Strategies towards enhancing the efficiency of BODIPY dyes in dye sensitized solar cells

Article abstract of DOI:10.1016/j.jphotochem.2019.01.021

The goal of this study is to reveal the effect of electron donating and anchoring group of the dye on the electron injection dynamics and photovoltaic performance of BODIPY dye sensitized solar cells. The effect of electron donating properties was studied

Full text of DOI:10.1016/j.jphotochem.2019.01.021

Accepted Manuscript
Title: Strategies towards Enhancing the Efficiency of BODIPY
Dyes in Dye Sensitized Solar Cells
Authors: Elif Akhuseyin Yildiz, Gokhan Sevinc, H. Gul
Yaglioglu, Mustafa Hayvali
PII:
S1010-6030(18)31394-7
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2019.01.021
JPC 11680
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
27 September 2018
18 January 2019
19 January 2019
Please cite this article as: Yildiz EA, Sevinc G, Yaglioglu HG, Hayvali M,
Strategies towards Enhancing the Efficiency of BODIPY Dyes in Dye Sensitized
Solar Cells, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019),
https://doi.org/10.1016/j.jphotochem.2019.01.021
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Strategies towards Enhancing the Efficiency of BODIPY Dyes in Dye Sensitized Solar
Cells
Elif Akhuseyin Yildiza, Gokhan Sevincb, H. Gul Yaglioglua,*, Mustafa Hayvalic,*
aDepartment of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Beşevler,
Ankara, Turkey
bDepartment of Chemistry, Science and Literature Faculty, Bilecik Şeyh Edebali University, 11230,
Bilecik, Turkey
cDepartment of Chemistry, Faculty of Science, Ankara University, 06100 Beşevler, Ankara, Turkey
*Corresponding authors. Tel./Fax:+90 (312) 2127343 (H. G. Y.), Tel./Fax: +90 (312)2126720/1436)]
(M. H.)
E-mail addresses: yoglu@eng.ankara.edu.tr (H.G. Yaglioglu); hayvali@science.ankara.edu.tr (M.
Hayvali)
Graphical Abstract

HIGHLIGHTS
-
New BODIPY dyes with catechol anchoring and electron donating moieties for
DSSCs
-
-
-
Correlation between structural modification and excited state dynamics
Correlation between excited state dynamics and power conversion efficiency
Enhancing light harvesting capability by laser ablation of anode electrode
Abstract
The goal of this study is to reveal the effect of electron donating and anchoring group of the
dye on the electron injection dynamics and photovoltaic performance of BODIPY dye
sensitized solar cells. The effect of electron donating properties was studied by choosing
methoxyphenyl and triphenylamine (TPA) as electron donor moieties. Therefore, four new
BODIPY derivatives were designed and sensitized to achieve these goals. In addition,
attachment of the dye to TiO2 was studied by altering the surface morphology of TiO2 with fs
laser ablation.
The steady state absorption and emission spectra, femtosecond transient absorption spectra,
incident photon-to-current conversion efficiency (IPCE) and I-V measurements of the studied
dyes were investigated, systematically. It was found that methoxyphenyl moiety gives better
performance than TPA moiety. BODIPY dye with methoxyphenyl moiety and one anchoring
group demonstrated the best photovoltaic performance due to greater excited state lifetime in
spite of the lower light harvesting ability, as compared to other studied dyes. Increasing the
number of anchoring groups did not increase the photovoltaic performance of BODIPY dye
with methoxyphenyl moiety. Anchoring efficiency, light-harvesting capability, and electron
injection rate to TiO2 were enhanced by femtosecond laser ablation treatment of TiO2 surface.
Experimental results proved that power conversion efficiency was increased 47% when
photoanode was laser ablation treated.
Key words: Borondipyrromethene, Catechol-BODIPY, Ultrafast charge injection, DSSC,
Laser ablation

1. Introduction
Dye sensitized solar cells (DSSCs) have drawn a great interest because of their increasing
efficiency and low-cost fabrication, since it was invented by Grätzel and O’Regan, in 1991[1].
This technology enables an alternative option to conventional semiconductor based
photovoltaic structures [2]. DSSCs are generally composed of molecular dyes to harvest
sunlight, semiconductor oxide materials such as TiO2 [3], ZnO [4], SnO2 [5], a redox couple
to regenerate oxidized dye [6], and a metal coated counter electrode such as Pt [7]. Light
harvesting capability, localization of HUMO and LUMO energy levels of dye molecules and
electron transfer dynamics between dye and TiO2 layer play a key role to determine
photovoltaic conversion efficiency of the cell [8-14]. Up to date, many organic dyes, such as
porphyrin [15-18], phtalocyanine [19], and ruthenium complexes with long lived excited state
lifetimes [20] including widely used carboxylic acid as an anchoring group have been
investigated for their photophysical properties and employed in DSSC applications. New
organic dyes optimized specifically for DSSC applications and their photovoltaic performance
characterizations are needed to increase photovoltaic performance of DSSCs.
Boron dipyrromethene (BODIPY) dyes derived from 4,4-difluoro-4-bora-3a,4a-diaza-s-
indacene, have strong absorption and fluorescence properties in the visible and near infrared
spectral regions, great molar absorption coefficients and photostability [21-23]. Diverse
modification on the core of BODIPY structure at all positions and with any desired
functionality, allows tuning their photophysical properties as well as electron transfer
mechanisms. In the past nine years, a variety of BODIPY derivatives have been designed and
sensitized for various applications. Unfortunately, their use in solar cell applications has been
limited [24-26]. There are only a few works studying structural modification of BODIPY dyes
for DSSC applications [27-30]. Our aim is to relate photovoltaic performance with the excited
state lifetimes and electron injection dynamics. These relationships are crucial to develop
strategies towards enhancing the efficiency of BODIPY dyes. Therefore, more systematic
studies are needed in this field. However, there are only a few works investigating the
relationship between structural modification and electron injection dynamics of BODIPY
dyes as well as their effect on the performance of DSSCs [24, 31] .BODIPY fluorophores in
the literature generally use cyanoacedic acid and carboxylic acid that act as an anchoring
moiety localized at C2, C6 and C8 positions of BODIPY core [24, 28, 29]. Catechol moiety
can also be used as an anchoring group for BODIPY dyes in DSSC application.

In this work, we plan to add electron donation functionality to the BODIPY dye with catechol
anchoring group and investigate the electron injection dynamics as well as photovoltaic
performance. Therefore, novel BODIPY derivatives with one or two catechol anchoring
groups and having different electron donating properties were designed and sensitized in
order to increase excited state lifetime in solution, systematically. Triphenylamine and
methoxyphenyl moieties were chosen as electron donor moieties that were directly linked at
C8 position. The catechol anchoring moiety was attach to C3 position only as well as C3 and
C5 positions of the BODIPY core to see the effect of the symmetry. These positions were
chosen because, C3 and C5 positions are susceptible to intramolecular electron transfer; hence
influence the electron transfer mechanism and DSSC performance. Electron injection
dynamics were investigated by ultrafast pump-probe spectroscopy technique to reveal photo
physical mechanism behind the performance differences in DSSC. Power conversion
efficiencies of DSSCs with the sensitized dyes were investigated by incident photon-to-
current conversion efficiency (IPCE) and I-V measurements.
As well as molecular engineering, structural engineering of the DSSC cells are needed to
improve the photovoltaic performance as well. Enhancing light harvesting capability of
photoanode material has been a research topic for DSSC applications. Until now, several
light trapping mechanism such as photonic crystals [32], plasmonic effects[33] and various
micro/nanostructuring fabrication processes [34] were improved. Patterning on the
semiconductor oxide surface can enable dye to harvest more light [34, 35]. In an attempt to
investigate the effect of anchoring capability on the photovoltaic performance TiO2 surface
morphology was modified by femtosecond laser ablation technique and dye with highest
photovoltaic performance was coated on to this modified TiO2 surface to form anode
electrode. The effect of micro ablation patterning of TiO2 surface on the excited state lifetime
of the dye molecule and charge transfer dynamics between dye and TiO2 layer were
investigated with ultrafast pump probe experimental technique.
2. Experimental section
2.1 General

Reagents were obtained from Merck and used as received without further purification. 4-
(diphenylamino) benzaldehyde was prepared according to the procedure in the literature [36].
Reactions were monitored by thin layer chromatography using Silica gel plates (Merck,
Kieselgel 60, 0.25 mm thickness) with F254 indicator. Column chromatography was carried
out on silica (230-400 mesh). Melting points were determined on a Barnstead Electrothermal
IA 9100 platform. Mass spectral analyses were performed on an Agilent 6224 TOF LC/MS
spectrometer. 1H-NMR spectra were recorded on a VARIAN Mercury 400 MHz
spectrometer. 1H and 13C NMR spectra were recorded in CDCl3 or DMSO-d6, chemical shifts
1
13
(δ) were given in ppm relative to solvent peaks (chloroform H: δ 7.26; C: δ 77.3 and
DMSO 1H: δ 2.50; 13C: δ 39.51) with TMS as internal reference.
2.2 Synthesis
2.2.1 4,4-Difluoro-8-(4-methoxyphenyl)-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-
indacene (8met)
4-methoxybenzaldehyde (0.98 mL, 8.2 mmol) and 2,4-dimethylpyrrole (1.86 mL, 18.2
mmol) were dissolved in dry CH2Cl2 (200 mL). One drop of trifluoroacetic acid (TFA) was
added to the reaction mixture, and the resulting mixture was stirred in the dark for 12 h under
argon atmosphere at room temperature. After the complete consumption of aldehyde, p-
Chloranil (tetrachloro-1,4-benzoquinone) (3.0 g, 12.4 mmol) was added to the reaction
mixture. When the mixture was stirred for further 12 h, DIPEA (N,N-Diisopropylethylamine)
(10.0 mL, 57.4 mmol) and then BF3·OEt2 (11.4 mL, 90.2 mmol) were added to the mixture.
The stirring was continued overnight. The resulting solution was washed with water and dried
over anhydrous Na2SO4, and concentrated under reduced pressure. The crude product was
purified by column chromatography (elution: chloroform-hexane, 70:30 v/v) to obtain orange-
red crystal solid 8met. (0.58 g, yield 20%). 1H-NMR (400 MHz, CDCl3): δ 1.43 (s, 6H), 2.55
(s, 6H), 3.87 (s, 3H), 5.97 (s, 2H), 7.01 (d, J = 8.4 Hz, 2H), 7.17 (d, J = 8.4 Hz, 2H). HRMS
(TOF-ESI) (m/z) Calculated as 354.17151, found: 335.17452 [M-F]+.
2.2.2 4,4-Difluoro-8-[4-(diphenylamino)phenyl)]-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-
s-indacene (8TPA)
4-(diphenylamino)benzaldehyde (1.45 g, 5.5 mmol) and 2,4-dimethylpyrrole (1.20 mL, 11.7
mmol) were dissolved in dry CH2Cl2 (250 mL). One drop of trifluoroacetic acid (TFA) was

added to the reaction mixture as catalyst, and the resulting mixture was stirred in the dark for
12 h under argon atmosphere at room temperature. After the complete consumption of
aldehyde, p-Chloranil (tetrachloro-1,4-benzoquinone) (1.96 g, 7.9 mmol) was added to the
reaction mixture. When the mixture was stirred for further 12 h, DIPEA (N,N-
Diisopropylethylamine) (6.50 mL, 37.0 mmol) and then BF3·OEt2 (7.47 mL, 60.5 mmol) were
added to the mixture. The stirring was continued overnight. The resulting solution was
washed with water and dried over anhydrous Na2SO4, and concentrated under reduced
pressure. The crude product was purified by silica gel column chromatography (elution:
chloroform-hexane, 80:20 v/v) to obtain orange-red crystal solid 8TPA. (0.66 g, yield 24%).
1H-NMR (400 MHz, CDCl3): δ 1.61 (s, 6H), 2.58 (s, 6H), 6.02 (s, 2H), 7.03-7.00 (m, 6H),
7.13-7.09 (m, 4H), 7.27-7.23 (m, 4H). HRMS (TOF-ESI) (m/z) Calculated as 491.23443,
found: 492.24221 [M+H]+.
2.2.3 4,4-Difluoro-8-(4-methoxyphenyl)-1,5,7-trimethyl-3-[(1E)-3,4-dihydroxyphenyl]-4-
bora-3a,4a-diaza-s-indacene (1) and 4,4-Difluoro-8-(4-methoxyphenyl)-1,7-dimethyl-3,5-
bis-[(1E)-3,4-dihydroxyphenyl]-4-bora-3a,4a-diaza-s-indacene (3)
Compound 8met (484 mg, 1.37 mmol) and 3,4-dihydroxybenzaldehyde (568 mg, 4.11 mmol)
were refluxed in a mixture of toluene (50 ml), glacial acetic acid (2.2 mL) and piperidine (2.7
o
mL), the mixture was stirred at 120 C for 6 h. Any water formed during the reaction, was
removed azeotropically by heating overnight in a Dean-Stark apparatus. After the reaction
was quenched with water at room temperature, the mixture was diluted with EtOAc and
washed with water. The organic layer was collected from separating funnel and solvent was
evaporated under reduced vacuum. The crude solid was dried in a vacuum oven, then purified
by silica gel column chromatography by using mobile phase (elution: first chloroform, then
chloroform-methanol, 70:30 v/v) to obtain monostyryl BODIPY dye 1 (105 mg, yield 16%)
as red-purple powder (the purple colored fraction) and distyryl BODIPY dye 3 (202 mg, yield
1
25%) as green powder (green colored fraction). The monostyryl BODIPY dye 1, H-NMR
(400 MHz, DMSO-d6): δ 1.38 (s, 3H), 1.43 (s, 3H), 2.45 (s, 3H), 3.81 (s, 3H), 6.14 (s, 1H),
6.63 (s, 1H), 6.77 (d, j= 8.0 Hz, 1H), 6.89-6.87 (m, 2H), 7.09 (d, j= 8.8 Hz, 2H), 7.23 (d, j=
16.4 Hz, 1H), 7.27 (d, j= 8.8 Hz, 2H), 7.38 (d, j= 16.4 Hz, 1H). HRMS (TOF-ESI) (m/z)
1
Calculated as 474.19265, found: 473.18782 [M-H]-. Distyryl BODIPY dye 3, H-NMR (400
MHz, DMSO-d6): δ 1.43 (s, 6H), 3.82 (s, 3H), 6.79 (d, j= 8.0 Hz, 2H), 6.90-6.87 (m, 4H),
7.07 (s, 2H), 7.09 (d, j= 8.8 Hz, 2H), 7.25 (d, j= 16.0 Hz, 2H), 7.29 (d, j= 8.8 Hz, 2H), 7.37
13
(d, j= 16.0 Hz, 2H). C-NMR (100 MHz, DMSO-d6) δ: 160.2, 152.5, 148.1, 146.3, 141.6,

138.0, 137.7, 133.2, 130.2, 128.3, 126.7, 121.2, 118.3, 116.5, 115.4, 114.9, 113.4, 55.7, 14.8.
HRMS (TOF-ESI) (m/z) Calculated as 594.21379, found: 593.21207 [M-H]-.
2.2.4 4,4-Difluoro-8-[4-(diphenylamino)phenyl)]-1,5,7-trimethyl-3-[(1E)-3,4-
dihydroxyphenyl]-4-bora-3a,4a-diaza-s-indacene (2) and 4,4-Difluoro-8-[4-
(diphenylamino)phenyl)]-1,7-dimethyl-3,5-bis-[(1E)-3,4-dihydroxyphenyl]-4-bora-3a,4a-
diaza-s-indacene (4)
Compound 8TPA (514 mg, 1.04 mmol) and 3,4-dihydroxybenzaldehyde (430 mg, 3.12
mmol) were refluxed in a mixture of toluene (40 ml), glacial acetic acid (1.7 mL) and
o
piperidine (2.1 mL), the mixture was stirred at 120 C for 6 h. Any water formed during the
reaction, was removed azeotropically by heating overnight in a Dean-Stark apparatus. After
the reaction was quenched with water at room temperature, the mixture was diluted with
EtOAc and washed with water. The organic layer was collected from separating funnel and
solvent was evaporated under reduced vacuum. The crude solid was dried in a vacuum oven,
then purified by silica gel column chromatography by using mobile phase (elution: first
chloroform, then chloroform-methanol, 60:40 v/v) to obtain monostyryl BODIPY dye 2 (90
mg, yield 14%) as red-purple powder (the purple colored fraction) and distyryl BODIPY dye
4 (130 mg, yield 18%) as green powder (green colored fraction). Monostyryl BODIPY dye 2,
1H-NMR (400 MHz, DMSO-d6): δ 1.55 (s, 3H), 1.60 (s, 3H), 2.47 (s, 3H), 6.17 (s, 1H), 6.79
(d, j= 8.0 Hz, 1H), 6.88 (d, j= 8.0 Hz, 1H), 6.93 (s, 1H), 7.12-7.06 (m, 10H), 7.23 (s, 1H),
13
7.26 (d, j= 16.4 Hz, 1H), 7.38-7.34 (m, 4H), 7.41 (d, j= 16.4 Hz, 1H). C-NMR (100 MHz,
DMSO-d6) : 160.0, 158.6, 153.4, 152.9, 148.0, 147.8, 146.8, 145.8, 142.4, 139.7, 139.2,
139.1, 138.0, 129.7, 129.3, 127.7, 127.5, 124.4, 123.6, 122.7, 121.0, 118.1, 116.0, 114.4,
112.7, 24.0, 14.5, 14.1. HRMS (TOF-ESI) (m/z) Calculated as 611.25557, found: 610.25491
1
[M-H]-.Distyryl BODIPY dye 4, H-NMR (400 MHz, DMSO-d6): δ 1.60 (s, 6H), 6.79 (d, j=
8.4 Hz, 2H), 6.92-6.90 (m, 4H), 7.21-7.07 (m, 10H), 7.25 (s, 2H), 7.28 (d, j= 8.4 Hz, 2H),
7.41-7,34 (m, 6H). 13C-NMR (100 MHz, DMSO-d6) : 159.7, 152.0, 147.6, 145.8, 141.1,
137.5, 137.2, 136.6, 132.7, 129.7, 128.8, 127.8, 126.2, 124.4, 120.7, 120.1, 117.8, 116.0,
114.9, 114.4, 112.9, 14.3. HRMS (TOF-ESI) (m/z) Calculated as 731.27671, found:
730.27704 [M-H]-.
2.3 Optical measurements
The UV-Vis absorption spectra of the BODIPY sensitizers were recorded using a Shimadzu
UV-1800 scanning spectrophotometer. Fluorescence spectra of the studied dyes were
measured on Perkin Elmer LS55 spectrophotometer. Ultrafast transient absorption

spectroscopy measurements were carried out in femtosecond timescale by using a Ti:
Sapphire laser amplifier and an optical parametric amplifier system with 52 fs pulse duration
and 1 kHz repetition rate (Spectra Physics, Spitfire Pro XP, TOPAS). A commercial pump
probe experimental setup (Spectra Physics, Helios) with a white light continuum probe was
used to examine charge injection dynamics of studied dyes in solution and on the film of
anode electrode. The pump wavelength for ultrafast pump probe spectroscopy experiments
was chosen based on the maximum absorption wavelength. The excited state dynamics were
measured between 0.1 ps to 3.2 ns timescale. Experimental data was analyzed by using
Surface Xplorer software that is provided by Ultrafast Systems.
2.4 Preparation of BODIPY sensitized solar cells
Fluorine doped tin oxide (FTO) (12 cm-2Ω) coated glasses were purchased from (Sigma
Aldrich) and used in both photoelectrodes. The semiconductor oxide paste (TiO2-DSL 18 NR-
T, Dyesol) was coated on conductive side of the glass substrates by doctor blade method.
0
0
TiO2 coated electrode was dried at 125 C and sintered at 500 C for 30 minutes, and it was
immersed into the 3x10-4 M solution of the 1, 2, 3 and 4 dyes in THF for 18 hours. In order to
remove unlinked dyes from the surface, the films were washed with THF solvent and then
-
dried on hot plate. Pt-coated glass (Solaronix) was used as a counter electrode and I3 /I- (EL-
HPE, Dyesol) solution was used as a redox couple to integrate cell components.
2.5 Dye adsorption measurements
In order to obtain the amounts of dye molecule adsorbed on TiO2 surface, transparent TiO2
films of surface area 1 cm2 were dipped into a 6 mL of 3x10-4 M dye solution for 18 hours.
Then dye coated anode electrode washed with 2 ml THF to get rid of unbound dye molecules
in the same beaker. The initial concentration and the concentration volume were known, so
dye mass can be calculated from these values. After adsorption and washing process
absorption spectra of the solution was measured and the concentration was determined by
utilizing absorption coefficient of dye. The mass of the dye not adsorbed on TiO2 can be
calculated from the concentration value and volume of the final solution. This mass value is
subtracted from the initial mass value and the mass of dye adsorbed on TiO2 is obtained.
2.6 Characterization of BODIPY dyes sensitized solar cells

The photocurrent density versus photovoltage of the solar cells were measured by recording J-
V curves using Keithley 2400 source meter under illumination of AM 1.5 G simulated solar
light (Newport with 300 W Xe lamp and AM 1.5 G filter). The incident light intensity was
adjusted to 100 mW/cm2 on the detector active area. The incident photon-to-current
conversion efficiency (IPCE) spectra of the cells were measured with a commercial setup
(Newport, Oriel). All device performance was carried out in ambient atmosphere, unless
specified otherwise.
2.7 Micro patterning fabrication
TiO2 coated film onto FTO glass was ablated by femtosecond pulse laser. The vertical
polarized laser with a wavelength of 400 nm, pulse duration of 52 fs and repetition rate of 1
kHz (Spectra Physics, Spitfire Pro XP, TOPAS) was used. Laser beam was passed through a
number of natural density filters to adjust power and sent a convex lens to focus the beam on
the surface of the sample. The laser energy per pulse was measures as 5.6 µJ. The film-coated
glass was mounted on a motorized xyz stage (Newport). The stage speed on one direction (x)
was adjusted to 20 mm s-1 to be able to have one pulse reaching to one spot of the sample,
which is placed at the focus and perpendicular to the laser beam. The film surfaces were
ablated periodically line by line, by moving stage along z direction. The ablated film
morphology was characterized by scanning electron microscopy (SEM, ZEISS EVO 40).
3. Results and Discussion
3.1 Steady state absorption and emission properties of BODIPY dyes and BODIPY-
sensitized TiO2 electrodes
Investigated dye molecules consist of an electron donor moiety and one or two anchoring
group(s) (Scheme 1). Here, electron donating moieties which are triphenylamine and
methoxyphenyl bound to meso (C8) position, while the catechol moiety which is the
anchoring group attached to both C3 and C3-C5 positions of the BODIPY core via
Knoevenagel condensation reaction. In order to evaluate the ground state interaction, linear
absorption spectra of the sensitizers were measured both in THF solution and on TiO2 surface.

Linear absorption properties of studied dyes (in THF) are given in Fig. 1. All studied dyes
demonstrate high molar extinction coefficients at their absorption maxima (Table 1).
Dye 1 has methoxyphenyl moiety at meso (C8) position and 2-(3,4-dihydroxyphenyl) ethenyl
moiety at C3 position of the BODIPY core and its absorption maximum wavelength is around
574 nm. Similarly, dye 2 which has triphenylamine (TPA) moiety at meso position, gives an
absorption maximum at 576 nm. This slight red-shifting is due to the orthogonal alignment
between meso substituent and the BODIPY core. An efficient way to red-shift the absorption
and emission maxima is to extend the conjugation of the 3 and 5 positions of the BODIPY
core [37]. Therefore, 2-(3,4-dihydroxyphenyl) ethenyl moieties bind to C3 and C5 positions
of the BODIPY core and molecule became symmetrical structure (dyes 3 and 4). When the
molecule becomes symmetric, the color of the dye solution changes from bright pink to bluish
and the absorption peaks are significantly red-shifted (about 75 nm) as compared to that of
unsymmetrical dyes (dye 1 and dye 2). The strong absorption band of studied BODIPY dyes
at longer wavelengths around 575 nm and 650 nm are correspond to S0→S1 transition, while
the absorption band at shorter wavelengths region correspond to S0→S2 transitions[38].
The sensitized BODIPY dyes show strong fluorescence spectra in THF solution, as illustrated
in Fig. 2. It is obviously seen from the figure that the fluorescence intensity depends on the
molecular symmetry. The fluorescence peaks are around 610 nm for dyes 1-2 and around 675
nm for dyes 3-4, respectively. The fluorescence intensity of dye 2 is lower than that of dye 1,
due to electron donating properties of the TPA group. The TPA is a well-known electron
donor moiety and affects the fluorescence intensity of the dye molecule. Furthermore, TPA
moiety has a stronger electron donating ability than the methoxyphenyl moiety. Therefore,
fluorescence intensity decreases in dyes containing TPA moiety due to the increasing
intramolecular charge transfer [39]. When molecule becomes symmetrical (dyes 3 and 4), the
fluorescence signals get considerably smaller as compared to that of unsymmetrical dyes
(dyes 1 and 2). This may be due to the formation of a charge transfer state (CTS) lower than
the excited singlet state and efficient intramolecular electron transitions to the formed CTS.
The small fluorescence signals appearing about 740 nm, may be another indication of the
formation CTS around this wavelength for dyes 3 and 4. In order to examine solvent effect on
CTS, we did steady state absorption and emission measurements both in polar (EtOH) and
nonpolar (Toluene) solution environments. The linear absorption spectra of the dyes in both
solutions are given Fig. S16. All studied dyes showed similar absorption characteristic
behaviors in both solutions. In the fluorescence spectra, there are significant differences

depending on the solvent polarity as shown in Fig. S17. Although all studied dyes
demonstrated similar emission behavior in polar solutions (THF and EtOH), the fluorescence
intensity decreases more in EtOH as compared to THF. That is because; intramolecular
charge transfer increases in polar environment and therefore, fluorescence intensity decreases.
In nonpolar solvent (Toluene), it is clearly seen that the fluorescence signal maxima shows a
blue-shifting due to less polarity. On the other hand, the fluorescence signal has a shoulder
that corresponds to emission from CTS in the lower energy side of the spectrum. These
experimental measurements showed that CTS clearly depends on the solvent polarity.
The steady state linear absorption spectra of the sensitizers anchored on the transparent TiO2
films are shown in Fig. 3. It is clearly seen from the figure that absorption of the dyes
adsorbed on TiO2 becomes significantly broadened compared to that of dyes in THF.
Broadening effect could be due to aggregation on the surface of TiO2 or bound formation
between the dye molecules and semiconductor TiO2 layer [24, 28, 40, 41]. In order to explain
the aggregation effect, we also performed fluorescence experiments and fluence dependence
ultrafast pump probe experiments for BODIPY chromophores adsorbed on TiO2. In an
attempt to understand the aggregation effect on emission characteristics, we recorded the
fluorescence measurements for all studied dyes in THF solution as well as adsorbed on TiO2
layer. It is observed that the fluorescence signals of the anode electrodes are quenched
significantly, while the dyes have strong fluorescence intensity in THF solution. The
quenching mechanism can be ascribed to aggregation effect since BODIPY compounds have
planar indacene skeletons and enable the form H or J type aggregates [40, 41]. Thus,
broadened absorption bands of the dyes on TiO2 might be a sign of intermolecular interactions
of the molecules that are aggregated on the TiO2 surfaces. On the other hand, in order to
examine the aggregation effect, ultrafast pump probe spectroscopy experiments were
performed with two pump beam fluences (about 8 J/m2 and 4 J/m2). The experimental results
showed that the excited state lifetime of the dye adsorbed on TiO2 decreases with increasing
pump beam fluence as given in Fig. S18. That is because, in an aggregation environment, the
excited singlet state of chromophores can act as a quenching system for singlet excitation of
the other molecules. This phenomena causes decreasing excited state lifetimes for higher
excitation intensities as well as fluorescence intensity [42]. The decreasing of the excited state
lifetime at higher excitation fluence indicated that the dye molecules aggregate on TiO2 layer.
The sensitizer dye molecule with catechol anchoring group have been known to bind to the
TiO2 surface through a bidentate mononuclear chelating or a bidantate dinuclear bridging

linkages [43, 44]. The solar cell that is fabricated with sensitizer including catechol group is
classified as type-II DSSC. The type-II DSSCs show a direct electron transition from dye to
conduction band of TiO2 via dye-to-TiO2 charge transfer (DTCT) bands. For this mechanism,
the dyes including anchoring catechol moieties show a strong absorption band in the lower
energy side of the spectrum upon binding on TiO2 layer. In Fig. 3, the dyes bounded on TiO2
layer show a dominant absorption peak around 580 nm and 660 nm wavelengths which
correspond to steady state absorption from HOMO to LUMO level of the dyes in solution. In
addition, these bands have a tail around near infrared region as compared to that of in THF.
Therefore, this weak absorption signals may be attributed to direct transition from dye to
conduction band of TiO2. In this study, transitions occur from HOMO level to LUMO level of
the dye then from LUMO level to conduction band of TiO2 (indirect transition). Therefore,
the indirect transition is dominant for the studied dye adsorbed on TiO2 surface.
In an attempt to understand the number of anchoring groups adsorbed on TiO2 we performed
absorption measurements for symmetrical and unsymmetrical dye molecules in THF and on
TiO2 surface. Anchoring groups in BODIPY dyes especially on position of 3 and 5 are
affected electron transfer from oxygen anion to BODIPY core in basic environment.
Therefore bathochromic shift is observed on the absorption spectra in basic environment. If
anchoring groups are covalently bounded to TiO2, oxygen anion does not form and therefore
bathochromic shift is not observed in the absorption spectra of the anode electrode. In order to
understand whether the mono or di-anchoring groups are bound to the TiO2, we conducted
absorption measurements of dye 2 in THF solution and on TiO2 surface of the anode electrode
with and without KOH. Firstly, the absorption spectrum of dye 2 solution with KOH solution
(in THF) was recorded and compared with dye 2 in THF (Fig. S19). It was seen that the
spectrum was broadened and red shifted significantly, as expected. After the anode electrode
with dye 2 was immersed in KOH solution, absorption spectrum was recorded and compared
with anode electrode with dye 2. If there was unbounded anchoring group side on TiO2, we
expect to see bathochromic shift in the absorption spectra. We did not see any shift in
wavelength on the absorption spectra as seen in Fig. S20. The same process was performed
for symmetrical dye 4 to see whether the di-anchoring groups are bound on TiO2. We did not
observe any spectral shift in the absorption spectra of anode electrode with KOH for dye 4 as
well (Fig. S21 and S22). We believe these results are the evidence that both mono and di-
anchoring groups are bounded to TiO2 surface for dye 2 and 4, respectively.

3.2 Ultrafast transient absorption spectroscopy measurements
In an attempt to investigate charge transfer dynamics and decay kinetics of the sensitizers,
ultrafast pump probe spectroscopy experiments were performed for dyes both in solution and
on TiO2 film. Fig. 4a-d indicates the transient absorption spectra at different time delays upon
pulsed laser excitation in THF for dyes 1-4, respectively. The wavelength dependent transient
absorption spectra are similar for dyes 1 and 2. There is a negative absorption signal
consisting of nested two bands, between 555 nm and 670 nm wavelengths. The negative
signals around 585 nm and around 630 nm, can be ascribed to ground state and charge
transfer state (CTS) bleaching, respectively. In addition, there are positive signals localized
below 555 nm and above 670 nm, which can be attributed to excited state absorption (ESA).
On the other hand, there is a negative signal (535 nm) buried in the ESA signal, causing to
ESA signal decrease. Even if this signal seems to be positive, it is negative and competes with
the positive ESA signals. This signal corresponds to shoulder of the main absorption peak in
the linear absorption spectra.
The ultrafast transient absorption spectroscopy experiments of symmetrical dyes (3 and 4)
were performed at 650 nm pump wavelength. In transient absorption spectra, there are two
distinct negative absorption signals that are localized around 660 nm and 730 nm. The former
can be attributed to ground state bleach signal and the latter can be ascribed to saturation of
the CTS. In addition, a broad ESA signal centered around 485 nm and a negative signal
around 600 nm buried in this broad ESA signal appear in the nonlinear absorption spectra for
both dyes 3 and 4. As mentioned above, this buried negative signal corresponds to shoulder of
the main absorption peak in the linear absorption spectra.
The singlet excited state lifetimes of the dyes in THF solutions decay with several processes,
which are transitions to lower vibrational states, charge transfer to the CTS, and decay to
ground state. Therefore, decay kinetics of the excited state of the dyes in THF (Fig. 5) was
fitted with three exponential decay functions to extract transient dynamics of the studied
compounds and given in Table 2. Among the dyes studied dye 1 has the longest excited state
lifetime (about 4 ns) in THF.
In an attempt to understand the charge injection dynamics between dye molecule and TiO2,
femtosecond transient absorption spectroscopy experiments were carried out for all the
photoelectrodes (dyes adsorbed on TiO2 film on FTO glass). The results are given in Fig. 6a-
d. There is a single negative signal ranged from 520 nm to 650 nm and ESA bands below 520

nm and above 650 nm for dyes 1 and 2. Similarly, in transient absorption spectra for dyes 3
and 4, there is a single negative signal localized at 650 nm and ESA bands below 600 nm and
above 720 nm. It is clear that the charge transfer states seen for dyes in THF, disappear for
adsorbed dyes due to binding of catechol moiety to TiO2 and fast electron transferring to the
conduction band of TiO2. The decay traces of the bleach signals (Fig. 7) were fitted to three
exponential signals for all studied compounds and their time components are given in Table
2, as well. Note that middle component correspond to electron transfer from LUMO level of
the dyes to conduction band of TiO2 for dyes on TiO2. It is obviously seen that time
components of dyes adsorbed on the TiO2 layer decrease as compared to that of dyes in THF,
due to faster electron transfers from LUMO level of dye to the conduction band of TiO2.
Table 2 shows that dye 1 shows fastest electron injection to TiO2.
The time and wavelength dependent transient absorption spectra of the dyes adsorbed on
TiO2 are shown in Fig. S23 and S24. It is clearly seen from the figure that dye 1 has longer
excited state lifetimes in solution and faster electron transfer to TiO2 and therefore, we expect
dye 1 to have better photovoltaic performance in DSSC.
3.3 Photovoltaic performance
BODIPY sensitized solar cells were fabricated in order to evaluate the photovoltaic
performances by using dye coated TiO2 as a photoanode, Iodide/Triiodide redox couple as an
electrolyte mediator and platinized FTO glass as a counter electrode. All device performance
was carried out in ambient atmosphere, unless specified otherwise.
Monochromatic IPCE spectra, measured as a function of wavelength ranging from 300 nm to
1100 nm are given in Fig. 8. Unsymmetrical dyes (1 and 2) with different electron donors
show external quantum efficiency between 500 nm and 750 nm. Among the unsymmetrical
BODIPY dyes, the cell based on methoxyphenyl moiety (dye 1) indicates greater IPCE
spectrum, with a peak value of 7.5% at 610 nm, than that of TPA moiety (dye 2) with a peak
value of 5% at 610 nm. That is because dye 1 with methoxyphenyl moiety has longer excited
state lifetime in solution and faster electron injection on photo anode film as compared to dye
2 with TPA moiety. This finding support our expectations based on ultrafast spectroscopy
data. Despite the fact that sensitizer with TPA moiety has greater molar absorption coefficient
and further light harvesting capability, it’s IPCE value is lower. Enhancement in IPCE value
could be attributed to differences on the electron donating properties of investigating dyes.

IPCE spectra of symmetrical dyes (3 and 4) showed similar characteristic behaviors to each
other between 580 nm and 830 nm wavelengths as seen in Fig. 8. Surprisingly, IPCE values
of dyes 3 and 4 are almost the same, despite the fact that they have different electron donating
moieties as contrary to dyes 1 and 2. Note that dyes 3 and 4 have two catechol moieties as
contrary to dyes 1 and 2, which have only one catechol moiety. Comparing investigated dyes
with the same TPA electron donating properties and different number of catechol moieties
(i.e. dyes 2 and 4), one could see that although both dyes show the same absorption intensity
(as seen in Fig. 3), dye 4 showed lower photo conversion efficiency. That is because, although
the anode electrodes with dye 2 and dye 4 have the same absorption value, the cell fabricated
with dye 2 showed better photovoltaic performance due to the faster electron injection from
excited state of the dye 2 to conduction band of the TiO2 than that of dye 4. The cell with dye
1 demonstrated better photovoltaic performance because its longer excited state lifetime in
solution and faster electron injection to the conduction band of the TiO2. These results showed
that the excited state lifetime is a very effective parameter to determine photovoltaic
performance. The amount of the dye adsorbed on TiO2 surface affect the photovoltaic
performance of DSSCs. The dyes with methoxyphenyl moiety were more adsorbed on TiO2
surface than those with TPA moiety. That is because the dyes with TPA moiety have more
bulky structure as compared to methoxyphenyl moiety. Therefore, the electron donating
property as well as number of anchoring group is a parameter that determines the amount of
dye adsorbed on TiO2.
The photocurrent density-photo voltage (J-V) curves of the BODIPY sensitized solar cells are
seen in Fig. 9 and the results are summarized in Table 3.
J-V results show that
unsymmetrical dye with methoxyphenyl moiety has more efficient electron injection process
than that of TPA moiety. Note that power conversion efficiency found for dye 1 (0.44%) and
dye 3 (0.11%) with methoxyphenyl are comparable with the BODIPY dyes including
trimethoxyphenyl (0.13%) in the literature [24]. Similarly, power conversion efficiency of dye
2 (0.35%) and dye 4 (0.09%) with TPA moiety are also comparable with BODIPY dyes with
TPA moieties in the literature [26, 28]. Among the unsymmetrical dyes, dye 1 with
methoxyphenyl moiety has a longer excited state lifetime in THF solution, and demonstrates
efficient electron transfer to anode electrode. J-V measurements of the investigated dyes are
in line with the IPCE measurements explained above.
The fabricated cells with symmetrical dyes (3 and 4) showed lower photovoltaic performance
as compared to that of asymmetrical dyes (1 and 2). It is clearly seen from the photocurrent

action spectra that both short-circuit photocurrent density (Jsc) and open circuit voltage (Voc)
are lower for symmetrical BODIPY dyes. The lower photocurrent efficiencies frequently
originate from inefficient electron injection from dye to TiO2 [10, 45]. The symmetrical dyes
have linear absorption appearing near infrared (NIR) region of the absorption spectrum of
anode electrode as compared to asymmetrical dyes. These signals can be attributed to
absorption from HOMO level to trap states of the TiO2 due to the direct transition. This result
leads to back electron transfer from trap states to the oxidized dye and decrease in the Jsc. On
the other hand, the symmetrical dyes in THF showed shorter excited state lifetime and slow
electron injection rate from LUMO of dye to conduction band of TiO2 as compared to
asymmetrical dyes. Similar studies revealed that the electron injection dynamics is much
slower for the dyes demonstrating inefficient electron transfer [45, 46]. Additionally, since the
absorption spectra of symmetrical dyes shift to longer wavelength region as compared to that
of unsymmetrical dye, electron transfer from the excited state of the dye to the trap states of
TiO2, is possible as well. On the other hand, the decrement of Voc is generally related to band
edge shift of TiO2 conduction band. Increasing the number of anchoring group leads more
protons to transfer to TiO2 upon dye molecules bounding to TiO2. Therefore, positively
charged TiO2 causes lowering the conduction band edges of TiO2 and therefore lowering Voc
[47, 48].
In an attempt to investigate the effect of anchoring capability on the photovoltaic performance
we modified the surface roughness of TiO2 film by using fs laser ablation technique. Four
anode electrodes were produced with dye 1 by using the same procedure. The anode
electrodes were placed perpendicular to the focused laser beam and scanned with a motorized
xyz stage such that each laser pulse hit one spot on the sample. The surface of the films
scanned line by line. While the first electrode had no scanning, the others were scanned with
about 20 µm, 40 µm, and 60 µm line separations as seen from the SEM images in Figure 8a-
d, respectively. The absorption properties of the scanned anode electrodes (given in Fig. 9)
show that anode electrode with 20 µm scanned line separation show higher absorption
properties. This may be the indication that the more the surface roughness, the better the
adsorption of the dye on anode electrode. Ultrafast pump probe experiments performed on
bare anode electrode and scanned with 20 µm line separation given in Fig. 10 revealed that
the scanned anode show shorter lifetime i.e. faster electron injection to the conduction band of
TiO2. IPCE and J-V measurements proved that scanning the anode electrode gives better
photovoltaic performance (Fig. 11, Fig. 12, and Table 4). Power conversion efficiency

improved 47% compared to the same cell without laser ablation. There might be two
explanations for these findings. One of them is the fact that scanning increases the surface
area of the material and therefore, increases anchoring capability of dye as seen in the
literature (13.5%) [34]. The other explanation could be that fs pulses may break the bonds
between Ti and oxygene atoms and produce Ti+ ions, which could be used by catechol moiety
to form covalent bonding. We leave this point open to discussion.
4. Conclusion
The goal of this study is to reveal the parameters affecting the photo conversion efficiencies
of BODIPY dye sensitized solar cells. Therefore, the effects of electron donating moieties as
well as molecular symmetry, on electron injection dynamics and photovoltaic performance
were studied. Attachment of the dye to TiO2 was studied by altering the surface morphology
of TiO2 with fs laser ablation. Four new BODIPY derivatives were designed and sensitized to
achieve these goals. Among the sensitized dyes, dye with methoxyphenyl electron donor
group and unsymmetrical structure (i.e. one anchoring group at C3 position) showed the best
photovoltaic performance despite of the lowest absorption spectra on TiO2 layer, comparing
to the other sensitizers. This dye showed the longer excited state lifetime in solution and faster
electron injection dynamics to TiO2 as compared to the other sensitizers. Therefore, while
designing new chromophores for DSSC applications one should consider not only the light
harvesting capability but also the longer excited state lifetime in solution and shorter electron
transfer time to TiO2 on the film of anode electrode. In addition, we show that increasing the
number of the anchoring group on the studied dye molecules may not result better
photovoltaic performance. On the other hand, fs laser ablation treatment of the TiO2 surface
enhances the anchoring capability, shortens the electron injection time to TiO2 conduction
band, and therefore, increases the DSSC performance by about 47%. We believe that our
results are useful while designing new BODIPY chromophores for DSSC applications.
Acknowledgment

We gratefully acknowledge the financial support by Scientific and Technical Research
Council of Turkey (TUBITAK) (No. 115F400)
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Figure Captions
Figure 1.Linear absorption spectra of BODIPY dyes, c=5x10-5 M in THF
Figure 2.Fluorescence spectra of BODIPY dyes 1, 2, 3 and 4, c=5x10-5 M in THF
Figure 3.Linear absorption spectra of the BODIPY sensitizers adsorbed on TiO2 layer
Figure 4.Transient absorption spectra of the a) 1, b) 2, c) 3 and d) 4 BODIPY dyes in THF.
Figure 5. Time evolution of the dyes 1, 2, 3 and 4 in THF at their bleach maxima
Figure 6. Transient absorption spectra of the a) 1, b) 2, c) 3 and d) 4 BODIPY dyes on TiO2
Figure 7. Time evolution of the dyes 1, 2, 3 and 4 on TiO2 at their bleach maxima
Figure 8. Incident photon to current conversion efficiencies as a function of wavelength for
BODIPY dyes sensitized solar cells
Figure 9. I-V curves of dyes 1, 2, 3 and 4 sensitized solar cells under A.M. 1.5 G illumination
Figure 10. SEM images of a) bare, b) 60 µm, c) 40 µm and d) 20 µm line pattern on the laser
ablated TiO2 film.
Figure 11. Linear absorption spectra of dye 1 adsorbed on bare and scanning TiO2 layers
Figure 12. Time evolution of the dye 1 on bare and 20 µm line patterning TiO2 at their bleach
maxima
Figure 13. Incident photon to current conversion efficiencies as a function of wavelength for
dye 1 sensitized solar cells using adsorbed on bare and scanning TiO2 layers
Figure 14. I-V curves of dye 1 sensitized solar cells using adsorbed on bare and scanning
TiO2 layers, under A.M. 1.5 G illumination

Figure 1
Figure 2

Figure 3
Figure 4

Figure 5
Figure 6

Figure 7
Figure 8

Figure 9
Figure 10

Figure 11
Figure 12

Figure 13
Figure 14

O
O
O
O
i, ii, iii
iv
+
+
N
H
N
N
N
N
B
B
N
N
F
F
F
F
O
B
F
F
HO
OH
HO
8met
1
3
HO
OH
HO
N
N
N
N
i, ii, iii
iv
+
N
N
+
N
H
N
N
B
B
F
F
F
F
N
N
B
O
F
F
HO
HO
OH
2
4
8TFA
HO
HO
OH
Scheme 1. Synthetic pathway of the BODIPY dyes; (i) cat. trifluoroacetic acid, rt, 12 h; (ii)
tetrachloro-1,4-benzoquinone, rt, 12 h; (iii) (1) N,N-diisopropylethylamine; (2) BF3.Et2O, rt,
24 h; (iv) 3,4-dihdyroxybenzaldehyde, piperidine, acetic acid, reflux, 6 h.