Small molecular push-pull donors for organic photovoltaics: effect of the heterocyclic π-spacer

Article abstract of DOI:10.1039/c5ra21958b

A series of (D-π-A) small push-pull molecules involving a triphenylamine electron-rich group (D) connected to a dicyanovinyl electron-deficient unit (A) through different chalcogenophene type π-connectors has been synthesized. Optical and electrochemical results reveal that the replacement of furan by thiophene and selenophene leads to a progressive decrease of the optical band gap of the material and to a parallel increase of hole mobility and power conversion efficiency (PCE). Thus, a PCE of 3.33% has been obtained for a simple air-processed solar cell involving the selenophene-based donor and the [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) as the acceptor.

Full text of DOI:10.1039/c5ra21958b

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DOI: 10.1039/C5RA21958B
Small molecular pushꢀpull donors for organic photovoltaics: effect of the
heterocyclic πꢀspacer
Antoine Labrunie, a § Yue Jiang,a,b, § François Baert,a Antoine Leliège,a Jean Roncali, a
,
Clément Cabanetos a* and Philippe Blanchard a*
a CNRS UMR 6200, MOLTECH-Anjou, University of Angers, 2 Bd Lavoisier, 49045 Angers, France
b South China University of Technology, 381 Wushan Rd, Tianhe, Guangzhou, Guangdong, China
§both authors contributed equally to this work
Corresponding authors: clement.cabanetos@univ-angers.fr; Philippe.blanchard@univ-angers.fr
Keywords: organic photovoltaics, push-pull molecule, triphenylamine, structure-properties
relationships, selenophene
Abstract: A series of (D-π-A) small push-pull molecules involving a triphenylamine
electron-rich group (D) connected to a dicyanovinyl electron-deficient unit (A) through
different chalcogenophene type π-connectors has been synthesized. Optical and
electrochemical results reveal that the replacement of furan by thiophene and selenophene
leads to a progressive decrease of the optical band gap of the material and to a parallel
increase of hole mobility and power conversion efficiency (PCE). Thus, a PCE of 3.33% has
been obtained for a simple air-processed solar cell involving the selenophene-based donor and
the [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor.
1. Introduction
Over the past decades, organic solar cells (OSCs) have generated a considerable research
effort due to the lightness, flexibility, low environmental impact and potential low cost of
organic materials.1, 2 Recent progress in device fabrication and material engineering have led
to remarkable progress with power conversion efficiencies (PCE) now exceeding the
symbolic value of 10%.3-5
However, beyond striving for improved PCEs, the development of active materials by simple,
clean, cost-effective, efficient and scalable syntheses is a mandatory condition for a future
industrial production.6-8 Owing to their well-defined chemical structure, molecular active
materials present the advantages of a better reproducibility of synthesis and purification and a
1

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more precise analysis of the structure-property relationships compared to polydisperse
polymers.9-11 Among them, Donor-Connector-Acceptor (D-π-A) type push-pull molecules
based on arylamines donor blocks represent interesting molecular donors that combine
12-15
synthetic accessibility, simplicity and efficiency.6,
Indeed, TPA-T-DCV (Figure 1),
prepared in good yields in a few synthetic steps, has been over the past few years an
interesting working structure to investigate the effects of various structural variations such as
(i) the covalent bridging of the dicyanovinyl (DCV) group,16 (ii) its replacement by other
electron-withdrawing units,17 (iii) the substitution of one phenyl ring of TPA by fused ring
systems or aliphatic chains,18, 19 or (iv) the drastic size reduction to assess the limit of
simplification.20
As a further step of this systematic analysis of structure-properties relationships, we
report here on the effect of the nature of the heterocyclic π-connector by the synthesis,
characterization and photovoltaic evaluation of the furan (TPA-F-DCV) and selenophene
(TPA-S-DCV) analogues of TPA-T-DCV. (Figure1).
Figure 1. Chemical structure of the D-π-A target molecules
Indeed, while several articles deal with the effect of the replacement of the thiophene ring by
other chalcogenophene in conjugated polymers for OSCs,21-25 only few examples of
molecular donors, especially incorporating the selenophene ring, have been reported.26-29
2. Results and discussions
The synthesis of the three molecules is depicted in Scheme 1.
2

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DOI: 10.1039/C5RA21958B
O
X
X
Sn
CN
NC
Br
X
X = S; R = Bu
X = Se; R = Me
Malononitrile, Et3N
CHCl3, reflux
X
POCl3, DMF
1,2-Dichloroethane, reflux
N
N
N
N
Pd(PPh3)4
Toluene, 110°C
4: X = O (84%)
5: X = S (79%)
6: X = Se (82%)
2: X = S (89%)
3: X = Se (92%)
TPA-F-DCV: X = O (76%)
TPA-T-DCV: X = S (79%)
TPA-S-DCV: X = Se (72%)
1
OH
OH
O
O
B
Pd(dppf)Cl2 , K2CO3
Dioxane, H2O, 105°C
Scheme 1. Synthetic route to TPAꢀFꢀDCV, TPAꢀTꢀDCV and TPAꢀSꢀDCV
5-Formylfuran-2-boronic acid was engaged in a Suzuki cross-coupling reaction with the
commercially available 4-bromotriphenylamine 1 affording the carbonyl compound 4.30 In
parallel, the thiophene and selenophene analogues 5 and 6 were prepared by a two-step
procedure involving a Stille cross-coupling of 4-bromotriphenylamine 1 and the appropriate
trimethyl stannylated chalcogenophene, followed by a Vilsmeier-Haack formylation with
POCl3/DMF.16 Finally, a Knoevenagel condensation between malononitrile and the
carbaldehydes 4, 5 or 6 led to the target products TPA-F-DCV, TPA-T-DCV and TPA-S-
DCV respectively.
The compounds show good solubility in common organic solvents and their thermal
stabilities were assessed by thermogravimetric analysis (Figure 2).
Figure 2. Thermogravimetric analysis of TPA-F-DCV (black), TPA-T-DCV (red) and TPA-S-DCV (blue)
recorded at 5°C/min under N2.
3

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The furan and thiophene derivatives exhibit a comparable decomposition temperature (Td) of
ca 255°C. However, introducing a selenium atom slightly increases Td since a 5 % weight
loss was recorded at ca 280 °C for TPA-S-DCV.
UV-Vis absorption spectra were recorded on chloroform solutions and on films spin-cast on
glass.
a)
b)
Figure 3. UV-Vis absorption spectra of TPA-F-DCV (black squares), TPA-T-DCV (red circles) and TPA-S-
DCV (blue triangles) in chloroform (a) and as thin film on glass (b).
The spectrum of all compounds shows an intense absorption band with a maximum (λmax
)
around 500 nm assigned to an internal charge transfer (ICT) from the TPA moiety to the
electron-deficient DCV group (Fig. 3). Replacement of the furan spacer by thiophene and
selenophene leads to a progressive bathochromic shift of λmax from 499 to 520 nm with a
parallel increase of the molar extinction coefficient (ε) from 33000 to 36800 M-1 cm-1 (Table
1), in agreement with previous observations (Figure 3).23, 31, 32 Broader absorption bands and
red-shifted absorption edges with onsets at ca 595 nm, 606 nm and 630 nm corresponding to
optical band gaps (Egopt) of 2.08 eV, 2.04 eV and 1.96 eV for TPA-F-DCV, TPA-T-DCV and
TPA-S-DCV respectively were observed on thin films (Figure 3, b).
Table 1. UV-Vis absorption data.
opt
λmax (nm)
λonset (nm)
Eg (eV)
ε
λmax (nm) λonset (nm)
Compound
(M-1 cm-1)
33000
35000
36800
film
517
523
535
film
595
606
630
solution
solution
film
499
505
520
560
572
587
2.08
2.04
1.96
TPA-F-DCV
TPA-T-DCV
TPA-S-DCV
4

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The electrochemical properties of the three molecules have been analyzed by cyclic
voltammetry in dichloromethane solution in the presence of Bu4NPF6 as the supporting
electrolyte. Data are summarized in Table 2.
Table 2. Electrochemical data for the target compounds (0.5 mM in 0.10 M Bu4NPF6/CH2Cl2, scan
rate 100 mV s-1, Pt working and counter electrodes, ref. SCE)
Epa
EOx/onset
Epc
ERed/onset
HOMO a) LUMO b)
1
1
ꢀ
Eelec
Compound
(V)
(V)
(V)
(V)
(eV)
(eV)
(eV)
0.98
0.86
0.87
0.87
-1.24
-1.07
-1.05
-1.01
-5.85
-3.92
1.93
TPA-F-DCV
TPA-T-DCV
TPA-S-DCV
1.01
-1.20
-5.86
-3.94
1.92
1.88
1.01
-1.20
-5.86
-3.98
a) HOMO (eV) = ꢀ(EOx/onset (V) + 4.99), b) LUMO (eV) = ꢀ(ERed/onset (V) + 4.99)33
The cyclic voltammogram (CV) of all compounds shows a reversible one-electron oxidation
wave, assigned to the formation of a stable radical cation (Figure 4). While the CVs of TPA-
T-DCV and TPA-S-DCV show comparable anodic peak potentials at Epa = 1.01 V, the furan
derivative presents a slight negative shift of Epa to 0.98 V indicative of an increase of the
HOMO level. In the negative potentials region, the CV of all donors exhibits an irreversible
reduction process with cathodic potential peak at Epc = -1.24 V for TPA-F-DCV and -1.20 V
for both the thiophene and selenophene compounds. HOMO and LUMO levels were
calculated respectively from the onsets of oxidation and reduction waves (Table 2).
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Figure 4. Cyclic voltammograms of TPA-F-DCV (black), TPA-T-DCV (red) and TPA-S-DCV (blue) 0.5 mM
in 0.1 M Bu4NPF6/CH2Cl2, scan rate 100 mV s-1, Pt working and counter electrodes, ref SCE.
A slight reduction of the electrochemical gap (
(1.93 eV) to the selenophene one (1.88 eV), in agreement with optical data. Electrochemical
data also show that this decrease of
Eelec is mainly associated to the lowering of the LUMO
ꢀ
Eelec) is observed from the furan derivative
ꢀ
energy levels when varying the heteroatom of the chalcogenophene from oxygen to selenium
whereas the HOMO energy levels remains relatively unaffected.
To gain further insight into the electronic properties, the frontier orbitals and energy levels of
the compounds have been investigated by theoretical calculations performed with Gaussian 09
program using Becke’s three-parameter gradient-corrected functional (B3LYP) with the 6-
31G(d,p) basis set (Figure 5).
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Figure 5. HOMO and LUMO energy levels and their representations for TPA-F-DCV (left), TPA-T-DCV
(middle) and TPA-S-DCV (right) after optimization with Gaussian 09 at the B3LYP/ 6-31G(d,p) level of theory.
The optimized geometries and computed electronic structures at the observed minima reveal
that the LUMO is strongly localized on the five-membered ring-DCV block whereas the
HOMO is mainly distributed on the TPA-spacer. In addition, the geometry of TPA-F-DCV
presents the smallest dihedral angle between the phenyl ring and the heterocycle (1.4° vs
19.6° and 17.6° for TPA-T-DCV and TPA-S-DCV respectively) ensuring a better planarity
of the structure. The calculated values of the HOMO and LUMO levels are consistent with
optical and electrochemical results.
The photovoltaic properties of the three donors have been evaluated in bulk heterojunction
solar cells of 0.27 cm2 of configuration: ITO/PEDOT: PSS (ca 40 nm)/ Donor: PC61BM
blend/ LiF (1 nm)/ Al (120 nm). Except for LiF and aluminum depositions, the cells were
fabricated in ambient atmosphere. Table 3 gathers the photovoltaic parameters of these
devices and Figure 6a shows the best current density-voltage (JꢀV) characteristics measured
under AM. 1.5 simulated solar illumination (80 mW cm-2).
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Table 3. Photovoltaic properties of TPAꢀFꢀDCV, TPAꢀTꢀDCV and TPAꢀSꢀDCV blended with PC61BM (1:2
w/w).
sc (mAcm-2)
FF (%) PCEmax/ave (%)
ꢁh (
cm2V-1s-1)
a)
Donor
Voc (V)
J
8.1 x 10-6
1.2 x 10-5
1.7 x 10-5
TPAꢀFꢀDCV
TPAꢀTꢀDCV
TPAꢀSꢀDCV
0.97
0.97
0.95
5.28
6.53
7.15
38.9
37.9
39.2
2.50/2.23
3.00/2.86
3.33/3.17
a) Average value recorded over 18 devices.
The high open-circuit voltages (Voc ~ 1.0 V) are consistent with the low-lying HOMO level of
the donors resulting in a large offset with the LUMO level of PC61BM. Although comparable
Voc and fill factors (FF) values are obtained for all cells, the short-circuit current-density (Jsc)
progressively increases from TPA-F-DCV to TPA-S-DCV leading to a parallel increase of
PCE from 2.50 to 3.33% (Table 3). This trend is confirmed by external quantum efficiency
(EQE) measurements performed under monochromatic irradiation (Figure 6b).
a)
b)
Figure 6. J –V characteristics and EQE curves of TPA-F-DCV (black squares), TPA-T-DCV (red circles) and
TPA-S-DCV (blue triangles) based OSCs.
Thus, varying the heteroatom from oxygen to selenium results in a slight extension of the
contribution of the donor in the long wavelengths region with a maximum photon-to-current
conversion of 52 % at 504 nm, 54% at 523 nm and 55% at 540 nm for TPA-F-DCV, TPA-T-
DCV and TPA-S-DCV respectively.
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The hole-mobility of the materials (ꢁh) was determined using the space-charge limited
current (SCLC) method on hole-only devices (Figure 7).
Figure 7. J–V characteristics of hole only devices ITO/PEDOT:PSS/ TPAꢀFꢀDCV (black squares), TPAꢀTꢀDCV
(red circles) or TPAꢀSꢀDCV (blue triangles)/Au.
As shown in Table 3, the furan-containing molecule exhibits the lowest mobility (of ca
8.1ꢀ10-6 cm2 V-1 s-1) and the selenophene derivative the highest (1.7ꢀ10-5 cm2 V-1 s-1). These
results, in agreement with the Jsc trend, can be correlated to the improved orbitals overlap of
heavier chalcogen heteroatoms resulting in enhanced intermolecular electronic interactions.
3. Conclusion
Triphenylamine based D-π-A type small molecular donors with three different five-
membered heterocycles as π-conjugating connector have been synthesized. Replacement of
furan by thiophene and selenophene produces a small reduction of the band gap and an
improvement of the hole-mobility and photovoltaic conversion efficiency of the donor. In
addition to the synthetic accessibility (ie, three steps from commercially available materials),
promising power conversion efficiencies above 3.0% were obtained with simple BHJ solar
cells processed in ambient atmosphere with PC61BM as acceptor. Further device optimization
(morphology, additives, and interfaces) can be expected to significantly improve these
preliminary results.
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4. Acknowledgments
The RFI LUMOMAT is also acknowledged for the PhD grant of A. Labrunie. The Chinese
Government Scholarship (CGC) program is acknowledged for the Ph-D grant of Y. Jiang and
the Ministère de la Recherche is thanked for the Ph-D grant of F. Baert. The PIAM
(Plateforme d’Ingénierie et Analyses Moléculaires) of the University of Angers is thanked for
the characterization of organic compounds. Finally, this paper is dedicated to Dr. Errol Blart
on the occasion of his 50th birthday.
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6. Graphical Abstract
11