Polymer solar cells based on diphenylmethanofullerenes with reduced sidechain length

Article abstract of DOI:10.1039/c0jm01160f

Diphenylmethanofullerenes (DPMs) show interesting properties as acceptors in polymer bulk heterojunction solar cells due to the high open circuit voltages they generate compared to their energy levels. Here we investigate the effect of reducing the alkane

Full text of DOI:10.1039/c0jm01160f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Polymer solar cells based on diphenylmethanofullerenes with reduced
sidechain length†‡
ad
a
a
a
*
*
Henk J. Bolink,
Salvatore Filippone and Nazario Martın
Eugenio Coronado, Alicia Forment-Aliaga, Martijn Lenes, Andrea La Rosa,b
b
bc
*
ꢀ
Received 22nd April 2010, Accepted 5th June 2010
DOI: 10.1039/c0jm01160f
Diphenylmethanofullerenes (DPMs) show interesting properties as acceptors in polymer bulk
heterojunction solar cells due to the high open circuit voltages they generate compared to their energy
levels. Here we investigate the effect of reducing the alkane sidechain length of the DPMs from C12 to
C6 in the properties of the solar cell. This change leads to an increase in the electron mobility, thus
allowing for a lower fullerene content, which in turn results in an increase in the short circuit current
and, finally, in an increase in the efficiency of the device (from 2.3 to 2.6%) due to the higher
concentration of the more absorbing polymer in the film. Atomic force microscopy images and external
quantum efficiencies suggest the absence of crystallization of the fullerene to be at the origin of the
slightly reduced performance of DPMs versus the standard fullerene [6,6]-phenyl-C61-butyric acid
methyl ester, implying that higher efficiencies could be possible with this class of fullerenes.
1,1-bis(4,40-dodecyloxyphenyl)-(5,6) C61 (DPM-12) as acceptor
Introduction
can be seen as a remarkable exception.10 Here, an increase in the
open circuit was observed compared to the standard PCBM,
despite the fact that DPM-12 has a similar LUMO level. The power
conversion efficiencies obtained using this novel acceptor material
however are slightly lower compared to PCBM due to a reduced
short circuit current. The probable origin of this reduced current
lies in the lower charge transport properties of DPM-12 leading to
a rather high volume ratio of the (less absorbing) acceptor material
in optimized devices. Furthermore, this lower charge carrier
mobility of DPM-12 may be related to the rather long C12 side
chains of the molecule. In order to check this point, we report here
the use of a diphenylmethanofullerene with a shorter (C6) alkane
sidechain (DPM-6) as the acceptor material in polymer solar cells.
The performance of polymer solar cells has grown steadily over
the last decade1 resulting in devices with power conversion
efficiencies over 6% for a number of different polymeric donor
materials.2 Remarkably, the diversity of the used acceptor
materials is extremely small. Even though other acceptors such as
n-type polymers3 or inorganic materials4 show promising results,
the small molecular weight material [6,6]-phenyl-C61-butyric acid
methyl ester [60]PCBM5 (or its more absorbing alternative
[70]PCBM)6 has shown superior properties in terms of charge
carrier transport, charge separation efficiency and the possibility
of creating the ideal morphology in these types of solar cells.
Thus, virtually all recorded efficiencies have been obtained by
optimizing the donor material with respect to the properties of
PCBM. Recently however, an alternative route towards higher
efficiencies was demonstrated by modification of the fullerene
acceptor material. Since the open circuit voltage of a bulk
heterojunction solar cell is ultimately limited by the difference in
energy between the HOMO of the donor and the LUMO of the
acceptor, modification of the latter should result in a very direct
increase in performance due to an increase in the output voltage.7
Examples, such as the use of fullerene bisadducts8 and
endohedral9 fullerenes show the viability of such an approach.
Given the above mentioned relation between the acceptor
LUMO level and the open circuit voltage, results obtained using
Results and discussion
Synthesis
ThesynthesisofDPM-6 hasbeencarriedoutinthree syntheticsteps
from commercially available 4,40-dihydroxybenzophenone (1).
Thus, reaction of dihexyloxybenzophenone hydrazone(3) prepared
from dihydroxybenzophenone (2)–reactedwith[60]fullereneunder
basic conditions (Bamford–Stevens reaction) in refluxing o-DCB –
afforded DPM-6 in moderate yield, according to the procedure
described in the literature for related compounds (Scheme 1).11
Cyclic voltammetry experiments carried out on DPM-6 in
o-DCB/MeCN (4 : 1) at room temperature showed three
reversible reduction waves at ꢀ1.084 V, ꢀ1.481 V and ꢀ1.996 V
vs ferrocene, similar to the values found for PCBM (E1 ¼ ꢀ1.077
V, E2 ¼ ꢀ1.469 V, E3 ¼ ꢀ1.980 V). These data confirmed similar
LUMO energy levels for both fullerene acceptors. (Fig. 1)
aInstituto de Ciencia Molecular, Universidad de Valencia, PO Box 22085,
ES-46071 Valencia, Spain. E-mail: henk.bolink@uv.es
bDepartamento de Quꢀımica Orgaꢀnica, Facultad de Quꢀımica, Universidad
Complutense, 28040 Madrid, Spain. E-mail: nazmar@quim.ucm.es
cIMDEA-Nanociencia, 28049 Madrid, Spain
dFundacion General de la Universidad de Valencia, Valencia
† This paper is part of a Journal of Materials Chemistry themed issue in
celebration of the 70th birthday of Professor Fred Wudl.
‡ Electronic supplementary information (ESI) available: Further
experimental details and characterisation data. See DOI:
10.1039/c0jm01160f
Photovoltaic devices
DPM-6 was tested for its use as the acceptor in combination with
the well-known semiconducting polymer poly(3-hexylthiophene)
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Scheme 1 The synthesis of DPM-6.
Fig.
2
Current density (J) vs voltage (V) characteristics for
a P3HT : PCBM (black squares) and P3HT:DPM-6 (red circles) solar
cell.
Fig. 1 Cyclic voltammetry [V vs Ag/AgNO3] of DPM-6 (/) and PCBM
(—). Working electrode: GCE; reference electrode: Ag/Ag+; counter
electrode: Pt; supporting electrolyte: 0.1 M Bu4NClO4; scan rate: 100 mV
sꢀ1; solvent: o-DCB/MeCN (4 : 1 v/v).
Fig. 3 The external quantum efficiency (EQE) for a P3HT : PCBM
(black squares) and P3HT:DPM-6 (red circles) solar cell.
the devices, including the data obtained from a device with
a suboptimal P3HT : DPM-6 weight ratio of 1 : 2 and the
previously published result from a P3HT : DPM-12 device. The
efficiencies obtained with DPM-6 (2.6%) are superior to the
previous results of DPM-12 (2.3%), mainly due to the higher short
circuit current. This higher short circuit current can be directly
related to the lower optimal weight ratio of DPM-6 in the blend
compared to DPM-12. In order to understand why less fullerene
content is required for optimal device performance in DPM-6
based devices, mobility measurements were performed.
(P3HT). For comparison, P3HT : PCBM devices were fabricated
in parallel. The optimal polymer to fullerene ratio of the active
layer blend depends on a large number of factors, including the
charge transport and absorbing capabilities of both materials
and the blend morphology. For P3HT : PCBM a ratio of 1 : 1
has been shown to be optimal, due to the balance in the charge
transport of the two materials and the phase separated nature of
the materials after thermal or solvent annealing (in contrast to
for example the PPV : PCBM blend, which shows a complicated
mixture of pure fullerene and blended regions). As stated above,
the previously reported P3HT : DPM-12 devices used a poly-
mer : fullerene ratio of 1 : 2 in the blend, likely originating from
the poorer transport capabilities of the fullerene. From a light
harvesting point of view, such a large fullerene content is unde-
sirable, due to its lower extinction coefficient. When reducing the
sidechain length from C12 to C6 a different behavior is observed.
Here a polymer : fullerene ratio of 1 : 1 is optimal. Figs 2 and 3
show the current density (J) to voltage (V) characteristics and
external quantum efficiencies (EQE) of a typical P3HT : DPM-6
cell together with a reference P3HT : PCBM cell, both with
a blend ratio of 1 : 1. Table 1 lists all the relevant parameters of
Mobility measurements
A high charge carrier mobility of donor and acceptor material is
of vital importance for charge generation and extraction in solar
cells. Previously, 40-fold reduced mobility of DPM-12 compared
to PCBM was observed.10 In order to determine the charge
carrier mobility of DPM-6, electron single carrier devices were
made by sandwiching
a pure DPM-6 layer in between
a PEDOT : PSS anode and a barium silver cathode.12 The deep
lying HOMO level (6 eV) ensures that no holes are injected from
the PEDOT : PSS anode (5.2 eV work function) and only
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Table 1 The solar cell characteristics
Blend
Weight ratio
Voc/V
Jsc/mA cmꢀ2
FF (%)
Max EQE (%)
Intensity/W mꢀ2
PCE (%)
P3HT : PCBM
P3HT : DPM-6
P3HT : DPM-6
P3HT : DPM-12a
1 : 1
1 : 1
1 : 2
1 : 2
0.64
0.69
0.60
0.65
9.72
8.47
3.04
4.7
59
50
58
58
59
60
30
54
1200
1200
1200
800
3.1
2.6
1.0
2.3
a
Taken from ref. 10.
PCBM it is in the same order as the hole mobility observed in
annealed P3HT, ensuring balanced transport between electrons
and holes and hence explaining the 1 : 1 optimal weight ratio of
P3HT : DPM-6.
Comparison between DPM-6 and PCBM
Despite the increased performance of DPM-6 over DPM-12,
PCBM is still the superior acceptor when combined with P3HT.
The lower power conversion efficiency of DPM-6 is mainly
caused by a lower short circuit current. From the EQE
measurements it is observed that the maximum quantum
efficiency for both acceptors is identical, yet PCBM cells show
a more pronounced shoulder at higher wavelengths. This
additional shoulder has been shown to originate from an increase
of the polymer crystallinity after thermal or solvent annealing.14
In fact, the phase separation and subsequent crystallization of
both polymer and fullerene is the main cause of the large increase
in device performance upon annealing of P3HT : PCBM solar
cells.15
Fig. 4 Current density (J) vs voltage (V) corrected for the built-in
voltage (Vbi) of a DPM-6 electron only device (symbols), fitted using eqn
(1) (red line).
electron transport is measured. The J–V characteristics of the
devices (see Fig. 4) show a quadratic dependence, typical for
space charge limited current, allowing the charge carrier mobility
(m) to be determined by using:13
Annealing effect of pure fullerene films
9
V2
L3
J ¼ 303rm
(1)
In order to get a first indication of the effect of a thermal treat-
ment on the P3HT : DPM-6 blend, we have obtained atomic
force microscopy (AFM) images of layers of the pure fullerene
materials before and after annealing. PCBM layers are known to
form large, needle-like structures after thermal treatment, which
8
An electron mobility of 6 ꢁ 10ꢀ8 m2 Vꢀ1 sꢀ1 was found, which
is 3 to 4 times lower than that obtained for PCBM using identical
methods.12 Even though this value is slightly lower compared to
Fig. 5 AFM images of a pure PCBM film, as-cast (left) and annealed at 150 ꢂC for 1 h (right).
1384 | J. Mater. Chem., 2011, 21, 1382–1386
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Fig. 6 AFM images of a pure DPM-6 film, as-cast (left) and annealed at 150 ꢂC for 1 h (right).
indicates at least partial crystallization of the material.16 Indeed,
we observe the formation of these structures, as indicated in
Fig. 5.
performance vs PCBM is explained by the lower tendency of the
fullerene to crystallize and phase separate. It is therefore not
unlikely that a further optimization of the processing conditions,
such as a higher annealing step or different (mixed) spin casting
solutions may result in an improved morphology and hence
efficiencies, even surpassing those obtained in PCBM devices due
to the higher achievable open circuit voltage of DPM-6.
Furthermore, an enhanced thermal stability is expected, due to
the more amorphous nature of the active layer, making this
fullerene a possible candidate as the acceptor in high perfor-
mance polymers that do not require any thermal annealing.19
For DPM-6 however, no significant change in the surface
morphology was found when annealing up to 150 ꢂC (see Fig. 6).
Since we observe no signs of crystallization of the pure material,
it is unlikely DPM-6 will do so in the blend of polymer and
fullerene. These results indicate further optimisation of the
processing conditions, such as a higher annealing step or going to
other (mixed solvent) spin casting solutions, can lead to a better
morphology and hence device performance. The amorphous
nature of these cells, conversely, might result in an enhanced
thermal stability, since ongoing crystallization and phase
separation will lead to a deteriorated device performance.17
Experimental details
Details on the materials synthesis as well as the material char-
acterization is presented in the ESI.‡ Pre-patterned ITO-covered
glass substrates were first cleaned using soap water, demineral-
ized water, propanol and an UV–ozone treatment. Subsequently,
a layer of PEDOT/PSS (Bayer AG) was spin-coated under
ambient conditions onto the cleaned substrates and ꢂthe layer was
dried by annealing the substrate for 30 min at 150 C.
Open circuit voltage
Similar to the original work with DPM-12, DPM-6 devices
generate a significantly higher open circuit voltage compared to
PCBM. In fact, the open circuit voltages typically reached with
DPM-6 of 0.69 V is 0.04 V higher compared to the initial work
with DPM-12. In this work, however, no low work function
interlayer at the cathode was used, as can be seen by the low open
circuit voltage of 0.55 V of their PCBM : P3HT reference device
vs literature values of around 0.6 V.1 Additionally, besides the
typically reached open circuit voltages of 0.69 V, in some cases
voltages as high as 0.73 V were observed. A detailed analysis of
these high open circuit voltages using impedance spectroscopy
will be published elsewhere.18
For solar cells, the blend of either P3HT : PCBM or
P3HT : DPM-6 was spincoated from chlorobenzene resulting in
layer thicknesses of around 200 nm. After a thermal annealing
step at 135 ꢂC for 20 min, the devices were completed by thermal
evaporation of a 5 nm barium/70 nm silver top contact under
vacuum (2 ꢁ 10ꢀ6 mbar) using a vacuum chamber integrated into
an inert atmosphere glovebox (1 ppm O2 and <1 ppm H2O).
For electron-only devices, a pure layer of DPM-6 was spin-
coated from chlorobenzene resulting in a layer thickness of
120 nm. Devices were finished with identical cathodes as the solar
cells.
Conclusions
A diphenylmethanofullerene with a C6 sidechain on the benzene
ring has been synthesized and used in polymer fullerene bulk
heterojunction solar cells. Due to the higher electron mobility
compared to previous DPMs with C12 sidechains, less fullerene
content is needed for proper device operation, resulting in
a higher short circuit current and device performance. The lower
All the devices were fabricated in air up to the spincoating of
the active layer, after which the samples were transferred to a N2
filled glovebox. Evaporation of the top contact and device
characterisation was carried out in the same glovebox system.
The solar cells were illuminated by a white light halogen lamp in
combination with interference filters for the EQE and J–V
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This work has been supported by the Spanish Ministry of Science
and Innovation (MICINN) (PS-120000-2007-1 (FOTOMOL)
2008, MAT2006-28185-E, MAT2007-61584, CSD2007-00010)
and the Generalitat Valenciana. We would especially like to
thank Alejandra Soriano for the careful device preparation.
ALR thanks the European Commision for the project FP7-
212942-1 (FUNMOLS) 2012. A.F.-A. thanks the Spanish Min-
isterio de Ciencia e Innovacion for a Juan de la Cierva contract.
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