, CH ), 2.64 (s, 3H, CH ), 2.60 (s, 3H, CH ), 2.45 (s, 3H, CH ), 2.42 (s, 3H,
2 3 3 3
CH ), 1.67 (m, 2H, CH ), 1.50–1.49 (m, 2H, CH ), 1.42–1.37 (m, 2H, CH ), 0.92 (t, J = 6.9 Hz, 3H,
3
2
2
2
13
CH ). C NMR (100 MHz, DMSO [D ]): δ 163.8, 155.5, 154.2, 150.8, 148.1, 147.5, 146.9, 146.6,
3
6
1
1
1
7
2
5
38.9, 137.4, 136.3, 134.4, 132.0, 131.1, 130.1, 129.6, 126.0, 125.8, 124.4, 123.4, 122.1, 120.3, 116.2,
-1
13.3, 31.7, 30.9, 28.1, 21.7, 14.4, 14.1, 13.8, 13.6, 13.4. IR (KBr, cm ): 2928 (m), 2219 (m), 1614 (m),
–
538 (s), 1232 (m), 1176 (s), 1015 (m), 801 (m). TOFMS (ESI) calcd for [M–H] C H BF N O :
4
4
40
2
4
3
21.3162, found: 721.3171.
.3.8.(E)-2-cyano-3-(5-(5,5-difluoro-1,3,7,9-tetramethyl-10-pentyl-8-(10-propyl-10H-phenoxazin-3-yl)-
4
4
H-5λ ,6λ -dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinin-2-yl)furan-2-yl)acrylic acid (POZ-B)
The synthetic method was similar to that of CBZ-B except that compound 5a (97 mg, 0.15 mmol)
was replaced with compound 5c (95 mg, 0.15 mmol). Dye POZ-B (91 mg, 86%) was isolated as a dark
1
solid. H NMR (400 MHz, DMSO[D ]): δ 8.10 (s, 1H, CH), 7.63 (s, 1H, ArH), 6.99 (s, 1H, ArH), 6.86
6
(
m, 1H, ArH), 6.78 (m, 2H, ArH), 6.72 (d, J = 7.9 Hz, 1H, CH ), 6.67 (m, 2H, ArH), 6.63 (s, 1H, ArH),
2
3
.55 (t, J = 7.5 Hz, 2H, CH ), 3.13 (m, 2H, CH ), 2.64 (s, 3H, CH ), 2.59 (s, 3 H, CH ), 2.51 (s, 3 H,
2 2 3 3
11

ACCEPTED MANUSCRIPT
CH ), 2.40 (s, 3 H, CH ), 1.66–1.58 (m, 4H, CH ), 1.50 (m, 2H, CH ), 1.41–1.37 (m, 2H, CH ), 1.00 (t,
3
3
2
2
2
1
3
J = 6.4 Hz, 3H, CH ), 0.91 (t, J = 6.8 Hz, 3H, CH ). C NMR (100 MHz, DMSO [D ]): δ 164.4, 156.0,
3
3
6
154.9, 151.2, 148.4, 147.9, 144.3, 139.4, 138.0, 136.6, 134.3, 133.0, 132.8, 132.5, 130.6, 126.9, 126.3,
125.0, 124.6, 121.3, 120.7, 116.9, 116.6, 115.5, 113.8, 112.5, 112.3, 96.9, 45.0, 32.3, 31.3, 28.5, 22.2,
-1
1
8.1, 14.8, 14.5, 14.3, 14.1, 13.8, 11.2. IR (KBr, cm ): 2962 (m), 2219 (m), 1682 (s), 1538 (s), 1494 (s),
–
1
261 (m), 1198 (m), 1014 (m), 801 (m). TOFMS (ESI) calcd for [M–H] C H BF N O : 701.3111,
41
40
2
4
4
found: 701.3121.
4
4
2
.3.9. (E)-2-cyano-3-(5-(5,5-difluoro-1,3,7,9-tetramethyl-10-pentyl-5H-5λ ,6λ -dipyrrolo[1,2-c:2',1'-f]
[
1,3,2]diazaborinin-2-yl)furan-2-yl)acrylic acid (R1)
The synthetic method was similar to that of CBZ-B except that compound 5a (97 mg, 0.15 mmol)
[
21]
was replaced with compound 3
(62 mg, 0.15 mmol). Dye R1 (63 mg, 88%) was isolated as a red
1
solid. H NMR (400 MHz, DMSO [D ]): δ 8.06 (s, 1H, CH), 7.60 (m, 1H, ArH), 6.96 (d, J = 2.8 Hz,
6
1
H, ArH), 6.41 (s, 1H, ArH), 3.06 (t, J = 6.8 Hz, 2H, CH ), 2.61 (s, 3H, CH ), 2.56 (s, 3H, CH ), 2.47
2 3 3
(
s, 6H, CH ), 1.62 (m, 2H, CH ), 1.51–1.46 (m, 2H, CH ), 1.40–1.35 (m, 2H, CH ), 0.90 (t, J = 7.0 Hz,
3 2 2 2
1
3
3
1
1
H, CH ). C NMR (100 MHz, DMSO [D ]): δ 164.4, 157.3, 154.8, 151.2, 148.5, 147.9, 144.0, 137.8,
3
6
36.7, 132.9, 131.9, 130.3, 126.4, 123.9, 120.6, 116.7, 113.7, 32.3, 31.9, 28.4, 22.2, 16.7, 14.8, 14.5,
-
1
4.3, 14.1. IR (KBr, cm ): 2924 (m), 2218 (s), 1698 (m), 1554 (s), 1261 (s), 1201 (m), 1085 (s), 803
–
(
m). MALDI-TOF calcd for [M–H] C H BF N O : 478.2114, found: 478.2121.
26
27
2
3
3
2.4. Fabrication of dye-sensitized solar cells
DSSCs were fabricated using fluorine tin oxide (FTO) coated glass as substrate for both photoanode
and counter electrode. The photoanode was fabricated using the TiO thin film coated on FTO glass
2
[
18]
substrate by electro-spray as envisaged in the literature . The film thickness was 10.5 ꢀm which is
12

ACCEPTED MANUSCRIPT
measured by a profilometer. The dye adsorption was performed by immersing the nanostructure films
of the TiO electrode into the anhydrous CHCl solution (0.3 mM) for 12 h at room temperature in the
2
3
dark. The electrode was then rinsed with CHCl and dried by air flow. The active area of the TiO film
3
2
2
electrode was 0.25 cm . To prepare the counter electrode, the Pt catalyst was deposited on cleaned FTO
glass by spraying with H PtCl solution, followed by heat treatment at 410 ˚C for 20 min. A hole was
2
6
drilled on the counter electrode using a drill press. The perforated sheet was cleaned with ultrasound in
an ethanol bath for 15 min. For the assembly procedure, the dye-covered TiO electrode and Pt-counter
2
electrode were assembled into a sandwich type cell and sealed together using a thermal melt polymer
film. The organic electrolyte solution consisting of 0.6 M 1, 2-dimethyl-3-propylimidazolium iodide
(
DMPII), 0.1 M LiI, and 0.1 M I in CH CN was introduced into cell via vacuum backfilling from the
2 3
hole in the back of the counter electrode. Lastly, the hole was sealed by a thermal adhesive film and a
cover glass. The losses of light reflection and absorption by the conducting glass were not corrected.
2.4. Characterization of dye-sensitized solar cells
The photo-electrochemical characterizations on the DSSCs were carried out using an Oriel Class
AAA solar simulator (Newport, Oriel 94043 A). The photocurrent density-photovoltage (J–V) curves of
DSSCs were recorded using a Keithley 2420 source meter (Keithley Instruments Inc.) controlled by
Testpoint software. The monochromatic incident photo-to-current conversion efficiency (IPCE) spectra
for the DSSCs were measured as a function of wavelength from 300 to 800 nm, which was recorded on
QE/IPCE measurement kit (Newport Corporation) using a 300 W Xe lamp light source.
Electrochemical impedance spectroscopy (EIS) was performed using an AUTOLAB PGSTAT 302N
analyzer (Metrohm) under dark at room temperature. The frequencies ranged from 0.1 Hz to 100 kHz,
and the applied voltage bias was –0.60 V.
3
Result and discussion
3.1 Synthesis of sensitizers
13

ACCEPTED MANUSCRIPT
As demonstrated, the n-amyl chain on Bodipy core of B3 can help in better solubility of the dyes in
common organic solvents and suppression of dye aggregation. Disappointingly, B3-based solar cells
exhibit a moderate photocurrent density, thus the electron-rich segment furan was introduced into dye
skeleton as a conjugated spacer, aiming to optimize the absorption profile and light-harvesting
efficiency. The synthetic approaches to novel sensitizers CBZ-B, TPA-B and POZ-B are presented in
Scheme 1. All the dyes were synthesized according to the traditional methods including N-alkylation,
bromination, Miyaura borylation, Suzuki coupling and Knoevenagel condensation. Iodo-exposed
[
21]
Bodipy 4, synthesized in an analogous method as reported earlier , was utilized as a starting core for
coupling with the donor/acceptor part for CBZ-B, TPA-B and POZ-B. Firstly, Miyaura borylation
reaction of the bromo-exposed arylamine 1 with B Pin was carried out using KOAc as a base and
2
2
Pd(PPh ) Cl as catalyst in toluene, resulting in intermediate 2. Subsequently, three electronic donors
3
2
2
(
MeO-CBZ, TPA and POZ boronic ester) were appended onto the Bodipy 4 by Pd(PPh ) -assisted
3 4
Suzuki coupling protocols to give formylated Bodipy 5 with a yield of over 80%. Finally, the obtained
carbaldehydes were reacted with cyanoacetic acid in a Knoevenagel condensation to afford target
organic photosensitizers in good yields. The counterpart dye R1 was also synthesized by similar
pathways described in Scheme 1. The chemical structures of all the key intermediates and target dyes
were characterized by NMR spectroscopy and mass spectrometry (see Experimental section).
Scheme 1 here
3.2 Optical properties
The influence of different arylamine electron donors on the light harvesting capacities was first
evaluated by measuring the UV-visible absorption spectra in diluted CHCl solutions at room
3
temperature, as presented in Fig. 1, and the corresponding spectroscopic parameters are also
14

ACCEPTED MANUSCRIPT
summarized in Table 1. The auxiliary electron-withdrawing group of Bodipy is expected to optimize
the molecular HOMO-LUMO energy gap and facilitate the intramolecular charge transfer from the
donor to the anchor. As expected, the absorption spectra of these dyes display two prominent bands,
appearing in the range of 450–700 (band I) nm and 280–440 (band II) nm, respectively. The former
bands can be assigned to overlap of the typical Bodipy absorption and charge transfer transition
absorption while the later one are ascribed to the localized aromatic π–π* electron transitions of
aromatic rings. How the absorbed photons populate in the absorption bands will be investigated by
employing theoretical calculation (vide infra, Table 2). It is noteworthy that the absorption maximum
wavelengths (λmax) of the dyes in CHCl are in the order of POZ-B (556 nm) > TPA-B (554 nm) >
3
CBZ-B (552 nm). We found that with the introducing of MeO-CBZ, TPA and POZ group to the
Bodipy unit, the absorption peaks of these dyes are dramatically red-shifted compared to the reference
[18]
dyes R1 (530 nm) and R2
(516 nm) (Fig. 2), which is propitious to improve the extent of electron
delocalization over the whole molecule and its light-harvesting efficiency. All three dyes exhibit large
–
1
–1
–1
molar extinction coefficients (ε) at their absorption maxima (59 000 M cm for CBZ-B, 71 000 M
–
1
–1
–1
cm for TPA-B, 69 000 M cm for POZ-B), with values ~4–5 times larger than those of typical
–
1
–1
[40]
Ru(II) polypyridyl complexes (14 000 M cm for N719 ). Therefore, these dyes are expected to
[
41]
efficiently harvest light and enable large photocurrents in a DSSC device . Compared to dye CBZ-B,
the incorporation of a TPA unit instead of MeO-CBZ moiety on the Bodipy conjugated bridge in dye
TPA-B slightly red-shifts the long wavelength band by 2 nm with a large enhancement of molar
extinction coefficients. In contrast, when replacing the donor group in TPA-B with POZ unit, the long
wavelength band of dye POZ-B is similarly red-shifted by 2 nm with little declination of molar
extinction coefficients. The significant difference in light harvesting efficiency for dyes CBZ-B,
15

ACCEPTED MANUSCRIPT
TPA-B and POZ-B indicates that their absorption behavior mainly relies on the donor chromophore
part as their consistency in both π spacer and anchoring group.
–
1
–1
Notably, compared with B3 (λmax = 533 nm, ε = 57 000 M cm ), incorporation of a furan unit in
TPA-B significantly red-shifted the absorption band by 21 nm along with obvious higher molar
extinction coefficients. In addition, there is a deep valley between 350 and 450 nm in the absorption
curve of B3, which may cause low IPCE response at this region, especially considering the strong light
absorption by triiodide in the electrolyte. As shown in Fig. 2, a shoulder absorption band is observed at
400 nm for TPA-B, such enhanced light-harvesting capability and the red-shift may be ascribed to the
electro-rich property of furan conjugated spacer, and excellent coplanarity between the furan and
Bodipy group, which benefits the capture of photons in an extended region.
Fig. 1 here
Fig. 2 here
Table 1 here
Fig. 3 shows the absorption spectra of the Bodipy dyes anchored onto a 3 µm transparent
nanocrystalline TiO films. When anchored onto TiO films, the sensitizers may bring forth interactions
2
2
between a dye molecule and the nanoparticle of TiO or among dye molecules. As a result, the
2
absorption spectra of sensitizer adsorbed on TiO are changed to some extent. It can be found that the
2
λmax values for CBZ-B, TPA-B and POZ-B on the TiO film are 536, 542 and 544 nm, respectively. As
2
compared to the solution spectra, the absorption peaks for the three dyes binding with TiO are slightly
2
blue-shifted by 16, 12 and 12 nm, respectively, indicating the formation of H-aggregation or
deprotonation of carboxylic acid groups on the TiO surface. As shown in Fig. 3, the tail of the
2
absorption spectrum extends to 700 nm and the absorption spectra of CBZ-B, TPA-B and POZ-B
16

ACCEPTED MANUSCRIPT
become broadened after adsorption on the surface of transparent mesoporous TiO films, which should
2
favor the light harvesting of solar cells and enhance the photocurrent response region. Furthermore, it is
found out that the dye loading is remarkably higher in the case of TPA-B with TPA group as donor
compared to CBZ-B and POZ-B with MeO-CBZ and POZ group as donors (Table 3). Without a
consideration of the aggregation on film, increasing the molar extinction coefficients and adsorbed
amount of dyes on the TiO surface will enhance the light absorption ability of the sensitizer, implying
2
that the dyes of TPA-B based DSSCs will own better photovoltaic performances.
Fig. 3 here
3.3 Electrochemical properties
For a typical cell sensitizer, suitable highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) levels are required to enable efficient dye regeneration and
electron injection processes. To ensure the energy level alignment of dyes CBZ-B, TPA-B and POZ-B
–
–
versus TiO and the I / I redox mediator, CV measurements were carried out in CH Cl solution with
2
3
2
2
ferrocene as external standard (Fig. 4). The ground state oxidation potentials (E ) corresponding to the
ox
HOMO levels of the dyes were compiled in Table 1. CV measurements indicate that the HOMO levels
of dyes CBZ-B, TPA-B and POZ-B are 1.22 V, 1.16 V and 0.88 V, respectively, versus the normal
hydrogen electrode (NHE, the same below). Phenothiazine (PTZ) electron group owns a similar
heteocyclic structure with POZ unit except replacing oxygen atom with sulphur atom, and the HOMO
[
21]
level of PTZ-B (UY7) in CH Cl solution is about 0.91 V . The obvious potential difference among
2
2
these dyes suggests that the strength of donating group has relatively large effect in the electronic
oxidation behavior of D–A–π–A based dyes. The HOMO level is mainly affected by electron donor,
and the electron donor with strong donating ability will shift the HOMO level more negative. Evidently,
17

ACCEPTED MANUSCRIPT
from the HOMO level data of these dyes, it can be concluded that the electron-donating power of these
electron donors in the order POZ > PTZ > TPA > MeO-CBZ. The observed potential levels lie below
–
–
the standard potential of the I / I redox couple (0.40 V vs. NHE) by 0.48–0.82 eV. Therefore, the
3
oxidized dye regeneration by the electrolyte should be energetically favorable. The least driving force
for the dye regeneration may be the cause of low cell performance of POZ-B (vide infra). All dyes
exhibit distinct and reversible oxidations without any reductive features, suggesting that the oxidized
[
42]
states of the dyes are stable . Furthermore, calculated from E value and the optical band gap (E )
ox
0-0
derived from the wavelength at 10% maximum absorption intensity for the dye in CH Cl solution, the
2
2
*
ox
excited-state oxidation potential (E ) corresponding to LUMO levels of dyes CBZ-B, TPA-B and
POZ-B are calculated to be –0.82, –0.86 and –1.09 V, respectively. The LUMO levels of the Bodipy
dyes are more negative than the Fermi level of TiO (–0.5 V vs. NHE), suggesting that a sufficient
2
driving force is provided for the efficient electron injection into TiO conduction band from the excited
2
dyes (Fig. 5). Hence, these prepared Bodipy dyes are suitable for utilization as DSSC sensitizers.
Fig. 4 here
Fig. 5 here
3.4 Molecular Calculations
To gain insight into the geometrical structures and electron density distribution, DFT calculations
with the Gaussian 09 package of the resulting Bodipy dyes are performed at B3LYP/6-31G(d) level of
theory. Optimized molecular structures and selected frontier molecular orbital topologies involving the
HOMO–3 to LUMO+3 are shown in Fig. 6. The HOMOs homogeneously lie on the arylamine cores
(
MeO-CBZ, TPA and POZ) with extension to the neighbored Bodipy fragments, that is to say, D-A
system. As most transitions move excited electrons to LUMO and LUMO+1 orbitals (vide infra, Table
), checking the electron distribution of above two orbitals is necessary for evaluating electron
2
18

ACCEPTED MANUSCRIPT
injection in view of orbital overlapping between sensitizer and TiO . DFT calculation on CBZ-B,
2
TPA-B and POZ-B indicates that the electrons of both LUMO and LUMO+1 are delocalized across
the entire A–π–A system with a large composition on the 2-cyanoacrylic acid subunit, where the
electron is close to the TiO surface and can be easily injected into the conduction band (CB) of the
2
TiO semiconductor. Hence, photons from both bands I and II are useful for electron injection and
2
photovoltaic conversion. In addition, the distributions of most ground and excited frontier orbitals
(such as HOMO–3, HOMO–2, HOMO–1, LUMO and LUMO+1) of the three sensitizers are
well-overlapped with the Bodipy unit. Thus, it can be predicted that the photoexcited electrons could be
successively transferred from the donor to the Bodipy unit, then transferred to anchoring site of the
cyanoacrylic acid anchoring group, and finally to TiO . It is consistent with our expected cascaded
2
electron-trap role of the introduced Bodipy unit.
The excitation energy characteristics of these compounds were calculated by TD-DFT method at the
same hybrid function and basis set, and corresponding of relevant parameters are summarized in Table
2. The contribution of electronic transition from HOMO to LUMO is about 70% in CBZ-B, TPA-B
and POZ-B. As a result, we confirm that electronic transitions of all sensitizers are originated from
HOMO to LUMO. The oscillator strength (f) of a transition is related to the molar absorption
coefficient. The larger the f between the ground state system and an excited state, the easier it is to
absorb the corresponding frequency of the electromagnetic wave and then jump to the excited state,
thus the corresponding absorption peak in the absorption spectrum is stronger. From the Table 2, it can
be seen that the electrons are transferred from HOMO to LUMO with f only about 0.08–0.20. On the
other hand, these dyes of CBZ-B, TPA-B and POZ-B have another excitation state with the electron
transferred from HOMO–1 to LUMO, and the f value is 0.81, 0.85 and 0.95, respectively. Since the f
value of the transition from HOMO to LUMO is exceedingly smaller than that of the transition from
HOMO–1 to LUMO, only one absorption peak is observed in the longer wavelength (band I) for all
three dyes. The calculated λmax with maximum oscillator strength arises out of electronic transition
19

ACCEPTED MANUSCRIPT
from the HOMO–1 to the LUMO, not from HOMO to the LUMO. Moreover, it can be seen that the
calculated data of λmax for dyes CBZ-B, TPA-B and POZ-B is 492, 491 and 495 nm, respectively, are
in good agreement with the measured data from UV-vis absorption spectra in CHCl solution (552 nm
3
for CBZ-B, 554 nm for TPA-B, 556 nm for POZ-B). As demonstrated, the functional electron transfer
in band I is from not only HOMO to LUMO, but also HOMO–1 to LUMO. The short band in the
2
3
80–440 nm (band II) is composed of several electron excitations calculated around 430, 380, 358 and
10 nm, which also corresponds well with experimental results from absorption spectra in solution.
Table 2 here
Fig. 6 here
3.5 Photovoltaic performance
It can be seen that dye R1 is the basic structure, and three different kinds of structural modification
on the 6-position of R1 core have been selected to examine the donor effects on the photovoltaic
performance. By using dye-coated TiO film as the working electrode, platinized FTO glass as the
2
counter electrode, and 0.6 M DMPII, 0.1 M LiI, and 0.1 M I in CH CN solution as the redox
2
3
electrolyte, DSSCs were fabricated to explore the potentials of new resulting Bodipy dyes as
photosensitizers. A black aperture mask was attached on the assembled devices during the
measurements to prevent additional illumination through the lateral space.
Fig. 7 here
Table 3 here
–2
Fig. 7 presents photocurrent–voltage (J–V) curves of cells measured at 100 mW cm under
2
simulated 1.5 air mass global solar light with 0.25cm active surface area, and detailed photovoltaic
parameters like short-circuit current density (J ), open-circuit voltage (V ), fill factor (ff), and
sc
oc
photovoltaic conversion efficiency (η) are listed in Table 3. Depending on the different electron
20

ACCEPTED MANUSCRIPT
donating groups, these dyes showed distinguishing and interesting results. Among these dyes, the
–
2
TPA-B sensitized solar cells shows the highest PCE of 4.76% (J = 13.95 mA cm , V = 557 mV, ff =
sc
oc
–2
0
.61). Under the same conditions, the cell based on sensitizer CBZ-B provided a J of 9.15 mA cm , a
sc
V of 487 mV, and an ff of 0.68, offering a PCE of 3.02%, while POZ-B based cell gave a J of 10.91
oc
sc
–
2
mA cm , a V of 546 mV, and an ff of 0.61, resulting in a PCE of 3.61%. It should be pointed out that
oc
after further optimizing the preparation conditions of the DSSCs, the PCE of 2.38% (J = 7.53 mA
sc
–
2
cm , V = 500 mV, ff = 0.63) for sensitizer B3 was obtained in this work, which was higher than the
oc
previous result.
Analyzing the photovoltaic results carefully, it has been demonstrated that the high PCE of TPA-B is
mostly due to the increased short-circuit photocurrent. As listed in Table 3, the J of TPA-B was higher
sc
than that of CBZ-B, POZ-B and B3 by ~52%, 28% and 85%, respectively. It is well-known that the
integral of the IPCE spectrum is generally related to the value of J in DSSC performance; IPCE =
sc
(
1240 × J ) 100 / (λ × P ) (in%), where λ and P are specific wavelength and fixed input power,
sc in in
respectively. In order to rationalize the abovementioned observations, IPCE spectra response in the
visible region for CBZ-B, TPA-B and POZ-B sensitized devices are measured.
Along with a better conjugated backbone, sensitizers CBZ-B, TPA-B and POZ-B are beneficial to
the harvest of photons from longer wavelength, which is expected to present an extended IPCE
response with respect to analogue B3. As shown in Fig. 8, all devices exhibited remarkably broad IPCE
spectra covering almost the whole visible spectrum with the initial response wavelengths of
photocurrent response stepwise red-shifted from B3 (699 nm) to CBZ-B (710 nm), TPA-B (730 nm)
and POZ-B (775 nm). Compared to those of their UV-vis absorption spectra in CHCl solutions, the
3
IPCE spectra are significantly broadened and match well with the threshold wavelength of the
absorption spectra of the dyes adsorbed on TiO film. Especially, sensitizers CBZ-B, TPA-B and
2
POZ-B exhibited recessed regions around 460 nm for D–A–π–A-based solar cells, as well as consistent
with the spectral responses. The device containing CBZ-B shows relatively low IPCE above 40% in
21

ACCEPTED MANUSCRIPT
the region from 375 nm to 625 nm and reaches its maximum value of 57% at around 515 nm.
Remarkably, when incorporation of TPA into the dye skeleton, the DSSCs based on TPA-B exhibited
unexpected higher efficiency. Note that the IPCE exceeds 60% in the spectral range 385–640 nm for
TPA-B, which reaches its maximum of 72% at ca. 510 nm. TPA-B possessed higher IPCE values and
larger integral compared to that of CBZ-B, which may be due to its augmented molar absorption
coefficient and adsorption amount on TiO film. Furthermore, TPA-B also has a broadened absorption
2
both in CH Cl solution and on TiO film caused by the enhanced electron-donating ability of the
2
2
2
triphenylamine group relative to the carbazole group, consequently leads to larger electron
delocalization and better photoelectric conversion properties to some extent.
The device based on POZ-B showed broader IPCE spectrum than that of TPA-B, which may be
attributed to its broader absorption in the visible light region (Fig. 1 and 3) and better electron-donating
power of POZ based on the HOMO level data (Fig. 5). In spite of this, the relatively lower adsorbed
dye density on TiO film for POZ-B (Table 3) reduces the light harvesting capacity, thus the IPCE of
2
POZ-B is inferior to that of TPA-B in most region of the spectrum (300~670 nm). Consistent with the
variation tendency of photo-generated current integrated from IPCE spectra, the J values exhibit an
sc
order of B3 < CBZ-B < POZ-B < TPA-B. As a result, TPA-B achieved the improved short-circuit
–
2
current density of 13.95 mA cm due to its superior IPCE action spectrum which might be mainly
arisen from the improvement of light harvesting efficiency.
Fig. 8 here
TPA-B also possessed relatively higher V , which is needed to be further verified by EIS studies
oc
(
vide infra). Ordinarily, the V of a DSSC is related to the electron transport at the interfaces or
oc
[
43]
electron lifetime in the cell . The best J and V for the device of TPA-B lead to the most efficient
sc
oc
final photoelectricity conversion yield. The above result strongly suggests that TPA can be an excellent
donor for 2,6-Bodipy system to improve the photovoltaic performance.
22

ACCEPTED MANUSCRIPT
To optimize the performance of TPA-B-based DSSCs, we carried out 4-tert-butylpyridine (TBP)
treatments with different concentrations. The corresponding data of photovoltaic performance are
tabulated in Table 4. TBP acts as electrolyte additive to modify the surface of TiO effectively and thus
2
can enhance the V . When the concentration of TBP increased from 0 to 0.5 M, the photovoltages
oc
increased limitedly from 557 mV to 616 mV, while photocurrent densities decreased sharply from
–2
–2
1
3.95 mA cm to 2.81 mA cm . Normally addition of TBP will shift the TiO conduction band edge
2
to more negative value, which leads less efficient electron injection and therefore smaller photocurrent,
as observed in this study. Despite the higher V obtained in the case of electrolyte with 0.5 M TBP, the
oc
loss in the J resulted in the lower power conversion efficiency (1.29%) compared to that of the
sc
electrolyte without TBP (4.76%). In our contrast tests, the 11% increase of V associated with the
oc
addition of 0.5 M TBP resulted in an 80% reduction of J . To be exact, the V increasing caused by the
sc
oc
introduction of the TBP cannot offset the J decreasing in Bodipy sensitizers. The pronounced
sc
reduction in J of the TPA-B-based DSSC with the presence of TBP indicates that either poor electron
sc
injection or charge-collection yield is involved. Similar phenomenon has been obtained for other
sensitizers.
Table 4 here
3.6 Electrochemical impedance analysis
EIS analysis was performed to elucidate the effect of structure modification on V and obtain more
oc
interfacial charge-transfer information (Fig. 9). Impedance spectra for DSSCs based CBZ-B, TPA-B
and POZ-B were measured under the dark condition with a forward bias of –0.60 V in the measured
frequency range of 0.1 Hz–100 kHz.
The Nyquist plots for DSSCs based on CBZ-B, TPA-B and POZ-B devices are displayed in Fig. 9a,
23

ACCEPTED MANUSCRIPT
and the corresponding parameters deduced from fitting curves by Z-view software using appropriate
equivalent circuit model (Fig. 9c) are listed in Table 5. Under dark condition, as there is no
photocurrent generation, electrons are transported through the mesoporous TiO network and then react
2
–
–
–
[44]
with I3 in the electrolyte. Simultaneously, I is oxidized to I at the counter electrode . The
3
resistance of the electron recombination can be estimated from the interfacial kinetic parameters. A
typical Nyquist diagram of DSSCs consists of three semicircles. The first semicircle in high frequency
region is assigned to charge transfer resistance at the Pt counter electrode/electrolyte interface (RCE),
the second large semicircle in the intermediate frequency region is accorded to charge transfer
resistance at the interface between TiO /dye/electrolyte (R ), whereas the third semicircle represents
2
ct
–
–
the impedance corresponding to Warburg diffusion process of I /I in the electrolyte (R ). Since the
3
w
semicircle for Warburg diffusion within the electrolyte was overlapped by the second large semicircle,
only two semicircles are observed in the Nyquist plots for TPA-B and POZ-B.
According to Table 5, both the series resistance (R , ascribed to the intercept of the first semicircle)
s
and RCE are similar to each other, as the same electrode material and electrolyte were used in all cases.
However, when it comes to R , we can find that the values change significantly. Here, although the R
ct
ct
value of TPA-B is smaller than that of POZ-B, chemical capacitance (C ) of TPA-B is almost 2.2-fold
ꢀ
with respect to POZ-B, which may be related to the dipole moment induced by the adsorbed dye
[
45]
molecules . The electron lifetime (τ) reflects the response time constant for recombination dynamics.
If the cells are fabricated and tested under the same conditions, only the difference of the organic
sensitizers can influence the electron lifetime, which has been calculated by fitting the equation τ = R
n
ct
[46]
×
Cꢀ
. A longer electron lifetime implies slower recombination rate at the dye-sensitized
[
47]
TiO /electrolyte interface and vice versa . The corresponding electron lifetimes for CBZ-B, TPA-B,
2
24

ACCEPTED MANUSCRIPT
POZ-B and B3 are 34 ms (R = 15.64 ꢁ, C = 2.18 mF), 109 ms (R = 29.89 ꢁ, C = 3.63 mF), 106
ct
ꢀ
ct
ꢀ
ms (R = 64.41 ꢁ, C = 1.64 mF) and 33 ms (R = 19.37 ꢁ, C = 1.68 mF) under the identical forward
ct
ꢀ
ct
ꢀ
bias (–0.60 V) condition, respectively. More planar arylamine donors (MeO-CBZ and POZ) seem to be
more easily recombination in dark condition. Pyramidal geometry of triphenylamine donor together
with higher dye amount for TPA-B may be the reason of longer electron lifetime. The electron lifetime
trend is positively in good accordance with the experimental results of the V . Obviously, the concerted
oc
contribution from both the interface charge transfer resistance and chemical capacitance determines the
remarkable difference in V values for the series of Bodipy dyes.
oc
A Bode phase plot is also related to the charge transfer resistance for DSSCs based on these dyes
(Fig. 9b). The peak located at middle frequency (fmax) is indicative of the electron recombination
between the TiO /dye/electrolyte interfaces. From Fig. 9b, it can be seen that the frequency of the
2
characteristic peak increased in the order of TPA-B < POZ-B < B3 < CBZ-B. The peak shift from
high to low frequencies reveals the slower charge recombination rate and longer electron lifetimes
–
1 [48]
because the reciprocal of the fmax is correlated with electron lifetime, τ = (2πfmax)
. We can
e
speculate that the tendency of the electronic lifetime for the three dyes is TPA-B > POZ-B > B3>
–
–
CBZ-B. Evidently, the generation of dark current and the back reaction of injected electron with I /I
3
redox couple in the electrolyte can be more effectively suppressed by introducing of TPA donor and
furan unit at the 2,6-site of Bodipy, thus obtaining accordingly higher V in TPA-B-based DSSCs
oc
compared to the other two dyes.
Fig. 9 here
Table 5 here
25

ACCEPTED MANUSCRIPT
4
. Conclusions
In summary, we have successfully synthesized three novel D–A–π–A-featured dyes CBZ-B, TPA-B
and POZ-B by incorporation an electron-withdrawing unit of Bodipy as the additional acceptor and
furan unit as spacer for exploring more efficient sensitizer through ingenious molecular engineering.
The spectral, electrochemical and photovoltaic properties of these dyes were investigated
systematically and compared according to the various arylamine chromophores of donors. Our results
reveal that all D–A–π–A-featured Bodipy sensitizers own higher molar extinction coefficient, whereas
dye B3 with D–π–A structure shows a litter lower response. Thus, incorporation of furan as a
conjugated bridge essentially facilitates electron delocalization over the whole molecule, and realizes
the enhancement of light-harvesting ability with a predominant increase in J . Furthermore, with
sc
respect to MeO-CBZ and POZ-based dyes CBZ-B and POZ-B, the TPA unit brings the sensitizer
superior spectral properties and improves light absorptivity. Investigated theoretically by DFT and
TDDFT, the introduction of both Bodipy and furan units can efficiently optimize broaden the
absorption band and the energy levels. Moreover, the functional electron transfer in the longer
wavelength (band I) is mainly from not only HOMO to LUMO but also HOMO-1 to LUMO. EIS
analysis reveals that the structure variation of electron donor has some degree of influence on the light
harvesting and the electron lifetime. Both J and V increased with the introduction of TPA and furan
sc
oc
at the 2, 6-position of Bodipy and the DSSC of dye TPA-B exhibited the maximum photoelectricity
–
2
conversion efficiency of 4.76% (J = 13.95 mA cm , V = 557 mV, ff = 0.61) under simulated AM
sc
oc
–
2
1
.5 irradiation (100 mW cm ). The conversion efficiency of TPA-B cell is almost 2 times of that of
reference B3 under the same test condition. These findings help us to understand the crucial importance
of molecular engineering and offer a useful strategy for the future development of other simple organic
26

ACCEPTED MANUSCRIPT
sensitizers for highly efficient DSSCs.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (Nos. 21406189,
41505127), the China Postdoctoral Science Foundation (No. 2016M591883), the Natural Science
Foundation of Jiangsu Province (No. BK20150901), the Postdoctoral Research Funding Plan in Jiangsu
Province (No. 1601262C), the Students Innovation and Entrepreneurship Training Program
(201510300022Z) and the Jiangsu Government Scholarship for Overseas studies.
References
[
1] Grätzel M, Janssen RAJ, Mitzi DB, Sargent EH. Materials interface engineering for
solution-processed photovoltaics. Nature 2012;488:304–12.
2] O’Regan B, Grätzel M. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal
TiO films. Nature 1991;353:737–40.
[
2
[
3] Han L, Islam A, Chen H, Malapaka C, Chiranjeevi B, Zhang S, et al. High-efficiency dye-sensitized
solar cell with a novel co-adsorbent. Energy Environ Sci 2012;5:6057–60.
4] O’Regan BC, Durrant JR. Kinetic and energetic paradigms for dye-sensitized solar cells: moving
from the ideal to the real. Acc Chem Res 2009;42:1799–808.
5] Hagfeldt A, Boschloo G, Sun L, Kloo L, Pettersson H. Dye-sensitized solar cells. Chem Rev
010;110:6595–663.
6] Liang M, Chen J. Arylamine organic dyes for dye-sensitized solar cells. Chem Soc Rev
013;42:3453–88.
7] Wu YZ, Zhu WH. Organic sensitizers from D-π-A to D-A-π-A: effect of the internal
[
[
2
[
2
[
27

ACCEPTED MANUSCRIPT
electron-withdrawing units on molecular absorption, energy levels and photovoltaic performances.
Chem Soc Rev 2013;42:2039–58.
[
8] Chai QP, Li WQ, Wu YZ, Pei K, Liu JC, Geng ZY, Tian H, Zhu WH. Effect of a long alkyl group
on cyclopentadithiophene as a conjugated bridge for D-A-π-A organic sensitizers: IPCE, electron
diffusion length, and charge recombination. ACS Appl Mater Interfaces 2014;6:14621–30.
[
9] Pei K, Wu YZ, Li H, Geng ZY, Tian H, Zhu WH. Cosensitization of D-A-π-A quinoxaline organic
dye: efficiently filling the absorption valley with high photovoltaic efficiency. ACS Appl Mater
Interfaces 2015;7:5296–304.
[
10] Wu YZ, Zhu WH, Zakeeruddin SM; Grätzel M. Insight into D-A-π-A structured sensitizers: A
promising route to highly efficient and stable dye-sensitized solar cells. ACS Appl Mater Interfaces
015;7:9307–18.
11] Xie YS, Wu WJ, Zhu HB, Liu JC, Zhang WW, Tian H, Zhu WH. Unprecedentedly targeted
2
[
customization of molecular energy levels with auxiliary-groups in organic solar cell sensitizers. Chem
Sci 2016;7:544–9.
[
12] Loudet A, Burgess K. BODIPY dyes and their derivatives:ꢂ syntheses and spectroscopic properties.
Chem Rev 2007;107:4891–932.
13] Ulrich G, Ziessel R, Harriman A. The chemistry of fluorescent Bodipy dyes: versatility
unsurpassed. Angew Chem Int Ed 2008;47: 1184–1201.
14] Hattori S, Ohkubo K, Urano Y, Sunahara H, Nagano T, Wada Y, et al. Charge separation in a
[
[
nonfluorescent donor-acceptor dyad derived from boron dipyrromethene dye, leading to photocurrent
generation. J Phys Chem B 2005;109:15368–75.
[
15] Erten-Ela S, Yilmaz MD, Icli B, Dede Y, Icli S, Akkaya EU. A panchromatic boradiazaindacene
28