High-performance solution-processed solar cells and ambipolar behavior in organic field-effect transistors with thienyl-BODIPY scaffoldings

Article abstract of DOI:10.1021/ja3072513

Green-absorbing dipyrromethene dyes engineered from bis-vinyl-thienyl modules are planar molecules, exhibiting strong absorption in the 713-724 nm range and displaying comparable electron and hole mobilities in thin films (maximum value 1 × 10^-3 cm²/(Vs)). Bulk heterojunction solar cells assembled with these dyes and a fullerene derivative (PC₆₁BM) at a low ratio give a power conversion efficiency as high as 4.7%, with short-circuit current values of 14.2 mA/cm², open-circuit voltage of 0.7 V, and a broad external quantum efficiency ranging from 350 to 920 nm with a maximum value of 60%.

Full text of DOI:10.1021/ja3072513

Communication
pubs.acs.org/JACS
High-Performance Solution-Processed Solar Cells and Ambipolar
Behavior in Organic Field-Effect Transistors with Thienyl-BODIPY
Scaffoldings
†
‡
§
§
§
⊥
́ ̂
Thomas Bura, Nicolas Leclerc, Sadiara Fall, Patrick Leveque, Thomas Heiser, Pascal Retailleau,
,†
†
†
Sandra Rihn, Antoine Mirloup, and Raymond Ziessel*
†
Laboratoire de Chimie Molec
au CNRS, 25 rue Becquerel, 67087 Strasbourg Cedex 02, France
Laboratoire d’Ingenierie des Polymeres pour les Hautes Technologies, Universite de Strasbourg, Ecole Europee
Polymeres et Materiaux, 25 rue Becquerel, 67087 Strasbourg, France
Institut d’Electronique du Solide et des Systemes, Universite de Strasbourg-CNRS, 23 rue du Loess, 67037 Strasbourg, France
Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles, CNRS, Bat. 27-1 avenue de la Terrasse, 91198
Gif-sur-Yvette Cedex, France
́ ́ ́ ̀ ́
ulaire et Spectroscopies Avancees LCOSA, Ecole Europeenne de Chimie, Polymeres et Materiaux,
UMR 7515 associe
‡
́
́
̀
́
́
nne de Chimie,
̀
́
§
̀
́
⊥
̂
*
S Supporting Information
13
used in BHJ solar cells. To be efficient in such devices, the
dyes must fulfill at least the following requirements: (i) strong
optical absorption in the wavelength range from 500 to 900
nm; (ii) a sufficiently large energy difference between the
LUMO of the dye and the LUMO of the acceptor (such as
PCBM); (iii) a deep HOMO of the dye to maximize the open-
circuit voltage (VOC); (iv) a near-planar molecular structure to
favor local organization and a good charge carrier mobility in
ABSTRACT: Green-absorbing dipyrromethene dyes en-
gineered from bis-vinyl-thienyl modules are planar
molecules, exhibiting strong absorption in the 713−724
nm range and displaying comparable electron and hole
−3
2
mobilities in thin films (maximum value 1 × 10 cm /
V·s)). Bulk heterojunction solar cells assembled with
these dyes and a fullerene derivative (PC BM) at a low
(
6
1
ratio give a power conversion efficiency as high as 4.7%,
the film to maximize the short-circuit current (J ) and fill
SC
2
with short-circuit current values of 14.2 mA/cm , open-
factor (FF); (v) adequate solubilizing side chains to ensure
good filmability when blended with PCBM; and finally (vi) the
dyes must be robust and easily prepared.
circuit voltage of 0.7 V, and a broad external quantum
efficiency ranging from 350 to 920 nm with a maximum
value of 60%.
Planar BODIPYs engineered from vinyl-thiophene modules
and with various solubilizing chains at the periphery are
14
unknown but accessible with dedicated protocols. Such dyes
possess most of the attributes listed above to produce high-
performance solar cells.
lastic electronic devices provide prototypes for many
electronic applications, including information technology,
P
renewable energy harvesting, life sciences, and biomedicine
It is now well established that solubilizing side chains deeply
1
1
5,16
(
(
(
“bioships”). In this regard, organic field-effect transistors
impact organic materials’ semiconducting properties.
In the
2
OFETs) and solution-processed polymer bulk-heterojunction
present work, we have synthesized a new series of thienyl-
3
BHJ) solar cells have been widely investigated, with
BODIPY dyes (TB in Chart 1) and investigated their physical
n
impressive successes concerning ambipolar charge transport
and power conversion efficiency (PCE), respectively. In this
rapidly advancing field, small conjugated organic molecules,
and optoelectronic properties as well as their performances as
active materials in both field-effect transistors and organic solar
cells, with a special focus on the influence of the solubilizing
chains. The dyes were prepared by Knovenagel condensation of
4
molecular assemblies, and gelators are very attractive
candidates, as they can be provided pure and in large amounts
through reproducible and well-established synthetic protocols.
Thin films with good characteristics can easily be produced by
spin-coating, casting, or printing under ambient conditions.
Spectacular results have been obtained with various chemical
the aldehyde T with a tetramethyl BODIPY derivative under
n
17
established conditions.
Differential scanning calorimetry (DSC) measurements
indicate that TB is a semicrystalline material with reproducible
2
melting and crystallization thermal transitions (mp = 245 °C
and cp = 220 °C). The other molecules remain amorphous,
showing a melting transition only during the first heating ramp
(see SI).
3
c,d
5
frameworks,
of which oligothiophenes, diketopyrrolopyr-
6
7
8
9
role (DPP), squaraine, hexabenzo-coronenes, merocyanine,
donor−acceptor oxoindane, and thiadiazolo-bithienyl dyes
lie at the forefront owing to their high PCEs, reaching about 7%
10
11
In order to investigate the molecular structure along with the
12
packing structure, we grew single crystals of TB . X-ray
in the best case. Boron dipyrromethene (BODIPY) dyes are
characterized by outstanding chemical and photochemical
stabilities, redox activity, and optical features that can easily
be tailored by chemical transformation, allowing them to be
2
Received: August 7, 2012
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ja3072513 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Chart 1. Thiophene-BODIPYs (TB ) Studied Herein
n
Figure 2. Packing view down the ac direction. Molecules with the
same color code lie within the same layer. Dotted cyan lines indicate
I···F interactions. Green-colored rings indicate π−π stacking between
molecules at x, y, z and in an adjacent layer.
diffraction establishes that the TB framework substituents are
essentially coplanar with the near-planar BODIPY core
2
(
0
maximum deviation from the mean least-squares plane is
.17(1) Å for the boron atom; Figure 1). There is a slight
and 2 − x, 3 − y, −z. The centroid−centroid distances involving
a pyrrole group and a thiophene group range from 3.710(4) to
3
.954(5) Å.
The TB2−4 dyes in THF display two intense absorption
peaks in the 430−450 and 560−750 nm ranges with extinction
−1
−1
coefficients of about 100 000 M cm (Figure 3 and Table
Figure 1. ORTEP view of compound TB . Displacement ellipsoids are
2
drawn at the 50% probability level, and H atoms are shown as small
spheres of arbitrary radii. On the right is shown a perpendicular view
along the BODIPY axis, highlighting the planar structure and the
perpendicular iodophenyl fragment.
Figure 3. Absorption of TB (red trace) and TB (blue trace) in THF
solution and thin films (dashed traces).
1
2
S1). The more energetic transition is due to the styryl residues
curvature of the overall structure with respect to the BODIPY
while the less energetic is the S →S of the indacene unit with a
0
1
−1
core. The orthogonally attached (83.8°) iodophenyl group
clear vibronic structure (Δν = 1330 cm ). In contrast, the
absorption of dye TB is blue-shifted by 47 nm. This could be
(∠B1−C8−I1 > 173.3°) on one side and the mean plane
1
constituted by the two vinyl-bithiophene arms (B and C,
dihedral angle 9.3°) on the other lead to a distortion that is
partially compensated by bending of the hexyl chains.
Interestingly, the inner thiophene groups are head-to-head,
whereas the outer thiophenes are head-to-tail, with dihedral
angles between the two pairs of thiophene rings of 7.8° (B) and
explained by the less conjugated styryl monothiophene arms
with regard to the bithiophene styryl units of TB2−4. All TBn
dyes exhibit fluorescence in the near-infrared (Table S1). In all
cases the excitation spectra perfectly match the absorption
spectra, excluding the presence of aggregates. In thin films
(thickness of about 110 nm) the absorption is enlarged,
peaking at 720 and 780 nm respectively for TB and TB
1
7.1° (C). Arm C is curved toward arm B, with the shortest
1
2
contact being C16C···C11B = 3.83 Å. The two arms define an
(Figure 3) and indicating an optical band gap of about 1.60 and
1.45 eV, respectively. Cyclic voltammetry was used to
determine the HOMO/LUMO levels of the dyes in solution
after calibrating them with respect to PC BM and ferrocene.
2
ellipsoidal area of approximately 10 × 7 Å , into which
protrudes the iodo atom of the iodophenyl substituent of a
neighboring molecule along the b axis so as to make a short
contact to F (I···F = 3.13 Å) (Figure 2). These interactions
cross-link the (3 1 3) planes in which the inversion-related
molecules lie in chains with the double-thiophene arms side by
side in the [1 0 −1] direction, producing zipper-like motifs of
the iodobenzene groups. π−π stacking interactions between
layers involve overlap of both thiophene-vinylpyrrole moieties
of a molecule at general position x, y, z with the equivalent
inverted moiety from molecules at positions 1 − x, 3 − y, 1 − z
61
The TB dye displays two reversible oxidation waves at about
2
+0.64 and +0.82 V respectively, assigned in light of previous
results to the BODIPY radical cation and dication. The two
reversible reduction waves at about −0.89 and −1.62 V are
assigned to the BODIPY radical anion and dianion (Figure 4,
18
Table S2).
The bis-thienyl module is not electroactive within the +1.40
to −1.40 V window. PC BM and ferrocene were used as
61
B
dx.doi.org/10.1021/ja3072513 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
state (Figure 2). This behavior opens the way to a wide range
of applications such as electroluminescent OFETs and
complementary metal oxide semiconductor (CMOS) devices
20
and circuits.
BHJ devices were investigated using the dyes, blended with
PC BM, as the active layer. The standard device structure was
61
glass/ITO/PEDOT:PSS(∼40 nm)/dyes:PC BM/Al-
6
1
(
∼120 nm). Various dye:PC BM mass ratios and different
61
film thicknesses have been explored. The device current versus
voltage (J−V) curves have been measured in darkness and
under solar light (AM1.5G solar simulator at 100 mW/cm ) on
2
2
9
mm diodes. More details on the elaboration and character-
Figure 4. (Left) Position of the frontier molecular orbital energies
ization procedures can be found in the SI. The photovoltaic
versus PC BM and ferrocene for all dyes. The electrochemical gaps
parameters achieved for cells with an optimized dye:PC BM
61
61
were determined in solution and the optical gaps on thin films are
ratio are shown in Figure 6 for TB and are summarized in
2
given in parentheses. (Right) Cyclic voltammetry of TB , ferrocene,
2
Table 1.
and PC BM: (top) cathodic window; (bottom) anodic window.
61
internal references, enabling establishment of the frontier
molecular orbital energies for each dye with respect to the
electron acceptor (PC BM) (Figure 4). The electrochemical
6
1
gap calculated from the HOMO/LUMO ranges from 1.45 to
.50 eV for TB2−4 and is equal to 1.65 eV for TB , which
1
1
matches fairly well with the optical gap determined in the thin
film. The energy data indicate that all four dyes are suitable
candidates to be used as electron donors when blended with
PC BM in BHJ solar cells. Importantly, the low-lying HOMO
Figure 6. (a) J−V characteristics for the best TB photovoltaic cells
2
6
1
with an active layer obtained from chloroform solution with a TB2
level is desirable for a high V , while the energy offset of the
oc
concentration of 5 mg/mL (black) and from chlorobenzene solutions
dye LUMO and PC BM LUMO is about 0.23 V, a value close
6
1
with a TB concentration of 40 mg/mL and an Al cathode (green) or a
1
6
2
to the ideal case.
2
Ca/Al cathode (red) under standard AM1.5G (100 mW/cm )
Charge carrier mobilities in the dyes were measured by using
them as a semiconductor layer in standard bottom-contact
OFETs. The experimental procedure used for the device
elaboration and the mobility measurements are given in Table
S3.
irradiation. (b) IPCE spectrum corresponding to the best TB2
photovoltaic cells with an active layer obtained from chlorobenzene
solutions with a TB concentration of 40 mg/mL and an Al cathode.
2
Table 1. Photovoltaic Characteristics for Solution-Processed
BHJ Solar Cells under Optimized Conditions
TB exhibited rather distinctive behavior in comparison to
2
the other molecules. Indeed, an unexpected ambipolar
character was observed together with relatively high and
comparable electron and hole mobilities (Figure 5).
Voc
Jsc (mA/ FF
PCE
(%)
annealing temp,
2
dye TB:PCBM (V)
cm )
(%)
time (°C, min)
a
TB1
TB2
TB2
TB3
TB4
1:2
0.76
0.74
0.7
5.84
14.2
14.3
5.1
31
43
47
30
32
1.4
4.5
4.7
0.9
1.5
70, 20
100, 10
120, 20
−
b
c
1:0.8
1:0.5
a
1:2
0.56
0.55
a
1:1
8.5
80, 20
a
From chloroform solutions with a TB concentration of 5 mg/mL
n
b
(active layer thickness: 110 ± 10 nm). From chlorobenzene solutions
with a TB concentration of 40 mg/mL and an Al cathode (active layer
n
c
thickness: 165 ± 10 nm). From chlorobenzene solutions with a TBn
concentration of 40 mg/mL and a Ca(20 nm)/Al(120 nm) bilayer
cathode (active layer thickness: 165 ± 10 nm).
Figure 5. I −V plots characteristic for TB OFETs obtained for (a)
ds
ds
2
TB exhibits a higher V value, in agreement with a deeper
1
oc
holes and (b) electrons.
19
lying HOMO level. On the other hand, TB -based devices
2
shows FF values about 45%. Moreover, increasing the
The other molecules exhibit only hole transport with
TB :PC BM device thickness improves the device perform-
2
61
−4
2
significantly lower mobilities (1 × 10 cm /(V·s) for TB , 1
ances, while the opposite occurs for the other dyes (see SI).
1
−8
2
−7
2
2
×
10 cm /(V·s) for TB , and 6 × 10 cm /(V·s) for TB ).
Similar OFET measurements on TB gave hole and electron
mobility in the 1 × 10 cm /(V·s) range, i.e., up to 5 orders of
magnitude higher hole mobility with respect to TB . These
With a J of 14.2 mA/cm and a V as high as 0.74 V, a PCE of
3
4
sc
oc
up to 4.5% could be reached. The J value is consistent with the
2
sc
−3
2
broad external quantum efficiency (EQE), which reaches a
maximum value of 60% and expands from 350 to 920 nm while
remaining above 40% at the absorption minimum close to 550
nm (Figures 3 and 6b). The remarkably high Jsc value is a
3
high mobility values are in line with the high propensity of this
planar dye to organize in three dimensions with strong
intermolecular interactions (I···F and π−π-stacking) favoring
short distances between neighboring molecules in the solid
consequence of both the TB band gap (1.45 eV), which
2
matches the optimum value predicted by Shockley and
C
dx.doi.org/10.1021/ja3072513 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
2
1
Queisser for a single-junction solar cell, and the large
extinction coefficient of BODIPY dyes, which allows significant
photon absorption even within the low absorption region.
REFERENCES
■
(
1) Handbook of Organic Electronics and Photonics, Vols. 1−3; Nalwa,
H. S., Ed.; American Scientific Publishers: Los Angeles, CA, 2008.
(2) Chua, L. L.; Zumseil, J.; Chang, J. F.; Ou, E. C. W.; Ho, P. K. H.;
Sirringhaus, H.; Richard, R. H. Nature 2005, 434, 194−199.
(3) (a) Boudreault, P.-L. T.; Najari, A.; Leclerc, M. Chem. Mater.
Additionally, the relatively high V is a result of the low-lying
HOMO level of the dye, as may be expected from the
previously established correlation between the donor-HOMO/
acceptor-LUMO offset and Voc. The significantly lower
performances achieved when using TB or TB dyes, whose
frontier orbital energy levels are similar to those of TB , are
most likely due to the much lower hole mobilities, causing
inefficient charge separation and collection. The rapid drop in
device performances with increasing active layer thickness
corroborates this conclusion. An unfavorable morphology of
the dye:PC BM blends may also contribute to the poor device
oc
19
2
011, 23, 456−469. (b) Walker, B.; Kim, C.; Nguyen, T.-Q. Chem.
Mater. 2011, 23, 470−482. (c) Beaujuge, P. M.; Frechet, J. M. J. J. Am.
erle, P.
́
3
4
Chem. Soc. 2011, 133, 20009−20029. (d) Mishra, A.; Bau
̈
2
Angew. Chem., Int. Ed. 2012, 51, 2020−2067. (e) Zhou, H.; Yang, L.;
You, W. Macromolecules 2012, 45, 607−632.
(
4) (a) Prasanthkumar, S.; Saeki, A.; Seki, S.; Ajayaghosh, A. J. Am.
Chem. Soc. 2010, 132, 8866−8867. (b) Babu, S. S.; Prasanthkumar, S.;
Ajayaghosh, A. Angew. Chem., Int. Ed. 2012, 51, 1766−1776.
(5) Fitzner, R.; Reinold, E.; Mishra, A.; Mena-Osteritz; Ziehlke, H.;
61
operation in these cases. Atomic force microscopy measure-
ments performed on annealed blends (Figure S34) show that
TB -, TB -, and TB -based blends exhibit an amorphous
Ko
Uhrich, C.; Pfeiffer, M.; Bau
10.
6) Walker, B.; Tamayo, A. B.; Dang, X.-D.; Zalar, P.; Seo, J. H.;
Garcia, A.; Tantiwiwat, M.; Nguyen, T.-Q. Adv. Funct. Mater. 2009, 19,
063−3069.
7) (a) Wei, G.; Wang, S.; Sun, K.; Thompson, M. E.; Forrest, S. R.
̈
rner, C.; Leo, K.; Riede, M.; Weil, M.; Tsaryova, O.; Weiss, A.;
̈
erle, P. Adv. Funct. Mater. 2011, 21, 897−
9
(
1
3
4
surface morphology without noticeable macrophase separation.
On the other hand, TB -based annealed blends show clear
2
3
(
lamella-like features. These observations highlight again the
very specific behavior of TB2. It is likely that the enhanced
Adv. Eng. Mater. 2011, 2, 184−187. (b) Ajayaghosh, A. Acc. Chem. Res.
molecular ordering of TB is at the origin of the observed
2
2005, 38, 449−459.
relatively high charge carrier mobilities and thereby contributes
to the good device performances.
(8) Wong, W. W. H.; Singh, T. B.; Vak, D.; Pisula, W.; Yan, C.; Feng,
X.; Williams, E. L.; Chan, K. L.; Mao, Q.; Jones, D. J.; Ma, C.-Q.;
In order to increase the FF of the TB devices, we reduced
Mu
27−938.
9) Kronenberg, N. M.; Deppisch, M.; Wu
W. A.; Deing, K.; Meerholz, K. Chem. Commun. 2008, 6489−6491.
10) Burckstummer, H.; Tulyakova, E. V.; Deppisch, M.; Lenze, M.
R.; Kronenberg, N. M.; Gsanger, M.; Stolte, M.; Meerholz, K.;
Wurthner, F Angew. Chem., Int. Ed. 2011, 4506, 11628−11632.
11) Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.;
̈ ̈
llen, K.; Bauerle, P.; Holmes, A. B. Adv. Funct. Mater. 2010, 20,
2
9
(
the TB :PC BM weight ratio in agreement with comple-
2
61
̈
rthner, F.; Ladermann, H.
mentary OFET experiments (Figure S43) and added a 20 nm
thick Ca layer to improve the electrical contact to the cathode.
Together, the bilayer cathode and the lower 1:0.5 TB :PC BM
(
̈
̈
2
61
̈
content allowed us to slightly increase the FF and achieve a
PCE of 4.7%. It is very likely that further optimization of the
metal/organic interfaces and layer thicknesses may allow us to
enhance the FF and EQE values and improve the PCE further.
In summary, we have designed new green-absorbing dyes
based on thienyl-BODIPY frameworks, which provide a
maximum power conversion efficiency of 4.7% in solution-
processed BHJ solar cells. The planar bis-thienyl-BODIPY
derivatives favor short contact distances between neighboring
molecules in the solid state and exhibit high charge mobility
and an ambipolar behavior. Challenging studies are now in
progress to collect more efficiently the photons in the 500−620
nm range in order to further improve the device efficiency.
̈
(
Heeger, A. J. Nat. Mater. 2011, 11, 44−48.
(12) (a) Ziessel, R.; Ulrich, G.; Harriman, A. New J. Chem. 2007, 31,
496−501. (b) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int.
Ed. 2008, 47, 1184−1201.
(13) (a) Rousseau, T.; Cravino, A.; Bura, T.; Ulrich, G.; Ziessel, R.;
Roncali, J. Chem. Commun. 2009, 1673−1675. (b) Lin, H.-Y.; Huang,
W.-C.; Chou, H.-H.; Hsu, C.-Y.; Lin, J. T.; Lin, H. W. Chem. Commun.
2
(
012, 48, 8913−8915.
14) (a) Benniston, A. C.; Copley, G.; Harriman, A.; Ryan, R. J.
Mater. Chem. 2011, 21, 2601−2608. (b) Chen, Y.; Zhao, J.; Guo, H.;
Xie, L. J. Org. Chem. 2012, 77, 2192−2206. (c) Zhang, X.; Yu, H.;
Xiao, Y. J. Org. Chem. 2012, 77, 669−673 and references therein.
(
15) (a) Biniek, L.; Fall, S.; Chochos, C. L.; Anokhin, D. V.; Ivanov,
D. A.; Leclerc, N.; Leveque, P.; Heiser, T. Macromolecules 2010, 43,
779−9786. (b) Fall, S.; Biniek, L.; Leclerc, N.; Leveque, P.; Heiser, T.
Appl. Phys. Lett. 2012, 101, 123301-1−123301-4.
16) Chu, T. Y.; Lu, J.; Beaupre, S.; Zhang, Y.; Pouliot, J. R.; Zhou, J.;
Najari, A.; Leclerc, M.; Tao, Y. Adv. Funct. Mater. 2012, 22, 2345−
351.
17) Ziessel, R.; Bura, T.; Olivier, J.-H. Synlett 2010, 2304−2310.
(18) Ziessel, R.; Bonardi, L.; Retailleau, P.; Ulrich, G. J. Org. Chem.
ASSOCIATED CONTENT
Supporting Information
́
̂
■
9
́
̂
*
S
General methods, synthetic experimental part, absorption and
fluorescence spectra, traces NMR spectra, DSC traces,
electrochemical data, devices preparation, and mobilities. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(
́
2
(
2
006, 71, 3093−3102.
AUTHOR INFORMATION
Corresponding Author
ziessel@unistra.fr
(19) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater. 2009, 21,
323−1338.
■
1
(
20) See, for instance: Anthopoulos, T. D.; Setayesh, S.; Smits, E.;
̈
Colle, M.; Cantore, E.; de Boer, B.; Blom, P. W. M.; de Leeuw, D. M.
Notes
Adv. Mater. 2006, 18, 1900−1904.
(21) Shockley, W.; Queisser, H. J. J. Appl. Phys. 1961, 32, 510−519.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(
We thank the Centre National de la Recherche Scientifique
CNRS) for financial support of this work. N. Zimmermann is
kindly acknowledged for is help in photovoltaic devices
elaboration.
D
dx.doi.org/10.1021/ja3072513 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX