COMMUNICATION
Improving the compatibility of fullerene acceptors with
fluorene-containing donor-polymers in organic photovoltaic devicesw
Jan-Carlos Kuhlmann,a Paul de Bruyn,bc Ricardo K. M. Bouwer,ac Auke Meetsma,b
Paul W. M. Blomb and Jan C. Hummelen*ab
Received 11th May 2010, Accepted 9th July 2010
DOI: 10.1039/c0cc01347a
Fluorene-containing PCBM analogs have been synthesized and
tested with a polyfluorene copolymer, PF10TBT, in organic
photovoltaic devices resulting in an increase of B130 mV in
the open circuit voltage compared to devices with PCBM as
acceptor material.
copolymers: bis-hexyl, bis-octyl and bis-decyl. The resulting
9,9-dialkyl-9H-fluoren-2-yl-C61-butyric-acid methyl esters are
abbreviated FnCBM where n is the number of carbons of the
alkyl chain. The synthesis of the FnCBM series is given in
Scheme 1 (described in more detail in the ESIw). The fluorene
moiety was dialkylated with the appropriate alkylbromide in
dimethylsulfoxide in the presence of potassium hydroxide.
Purification with column chromatography (SiO2/heptane)
yielded the products as colorless oils. The dialkylated fluorenes
1–3 were further functionalized by Friedel–Crafts acylation
with methyl 5-chloro-5-oxopentanoate. Column chromato-
graphy (SiO2/toluene) yielded the pure keto esters 4–6. These
keto esters were then reacted with tosylhydrazide in toluene
under Dean–Stark conditions. Subsequent recrystalli-
sation from hot methanol yielded the tosylhydrazones 7–9 as
pale yellow crystals. Standard diazo addition5 of 7–9 to C60
followed by column chromatography (SiO2/CS2 to remove
unreacted C60, then 1 : 1 toluene/cyclohexane) yielded the
pure adducts 10–12.
This communication describes the synthesis and characteri-
zation of fluorene-containing phenyl-C61-butyric-acid methyl
ester (PCBM) analogs and preliminary results of their use in
photovoltaic devices containing the fluorene-bearing polymer,
poly[2,7-(9,9-didecylfluorene)-alt-5,5-(40,70-di-2-thienyl-
20,10,30-benzothiadiazole)] (PF10TBT, Scheme 1: 13). Power
conversion efficiencies (PCE) of up to 4.2% in bulk-heterojunction
(BHJ) organic photovoltaic devices (OPVs) of PF10TBT with
PCBM as acceptor have been reported.1,2
The morphology of the active layer is crucial for highly
efficient OPVs. One way to improve the morphology of the
active layer is by improving the compatibility of donor and
acceptor by increasing their structural similarity. This approach
of incorporating groups that are structurally similar to the
donor has been reported previously by our group.3 In this
previous work, a thiophene unit was included in the fullerene
acceptor to improve the miscibility with polythiophene donors,
particularly regioregular poly(3-hexylthiophene) (rr-P3HT).
This improved the morphology of the BHJ film after thermal
annealing by increasing the ability of the P3HT to form crystal-
line domains. PCEs of 3.0% were measured in these devices.
Recently a fluorene-copolymer containing polar side groups
to improve the chemical compatibility with fullerenes was
reported, but this (opposite) approach resulted in slightly
lower PCEs as compared to the unmodified copolymer.4
In this communication we report on our efforts to improve
the compatibility of the fullerene acceptor with fluorene-
containing polymers by incorporating fluorene moieties into
three different fullerene acceptors, differing by the substituents
at the 9-position of the fluorene. We chose the three alkyl
chains that are the most commonly used in fluorene-based
Acylation of fluorenes can occur at different positions of the
fluorene moiety.6 Since unambiguous structural proof could not
be obtained by NMR spectroscopy, crystal structures were
obtained for 7 to confirm the proposed structure (Fig. S3, ESIw).
Cyclic voltammetry (CV) of the FnCBMs shows only
marginal differences (7–16 mV more negative) in the first
half-wave reduction potentials as compared to PCBM (Table 1).
All reductions were reversible. To evaluate the effect of the
fluorene moieties on the performance of OPVs we fabricated a
series of photovoltaic devices with the FnCBMs as acceptors
and PF10TBT as the donor and one device with PCBM as
the acceptor for reference. We fabricated these devices by spin-
coating PF10TBT and the fullerene acceptor from chloro-
benzene solutions on PEDOT : PSS-coated ITO glass substrates
and evaporating LiF and aluminium on top of the active layer.
In a first series the devices were fabricated with a 1 : 4
polymer : fullerene ratio (by weight). These are the optimal
conditions for maximizing PCEs with PCBM but not necessarily
the best for the FnCBMs. The results obtained from the best of
these devices with active layer thicknesses of B120 nm are
summarized in Table 1.
a Stratingh Institute for Chemistry, University of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands.
E-mail: j.c.hummelen@rug.nl; Fax: +31 50 363 8751;
Tel: +31 50 363 5553
Compared to PCBM, the devices that have FnCBMs as
acceptor showed increased open-circuit voltages (VOC) of up
b Zernike Institute for Advanced Materials, University of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
c Dutch Polymer Institute, P.O. Box 902, 5600 AX Eindhoven,
The Netherlands
to 130 mV (for F8CBM and F10CBM). This increase in VOC
,
compared to devices comprising PF10TBT : PCBM active
layers, cannot be explained by the energy levels of the polymer
and that of the fullerene measured with CV. However, it is
known that in OPVs comprising PF10TBT : PCBM active
layers VOC increases with decreasing concentrations of
w Electronic supplementary information (ESI) available: Details on
synthetic procedures, solar cells preparation, and crystal structure of
compound 7. CCDC 776856. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0cc01347a
ꢀc
This journal is The Royal Society of Chemistry 2010
7232 | Chem. Commun., 2010, 46, 7232–7234
Kuhlmann, Jan-Carlos
