Synthesis and dye sensitized solar cell applications of Bodipy derivatives with bis-dimethylfluorenyl amine donor groups

Article abstract of DOI:10.1039/c4nj02393e

Three Bodipy dyes with strong absorptivities in the visible and near infrared regions were designed, synthesized and their potential as photosensitizers for liquid electrolyte-based dye sensitized solar cells have been evaluated. For the first time Bodipy derivatives with bis-dimethylfluorenyl amine donor groups which were known for their bulky structures as donor groups have been used together. We altered our mostly used triphenylamine group with these and investigated the dye-sensitized solar cell efficiencies of this new class of Bodipy dyes.

Full text of DOI:10.1039/c4nj02393e

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Synthesis and dye sensitized solar cell
applications of Bodipy derivatives with
bis-dimethylfluorenyl amine donor groups†
Cite this: New J. Chem., 2015,
39, 4086
Yusuf Çakmak,*a Safacan Kolemen,a Muhammed Buyuktemiz,b Yavuz Dedeb and
Sule Erten-Ela*c
Three Bodipy dyes with strong absorptivities in the visible and near infrared regions were designed,
synthesized and their potential as photosensitizers for liquid electrolyte-based dye sensitized solar cells
have been evaluated. For the first time Bodipy derivatives with bis-dimethylfluorenyl amine donor groups
which were known for their bulky structures as donor groups have been used together. We altered our
mostly used triphenylamine group with these and investigated the dye-sensitized solar cell efficiencies
of this new class of Bodipy dyes.
Received (in Victoria, Australia)
24th December 2014,
Accepted 12th March 2015
DOI: 10.1039/c4nj02393e
www.rsc.org/njc
characteristics of the parent chromophore can be altered in the
desired direction, for example, it is possible to shift the absorption
Introduction
The dye-sensitized solar cell (DSSC) concept is thought of as an wavelength through the red end of the electromagnetic spectrum
important alternative to the traditional semiconductor based by just simple chemical transformations.8b,9 In addition to these,
solar cells because of the limits and problems of these widely solubility and aggregation characteristics of these dyes can also be
used constructs.1 An important part of this emerging field modulated as needed.3
is the choice of sensitizer.2 Bodipy dyes which we use in our
Thiophene is known to be used frequently in DSSCs because
studies have a strong reputation as fluorophores3 and in recent of its electron transfer ability.10 Since electron delocalization
years a number of DSSCs have been studied using these through the p-system is an important factor in DSSCs, we have
compounds. The first rationally designed example of a Bodipy- appended the thiophene unit to the Bodipy structure. The
based sensitizer was reported a few years ago,4 followed by a few thiophene carboxylic acid unit has been attached to the meso
more recent articles including both liquid electrolyte and solid (8À) position of the Bodipy core, since this position was
state based DSSCs.5 A few years ago, we have achieved an overall calculated be more feasible for electron injection.5a,6 Direct
efficiency of 2.46 using a long wavelength-absorbing derivative.6 attachment of the thiophene unit to the core was also thought
Lately, more efficient liquid electrolyte-based DSSCs based on to enhance the efficiency of electron transfer. As an anchoring
Bodipy structures have been achieved with record values of overall group, the carboxylic acid was used due to the synthetic
power-to-current conversion efficiency of 4.89% and 5.31% by problems we faced while trying to attach the cyanoacetic acid
Zhang’s group8 and 6.06% by Kubo’s group.9 It is clear that the moiety, which is also known to bind efficiently to the TiO2
optimal solar cell performance of a photosensitizer is dependent surface and increases the electron withdrawing ability of the
on a large number of parameters; however, the absorption range, electron-injecting unit. The electron donor part is also an
anchoring groups and the direction of electronic reorganization important parameter in DSSC design and efficient usage of
upon excitation should be among the most important ones.7 the donor groups has a substantial role in increased cell
Recent developments in Bodipy chemistry allow diverse modifica- efficiencies. In this study, we altered our mostly used triphenyl-
tions on the core structure.3,8 Through these modifications, many amine group with the bis-dimethylfluorenyl amine group.
This group has been successfully utilized in some literature
reports11 and due to its bulky structure it has been widely
proposed as a donor group. It has also been suggested that
using bulky structures increases the stability of oxidized dye
molecules. Hence, in Bodipy dyes we have designed and
synthesized bis-dimethylfluorenyl amine donor groups for
DSSC application for the first time to our knowledge. Fig. 1
shows the synthesized photosensitizers for DSSC application.
a National Nanotechnology Research Center (UNAM), Bilkent University,
Bilkent Ankara, 06800, Turkey
b Department of Chemistry, Faculty of Science, Gazi University, Teknikokullar,
Ankara 06500, Turkey
c Institute of Solar Energy, Ege University, Izmir, 35100, Turkey.
E-mail: suleerten@yahoo.com; Tel: +90 232 3111231
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4nj02393e
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Fig. 1 Synthesized photosensitizers TB5, TB6, and TB7.
Fig. 3 Synthesis of the final PSs TB6 and TB7.
Chromatography was performed over Merck Silica gel 60 (particle
size: 0.040–0.063 mm, 230–400 mesh ASTM).
Characterization
1H NMR and 13C NMR spectra were recorded at room tempera-
ture on Bruker DPX–400 (operating at 400 MHz for 1H NMR and
100 MHz for 13C NMR) in MeOD-d4 and DMSO-d6. Coupling
constants ( J values) are given in Hz and chemical shifts are
reported in parts per million (ppm). Splitting patterns are
designated as s (singlet), d (doublet), t (triplet), q (quartet),
m (multiplet), and p (pentet). Absorption spectra were acquired
using a Varian Cary-100 spectrophotometer. Mass spectra were
recorded on Agilent Technologies 6530 Accurate-Mass Q-TOF
LC/MS. Spectrophotometric grade solvents were used for spectro-
scopy experiments.
Fig. 2 Synthesis scheme of the precursor molecules.
In the synthesis part (Fig. 2), initially TB1–TB4 have been
synthesized.12 We first iodinated the fluorene molecule to yield
TB1 in 90% yield. In the second reaction dimethylation reaction
was accomplished by using a modified procedure from litera-
ture with MeI and KOMe in DMF. Then, in the third step we
synthesized N,N-bis(9,9-dimethylfluoren-2-yl)aniline under the
Ullman reaction conditions by using aniline and coupling
reagents at high temperatures. Then, in order to synthesize
TB4, which was needed for the Knoevenagel reaction, a typical
Vilsmeier formylation reaction in CHCl3 was carried out. After
synthesis of the donor part, we moved to the synthesis of the
Bodipy dye core. 5-Formyl-2-thiophenecarboxylic acid was
obtained from suppliers and reacted with 2-methylpyrrole to
get TB5 and upon reaction with the synthesized aldehyde TB4,
we obtained both TB6 and TB7 which were tested as photo-
sensitizers in DSSCs (Fig. 3).
Synthetic details
TB1, TB2, TB3 and TB4 were synthesized according to the
literature.12
Compound TB5. 400 mL of argon-degassed CH2Cl2 was
added into 2-methyl pyrrole (11.5 mmol, 0.93 g) and 5-formyl-
2-thiophenecarboxylic acid (5.46 mmol, 0.85 g). One drop of
TFA was added and the solution was stirred under N2 at room
temperature for 1 day. After addition of p-chloranil (5.46 mmol,
1.34 g) to the reaction mixture, stirring was continued for
30 min. 5 mL of Et3N and 5 mL of BF3.OEt2 were successively
added and after 30 min, the reaction mixture was washed three
times with water (3 Â 100 mL) which was then extracted into
the DCM (3 Â 100 mL) and dried over anhydrous Na2SO4. The
solvent was evaporated and the residue was purified using
silica gel column chromatography employing DCM : MeOH =
80 : 20 as an eluant. Red solid (0.75 g, 40%). 1H NMR (400 MHz,
MeOD-d4): dH 7.79 (1H, d, J = 3.8 Hz, ArH), 7.50 (1H, d, J =
3.9 Hz, ArH), 7.12 (2H, d, J = 4.1 Hz, ArH), 6.44 (2H, d, J = 4.4 Hz,
Experimental section
Materials
All chemicals and solvents obtained from suppliers were used ArH), 2.62 (6H, s, CH3). MS (TOF-ESI): m/z: calcd: 301.0788,
without further purification. Reactions were monitored using found: 301.0798 [M–COOH]À, D = 3.3 ppm. eabs = 43 700
thin layer chromatography over Merck TLC Silica gel 60 F254
.
(in MeOH).
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Compounds TB6 and TB7. TB5 (0.29 mmol, 0.10 g) and TB4 Results and discussion
(0.38 mmol, 0.19 g) were added into a 100 mL round-bottomed
flask containing 50 mL of benzene and to this solution was
added piperidine (0.3 mL) and acetic acid (0.3 mL). The mixture
was heated under reflux by using a Dean Stark trap and the
reaction was monitored by TLC (DCM : MeOH = 80 : 20). When
all the starting material had been consumed, the mixture is
concentrated and directly added to a chromatographic column
by using DCM : MeOH/80 : 20 and TB7 was separated as a pure
product in green color in 52% yield (0.152 mmol, 0.2 g). To
separate the mono Knoevenagel product TB6 a second column
chromatography was done with DCM : MeOH/90 : 10 and TB6
was separated as a pure product in 15% yield in deep blue color
(0.04 mmol, 0.035 g).
Optical properties
After the synthesis of TB6 and TB7 along with the sole Bodipy
TB5, we performed photophysical analysis, CV measurements,
and theoretical studies in order to determine the suitability of
these units for constructing a cell with TiO2 and redox couple
IÀ/I3À. Absorbance spectra of these dyes in solution (Fig. 4) and
on the TiO2 (Fig. S5, ESI†) surface was investigated and it was
found that TB5 exhibited a narrow absorption spectrum with a
lmax at 520 nm in solution, which is characteristic for Bodipy
dyes and the molar absorptivity of this compound was 43 700 in
MeOH at this wavelength. This compound was barely soluble in
DCM or CHCl3. TB6 exhibited a broader absorption spectrum
with an absorption maximum at 665 nm in DCM and fluorene
moieties were are also found to absorb strongly in the UV
region of the spectrum at around 370 nm. The molar absorp-
tivity of this compound was 38 800 at 665 nm. Since the
fluorene moieties that inhibit p–p stacking of the compound
increases the solubility; we were able to dissolve them in DCM.
TB7 exhibited a strongly red-shifted absorbance maximum at
773 nm in DCM. This compound was found to be promising in
this respect because absorption in the near IR end of the
spectrum is highly desired due to strong sunlight illumination
in this wavelength region. For example, one of our previously
studied sensitizers, PS1, which had a record efficiency among
the Bodipy dyes when published in DSSC applications, exhib-
ited an absorption maximum at 724 nm.13 TB7 had a molar
absorptivity of 53 700 and an absorption maximum at 773 nm.
All the three compounds exhibited very weak emission, which
was due to the charge transfer events because of the thiophene-
carboxylic acid group, which is usual for these types of dyes.
TB6: 1H NMR (400 MHz, DMSO-d6): dH 7.80 (2H, d, J =
8.0 Hz, ArH), 7.77 (2H, d, J = 7.3 Hz, ArH), 7.72 (1H, d, J = 16.3 Hz,
CH), 7.62 (1H, d, J = 3.8 Hz, ArH), 7.49–7.60 (5H, m, ArH), 7.43
(1H, d, J = 16.3 Hz, CH), 7.25–7.38 (9H, m, ArH), 7.05–7.15 (4H, m,
ArH), 6.48 (2H, d, ArH), 2.56 (3H, s, ArCH3), 1.38 (12H, s, CCH3).
13C NMR (100 MHz, DMSO-d6): dC 163.6, 156.7, 155.4, 153.8,
149.4, 146.4, 139.1, 138.6, 135.2, 133.6, 133.0, 131.3, 129.8, 129.4,
127.6, 127.4, 124.4, 123.2, 122.4, 121.7, 120.2, 119.8, 118.5, 116.2,
47.0, 30.1, 27.1 ppm. MS (TOF-ESI): m/z: calcd: 833.3059, found:
833.2955 [M], D = 12.5 ppm. eabs = 38 800 (in DCM). TB7: 1H NMR
(400 MHz, DMSO-d6): dH 7.76 (4H, d, J = 8.0 Hz, ArH), 7.72 (4H, d,
J = 6.8 Hz, ArH), 7.65 (2H, d, J = 16.6 Hz, CH), 7.40–7.50 (12H, m,
ArH; CH), 7.20–7.35 (16H, m, ArH), 7.07 (8H, m, ArH), 1.34 (24H,
s, CCH3). 13C NMR (100 MHz, DMSO-d6): dC 163.4, 155.4, 154.8,
153.7, 149.2, 146.3, 138.5, 137.8, 135.3, 135.2, 132.6, 131.8, 130.0,
129.7, 129.2, 129.0, 127.5, 127.4, 124.3, 123.1, 122.3, 121.7, 120.1,
119.7, 117.9, 116.6, 46.9, 27.1 ppm. MS (TOF-ESI): m/z: calcd:
1320.5359, found: 1320.5085 [M], D = 20.7 ppm. eabs = 53700
(in DCM).
Electrochemical properties
Cyclic voltammetry results of the synthesized compounds are
shown in Fig. 5. The results obtained from these data are
summarized in Table 1, showing explicitly the oxidation/
reduction bands of these compounds and the HOMO–LUMO
Electrochemistry of Bodipy dyes
CV measurements were made by using CH-Instrument 660 B
Model potentiostat equipment. Solution was prepared in
chloroform (10À3 M). A three-electrode cell was used consisting
of a glassy carbon working electrode, a Pt wire counter electrode
and an Ag/AgCl reference electrode, all placed in a glass vessel.
Tetrabutylammonium hexafluorophosphate (TBAPF6), 0.1 M,
was used as a supporting electrolyte. Ferrocene was used as
an internal reference electrode. HOMO/LUMO values were
calculated according to literature.6
Dye sensitized solar cell device characterization
The current density versus voltage (I–V) characteristics of the
devices were measured using a Keithley 2400 source measure-
ment unit. The device performance was characterized under
AM 1.5 G conditions with an illumination intensity of 100 mW
cmÀ2 using a solar simulator. A special mask was used to define
the active area in the devices and measurements were carried
out using this special mask. Reproducibility of measurements
was checked several times for the accuracy and precision.
Fig. 4 Absorbance spectra of the compounds synthesized. TB5 in MeOH
and TB6 and TB7 in DCM.
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Fig. 6 Energy level diagrams of TB5, TB6 and TB7 from the electro-
chemical data summarized in Table 1.
Fig. 5 Cyclic voltammetry measurements of TB5 (blue), TB6 (green) and
TB7 (red).
Table
1
Redox potentials and EHOMO and ELUMO levels of Bodipy
derivatives
a
b
c
d
e
Eox
Ered
Efer
EHOMO
ELUMO
f
(Volt)
(Volt)
(Volt)
(eV)
(eV)
EBand gap
TB5
TB6
TB7
0.54
0.87
0.83
À0.70
À0.72
À0.63
0.58
0.57
0.61
4.84
5.10
5.02
3.52
3.51
3.56
1.32
1.59
1.46
a
b
Oxidation potential of Bodipy derivatives. Reduction potential of
c
Bodipy derivatives. Potential of ferrocene, the internal reference electrode.
d HOMO energy level of Bodipy derivatives. LUMO energy level of Bodipy
e
derivatives. f Energy band gap of Bodipy derivatives.
Fig. 7 Schematic representation of HOMOs and LUMOs of TB6. The
UB3LYP/CEP-31G level of theory.
band levels and band gaps. Energy levels of the HOMO and
the LUMO are highly important in the DSSC concept. To
summarize, upon photoexcitation of the dye molecules,
Theoretical studies
In order to understand the structure–performance relationship
electrons are excited from the HOMO to the LUMO of the of the photosensitizers, we calculated the HOMO–LUMO
dye and then electron injection from the LUMO to the diagrams (for calculation details, see the ESI†). The HOMO–
conduction band of TiO2 takes place. Hence, the LUMO LUMO of TB6 are shown in Fig. 7, in which the HOMO
molecular orbital electron density can be seen on the bis-
dimethylfluorenyl group and the LUMO electron density on
the Bodipy and the thiophene unit. It was apparent that upon
photoexcitation, electrons moved from the bis-dimethyl-
fluorenyl unit to the anchoring thiophene unit with p–p* type
energy level should be higher in energy than that of the
conduction band of TiO2.
The energy level of the conduction band was estimated to be
4.2 eV.2a In all three Bodipy dyes; TB5, TB6, and TB7, their
LUMO levels were convenient for electron injection with 3.51–
3.56 eV values which were higher in energy, as shown in Fig. 6. excitation. However, in the LUMO p-electron localization on the
HOMO levels of the Bodipy dyes should also be considered Bodipy core was not desired and this may be responsible for the
since their energy level should be lower than the redox couple low electron injection efficiency. In optimized geometry of
energy level (IÀ/I3À), which was also calculated to be 4.9 eV. In
the molecule, the angle between the Bodipy plane and the
thiophene carboxylic acid unit plane was 46.61, which seemed
to be enough to achieve an effective conjugation between these
two units. Although this angle seemed to be large, it was
smaller when compared to that of the 8-aryl Bodipy molecules
Table 1, we can observe that only TB5 was not convenient for
reducing the Bodipy dye by the redox couple. On the other
hand, TB6 and TB7 had the desired energy levels of 5.10 and
5.02 eV, respectively. However, it was emphasized in a recent
review by Ooyama and Harima that, the energy difference (almost 901) synthesized from 2,4-dimethylpyrrole due to the
between these levels should be at least 0.2–0.3 eV for an steric effect of the extra methyl units and in this case, the
efficient electron regeneration.13 In our molecules, these p-electron conjugation was limited.
The electron localization of the HOMO–LUMO of TB7 (Fig. 8)
was similar to that of TB6 according to the computational
studies. The HOMO was localized mainly on the donor group,
were 0.2 and 0.12 eV for TB6 and TB7, respectively. Although,
these values seem to be appropriate, a larger gap would be
better suited.
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Table 3 Photovoltaic performance of TiO2-based dye-sensitized solar
cells
Jsc (mA cmÀ2) Voc (mV) ff
Z (%) Adsorbed dye (Â10À7 mol cmÀ2
)
TB5 1.12
TB6 2.61
TB7 2.12
370
400
400
0.48 0.19 1.7
0.57 0.59 3.9
0.58 0.49 3.0
and the incident photon-to-current conversion efficiency (IPCE)
(Fig. S6, ESI†) and by the help of these parameters, we calcu-
lated the solar energy-to-electricity conversion yield (Z). These
values are tabulated in Table 3. TB6 produced more photo-
current with 2.61 mA cmÀ2 under AM 1.5 simulated sunlight
(100 mW cmÀ2). The cell constructed from TB6 had an open-
circuit voltage of 400 mV and a fill factor of 0.57 and the
calculated total sunlight-to-electricity conversion yield (Z) was
0.59 (Table 3). This made this dye more efficient compared to
TB5 and TB7 without using chenodeoxycholic acid as a coad-
sorbent. Herein, it is worth noting that the adsorbed amount of
TB6 on the TiO2 surface was more (see Table 3) so this may also
be a reason for this result. Also, the incident photon-to-current
conversion efficiency (IPCE) spectrum gave us information
about the response of the dye molecules upon illumination
with the monochromatic light over a wide range of wavelengths.
This spectrum resembles the absorbance of the dyes, interestingly
large amounts of photocurrent were observed for TB6 and TB7 in
the range 300–400 nm (Fig. S5 and S11, ESI†). However, the
irradiation of sunlight in this range is not strong and its reflection
to the solar-light photocurrent is limited. For TB6 we observed
effective photocurrent in the range 600–700 nm.
Fig. 8 Schematic representation of HOMOs and LUMOs of TB7. The
UB3LYP/CEP-31G level of theory.
Table 2 TD-DFT transitions for TB6 and TB7
Dye S0 - S1 Transition Absorption (nm) Oscillator strength LHE (%)
TB6 HOMO - LUMO 869
TB7 HOMO - LUMO 975
0.9619
0.6896
89
79
while the LUMO on thiophene and Bodipy. The angle between
the Bodipy plane and thiophene (49.11) was slightly wider
compared to that of TB6.
The light-harvesting efficiency (LHE)14 of the dyes were
calculated using LHE = 1–10Àf, where f is the oscillator strength
of the dyes. LHE values were 89% and 79% for TB6 and TB7,
respectively. TD-DFT results are summarized in Table 2.
Besides an acceptable deviation of the computed absolute
numerical values from the measurements, the TD-DFT results
are in very good agreement with the prediction of the red
shifting (106 nm computed) of the main S0 - S1 transition
and the LHE trend (in favor of TB6).
We also constructed the cell from TB5 for comparison and
we reached lower solar cell parameter values using this dye with
Z = 0.19. For TB7, we acquired similar but also lower values
compared to TB6. The critical parameters for this dye exhibited
a highly panchromatic nature of the IPCE spectrum (Fig. S5,
ESI†). Although the photocurrent was low, it was generated in a
wide spectrum, ranging 500–850 nm. The open-circuit voltage
(Voc) was 400 mV, the photocurrent density was 2.12 mA cmÀ2
Photovoltaic performances
After careful investigation of CV results, we constructed the
cells by using typical methods as mentioned in the experi-
mental section. Firstly, we measured the short-circuit photo-
current density ( Jsc), the open-circuit photovoltage (Voc), the
photocurrent–voltage curve ( J–V curve) (Fig. 9), the fill factor (ff)
Fig. 10 Current vs. voltage graphs of the photosensitizers after adding
Fig. 9 Current vs. voltage graphs of the photosensitizers.
Cheno (2 mM).
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Table 4 Photovoltaic performance of TiO2-based dye-sensitized solar
cells after adding Cheno (2 mM)
coadsorbent was still needed to inhibit the stacking of the dye
molecules on TiO2. The second parameter we used in these
molecules was the p-electron linker unit, thiophene. This group
has been widely used in various dye-sensitized solar cells due to
its high p-electron localization ability and when we inspected
the LUMOs we observed that the electron density moved
successfully from the donor group to the acceptor unit. The
main obstacle may be the fact that the electron density was not
fully transferred to the acceptor and there was some residual
density on the Bodipy core. This complication may be resolved
by using the oligothiophene unit as a p-electron linker for a
more efficient electron transfer. Further studies to solve the
addressed problems and develop more efficient DSSCs suitable
for near-IR sensitization are underway.
Jsc (mA cmÀ2
)
Voc (mV)
ff
Z (%)
TB5
TB6
TB7
3.60
3.47
5.32
450
450
450
0.38
0.47
0.52
0.61
0.73
1.25
Acknowledgements
We acknowledge Prof. Engin U. Akkaya for fruitful discussions,
¨
˘
and Tugba Ozdemir Ku¨tu¨k for help in synthesis. We acknowl-
edge the Alexander von Humboldt Foundation (AvH), the
Turkish Scientific and Technological Research Council (TUBITAK),
Grant No. 114F161, the UNESCO–LOREAL Foundation and the
Turkish Academy of Sciences (TUBA).
Fig. 11 Incident photon-to-current conversion efficiencies of the liquid
electrolyte based-DSSCs as a function of wavelength after adding Cheno.
Notes and references
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electricity conversion yield (Z) was 0.49.
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but this time we used the coadsorbent chenodeoxycholic acid
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shown in Fig. 11 and 10, respectively. Table 4 shows the
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The novelties of this study include a bulky donor group and a
thiophene p-electron linker. As a donor group, we used the bis-
dimethylfluorenyl amine group which was used for the first
time with Bodipy dyes as far as we know and due to its sterically
bulky structure it is expected to inhibit the aggregation of the
dyes on the TiO2 nanocrystalline surface. After constructing the
cells using the Bodipy dyes that we have synthesized, the overall
efficiency (Z) was calculated to be 0.59 for TB6 and 0.49 for TB7.
By using chenodeoxycholic acid as a coadsorbent, we observed
an increase in efficiency of all the photosensitizers tested. And
the efficiency of TB7 was observed to increase from 0.49 to 1.25.
Although we used the bulky donor group, it seemed that using a
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