Small molecule BODIPY dyes as non-fullerene acceptors in bulk heterojunction organic photovoltaics

Article abstract of DOI:10.1039/c3cc49648a

A series of acceptor-donor-acceptor molecules containing terminal BODIPY moieties conjugated through the meso position were synthesized. Deep LUMO energy levels and good visible absorption led to their use as acceptors in bulk heterojunction solar cells. Inverted devices were fabricated, reaching efficiencies as high as 1.51%.

Full text of DOI:10.1039/c3cc49648a

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Small molecule BODIPY dyes as non-fullerene
acceptors in bulk heterojunction organic
photovoltaics†
Cite this: Chem. Commun., 2014,
50, 2913
Received 20th December 2013,
Accepted 21st January 2014
Ambata M. Poe,a Andrea M. Della Pelle,a Ayyagari V. Subrahmanyam,a
William White,a Guillaume Wantzb and S. Thayumanavan*a
DOI: 10.1039/c3cc49648a
www.rsc.org/chemcomm
A series of acceptor–donor–acceptor molecules containing terminal is conjugated through its meso position using a 3-hexylthiophene linker
BODIPY moieties conjugated through the meso position were synthe- to 4,8-bis(5-(2-ethylhexyl) thiophen-2-yl)benzo[1,2-b:4,5-b0]dithiophene,
sized. Deep LUMO energy levels and good visible absorption led to their 4,4-bis(2-ethylhexyl)-4H-cyclopenta-[2,1-b:3,4-b]dithiophene or
use as acceptors in bulk heterojunction solar cells. Inverted devices N-(2-ethylhexyl)-dithieno [3,2-b:20,30-d]pyrrole to afford BDP-BDT,
were fabricated, reaching efficiencies as high as 1.51%.
BDP-CPDT, and BDP-DTP respectively (Scheme 1). These molecules
show strong visible absorption with low lying LUMO levels, making
The field of organic photovoltaics (OPV) has advanced in recent years them electronically suitable as acceptors for many donor materials.
through extensive investigation into various conjugated polymer and To examine their potential as acceptors in BHJ OPVs, devices were
small molecule structures. Most of these accomplishments have been fabricated using P3HT as the donor.
focused on optimizing donors in bulk heterojunction (BHJ) organic
Syntheses of the molecules include the formation of a terminal
solar cells, where the acceptor is primarily based on fullerenes.1 unit composed of a BODIPY core substituted at the meso position with
While fullerene derivatives have several obviously attractive features a brominated hexylthiophene, providing an additional solubilizing
for BHJ OPV devices,2 the search for non-fullerene acceptors has group and a functional handle on the a-position of the thiophene for
gained significant attention in recent years, mainly driven by the subsequent palladium catalyzed cross coupling reactions. The lack of
possibility that the new acceptors could exhibit stronger absorption in b-substituents on the BODIPY moiety is expected to greatly benefit the
the visible region of the electromagnetic spectrum.3 Concurrently, overall planarity of the molecule due to the limited steric interactions
there has also been a surge in the development of small molecules as that often force a twist between the thiophene and the BODIPY cap.7
active materials for OPVs, because of the reproducibility and the This molecule is synthesized through Liebeskind–Srogl coupling
potential for frontier orbital energy level controls that they offer.1c,3c between 8-(thiomethyl)-BODIPY and 2-bromo-3-hexylthiophene-5-
The combination of these two interests has triggered the search for boronic acid.8 Stille coupling between the bistannylated donors
small molecule non-fullerene acceptors for BHJ OPV.1b,3b,4
and the previously synthesized terminal unit afforded the targeted
Considering that 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene molecules in good yields. For detailed syntheses and characterization,
(BODIPY) based molecules exhibit a strong absorption in the visible refer to the ESI.†
region (>500 nm) and variable redox chemistry, we conceived the use of
these molecules as acceptors in BHJ OPV devices.5,6 Specifically, we
hypothesized that acceptor–donor–acceptor (A–D–A) type molecules
with BODIPY as the acceptor moiety will provide convenient access
to low band-gap small molecules that could be tested as acceptors in
BHJ devices. Herein, we report three such molecules, in which BODIPY
a Department of Chemistry, University of Massachusetts, Amherst,
Massachusetts 01003, USA. E-mail: thai@chem.umass.edu; Fax: +1-413-545-4490
b
´
Universite de Bordeaux, CNRS, IMS UMR 5218,
Institut Polytechnique de Bordeaux, ENSCBP, 33607 Pessac, France.
E-mail: guillaume.wantz@ims-bordeaux.fr
† Electronic supplementary information (ESI) available: Detailed synthetic pro-
cedures, NMR spectra, cyclic voltammograms, OFET device preparation proce-
dure and output and transfer characteristics, photovoltaic device preparation
procedure and related data. See DOI: 10.1039/c3cc49648a
Scheme 1 Molecular structures of the BODIPY-based A–D–A molecules.
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HOMO energy levels (Table 1). Decreasing the donor strength of the
core moiety stabilized the HOMO energy level, as the weakest donor
(BDP-BDT) possesses the most stable HOMO (ꢀ5.40 eV). Ered values
are similar throughout the series, indicating that the reduction of
the BODIPY moiety within the molecule is relatively unaffected by
the donor strength of the core moiety. Since each of these molecules
possesses a LUMO significantly lower than that of P3HT, exciton
dissociation is thermodynamically possible, allowing them to be
considered as candidates for acceptors in BHJ devices.
To investigate the charge transport properties of these molecules,
bottom contact organic field effect transistors (OFET) were fabri-
cated. Output and transfer characteristics are displayed and
summarized in the ESI.† All molecules exhibit only n-type character-
Fig. 1 Solution (left) and thin film (right) absorption spectra of BDP-BDT,
BDP-CPDT and BDP-DTP.
Donor incorporation through the a and b positions of the istics with mobilities on the order of 10ꢀ5 cm2 Vꢀ1 sꢀ1 (Table S1,
BODIPY core is known to extend conjugation and, in turn, red shift ESI†). Of the devices, BDP-CPDT exhibited the highest electron
the absorption and reduce the band gap of the resulting system. mobility (5.77 ꢁ 10ꢀ5 cm2 Vꢀ1
s
ꢀ1), while that of BDP-BDT was
However, conjugation through the meso position does not typically the lowest (3.30 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1). Although the directionality of
provide the same dramatic shift in absorption due to minimal charge transport in OPV devices and OFET devices is orthogonal, the
communication between the core and terminal BODIPY in the consistent n-type transport in these BODIPY-capped molecules does
ground state.9 Consequently, through selective tuning of the core, a suggest that this functionality could be a promising building block
system possessing a predictable and well-defined absorption profile of low band gap, small molecule acceptors for OPVs.
with significant absorbance within the visible region of the solar
Inverted OPV devices were fabricated with the architecture
spectrum can be synthesized with ease. This feature is important ITO/ZnO/P3HT:acceptor (40 nm)/MoO3 (7 nm)/Ag (100 nm). The
for tuning the absorption properties of the acceptor to complement active layer was spun cast from 15 mg mlꢀ1 solutions in o-dichloro-
the donor material with which it is paired in BHJ devices.
benzene (o-DCB). An optimal D : A ratio was determined to be 1 : 1.5
Fig. 1 shows the UV-visible absorption spectra of chlorobenzene for each acceptor (ESI†) with BDP-BDT, BDP-CPDT and BDP-DTP
solutions and thin films spun from dichloromethane solutions; the yielding efficiencies of 1.21%, 1.02% and 0.84%, respectively.
optical properties are summarized in Table 1. The three molecules To assess whether the light absorption characteristics at wave-
exhibit an absorption maximum that is characteristic of the p–p* lengths complementary to that of P3HT contribute to the overall
transition in the BODIPY moiety,10 with an extinction coefficient on efficiency, we obtained the external quantum efficiency (EQE)
the order of 104 Mꢀ1 cmꢀ1 (Table 1). This absorption slightly red for these devices (Fig. 2). EQE measurements show that the
shifts in thin films. All molecules also exhibit a lower energy devices containing acceptors with red shifted charge transfer
shoulder that is attributed to an intramolecular charge transfer bands (BDP-CPDT, BDP-DTP) significantly benefit from the
process, as its position in both the solution and solid states varies
with the donor strength of the core moiety. While this band exists as
a shoulder in BDP-BDT, both BDP-CPDT and BDP-DTP possess
significantly red shifted peaks with absorption onsets at 805 and
845 nm respectively. The significant shift in the latter two molecules
is attributed to the enhanced donor strength of the core moiety.11
The optical band gaps for these molecules were estimated from the
absorption onset in thin films (Table 1).
Next, the electrochemical properties of the molecules were
examined using cyclic voltammetry.12 Each molecule shows ampho-
teric behavior, exhibiting two reversible oxidations and at least one
reduction (Fig. S1, ESI†). The onset of the first oxidation potential for
BDP-CPDT and BDP-DTP are very similar, leading to comparable
Fig. 2 J–V curves (left) and EQE spectra (right) of devices with various
acceptors where the P3HT : acceptor ratio is 1 : 1.5.
Table 1 Optical and electrochemical properties of synthesized molecules
solution
film
max
film
l
(nm)
l
(l
(nm)
(nm))b
max
(e (ꢁ 104 Mꢀ1 cmꢀ1))a
Eogptc (eV)
Eox d (V)
HOMOe (eV)
Ered d (V)
LUMO f (eV)
onset
BDP-BDT
BDP-CPDT
BDP-DTP
518 (6.2)
516 (7.6)
516 (2.5)
532 (717)
530 (805)
532 (845)
1.73
1.54
1.47
0.60
0.36
0.34
ꢀ5.40
ꢀ5.16
ꢀ5.14
ꢀ0.91
ꢀ0.98
ꢀ1.06
ꢀ3.79
ꢀ3.82
ꢀ3.74
a
film
Measured in chlorobenzene solutions. b Determined from thin films spun cast from dichloromethane solutions and annealed at 150 1C. c Egopt = 1240/l
.
onset
d Determined from the onset of oxidation and reduction extracted from cyclic voltammograms measured in anhydrous dichloromethane with 0.1 M Bu4NPF6
as the supporting electrolyte and corrected with respect to the Fc/Fc+ redox couple. e HOMO = ꢀ(Eox + 4.8). f LUMO = ꢀ(Ered + 4.8).
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Table
2
Photovoltaic cell performance (ITO/ZnO/P3HT : acceptor
morphological studies of the blends are currently underway. Also, we
are further investigating the effect of both core and cap modifications
on the electronic properties of the resulting materials for optimal
pairing with donors other than P3HT. Through fine-tuning of the
molecular structure vs. device performance and morphological char-
acteristics, we hope to achieve a library of BODIPY based electron
acceptors with robust electronic properties for OPV applications.
(1 : 1.5)/MoO3/Ag) with various amounts of the CN solvent additive
Acceptor
CN (vol%) VOC (V) JSC (mA cmꢀ2
)
FF
PCE (%)
BDP-BDT
0
1
3
5
0.65
0.39
0.57
0.14
3.09
2.73
3.20
3.03
0.60 1.21
0.30 0.33
0.56 1.01
0.26 0.11
´ ´
´
We thank Frederic Guillain, Lionel Derue, Clemence Lecourtier
and Elodie Destouesse of the ELORGA research team of the IMS
laboratory at the University of Bordeaux for their help with photo-
voltaic device fabrication. We acknowledge the support from the
U.S. Department of Energy through the EFRC at UMass Amherst
(DE-SC0001087). We also thank the National Science Foundation for
partial support through the Collaborative Undergraduates Research
in Energy (CURE) REU program to WW, along with the cluster of
excellence LabEx AMADEus of the University of Bordeaux and the
Northeast Alliance for Graduate Education and the Professoriate
(NSF9978878) to AMP.
BDP-CPDT
BDP-DTP
0
1
3
5
0.60
0.60
0.62
0.61
3.28
3.04
3.90
3.67
0.61 1.20
0.61 1.11
0.63 1.51
0.63 1.41
0
1
3
5
0.60
0.56
0.57
0.57
2.18
2.64
3.28
1.60
0.65 0.84
0.62 0.92
0.63 1.18
0.62 0.56
Active layers spun cast from 1,2-dichlorobenzene solutions (15 mg mlꢀ1
at 1500 RPM for 60 s and annealed at 150 1C for 2 min.
)
acceptor absorption above 650 nm. The weaker performance of the
BDP-DTP molecule is due to the relatively low JSC (2.18 mA cmꢀ2),
which is likely due to non-optimal active layer morphology. Use of
solvent additives has improved the efficiency of PCBM-based BHJ
devices, which has been attributed to the ability of certain solvents to
promote PCBM aggregation by selectively solubilizing the acceptor.13
Since these acceptors have good solubility in most organic solvents,
we decided to use 1-chloronaphthalene (CN) as a solvent additive.
The J–V curves and EQE spectra are shown in the ESI;† the device
characteristics are listed in Table 2.
Notes and references
1 (a) Z. He, C. Zhong, S. Su, M. Xu, H. Wu and Y. Cao, Nat. Photonics,
2012, 6, 593; (b) C. L. Chochos and S. A. Choulis, Prog. Polym. Sci.,
2011, 36, 1326; (c) Y. Lin, Y. Li and X. Zhan, Chem. Soc. Rev., 2012,
41, 4245; (d) Y. Li, Acc. Chem. Res., 2012, 45, 723; (e) L. Huo, S. Zhang,
X. Guo, F. Xu, Y. Li and J. Hou, Angew. Chem., Int. Ed., 2011, 50, 9697;
( f ) H. Zhou, L. Yang and W. You, Macromolecules, 2012, 45, 607.
2 (a) B. C. Thompson and J. M. Frechet, Angew. Chem., Int. Ed., 2008,
47, 58; (b) T. Liu and A. Troisi, Adv. Mater., 2013, 25, 1038.
3 (a) Y. Zhou, Y. Z. Dai, Y. Q. Zheng, X. Y. Wang, J. Y. Wang and J. Pei, Chem.
Commun., 2013, 49, 5802; (b) J. T. Bloking, X. Han, A. T. Higgs, J. P. Kastrop,
´
The use of CN as a solvent additive had positive effects on the BDP-
CPDT and BDP-DTP devices, as the device efficiency increased to 1.51%
and 1.18%, respectively, when 3% CN was added. However, CN had a
detrimental effect on BDP-BDT devices, yielding decreased VOC and FF
values (Table 2). To examine the effect of CN on the morphology of the
P3HT:BDP-CPDT devices, atomic force microscopy (AFM) was used.
AFM height and phase images of the P3HT:BDP-CPDT active layer are
shown in Fig. S9 (ESI†). Addition of 3% CN increased the surface
roughness from 5.9 nm to 7.2 nm; the additive-based device also shows
a much more uniform and well-ordered network, which is desired for
efficient charge transport.
L. Pandey, J. E. Norton, C. Risko, C. E. Chen, J.-L. Bredas, M. D.
McGehee and A. Sellinger, Chem. Mater., 2011, 23, 5484; (c) C. L. Chochos,
N. Tagmatarchis and V. G. Gregoriou, RSC Adv., 2013, 3, 7160.
4 (a) Y. Zhou, L. Ding, K. Shi, Y. Z. Dai, N. Ai, J. Wang and J. Pei, Adv.
Mater., 2012, 24, 957; (b) Y. Lin, P. Cheng, Y. Li and X. Zhan, Chem.
Commun., 2012, 48, 4773; (c) J. E. Anthony, Chem. Mater., 2011,
23, 583; (d) A. F. Eftaiha, J.-P. Sun, I. G. Hill and G. C. Welch,
J. Mater. Chem. A, 2014, 2, 1201.
5 A. C. Benniston and G. Copley, Phys. Chem. Chem. Phys., 2009, 11, 4124.
6 Y. Hayashi, N. Obata, M. Tamaru, S. Yamaguchi, Y. Matsuo, A. Saeki,
S. Seki, Y. Kureishi, S. Saito, S. Yamaguchi and H. Shinokubo, Org.
Lett., 2012, 14, 866.
7 (a) A. C. Benniston, G. Copley, A. Harriman, D. B. Rewinska, R. W.
Harrington and W. Clegg, J. Am. Chem. Soc., 2008, 130, 7174; (b) C.-C. Chi,
Y.-J. Huang and C.-T. Chen, J. Chin. Chem. Soc., 2012, 59, 305.
8 (a) E. Lager, J. Liu, A. Aguilar-Aguilar, B. Z. Tang and E. Pena-Cabrera,
J. Org. Chem., 2009, 74, 2053; (b) E. Pena-Cabrera, A. Aguilar-Aguilar,
M. Gonzalez-Dominguez, E. Lager, R. Zamudio-Vazquez, J. Godoy-Vargas
and F. Villanueva-Garcia, Org. Lett., 2007, 9, 3985.
9 (a) J. C. Forgie, P. J. Skabara, I. Stibor, F. Vilela and Z. Vobecka,
Chem. Mater., 2009, 21, 1784; (b) B. C. Popere, A. M. Della Pelle,
A. Poe, G. Balaji and S. Thayumanavan, Chem. Sci., 2012, 3, 3093;
(c) E. H. A. Beckers, P. A. van Hal, A. Dhanabalan, S. C. J. Meskers,
J. Knol, J. C. Hummelen and R. A. J. Janssen, J. Phys. Chem. A, 2003,
107, 6218.
In summary, a series of n-type small molecules consisting of a
thiophene-based donor core conjugated to a BODIPY cap through the
meso position were synthesized. These molecules exhibited good
visible absorption with a common peak between 530 and 532 nm
in thin films, independent of the donor core. Introduction of a
stronger donor (CPDT, DTP) to the core of the molecule led to
a significant red shift in the charge transfer band, resulting in an
optical band gap as low as 1.47 eV. Each of the molecules possessed a
low-lying LUMO, between ꢀ3.7 eV and ꢀ3.9 eV, and a decent electron
10 (a) H. Y. Lin, W. C. Huang, Y. C. Chen, H. H. Chou, C. Y. Hsu,
J. T. Lin and H. W. Lin, Chem. Commun., 2012, 48, 8913; (b) S. Yin,
V. Leen, C. Jackers, D. Beljonne, B. Van Averbeke, M. Van der
Auweraer, N. Boens and W. Dehaen, Chem.–Eur. J., 2011, 17, 13247.
11 D. Collado, J. Casado, S. Rodriguez Gonzalez, J. T. Lopez Navarrete,
R. Suau, E. Perez-Inestrosa, T. M. Pappenfus and M. M. Raposo,
Chem.–Eur. J., 2011, 17, 498.
mobility, B10ꢀ5 cm2 Vꢀ1 ꢀ1, making them potential acceptors for
s
many donor materials. Inverted bulk heterojunction photovoltaic
devices were successfully fabricated, combining these molecules
with P3HT as the donor. The P3HT:BDP-BDT active layer yielded
an efficiency of 1.21%. The use of 1-chloronaphthalene as a solvent 12 C. M. Cardona, W. Li, A. E. Kaifer, D. Stockdale and G. C. Bazan, Adv.
Mater., 2011, 23, 2367.
13 (a) M. T. Dang, L. Hirsch and G. Wantz, Adv. Mater., 2011, 23, 3597;
additive improved the device performance in BDP-CPDT and BDP-
DTP based devices, with the best performing cell yielding a PCE of
(b) J. Peet, A. J. Heeger and G. C. Bazan, Acc. Chem. Res., 2009,
1.51% (P3HT:BDP-CPDT + 3% CN). Further device optimization and
42, 1700; (c) A. Facchetti, Chem. Mater., 2011, 23, 733.
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Chem. Commun., 2014, 50, 2913--2915 | 2915