Structural modulation of internal charge transfer in small molecular donors for organic solar cells

Article abstract of DOI:10.1039/c2cc33921h

Donor-acceptor molecules with small chain extension have been synthesized and used as active material in organic solar cells. The effect of fusion of a phenyl group on the end dicyanovinylene acceptor is discussed.

Full text of DOI:10.1039/c2cc33921h

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Structural modulation of internal charge transfer in small molecular
donors for organic solar cells
Antoine Leliège, Charles-Henri Le Régent, Magali Allain, Philippe Blanchard* and Jean Roncali*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The synthesis of compounds 119 and 2 is shown in Scheme 1.
Donor-acceptor molecules with small chain extension have
50 A Stille coupling between the commercial bromo derivative of
triphenylamine (TPA) (5) and 2-(tri-n-butylstannyl)thiophene
gave compound 4 (52% yield) which was subsequently subjected
to Vilsmeir formylation to give aldehyde 3 in 74% yield. The
been synthesized and used as active material in organic solar
cells. The effect of fusion of a phenyl group on the end
dicyanovinylene acceptor is discussed.
target compound
55 Knoevenagel condensation of 3 with malononitrile.
1 was then obtained in 75% yield by
10 Organic photovoltaic cells (OPV) offer the possibility to develop
lightweight and cost effective solar cells by means of low
environmental impact technology. Although low band gap
processable conjugated polymers remain a major class of donor
materials for OPV,1-4 the past few years have witnessed the
15 growing importance acquired by donors based on small
molecules.5,6 Besides the obvious advantages of perfectly defined
chemical structures in terms of reproducibility of synthesis,
purification and performances, molecular donors allow a more
straightforward analysis of structure-properties relationships than
20 polydisperse polymers. In recent years this approach has
generated a huge amount of work and many new classes of
molecular donors have been synthesized and evaluated.5-18
The structure of efficient molecular donors generally involves
a combination of donor and acceptor blocks in order to create an
25 internal charge transfer (ICT) which produces at the same time an
extension of the absorption spectrum towards longer wavelengths
and a decrease of the HOMO level which results in an increase of
the open-circuit voltage (Voc) of the OPV cell.7 Although
interesting results have been obtained with different classes of
30 molecular
donors5-18
such
as
triphenylamines,7,8
oligothiophenes,9-12 squaraines,13 diketopyrrolopyrroles14 or
borondipyrromethenes,15 these materials are in general based on
rather large molecules of high molecular weight.
Scheme 1 Synthesis of compounds 1 and 2
A Stille coupling of the stannyl compound 6, obtained by
reacting bromo-TPA (5) with n-BuLi at low temperature followed
60 by addition of Bu3SnCl,20 with the key bromo derivative 7 (see
ESI) in the presence of a palladium catalyst gave the target
compound 2 in 87% yield.
The UV-Vis absorption spectrum of 1 shows two maxima in
the 300-400 nm region and an intense absorption band with a
65 maximum at 501 nm assigned to an ICT band (Fig. 1).7
Comparison with the spectrum of 2 shows that introduction of a
fused phenyl ring in the acceptor block produces a red shift of the
maximum of the ICT band to 610 nm together with a decrease of
its relative intensity with a ca twofold decrease of the molecular
70 absorption coefficient () (Table 1).
In this context, the question of the size of the donor molecule
35 appears as an interesting problem. In fact, recent work has shown
that interesting photovoltaic performances could be obtained with
molecular donors of limited spatial extension and molecular
weight.17,18 In fact, the question of the minimal size of the donor
is not trivial and could have important implications regarding
40 some economic aspects of future industrial production of active
materials for OPV
We report here preliminary results on small molecular donors
1 and 2 combining a triphenylamine donor block (TPA) with a
dicyanovinyl thiophene electron acceptor group. In an attempt to
45 modulate the characteristics of the ICT,7 the dicyanovinyl group
has been fused to the thiophene by a phenyl ring (2). The
synthesis of these compounds is described and a comparative
analysis of their performances in simple OPV cells is presented.
Thermal analysis by DSC and TGA measured at 10°C/min
under N2 shows that the bridging of the acceptor unit in 2 leads to
a significant increase of melting point (127-129°C for 1 and 249-
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251°C for 2) and improves thermal stability as deduced from the
huge decrease of the LUMO level of the molecule (Table 1). This
decomposition temperatures determined at 5% weight loss
(292°C for 1 and 334°C for 2, see ESI).
effect could be related to the stabilization of the anion-radical of 2
by aromatization of the cyclopentadiene ring.
35
Compounds 1 and 2 have been used as donor material in
bilayer planar heterojunction solar cells. The cells of 0.28 cm2
area were fabricated using the already described procedures20 (see
ESI). In spite of the absence of solubilising groups, the donors
were soluble enough for solution-processing. The donor layer of
40000
30000
20000
10000
0
40 35 nm thickness was spun-cast from chloroform solutions of 1 or
2 (10 mg/mL) on ITO substrates pre-coated by a 40 nm layer of
PEDOT:PSS. A 30 nm layer of C60 was then deposited by
thermal evaporation under vacuum and the devices were
completed by vacuum deposition of a 100 nm aluminium
45 electrode.
50
40
30
20
10
0
300
400
500
600
700
800
Wavelength (nm)
5
Fig. 1 UV-Vis absorption spectra of 1 (solid line) and 2 (dotted line) in
CH2Cl2
The crystal structure of compound 2 shows that except for the
two phenyl groups of the TPA unit, the conjugated system adopts
a planar geometry, whereas the molecular packing exhibits a
10 head-to-tail arrangement as generally observed for strongly
dipolar molecules17b,17c,18 with a distance of 3.40 Å between the
planes of two neighbouring molecules (Fig. 2).
400
500
600
700
800
900
Wavelength (nm)
Fig. 3 External quantum efficiency spectra of the bilayer cells based on
donor 1 (solid line) and 2 (dotted line)
Fig. 3 shows the external conversion efficiency (EQE) spectra
50 of the two types of cells under monochromatic irradiation. The
spectrum of the cell based on donor 1 shows a broad wave
extending from 350 to 650 nm with a maximum of 45% around
530 nm. For the cell based on 2 the spectrum shows a first
maximum at ca 460 nm followed by a second broad band
55 extending from 550 to ca 820 nm. The EQE spectrum is
consistent with the red shifted ICT band observed in the UV-Vis
spectrum of donor 2 (Fig. 1), although the relative contribution of
the ICT band to the photocurrent is smaller than expected. The
current density vs voltage curves recorded in the dark and under
60 simulated solar irradiation in AM 1.5 conditions are shown in
Fig. 4. The cell based on donor 1 gave a short-circuit current
density (Jsc) of 5.03 mA cm-2 and a Voc of 0.85 V. Combined with
a filling factor (FF) of 43%, these data lead to a power
conversion efficiency (PCE) of 2.04%. A five minutes annealing
65 at 120°C produced a substantial increase of Jsc and Voc leading to
a PCE of 2.53 % (Table 2).
15
Fig. 2 Crystallographic structure of compound 2
Cyclic voltammetry performed in CH2Cl2 in the presence of
Bu4NPF6 as supporting electrolyte shows that compounds are
reversibly oxidized into the corresponding cation-radical with
identical anodic peak potential (Epa) of 1.00 V vs SCE (see ESI,
20 Fig. S4).
Table 1 UV-Vis absorptiona and cyclic voltammetricb data for
compounds 1 and 2
b
b
c
c
d
a
a
Compd
Epa Epc EHOMO ELUMO
eV eV
Eg
eV
maxICT maxICT
E
eV
V
V
nm
501
610
M-1cm-1
1
2
33900 1.00 -1.23 -5.96 -3.79 2.17 2.03
16200 1.00 -0.80 -5.96 -4.22 1.74 1.59
The results obtained with donor 2 show that the as-fabricated
cell presents the same Voc of 0.85 V, as expected from the
identical HOMO level, but smaller initial value of Jsc, FF and
70 PCE. However, a five minutes thermal treatment produces a large
increase of these values with a PCE close to 3.00%.
a in CH2Cl2; b 1 mM in 0.10 M Bu4NPF6/CH2Cl2, 100 mV s-1, Pt working
electrode, Ref. SCE; c using an offset of -4.99 eV vs vacuum level for
25 SCE; 21 d from the low energy absorption edge of films spun-cast on glass
(see ESI, Fig. S5).
To summarize we have shown that the covalent bridging of the
Compound 1 undergoes an irreversible reduction with a
cathodic peak potential Epc at -1.23 V while compound 2 is
30 reversibly reduced with an Epc of -0.80 V. This 430 mV positive
shift of Epc shows that the fusion with a phenyl ring produces a
dicyanovinyl thiophene acceptor group by
a phenyl ring
represents an efficient synthetic approach for engineering the ICT
75 and the LUMO energy level of donor materials based on dipolar
small donor-acceptor molecules.
2
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4
35
R. Po, M. Maggini and N. Camaioni, J. Phys. Chem. C, 2010, 114,
695.
5
J. Roncali, P. Frère, P. Blanchard, R. de Bettignies, M. Turbiez, S.
Roquet, P. Leriche, Y. Nicolas, Thin Solid Films, 2006, 511–512,
567.
6
For reviews see: (a) J. Roncali, P. Leriche and A. Cravino, Adv.
Mater., 2007, 19, 2045; (b) J. Roncali, Acc. Chem. Res., 2009, 42,
1719; (c) W. Tang, J. Hai, Y. Dai, Z. Huang, B. Lu, F. Yuan, J. Tang
and F. Zhang, Solar Energy Materials & Solar Cells, 2010, 94, 1963;
(d) B. Walker, C. Kim and T.-Q. Nguyen, Chem. Mater. 2011, 23,
470; (e) A. Mishra and P. Bäuerle, Angew. Chem. Int. Ed., 2012, 51,
2020.
40
45
7
S. Roquet, A. Cravino, P. Leriche, O. Alévêque, P. Frère and J.
Roncali, J. Am. Chem. Soc., 2006, 128, 3459.
8
H. Shang, H. Fan, Y. Liu, W. Hu, Y. Li, X. Zhan, Adv. Mater., 2011,
23, 1554.
50 9 (a) C. Uhrich, R. Schueppel, A. Petrich, M. Pfeiffer, K. Leo, E. Brier,
P. Kilickiran, P. Bäuerle, Adv. Funct. Mater., 2007, 17, 2991; (b) K.
Schulze, M. Riede, E. Brier, E. Reinold, P. Bäuerle and K. Leo, J.
Appl. Phys., 2008, 104, 074511.
10 (a) Y. Liu, X. Wan, F. Wang, B. Yin, J. Zhou, G. Long, S. Yin and
55
60
65
Y. Chen, J. Mater. Chem. 2010, 20, 2464; (b) Y. Liu , X. Wan , F.
Wang , J. Zhou , G. Long , J. Tian and Y. Chen, Adv. Mater., 2011,
23, 5387; (c) Z. Li, G. He, X. Wan, Y. Liu, J. Zhou, G. Long, Y. Zuo,
M. Zhang and Y. Chen, Adv. Energy Mater., 2012, 2, 74.
11 Y. Sun, G. C. Welch, W. L. Leong, C. J. Takacs, G. C. Bazan and A.
J. Heeger, Nat. Mater., 2011, 11, 44.
Fig. 4 Current density vs voltage curves for thermally annealed bilayer
cells donor/C60 (s = 0.28 cm2), in the dark () and under white light
illumination at 90 mW cm-2 (). Donor 1 (top), donor 2 (bottom)
12 P. F. Xia, X. J. Feng, J. Lu, S.-W. Tsang, R. Movileanu, Y. Tao and
M. S. Wong, Adv. Mater., 2008, 20, 4810.
5 Table 2 Electrical characteristics of bilayer heterojunction solar cells
13 (a) F. Silvestri, M. D. Irwin, L. Beverina, A. Facchetti, G. A. Pagani
and T. J. Marks, J. Am. Chem. Soc., 2008, 130, 17640; (b) S. Wang,
L. Hall, V. V. Diev, R. Haiges, G. Wei, X. Xiao, P. I. Djurovich, S.
R. Forrest and M. E. Thompson, Chem. Mater., 2011, 23, 4789.
14 B. Walker, A. B. Tamayo, X.-D. Dang, P. Zalar, J. H. Seo, A. Garcia,
M. Tantiwiwat and T.-Q. Nguyen, Adv. Funct. Mater., 2009, 19,
3063.
under AM 1.5 simulated solar irradiation at 90 mW cm-2
Jsc
(mA cm-2)
5.03
5.77
3.57
Voc
(V)
0.85
0.92
0.85
0.97
FF
(%)
43
42
41
PCE
(%)
2.04
2.53
1.37
2.97
Compd
1
1a
2
70 15 (a) T. Rousseau, A. Cravino, T. Bura, G. Ulrich, R. Ziessel and J.
Roncali, Chem. Commun., 2009, 1673; (b) T. Rousseau, A. Cravino,
E. Ripaud, P. Leriche, S. Rihn, A. De Nicola, R. Ziessel and J.
Roncali, J. Chem. Commun., 2010, 46, 5082.
2a
5.32
52
a After 5 minutes annealing at 120°C.
16 J.A. Mikroyannidis, A.N. Kabanakis, P. Balraju and G. D. Sharma,
Taking into account for the simplicity of the devices used for
10 evaluation, these preliminary results suggest that the use of C70 as
acceptor and of more advanced technology for device fabrication
can be expected to lead to higher performances. Work aiming at
the extension of these ideas is now in progress and will be
reported in future publications.
75
80
85
90
95
Langmuir, 2010, 26, 17739.
17 (a) N. M. Kroneneberg, M. Deppisch, F. Würthner, H. W. A.
Lademann, K. Deing and K. Meerholz, Chem. Commun. 2008, 6489;
(b) F. Würthner and K. Meerholz, Chem. Eur. J., 2010, 16, 9366; (c)
H. Bürckstümmer, E. V. Tulyakova, M. Deppisch, M. R. Lenze, N.
M. Kronenberg, M. Gsänger, M. Stolte, K. Meerholz and F.
Würthner, Angew. Chem. Int. Ed., 2011, 50, 11628.
18 (a) L.-Y. Lin, Y.-H. Chen, Z.-Y. Huang, H.-W. Lin, S.-H. Chou, F.
Lin, C.-W. Chen, Y-H. Liu and K-T. Wong, J. Am. Chem. Soc., 2011,
133, 15822; (b) H.-W. Lin, L.-Y. Lin, Y.-H. Chen, C.-W. Chen, Y.-
T. Lin, S.-W. Chiu and K.-T. Wong, Chem. Commun., 2011, 47,
7872; (c) S-W. Chiu, L-Y. Lin, H-W. Lin, Y-H. Chen, Z-Y Huang,
Y-T. Lin, F. Lin, Y-H. Liu and K-T. Wong, Chem. Commun. 2012,
48, 1857.
15
The Ministère de la Recherche is acknowledged for the PhD
grant to A. Leliège. We thank the PIAM of the University of
Angers for characterization of organic compounds.
Notes and references
19 Another synthesis of 1 has been recently reported: C. Sissa, V.
Parthasarathy, D. Drouin-Kucma, M. H. V. Werts, M. Blanchard-
Desce and F. Terenziani, Phys. Chem. Chem. Phys., 2010, 12, 11715.
20 A. Leliège, P. Blanchard, T. Rousseau and J. Roncali, Org. Lett.,
2011, 13, 3098.
20 a LUNAM, University of Angers, CNRS UMR 6200, MOLTECH-Anjou,
Group Linear Conjugated Systems, 2 Bd Lavoisier, 49045 Angers,
France. Fax: 33 2 41735405; Tel: 33 2 41735059; E-mail:
philippe.blanchard@univ-angers.fr, jean.roncali@univ-angers.fr
† Electronic Supplementary Information (ESI) available: Synthesis and
25 characterization of new compounds. Crystallographic data, cyclic
voltammetry, absorption spectroscopy, devices fabrication and thermal
analysis . See DOI: 10.1039/b000000x/
21 C. M. Cardona, W. Li, A. E. Kaifer, D. Stockdale and G. C. Bazan,
Adv. Mater., 2011, 23, 2367.
1
S. Günes, H. Neugebauer and N. S. Sariciftci, Chem. Rev., 2007, 107,
1324.
30 2 B. C. Thompson and J. M. J. Fréchet, Angew. Chem. Int. Ed., 2008,
47, 58.
3
G. Denler, M. C. Scharber and C. J. Brabec, Adv. Mater., 2009, 21,
1323.
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