Cross-linkable azido C60-fullerene derivatives for efficient thermal stabilization of polymer bulk-heterojunction solar cells

Article abstract of DOI:10.1039/c4tc01178c

Original [60]PCBM-inspired fullerene derivatives bearing an azido functional group were synthesized. By incorporating an optimized quantity of this thermal cross-linker additive in the P3HT:[60]PCBM photoactive layer, an impressive stabilization of the bu

Full text of DOI:10.1039/c4tc01178c

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Materials Chemistry C
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Cross-linkable azido C -fullerene derivatives for
6
0
efficient thermal stabilization of polymer bulk-
heterojunction solar cells†
Cite this: J. Mater. Chem. C, 2014, 2,
163
7
Received 4th June 2014
Accepted 11th July 2014
a
b
bc
d
Aurel Diacon,‡ Lionel Derue,‡ Clemence Lecourtier, Olivier Dautel,
´
b
a
Guillaume Wantz* and Pietrick Hudhomme*
´
DOI: 10.1039/c4tc01178c
www.rsc.org/MaterialsC
Original [60]PCBM-inspired fullerene derivatives bearing an azido [60]PCBM has been the most widely investigated with a repro-
14
functional group were synthesized. By incorporating an optimized ducible power conversion efficiency (PCE) approaching 5%.
quantity of this thermal cross-linker additive in the P3HT:[60]PCBM Besides the need for a high PCE, the stability of organic solar
photoactive layer, an impressive stabilization of the bulk-heterojunc- cells constitutes a limiting parameter for attaining large scale
15,16
tion morphology at its optimal photovoltaic performance was commercialization.
Improvements of the active layer nano-
achieved.
morphology were obtained with an appropriate annealing
treatment and/or the addition of a processing additive.
17
18,19
During thermal annealing, both the polymer and [60]PCBM
crystallize to form a nanoscale interpenetrating network, but
this morphology is not thermally stable since each partner
thermodynamically prefers to phase segregate. Particularly, it is
well-established that [60]PCBM forms crystalline domains in
the lms, resulting in a signicant decrease of the long-term
Polymer:fullerene-based bulk-heterojunction (BHJ) solar cells
have attracted widespread interest in academic and industrial
communities leading nowadays to one of the hottest elds of
1–5
research. This particular attention is due to the high potential
of organic solar cells which presents the advantages of low cost,
6
light weight and processability on exible substrates with new
20
solar cell performance. Consequently the control of the
nanostructure and the stability of the polymer:fullerene
7,8
market areas. The BHJ concept based on an interpenetrating
network between the electron donor and acceptor materials
network morphology are the most important challenges in BHJ
9
offers an efficient strategy to maximize the interfacial area. The
21,22
solar cell development.
Chemical strategies now explored to
choice of these materials and the control of the blend
morphology are determinant parameters that govern the phys-
ical interaction inside the BHJ affecting the whole photocurrent
process. While extensive studies have been aimed at designing
optimal conjugated polymers as electron donors, fullerenes
were rapidly considered to be the ideal acceptors for BHJ based
devices thanks to their photophysical and electrochemical
freeze this nanomorphology are based on the utilization of
fullerene-containing nucleating agents which allow to control
the crystallization process of [60]PCBM, or cross-linking
23
reactions which are investigated to increase the lifetime of the
device by locking the morphology thus hindering the phase
24
separation. Previous efforts have focused on cross-linked
25–30
31,32
polymers,
small molecules,
and only very few examples
10–12
properties combined with their high electron mobility.
Since the advent of [6,6]-phenyl-C61-butyric acid methyl ester
33
are using cross-linked polymer-fullerene complexes or cross-
linked fullerenes. In this last topic, interesting approaches were
13
(
[60]PCBM), the blend of poly-(3-hexylthiophene) (P3HT) and
34
reported to modify [60]PCBM with the introduction of epoxide
3
5–38
or styryl
cross-linkable groups and to apply these n-type
a
Universit ´e d'Angers, Laboratoire MOLTECH-Anjou, UMR CNRS 6200, 2 Bd Lavoisier, materials by replacement of [60]PCBM in the active layer. The
4
9045 Angers, France. E-mail: pietrick.hudhomme@univ-angers.fr
cross-linking phenomenon induced by the epoxide group was
shown to be incomplete by addition of a Lewis acid initiator or
b
Universit ´e de Bordeaux, Laboratoire IMS, UMR CNRS 5218, Ecole Nationale
Sup ´e rieure de Chimie, Biologie et Physique, 16 Av. Pey Berland, 33607 Pessac, France
ꢀ
heating at 140 C, whereas the corresponding process with the
c
Laboratoire LOF, Solvay, 178 Av. Albert Schweitzer, 33608 Pessac, France
ꢀ
styryl group occurred at 160 C. Another strategy was investi-
d
Laboratoire AM2N, Institut Charles Gerhardt Montpellier, UMR CNRS 5253, Ecole
gated by working with a [60]PCBM derivative bearing an oxetane
group as an electron extracting interlayer for improving the
Electronic supplementary information (ESI) available: Experimental procedures, device performance.
Nationale Sup ´e rieure de Chimie de Montpellier, 8 rue de l’Ecole Normale, 34296
Montpellier cedex 5, France
39
†
characterization data, and characteristics of devices. This material is available free
In this study, we successfully developed an original and
powerful approach to introduce an azido cross-linkable group
of charge on the internet. See DOI: 10.1039/c4tc01178c
‡
Both A. D. and L. D. contributed equally to this work.
This journal is © The Royal Society of Chemistry 2014
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onto a [60]PCBM-based system (Scheme 1). The key point of this
This cross-linking temperature appears to be close to that
innovative strategy was to develop a straightforward synthesis of used for the annealing of P3HT:[60]PCBM lms. The DSC
highly soluble cross-linkable PCBM-based derivatives in order analysis of P3HT and [60]PCB-C -N in a physical mixture
6
3
to limit the modication of the morphology in common organic revealed an exothermic peak during the rst heating cycle and
solar cells. Taking advantage of the highly well-known reactivity no modication of P3HT crystallization temperature. This could
40
of the azido group towards C60
,
an efficient cross-linking be interpreted as a selective reaction of the azido group towards
process was attained during the thermal annealing treatment the fullerene moiety. Thus, a fast cross-linking reaction could
resulting in an enhanced stability with suppression of macro- effectively freeze the BHJ morphology, without causing any
phase separation.
60]PCBM was synthesized from a commercial fullerene C60
segregation due to the reaction between fullerene derivatives.
[
The effect of such cross-linkable materials on the BHJ
according to the literature procedure, then hydrolyzed into morphology was studied on the P3HT:[60]PCBM blend used as a
1
3
[
60]PCBA. Considering that the reaction of the azido group model. Aer deposition on glass/ITO/TiOx (Fig. 1a), thermal
ꢀ
41
ꢀ
onto C60 was susceptible to occur at a temperature up to 50 C,
a one-step synthetic strategy was driven using an esterication many dendrites, characterizing severe aggregation by crystalli-
with 3-azidopropan-1-ol or 6-azidohexan-1-ol under activa- zation of [60]PCBM (Fig. 1b). In contrast, the blend with
annealing at 150 C for 24 h induced the formation of
42
43
45
0
tion conditions with N-(3-dimethylaminopropyl)-N -ethyl- P3HT:[60]PCB-C
3
-N
3
or P3HT:[60]PCB-C
6
-N
3
showed a homo-
carbodiimide (EDC) in the presence of 1-hydroxybenzotriazole geneous lm (Fig. 1c and d). This result could be explained from
HOBt) and 4-N,N-dimethylaminopyridine (DMAP). Corre- the fullerene cross-linking phenomenon inhibiting the macro-
(
sponding azidofullerenes [60]PCB-C -N and [60]PCB-C -N
3
phase separation.
3
3
6
were puried by chromatography on silica gel using CH Cl /
Nanoscale imaging using atomic force microscopy (AFM)
2
2
pentane (7/3) as the mixture of eluents and isolated in 80% yield further demonstrated that the typical brillar structure of the
(
1
Scheme 1). Both compounds were immediately dissolved in non-aged P3HT:[60]PCBM layer (Fig. 2a) completely changed
ꢁ
1
ꢀ
,2-dichlorobenzene at the concentration of 40 mg mL and aer 24 h at 150 C (Fig. 2b). However, the addition of an
ꢀ
kept at 4 C without any degradation.
optimized content of the [60]PCBM-C
6 3
-N cross-linker in the
1
These materials were unambiguously characterized by
H
P3HT:[60]PCBM:[60]PCB-C -N (1 : 0.8 : 0.2 weight ratio) blend
6
3
NMR spectroscopy with the CH N group at 3.26 ppm, and by enabled the stabilization of the morphology (Fig. 2c and d).
2
3
FT-IR measurements which conrmed the presence of the azido
BHJ solar cells based on composite lms of P3HT:[60]PCBM,
P3HT:[60]PCB-C -N and P3HT:[60]PCBM:[60]PCB-C -N3
Mass spectrometry (MALDI-TOF) showed the molecular peaks mixtures were fabricated with an inverted device structure of
at m/z ¼ 979 and 1021 for [60]PCB-C -N and [60]PCB-C -N ITO/TiOx/BHJ/MoO /Ag and characterized on a AM1.5 solar
respectively (see ESI†). The differential scanning calorimetry simulator set at 100 mW cmꢁ (S.I.). When [60]PCB-C
DSC) measurements of both azido-functionalized fullerenes used as the only acceptor in devices in place of [60]PCBM, then
ꢁ1
functionality with a strong nN stretching band at 2092 cm
.
3
6
3
6
3
3
6
3
,
3
2
6
3
-N was
(
ꢀ
ꢀ
presented an exothermic peak with a maximum around 130 C, PCE was dramatically reduced aer thermal ageing at 150 C
thus clearly suggesting the cross-linking reaction. The second
cycle of heating does not exhibit any exothermic peak, which
can be interpreted as a rapid and complete cross-linking reac-
tion. For interpretating this process, a solution of PCB-C
3
-N
3
ꢀ
was heated at 150 C in 1,2-dichlorobenzene and the resulting
material was further analyzed by MALDI-TOF mass spectrom-
etry. Some dimer and trimer structures containing aziridine
heterocycles were clearly evidenced resulting from the
1
,3-dipolar cycloaddition of the azido group onto the C60 core
44
followed by loss of N from the triazoline intermediate (S.I.).
2
Fig. 1 Optical microscopy images of (a) P3HT:[60]PCBM after solvent
ꢀ
annealing (b) the same blend after 24 h at 150 C showing dendrites of
[
60]PCBM (in brown) surrounded by depletion zones (purple areas). (c)
and [60]PCB- P3HT:[60]PCB-C -N and (d) P3HT:[60]PCB-C -N after the same
thermal ageing. Scale-bar ¼ 100 mm.
Scheme 1 Synthesis of azidofullerenes [60]PCB-C
-N
-N
3 3
3
3
6
3
C
6
3
.
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Fig. 2 AFM phase images of P3HT:[60]PCBM (1 : 1 weight ratio), (a) after solvent annealing not thermally treated and (b) after thermal ageing at
ꢀ
1
50 C for 24 h the nano-sized domains are totally destructed. AFM phase images of P3HT:[60]PCBM:[60]PCB-C
6
-N
3
(1 : 0.8 : 0.2 weight ratio)
ꢀ
are shown in (c) after solvent annealing not thermally treated and (d) after thermal ageing at 150 C for 24 h. Nano-domains of P3HT and of PCBM
are preserved thanks to the cross-linker. All image sizes are 2 ꢂ 2 mm .
2
during 24 h (S.I.). This could be explained by a non-controlled
multiple functionalization of fullerene. Such formation of
multi-adducts could inhibit acceptor properties of the fullerene
phase in the BHJ with decrease of the electron mobility.
Consequently, [60]PCB-C
6 3
-N was then used as an additive
together with [60]PCBM to form a P3HT:[60]PCBM:[60]PCB-C
6
-
46
N ternary blend. The concentration of [60]PCBM-C -N was
3
6
3
carefully optimized and the 1 : 0.8 : 0.2 weight ratio for
P3HT:[60]PCBM:[60]PCB-C -N , respectively, provided the best
6
3
stabilization without affecting initial performances (S.I.). The
current–voltage (J–V) curves of these solar devices are plotted in
Fig. 3 and the relevant photovoltaic characteristics are listed in
Table 1. Pristine devices exhibited similar performance, indi-
cating that the incorporation of the azido cross-linker in addi-
tion to the [60]PCBM-type acceptor did not signicantly affect
the PCE of solar cells. Whereas the PCE of P3HT:[60]PCBM
devices decreased rapidly upon annealing at 150 C to attain
1.4% PCE aer 24 h, the devices with a cross-linker stabilized
their efficiency at 2.8%. This stabilized efficiency remains even
Fig. 3 J–V curves of P3HT:[60]PCBM solar cells (black symbols) and
after 24 h of thermal ageing at 150 C (red symbols). J–V curves of
ꢀ
P3HT:[60]PCBM:[60]PCB-C
6 3
-N (in a 1 : 0.8 : 0.2 ratio, respectively)
ꢀ
solar cell pristine (blue symbols) and after 24 h of thermal ageing at
ꢀ
1
50 C (green symbols).
Table 1 Photovoltaic characteristics of solar cells
ꢁ2
PCE (%)
Jsc (mA cm
)
FF (%)
Voc (V)
P3HT:[60]PCBM (1 : 1 weight ratio)
P3HT:[60]PCBM (1 : 1 weight ratio)
Pristine
24 h @ 150 C
Pristine
3.6 ꢃ 0.1
1.4 ꢃ 0.1
3.7 ꢃ 0.2
11.8 ꢃ 0.1
8.4 ꢃ 0.2
12.5 ꢃ 0.4
0.59 ꢃ 0.01
0.39 ꢃ 0.01
0.59 ꢃ 0.01
0.51 ꢃ 0.01
0.42 ꢃ 0.02
0.50 ꢃ 0.01
ꢀ
P3HT:[60]PCBM:[60]PCB-C
1 : 0.8 : 0.2 weight ratio)
P3HT:[60]PCBM:[60]PCB-C
1 : 0.8 : 0.2 weight ratio)
6
-N
3
(
ꢀ
6
-N
3
24h @ 150 C
2.8 ꢃ 0.2
10.1 ꢃ 0.5
0.52 ꢃ 0.02
0.53 ꢃ 0.01
(
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aer longer thermal treatments. One can note the increased Voc 12 Y. Lai, Y. Cheng and C. S. Hsu, Energy Environ. Sci., 2014, 7,
aer thermal treatment which is in agreement with the 1866.
formation of fullerene bis- or tris-adducts raising directly the 13 J. C. Hummelen, B. W. Knight, F. LePeq, F. Wudl, J. Yao and
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Conclusions
The approach described here provides a powerful tool to effi-
ciently enhance the stability of P3HT:[60]PCBM-based organic 18 J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses,
solar cells using the addition of a cross-linkable azido func- A. J. Heeger and G. C. Bazan, Nat. Mater., 2007, 6, 497.
tionalized fullerene derivative. This compatibilizer effect dras- 19 K. Schmidt, C. J. Tassone, J. R. Niskala, A. T. Yiu, O. P. Lee,
tically suppresses the formation of [60]PCBM crystals in the BHJ
and macro-phase separation leading to an extremely stable
T. M. Weiss, C. Wang, J. M. J. Fr ´e chet, P. M. Beaujuge and
M. F. Toney, Adv. Mater., 2014, 26, 300.
morphology and devices. The proposed acceptor–acceptor 20 W. L. Ma, C. Y. Yang, X. Gong, K. Lee and A. J. Heeger, Adv.
cross-linking strategy using the azidofullerene derivative Funct. Mater., 2005, 15, 1617.
provides a versatile tool to overcome the morphology instability 21 J. Peet, M. L. Senatore, A. J. Heeger and G. C. Bazan, Adv.
for the fabrication of more stable fullerene-based solar cells. Mater., 2009, 21, 1521.
These fullerene materials can now be expected to be universally 22 J. Peet, A. J. Heeger and G. C. Bazan, Acc. Chem. Res., 2009,
applied as additives and morphology stabilizers in all kinds of
fullerene-based BHJ solar cells.
42, 1700.
23 C. Lindqvist, J. Bergqvist, C.-C. Feng, S. Gustafsson,
O. B ¨a cke, N. D. Treat, C. Bounioux, P. Henriksson,
R. Kroon, E. Wang, A. Sanz-Velasco, P. M. Kristiansen,
N. Stingelin, E. Olsson, O. Ingan ¨a s, M. R. Andersson and
C. M u¨ ller, Adv. Energy Mater., 2014, 1301437.
Acknowledgements
This research was supported by the Agence Nationale de la
Recherche (ANR) with the ANR-2010-HABISOL-003 (PROGELEC) 24 G. Wantz, L. Derue, O. Dautel, A. Rivaton, P. Hudhomme and
program CEPHORCAS. The authors are thankful to Lionel C. Dagron-Lartigau, Polym. Int., 2014, 63, 1346.
Hirsch, Sylvain Chambon, Bertrand Pavageau, Christine Dag- 25 B. J. Kim, Y. Miyamoto, B. Ma and J. M. J. Fr ´e chet, Adv. Funct.
ron-Lartigau, Roger Hiorns, Agn `e s Rivaton, Martin Drees and
Antonio Facchetti for fruitful discussions.
Mater., 2009, 19, 2273.
26 S. Miyanishi, K. Tajima and K. Hashimoto, Macromolecules,
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