Small molecular donors for organic solar cells obtained by simple and clean synthesis

Article abstract of DOI:10.1002/cssc.201301339

A small donor-acceptor molecule is synthesized in a two-step procedure involving reaction of N,N-diphenylhydrazine on 2,5-diformylthiophene and Knoevenagel condensation. Results of UV/Vis absorption spectroscopy and cyclic voltammetry show that replacement of the phenyl ring bridge of a reference compound 2 by an azo group produces a slight red-shift of λ _max, an enhancement of the molecular absorption coefficient, and a decrease of the energy level of the frontier orbitals. A preliminary evaluation of the potentialities of compound 1 as donor material in a basic bilayer planar heterojunction cell of 28 mm² active area using C₆₀ as acceptor gave a short-circuit current density of 6.32 mA cm^-2 and a power conversion efficiency of 2.07 %. Keep it simple: A simple donor-acceptor molecule is synthesized from 2,5-thiophene dialdehyde using two condensation reactions with water as sole by-product. A 30 mm² simple bilayer solar cell fabricated with a spun-cast film of donor and vacuum-deposited C ₆₀ shows a power conversion efficiency >2.0 % under simulated solar light.

Full text of DOI:10.1002/cssc.201301339

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DOI: 10.1002/cssc.201301339
Small Molecular Donors for Organic Solar Cells Obtained
by Simple and Clean Synthesis
Dora Demeter,[a] Salma Mohamed,[a] Andreea Diac,[a, b] Ion Grosu,[b] and Jean Roncali*[a]
A small donor–acceptor molecule is synthesized in a two-step
procedure involving reaction of N,N-diphenylhydrazine on 2,5-
diformylthiophene and Knoevenagel condensation. Results of
UV/Vis absorption spectroscopy and cyclic voltammetry show
that replacement of the phenyl ring bridge of a reference com-
witnessed rapid progress and solution-processed BHJ cells
with PCE values comparable to those obtained with the best
polymer-based devices have been recently reported by several
groups.[6] However, these high performances have been ob-
tained with complex molecules of high molecular weight, pre-
pared by multistep syntheses involving metal-catalyzed cou-
pling reactions and with overall yields sometimes limited to
a few percents.
pound 2 by an azo group produces a slight red-shift of lmax
,
an enhancement of the molecular absorption coefficient, and
a decrease of the energy level of the frontier orbitals. A pre-
liminary evaluation of the potentialities of compound 1 as
donor material in a basic bilayer planar heterojunction cell of
28 mm2 active area using C60 as acceptor gave a short-circuit
current density of 6.32 mAcmÀ2 and a power conversion effi-
ciency of 2.07%.
In addition to problems of the cost of the synthesis, the
presence of remnant traces of metals such as palladium or
nickel can have deleterious effects on the performances of or-
ganic semi-conductors.[7] Recently, several authors have dis-
cussed issues associated with the industrial development of
OPV and emphasized the key role of the cost, scalability, and
environmental impact of the synthesis of active material.[8] In
this context, our group has undertaken a project aimed at
a drastic reduction of the size and complexity of molecular
donors.[9,10] This approach is expected to contribute to de-
crease the cost of active OPV materials by improving the over-
all yield and scalability of the synthesis and to limit its environ-
mental impact by reducing the number of synthetic steps and
avoid as much as possible, the use of metal-catalysts.
In recent years several groups have described efficient OPV
cells fabricated by vacuum-deposition of small molecular
donors. Thus, Wꢀrthner et al. have reported cells with PCE of
6.10% based on merocyanines donors.[11] Wong et al. have syn-
thesized donor–acceptor molecules based on triphenylamine
and fabricated solar cells with a maximum PCE of 6.80% using
C70 as acceptor.[12]
Organic photovoltaics (OPVs) are attracting increasing interest
motivated by the technological opportunities offered by the
lightness, plasticity, and flexibility of organic materials.[1–6] How-
ever, besides scientific and technical interest, the considerable
development of OPVs is mainly motivated by the expected
drastic reductions of cost and environmental impact compared
to the already established silicon solar cells.
In essence, designing an OPV cell involves the creation of
a heterojunction by interfacing an electron donor material
with an electron acceptor material.[1] Soluble conjugated poly-
mers constitute the main class of donor materials and high
power conversion efficiencies (PCEs) of 8–9% have been re-
ported for single-junction devices.[2] However, the inherent
polydispersity of polymers poses the problem of the reproduci-
bility of the synthesis, purification, and, hence, composition
and electronic properties of the active material. An alternative
approach consists in replacement of the polymers by molecu-
lar donors. Molecular donors have been used in multilayer
planar heterojunction (PHJ) cells fabricated by vacuum deposi-
tion for a long time, and the field is still very active.[3] Soluble
molecular donors introduced in 2005 in the fabrication of solu-
tion-processed bulk heterojunction (BHJ) solar cells[4] have rap-
idly gained considerable interest and given rise to the synthe-
sis of a huge number of new chromophores.[5] The field has
We have recently described
a cell of 4.0% PCE fabricated by
co-evaporation of compound
2
(Scheme 1) and C60 as acceptor.[9-
ab,13] Furthermore, an even a smaller
molecule such as compound 3 can
still produce a short-circuit current
density (Jsc) of almost 7.0 mAcmÀ2
and a PCE close to 2.0%.[10b] How-
ever the light-harvesting proper-
ties of this molecule are limited
(lmax =473 nm, lonset =550 nm).
[a] D. Demeter, S. Mohamed, A. Diac, J. Roncali
Group Linear Conjugated Systems
Grazulevicius et al. have widely
developed hole-transporting mate-
rials synthesized by condensation
of diarylhydrazines with various al-
dehydes.[14] More recently this ap-
proach was used by Shen et al. for
the synthesis of sensitizers for dye-
sensitized solar cells (DSSCs).[15] As
CNRS, Moltech-Anjou, University of Angers
2 Bd Lavoisier 49045 Angers (France)
E-mail: jeanroncali@gmail.com
[b] A. Diac, I. Grosu
Babes Bolyai University
Center of Supramolecular Organic and Organometallic Chemistry
(CSOOMC)
Scheme 1. Chemical structure
of the target donor 1 and ref-
erence compounds.
Cluj-Napoca, 11 Arany Janos St., 400028 Cluj-Napoca (Romania)
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a further step in the development of donor molecules for OPV
combining simplicity, low molecular weight, and clean synthe-
sis we report here the preparation of compound (1) with a di-
phenylhydrazone donor block and a preliminary evaluation of
its potentialities as donor in basic bilayer PHJ cells.
The synthesis of compound 1 is depicted in Scheme 2. Shen
et al. have synthesized sensitizers by a three-step synthesis in-
volving reaction of N’N-diphenylhydrazine with 2-thiophene
carboxaldehyde followed by formylation of the free a-position
Figure 1. Top: UV/Vis absorption spectrum of compound 1 in methylene
chloride. Bottom: normalized UV/Vis absorption spectra of thin films of com-
pound 1 spun-cast on glass from chloroform solution (dashed line) and
chlorobenzene solution (solid line).
Scheme 2. Synthesis of the target compound 1.
of thiophene of the resulting hydrazone and condensation
with cyanoacrylic acid.[15] In an attempt to simplify the synthe-
sis by skipping the formylation step, we have directly reacted
N’N-diphenylhydrazine (7) with 2,5-thiophene dicarboxalde-
hyde (6). Reaction of 6 with one equivalent of 7 in methanol
at low temperature gives as major product the bis-adduct 5
while the target monoaldehyde 4 was isolated in 12% yield. In
a second series of experiments, one equivalent of 7 was react-
ed with 0.5 equivalent of dialdehyde 6 using a more dilute re-
action medium. Under these conditions aldehyde 4 was isolat-
ed in 62% yield together with 21% of the bis-adduct 5 while
18% of the starting dialdehyde 6 were recovered. A Knoevena-
gel condensation of aldehyde 4 and malonodinitrile gave the
target compound 1 in 70% yield (Scheme 2).
The UV/Vis absorption spectrum of compound 1 in methyl-
ene chloride shows an absorption maximum (lmax) at 504 nm
(Figure 1), which corresponds to a 31 nm bathochromic shift
compared to compound 3 and a 3 nm red-shift compared to
compound 2. Compound 1 shows a good solubility in chloro-
form (25 mgmLÀ1) and in chlorobenzene although the solubili-
ty is lower in this solvent (12.5 mgmLÀ1). The UV/Vis absorp-
tion spectra of thin-films cast on glass from solutions in these
two solvents show in both cases a bathochromic shift of lmax
to ca. 525 nm and a broadening of the absorption band due
to intermolecular interactions in the solid state.
Figure 2. Cyclic voltammogram of compound 1 in 0.10m Bu4NPF6/CH2Cl2, Pt
electrodes, scan rate 100 mVsÀ1
.
a more extended conjugated system by coupling of the cation
radical, as frequently observed in the electrochemistry of TPA
derivatives.[16] In the negative potential region (not shown), the
CV shows an irreversible reduction wave peaking at À0.91 V in-
dicative of unstable anion radical. Comparison of the data for
compound 1 to those for 2 and 3 shows that compound
1 presents the most red-shifted lmax, the highest molecular ab-
sorption coefficient (e), and the lowest highest occupied mo-
lecular orbital (HOMO) and lowest unoccupied molecular orbi-
tal (LUMO) energy levels (Table 1). Considering the fact that
the open-circuit voltage (Voc) of OPV cells depends on the dif-
ference between the LUMO of the acceptor and the HOMO of
the donor, this deep HOMO level appears interesting.[17] These
results show that replacement of the phenyl bridge of com-
pound 2 by an azo group produces interesting modifications
of the electronic properties of the molecule regarding a possi-
ble use as donor in OPV cells. It is worth noting that these
modifications are associated with a decrease of the molecular
A cyclic voltammogram (CV) of 1 recorded in CH2Cl2 with
Bu4NPF6 as supporting electrolyte is shown in Figure 2. This CV
exhibits a quasi-irreversible oxidation wave with an anodic
peak potential (Epa) at 1.32 V. The weak cathodic peak observed
around 1.0 V in the reverse scan suggests the formation of
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have no explanation for this phenomenon it can be hypothe-
sized that the thermally-induced rearrangement of the active
donor/acceptor (D/A) interfacial zone is different for the two
acceptors. Application of higher annealing temperature
(1108C) produces a small further increase of Jsc but begins to
deteriorate Voc and FF values. Figure 3 shows the J–V curves for
the best device of the series, which gave a Jsc of 6.32 mAcmÀ2,
a Voc of 0.79 V, and a PCE of 2.07%.
Table 1. Data of UV/Vis spectroscopy and cyclic voltammetry in the con-
ditions of Figures 1 and 2.
[a]
Donor MW
[gmolÀ1
lmax
emax
Epa
[V]
Epc
[V]
EHOMO
[eV]
ELUMO
[eV]
]
[nm] [MÀ1 cmÀ1
]
1
354
403
327
504
501
473
39000
33900
27000
1.32 À0.91 À6.20
1.01 À1.23 À5.89
1.20 À1.17 À6.08
À4.35
À4.06
À4.12
2[b]
3[c]
[a] Using an offset of À4.99 eV for SCE vs. the vacuum level.[18] [b] From
Ref. [9a]. [c] From Ref. [10b].
weight and a simplification of the synthesis with in particular
the elimination of the Stille coupling involved in the synthesis
of compound 2.[9]
A preliminary evaluation of potentialities of compound 1 as
donor for OPV has been carried out on bilayer PHJ cells fabri-
cated by spin-casting a ca. 20 nm thick film of 1 from a chloro-
form solution onto indium tin oxide (ITO) substrates precoated
with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS). The substrates were then introduced into
a vacuum chamber, a 30 nm thick layer of C60 or C70 fullerene
acceptor was deposited by thermal evaporation under a pres-
sure of 2ꢂ10À6 mbar, and the devices were completed by dep-
osition of a 100 nm layer of aluminum. Although the solubility
of compound 1 could allow the fabrication of solution-pro-
cessed BHJ cells, no attempt at that direction was made at this
stage. In fact BHJ cells generally require tedious optimization
work whereas simple PHJ cells, although less efficient, are
more suitable for a first evaluation of new materials as they
give more reproducible and more reliable results.
As shown in Table 2, as-fabricated cells show short-circuit
current densities (Jsc) in the range of 2.50 to 3.50 mAcmÀ2 and
PCE of 0.60 to 0.80%. Devices containing C70 as acceptor give
slightly better Jsc and PCE values but lower fill factors (FF). Ap-
plication of a 10 min thermal treatment at 1008C strongly im-
proves the photovoltaic characteristics with in particular
a large increase of Jsc to values exceeding 6.00 mAcmÀ2. After
annealing, the best results are obtained with C60. Although we
Figure 3. a) Current density–voltage curves for a bilayer solar cell (ITO/PE-
DOT:PSS/1/C60/aluminium. In the dark (open circles) and under simulated
solar light with incident power light of 90 mWcmÀ2 (black circles). b) Exter-
nal quantum efficiency spectra of the bilayer cells 1/C60.
Figure 3(bottom) shows the external quantum efficiency
(EQE) action spectrum of the best device under monochromat-
ic irradiation. The spectrum shows a broad maximum of ca.
55–60% in the 400–600 nm region with an onset of photocur-
rent at ca. 620 nm. Although these results are encouraging
they also underline the need for improving the light-harvesting
properties of the donor by further structural modification.
When evaluated in the above conditions, reference com-
pound 2 gives a PCE of 2.50%. In a recently published paper,
a thorough optimization of the device architecture has allowed
to increase this value to 4.00% for cells based on co-evaporat-
ed bulk heterojunction of donor and C60 layer.[13] This result
suggests that there is still room for improvement in the case
of donor 1.
Table 2. Photovoltaic characteristics of 1/acceptor bilayer solar cells
under AM 1.5 simulated solar light with an incident power light of
90 mWcmÀ2
.
Cell
Acceptor
Ann. temp
[8C]
Jsc
Voc
[V]
FF
[%]
PCE
[%]
[mAcmÀ2
]
1
2
3
4
1
2
3
4
1
2
3
4
C60
C60
C70
C70
C60
C60
C70
C70
C60
C60
C70
C70
ambient
ambient
ambient
ambient
100
100
100
100
110
2.48
2.05
3.17
3.46
6.32
5.72
4.64
4.16
6.40
6.50
4.73
5.27
0.84
0.89
0.86
0.88
0.79
0.82
0.87
0.89
0.75
0.64
0.77
0.77
27.4
28.1
23.9
25.5
37.4
35.3
29.3
29.1
33.1
37.3
27.9
27.4
0.63
0.57
0.72
0.85
2.07
1.83
1.31
1.19
1.76
1.72
1.13
1.23
To summarize, a soluble push–pull molecule has been syn-
thesized using two condensation reactions with water as by-
product. Comparison of the electronic properties to those of
a reference compound shows that replacement of thiophene
110
110
110
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153.8, 150.0, 142.3, 139.1, 133.8, 130.0, 127.3, 126.4, 125.8, 122.5,
122.4, 114.5, 113.5, 75.7 ppm. MS MALDI-TOF: 354.2.
by an azo bridge leads at the same time to a decrease of mo-
lecular weight and to improved optical properties and energy
levels a priori favorable for photovoltaic conversion. Prelimina-
ry tests with simple bilayer solar cells show that performances
comparable to those reported for much larger molecules eval-
uated in optimized devices of smaller active area can be ob-
tained with a small molecule obtained by simple and clean
chemistry.
Device fabrication and testing
Indium-tin oxide coated glass slides of 24 mmꢂ25 mmꢂ1.1 mm
dimensions with a surface resistance of 10 W/& were purchased
from Kintec company. Part of the ITO layer was etched away with
37% HCl. The ITO electrodes were then cleaned in ultrasonic bath
(successively Deconex (from VWR international GmbH), distilled
water (15.3 MWcmÀ1), acetone, ethanol and distilled water again
for 10 min each and dried in an oven at 1008C. The dried electro-
des 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 choloform solutions containing 4 mg donor/mL.
After film deposition the devices were introduced in an argon glo-
vebox (200B, MBraun) equiped with a vacuum chamber and
a 25 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 diameter (0.28 cm2) on each
ITO electrode.
The J vs V curves of the devices were recorded in the dark and
under illumination using a Keithley 236 source-measure unit and
a home-made acquisition program. The light source was an AM1.5
Solar Constant 575 PV simulator (Steuernagel Lichttechnik,
equipped with a metal halogen lamp). The light intensity was mea-
sured by a broad-band power meter (13PEM001, Melles Griot). The
devices were illuminated through the ITO electrode side. The effi-
ciency values reported here are not corrected for the possible
spectral mismatch of the solar simulator. External quantum efficien-
cy (EQE) was measured using a halogen lamp (Osram) with
an Action Spectra Pro 150 monochromator, a lock-in amplifier
(PerkinElmer 7225) and a S2281 photodiode (Hamamatsu).
Work aiming at the extension of this synthetic approach
with in particular optimization of the selectivity of the reaction
of diphenylhydrazine with dialdehydes, the design of mole-
cules with improved light-harvesting properties, and further
evaluation in BHJ cells is now underway and will be reported
in future publications
Experimental Section
General
NMR spectra were recorded with a Bruker AVANCE III 300 (1H,
300 MHz and 13C, 75 MHz) or Bruker AVANCE DRX 500 (1H, 500 MHz
and 13C, 125 MHz). Chemical shifts are given in ppm relative to
TMS. Infrared spectra were recorded on a Bruker spectrometer
Vertex 70 and UV/Vis spectra 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 dithranol as matrix.
Cyclic voltammetry was performed in dichloromethane solutions
purchased from SDS (HPLC grade). Tetrabutylammonium hexafluor-
ophosphate (0.10m as supporting electrolyte) was purchased from
Acros and used without purification. Solutions were deaerated by
nitrogen bubbling prior to each experiment. Experiments were car-
ried out in a one-compartment cell equipped with platinum elec-
trodes and saturated calomel reference electrode (SCE) with a Bio-
logic SP-150 potentiostat with positive feedback compensation.
Synthesis
Keywords: condensations
·
donor–acceptor
·
energy
5-((2,2-diphenylhydrazono)methyl)thiophene-2-carbaldehyde (4).
A
conversion · photovoltaics · solar cells
solution of thiophene-2,5-dicarbaldehyde (6) (1.27 g, 9.06 mmol) in
50 mL of dry THF is added dropwise to a mixture of N,N-diphenyl-
hydrazine hydrochloride (7) (1.0 g, 4.53 mmol) and sodium acetate
(1.40 g, 18.12 mmol) in 15 mL of dry methanol under argon atmos-
phere at room temperature. The reaction mixture is stirred over-
night at room temperature and then diluted with methylene chlo-
ride, washed with water and brine. After removal of the solvent
the residue is chromatographed on silica gel using methylene chlo-
ride as eluent to afford a yellow powder (0.85 g, 62%). 1H NMR
(300 MHz, CDCl3): d=9.85 (s, 1H), 7.61 (d, 1H, J=3.9 Hz), 7.47–7.42
(m, 5H,) 7.27–7.18 (m, 6H,) 6.19 ppm (d, 1H, J=3.9 Hz). MALDI-
TOF: 306.2.
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Received: December 12, 2013
Revised: January 18, 2014
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