Indan-C60: From a crystalline molecule to photovoltaic application

Article abstract of DOI:10.1039/c3cc46252h

Crystalline Indan-C₆₀ and its photovoltaic application were studied. Microsheets and aloe-like micro-nano superstructures can be assembled from Indan-C₆₀. Indan-PC₆₁BM derived from Indan-C ₆₀ was further investigated as an acceptor for OPV devices, which shows a higher V_oc, FF, and PCE than those obtained using PC ₆₁BM.

Full text of DOI:10.1039/c3cc46252h

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Indan-C60: from a crystalline molecule to photovoltaic
application†
Cite this: Chem. Commun., 2013,
49, 9923
Taishan Wang,ab Xiaxia Liao,b Jizheng Wang,b Chunru Wang,b Mei-Yee Chana and
Vivian Wing-Wah Yam*a
Received 15th August 2013,
Accepted 30th August 2013
DOI: 10.1039/c3cc46252h
www.rsc.org/chemcomm
Crystalline Indan-C60 and its photovoltaic application were studied. A new fullerene derivative of Indan-C60 was synthesized and character-
Microsheets and aloe-like micro-nano superstructures can be assembled ized. Surprisingly, Indan-C60 readily crystallizes in organic solvents and
from Indan-C60. Indan-PC61BM derived from Indan-C60 was further its crystalline character was analyzed using single crystal X-ray
investigated as an acceptor for OPV devices, which shows a higher crystallography. The self-assembly of Indan-C60 was investigated
Voc, FF, and PCE than those obtained using PC61BM.
by a solvent evaporation method. Moreover, the Indan-PC61BM
derivative based on Indan-C60 was employed as an acceptor for
Fullerene (C60) is a significant molecule due to its excellent chemical solar cells, and the OPV device based on P3HT and Indan-PC61BM
properties, appealing self-assembly performance, unique physical shows a higher Voc of 0.80 V, an enhanced fill factor (FF) of 67.2%,
properties, and extensive applications in semiconductors and opto- and an improved PCE of 4.33%, while the OPV device based on
electrical devices.1 Molecular self-assembly of C60 and its derivatives P3HT/PC61BM displays a Voc of 0.63 V and a PCE of 3.8%.
has attracted much interest as the well-ordered thin films or bulk
The synthesis of Indan-C60 and Indan-PC61BM is shown in
materials have been shown to find applications in catalysis, sensor Scheme 1. Briefly, 1-indanone was first converted to the corre-
technology, and interfacial heterojunctions.2 Moreover, the crystal- sponding tosylhydrazone moiety, which was then reacted
linity of C60 and its derivatives imparted by p-stacking interactions with C60 to yield Indan-C60.7 Indan-C60 was isolated by high-
play an important role in material chemistry as this kind of performance liquid chromatography (HPLC) using a Buckyprep
crystallization-driven self-assembly can result in better optoelectronic column. Indan-PC61BM was synthesized from Indan-C60 with
materials, such as those with higher electron mobility.3 In addition, a tosylhydrazone moiety used for PC61BM and the resulted bis-
the electron deficiency of fullerene C60 for an n-type organic adducts were also isolated by HPLC.7 The Indan-PC61BM pro-
semiconductor, i.e., electron accepting character, has been widely ducts contain a mixture of regioisomers and they were not
utilized in combination with suitable electron donors for the devel- further purified. Indan-C60 and Indan-PC61BM were confirmed
opment of organic photovoltaic (OPV) devices.4 A representative by the matrix assisted laser desorption ionization-time of flight
polymeric bulk heterojunction (BHJ) device, containing poly(3-hexyl- (MALDI-TOF) mass spectrometry and 1H NMR spectroscopy
thiophene) (P3HT) and ([6,6]-phenyl-C61-butyric acid methyl ester analysis (see ESI†).
(PC61BM)), is widely studied as an organic solar cell.5 However, the
low open-circuit voltage (Voc) of ca. 0.6 V for the P3HT:PC61BM-based
device has limited the power conversion efficiency (PCE).6 Therefore,
it is important to explore new fullerene structures and to study their
material chemistry for promising applications.
Herein, we focus on the exploration of new fullerene materials
towards molecular self-assembly and applications in OPV devices.
a Department of Chemistry, The University of Hong Kong, Pokfulam Road,
Hong Kong, P.R. China. E-mail: wwyam@hku.hk; Fax: +852 2857 1586;
Tel: +852 2859 2153
b Beijing National Laboratory for Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing, P.R. China
† Electronic supplementary information (ESI) available: Synthesis details, MS,
NMR and UV-vis spectra, OPV experiments, AFM images, and CV results. CCDC
938648. For ESI and crystallographic data in CIF or other electronic format see
Scheme 1 Structures and synthetic routes of Indan-C60 and Indan-PC61BM.
Chem. Commun., 2013, 49, 9923--9925 9923
DOI: 10.1039/c3cc46252h
c
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Fig. 2 SEM images of assembled Indan-C60 by a solvent evaporation method,
using o-DCB (a) and chloroform (b), respectively.
micro-nano superstructures are composed of leaf-like substructures
assembled from Indan-C60. For these aloe-like micro-nano struc-
tures, there is no observable XRD peak (see ESI†). This result reveals
that in chloroform solution the Indan-C60 molecules undergo
assembly and are deposited very quickly due to the highly volatile
nature of chloroform. The sizes of these aloe-like superstructures
are ca. 2 mm and these micro-nano structures may have potential
applications in catalysis or as interfacial materials.
The electrochemical property of Indan-C60 was investigated using
cyclic voltammetry (CV). Compared with that of PC61BM which is
widely used as a good acceptor in polymer solar cells, reduction
potentials of Indan-C60 show cathodic shifts and the estimated
LUMO level of Indan-C60 is 0.06 eV higher in energy than that of
PC61BM. It is known that the high LUMO levels of fullerene
acceptors are favourable for providing a high Voc for solar cells since
Voc is proportional to the energy level difference between the donor
HOMO and the acceptor LUMO. Therefore, we further modified
Indan-C60 and prepared Indan-PC61BM. Unlike the highly crystalline
Indan-C60, Indan-PC61BM has greatly improved solubility in organic
solvents. As listed in Table 1, the half-wave reduction potentials and
the estimated LUMO levels of Indan-C60, Indan-PC61BM, and
PC61BM are compared. Moreover, Indan-PC61BM is found to have
a higher LUMO level than Indan-C60 and PC61BM, and such an
enhanced LUMO level may be attributed to the 56 p-electron
fullerene system, similar to that found in other bis-adducts such
as ICBA,6a DMPCBA,6c OXCBA,6d and OQMF.6f
Fig. 1 Indan-C60 structure and unit cell (a) as determined by X-ray crystal-
lography. A view of the packing mode along the a axis (b).
Interestingly, Indan-C60 is found to readily crystallize to form
microcrystals when the purified sample in toluene solution was left
to stand for hours (see ESI†). These microcrystals have a plate shape
and a high degree of crystallinity as revealed by scanning electron
microscopic (SEM) and XRD characterization. In addition, the
Indan-C60 microcrystals have lower solubility in toluene, chloroform,
and 1,2-dichlorobenzene (o-DCB) than in carbon disulfide (CS2). A
single crystal of Indan-C60 with a plate-shape structure was obtained
from CS2 and the X-ray diffraction analysis results,8 as shown in
Fig. 1a, revealed that the adduct site of carbene is a 6,6-ring juncture
of C60 with a closed quaternary-carbon-bridged transannular
bond (1.628 Å), which is similar to the methano-bridged bond
in the well-known PC61BM.7
The obtained single crystal has a unit cell comprised of
2C69H8Á5CS2, which is packed in a triclinic space group with unit cell
parameters of a = 9.857 Å, b = 14.021 Å, c = 14.292 Å and a = 89.901, b =
87.031, g = 84.221. Fig. 1b shows the packing mode along the a axis
and the nearest distances between these two molecules were labeled.
The relatively short distances (3.228 and 3.279 Å) between C60 moieties
reveal strong p–p interactions. Moreover, there is a strong interaction
between the benzene ring and the neighboring C60 moiety as their
nearest distance is 3.398 Å. These results indicate that p–p interactions
between C60 moieties combined with arene–C60 interactions are
involved in the first step of their self-organization, and this probably
is the main reason for the high crystallinity of Indan-C60.
Considering the excellent crystalline character of Indan-C60, the
self-assembly of Indan-C60 was then investigated by the solvent
evaporation method that is widely used to prepare various unusual
crystal shapes of pristine C60 solids.3 Fig. 2 shows the SEM images of
Indan-C60 prepared in o-DCB and in chloroform solutions, respec-
tively. Fig. 2a shows the assembled Indan-C60 in o-DCB solution, and
a large number of microsheets were observed. The resulting full-
erene sheets have large sizes with lengths of ca. 10 mm. The powder
XRD analysis (see ESI†) revealed that these microsheets have a high
degree of ordering and these microsheets with a 2-dimensional
layered structure may offer them advantages in the fabrication of
electronic and optoelectronic devices. Fig. 2b shows the SEM images
of aloe-like micro-nano structures prepared from a chloroform
solution of Indan-C60 and it is interesting to observe that these
Furthermore, we fabricated a bulk heterojunction solar cell
device using Indan-PC61BM with a typical structure of ITO/
PEDOT:PSS/P3HT:Indan-PC61BM/Ca/Al, and PC61BM was used
as a reference. The concentration of Indan-PC61BM in o-DCB was
12 mg mLÀ1 and the donor/acceptor ratio was optimized to be
1 : 0.7. The annealing temperature of the devices was 110 1C for
10 min. Voc, short-circuit current density (Jsc), FF, and PCE for the
Table 1 Half-wave reduction potentialsa and LUMO energy levelsb of Indan-C60
,
Indan-PC61BM and PC61BM
Compound
redE1 (V)
redE2 (V)
LUMO (eV)
Indan-C60
Indan-PC61BM
PC61BM
À1.20
À1.26
À1.14
À1.64
À1.70
À1.55
À3.60
À3.54
À3.66
Versus Fc/Fc+. LUMO energy levels were estimated using the follow-
a
b
ing equation: LUMO level = À (E1 + 4.8) eV.
c
9924 Chem. Commun., 2013, 49, 9923--9925
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Table 2 Performance of the solar cells based on P3HT and two fullerene
New Materials. This work was fully supported by the Theme-
Based Research Scheme of the Research Grants Council of
Hong Kong, China (T23-713/11). T.S.W. thanks the China
Post-doctoral Science Foundation (201104153) and the Hong
Kong Scholars Program.
acceptors (Indan-PC61BM and PC61BM)
Acceptor
Voc (V)
Jsc (mA cmÀ2
)
FF (%)
PCE (%)
Indan-PC61BM
PC61BM
0.80
0.63
8.08
9.34
67.2
64.9
4.33
3.80
Notes and references
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In conclusion, a new fullerene derivative of Indan-C60, which readily
crystallizes in organic solvents and can be used as a precursor for good
fullerene acceptors, was reported. The self-assembly of Indan-C60 was
investigated, and microsheets as well as the aloe-like micro-nano
structures of Indan-C60 were prepared. The Indan-PC61BM derivative
based on Indan-C60 was employed as an acceptor in OPV devices, and
the OPV device based on P3HT and Indan-PC61BM shows a higher Voc
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of 0.80 V and a higher PCE of 4.33%. Further work on its assembly and 8 Crystal data for 2C69H8Á5CS2: black plate, crystal size 0.04 Â 0.29 Â
%
0.41 mm, triclinic, space group P1, a = 9.857 Å, b = 14.021 Å, c = 14.292 Å
acceptor derivatives is underway.
and a = 89.901, b = 87.031, g = 84.221, V = 1966.1, Z = 2, T = 173 K;
R1 = 0.043 for 7170 reflections with I > 2s(I), wR2 = 0.1556 for all data.
V.W.-W.Y. acknowledges support from The University
of Hong Kong under the URC Strategic Research Theme on
CCDC number: 938648.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 9923--9925 9925