Simple and versatile molecular donors for organic photovoltaics prepared by metal-free synthesis

Article abstract of DOI:10.1002/chem.201405425

Donor-acceptor molecules (D-π -A) built by connecting a diphenylhydrazone block to a dicyanovinyl acceptor group via various thiophene-based π-conjugating spacers (1-5 ) were synthesized from mono- or dialdehydes by a simple metal-free procedure. Cyclic v

Full text of DOI:10.1002/chem.201405425

DOI: 10.1002/chem.201405425
Full Paper
&
Photochemistry
Simple and Versatile Molecular Donors for Organic Photovoltaics
Prepared by Metal-Free Synthesis
Andreea Diac,[a, b] Dora Demeter,[a] Magali Allain,[a] Ion Grosu,[b] and Jean Roncali*[a]
Abstract: Donor–acceptor molecules (D-p-A) built by con-
necting a diphenylhydrazone block to a dicyanovinyl accept-
or group via various thiophene-based p-conjugating spacers
(1–5) were synthesized from mono- or dialdehydes by
a simple metal-free procedure. Cyclic voltammetry and UV/
Vis absorption spectroscopy show that the extension and/or
increase of the donor strength of the spacer produces a de-
crease of the HOMO and LUMO energy level, a red shift of
the absorption spectrum and an increase of the molecular
absorption coefficient. Compared to solutions, the optical
spectra of spin-cast thin films of compounds 1–3 show
a broadening and red shift of the absorption bands, consis-
tent with the formation of J-aggregates. In contrast the blue
shift observed for the EDOT-containing compounds 4 and 5
suggests the presence of H-aggregates. Solution-cast and
vacuum-deposited films of donors 1–5 were evaluated in
solar cells with fullerene C60 as acceptor. A power-conversion
efficiency among the highest reported for bilayer devices of
basic configuration was obtained with compound 2. On the
other hand, the results obtained with 4 and 5 suggest that
the presence of EDOT in the structure can have deleterious
effects on the organization and performances of the donor
material.
Introduction
vacuum depositions seems to imply the production of durable
solar modules, while the production of cheap devices using,
for example, ink-jet printing on flexible substrates suggests
possible applications in short-lifetime products such as packag-
ing. On the other hand, these two different technologies imply
that besides light-absorption and energy levels, the design of
active materials takes also into account physical parameters
such as melting and decomposition temperature, solubility
and film-forming ability.
The design and synthesis of active materials for organic photo-
voltaic cells (OPV) is a focus of considerable current interest
motivated by the possibility of producing solar electricity using
light-weight, cost-effective and environmentally friendly solar
cells.[1] The heterojunction generated by contacting an electron
donor material (D) with an electron acceptor (A) is the heart of
an OPV cell.[2–5] In such a system excitons resulting from the
absorption of solar photons are dissociated into positive and
negative charges by the electrical field at the D–A interface.
Over the past two decades this concept has been developed
along two main lines namely planar heterojunctions (PHJs)[4]
initially described by Tang,[4a] and bulk heterojunctions (BHJs)
in which the D–A interface is distributed in the whole volume
of the active layer as interpenetrated networks of D and A ma-
terials.[5] The fabrication of PHJs and BHJs basically resorts to
two different technologies namely thermal evaporation under
high vacuum for PHJs and solution-process for BHJs. While
these two techniques present specific advantages and draw-
backs, they also suggest two different viewpoints on OPV. The
relatively expensive fabrication of multilayer cells by successive
Although soluble conjugated polymers remain a major class
of donor materials for solution-processed BHJs,[1–3,5] molecular
donors have recently emerged on the forefront of the chemis-
try of OPV materials due to the advantages of well-defined
chemical structures in terms of reproducibility of synthesis, pu-
rification and properties combined with possible analyses of
structure–properties relationships.[6] Furthermore, with ade-
quate structural design, molecular donors present the advant-
age of being compatible with both processing techniques. In
recent years the performances of OPV cells have increased rap-
idly to reach values of approximately 9.0% for solution-pro-
cessed single-junction BHJs[7] and about 7.0–8.0% for vacuum-
deposited PHJs.[8] This progress results from a multidisciplinary
research efforts involving both device technology (introduction
of multiple optimized buffer layers,[8b] additives,[9] replacement
of fullerene C60 by C70-based acceptors[7–9]) as well as the devel-
opment of new donor materials.[1–3,6–8] Although these results
have clearly established the scientific credibility of OPV, the in-
dustrial future of this technology will depend for a large part
on the possibility to offer decisive economic and environmen-
tal advantages over the well-established silicon solar cells. In
this context, the synthesis of active materials by methods com-
[a] A. Diac, Dr. D. Demeter, M. Allain, Dr. J. Roncali
Group Linear Conjugated Systems, CNRS Moltech-Anjou
University of Angers
2 Boulevard Lavoisier 49045 Angers (France)
E-mail: jeanroncali@gmail.com
[b] A. Diac, Prof. I. Grosu
Babes Bolyai University, Center of Supramolecular Organic
and Organometallic Chemistry (CSOOMC)
Cluj-Napoca, 11 Arany Janos Street, 400028 Cluj-Napoca (Romania)
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bining high yields, scalability, compatibility with industrial pro-
cesses, low environmental impact and low cost represents
a challenging task for chemists interested in OPV.[10]
Results and Discussion
The synthesis of the target compounds is depicted in
Scheme 1. Treatment of 0.5 equivalent of N’,N-diphenylhydra-
zine (9) with one equivalent of 2,5-thiophene dicarboxalde-
hyde (11 a) in THF/methanol at low temperature gave alde-
hyde 6a in 62% yield (based on 9) together with 21% of the
bis-adduct 10a while 18% of the starting dialdehyde 11 a was
recovered.[12d] Application of the same procedure to diformylbi-
thiophene 11 b[13] gave the bis-adduct 10b and aldehyde 6b
in 17 and 37% yield, respectively. Condensation of N’N-diphe-
nylhydrazine (9) with monoaldehydes of thienothiophene (8c),
EDOT (8d) and bis-EDOT (8e) gave compounds 7c, 7d and 7e
in 51 to 63% yields. Finally, a Knoevenagel condensation of al-
dehydes 6a–e with malonodinitrile gave the target com-
pounds 1–5 in 62 to 96% yields.
The most efficient polymeric or molecular donor materials
known to date are generally obtained by multistep syntheses
involving cross-coupling reactions of aromatic organometallic
reagents containing magnesium, zinc or tin with metal cata-
lysts such as nickel or palladium.[10c] Besides the toxicity and
cost of some of these reagents and catalysts, the presence of
remnant traces of metal is known to have deleterious effects
on the performances of organic semiconductors,[11] and thus
implies additional purification steps. In this context, the limita-
tion of the overall number of synthetic steps and in particular
of those involving metal-catalysed coupling reactions appears
as an interesting goal.
It has been shown recently that efficient PHJ cells can be
fabricated with push–pull molecules of relatively simple struc-
ture such as merocyanines,[8a] or triarylamine derivatives.[8c,d] In
our continuing interest in the design of molecular donors com-
bining structural simplicity, high overall yield and clean synthe-
sis,[12] we report here on a series of new D–A molecules in
which a diphenylhydrazone donor block is connected to a di-
cyanovinyl acceptor through p-conjugating spacers based on
thiophene (1), bithiophene (2), thieno[3,4b]thiophene (3), 3,4-
ethylenedioxythiophene (EDOT; 4) and Bis-EDOT (5) (Figure 1).
These compounds combine the advantage of a simple, clean
and efficient synthesis with physical properties that allow their
implementation into devices using both vacuum deposition
and solution process. The synthesis and characterization of the
electronic properties of the molecules by UV/Vis spectroscopy
and cyclic voltammetry are described and the results of a first
evaluation of their potential as donor material in PHJ cells are
discussed in terms of structure–properties relationships.
X-ray diffraction
Deep red single crystals for compounds 1, 2 and 3 were
grown by slow evaporation from chloroform solutions. The re-
sults of single crystal X-ray diffraction analysis are presented in
Table 1.
Table 1. Crystallographic data, details of data collection, and structure re-
finement parameters for compounds 1–3.[22]
Compd
1
2
3
formula
M [gmolÀ1
T [K]
C21H14N4S
354.42
293 (2)
monoclinic
P21/n
11.802 (1)
8.7604 (8)
18.7063 (8)
90
106.836 (5)
90
1851.2 (2)
4
C51H33Cl3N8S4
992.44
293 (2)
triclinic
P1
5.4939 (5)
14.717 (1)
16.805 (1)
86.677 (7)
82.864 (7)
85.485 (7)
1342.44 (17)
1
C23H14N4S2
410.50
293 (2)
monoclinic
C2/c
35.046 (7)
8.817 (2)
13.733 (2)
90
101.81 (1)
90
4153.7 (14)
8
]
crystal system
space group
a [A]
b [A]
c [A]
¯
a [8]
b [8]
g [8]
V [A3]
Z
1calcd [gcmÀ3
]
1.272
0.186
1.111
1.228
0.367
0.976
1.313
0.273
1.068
m [mmÀ1
GoF on F2
]
Final R1/wR2 [I>2s(I)]
R1/wR2 (all data)
0.0443/0.0874 0.0742/0.1940 0.0599/0.0849
0.0779/0.1011 0.1396/0.2208 0.1505/0.1074
As shown in Figure 2, for the three compounds the C=N
double bond connecting the hydrazone donor block to the thi-
ophenic spacer presents an E configuration.
The three compounds crystallize in centrosymmetric space
groups, with one independent molecule in the asymmetric
unit, compounds 1 and 3 in the monoclinic system and 2 in
the triclinic system. The planarity of the three molecules was
evaluated by comparing the dihedral angles between the
least-square planes of the three fragments: the donor, the p-
conjugated spacer and the acceptor (Figure 3).
In the crystal the molecules of 1 are interconnected into 2D
sheets by hydrogen bonding between the hydrogen atom of
Figure 1. Chemical structure of the target D–A compounds.
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on glass. The spectrum of all
compounds presents a first ab-
sorption band around 350–
400 nm followed by a more in-
tense band in the 500–600 nm
region attributed to an internal
charge transfer (ICT).
As shown by the data in
Table 2, the increase of the con-
jugation length and/or donor
strength of the conjugating
spacer produces a bathochromic
shift of the absorption maximum
(lmax) of the ICT band from 507
to 574 nm for 1 and 5, respec-
tively, with an increase of the
molecular absorption coefficient
(e). Comparison of these spectra
to those of the corresponding
spin-cast films reveals two differ-
ent behaviours. For compounds
1–3, the solid-state spectrum
presents a broadening of the ab-
sorption bands with a bathochro-
mic shift of lmax. These phenom-
ena commonly observed for
many classes of molecular or
polymeric conjugated systems
are generally attributed to inter-
molecular interactions with for-
mation of J-aggregates. In con-
Scheme 1. Synthesis of the target compounds 1–5. i) AcONa, MeOH/THF; ii) POCl3, DMF, 1,2-dichloroethane;
iii) CH2(CN)2, Et3N, CHCl3.
the dicyanovinyl group and the nitrogen atom of the same
group of a neighbouring molecule (N4···H4 2.619(2) ꢁ) and by
CÀH···p interactions between aromatic ÀCH and neighbouring
phenyl ring (CÀH19···centroid 3.006(1) ꢁ, CÀH14···centroid
3.420(1) ꢁ ( Figure 4).
trast the solid-state spectra of the EDOT-containing com-
pounds 4 and 5 show narrower absorption bands with a hypso-
chromic shift of lmax in particular for compound 5 (14 nm;
Table 2). This peculiar behaviour which suggests the presence
of H-aggregates has been previously observed for other EDOT-
containing systems such as hybrid EDOT–thiophene oligo-
mers[14] or push–pull molecules.[15] Although these results pro-
vide a further illustration of the major contribution of EDOT
units on the formation of H-aggregates, a general explanation
of this process is still lacking and would require further analy-
ses of structure–properties relationships.
The crystal packing of compound 2 reveals the presence of
two types of H bonds between the nitrogen atoms of the CN
groups and one hydrogen of the vinyl group (N2···H4
2.723(3) ꢁ) and the other with the iminic hydrogen (N1···H13
2.650(4) ꢁ) (Figure 5).
For compound 3, the crystallographic structure reveals three
types of intermolecular interactions namely sulfur–sulfur inter-
actions between thiophene rings (S2···S2 3.376(1) ꢁ), H bonds
between one nitrogen atom of the CN groups with one H of
the vinyl group (N4···H20 2.68 ꢁ) and the other H atom of the
thienothiophene ring (N4···H18 2.688(3) ꢁ) and p stacking (cen-
troid···centroid 3.788(1) ꢁ, centroid S2/C16-C19; Figure 5). In
the crystal lattice supramolecular dimers with head-to-tail ar-
rangement sustained by p stacking are observed. These dimers
are further interconnected by H bonds and S···S interactions
into supramolecular chains (Figure 6).
Cyclic voltammetry
Figure 8 shows the oxidative cyclic voltammograms (CV) of
compounds 1–5 in CH2Cl2 in the presence of tetrabutylammo-
nium hexafluorophosphate as supporting electrolyte. The CV
of all compounds presents a first oxidation peak corresponding
to the formation of the cation radical.
As expected, the extension of the spacer or the increase of
its donor strength by insertion of EDOT units produces a large
negative shift of the anodic peak potential (Epa) from 1.32 V for
compound 1 to 0.79 V for compound 5 (Table 2). In this latter
case the CV shows two reversible oxidation waves indicative of
stable cation-radical and dication. A closer examination of the
CVs shows that except for compound 5, the oxidation process
UV/Vis absorption spectroscopy
Figure 7 shows the UV/Vis absorption spectra of compounds
1–5 in methylene chloride solution and as thin films spin-cast
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Figure 4. Crystal packing in 1: 2D layer generated by hydrogen bonds (red
dashed line) and CÀH···p interactions (cyan dashed line).
Figure 5. Supramolecular chain generated by hydrogen bonds in the crystal
of compound 2.
Figure 2. Crystallographic structure of compounds 1 (top), 2 (middle) and 3
(bottom).
is not fully reversible and that a cathodic wave of weak intensi-
ty is observed in the reverse scan. This wave, particularly visi-
ble in the CV of compound 1 and hardly discernible in the
case of compound 2 can be attributed to the reduction of the
product resulting from the coupling of the cation radical, a pro-
cess frequently observed in the electrochemistry of TPA deriva-
tives.[17] In the negative potential region, the CV of all com-
pounds (data not shown) present an irreversible reduction
wave with a cathodic peak potential (Epc) varying from À0.91 V
for compound 1 to À1.17 V for compound 5 (Table 2). The
Figure 6. Crystal packing of compound 3 showing S···S interactions (yellow
dashed line), hydrogen bonding (red dashed line) and p stacking (green
dashed line).
energy levels of the frontier orbitals were estimated from the
oxidation potentials and the optical data. For all compounds
the level of the LUMO is compatible with a photoinduced elec-
tron transfer into the LUMO of C60.
Evaluation of photovoltaic
performances
Compounds 1–5 were evaluated
as donor materials in bilayer
PHJs of 0.28 cm2 active area with
vacuum-deposited C60 as elec-
tron acceptor. Although solu-
Figure 3. Dihedral angles between the three constitutive blocks of the conjugated system of compounds 1–3.
tion-processed BHJs are known
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to reach much higher efficien-
cies, especially when using small
active areas, optimized interfacial
layers, additives and C70 deriva-
tives, such cells require lengthy
optimization and hence larger
quantities of donor material and
they show in general a larger
dispersion of results than PHJ
cells. In this context, basic PHJs
cells appear more appropriate
for an analysis of structure–prop-
erties relationships. In other
words, the purpose of this work
is not to produce champion de-
vices but rather to analyse the
effects of subtle structural
modifications.
The bilayer cells were fabricat-
ed by deposition of a donor
layer on ITO substrates precoat-
ed with a 40 nm thick film of
spun-cast PEDOT:PSS. The sub-
strates were then introduced in
Figure 7. Normalized UV/Vis absorption spectra of compounds 1–5. Dotted lines: in CH2Cl2. Solid lines: thin films
spin-cast on glass from chloroform solutions.
Table 2. Data of UV/Vis spectroscopy ((s): ~1ꢂ10À5 m in CH2Cl2); (f): films spin-cast on glass), and cyclic voltam-
metry (in 0.10m Bu4NPF6/CH2Cl2, scan rate 100 mVsÀ1, Pt electrodes, reference SCE).
a
vacuum chamber, a 30 nm
thick layer of C60 was deposited
by thermal evaporation and the
devices were completed by dep-
osition of a 100 nm layer of alu-
minium. Each ITO substrate con-
tains two circular cells and each
batch typically involves 6–8 cells.
After fabrication the cells were
subjected to a 10 min thermal
treatment at a temperature of
110 to 1608C.
Donor
lmax (s)
[nm]
DE
[eV]
emax
[mÀ1 cmÀ1
lmax (f)
[nm]
Eg
[eV]
Epa
[V]
Epc
[V]
EHOMO
ELUMO
]
[eV][b]
[eV][c]
1[a]
2
3
4
507
533
518
525
574
2.44
2.32
2.39
2.35
2.15
39000
32000
29000
36000
47000
520
542
532
520
560
2.00
1.86
1.92
2.01
1.75
1.32
1.02
1.15
1.11
0.79
À0.91
À1.02
À1.02
À1.11
À1.17
À6.01
À5.71
À5.84
À5.80
À5.48
À3.57
À3.39
À3.45
À3.45
À3.33
5
[a] From ref. [12d]. [b] Using E0 with an offset of À4.99 eV for SCE versus the vacuum level.[16] [c] Determined
ox
by EHOMOÀDE.
An interesting specificity of
the new donors lies in the fact
that their solubility and melting
and decomposition tempera-
tures are compatible with both
solution-process and vacuum
deposition. It was therefore in-
teresting to take advantage of
this processing versatility to
compare the performances of
OPV cells fabricated by the two
techniques. Figure 9 shows the
current density versus voltage
curves for the cells based on
compounds 1–3 and the corre-
sponding photovoltaic parame-
ters are listed in Table 3.
As already reported in a short
communication, a cell based on
a spin-cast film of 1 (entry 1s)
gave a short-circuit current–den-
Figure 8. Cyclic voltammograms corresponding to the oxidation of compounds 1–5 in 0.10m Bu4NPF6/CH2Cl2, Pt
electrodes, scan rate 100 mVsÀ1
.
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(Figure 10). Comparison of the
data for compounds 1 and 4 re-
veals a decrease of PCE for both
solution-cast and vacuum-depos-
ited donor layers due essentially
to a decrease of Voc associated
with the higher HOMO level of
compound 4. On the other
hand, the results obtained with
donors 2 and 5 show that the
replacement of bithienyl spacer
by a bis-EDOT produces an even
larger decrease of Jsc with a two-
fold decrease of PCE for solu-
tion-processed devices while Jsc
decreases
from
8.40
to
2.17 mAcmÀ2 and PCE from 3.22
to 0.65% for the cells based on
the vacuum-deposited donors
layers.
Comparison of the results ob-
tained with solution-cast and
vacuum-deposited donor layers
reveals two contrasted situa-
tions. For compounds 1 and 2,
vacuum deposition improves Jsc
Figure 9. Current density versus voltage curves for bilayer photovoltaic cells ITO/PEDOT:PSS/D/C60/Al based on
spin-cast (s) and 15 nm vacuum-deposited (e) layers of donors 1, 2 and 3. Empty circles: in the dark; filled circles:
under AM 1.5 simulated solar illumination with an incident power light of 90 mWcmÀ2
.
sity (Jsc) of 6.32 mAcmÀ2 and a power conversion efficiency
(PCE) of 2.07%.[12d] As shown in Table 3, the cells based on
vacuum deposited donor 1 (entry 1e) show a slight improve-
ment of Jsc from approximately 6.30 to 6.90 mAcmÀ2 but a de-
crease of Voc and FF with as net result a decrease of PCE to
about 1.70%. The results obtained with vacuum-deposited
donor 3 reveal a higher of Jsc of 8.50 mAcmÀ2, however the
maximum PCE is limited to 2.30% due to the decrease of Voc
to 0.60 V, in agreement with the higher HOMO level of com-
pound 3. In spite of a correct solubility, the film-forming prop-
erties of compound 3 were insufficient to produce a good
quality spin-cast film.
while the reverse effect is observed for the EDOT-containing
donors in particular for compound 5 for which vacuum-depo-
sition leads to
performances.
a considerable deterioration of the PV
In order to complete these observations, the external quan-
tum efficiency (EQE) spectra of the cells were recorded under
monochromatic irradiation. The spectra of all devices show
Table 3. Photovoltaic characteristics of PHJ bilayer cells ITO/PEDOT:PSS/
D/C60/Al under AM 1.5 simulated solar irradiation with an incident power
light of 90 mWcmÀ2 [a]
.
Compd
An. temp.
[8C]
Jsc
Voc
[V]
FF
[%]
PCE
[%]
Comparison of the data for donors 1 and 2 shows that the
insertion of a bithienyl bridge produces an increase of PCE to
2.55% for the spin-cast film and 3.22% for the vacuum depos-
ited one. This result is due essentially to a better FF and to an
increase of Jsc above 8.0 mAcmÀ2. The relatively high value of
Voc in spite of the higher HOMO level of compound 2 could be
due to a higher interfacial dipole or to a decrease of charge re-
combination. Further work is needed to clarify this point. Com-
parison of entries 2s and 2e in Table 3 shows that the cells
made by vacuum deposition present higher Jsc and Voc and
hence higher PCE. It is worth noting that the 3.22% PCE is the
highest value obtained so far with our standard 0.28 cm2 bilay-
er device. Taking into account the large increase of PCE ach-
ieved by optimization of cells based on other small TPA-based
donors,[18] these first results suggest that there is still much
room for improvement.
[mAcmÀ2
]
1s[a]
1s[a]
1e
1e
2s
110
110
110
110
120
120
120
120
140
140
150
150
160
160
160
160
160
160
6.40
6.32
6.83
6.87
7.32
7.14
8.73
8.42
7.71
8.53
5.68
6.19
4.78
5.25
5.65
5.30
2.12
2.17
0.75
0.79
0.68
0.69
0.74
0.76
0.77
0.80
0.61
0.60
0.66
0.63
0.69
0.69
0.63
0.54
0.66
0.70
33
37
32
32
40
42
40
43
41
40
29
35
29
30
32
41
39
39
1.76
2.07
1.64
1.68
2.41
2.55
3.06
3.22
2.15
2.28
1.28
1.47
1.05
1.20
1.22
1.30
0.63
0.65
2s
2e
2e
3e
3e
4s
4s
4e
4e
5s
5s
5e
5e
The results obtained with the cells based on donors 4 and 5
show that the replacement of the thiophenes by EDOT units
has deleterious consequences for the PV performances
[a] Data in italics are the average of 6–8 cells, data in bold are the best re-
sults for each series. An. temp=annealing temperature.
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mic shift of the onset of photo-
current
to
approximately
650 nm. The EQE spectra of the
cells based on solution-cast and
vacuum-deposited donor layers
are rather similar except for
a slightly higher intensity for the
vacuum-deposited
(Figure 11).
donor
As shown in Figure 12, the
processing technique has
a much stronger effect in the
case of EDOT-containing donors.
For compound 4 the EQE spectra
of both types of cells have
almost identical shape with
a lower photocurrent for the
vacuum-deposited donor. For
compound 5, the spectra of the
solution-processed and vacuum-
deposited donors present as ex-
pected, an extension of the pho-
Figure 10. Current density versus voltage curves for bilayer solar cells (ITO/PEDOT:PSS/D/C60/aluminium. In the
dark (open circles) and under simulated solar light with incident power light of 90 mWcmÀ2 (black circles).
toresponse
towards
longer
a first sharp peak at 370–380 nm attributed to the contribution
of C60 to the photocurrent. The spectrum of the cell based on
vacuum-deposited compound 1 shows a first band at 400 nm
followed by a broader band of about 50% extending to about
600 nm (Figure 11). The response of compound 3e is rather
similar to that of 1e and both are consistent with the absorp-
tion spectra of the donors.
wavelengths. However, a marked decrease of EQE is observed
in the 500–700 nm region for the vacuum-deposited donor.
These EQE spectra agree well with the results obtained under
white-light illumination and clearly confirm the negative influ-
ence of the bis-EDOT spacer on the conversion efficiency of
the donor molecule.
Previous work on hybrid thiophene–EDOT oligomers has
shown that the number and position of EDOT units in the con-
jugated structure exerts a determining influence on the molec-
ular organization and in particu-
The spectrum of the cells based on donor 2 shows, as ex-
pected, an extension of the photoresponse with a bathochro-
lar on the formation of J- or H-
aggregates.[14,15] In fact, results
on OFETs and OPV cells have
shown that the spectral signa-
ture of H-aggregates is associat-
ed with a higher hole-mobility in
OFETs but with a dramatic loss
of the light-harvesting properties
and hence PCE of OPV cells.[14] In
this context the results obtained
with compounds 4 and 5 pro-
vide a further illustration of the
problems posed by use of the
EDOT building block for the
design of OPV materials, even if
interesting positive effects have
been demonstrated in some
cases.[15]
Although our results confirm
the versatile processibility of this
class of molecular donors, they
also show that depending on
the molecular structure, the
processing technique can exert
Figure 11. Spectra of external quantum efficiency of bilayer cells based on donors 1, 2 and 3 under monochro-
matic irradiation.
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cells using the most promising
compounds of this study is now
underway and will be reported
elsewhere.
Experimental Section
General
NMR spectra were recorded with
a
Bruker AVANCE III 300 (1H,
300 MHz and 13C, 75 MHz). Chemi-
cal shifts are given in ppm relative
to TMS. UV/Vis spectra were re-
corded with
a
Perkin–Elmer
Lambda 19 or 950 spectrometer.
Melting points are uncorrected.
Matrix-assisted laser desorption/
ionization was performed on
MALDI-TOF MS BIFLEX III Bruker
Daltonics spectrometer using di-
thranol as matrix.
Cyclic voltammetry was performed
in dichloromethane solution pur-
chased from SDS (HPLC grade).
Tetrabutylammonium hexafluoro-
phosphate (0.10m as supporting
Figure 12. Spectra of external quantum efficiency of bilayer cells based on donors 4, and 5 under monochromatic
irradiation. Spun-cast (s) donor layer and vacuum-deposited (e) donor layer are shown.
electrolyte) was purchased from Acros and was used without pu-
rification. Solutions were deaerated by nitrogen bubbling prior to
each experiment. Experiments were carried out in a one-compart-
ment cell equipped with platinum electrodes and saturated calo-
mel reference electrode (SCE) with a Biologic SP-150 potentiostat
with positive feedback compensation.
a significant influence on the efficiency of the resulting OPV
cell. Thus, whereas for compound 1 both methods lead to
rather similar results, for compound 2 vacuum deposition
leads to a about 25% higher PCE due essentially to an increase
of Jsc. In contrast, for compounds 4 and 5 vacuum-deposition
seems to exalt the deleterious influence of the EDOT unit on
molecular packing and hence PV performances.
Synthesis
Compounds 1,[12d] 11 a,[13] 11 b,[13] 8c,[19] 8d[20] and 8e[21] were ob-
tained using known procedures. While all final compounds are ob-
tained by Knoevenagel condensation of malonodinitrile with the
appropriate aldehyde, these aldehydes were obtained along two
main routes. In the case of compounds 1 and 2 treatment of di-
phenylhydrazine 9 with dialdehydes 11 a and 11 b gives a mixture
of the double condensation product 10a and 10b and of the ex-
pected aldehyde 6a or 6b. The use of a twofold excess of dialde-
hyde and of diluted reaction medium allows the orientation of the
reaction towards the desired aldehyde and limitation of the forma-
tion of by-products 10a and 10b while a large part of the starting
dialdehyde can be recovered.
Conclusion
To summarize, new push–pull molecules have been synthe-
sized in good yields by a simple and straightforward metal-free
procedure. Electrochemical and optical results show that the
nature and length of the conjugating spacer exert a determin-
ing influence on the energy levels and light-harvesting proper-
ties of the molecule. A first evaluation of the potential of the
compounds as donor material in simple bilayer cells has con-
firmed the determining role of the conjugating bridge on con-
version efficiency. The results obtained with EDOT-containing
molecules have confirmed the difficulty to predict the effects
of this building block on the organization of the material and
hence on its electronic properties and photovoltaic performan-
ces. On the other hand, the results obtained with donors
based on thiophenic spacers and in particular bithiophene
suggest that these materials can represent an interesting
trade-off when considering the balance of conversion efficien-
cy and the cost, simplicity, cleanness, yield and scalability of
the synthesis and the processing versatility. Work devoted to
the optimization of the synthetic steps, to the extension of this
synthetic approach, to molecules with improved photovoltaic
parameters and to the fabrication of optimized PHJ and BHJ
5,5’-Bis((E)-(2,2-diphenylhydrazono)methyl)-2,2’-bithiophene
(10b)
To
a solution of [2,2’-bithiophene]-5,5’-dicarbaldehyde (11 b;
470 mg, 2.11 mmol) in 120 mL dry THF warmed to 258C was
added a solution of N,N’-diphenylhydrazine hydrochloride (9;
233 mg, 1.05 mmol) and sodium acetate (346 mg, 4.22 mmol) in
30 mL anhydrous methanol. The reaction mixture was stirred for
three days at 258C under argon atmosphere. After being cooled to
room temperature, the mixture was diluted with methylene chlo-
ride and washed with water and brine. After removal of solvent
the residue was chromatographed on silica gel using methylene
chloride as eluent to give 100 mg (17%) of a yellow powder. M.p:
1
268–2698C; H NMR (300 MHz, CDCl3): d=7.46–7.41 (m, 8H), 7.24–
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7.18 (m, 14H), 7.08 (d, 2H, J=3.6 Hz), 6.80 ppm (d, 2H, J=3.9 Hz);
13C NMR (75 MHz, CDCl3): d=143.4, 141.0, 137.3, 130.2, 130.0,
127.3, 124.9, 124.0, 122.6 ppm; MS (MALDI-TOF): 554.4 [M]+; HRMS
(FAB): elemental analysis calcd (%): 554.1599; found: 554.1601.
0.6 mmol) in 10 mL 1,2-dichloroethane. The reaction mixture was
heated to 878C for 6 h. After being cooled to room temperature,
a saturated aqueous solution of sodium acetate and dichlorome-
thane were added and the mixture was additionally stirred for 2 h.
After extraction with CH2Cl2 (3ꢂ50 mL), the combined organic
phases were washed with water and brine, dried over MgSO4 and
concentrated. The residue was purified by column chromatography
on silica gel (eluent: CH2Cl2/EP=2:1) to afford 140 mg (66%) of an
(E)-5’-((2,2-Diphenylhydrazono)methyl)-[2,2’-bithiophene]-5-
carbaldehyde (6b)
1
The same reaction gave after chromatography 150 mg (37%) of an
orange powder. M.p.: 140–1418C); 1H NMR (300 MHz, CDCl3): d=
9.85 (s, 1H), 7.67 (d, 1H, J=3.9 Hz), 7.47–7.42 ( m, 4H), 7.28 (d, 1H,
J=3.9 Hz), 7.25–7.18 (m, 8H), 6.83 ppm (d, 1H, J=3.9 Hz); 13C NMR
(75 MHz, CDCl3): d=182.6, 147.5, 144.0, 143.1, 141.7, 137.6, 135.2,
130.1, 129.3, 127.1, 126.5, 125.2, 124.3, 122.6 ppm; MS (MALDI-
TOF): 388.2 [M]+; HRMS (MALDI-TOF): elemental analysis calcd (%):
388.0704; found: 388.0702.
orange solid. M.p.: 145–1478C; H NMR (300 MHz, CDCl3): d=9.92
(s, 1H), 7.86 (s, 1H), 7.48–7.43 ( m, 4H), 7.28 (s, 1H), 7.25–7.19 (m,
6H), 7.06 ppm (s, 1H); 13C NMR (75 MHz, CDCl3): d=183.2, 151.0,
146.3, 145.0, 142.9, 138.5, 130.1, 129.4, 129.1, 125.5, 122.6,
118.4 ppm; MS (MALDI-TOF): 362.2 [M]+; HRMS (MALDI-TOF): ele-
mental analysis calcd (%): 362.0548; found: 362.0550.
(E)-2-((5-((2,2-Diphenylhydrazono)methyl)thieno[3,2-b]thio-
phen-2yl)methylene)-malononitrile (3)
(E)-2-((5’-((2,2-Diphenylhydrazono)methyl)-[2,2’-bithiophen]-
5-yl)methylene)malononitrile (2)
To a solution of (E)-5-((2,2-diphenylhydrazono)methyl)thieno[3,2-
b]thiophene-2-carbaldehyde (6c; 120 mg, 0.33 mmol) in 30 mL
CHCl3, malonodinitrile (50 mg, 0.75 mmol) and few drops of trie-
thylamine were added. The mixture was stirred under argon at-
mosphere, at room temperature for 4 h. After removal of solvent,
the residue was solubilized in dichloromethane, washed with water
and brine, and dried over magnesium sulfate. The resulting solid
was washed with cold ethanol, giving 120 mg (89%) of a deep-
Malonodinitrile (69 mg, 1.04 mmol) and few drops of triethylamine
were added to a solution of aldehyde 6b (200 mg, 0.52 mmol) in
70 mL chloroform and the mixture was heated at 628C for 15 h.
After cooling to room temperature, the reaction mixture was dilut-
ed with methylene chloride, washed with water and brine and
dried over anhydrous MgSO4. After solvent removal, the residue
was purified by column chromatography (silica gel, eluent: di-
chloromethane/petroleum ether=4:1) to give 140 mg (62%) of
1
violet powder. M.p.: 262–2648C; H NMR (300 MHz, CDCl3): d=7.81
(d, 2H, J=8.4 Hz), 7.49–7.44 ( m, 4H), 7.30–7.19 (m, 7H), 7.05 ppm
(s, 1H); 13C NMR (75 MHz, CDCl3): d=153.4, 150.8, 148.7, 142.6,
139.3, 136.7, 130.6, 130.2, 128.6, 125.8, 122.6, 117.8, 114.6, 113.8,
75.5 ppm; MS (MALDI-TOF): 410.2 [M]+; HRMS (MALDI-TOF): ele-
mental analysis calcd (%): 410.0660; found: 410.0650.
1
a deep violet powder. M.p.: 206–2088C; H NMR (300 MHz, CDCl3):
d=7.75 (s, 1H), 7.64 (d, 1H, J=4.2 Hz), 7.50–7.45 ( m, 4H), 7.32 (d,
1H, J=4.2 Hz), 7.30 (d, 1H, J=3.9 Hz), 7.28–7.27 (m, 1H), 7.25–
7.20 ppm (m, 7H); 13C NMR (75 MHz, CDCl3): d=150.1, 149.8, 145.5,
143.0, 140.5, 134.2, 133.5, 130.1, 128.9, 127.8, 127.3, 125.3, 124.6,
122.5, 114.5, 113.7, 75.7 ppm; MS (MALDI-TOF): 436.1 [M]+; HRMS
(MALDI-TOF): elemental analysis calcd (%): 436.0816; found:
436.0804.
(E)-2-((2,3-Dihydrothieno[3,4-b][1,4]dioxin-5-yl)methylene)-1,1-
diphenylhydrazine (7d)
A
mixture of N,N’-diphenylhydrazine hydrochloride (1.43 g,
6.46 mmol) and sodium acetate (1.06 g, 12.9 mmol) in 25 mL
MeOH was added dropwise to a solution of aldehyde 8d (1.1 g,
6.46 mmol) in 50 mL dry THF, under argon atmosphere. The reac-
tion mixture was stirred, overnight, at room temperature under
inert atmosphere. The mixture was quenched with water and ex-
tracted with dichloromethane. The combined organic extracts
were washed with water, brine and dried over MgSO4. After solvent
removal, the crude was purified by column chromatography on
silica gel using a mixture of petroleum ether/dichloromethane=
2:1 as eluent to give 1.26 g (58%) of a yellow solid. M.p.: 146–
1478C; 1H NMR (300 MHz, CDCl3): d=7.43–7. 37 (m, 4H), 7.27 (s,
1H), 7.19–7.14 (m, 6H), 6.24 (s, 1H), 4.14 ppm (s(br), 4H); 13C NMR
(75 MHz, CDCl3): d=143.6, 141.7, 140.0, 129.9, 127.7, 124.6, 122.5,
116.1, 99.4, 64.9, 64.8 ppm; HRMS (MALDI-TOF): elemental analysis
calcd (%): 336.0932; found: 336.0925.
(E)-1,1-Diphenyl-2-(thieno[3,2-b]thiophen-2-ylmethylene)hy-
drazine (7c)
A solution of N,N’-diphenylhydrazine hydrochloride (9; 262 mg,
1.19 mmol) and sodium acetate (195 mg, 2.38 mmol) in 8 mL dry
methanol was added dropwise to a solution of thieno[3,2-b]thio-
phene-2-carbaldehyde (8c; 200 mg, 1.19 mmol) in 20 mL anhy-
drous THF. The reaction mixture is stirred, overnight, at room tem-
perature, under argon atmosphere and then it was heated to 708C
for 4 h. After being cooled to room temperature, the solution was
diluted with methylene chloride, washed with water and brine and
dried over MgSO4. After solvent removal, the residue was chroma-
tographed on silica gel, using as eluent a mixture of dichlorome-
thane/petroleum ether=2:1 to give 250 mg (63%) of a yellow
1
solid. M.p.: 170–1718C; H NMR (300 MHz, CDCl3): d=7.46–7.41 (m,
4H), 7.35 (d, 1H, J=5.1 Hz), 7.30 (s, 1H), 7.23–7.19 (m, 7H),
7.04 ppm (s, 1H); 13C NMR (75 MHz, CDCl3): d=144.4, 143.4, 139.2,
139.0, 130.6, 130.0, 127.6, 124.9, 122.6, 120.0, 118.6 ppm; MS
(MALDI-TOF): 334.2 [M]+; HRMS (MALDI-TOF): elemental analysis
calcd (%): 334.0598; found: 334.0588.
(E)-7-((2,2-Diphenylhydrazono)methyl)-2,3-dihydrothieno[3,4-
b][1,4]dioxine-5-carbaldehyde (6d)
To a solution of hydrazone derivative 7d (0.6 g, 1.78 mmol) in
50 mL 1,2-dichloroethane cooled to 08C anhydrous DMF (0.2 mL,
2.5 mmol) and phosphoryl chloride (0.2 mL, 2.14 mmol) were
added. The reaction mixture was stirred, overnight, at room tem-
perature. Aqueous solution of sodium acetate and dichlorome-
thane are added and the solution was additionally stirred for 1 h.
The organic layer was washed twice with water and dried over
MgSO4. After removal of solvent the crude was chromatographed
(E)-5-((2,2-Diphenylhydrazono)methyl)thieno[3,2-b]thiophene-
2-carbaldehyde (6c)
Anhydrous DMF (0.08 mL, 1.08 mmol) and phosphorus oxychloride
(0.08 mL, 0.9 mmol) were added to a solution of (E)-1,1-diphenyl-2-
(thieno[3,2-b]thiophen-2-ylmethylene)hydrazine
(7c;
200 mg,
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on silica gel using dichloromethane as eluent to afford 0.5 g (77%)
of a yellow powder. M.p.: 263–2648C; 1H NMR (300 MHz, CDCl3):
d=9.87 (s, 1H), 7.46–7.40 (m, 4H), 7.25–7.16 (m, 7H), 4.33–4.30 (m,
2H), 4.21–4.19 ppm (m, 2H); 13C NMR (75 MHz, CDCl3): d=179.8,
148.4, 142.8, 138.8, 130.1, 127.6, 125.4, 125.2, 122.5, 116.5, 65.5,
64.6 ppm; HRMS (MALDI-TOF): elemental analysis calcd (%):
364.0882; found: 364.0875.
108.7, 65.5, 65.4, 65.0, 64.7 ppm; HRMS (MALDI-TOF): elemental
analysis calcd (%): 504.0814; found: 504.0812.
(E)-2-((7’-((2,2-Diphenylhydrazono)methyl)-2,2’,3,3’-tetrahydro-
[5,5’-bithieno[3,4-b][1,4]dioxin]-7-yl)methylene)malononitrile
(5)
Malonodinitrile (21 mg, 0.64 mmol) and a few drops of triethyla-
mine were added to
a solution of aldehyde 6e (160 mg,
(E)-2-((7-((2,2-Diphenylhydrazono)methyl)-2,3-dihydrothie-
0.32 mmol) in 30 mL chloroform and the mixture was stirred, over-
night, at ambient temperature. After solvent removal the residue
was dissolved in dichloromethane, washed with water, brine and
dried over MgSO4. The crude was chromatographed on silica gel
using dichloromethane as eluent yielding 170 mg (96%) of a deep
no[3,4-b][1,4]dioxin-5-yl)methylene)malononitrile (4)
A few drops of triethylamine was added to a mixture of 37 (0.46 g,
1.26 mmol) and malononitrile (0.125 g, 1.89 mmol) in 50 mL CHCl3.
The reaction mixture was heated to 508C under argon atmosphere
for 2 h. The solvent was evaporated and the residue was dissolved
in dichloromethane, washed with water and brine and dried over
MgSO4. After solvent removal, the crude was purified on silica gel
using dichloromethane as eluent to give 0.44 g (85%) of a violet
powder. M.p.: 229–2308C; 1H NMR (300 MHz, CDCl3): d=7.77 (s,
1H), 7.48–7.42 (m, 4H), 7.28–7.16 (m, 7H), 4.35–4.33 (m, 2H), 4.22–
4.19 ppm (m, 2H); 13C NMR (75 MHz, CDCl3): d=148.8, 144.8, 142.5,
138.4, 130.2, 130.0, 125.9, 124.6, 122.6, 115.7, 114.5, 111.6, 71.1,
66.0, 64.6 ppm; HRMS (MALDI-TOF): elemental analysis calcd (%):
412.0994; found: 412.0987.
1
violet powder. M.p.: 308–3098C; H NMR (300 MHz, CDCl3): d=7.76
(s, 1H), 7.47–7.41 (m, 4H), 7.24–7.17 (m, 7H), 4.49–4.47 (m, 2H),
4.43–4.38 (m, 4H), 4.22–4.19 ppm (m, 2H); 13C NMR (75 MHz,
CDCl3): d=148.5, 144.2, 143.1, 141.2, 139.6, 136.2, 130.1, 126.5,
125.2, 122.5, 120.3, 116.2, 115.3, 111.4, 110.7, 108.4, 68.3, 65.9, 65.7,
64.9, 64.7 ppm; HRMS (FAB): elemental analysis calcd (%):
552.0926; found: 552.0917.
Device fabrication and testing
Indium–tin oxide coated glass slides of 24 mmꢂ25 mmꢂ1.1 mm
dimensions with a surface resistance of 10 W per square were pur-
chased from Kintec company. Part of the ITO layer was etched
away with 37% HCl. The ITO electrodes were then cleaned in an ul-
trasonic bath with, successively, Deconex (VWR international
GmbH), distilled water (15.3 MWcmÀ1), acetone, ethanol and dis-
tilled water for 10 min each wash, and dried in an oven at 1008C.
The dried electrodes were then modified by a spun-cast layer of
PEDOT:PSS (Clevios P VP. AI 4083 (HC-Starck) filtered through
a 0.45 mm membrane just prior use). Spin-casting was achieved at
5000 rpm (r=10 s, t=60 s), and the electrode was then dried at
1308C for 15 min. Films of donor materials (ca. 20 nmnm) were
spun-cast in atmospheric conditions from chloroform solutions
containing 4 mg donor per mL. After film deposition the devices
were introduced in an argon glovebox (200B, MBraun) equipped
with a vacuum chamber and a 30 nm film of Fullerene C60 (99+%;
MER Corporation) and a 100 nm thick aluminium electrode were
thermally evaporated on top of the donor film under a pressure of
2ꢂ10À6 mbar through a mask defining two cells of 6.0 mm diame-
ter (0.28 cm2) on each ITO electrode.
(E)-1,1-Diphenyl-2-((2,2’,3,3’-tetrahydro-[5,5’-bithieno[3,4-b]
[1,4]dioxin]-7-yl)methylene)hydrazine (7e)
To a solution of aldehyde 8e (0.35 g, 1.13 mmol) in 100 mL dry
THF heated to 668C was added dropwise a solution of N,N’-diphe-
nylhydrazine hydrochloride (0.25 g, 1.13 mmol) and sodium acetate
(0.18 g, 2.26 mmol) in 8 mL anhydrous methanol. The reaction mix-
ture was refluxed during 24 h. After being cooled to room temper-
ature, the mixture was quenched with water and dichloromethane.
The aqueous layer was extracted twice with CH2Cl2 and the com-
bined organic extracts were dried over MgSO4. After solvent re-
moval, the crude was purified by column chromatography on silica
gel, using dichloromethane as eluent to afford 0.27 g (51%) of
a yellow solid. M.p.: 252–2538C; 1H NMR (300 MHz, CDCl3): d=
7.44–7.37 (m, 4H), 7.32 (s, 1H), 7.20–7.15 (m, 6H), 6.29 (s, 1H),
4.41–4.38 (m, 2H), 4.29–4.25 (m, 4H), 4.18–4.15 (m, 2H); 13C NMR
(75 MHz, CDCl3): d=143.7, 141.5, 139.7, 137.6, 136.8, 129.9, 128.1,
124.5, 122.6, 113.6, 98.2, 65.2, 64.8 ppm; HRMS (MALDI-TOF): ele-
mental analysis calcd (%): 476.0864; found: 476.0859.
The J versus V curves of the devices were recorded in the dark and
under illumination using a Keithley 236 source-measure unit and
a homemade acquisition program. The light source was an AM1.5
Solar Constant 575 PV simulator (Steuernagel Lichttecknik,
equipped with a metal halogen lamp). For each measurement the
incident light intensity (ca. 90 mWcmÀ2) was measured by a broad-
band power meter (13PEM001, Melles Griot). The devices were illu-
minated through the ITO electrode side. The efficiency values re-
ported here are not corrected for the possible spectral mismatch
of the solar simulator. External quantum efficiency (EQE) was mea-
sured using a halogen lamp (Osram) with an Action Spectra Pro
150 monochromator, a lock-in amplifier (PerkinElmer 7225) and
a S2281 photodiode (Hamamatsu).
(E)-7’-((2,2-Diphenylhydrazono)methyl)-2,2’,3,3’-tetrahydro-
[5,5’-bithieno[3,4-b][1,4]dioxine]-7-carbaldehyde (6e)
At 08C, anhydrous DMF (0.06 mL, 0.76 mmol) and phosphorus oxy-
chloride (0.06 mL, 0.65 mmol) were added to a solution of hydra-
zone derivative 7e (0.26 g, 0.54 mmol) in 30 mL 1,2-dichloroethane.
The mixture was stirred, overnight, at room temperature under
argon atmosphere. A saturated aqueous solution of sodium ace-
tate and dichloromethane were added and the solution was addi-
tionally stirred for 2 h. The aqueous layer was extracted twice with
CH2Cl2; the combined organic extracts were washed with water
and brine, and then dried over MgSO4. Solvent removal and
column chromatography on silica gel (eluent: dichloromethane)
gave 0.18 g (65%) of an orange solid. M.p.: 276–2778C; 1H NMR
(300 MHz, CDCl3): d=9.90 (s, 1H), 7.45–7.39 (m, 4H), 7.28 (s, 1H),
7.22–7.17 (m, 6H), 4.48–4.42 (m, 4H), 4.34–4.31 (m, 2H), 4.20–
4.18 ppm (m, 2H); 13C NMR (75 MHz, CDCl3): d=179.6, 148.1, 143.4,
140.0, 139.5, 136.6, 130.0, 127.2, 124.8, 122.6, 122.1, 117.9, 115.4,
Acknowledgements
The authors thanks the French “Ministꢃre des affaires ꢄtran-
gꢃres” for the Eiffel grant for A.D.
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Keywords: basic condensation · clean chemistry · donor–
acceptor systems · organic solar cells · photochemistry
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Received: September 26, 2014
Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
11
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
Finding the right partner: Small push–
pull molecules were synthesized in
&
Photochemistry
A. Diac, D. Demeter, M. Allain, I. Grosu,
J. Roncali*
good yields by a three-step metal-free
procedure. Thin films produced by both
solution process and vacuum deposition
led to interesting photovoltaic perform-
ances in simple bilayer heterojunction
solar cells with C60 as acceptor material.
&& – &&
Simple and Versatile Molecular Donors
for Organic Photovoltaics Prepared by
Metal-Free Synthesis
&
&
Chem. Eur. J. 2014, 20, 1 – 12
www.chemeurj.org
12
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!