Pyrrolo[3,2-b]pyrrole as the Central Core of the Electron Donor for Solution-Processed Organic Solar Cells

Article abstract of DOI:10.1002/cplu.201700158

A series of three small-molecule acceptor–donor–acceptor (A-D-A) compounds with a tetraaryl-1,4-dihydropyrrolo[3,2-b]pyrrole as the central building block were synthesized and fully characterized. These molecules present high thermal stability and suitable HOMO–LUMO energy levels making them feasible electron-donor materials in bulk heterojunction organic solar cells (BHJ-OSC). Moreover, theoretical work predicts of a lack of planarity and no π–π stacking, furthermore. The electron density of the HOMO is distributed on the pyrrole–pyrrole moiety and that of the LUMO is delocalized, in contrast, on the phenyl groups. The organic solar cells deliver an open-circuit voltage of 0.99 V. However, the overall efficiency is limited due to the low charge mobility measured for holes, 10^?9 cm² V^?1 s^?1.

Full text of DOI:10.1002/cplu.201700158

Accepted Article
Title: Pyrrolo[3,2-b]pyrrole as Electron-Donor Central Core for Solution-
Processed Organic Solar Cells
Authors: Rocio Dominguez, Nuria F Montcada, Pilar de la Cruz, Emilio
Palomares, and Fernando Langa
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemPlusChem 10.1002/cplu.201700158
Link to VoR: http://dx.doi.org/10.1002/cplu.201700158
A Journal of
www.chempluschem.org

10.1002/cplu.201700158
ChemPlusChem
FULL PAPER
Pyrrolo[3,2-b]pyrrole as Electron-Donor Central Core for Solution-
Processed Organic Solar Cells
Rocío Domínguez,[a] Núria F. Montcada,*[b] Pilar de la Cruz,[a] Emilio Palomares[b,c] and Fernando
Langa*[a]
Abstract: A series of three acceptor-donor-acceptor (A-D-A) small
molecules using a tetraaryl-1,4-dihydropyrrolo[3,2-b]pyrrole in their
central building block were synthesized and fully characterized.
These molecules present high thermal stability and suitable HOMO-
LUMO energy levels making them feasible electron donor materials
in Bulk Heterojunction Organic Solar Cells (BHJ-OSC). Moreover,
theoretical work was also performed obtaining a prediction of a lack
of planarity and no p-p stacking, furthermore, the probability of the
Highest Occupied Molecular Orbital (HOMO) is distributed on the
pyrrole-pyrrole moiety and the probability of the Lowest Unoccupied
Molecular Orbital (LUMO) is delocalized, on the contrary, on the
phenyl groups. The organic solar cells deliver an open-circuit voltage
of 0.99V. However, the overall efficiency is limited due to the low
charge mobility measured for holes, 10-9 cm2V-1s-1.
applications. Moreover, these molecules can be easily tuned
combining different donor and acceptors and bridge elements in
order to widen their absorption overlap with the solar spectrum,
better control of the HOMO and LUMO energy levels and
improve charge-carrier mobilities and transport pathways to the
electrodes. All of these advantages have been widely explored
finally reaching the same efficiencies as polymer-based solar
cells. For instance, electron-donor central cores moieties has
been studied and reported, and fused-ring derivatives have been
the most used moieties to obtain the highest efficiencies.[4]
Regarding this, heteropentalenes (10π-electron aromatic
compounds) such as thieno[3,2-b]thiophenes and thieno[3,2-
b]pyrroles are considered a promising candidates as electron
donor moieties for BHJSCs that can be easily modified by the
introduction of fused rings as a structural element, [5] achieving in
the first case efficiencies up to 8.1%.[6]
On the other hand, families of furo-, thieno-, and seleno[3,2-
b]pyrroles are also well-known in the scientific literature and their
chemistry has been well studied.[7] In contrast, the electronic
properties of 1,4-dihydropyrrolo[3,2-b]pyrroles are almost
unknown, probably due to its quite higher complex synthesis
compared to others.[8] Nevertheless, a three components direct
synthesis of this 1,4-dihydropyrrolo[3,2-b]pyrrole[9] made this
core accessible and viable to be studied and included as
possible donor moiety in BHJOSC. Moreover, the synthesis and
optical properties of new pentaaryl- and hexaaryl-1,4-
dihydropyrrolo[3,2-b]pyrroles prepared via direct arylation from
tetraaryl-1,4-dihydropyrrolo[3,2-b]pyrroles were reported by D. T.
Gryko and co-workers.[10] These new derivatives exhibited
interesting optical properties, such as strong blue fluorescence,
moderate to large Stokes shifts and fluorescence quantum
yields up to 88%, making them a good candidates for many
optoelectronic applications.
Introduction
Bulk heterojunction solar cells based on organic conjugated
semiconductors as electron-donor materials blended with
fullerene derivatives as electron-acceptor materials are
technologically appealing for light-weight, flexible, low-cost and
roll-to-roll solar cells. Either semiconductor polymers or small
molecule donor materials have been widely studied in organic
solar cells (OSC) achieving efficiencies above 10%.[1]
Small organic semiconductor molecules were the first choice for
organic solar cells[2], however, a breakthrough was carried out
with semiconducting polymers which hindered the use of
solution processed small organic molecules in solar cells until
the solar-to-energy conversion efficiency reached values close
to 5% at 1 sun.[3] The well-defined molecular structures with
monodisperse nature and negligible batch-to-batch variations
makes small molecules of particular interest for solar cell
Taking this into account, we present herein the synthesis and
characterization of a new series of acceptor-donor-acceptor (A-
D-A) small molecules (1-3) based on tetraaryl-1,4-
dihydropyrrolo[3,2-b]pyrrole and their possibility to be used as
electron-donor central building block in solution-processed BHJ-
OSCs.
[a]
R. Domínguez, P. de la Cruz, Prof. F. Langa
Institute of Nanoscience, Nanotechnology and Molecular Materials
(INAMOL)
Universidad de Castilla-La Mancha
Avda. Carlos III s/n, 45071 Toledo (Spain)
E-mail: Fernando.Langa@uclm.es
[b]
[c]
N. F. Montcada, Prof. E. Palomares
Institute of Chemical Research of Catalonia (ICIQ)
The Barcelona Institute of Science and Technology
Avda. Països Catalans, 16. Tarragona. E-43007 (Spain)
E-mail: epalomares@ICIQ.es
Prof. E. Palomares
ICREA
Passeig Luis Companys, 23. Barcelona. E-08010 (Spain)
Figure 1. Chemical structure of 1, 2 and 3 pyrrole-based small molecules.
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/cplu.201700158
ChemPlusChem
FULL PAPER
Figure 2. (Left) TGA curves of derivatives 1 (green), 2 (red) and 3 (black)
under nitrogen atmosphere (scan rate of 10ºC min-1); (right) DSC curves of
derivatives 1 (green), 2 (red) and 3 (black) under nitrogen atmosphere.
Results and Discussion
Synthesis and thermal properties
The synthetic route for preparing the new family of small
molecules (1-3) is outlined in Scheme 1. First, 2,6-di(4-
cyanophenyl)-1,5-di(4-methylphenyl)-3,4-dihydropyrrolo[3,2-
b]pyrrole (4)[9] and 2,6-di(4-cyanophenyl)-1,5-di(4-octylphenyl)-
For the DSC experiments, the samples were heated from 25 to
360 ºC and then cooled to -10 ºC with a heating rate of 20
ºC/min and a cooling rate of 10 ºC/min under nitrogen flow. 1
showed an endothermic peak at 346 ºC corresponding to the
melting point of the molecule and an exothermic peak at 265 ºC
associated to a crystallization process. Similarly, molecules 2
and 3 exhibited an endothermic peak at 208 ºC and 344 ºC,
respectively, ascribed to its corresponding melting point.
Nevertheless, no exothermic processes were found when
samples 2 and 3 were cooled (Figure 2 (right)). These results
show that the new family of molecules displays high thermal
stability and good enough for optoelectronic applications.
Optical properties
3,4-dihydropyrrole[3,2-b]pyrrole (5) were synthesized by
a
Debus-Radziszewski reaction. Cyano groups were then reduced
to the corresponding aldehydes by treatment with
diisobutylaluminium hydride (DIBAL-H) in anhydrous toluene at
0 ºC, affording derivatives 6 and 7 in excellent yields (95% and
98%, respectively). Finally, the target molecules 1-3 were
obtained by Knoevenagel reaction using malononitrile (for 1 and
2) or ethyl cyanoacetate (for 3), triethylamine as base and
chloroform as solvent at room temperature.
The absorption and emission properties of the new family of
sensitizers 1-3 have been investigated by UV-Visible and
fluorescence
spectroscopy.
Absorption
spectra
in
dichloromethane solution and on film are shown in Figure 3, and
the most relevant data are summarized in Table 1. Absorption
spectra of 1-3 sensitizers are dominated by an intense band
around 450-650 nm assigned to the intra-molecular charge
transfer (ICT) transition as consequence of the interplay of the
donor-acceptor architecture between the electron-rich inner part
of the molecule (tetraaryl-1,4-dihydropyrrolo-[3,2-b]pyrrole) and
the electron-deficient peripheral part (dicyanovinylene or ethyl
cyanoacetatevinylene). The maximum in
3 is blue-shifted
respecting to 1 and 2 as ethyl cyanoacetatevinylene acceptor
group exhibits less electrophilic character than dicyanovinylene.
Scheme 1. Synthetic route to 1-3.
1
All new compounds were characterized by H-NMR, 13C-NMR,
FT-IR and MALDI-TOF MS in order to confirm the expected
structures (see Supporting Information). While 1 is rather
insoluble, 2 and 3 show good solubility in organic solvents.
Thermogravimetric (TGA) and differential scanning calorimetry
(DSC) analyses were used in order to investigate the thermal
properties of the molecules. The decomposition temperature
was estimated from the TGA, as 390 ºC for 1, 374 ºC for 3 and
450ºC for 2 (Figure 2 (left)).
Figure 3. UV-Vis absorption spectra of derivatives 1 (green), 2 (red) and 3
(black) in dichloromethane solution (c ≈ 3 x 10-6 M at 25ºC) (dashed lines) and
on film (solid lines).
The UV-Visible absorption spectra recorded on film showed, in
all cases, a bathochromic shift on the maximum absorption
wavelength and broader absorption bands compared to the
spectra registered in solution owing to higher order of the
molecules in solid state. Additionally, the spectra recorded on
film exhibited a new absorption band at longer wavelengths
(especially in 2 and 3) due to the aggregation triggered on film
An increase of the planarity is shown in 2 due to the presence of
larger alkyl chains.[11] In case of molecule 3, the film present a
This article is protected by copyright. All rights reserved.

10.1002/cplu.201700158
ChemPlusChem
FULL PAPER
certain degree of molecular order due to the reduction of the
molecular distances for the film formation, however the widening
of the spectra also indicate the formation of different packing
domains. For molecule 1 an important decrease of the absoption
intensity is shown, an indication of a totally amorphous domains
formation.[12]
These oxidation processes were ascribed to the central electron-
donor building block (tetraaryl-1,4-dihydropyrrolo-[3,2-b]pyrrole),
whereas the unique reduction process observed in the
observation window was associated to the acceptor groups
(Figure 4).
The molecule
3
(cyanoacetatevinylene-based derivative)
The emission spectra were also recorded in diluted
dichloromethane solution (c ≈ 3 x 10-6 M at 25ºC) by excitation
at the corresponding maximum wavelength (see Figure S20,
Supporting Information). As expected, the emission wavelength
of derivatives 1 and 2 (with dicyanovinylene moiety) is higher
than that of dye 3 (Table 1).
displayed slightly lower first oxidation potential than 1 and 2
(DCV-based derivatives) because ethyl cyanoacetatevinylene
acceptor group is less electrophile than dicyanovinylene group.
The HOMO and LUMO energy levels were calculated from the
first oxidation and reduction potentials according to equations
EHOMO = -5.1 – EOX1(OSWV) (eV) and ELUMO = -5.1 – ERED1(OSWV)
(eV), respectively. 1-3 sensitizers showed relatively low
Electrochemical properties
The Cyclic voltammetry (CV) and Osteryoung square wave
voltammetry (OSWV) experiments were carried out in order to
determine the oxidation and reduction potentials, as an
approach to estimate the HOMO and LUMO energy levels since
the alignment of the HOMO values is very important in order to
achieve high open circuit voltage (VOC) (this energy is closely
related to the energy difference between the HOMO of the
electron-donor and the LUMO of the electron-acceptor
materials). The relevant electrochemical data is listed in Table 1.
All derivatives revealed two reversible oxidation waves with
similar intensities (determined by CV measurements) and an
irreversible one at higher voltage (see Figure S21-23 Supporting
Information).
electrochemical band gap (calculated according to equation
elec
Eg
= ELUMO – EHOMO (eV)) and suitable HOMO-LUMO energy
values, enabling their use as electron-donor and using PC61BM
or PC71BM as acceptor materials (Figure 5). It is worth
emphasizing that the low HOMO levels exhibited by this series
of molecules predict relatively high VOC, particularly for 1 and 2.
Figure 5. FMO energy levels and band gap of dyes 1-3 and the components
that form the studied BHJ devices.
Figure 4. OSWV plots of derivatives 1 (green), 2 (red) and 3 (black) (referred
to Fc/Fc+).
Table 1. Optical and electrochemical data of 1-3 resulted from UV-Vis absorption spectrum, emission spectrum and Osteryoung square wave voltammetry
(OSWV) curves.
Eox1(OSWV)
(V)
Ered1(OSWV)
(V)
λmaxsol (nm)
λem (nm)
log ε
λmaxfilm (nm)
EHOMOa (eV)
ELUMOa (eV)
Egelec b (eV)
1
2
3
545
548
513
655
654
639
4.83
4.90
4.82
593
596
527
0.42
0.43
0.36
-1.52
-1.51
-1.68
-5.52
-5.53
-5.46
-3.58
-3.59
-3.42
1.94
1.94
2.01
[a] Solution in 0.1 M in ODCB/Acetonitrile 4:1/n-Bu4NClO4 versus ferrocenium/ferrocene. HOMO and LUMO energy levels were estimated from OSWV oxidation
and reduction curves, respectively, assuming the absolute energy level of ferrocene/ferrocenium to be 5.1 eV below vacuum. [b] Egelec (eV) = ELUMO – EHOMO
.
Theoretical calculations
31G* level in vacuo with Gaussian 09W. As expected, these
Ground-state geometry of the new family of sensitizers was fully
optimized by density functional theory (DFT) at the B3LYP 6-
molecules displayed two different molecular planes: that one
formed between the pyrrole-pyrrole unit and the phenyl groups
This article is protected by copyright. All rights reserved.

10.1002/cplu.201700158
ChemPlusChem
FULL PAPER
linked to position 2, and that between the pyrrole-pyrrole unit
and the phenyl groups linked to position 1 with a dihedral angle
around 30º (Figure 6). Thereby, the lack of planarity hinders the
π-π stacking between molecules, which is inexistent for 1 as
observed in the UV-Vis spectra (Figure 3).
current density) and a poor FF (fill factor). From the solar cells
parameters, we can observe an outstanding Voc for 2-based
devices of 0.99V as it was predicted from the theoretical
energetics (the difference between the PCBM LUMO energy and
the 2 HOMO energy value), however for 1 and 3 the Voc is
decreased drastically respecting to the predicted value,
indicating an important charge loss maybe due to faster charge
recombination kinetics. In addition, the low JSC obtained in case
of 1-based devices is due to the combination of two factors: on
the one hand, the apparent lower light harvesting properties in
film as shown previously (Figure 3) and, on the other hand,the
extended trap points due to the amorphous domains, that
provoked an increase in transport resistance and, thus, a
leakage of Jsc by recombination. The lower FF also was in
agreement with this last point, where the resistance throughout
the film increase noticeably. Moreover, the deficient measured
Voc in device 1 emphasize a strong resistance at the interfaces
induced also by the higher lack of crystallinity of the active layer.
On the contrary, for device 3, it is evidence that there is no
limitation over the light harvesting properties (Figure 3), however,
even the molecule becomes more crystalline in film than using
molecule 1, the intercalation with fullene were clearly not proper
creating larger donor domains, greater than the diffusion length,
and the charges are not able to reach the donor-acceptor
interface, producing also a drastic drop of the FF.[12]
Figure 6. Optimized ground-state geometries of a) 1, b) 3 and c) 2.
Electronic distribution of the HOMO and LUMO energy levels
was also calculated by quantum chemical calculations, revealing
that the electron density of the HOMO in these molecules is
mainly distributed on the pyrrole-pyrrole moiety, whereas the
electron density of the LUMO is mainly delocalized on the
phenyl groups linked by position 2 to the pyrrole-pyrrole unit and
on the acceptor groups (Figure 7). Phenyl groups linked by
position 1 to the pyrrole-pyrrole unit practically do not contribute
to the electronic distribution of the HOMO and LUMO energy
levels.
With the aim to improve the unfavorable molecular packing,
solvent vapor annealing was carried out, obtaining only a
favorable result for 2-based devices reaching 1.06%; in this case
the FF was increased from 31 to 46 % indicating and
improvement on the nanomorphology which lead to less
resistance to the charge transport to the electrodes and, thus,
we found also an 82% increase of the photocurrent (from 1.4 to
2.5 mA/cm-2) but a slight decrease of the VOC to 0.92 V, in good
agreement with previous reports that used solvent annealing to
improve the solar cell performance. [13-14]
Figure 7. Electronic distribution of the HOMO and LUMO energy levels in
derivatives 1-3.
The theoretical HOMO and LUMO energy levels are similar to
the experimental values (see Table S1, Supporting Information),
also following the same trend; both energy levels are lower (in
absolute terms) for 3 than those of 1 and 2 due to the lower
electrophilic character of the ethyl cyanoacetatevinylene
acceptor group.
Photovoltaic performance
All
complete
devices
were
fabricated
using
the
Figure 8. Photocurrent density vs. voltage (J-V) curves for optimized 1-
3:PC71BM solar cells at 1 sun (sun simulated 100 mW cm-2 light intensity).
ITO/PEDOT:PSS/sm(1,2 or 3):PC71BM/Al configuration and for
hole only devices ITO/PEDOT:PSS/2:PC71BM/Au. The
fabrication methods are described in the experimental section.
In Figure 8 the J-V curves of 1,2 and 3 based complete devices
are shown and the main performance parameters are listed in
Table 2. The performance parameters obtained in the device
optimization before and after SVA process are collected in Table
S2 (Supporting Information). The 1, 2 and 3 devices without any
SVA treatment shown low device photocurrent efficiencies in all
cases, limited in the three cases for the lower JSC (short-circuit
Based on the results shown before, we will focus the
characterization study on complete devices using molecule 2 as
this is the most promising molecule among the others.
The hole mobility of the 2-based devices was calculated using
the space charge limited current method (SCLC). The hole only
devices are forced to work at higher potentials as can be seen in
Figure 9, at 7 V in this case.
This article is protected by copyright. All rights reserved.

10.1002/cplu.201700158
ChemPlusChem
FULL PAPER
change was observed (Figure 10 (right)) that is a result of a
higher reorganization of the photoactive domains. The
roughness increased up to 20 nm and
a
flake-form
nanomorphology was achieved, resulting in a better phase
separation and more distinct network of donor/acceptor domains.
Table 2. Main performance parameters of 1-3-based devices after SVA
process.
JSC
(mA/cm2)
Device
VOC (V)
FF (%)
PCE (%)
Conclusions
1
2
3
0.43
0.12
0.92
0.52
28
46
19
0.01
1.06
To summarize, a series of novel symmetrical A-D-A sensitizers,
1-3, based on tetraphenyl-1,4-dihydropyrrole-[3,2-b]pyrrole as
electron-donor central building block and different aliphatic chain
lengths and acceptor groups has been successfully synthesized
and characterized. These derivatives are soluble in common
organic solvents (except 1 that displays lower solubility), exhibit
wide absorption in a broad part of the solar spectrum (range 400
- 650 nm) and show thermal stability and good miscibility with
PC71BM. The 2-based devices display high VOC reaching values
up to 0.92 V under standard sun-simulated conditions of AM
1.5G 100 mW cm-2 after SVA post-treatment, however a low JSC
(2.5 mA cm-2) and moderate FF (46%) were obtained and
therefore a limited PCE of 1.06%. The hole mobility was
measured obtaining values in the order of 10-9 cm2V-1s-1 that is
clearly one of the main limitation of the device. According to the
theoretical calculations, the conjugation could be increased
through the groups linked to the pyrrolic nitrogen atoms, thus
enhancing and shifting the absorption to higher wavelengths but
maintaining the voltage since the HOMO energy level of the
molecules is not electronically involved. In addition, all the
molecules of this set present lack of planarity that do not favors
the electron hopping to the acceptor and efficient charge
transport to the contacts. In case of the 2-based devices the
improvement after the SVA treatment points out to a higher
reorganization of the nanodomains that makes this molecule the
best candidate to be further optimized work in the future focused
on optimizing the charge mobility.
2.5
0.044
0.004
Figure 9. Representative J-V curves for a hole only device of 2 at 1 sun (Red)
and dark (Blue); the solid lines represents the fitting to the SCLC equation.
The measured mobility is an average value of 27 indepndent diodes.
The mobility average for this molecule was 10-9 cm2V-1s-1. That
is clearly the major loss mechanism for this devices and the
main cause for the limited efficiencies.
Experimental Section
Surface morphology characterization
The surface morphology of 2:PC71BM blend film was
investigated by atomic force microscopy (AFM, tapping mode).
The sample was prepared reproducing the photoactive layer of
the optimized device as described in the experimental section.
Materials and instruments
The synthetic routes for the preparation of 1-3 derivatives are shown in
Scheme 1. 4-formylbenzonitrile, 4-methylaniline, 4-octylaniline, butane-
2,3-dione, DIBAL-H, malononitrile and ethyl cyanoacetate were
purchased from commercial sources (Aldrich, Acros). All other solvents
and chemicals used in this work were analytical grade and used without
further purification. Column chromatography was performed on silica gel
60, 0.06 - 0.2 mm (70 – 230 mesh ASTM) or 0.04 – 0.06 mm (230 – 400
mesh ASTM) from Scharlau.
1H NMR and 13C NMR spectra were recorded on a Bruker TopSpin AV-
400 (400 MHz) spectrometer. The reported chemical shifts were against
7.27 ppm in 1H NMR experiments and against 77.0 ppm in 13C NMR
experiments (CDCl3). Mass spectra were achieved from a VOYAGER
DETM STR mass spectrometer (MALDI-TOF) using dithranol as matrix.
FT-IR spectra were recorded in a Fourier Transform IR spectrometer
Avatar 370 Thermo Nicolet, using pellets of dispersed samples of the
corresponding compounds in dried KBr or ATR accessory. TGA analyses
were performed using a TGA/DSC Linea Excellent instrument by Mettler-
Toledo and collected under inert atmosphere of nitrogen with a rate of
10ºC min-1. Weight changes were recorded as a function of temperature.
DSC analyses were performed using a DSC Linea Excellent instrument
Figure 10. AFM topography images of 2:PC71BM thin films on PEDOT:PSS-
coated ITO substrates, (left) before SVA and (right) after SVA.
The topography roughness of the blend films was obtained from
the height images. The surface morphology before SVA
displayed a smooth roughness of around 2 nm (Figure 10 (left)).
Nevertheless, after solvent vapor annealing process, a notable
This article is protected by copyright. All rights reserved.

10.1002/cplu.201700158
ChemPlusChem
FULL PAPER
by Mettler-Toledo and collected under inert atmosphere of nitrogen at a
heating rate of 20 K min-1 and cooling rate of 10 K min-1. Melting points
were taken from the DSC experiments. AFM images were acquired in
tapping mode using a Multimode V8.10 (Veeco Instruments, Santa
Barbara, USA). Silicon cantilevers were used (RTESP from Bruker
Probes) with a resonance frequency of 300 kHz and a nominal force
constant of 40 N m-1.
Synthesis of 5
4-Octylaniline (800 mg, 3.90 mmol, 2 eq) and 4-formylbenzonitrile (511
mg, 3.90 mmol, 2 eq) were stirred in glacial acetic acid (6 mL) at 100 ºC
for 30 min. Then butane-2,3-dione (0.2 mL, 1.95 mmol, 1 eq) was added
and the resulting mixture was stirred at 100 ºC for 3h. After cooling, the
solvent was removed under reduced pressure. The crude product was
purified by chromatography column (silica-flash, CH2Cl2) and
recrystallization from hexane. 5 was achieved in 18 % of yield (240 mg,
0.35 mmol) as a dark yellow solid. 1H-NMR (400 MHz, CDCl3) δ/ppm:
Optical and electrochemical measurements
3
3
3
7.49 (d, 4H, J = 8.2 Hz), 7.29 (d, 4H, J = 8.2 Hz), 7.24 (d, 4H, J = 8.2
Hz), 7.18 (d, 4H, 3J = 8.2 Hz), 6.49 (s, 2H), 2.67 (t, 4H, 3J = 7.7 Hz), 1.67-
1.65 (m, 4H), 1.36-1.31 (m, 20H), 0.93-0.90 (m, 6H); 13C-NMR (100 MHz,
CDCl3) δ/ppm: 141.6, 137.7, 136.9, 135.0, 133.4, 131.9, 129.4, 127.8,
125.2, 119.1, 109.0, 95.9, 35.5, 31.9, 31.3, 29.4, 29.3, 29.2, 22.7, 14.1;
FT-IR (KBr) υ/cm-1:; UV-Vis (CH2Cl2) λmax/nm (log ε): 408 (4.85); MS
(m/z) (MALDI-TOF): calculated C48H52N4: 684.42; found: 684.58 (M +).
UV-Vis absorption measurements were carried out in a Shimadzu UV
3600 spectrophotometer using
Fluorescence spectra were recorded in a Cary Eclipse fluorescence
spectrophotometer using 1 cm standard quartz cuvettes.
1 cm standard quartz cuvettes.
Cyclic voltammetry (CV) and Osteryoung square wave voltammetry
(OSWV) experiments were carried out in a potentiostat/galvanostat
AUTOLAB using a one-compartment cell equipped with a glassy working
electrode (Ø = 2 mm) and a platinum wire counter electrode. An
Ag/AgNO3 (0.01 M in the supporting electrolyte) electrode was used as
reference and checked against the ferrocene/ferrocenium couple (Fc/Fc+)
before each experiment. Measurements were performed in o-
dichlorobenzene/acetonitrile 4:1 solutions and tetrabutylammonium
perchlorate (purchased from Sigma-Aldrich and used without purification)
(0.1 M) as background electrolyte. Solutions were deoxygenated by
argon bubbling prior to each experiment, which runs under argon
atmosphere.
General procedure for the synthesis of 6 and 7
The corresponding cyano derivative (4 or 5) (1 eq) was dissolved in
anhydrous toluene and cooled to 0 ºC. DIBAL-H (1.2 M in toluene, 2.1
eq) was added dropwise under Ar. The solution was stirred for 8 h at 0 ºC.
A 20 mL sample of chloroform was then added followed by 30 mL of
10 % HCl, and the solution was stirred at room temperature for 15 min.
The organic layer was separated and dried over anhydrous MgSO4. The
solvent was removed under vacuo and the crude product was purified by
chromatography column (silica flash, CH2Cl2).
Device fabrication and characterization
6, 1.35 g of 4 (2.76 mmol) in 90 mL of toluene. Yellow solid, 95% yield
(1.3 g, 2.63 mmol). 1H-NMR (400 MHz, CDCl3) δ/ppm: 9.93 (s, 2H), 7.72
The ITO coated glass substrates (5 Ω/cm2) were ultrasonically cleaned
consecutively in acetone, aqueous detergent, de-ionized water and
isopropyl alcohol, and finally dried under ambient conditions, the
substrates were treated in a UV-ozone cleaner for 20 min in order to
remove remaining organic particles and water. An aqueous solution of
PEDOT:PSS (HC StarckBaytron P.) was spin-coated over the ITO
substrates to obtain a film obtaining thickness of 25 nm (30 seconds at
4500 rpm and 30 seconds at 3500 rpm). The PEDOT:PSS film was
subsequently dried at 120ºC 15 minutes. Initially, the donor:acceptor ratio
(w/w) was varied from 1:3 to 2:1 in order to identify the most suitable ratio
for the system using PC61BM and PC71BM. The fabrication conditions
were further explored by varying the concentration and solvent of the
donor:acceptor solution, and spin-coating rates. Among all conditions,
the optimal performance was observed by using 20 mg mL-1 blend
solutions in chloroform as solvent for 1 and 3 derivatives and in
chlorobenzene for 2. 1:1 donor:acceptor ratio provided the higher results
for DCV-based sensitizers (1 and 2) whereas 2:1 D:A ratio yielded the
best results for 3-based devices. Subsequently, the solution of the
sensitizer was spin-coated over the PEDOT layer at 2000 rpm obtaining
a thickness of approximately 100 nm, the solvent annealing was carried
out immediately after the active layer deposition, this treatment was
performed by exposing the films to a saturated vapor atmosphere of
dichloromethane (10 mL) in a controlled volume closed vessel (100 mL)
for different periods of time. Finally 80 nm of aluminum (Al) top electrode
was thermally deposited on the active layer using a high vacuum thermal
evaporator (1·10-6 mbar) through a 0.09 cm2 shadow mask. For hole only
devices a 150 nm of Au electrode were evaporated instead.
3
3
3
(d, 4H, J = 8.1 Hz), 7.36 (d, 4H, J = 8.1 Hz), 7.22 (d, 4H, J = 8.1 Hz),
3
7.18 (d, 4H, J = 8.1 Hz), 6.52 (s, 2H), 2.41 (s, 6H); 13C-NMR (100 MHz,
CDCl3) δ/ppm: 191.6, 139.3, 137.0, 136.4, 135.7, 133.8, 133.6, 130.1,
129.7, 127.8, 125.2, 96.0, 21.1; UV-Vis (CH2Cl2) λmax/nm (log ε): 435
(4.62); FT-IR (KBr) ν/cm-1: 2920, 2816, 2733, 1693, 1599, 1512, 1458,
1375, 1215, 1167, 839, 737; MS (m/z) (MALDI-TOF): calculated
C34H26N2O2: 494.20; found: 494.35 (M+).
7, 0.24 g of 5 (0.35 mmol) in 30 mL of toluene. Yellow solid, 98% yield
(0.24 g, 034 mmol). 1H-NMR (400 MHz, CDCl3) δ/ppm: 9.93 (s, 2H), 7.72
(d, 4H, 3J = 8.2 Hz), 7.36 (d, 4H, 3J = 8.2 Hz), 7.23-7.18 (m, 8H), 6.54 (s,
2H), 2.65 (t, 4H, 3J = 7.7 Hz), 1.66-1.64 (m, 4H), 1.35-1.30 (m, 20H),
0.91-0.88 (m, 6H); 13C-NMR (100 MHz, CDCl3) δ/ppm: 191.6, 141.4,
139.3, 137.1, 135.7, 133.7, 133.6, 129.7, 129.4, 127.8, 125.2, 96.0, 35.5,
31.9, 31.4, 29.5, 29.4, 29.3, 22.7, 14.2; FT-IR (KBr) υ/cm-1:; UV-Vis
(CH2Cl2) λmax/nm (log ε): 436 (4.75), 252 (4.58); MS (m/z) (MALDI-TOF):
calculated C48H54N2O2: 690.42; found: 690.59 (M +).
General procedure for the synthesis of 1 and 2
In a round-bottomed flask under argon atmosphere, three drops of Et3N
were added to a solution of malononitrile (5 eq) and the corresponding di-
aldehyde (6 or 7) (1 eq) in CHCl3. The reaction mixture was stirred
overnight at 80 ºC and treated with brine after consumption of the starting
material (monitored by TLC). The organic phase was dried over MgSO4
and the crude product was purified by chromatography column (silica gel,
CH2Cl2) and recrystallization from methanol and hexane.
The J-V curves were measured using a Sun 2000 Solar Simulator (150
W, ABET Technologies) at 100 mW cm-2 calibrated silicon photodiode
(NREL) simulating the AM 1.5G spectrum. The applied potential and cell
current were measured with a Keithley 2400 digital source meter. The
thickness of the films was measured with a stylus profilometer Ambios
Tech. XP-1.
1, 0.21 g of 6 (0.42 mmol) in 50 mL of CHCl3. Purple solid, 24% yield (58
mg, 0.098 mmol). H-NMR (400 MHz, TCE-d2) δ/ppm: 7.11 (d, 4H, J =
1
3
3
3
8.1 Hz), 7.01 (s, 2H), 6.70 (d, 4H, J = 8.1 Hz), 6.62 (d, 4H, J = 8.1 Hz),
3.55 (d, 4H, J = 8.1 Hz), 5.94 (s, 2H), 1.79 (s, 6H); 13C-NMR (100 MHz,
3
TCE-d2) δ/ppm: low solubility; UV-Vis (CH2Cl2) λmax/nm (log ε): 545
(4.83); FT-IR (KBr) ν/cm-1: 3113, 3032, 2922, 2224, 1599, 1572, 1541,
1516, 1417, 1379, 1188, 1144, 825; MS (m/z) (MALDI-TOF): calculated
C40H26N6: 590.22; found: 590.88 (M+).
Synthesis of target molecules
The synthetic details of dyes 1-3 are described as follows.
This article is protected by copyright. All rights reserved.

10.1002/cplu.201700158
ChemPlusChem
FULL PAPER
2, 0.27 g of 7 (0.39 mmol) in 20 mL of CHCl3. Purple solid, 81% yield
(0.25 g, 0.32 mmol). 1H-NMR (400 MHz, CDCl3) δ/ppm: 7.76 (d, 4H, 3J =
[6]
[7]
Q. Zhang, Y. Wang, B. Kan, X. Wan, F. Liu, W. Ni, H. Feng, T. P.
Russell, Y. Chen, Chemical Communications 2015, 51, 15268-15271.
a) S. Srinivasan, G. B. Schuster, Organic Letters 2008, 10, 3657-3660;
b) I. Alkorta, F. Blanco, J. Elguero, Journal of Molecular Structure:
THEOCHEM 2008, 851, 75-83; c) I. Kim, D. Kim, S. Sambasivan, K. H.
Ahn, Asian Journal of Organic Chemistry 2012, 1, 60-64; d) H.
Behringer, E. Meinetsberger, Liebigs Annalen der Chemie 1982, 1982,
315-341.
3
3
8.1 Hz), 7.63 (s, 2H), 7.34 (d, 4H, J = 8.1 Hz), 7.27 (d, 4H, J = 8.1 Hz),
7.21 (d, 4H, 3J = 8.1 Hz), 6.57 (s, 2H), 2.69 (t, 4H, 3J = 7.7 Hz), 1.71-1.67
(m, 4H), 1.37-1.30 (m, 20H), 0.93-0.90 (m, 6H); 13C-NMR (100 MHz,
CDCl3) δ/ppm: 158.6, 142.0, 139.4, 136.8, 136.2, 134.8, 131.0, 129.6,
128.3, 127.8, 125.3, 114.3, 113.2, 96.7, 80.0, 35.5, 31.9, 31.3, 29.5, 29.4,
29.3, 22.7, 14.2; UV-Vis (CH2Cl2) λmax/nm (log ε): 548 (4.90); FT-IR (KBr)
ν/cm-1: 2953, 2924, 2852, 2224, 1601, 1574, 1516, 1456, 1419, 1379,
1182, 835, 781, 611, 532; MS (m/z) (MALDI-TOF): calculated C54H54N6:
786.44; found: 786.44 (M+). Melting point: 208 ºC.
[8]
[9]
A. Janiga, D. T. Gryko, Chemistry – An Asian Journal 2014, 9, 3036-
3045.
A. Janiga, E. Glodkowska-Mrowka, T. Stoklosa, D. T. Gryko, Asian
Journal of Organic Chemistry 2013, 2, 411-415.
[10] M. Krzeszewski, B. Thorsted, J. Brewer, D. T. Gryko, The Journal of
Organic Chemistry 2014, 79, 3119-3128.
Synthesis of 3
[11] N. S. S. Kumar, S. Varghese, C. H. Suresh, N. P. Rath, S. Das, The
Journal of Physical Chemistry C, 2009, 113, 11927-11935.
In a round-bottomed flask under argon atmosphere, three drops of Et3N
were added to a solution of ethyl cyanoacetate (5 eq) and 6 (0.53 g, 1.07
mmol, 1 eq) in 100 mL of CHCl3. The reaction mixture was stirred
overnight at 80 ºC and treated with brine after consumption of the starting
material (monitored by TLC). The organic phase was dried over MgSO4
and the crude product was purified by chromatography column (silica gel,
CHCl3) and recrystallization from hexane. Pure product 3 was obtained
[12] a) A. Viterisi, F. Gispert-Guirado, J. W. Ryan, E. Palomares, Journal of
Materials Chemistry, 2012, 22, 15175; b) G. Li, Y. Yao, H. Yang, V.
Shrotriya, G. Yang, Y. Yang, Advanced Functional Materials, 2007, 17,
1636-1644.
[13] K. C. Deing, U. Mayerhoffer, F. Wurthner, K. Meerholz, Physical
Chemistry Chemical Physics 2012, 14, 8328-8334.
1
as red solid in 79% of yield (0.58 g, 0.85 mmol). H-NMR (400 MHz,
[14] a) M. Schrader, R. Fitzner, M. Hein, C. Elschner, B. r. Baumeier, K.
Leo, M. Riede, P. Bäuerle, D. Andrienko, Journal of the American
Chemical Society 2012, 134, 6052-6056; b) D. Credgington, R.
Hamilton, P. Atienzar, J. Nelson, J. R. Durrant, Adv. Funct. Mater.
2011, 21, 2744-2753; c) A. Sánchez-Díaz, M. Izquierdo, S. Filippone,
N. Martin, E. Palomares, Adv. Funct. Mater. 2010, 20, 2695-2700.
CDCl3) δ/ppm: 8.15(s, 2H), 7.86 (d, 4H, 3J = 8.1 Hz), 7.31 (d, 4H, 3J = 8.1
Hz), 7.26-7.19 (m, 8H), 6.52 (s, 2H), 4.38 (q, 4H, 3J = 7 Hz), 2.43 (s, 6H),
1.40 (t, 6H, 3J = 7 Hz); 13C-NMR (100 MHz, CDCl3) δ/ppm: 162.9, 154.3,
138.2, 136.9, 136.7, 136.1, 134.1, 131.4, 130.2, 128.8, 127.7, 125.3,
116.1, 100.7, 96.1, 62.6, 21.1, 14.2; UV-Vis (CH2Cl2) λmax/nm (log ε): 513
(4.82); FT-IR (KBr) ν/cm-1: 3026, 2983, 2926, 2218, 1722, 1581, 1514,
1460, 1379, 1265, 1192, 827, 768; MS (m/z) (MALDI-TOF): calculated
C44H36N4O4: 684.27; found: 684.40 (M+).
Acknowledgements
This research was financially supported by the Spanish Ministry
of Economy and Competitiveness of Spain (CTQ2013-48252-P),
Junta de Comunidades de Castilla-La Mancha (PEII-2014-014-
P). RD thanks the Ministerio de Educación, Cultura y Deporte of
Spain for a FPU grant.
Keywords: bulk heterojunction solar cells · heteropentalenes ·
sensitizers · fused-ring systems
[1]
a) Z. He, B. Xiao, F. Liu, H. Wu, Y. Yang, S. Xiao, C. Wang, T. P.
Russell, Y. Cao, Nat Photon 2015, 9, 174-179; b) J. Zhao, Y. Li, G.
Yang, K. Jiang, H. Lin, H. Ade, W. Ma, H. Yan, Nature Energy 2016, 1,
15027; c) M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E. D.
Dunlop, Progress in Photovoltaics: Research and Applications 2014,
22, 1-9; d) Q. Zhang, B. Kan, F. Liu, G. Long, X. Wan, X. Chen, Y. Zuo,
W. Ni, H. Zhang, M. Li, Z. Hu, F. Huang, Y. Cao, Z. Liang, M. Zhang, T.
P. Russell, Y. Chen, Nat Photon 2015, 9, 35-41.
[2]
[3]
C. W. Tang, Applied Physics Letters 1986, 48, 183-185.
M. T. Dang, L. Hirsch, G. Wantz, Advanced Materials 2011, 23, 3597-
3602.
[4]
a) Y. Liu, C.-C. Chen, Z. Hong, J. Gao, Y. Yang, H. Zhou, L. Dou, G. Li,
Scientific Reports 2013, 3, 3356; b) B. Kan, Q. Zhang, M. Li, X. Wan,
W. Ni, G. Long, Y. Wang, X. Yang, H. Feng, Y. Chen, Journal of the
American Chemical Society 2014, 136, 15529-15532.
[5]
a) L. S. Fuller, B. Iddon Mp, K. A. Smith, Chemical Communications
1997, 2355-2356; b) M. Kuhn, F. C. Falk, J. Paradies, Organic Letters
2011, 13, 4100-4103; c) A. Capan, H. Veisi, A. C. Goren, T. Ozturk,
Macromolecules 2012, 45, 8228-8236; d) J. T. Henssler, A. J. Matzger,
Organic Letters 2009, 11, 3144-3147; e) Y. Liu, Q. Liu, X. Zhang, L. Ai,
Y. Wang, R. Peng, Z. Ge, New Journal of Chemistry 2013, 37, 1189-
1194; f) Z. Yuan, Y. Xiao, Y. Yang, T. Xiong, Macromolecules 2011, 44,
1788-1791.
This article is protected by copyright. All rights reserved.

10.1002/cplu.201700158
ChemPlusChem
FULL PAPER
Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
Small Molecules for Organic Solar
Cells: Three new acceptor-donor-
acceptor (A-D-A) small molecules
using a tetraaryl-1,4-
R. Domínguez, N. F. Montcada,* P. de
la Cruz, E. Palomares, F. Langa*
Page No. – Page No.
dihydropyrrolo[3,2-b]pyrrole in their
central building block were
Pyrrolo[3,2-b]pyrrole as Electron-
Donor Central Core for Solution-
Processed Organic Solar Cells
synthesized and fully characterized.
These molecules present high
thermal stability and suitable HOMO-
LUMO energy levels.The bulk
heterojunction organic solar cells
deliver an open-circuit voltage of
0.99V.
This article is protected by copyright. All rights reserved.