(Jsc) and fill factor (FF) are fixed. Successful examples such
as [6,6]-phenyl-C61-butyric acid methyl ester bisadduct
(bis-PC61BM), indene-C60 bisadduct (ICBA), dihydro-
naphthyl-C60 bisadduct (NCBA), and di(4-methylphenyl)-
methano-C60 bisadduct (DMPCBA) have been reported.4
With proper device optimization, up to ∼50% increments
in both Voc and PCE have been achieved for these bisadduct-
based solar cells in comparison to those standard PC61BM-
based solar cells with poly(3-hexylthiophene) (P3HT) as the
donor.
derivatives were prepared through the [2 þ 4] cycloaddi-
tion reaction of C60 and the in situ generated thieno-o-
quinodimethane dienes by a published method (Scheme 1).6
Pristine (R = H) and substituted (R = hexanoyl or
2-ethylhexanoyl) 2,3-bis(chloromethyl)thiophene were
used as the precursors for thieno-o-quinodimethane and
were prepared according to the literature.7 To obtain
both mono- and bisadducts of TOQC, we employed 2
equiv of precursor for all reactions. In comparison to
o-quinodimethane, thieno-o-quinodimethane is more re-
active, and all reactions were completed within 30 min.
The yields for the mono- and bisadducts of TOQC are
15ꢀ32 and 40ꢀ52%, respectively.
Scheme 1. Syntheses of TOQC Derivatives
The NMR spectra indicate that the monoadducts of
TOQC are isomerically pure compounds with Cs symme-
try while the bisadducts of TOQC are isomers as expected
(Figures S1ꢀS12 in Supporting Information). The MAL-
DI-TOF mass spectra show the expected molecular ion
peaks for all compounds. The UVꢀvis spectra of the
pristine mono-TOQC, bis-TOQC, and the reference
PC61BM are shown in Figure 1. Both mono-TOQC and
PC61BM show the characteristic absorption of the full-
erene 1,2-adduct at 433 and 428 nm, respectively, while bis-
TOQC shows enhanced absorption in the 440ꢀ510 nm
region. The side-chain-substituted derivatives, mono-
TOQC-H, bis-TOQC-H, mono-TOQC-EH, and bis-
TOQC-EH, show similar absorption spectra as their pris-
tine analogues (Figure S13). The side-chain-substituted
TOQCs are highly soluble in common organic solvents.
For example, their solubilities are over 3 wt % in chloro-
form and over 6 wt % in o-dichlorobenzene (ODCB). The
solubility is also good for the pristine bis-TOQC, about
1.5 wt % in ODCB but is quite low for the pristine mono-
TOQC, only 0.4 wt % in ODCB.
Although remarkable progress has been made, the rules
for designing efficient fullerene materials are obscure and
rarely discussed. Meanwhile, there is plenty of room for
developing new efficient acceptors through the chemical
modification of fullerene. In this work, we synthesized a
series of novel thiophene-containing fullerene derivatives,
the thieno-o-quinodimethane-C60 (TOQC) mono- and
bisadducts, through the DielsꢀAlder reaction and investi-
gated the synergistic influence of the LUMO energy levels
and the solubilizing side chains to the final photovoltaic
performance of the fullerene acceptors. Our results indi-
cate that not all bisadducts of fullerene with high LUMO
levels are efficient acceptors for OPVs. The acceptor based
on pristine TOQC bisadduct (no side chains) gave the
highest performance (PCE = 5.1%), while the side-chain-
substituted TOQC bisadducts show much lower efficiency
due to low Jsc and FF. On the contrary, the side-chain-
substituted TOQC monoadducts show much higher PCE
than the pristine one due to their improved solubility.
The design for TOQC is based on the following con-
siderations: first, the introduction of a thiophene moiety to
fullerenecan help to improve the miscibility between donor
and acceptor;5 second, the 5-position of the thiophene ring
is easy to functionalize with solubilizing chains. TOQC
Figure 1. UVꢀvis absorption spectra for mono-TOQC, bis-
TOQC, and PC61BM in chloroform (10ꢀ5 mol/L).
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(6) (a) Belik, P.; Gugel, A.; Spickermann, J.; Mullen, K. Angew.
Chem., Int. Ed. Engl. 1993, 32, 78. (b) Fernandez-Paniagua, U. M.;
Illescas, B. M.; Martin, N.; Seoane, C. J. Chem. Soc., Perkin Trans. 1996,
1, 1077.
(5) Popescu, L. M.; van’t Hof, P.; Sieval, A. B.; Jonkman, H. T.;
Hummelen, J. C. Appl. Phys. Lett. 2006, 89, 213507.
Org. Lett., Vol. 14, No. 6, 2012
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