Towards polymerizable fullerene derivatives to stabilize the initially formed phases in bulk-heterojunction solar cells

Article abstract of DOI:10.1039/b517022m

Photovoltaic cells have been prepared with blends of a polymerizable methanofullerene derivative bearing four butadiyne subunits and MDMO-PPV; annealing at 100°C for 2 hours resulted in an improvement of the performances of the devices and AFM studies sug

Full text of DOI:10.1039/b517022m

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Towards polymerizable fullerene derivatives to stabilize the initially
formed phases in bulk-heterojunction solar cellsw
Jean-Franc¸
ois Nierengarten*a and Sepas Setayesh*b
Received (in Montpellier, France) 11th November 2005, Accepted 20th January 2006
First published as an Advance Article on the web 17th February 2006
DOI: 10.1039/b517022m
Photovoltaic cells have been prepared with blends of a polymer-
izable methanofullerene derivative bearing four butadiyne sub-
units and MDMO-PPV; annealing at 100 1C for 2 hours
resulted in an improvement of the performances of the devices
and AFM studies suggested that cross-linking polymerization of
the fullerene derivative is capable of stabilizing the initially
formed phases.
first, then the initially formed phases can be fixed by cross-
linking polymerization of compound 1. In this way, the
tedious preparation of fullerene-rich polymers is avoided and
the size of the solubilizing groups can be dramatically reduced.
In the design of compound 1, it was decided to use butadiyne
functions as polymerisable units since the fullerene should not
be affected by the polymerization reaction.8
Following the observation of ultrafast photoinduced elec-
tron transfer from conjugated polymers to C60,1 a novel
approach to the preparation of solar cells has been proposed.2
It is based on an interpenetrating blend of donor (conjugated
polymer) and acceptor (C60) sandwiched between two asym-
metric contacts. Remarkably, the efficiency of the bulk hetero-
junction devices consisting of poly[2-methoxy-5-(3,
7-dimethyloctyloxy)]-p-phenylene vinylene (MDMO-PPV)
and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM)
have been increased up to 3% by the group of Sariciftci.3 It is
worth noting that the performance of this type of solar cell is
very sensitive to the morphology of the blend.3–5 In particular,
phase separation and clustering of the fullerene can occur due
to the operational heat through illumination, thus reducing the
effective donor/acceptor interfacial area and the efficiency of
the devices. In order to prevent such undesirable effects, we
have recently proposed an all-polymer approach for the pre-
paration of donor/acceptor bulk heterojunction photovoltaic
cells.6 The initially formed phases obtained from the polymer/
polymer mixtures are effectively more stable; however, the
short circuit current and the open circuit voltage of the first
devices are low when compared to the optimized MDMO-PPV
/PCBM solar cells. This has been ascribed to the low con-
ductivity of the fullerene polymer due to the presence of the
large solubilizing groups necessary to obtain soluble material.6
As part of this research, we now report the preparation of the
fullerene-functionalized monomer 1 allowing polymerization
in the bulk and its incorporation in photovoltaic devices.7
Ideally, a blend of the monomer and MDMO-PPV is prepared
The strategy employed for the preparation of the functio-
nalized methanofullerene derivative 1 is based upon Bingel
type chemistry.9 To this end, malonate 9 was prepared in six
steps from 1-heptyne (2) and undec-10-ynoic acid (3) (Scheme
1). Reaction of 2 and 3 in CH2Cl2 under dry air in the presence
of an excess of the Hay catalyst (CuCl, N,N,N0,N0-tetramethy-
lethylenediamine ¼ TMEDA, dry air) furnished compound 4
in 40% yield. LiAlH4 reduction of 4 followed by treatment of
the resulting 5 with p-toluenesulfonyl chloride (TsCl) in the
presence of pyridine and 4-dimethylaminopyridine (DMAP) in
CH2Cl2 yielded tosylate 6. Subsequent reaction with methyl
3,5-dihydroxybenzoate under classical Williamson conditions
afforded ester 7 in 71% yield. Reduction with LiAlH4 then
gave benzylic alcohol 8 in 75% yield. Treatment of 8 with
malonyl chloride in CH2Cl2/pyridine finally afforded malonate
a
`
`
´
Groupe de Chimie des Fullerenes et des Systemes Conjugues,
Laboratoire de Chimie de Coordination du CNRS, 205 Route de
Narbonne, 31077 Toulouse Cedex 4, France. E-mail:
jfnierengarten@lcc-toulouse.fr; Fax: þ33 561 553003;
Tel: þ33 561 333151
b Philips Electronics Netherland B.V., Prof. Holstlaan 4 (WB62),
5656 AA Eindhoven, The Netherlands. E-mail:
sepas.setayesh@philips.com; Fax: þ31 402 746505;
Tel: þ31 402 74594
w Part of this work has been reported in a patent (UK patent,
application 0413398.9, June 2004).
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Fig. 1 Current–voltage (I/V) characteristics in the dark (black circles)
and under illumination (open circles) of a MDMO-PPV/1 device with
a Ca/Al cathode.
9. The reaction of C60 with compound 9, iodine and
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) under Bingel con-
ditions then gave methanofullerene 1 in 53% yield. The
1
structure and purity of compound 1 were confirmed by H-
and 13C-NMR spectroscopy, mass spectrometry and elemental
analysis. Compound 1 could be stored for several months in
the freezer without significant degradation. It is also reason-
ably stable at room temperature in the neat state. In contrast,
when compound 1 is heated at 100 1C for 1 h, an insoluble
dark red material is obtained suggesting that polymerization
occurred. It can be noted that the UV/Vis spectra of a spin
coated thin film of compound 1 before and after heating at 100
1C for one hour are similar. The latter observation shows that
the fullerene chromophore is not affected by the polymeriza-
tion of the butadiyne subunits. In addition, it also reveals that
the solid state reaction of the butadiyne moieties does not
produce polydiacetylene (PDA) polymers. This is expected
since the reactive subunits are not pre-organized to allow the
formation of extended conjugated PDA chains.
In order to test the potential of fullerene derivative 1 for
photovoltaic applications, solar cells were prepared with
blends of 1 and MDMO-PPV, then characterized before any
thermal treatment. A mixture of MDMO-PPV and 1 (1 : 5 by
weight) was spin coated from a chlorobenzene solution on top
of a 100 nm (surface profiler (Veeco Dektak 3ST)) thick layer
of poly(ethylene dioxythiophene)-polystyrene sulfonate (PED-
OT:PSS, Baytron P)-covered indium-tin oxide (ITO) sub-
strate. The resulting thickness of the layer was determined
with a surface profiler to be 90 nm. Calcium (15 nm) and
aluminium (150 nm) were evaporated on the composite layer.
The devices were measured under nitrogen in a glove box. The
current/voltage (I/V) characteristics in the dark and under
illumination (standard AM1.5 conditions) are depicted in
Fig. 1. These cells exhibit a short circuit current (Isc) of 0.1
mA cmꢁ2, an open circuit voltage (Voc) of 0.59 V and a fill
factor (FF) of 0.27.
Scheme 1 Preparation of methanofullerene derivative 1. Reactant
and conditions: (a) CuCl, TMEDA, dry air, CH2Cl2, room temp., 24 h
(40%); (b) LiAlH4, THF, 0 1C, 4 h (95%); (c) TsCl, pyridine, DMAP,
CH2Cl2, 0 1C, 18 h (64%); (d) methyl 3,5-dihydroxybenzoate, K2CO3,
DMF, 70 1C, 24 h (71%); (e) LiAlH4, THF, 0 1C, 3 h (75%); (f)
malonyl chloride, pyridine, DMAP, CH2Cl2, room temp., 20 h (62%);
(g) C60, I2, DBU, toluene, room temp., 14 h (53%).
In a second set of experiments, the Ca/Al cathode was
replaced by silver (150 nm). To study their thermal stability,
these devices were annealed at 100 1C for 2 hours in the glove
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314 | New J. Chem., 2006, 30, 313–316

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this compound is able to form a polymeric film upon heating
without affecting the fullerene chromophore. The potential of
this new fullerene derivative for the preparation of bulk
heterojunction photovoltaic cells has been evaluated. The
results obtained with our first devices are quite promising.
Further investigations are still underway for a complete
characterization of the polymer obtained after thermal treat-
ment and to optimize the performances of these solar cells.
Experimental
Compound 9
Malonyl dichloride (85 mg, 0.60 mmol) was added to a stirred
degassed solution of 8 (0.76 g, 1.20 mmol), pyridine (0.098 mL,
1.21 mmol) and DMAP (40 mg, 0.30 mmol) in CH2Cl2 (20
mL) at 0 1C. The solution was warmed slowly to room
temperature (over 1 h) and stirred for 20 h. The resulting
CH2Cl2 solution was washed with water, dried (MgSO4),
filtered and evaporated. Column chromatography (SiO2,
CH2Cl2) yielded 9 (490 mg, 62% yield) as a colourless glassy
product. 1H NMR (CDCl3, 200 MHz): d ¼ 0.90 (t, 3J ¼ 6 Hz,
12H), 1.20–1.85 (m, 80H), 2.25 (t, 3J ¼ 6 Hz, 16H), 3.49
(s, 2H), 3.91 (t, 3J ¼ 6 Hz, 8H), 5.10 (s, 4H), 6.40 (s, 2H), 6.46
(s, 4H); 13C NMR (CDCl3, 50 MHz): d ¼ 13.88, 19.11, 22.11,
25.96, 27.99, 28.27, 28.75, 28.98, 29.18, 29.28, 29.33, 30.94,
41.44, 65.19, 65.23, 67.16, 67.98, 77.41, 77.49, 101.08, 106.28,
137.14, 160.40, 166.17.
Compound 1
DBU (0.3 mL, 1.8 mmol) was added to a stirred solution of
Fig. 2 AFM pictures of MDMO-PPV/1 before (a) and after anneal-
C60 (325 mg, 0.45 mmol), I2 (177 mg, 0.7 mmol) and 9 (600 mg,
ing (b).
0.45 mmol) in toluene (450 mL). The solution was stirred for
14 h, then filtered through a short plug of SiO2, eluting first
with toluene (to remove unreacted C60) and then with CH2Cl2.
Column chromatography (SiO2, toluene–hexane 7 : 3) yielded
1 (490 mg, 53%). Dark red glassy product. 1H-NMR (CDCl3,
box. During this treatment, polymerization of 1 might occur.
In addition, the influence of the remaining solvent present in
the active layer on the cathode is thus reduced. Interestingly,
the annealing resulted in a slight improvement of the perfor-
mances (before annealing: Isc ¼ 0.034 mA cmꢁ2, Voc ¼ 0.6 V,
FF ¼ 0.22; after annealing Isc ¼ 0.037 mA cmꢁ2, Voc ¼ 0.61 V,
FF ¼ 0.26). This effect has also been observed for polythio-
phene/PCBM composites,10 but it can be noted that the
MDMO-PPV/PCBM devices degrade upon annealing.
In order to gain more insight into the morphological
changes occurring during the thermal treatment, tapping mode
atomic force microscopy (TM-AFM) studies have been carried
out. The TM-AFM pictures of the MDMO-PPV/1 film (see
Fig. 2) show features of 0.2–0.6 mm widths and a height
difference of 60 nm. Through annealing the roughness of the
surface increases (height difference of 70 nm). Additionally,
new features with a diameter of 0.1–0.4 mm can be observed in
the height and phase picture. The morphological changes
observed after annealing the MDMO-PPV/1 are however
much smaller than the observed separated phases in the
MDMO-PPV/PCBM composites. This is a clear indication
that cross-linking of compound 1 is capable of stabilizing the
initially formed phases.
3
200 MHz): d ¼ 0.90 (t, J ¼ 6 Hz, 12H), 1.20–1.85 (m, 80H),
2.24 (t, 3J ¼ 6 Hz, 16H), 3.89 (t, 3J ¼ 6 Hz, 8H), 5.43 (s, 4H),
4
6.40 (s, 2H), 6.58 (d, J ¼ 2 Hz, 4H); 13C-NMR (CDCl3, 50
MHz): d ¼ 13.96, 19.10, 22.08, 26.01, 27.96, 28.24, 28.76,
28.96, 29.18, 29.29, 29.34, 30.91, 51.75, 65.27, 65.31, 67.93,
68.71, 71.30, 77.36, 77.45, 101.48, 106.70, 136.49, 138.89,
140.69, 141.71, 142.00, 142.78, 142.86, 143.65, 144.31,
144.39, 144.49, 144.71, 144.82, 144.95, 145.00, 145.05,
160.37, 163.18; IR (CH2Cl2): 1749 cmꢁ1 (CQO); Elemental
analysis calc. for C149H126O8: C 87.53%, H 6.21%; found C
82.97%, H 5.96%; FAB (MS): 2044.8 (10%, M1, calc. for
C
149H126O8: 2044.63); 720.0 (100%, [C60]1, calc. for C60:
720.00).
Acknowledgements
This work was supported by the CNRS, the French Ministry
of Research (ACI Jeunes Chercheurs) and ECODEV. We are
grateful to Dr D. Felder for the initial preparation of com-
pound 7 and to L. Oswald (IPCMS, Strasbourg) for technical
help. S. Setayesh thanks E.E.T. project 97115 for financial
support.
A new methanofullerene derivative bearing four butadiene
subunits has been prepared. Preliminary studies indicate that
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Bourgogne, Y. Rio, G. Accorsi, N. Armaroli and S. Setayesh,
Carbon, 2004, 42, 1077.
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316 | New J. Chem., 2006, 30, 313–316