Rapid and green synthesis of complementary D-A small molecules for organic photovoltaics

Article abstract of DOI:10.1016/j.orgel.2016.12.046

Two push-pull molecules with close molecular structures have been synthesized through two green steps, direct heteroarylation and Knoevenagel condensation. The electronic properties and the basic bilayer heterojunction solar cells demonstrate that the two

Full text of DOI:10.1016/j.orgel.2016.12.046

Accepted Manuscript
Rapid and green synthesis of complementary D-A small molecules for organic
photovoltaics
Jérémie Grolleau, Frédéric Gohier, Magali Allain, Stéphanie Legoupy, Clément
Cabanetos, Pierre Frère
PII:
S1566-1199(16)30585-7
10.1016/j.orgel.2016.12.046
ORGELE 3895
DOI:
Reference:
To appear in: Organic Electronics
Received Date: 16 October 2016
Revised Date: 19 December 2016
Accepted Date: 22 December 2016
Please cite this article as: J. Grolleau, F. Gohier, M. Allain, S. Legoupy, C. Cabanetos, P. Frère,
Rapid and green synthesis of complementary D-A small molecules for organic photovoltaics, Organic
Electronics (2017), doi: 10.1016/j.orgel.2016.12.046.
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ACCEPTED MANUSCRIPT
Rapid and green synthesis of complementary D-
A small molecules for organic photovoltaics
Jérémie Grolleau, Frédéric Gohier, Magali Allain, Stéphanie Legoupy,* Clément Cabanetos
and Pierre Frère*
University of Angers, MOLTECH-Anjou UMR CNRS 6200, SCL group, 2 boulevard
Lavoisier, 49045 Angers cedex
*
Corresponding authors. Fax +33 2 41 73 54 05
E-mail: pierre.frere@univ-angers.fr, stephanie.legoupy@univ-angers.fr
KEYWORDS. Organic Photovoltaics, Direct Arylation, Knoevenagel reactions, Green
chemistry.
ABSTRACT. Two push-pull molecules with close molecular structures have been
synthesized through two green steps, direct heteroarylation and Knoevenagel condensation.
The electronic properties and the basic bilayer heterojunction solar cells demonstrate that the
two molecules are complementary in term of light absorption. Thus solar cells using a blend
of the two molecules present higher power conversion efficiencies, reaching 3%, than the
cells made from each compound alone.

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Introduction
Organic photovoltaics (OPV) represent a promising source of low-cost electricity production
with expected lower environmental impact than silicon based solar cells.[1, 2] Up until
recently, most of research efforts on organic solar cells (OSCs) has been devoted/focusing on
the fabrication of bulk heterojunction (BHJ) organic solar cells, prepared from a blend of a p-
type conjugated materials, polymers or small molecules, and a n-type soluble fullerene
derivative.[3, 4] Optimization of donor materials from the chemical point of view, ie, by
studying structure/property relationships, has led to photovoltaic devices with high
performances. However, these improvements are often associated with increased complexity
of both the device and the structure of the donor materials, hence inducing longer tedious
synthetic procedures.[5] The design of donor materials based on the donor – acceptor (D-A)
concept consisting of alternating an electron rich block and an electron deficient unit in the π-
conjugated backbone to tune the electronic properties, has allowed to reach the highest power
conversion efficiencies (PCE) above 10%.[6-8] Nevertheless, considering the time-consuming
of multistep synthesis, often involving organometallic coupling reactions that require a high
degree of purification with manipulation of large amount of solvents,[9] it still falls far short
of the prerequisites to assume the green and low-cost criteria for the conception of most
conjugated materials.
Recent articles focused on the necessity of developing cheaper and cleaner routes for active
materials production for future industrial development of organic photovoltaic cells.[9-12]
Cheaper route towards organic semiconductors can be achieved by a simplification of the
conjugated structures. Recently, several groups have reported new small push-pull molecules
with chemical structures based on arylamine donor moiety, such as triphenylamine, linked to
small acceptor unit through a more or less short conjugated spacer.[7, 13-22] Depending on

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the latter, the relative simplicity of the molecular architectures allows an access to the final
compounds within two or maximum four steps with global yields of ca 65 %. Additionally,
simple modifications of the donor and/or acceptor block induce a fine tuning of their
electronic and photovoltaic properties affording both high open circuit voltage (V ) and
OC
short circuit current density (JSC).
In our continuous effort to design simple and easily accessible molecular architectures for
developing cleaner conjugated materials for OSCs,[23, 24] we have focused our efforts on the
green and rapid synthesis of the push-pull molecules DA1 [17] and DA2. Regarding DA2, the
small structural modification of DA1 by grafting a cyano group on the thiophene connector,
has been designed to strengthen the acceptor character of the push-pull molecule and thus to
decrease the band gap of the corresponding material.
Figure 1. Structure of DA1 and DA2
Experimental
Synthesis
4
-bromo-N,N-diphenylaniline and 2-formylthiophene are commercially available. 2-formyl-3-
cyanothiophene 2 was obtained by the regioselective deprotonation of 3-cyanothiophene
followed by treatment with DMF.
2
-formyl-3-cyanothiophene 2
Under an argon atmosphere, solution of n-BuLi 2.5M in hexane (9 mL, 22.50 mmol) was
added dropwise to a solution of diisoproylamine (3.6 mL, 25.69 mmol) in dry THF (20 mL)

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cooled to -78°C. After stirring for 20 min, lithium diisopropylamine was added at -78°C to a
solution of thiophene-3-carbonitrile (2.1 mL, 23.09 mmol) in anhydrous THF (40 mL), after
3
0 min was added dimethylformamide (2.8 mL, 36.16 mmol). The mixture was slowly
warmed to room temperature and stirred for 16h. The organic layer was washed with water
and solution of HCl (2 M), dried over MgSO and the solvent was evaporated. The crude was
4
purified by chromatography over silica gel (Pentane/AcOEt: 95/5) to give a white powder
(1.39 g, yield: 44%).
MP: 84-86°C
-1
-1
IR: υ = 2233 cm (C≡N), 1672 cm (C=O).
1
NMR H (CDCl ): 10.14 (d, 1H, J=1.2 Hz), 7.84 (dd, 1 H, J=1.2 Hz, J=5.0 Hz), 7.44 (d, 1H,
3
J=5.0 Hz).
1
3
NMR C (CDCl ): 180.5, 149.2, 135.0, 131.0, 116.5, 112.8.
3
+
HRMS (EI) calcd for C H NOS [M ]: 136.9935, found: 136.9932.
6
3
Direct coupling arylation
To a solution of 4-bromo-N,N-diphenylaniline (709 mg, 2.19 mmol), potassium carbonate
(458 mg, 3.32 mmol, 1.5 eq), palladium acetate (30.80 mg, 6%mol), pivalic acid (70 mg, 0.69
mmol, 30%mol) and tricyclohexylphosphine tetrafluoroborate (104 mg, 0.28 mmol, 12%mol)
in toluene (30 mL) degassed with argon during 20 min was added 2 formylthiophene (500
mg, 4.45 mmol) or 2-formylthiophene-3-carbonitrile (600 mg, 4.38 mmol). The mixture was
stirred at 100°C during 16h, cooled and poured into water. The organic layer was extracted
with ethyl acetate (2 times), dried over MgSO , filtered, and solvent was removed. The
4
residue was purified by chromatography on silica gel (Hexane / AcOEt: 60/40).
5
-(4-(diphenylamino)phenyl)thiophene-2-carbaldehyde: 3
7
15 mg, 92% yield. Red solid.
MP: 88-90 °C[17, 25]

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1
NMR H (CDCl ): 9.85 (s, 1H); 7.71 (d, 1H, J=4.0Hz); 7.52 (d, 2H, J=8.8Hz); 7.34-7.26 (m,
3
5
H); 7.17-7.05 (m, 8H).
1
3
NMR C (CDCl ): 182.8, 154.8, 149.4, 147.2, 141.6, 137.9, 129.7, 127.5, 126.4, 125.4,
3
1
24.1, 123.1, 122.6.
+
HRMS (FAB) calcd for C H NOS [M ]: 355.1031, found: 355.1028
23
17
5
-(4-(diphenylamino)phenyl)-2-formylthiophene-3-carbonitrile: 4
7
73 mg, 93 % yield. Red solid.
MP: 159-161°C.
-1
-1
IR: υ = 2230 cm (C≡N), υ = 1658 cm (C=O).
1
NMR H (CDCl ): 10.07 (s, 1H), 7.48-7.43 (m, 3H), 7.35-7.30 (m, 4H), 7.16-7.11 (m, 6H),
3
7
.07-7.04 (m, 2H).
1
3
NMR C (CDCl ): 180.3, 154.8, 150.3, 146.7, 145.4, 129.8, 127.5, 125.7, 124.6, 124.5,
3
1
23.7, 121.7, 117.5, 113.0.
+
HRMS (FAB) calcd for C H N OS [M ]: 380.0983, found: 380.0987.
24
16
2
Knoevenagel reaction by ball milling process
General procedure. A mixture of (diphenylamino)phenyl)thiophene-carbaldehyde 3 or 4 (1
mmol) and malononitrile (1 mmol) with solid potassium acetate (10 % mol) were ball-milled
at room temperature during 30 min. with frequency of 30 Hz. The red or purple solid was
recovered then washed with a mixture of EtOH – Pentane (20 mL, 6/4). The solid was
purified by flash chromatography on silica gel (Hexane/AcOEt: 8/2) to give a red-brown or a
purple powder.
2
-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)malononitrile: DA1
3
83 mg, 95% yield. Red brown solid.
MP = 185-188°C
1
NMR H (CDCl ): 7.74 (s, 1H); 7.68 (d, 1H, J=4.0 Hz); 7.53 (d, 2H, J=8.8 Hz); 7.34-7.29 (m,
3
5
H); 7.16-7.10 (m, 6H); 7.05 (d, 2H, J=8.8Hz).
1
3
NMR C (CDCl ): 157.3, 150.4, 150.1, 146.7, 140.5, 133.2, 129.7, 127.7, 125.6, 125.0,
3
1
24.5, 123.4, 121.8, 114.7, 113.8, 75.1.

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+
HRMS (FAB) calcd for C H N S [M ]: 403.1143, found: 403.1137.
26
17
3
Elemental Anal. calcd for C H N S: C, 77.39; H, 4.25; N, 10.41 found: C, 77.13; H, 4.26;
2
6
17
3
N, 10.44.
2
4
-((3-cyano-5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)malononitrile: DA2
06 mg, 95% yield. Purple solid.
MP: 215-217°C.
-1
IR: υ = 2226 cm (C≡N).
1
NMR H (CDCl ): 7.99 (s, 1H), 7.51-7.48 (m, 3H), 7.37-7.31 (m, 4H), 7.18-7.14 (m, 6H),
3
7
.05-7.03 (m, 2H).
1
3
NMR C (CDCl ): 156.4, 151.1, 146.5, 146.3, 137.5, 129.9, 127.9, 126.0, 125.1, 124.0,
3
1
22.7, 121.1, 120.5, 113.5, 112.8, 79.9.
+
HRMS (FAB) calcd for C H N S [M ]: 428.1096, found: 428.1087.
27
16
4
Elemental Anal. calcd for C H N S: C, 75.68; H, 3.76; N, 13.07; found: C, 75.70; H, 3.76;
2
7
16
4
N, 12.99.
Crystal Data and Structure Refinement
Single crystals of DA2 suitable for X-ray diffraction analysis were obtained by slow
evaporation of a mixture of ethanol- chloroform solution. The compound crystallizes in the
monoclinic C2/c space group. X-ray single-crystal diffraction data were collected at 293 K on
a BRUKER KappaCCD diffractometer, equipped with a graphite monochromator utilizing
Mo Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods and refined on
F2 by the full-matrix least-squares method using the SHELX97 package. All non-H atoms
were refined anisotropically, and absorption was corrected by the SADABS program. The H
atoms were included in the calculation without refinement.
Crystallographic Data for DA2.
3
C H N S, M = 428.5, red plate, 0.30 × 0.25 × 0.06 mm , monoclinic, space group C2/c, a =
27
16
4
3
1.315(3) Å, b = 8.1924(5) Å, c =17.832(2) Å, α = 90°, β = 103.555(9)°, γ = 90°, V

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3
3
−1
=
4447.3(7) Å , Z =8, ρ =1.280 g/cm , ꢀ(Mo Kα) = 0.167 mm , F(000) = 1776, θmin =
calc
3
.20°, θmax = 27.49°, 23142 reflections collected, 5068 unique (Rint = 0.0535),
parameters/restraints = 289/0, R1 = 0.0603 and wR2 = 0.0975 using 2951 reflections with I >
σ(I), R1 = 0.1293 and wR2 = 0.1188 using all data, GOF = 1.057, −0.248 < ꢁρ < 0.208
2
−
3
e.Å . CCDC- 1486170.
Preparation of the solar cells
Indium-tin oxide coated glass slides of 24×25×1.1 mm with a sheet resistance of RS = 10
ꢂ/sq were purchased from VisionTek Systems Ltd. The substrates were scrubbed using
dishwashing soap before being cleaned by a series of ultrasonic treatments for 15 min in
distilled water, acetone, and isopropanol. Once dried under a steam of nitrogen, a UV-ozone
plasma treatment (UV/Ozone ProCleaner Plus, Bioforce Nanosciences) was performed for 15
min. A filtered aqueous solution of poly(3,4-ethylenedioxy-thiophene)-poly(styrenesulfonate)
(PEDOT:PSS; Clevios P VP. AI 4083) through a 0.45 ꢀm RC membrane (Millex®) was
spun-cast onto the patterned ITO surface at 5000 rpm for 40 s before being baked at 115°C
for 15 min. The best devices were obtained with films spun-cast from chloroform solutions at
6
000 rpm for DA1, DA2 and Blend containing 10 mg/mL of material(s). Finally, OSCs were
completed by the successive thermal deposition of C (30 nm) and aluminum (80 nm) at a
6
0
-6
2
pressure of 10 Torr through a shadow mask defining two cells of 27 mm each. J vs V curves
were recorded in the dark and under illumination using a Keithley 236 source-measure unit
and a home-made acquisition program. The light source is an AM1.5 Solar Constant 575 PV
simulator (Steuernagel Lichttecknik, equipped with a metal halogen lamp). The light intensity
was measured by a broad-band power meter (13PEM001, Melles Griot). EQE was recorded
under ambient atmosphere using a halogen lamp (Osram) with an Action Spectra Pro 150

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monochromator, a lock-in amplifier (Perkin-Elmer 7225) and a S2281 photodiode
(Hamamatsu).
Space charge limited current (SCLC) measurements
A solution of molecular donor (10 mg/mL) in chloroform was spun cast at 1500 rpm on the
above described PEDOT: PSS substrates to provide organic layers of ca 80 nm. Gold cathodes
-6
(
150 nm) were thermally evaporated under a vacuum of 10 Torr, through a shadow mask
defining actives area of 12.60 mm², 3.10 mm² and 0.78 mm² per substrates. Hole mobilities µh
2
3
were evaluated using the Mott-Gurney law, ie, JSCLC = (9/8)ε ε µ (V /d ) where ε is the
0 r
h
0
permittivity of free space, ε is the static dielectric constant of the medium (ε = 3), V is the
r
r
voltage applied and d the thickness of the active layer.[26]
Results and discussion
As depicted in scheme 1, the synthesis of the titled derivatives has been carried out in two
green steps. First, the commercially available 4-bromo-triphenylamine (TPA-Br) was engaged
in a direct arylation reaction with the aldehydes 1 or 2 affording the extended derivatives 3
and 4. The latter were then engaged in a Knovenagel condensation with malononitrile
yielding the titled donors DA1 and DA2. In addition, it is worth noting that greener
procedures have been studied for these two steps.
Scheme 1: Green synthesis of compounds DA1 and DA2.
Regarding the direct arylation coupling, several procedures have been examined. All the
results are summarized in Table 1. The procedure described by Fagnou et al.[27] has been
first applied (entries 1 and 2) by using 1 equivalent TPA-Br and 2 equivalents of thiophene

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aldehyde (1 or 2), K CO (3 equiv.), Pd(OAc) (6 mol %), pivalic acid (PivOH, 30 mol %),
2
3
2
phosphine derivative PCy HBF (10-12 mol %) at 100°C in toluene for 16 h. Compounds 3
3
4
and 4 were isolated in 92 and 93 % yield respectively. As the excesses of aldehyde reactants 1
and 2 were not recurved, attempts were also performed by using a stoichiometric amount of
reactants (entries 3 and 4) to give an acceptable 72 % yield for 3, but a very low yield of ca
8
% for 4.
Table 1. Direct arylation between thiophene aldehydes 1 and 2 with TPA-Br using various
condition
a
Entry Thiophene TPA-Br
aldehyde Amount
equiv
Additives
Yield
(%)
PCy HBF
3
4
1
2
3
4
1
2
1
2
0.5
0.5
1.0
1.0
(12%)
PivOH (30%)
92
93
72
8
PCy HBF
3
4
(12%)
PivOH (30%)
PCy HBF
3
4
(12%)
PivOH (30%)
PCy HBF
3
4
(12%)
PivOH (30%)
a
Yields are isolated,
The Knoevenagel condensation between the corresponding aldehydes (3 and 4) and
malononitrile were then carried out, affording DA1 and DA2 in simplified green procedures.
By contrast with the usual method using chloroform in presence of piperidine or Et N,[17, 21,
3
2
8, 29] condensations without solvent by ball-milling technique have been performed.[30, 31]
The reactions proceeded with quantitative yields after 30 min of stirring at room temperature
of a blend of aldehyde (3 or 4), malononitrile (1 equivalent) and potassium acetate (10 mol
%). Highly pure materials were isolated in 95 % yield after a flash chromatography on silica
gel. The condensation can be also performed by reflux of ethanol solution during 16h with an
excess of malononitrile (2 equivalents) to give DA1 and DA2 in 90% yields. At room
temperature in ethanol in presence of catalytic amount of tBuONa, the yields for obtaining the

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two compounds decrease to 70 %. Finally, the ball- milling technique gives the best results in
terms of efficiency and rapidity while handling a minimum of solvent.
Thus the two compounds were prepared in two steps with a global yield of ca 85%. To
evaluate the environmental impact of these new synthesis of DA1, the amount of waste
produced per gram (E factor)[32] has been calculated and compared with the conventional
synthetic routes using organometallic coupling reactions (Stille and Suzuki cross couplings)
for the first step. For the calculation of the E factor, the criteria defined by Osedach et al. have
been used (see Figure S1-S4 in supplementary materials).[12] The access to DA1 from TPA-
Br and the commercially available thiophene carbaldehyde 1 via the direct C-H arylation
gives an E factor of 903. The greener method of this procedures, including the Knoevenagel
reaction in ball mills, is clearly demonstrated by comparing with the E factors obtained when
Suzuki coupling reaction was used (E = 1636).[25] In this procedure, if the coupling reaction
did not produce more wastes than for the direct-arylation way, the synthesis of the boronic
derivative in two steps from thiophene carbaldehyde[33] participates for 30 % in the E factor.
The synthesis of DA1 via Stille coupling reaction[17] leads to an even higher E factor (E =
3
445). The preparation and the use of the stannyl derivative of thiophene participates for 50%
in the E factor. The step of formylation by Vilsmeir-Haack reaction also generates a large
amount of wastes.
Finally, the synthesis of compound DA2, results in an E factor of 1572 including the
preparation of the 3-cyano-2-formylthiophene 2 from the commercially available 3-
cyanothiophene.
The crystallographic structures of DA2 single crystals, obtained from a slow evaporation of
ethanol-chloroform solutions, have been analysed by X-Ray diffraction. The latter crystallizes
in the monoclinic C1 2/c1 space group. It is important to note that the conformation adopted
by DA2 (Figure 2a) is similar to that obtained for DA1.[17, 18] The dicyanovinyl (DCV)

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moiety is in the plane defined by the 3-cyanothiophene unit and adopts a cis conformation
relative to the sulfur atom. The two outermost phenyl rings of the TPA unit are out of the
plane defined by the thiophene and DCV systems while the torsional angle between the
connected phenyl and thiophene cycles is of 19.1°. The structure is characterized by two axes
of stacking which are quasi perpendiculars (Figure 2b). In each column, the stacking of the
molecules is done so that each molecule presents several short interatomic distances less than
3
.6 Å with three other molecules as shown by dotted blue lines in Figure 2c.
Figure 2: Single-crystal X-ray structure of DA2.
The influence of the cyano group borne by the thiophene connector on the electronic
properties has been evaluated by theoretical calculations performed at the ab initio density
functional level with the Gaussian 09 package. Becke’s three-parameter gradient-corrected
functional (B3LYP) with a polarized 6-31G (d,p) basis was used for full geometry. The
contours of the HOMO and LUMO orbitals and a schematic evolution of their levels are

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illustrated in Figure 3. The optimized structures of the two compounds are almost identical
and, they present planar structures of the conjugated backbones and a torsion of the outermost
phenyl units of TPA. The HOMO are mainly allocated on the TPA – Thiophene units and the
carbon atoms of thiophene internal at TPA whereas LUMO are localized on the thiophene -
dicyanovinyl moieties as well as the extra cyano group borne by DA2. Such contribution of
this cyano group led to a strong stabilisation of the LUMO level and a moderate stabilisation
of the HOMO level for DA2 compared to DA1, resulting in a reduction of the band gap of ca
0
.3 eV for DA2. Thus, both a red shift of the absorption band and a positive shift of the
reduction potential are expected for DA2 compared to DA1.

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Figure 3: Calculated HOMOs and LUMOs and schematic representation of the energy levels
for DA1 and DA2.
The UV-vis absorption spectra of DA1 and DA2 have been recorded in methylene chloride
and from thin film spun casted on glass sheets (Figure 4 and Table 2). Both compounds
exhibit an intense absorption band in the 400 – 700 nm region assigned to an internal charge
transfer (ICT transition). In solution, DA1 presents a maximum wavelength λmax at 500 nm
-1
-1
with a molecular absorption coefficient εDA1 of 34232 L.mol cm . It turns out that the
insertion of the cyano group on the thiophene connector (DA2) both leads to an increase of ε

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and a red shift of the absorption band of ca 50 nm. In solid state, the absorption bands of titled
compounds are broadened and red shifted with maxima of the absorption bands around 521
and 573 nm corresponding to band gaps, calculated from the foot edge of the absorption
bands, of ca 1.95 and 1.77 eV for DA1 and DA2 respectively. This difference of ca 0.18 eV,
due to the electro-deficient cyano group grafted on the thiophene, is in good agreement with
the theoretical calculation.
-5
Figure 4 : Left) UV/Vis absorption spectra in solution (10 M in CH Cl ) of DA1 (black)
2
2
and DA2 (blue; Right) Normalized absorption spectra of films of DA1 (black) and DA2
blue).
(
Cyclic voltammograms (CVs) of DA1 and DA2, recorded in dichloromethane using Bu NPF6
4
as supporting electrolyte, show very close reversible oxidation peaks at 1.00 and 1.07 V
respectively (Figure 5 and Table 2). In contrast, the irreversible reduction peak of DA2 is
positively shifted by 380 mV compared to DA1. Considering the onsets of the oxidation and
reduction waves, both materials display very close experimental HOMOs (–5.89 eV and -5.90
eV for DA1 and DA2 respectively), whereas the LUMOs are well differenced (-3.95eV vs -
4
.35 eV for DA1 and DA2 respectively). Once again, these results agree with the theoretical
calculations since the insertion of the cyano group mainly contributes in stabilizing the
LUMO level of DA2.

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-3
Figure 5: CVs of DA1 (black) and DA2 (blue) 10 M in 0.1 M Bu NPF in CH Cl , v = 100
mV.s
4
6
2
2
-1
Table 2: Optical and electrochemical data of DA1 and DA2
a
a
b
c
c
d
d
Compd.
λmax
nm
ε
Eg
Eox
V
Ered
V
EHOMO
V
-5.89
-5.90
ELUMO
V
-3.95
-4.35
∆Eelec
eV
max
-
1
-1
Lmol cm
34232
eV
1.94
1.77
DA1
DA2
500
550
1.00
1.07
-1.15
-0.77
1.94
1.55
43217
a
-5
b
c
-3
1
0 M in CH Cl . From the foot edge bands absorption of films spun-cast on glass. 10 M
2 2
-
1
d
in 0.1 M Bu NPF –CH Cl , 100 mV.s , Pt working electrode, ref. SCE. Using an offset of -
4
6
2
2
4
.99 eV vs. vacuum level for SCE.[34]
The potential of DA2 as donor material has been evaluated and compared to DA1 in basic
bilayer planar heterojunction solar cells (PHJ). The cells were prepared by spin-casting a
chloroform solution of donor onto indium tin oxide (ITO) precoated with poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT/PSS). Then a 30 nm layer of C60
was deposited by thermal evaporation under vacuum, followed by the deposition of an 80 nm
aluminium electrode (27 mm² of active area). Annealing tests of the films have been

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performed at different temperatures. Results are gathered in Table 3 and the current voltage of
the cells are presented in Figure S5 in supplementary materials.
Solar cells fabricated with the reference compound DA1 are characterized by a moderate
average open-circuit voltage (V ) of ca 0.72 V, a fill factor (FF) of 0.36 and a relatively high
OC
-2
short-circuit current density (J ) for a bilayer cells of 6.91 mA.cm , resulting in an average
SC
PCE of 2.26 % (best of 2.46 %). In comparison, compound DA2 based OSCs are slightly less
efficient with a PCE close to 2% due to a decrease of both the VOC (0.66 V) and the FF (0.31).
-2
However, it is noteworthy that high JSC up to 7.5 mA.cm , were reached. In parallel,
h
measurements of the hole mobility (µ ), using the space charge limited current method using
h
-5
-5
2
-1 -1
the Mott-Gurney law, show very close results µ of ca 1.1x10 and 0.9x10 cm V s for
DA1 and DA2 respectively (Figure S6 in SI). To gain further insights, comparison of the
external quantum efficiency (EQE) spectra were performed on best devices (Figure 6). The
latter, in consistency with the UV-visible spectra and the Jsc parameters, reveal that grafting
the cyano on the thiophene connector results in a significant extension of the contribution of
the donor in the long wavelengths region. From these results, it turns out that blending these
two complementary donors should increase the charge carrier generation.[35-39]
Indeed, this efficient strategy has already been demonstrated in ternary blends for bulk
heterojunction solar cells where two donors were successfully mixed with the fullerene
derivative.[40-42]
Thus, the similar crystallinity, suggested by the X-ray structures, the good complementarity in
absorption coupled to comparable hole mobility, suggest that the two donors DA1 and DA2
should be compatible and would even team up to improve the overall efficiency. In this
context, bilayer solar cells based on an equimolar blend of DA1 and DA2 for the donor
materials and C60 for the acceptors materials were prepared (Figure S5 in supplementary
materials). It turns out that even if the FF slightly decreased the VOC overtook +0.84 V and the

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-2
Jsc parameter rose to 8.4 mA.cm achieving a maximum PCE of ca 3.1%. Although we have
no rational explanation for the increasing of the VOC for the blend of DA1 and DA2, that
present similar HOMO level, it is worth noting that the origin of the theoretical VOC of
organic photovoltaic devices has yet to be determined. Recent works demonstrated the
influence on the VOC values of the donor – acceptor interactions at the interface between small
donor molecules and acceptors.[43, 44] The EQE spectrum for the blend confirms a synergic
contribution of the absorption of the two compounds in the 650 – 400 nm region. This result
confirms the compatibility of the molecules through aggregation and crystallization to give
well organized active layers favourable to increase the J value.
SC
Table 3. Photovoltaic characteristics of PHJ cells based on donor DA1 and DA2 and under
-2
1
.5-simulated solar illumination at 90 mW cm after 5 min annealing
Compound
Tannealed
°C)
120
140
120
Average
VOC (V)
0.72
0.66
0.84
Average -
Average Average
Best
PCE (%)
2.46
2
(
Jsc (mA cm )
6.91
FF
PCE (%)
2.26
DA1
DA2
DA1 + DA2
0.36
0.31
0.30
7.52
8.38
1.91
1.96
3.06
2.66

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Figure 6. External quantum efficiency (EQE) of the cells produced with donors DA1 (black),
DA2 (blue) and a blend of DA1 and DA2 (green)
Conclusion
In summary, we have prepared two analogous donors whose syntheses have been performed
by the greenest routes as possible, i.e., through direct arylation using C-H bond activation and
Knoevenagel condensation without any solvent. We have examined the effects of a small
structural modification on the electronic properties and on the photovoltaic performances of
basic bilayer solar cells. An efficient improvement of the performance of the cells was
demonstrated when active layers were built with blend of donors. We demonstrate that the
minor molecular change for the donor materials allowed a sufficient variation of the electronic
properties without strong modification in the supramolecular organizations in the solid state.
Thus, the advantage of the binary active layer for enhancing the harvesting of photons is

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related to the synergistic effects of similar self-assembly properties, hole mobility and
complementary absorption of the titled materials.
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Graphical abstract