Functional panchromatic BODIPY dyes with near-infrared absorption: Design, synthesis, characterization and use in dye-sensitized solar cells

Article abstract of DOI:10.3762/bjoc.15.169

We report two novel functional dyes based on a boron-dipyrromethene (BODIPY) core displaying a panchromatic absorption with an extension to the near-infrared (NIR) range. An innovative synthetic approach for preparing the 2,3,5,6-tetramethyl-BODIPY unit is disclosed, and a versatile way to further functionalize this unit has been developed. The optoelectronic properties of the two dyes were computed by density functional theory modelling (DFT) and characterized through UV-vis spectroscopy and cyclic voltammetry (CV) measurements. Finally, we report preliminary results obtained using these functional dyes as photosensitizers in dyesensitized solar cells (DSSCs).

Full text of DOI:10.3762/bjoc.15.169

Functional panchromatic BODIPY dyes with near-infrared
absorption: design, synthesis, characterization and use
in dye-sensitized solar cells
Quentin Huaulmé1, Cyril Aumaitre1, Outi Vilhelmiina Kontkanen2, David Beljonne2,
Alexandra Sutter3, Gilles Ulrich3, Renaud Demadrille*1 and Nicolas Leclerc*3
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 1758–1768.
1CEA, Univ. Grenoble Alpes, CNRS, IRIG, SyMMES, F-38000
Grenoble, France, 2Chimie des Matériaux Nouveaux & Centre
d'Innovation et de Recherche en Matériaux Polymères Université de
Mons - UMONS / Materia Nova Place du Parc, 20, B-7000 Mons,
Belgium and 3ICPEES – UMR7515, CNRS-Université de Strasbourg,
25 rue Becquerel, 67087 Strasbourg CEDEX 02, France
doi:10.3762/bjoc.15.169
Received: 12 April 2019
Accepted: 10 July 2019
Published: 24 July 2019
This article is part of the thematic issue "Dyes in modern organic
chemistry".
Email:
Renaud Demadrille* - renaud.demadrille@cea.fr; Nicolas Leclerc* -
leclercn@unistra.fr
Guest Editor: H. Ihmels
* Corresponding author
© 2019 Huaulmé et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
boron-dipyrromethene; BODIPY; dye-sensitized solar cells;
near-infrared absorbers; organic dyes
Abstract
We report two novel functional dyes based on a boron-dipyrromethene (BODIPY) core displaying a panchromatic absorption with
an extension to the near-infrared (NIR) range. An innovative synthetic approach for preparing the 2,3,5,6-tetramethyl-BODIPY unit
is disclosed, and a versatile way to further functionalize this unit has been developed. The optoelectronic properties of the two dyes
were computed by density functional theory modelling (DFT) and characterized through UV–vis spectroscopy and cyclic voltam-
metry (CV) measurements. Finally, we report preliminary results obtained using these functional dyes as photosensitizers in dye-
sensitized solar cells (DSSCs).
Introduction
The past two decades have witnessed tremendous efforts to as perovskite solar cells. DSSCs can display satisfactory power
develop alternative photovoltaic (PV) technologies. Among conversion efficiencies (PCE) in the range of 10 to 14% [1-3]
them, dye-sensitized solar cells (DSSCs) display numerous but also a long-term stability when specific electrolytes based
advantages compared to its fully organic counterpart, i.e., bulk on ionic liquids are employed [4,5]. Besides, this technology
heterojunction solar cells, or other hybrid PV technologies such enables the fabrication of solar panels that can be prepared
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Results and Discussion
semi-transparent, colorful, and out of non-toxic constituents [6].
Historically, Ru(II)–polypyridyl complexes were the most used 1. Design and DFT calculations
dyes as photosensitizers in DSSCs (N719 or N749 Black Dye) In many opto-electronic devices the light-absorption properties
[7] but these molecules reveal low absorption coefficients over of the semiconductors are a critical parameter. This is particu-
the visible range. Furthermore, they contain a rare and a rela- larly the case when solar energy conversion applications are
tively high-cost element (Ru) and their plausible toxicity targeted. For instance, in order to maximize the photocurrent
restrains their development at the industrial level. For these density in a DSSC device, the sensitizer has to display high
reasons, in the last decade, metal-free organic dyes based on molar absorption coefficients, ideally along the entire visible
donor–(π-spacer)–acceptor structure have hence been extensive- range. Additionally, it must display appropriate positioning of
ly investigated and screened as sensitizers in DSSCs. Among frontier molecular orbitals (highest occupied molecular orbital/
the hundreds of dyes developed so far for this application, only lowest unoccupied molecular orbital, HOMO/LUMO) energy
few can show panchromatic absorption [8,9]. To improve the levels with respect to the conduction band (CB) of the metal
photogeneration of current, and hence the efficiency of DSSCs, oxide and the redox potential of the electrolyte. Abundant liter-
the development of new dye molecules better matching the ature on BODIPY derivatives allowed us to identify a chemical
solar emission spectrum or exhibiting absorption in the near-in- approach to fulfill those criteria, i.e., the introduction of an elec-
frared (NIR) range could be a fruitful strategy.
tron rich unit at the 3rd and 5th positions of the BODIPY core
via a vinyl spacer [14,21,23,24]. Introduction of such units on
Several families of molecules displaying an infrared absorption those positions is known to lead to a larger bathochromic shift
have been investigated in the last decade, such as phthalo- of the S0→S1 absorption band than the same substitution on its
cyanines [10,11], organic push–pull compounds [12], and 2nd and 6th positions [25]. A thiophene-triphenylamine unit
boron-dipyrromethene [13] (BODIPY®). BODIPY dyes are one was selected among the reported electron-donating units, due to
of the most extensively studied class of fluorophores due to its reversible redox properties and high electron-donating
their unique properties, including high absorption coefficients strength (see Figure 1) [26]. In order to promote the delocaliza-
in the visible and NIR ranges, high fluorescence quantum tion from the electron-donating unit to the electron-with-
yields, and high stability in various media. More importantly, drawing and anchoring group located on its 8 position, a
they display a very versatile chemistry, allowing the fine tuning 2,3,5,6-tetramethyl-BODIPY derivative has been designed (see
of all their physical and optical properties [14]. They hence Figure 1). Indeed, most of the BODIPY based materials, used in
have found applications in various fields, such as lasers organic semiconducting applications, exhibit methyl groups in
dyes [15], (bio)-labeling [16,17], photodynamic therapy [18], the 1,7-positions. However, such groups in these positions
or even bulk heterojunction solar cells [19,20] or DSSCs hinder sterically the position 8 of BODIPY cores and therefore
[21,22].
lead to a severe twist of the aromatic unit grafted on. Removing
these methyl groups is an effective way to reduce the dihedral
In this articl we report an innovative synthetic approach for angle between the meso substituent and the BODIPY core by
synthesizing 2,3,5,6-tetramethyl-BODIPY compounds and a lowering the steric hindrance between the two latter. It has
way to further functionalize such cores has been developed. The been previously reported that a better molecular planarity
optoelectronic properties of the functional molecules were in- usually originates a higher charge injection efficiency [27].
vestigated using UV–vis spectroscopy, and we show that they Furthermore, it has been reported that introduction of hydro-
can absorb light up to 900 nm in solution and 1000 nm in solid phobic alkyl chains on a sensitizer is a way to improve the
state, after grafting on anatase-TiO2 mesoporous films. The open-circuit voltage (Voc), by reducing the electronic recombi-
cyclic voltammetry (CV) measurements indicate that the com- nation rate at the electrolyte/semi-conducting oxide interface
pounds have HOMO and LUMO energy levels suitable for an [28]. To probe the validity of this concept for a BODIPY deriv-
application in DSSCs, in rather good agreement with the values ative, we decided to synthetize the two molecular structures
obtained from DFT calculations. Finally, we report preliminary disclosed in Figure 1, namely BOD-TTPA-alk and BOD-
results employing these molecules as photosensitizers in dye TTPA.
solar cells with iodine-based liquid electrolytes. We show that
the limited performances of these new BODIPY derivatives To support our molecular design, and especially to have better
arise from their deep LUMO energy level. In terms of energy, insight on the electronic structure and optical properties of the
the latter lies close to the conduction band of the electron-trans- dyes, we performed (time-dependent) density functional theory
porting oxide, limiting therefore the driving force of electronic (TD-DFT) calculations on the four representative molecules
injection, and hence the overall efficiency of the resulting solar displayed in Figure 2: Dyes (1 and 3) vs (2 and 4) differ by the
cell.
position (2,6 or 1,7) of the methyl groups on the BODIPY core,
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Figure 1: Molecular structures of the two target compounds BOD-TTPA-alk and BOD-TTPA, and the chemical structure of a BODIPY core displaying
its positions numbering.
while dyes (1 and 2) vs (3 and 4) differ by the presence (or not) reduced energy driving force for charge separation at the TiO2
of a triphenylamine donor group on the thiophene ring. As we surface. To check this hypothesis, we performed TD-DFT simu-
are dealing with charge-transfer electronic excitations, we have lations of the optical absorption spectra of the 4 representative
adopted a tuned range-separated CAM-B3LYP functional and dyes. The results reported in Figure 3 are fully consistent with
the polarizable continuum model (PCM) to account for the predictions from the one-electron picture. There is a sub-
(implicit) solvent effects (in THF).
stantial red shift of the lowest optical absorption band, from
≈750 nm in 2 to ≈850 nm in 1 by moving the methyl groups
The one-electron energy diagram reported in Figure 2 shows from (1,7) to (2,6) positions, in line with the increased elec-
that: (i) Grafting the triphenylamine donor moieties on the thio- tronic delocalization over the anchoring groups. An additional
phene-vinylene bridge only slightly (by ca. 0.1 eV) lowers the virtue of the placement of the methyl groups in positions (2,6)
ionization potential of the dyes, compare (1,2) to (3,4); this is is to extend the absorption range of the dyes across the whole
explained by the fact that the HOMO molecular orbital mostly visible range. Very similar effects are observed for the TPA-
spreads on the BODIPY-vinylthiophene core of the molecules, free molecules, except for an overall blue shift of the main
with only slight contributions from the TPA units (ii) The posi- absorption bands resulting from the reduced size of the π-elec-
tion of the methyl groups has by far a larger impact on tronic system.
the electron affinity of the molecules, which is substantially
increased when going from 2 to 1, and from 4 to 3. This is 2. Synthesis
clearly not a direct electronic effect but rather results from the The synthesis of the target materials is schemed in Figure 4. To
close to orthogonal orientation of the anchoring groups induced introduce the selected substitution pattern, 2,3-dimethylpyrrole
by steric effects in 2 and 4. As a result, while the LUMO orbital 1 is first synthesized through a Trofimov reaction in a one-pot
largely extends through the BODIPY unit towards the cyano- two-step reaction [29]. The condensation of the aforementioned
acrylic acid anchors in the case of 1 and 3, it is completely pyrrole on para-iodobenzoyl chloride affords the correspond-
confined to the BODIPY-vinylthiophene core in 2 and 4, ing dipyrromethenium chloride, which was then converted into
explaining the deeper unoccupied levels in the former mole- its BODIPY analogue 2 through complexation by BF3·OEt2 in
cules.
basic media. Regioselective introduction of distyryl substitu-
ents is achieved via Knoevenagel-type condensation in the pres-
Thus, our calculations suggest that, in case of an allowed ence of piperidine using aldehyde derivatives 3 and 4 whose
HOMO–LUMO transition, 1 and 3 should show a bathochromi- synthesis are described in Supporting Information File 1. This
cally shifted optical absorption spectrum, thereby hopefully condensation reaction affords compounds 5 and 6 with a slight
allowing for a more efficient sunlight absorption, yet also a variation of their pendant alkyl chains. The stereoisomerism of
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Figure 2: a) Geometrical optimization of four representative BODIPY-based materials for DSSCs application. b) Frontiers molecular orbitals reparti-
tion and their predicted energy values for the selected four representative BODIPY-based dyes.
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the resulting distyryl compound (trans) has been unambiguous-
ly attributed by 1H NMR (see Figures S9 and S11 in Support-
ing Information File 1), whose spectra feature characteristic
constant couplings of 16.2 and 16.1 Hz. It is worth mentioning
that the fluorine substitution was performed on the boron center
after introduction of the styryl residues, introduction prior
to the Knoevenagel reaction is known to impede it [30]. The
introduction of the aldehyde moiety is carried out through
catalytic carbo-palladation reaction using molecular carbon
monoxide in the presence of methyl formiate as hydrogen
source in modest yield (35% and 19% for 8 and 10, respective-
ly) due to the formation of a relatively large amount of dehalo-
genation side-product. Finally, a Knoevenagel condensation in
the presence of cyanoacetic acid and piperidine is performed to
lead to the targeted compounds BOD-TTPA-alk and BOD-
TTPA.
Figure 3: Predicted absorption spectra of the four dyes.
Figure 4: Synthetic scheme of the selected materials. a) hydroxylamine hydrochloride, NaHCO3, DMSO, 60 °C then acetylene, KOH, DMSO, 110
°C, 24% over the two steps; b) 4-iodobenzoyl chloride, DCM, rt then DDQ, DCM, rt then NEt3, BF3·OEt2, 0 °C to rt, 35%; c) piperidine, cat. PTSA,
toluene, 130 °C, 5: 35%, 6: 30%; d) (3-(2-methoxyethoxy)prop-1-yn-1-yl)magnesium bromide, THF, 60 °C, 85%; e) carbon monoxide, sodium
formiate, [Pd(PPh3)2Cl2], anhydrous DMF, 100 °C, 8: 19%, 9: 35%; f) cyanoacetic acid, piperidine, MeCN, CHCl3, 80 °C, BOD-TTPA: 34%, BOD-
TTPA-alk: 26%.
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3. Optical properties
900 nm. The absorption in the NIR region is a peculiar prop-
The optical properties of compounds BOD-TTPA-alk and erty that is of course a considerable advantage for solar cell ap-
BOD-TTPA were first evaluated in diluted (≈10−6 M in THF) plications.
solution (see Figure 5). They both display two intense absorp-
tion bands. At lower energy, the absorption band displays a With the aim of investigating the behavior of those dyes once
rather large and high extinction coefficient (5·10−4 M−1cm−1) adsorbed on mesoporous TiO2, sensitization has been per-
band attributed to the S0→S1 transition of the BODIPY core, formed on 2 µm thick TiO2 mesoporous layer (see Figure 5b
and still displays its characteristic shoulder at higher energy and c). To prevent dye aggregation on the surface of the semi-
[14]. Another intense absorption band is located around 400 nm conducting oxide, and to mimic the real conditions of use in
and is attributed to the π→π* transition of the triphenylamine solar cells, chenodeoxycholic acid (CDCA) was employed as
thiophene residues. It is worth noting that almost no difference co-sensitizer and hence added to the sensitization solution [31].
is observed between those two materials, which is consistent For the dyeing solutions the concentration of the dye was 1 mM
with their identical electronic structure. As often with such and the concentration of CDCA was 10 mM in a chloroform/
distyryl-BODIPY derivatives [19,25], no clear cut-off of absor- ethanol mixture (1:1 in volume). The absorbance spectra of the
bance is observed in the whole part of the visible range, leading resulting sensitized TiO2 layers are recorded and presented in
to a panchromatic absorption. The absorption is not limited to Figure 5b.
the visible domain, indeed the compound BOD-TTPA-alk and
BOD-TTPA exhibit a λmax in solution located at 771 nm For both dyes, grafting them on the surface clearly impacts the
and 768 nm, respectively, with an absorption edge close to absorption bands attributed to the BODIPY part of the systems.
Figure 5: a) Absorption spectra of compounds BOD-TTPA-alk and BOD-TTPA (THF, ≈10−6 M, 25 °C). b) Absorbance spectra of 2 µm thick meso-
porous TiO2 sensitized in a solution of dye/CDCA (1:10, molar ratio). c) Normalized absorbance spectra of BOD-TTPA-alk (red traces) and BOD-
TTPA (blue traces), both in diluted solution (aerated THF, ≈10−6 M, 25 °C – dashed lines) and once adsorbed on 2 µm thick mesoporous TiO2 (solid
lines).
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The absorption coefficient of the lower energy absorption band, their highest occupied molecular orbital (HOMO) and lower
attributed to the S0→S1 transition, is reduced and a higher unoccupied molecular orbital (LUMO). All the redox potentials
energy band around 700 nm increases in intensity. This last one were calibrated with respect to Ferrocene (Fc), assuming that
is more likely attributed to a conformationally restricted form of E(Fc/Fc+) = 0.40 V with respect to SCE (see experimental
the dye once adsorbed which shows an absorption value closed details). The low solubility of the BOD-TTPA derivative makes
to mono-styryl species, thus reinforcing the panchromatic this material much more difficult to characterize than its alky-
absorption of this dye. As long as BOD-TTPA-alk is lated counterpart. However, both dyes exhibit similar first oxi-
concerned, no significant shift of the lower energy band is ob- dation potentials about 0.6 V and 0.55 V for BOD-TTPA and
served (see Table 1 and Figure 5a). This suggests no self-aggre- BOD-TTPA-alk, respectively. Both processes are reversible. It
gation of the alkylated dye occurs on the oxide surface, which worth noting that the BOD-TTPA-alk exhibits also a second
can be explained by the presence of the co-adsorbent CDCA, reversible oxidation process at higher voltage of 0.8 V. Finally,
and of the multiple alkyl chain substitution of the TPA units. for both dyes, a quasi-reversible reduction process could be ob-
On the other hand, a bathochromic shift of the absorption served at −0.6 V and −0.8 V for BOD-TTPA and BOD-TTPA-
profile of BOD-TTPA is observed from diluted solution to the alk, respectively.
anchored dye (see Figure 5a). Despite the same amount of
CDCA in the dyeing solution (10 molar equivalents), this result The HOMO and LUMO levels were determined by using the
suggests a higher tendency for π stacking interactions. Once following equations (HOMO = Eox + 4.4 eV) and (LUMO =
anchored, BOD-TTPA exhibits consequently a broader absorp- Ered + 4.4 eV) where the onset potentials are in volts (vs SCE)
tion profile than its alkylated counterpart BOD-TTPA-alk.
and HOMO and LUMO are in electron volts [32]. We thus
calculated HOMO and LUMO levels of −5.0 eV and −3.8 eV
Furthermore, the measured intensity of absorbance of the sensi- for BOD-TTPA against −4.95 eV and −3.6 eV for BOD-
tized TiO2 layer is much higher for BOD-TPA than BOD- TTPA-alk. The small change of the HOMO energy levels is in
TTPA-alk in spite of the fact that the dyeing was performed line with the performed calculations (see section 1), where the
with solutions containing exactly the same concentration of the HOMO molecular orbital is shown to be mostly spread on the
dyes. The thickness of the aforementioned TiO2 layer being BODIPY-vinylthiophene moieties of the dyes, with only slight
identical and the two dyes BOD-TTPA-alk and BOD-TTPA contributions from the TPA units. The significant decrease in
displaying very similar absorption coefficient (see Table 1), electronic affinity observed in BOD-TTPA-alk is likely due to
sensitization of mesoporous TiO2 will be more effective for the boron center alkylation. Similar effects have been already
BOD-TTPA than BOD-TTPA-alk. This lower absorption orig- shown after fluorine substitution of BODIPY [33].
inating from a lower grafting level may arise from the overall
5. DSSCs fabrication and device
size of the molecule BOD-TTPA-alk that displays 4 alkyl
chains and 2 ethylene glycol chains. The bigger size of BOD- performances
TTPA-alk can eventually prevents its diffusion through all the In order to investigate the photovoltaic performances of the two
pores of the TiO2 layer or decreases the density of grafted mole- functional dyes BOD-TTPA-alk and BOD-TTPA, a set of
cules due to steric hindrance.
DSSCs were fabricated following a procedure reported previ-
ously [4]. The J(V) characteristics of the devices were recorded
in dark and upon irradiation with a mask. For the measure-
4. Electrochemical characterization
Cyclic voltammetry of both BODIPY dyes was carried out in ments a solar simulator with AM 1.5G filter was used after cali-
deoxygenated DCM solutions containing tetrabutylammonium bration with a Silicon cell at 1000 W m−2. For a direct compari-
hexafluorophosphate as salt (see Figure S24 and S25 in Sup- son, we fabricated solar cells with the same photoelectrode
porting Information File 1), to investigate their oxidation and composition consisting of a double layer TiO2 (a 12 µm-thick
reduction processes as well as to determine the energy levels of transparent layer and a 4 µm-thick scattering layer) purchased
Table 1: Selected optical properties of compounds BOD-TTPA-alk and BOD-TTPA.
Compounds
(nm)
ε (L·mol−1·cm−1) at
(nm)
(nm)
(cm)
BOD-TTPA-alk
BOD-TTPA
409, 771
411, 768
6.49·104, 4.46·104
6.52·104, 5.00·104
841
856
393, 700, 771
389, 801
867
907
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from Solaronix. This thickness was selected on the basis of a short-circuit current (Jsc) of 4.70 mA·cm−2, an open-circuit
previous study showing that thicker electrodes (typically above voltage (Voc) of 0.37 V and a fill factor (FF) of 65%. On the
12 µm) give rise to highest photocurrent density.
other hand, BOD-TTPA displays a power conversion effi-
ciency of 1.22%, along with a Jsc of 7.33 mA·cm−2, a Voc of
When dyes with low-lying LUMO energy levels are employed 0.33 V and a FF of 51%. Despite being rather low compared to
as sensitizers together with TiO2 electrodes, the choice of the the performances of more conventional dyes, these results are
electrolyte is crucial. From CV experiments (see previous quite consistent with previous reports on BODIPY sensitizers
section) we found that the LUMO levels of the dyes are at −3.9 with absorption in the NIR region [36].
and −3.6 eV. In other words, they are located roughly
0.1–0.4 eV above the energy level of the conducting band of the
oxide (which is around −4 eV). This alignment of the energy
levels could be damaging to the electron injection process.
Indeed, it is known that a minimum driving force of 0.15 eV
(neglecting entropy changes during the light absorption) is re-
quired to efficiently inject photo excited electrons from the
LUMO of the dyes in the CB of the oxide [34].
Usually iodine-based liquid electrolytes are comprising addi-
tives such as tert-butylpyridine (tBP) which is known to shift
the CB band of the oxide positively by creating a dipole effect
at the surface of TiO2. By suppressing this dipole effect, the CB
of TiO2 could be relocated deeper and this could facilitate the
photo-injection process. Consequently, for preliminary investi-
gations we prepared an electrolyte with a formulation close to
the one of HI-30 commercialized by Solaronix. HI-30 elec-
trolyte is known to be compatible with a large variety of organic
dyes. We therefore designed a modified electrolyte inspired
from HI-30 with the following composition: 0.5 M of butyl-
methylimidazolium iodide (BMII), 0.03 M of I2 0.5 M of LiI,
0.1 M of guanidinium thiocyanate in a mixture of acetonitrile
and 3-methoxyproprionitrile (85:15). Compared to classical
electrolytes, in this formulation we removed tBP, hoping that
Figure 6: J(V) curves of the best performing DSSCs devices sensi-
tized with compounds BOD-TTPA-alk (blue traces) and BOD-TTPA
(red traces) in dark (dashed lines) and under AM1.5G conditions (solid
lines). Light source: AM1.5G at 100 mW·cm−2, electrolyte composition:
0.5 M of BMII, 0.03 M of I2, 0.5 M of LiI, 0.1 M of guanidinium thio-
cyanate in a mixture of acetonitrile and 3-methoxyproprionitrile (85:15,
v/v), electrodes: 12 µm mesoporous anatase TiO2 + 4 µm TiO2 scat-
tering layer, dyeing bath: [Dye] = 0.2 mM, [CDCA] = 2 mM in CHCl3/
EtOH 1:1 (v/v)).
the suppression of the interface dipole would shift the CB First, comparing the photovoltaic behavior of the two dyes one
energy level of TiO2 and, consequently, enhance injection should note that the Jsc delivered by the solar cells are rather
process [35].
different (4.70 mA·cm−2 for BOD-TTPA-alk versus
7.33 mA·cm−2 for BOD-TTPA). The higher Jsc obtained with
The J(V) curves of the devices containing compounds BOD- BOD-TTPA can be explained with the broader and more
TTPA-alk and BOD-TTPA are reported in Figure 6. Selected intense absorption in the NIR range once grafted on TiO2 com-
photovoltaic parameters obtained from three different measure- pared to its alkylated counterpart. Second, one can notice that
ments for each dye are gathered in Table 2. BOD-TTPA-alk the Voc and FF recorded for devices sensitized with BOD-
exhibits a maximum power conversion efficiency of 1.12% a TTPA-alk are slightly higher compared to the ones prepared
Table 2: Photovoltaic parameters of compounds BOD-TTPA-alk and BOD-TTPA (light source: AM1.5G at 100 mW·cm−2, electrolyte composition:
0.5 M of BMII, 0.03 M of I2, 0.5 M of LiI, 0.1 M of guanidinium thiocyanate in a mixture of acetonitrile and 3-methoxyproprionitrile (85:15, v/v), elec-
trodes: 12 µm mesoporous anatase TiO2, + 4 µm TiO2 scattering layer, dyeing bath: [Dye] = 0.2 mM, [CDCA] = 2 mM in CHCl3/EtOH 1:1, v/v).
Highest value and mean values over three measurements in parenthesis.
Dye
Jsc (mA·cm-2)
Voc (V)
FF
PCE (%)
BOD-TTPA-alk
BOD-TTPA
4.70 (4.64)
7.33 (7.10)
0.37 (0.37)
0.33 (0.33)
0.65 (0.64)
0.51 (0.50)
1.12 (1.12)
1.22 (1.20)
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out of BOD-TTPA. This could be explained by the presence of conductor, which is correlated with its CB edge and electron
the alkyl chains on the TPA units that are known to prevent the density (see Figure 7).
redox mediator to interact with the TiO2 surface thus reducing
the probability to observe a recombination process between the Solar cells fabricated with various amount of tBP in the elec-
photo-injected electrons and the iodide.
trolyte were investigated (see Table S1 in Supporting Informa-
tion File 1). It appears from Figure 7 that increasing the concen-
To confirm that the main factor limiting the device perfor- tration of tBP implies, as expected, an increase of Voc, from
mances is the position of the LUMO level of these dyes with 328 mV to 460 mV for BOD-TTPA-alk and from 360 mV to
respect to the CB of the oxide, we investigated the effect of tBP 420 mV for BOD-TTPA. On the other hand, the Jsc values for
content in the electrolyte. By adding tBP in the electrolyte one BOD-TTPA-alk and BOD-TTPA all decrease from 7.33 to
can tune the electronic properties, i.e., the CB energy level posi- 0.88 mA·cm−2 and from 4.70 to 0.41 mA·cm−2, respectively.
tion of TiO2 [31,37-39]. The effect is well-known, a negative This result unambiguously highlights that the shift of TiO2 CB
dipole-related shift of the TiO2 Fermi level occurs by tBP impedes the photo-injection of electrons [40]. This proves that
adsorption on the surface. As the LUMO levels of the dyes are the LUMO energy levels of the dyes BOD-TTPA-alk and
estimated close to the CB Fermi level, the use of tBP should BOD-TTPA are too close from the CB of the oxide. Despite a
lead to a drop of the Jsc because of the reduction of the driving higher Voc, the loss of Jsc is responsible for the overall decrease
force for the injection. Simultaneously a rise of the Voc is ex- of the power conversion efficiencies.
pected. A shift of CB energy level toward the vacuum level
(i.e., away from the redox potential of the electrolyte) results in Conclusion
a higher open-circuit voltage (Voc). Indeed, in DSSCs the Voc is We have designed, synthesized and characterized two novel
determined by the quasi-Fermi-level of the metal oxide semi- functional BODIPY-based dyes showing intense panchromatic
Figure 7: Photovoltaic parameters evolution with the increasing concentration of tBP in the electrolyte.
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Experimental part and copies of NMR spectra.
[https://www.beilstein-journals.org/bjoc/content/
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Acknowledgements
The authors acknowledge financial support from the French
Research Agency ANR (grants ANR ANR-14-OHRI-0003
ODYCE). DB is a FNRS Research Director.
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ORCID® iDs
Quentin Huaulmé - https://orcid.org/0000-0003-2888-9887
Outi Vilhelmiina Kontkanen - https://orcid.org/0000-0002-2892-2141
David Beljonne - https://orcid.org/0000-0001-5082-9990
Gilles Ulrich - https://orcid.org/0000-0002-6980-6091
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