Mendeleev Commun., 2007, 17, 175–177
responsible for the different PL quenching abilities of these
20
15
10
5
compounds. We assume that long and non-branched alkyl chains
like in 1e, 1d and 1b interact quite efficiently with MDMO-PPV
side chains through the van der Waals bonding; hence, the
activation energy of the photoinduced electron transfer can be
significantly lowered for these compounds that is reflected in
their superior PL quenching abilities. We believe that such a
supramolecular approach to control the efficiency of photo-
induced charge separation in the polymer–fullerene blends will
allow one to improve further power conversion efficiencies in
the plastic solar cells.
According to the preliminary results, the substituent modi-
fications of fullerene acceptors 1a–f scales the power conversion
efficiencies of solar cells from very low (0.1%) to appreciably
high values (> 3.5%). Some of the materials showed a better
performance than PCBM in the tests under identical conditions.
1b
1e
1d
0
20
1a
1c
1f
15
10
5
)
I
/
0
I
(
0
20
1e (k = 4.75)
C60
PCBM
We are grateful to Mr. R. Koeppe and Professor S. Sariciftci
(Linz Institute for Organic Solar Cells) for the sample of
MDMO-PPV provided for the studies. This work was supported
by the RAS programme ‘Fundamental Problems of Physics
and Chemistry of Nano-Scale Systems and Materials’, INTAS
(grant no. 04-83-3733), the Russian Foundation for Basic
Research (project no. 04-03-32870) and the Russian Science
Support Foundation.
15
10
5
1f (k = 2.13)
0
0
2
4
0
2
4
0
2
4
Fullerene concentration (wt%)
Figure 2 Stern–Volmer plots for polymer PL quenching by fullerene
derivatives.
Online Supplementary Materials
Supplementary data associated with this article [list of 1H and
13C NMR shifts for all compounds, plots of 13C NMR spectra
for 1a–f, 2D COSY and HSQC NMR spectra for 1e, the plot
I0/I against C (fullerene) illustrating PL quenching abilities of
different compounds] can be found in the online version at
doi:10.1016/j.mencom.2007.05.015.
intensity decreases very strongly when just 0.2% of a fullerene
or its derivative is mixed with the polymer (Figure 2). A further
increase in the concentration of the fullerene acceptor in the
blends results in more complete PL quenching; the MDMO-PPV
luminescence becomes virtually undetectable by our instrument
in the films with the fullerene content above 7%.
The relative intensities of the band at 580 nm (I/I0, where I0
is the intensity of this band in the spectrum of pristine polymer)
in the emission spectra of the films with different fullerene
contents are listed in Table 2 (the I/I0–fullerene concentration
plot is shown in supplementary materials). These data indicate
that [60]fullerene and its derivatives exhibit considerably dif-
ferent PL quenching efficiencies.
For a quantitative evaluation of the quenching ability of
various fullerene acceptors, we used the Stern–Volmer plot
(Figure 2). The quenching factor Q defined by the equation
Q = (I0/I) – 1 was plotted against the fullerene concentration in
the blends (C, wt%). The experimental points can be linearly
fitted by the function Q = kC. The resulting linear fits are
shown in Figure 2. The slopes k of such linear fits can be used
for the quantitative characterization of the PL quenching ability
of the fullerene compounds; higher values of k correspond to
more efficient PL quenching. The calculated values of k are
given in Table 3. Compounds 1e, 1d and 1b (R = Bu, Pr and Et,
respectively) are the most efficient quenchers of MDMO-PPV
photoluminescence, while other compounds show significantly
lower k values.
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The electronic properties (electron affinities etc.) of 1a–f
and PCBM are very similar, as can be concluded from cyclic
voltammetry data (Table 1). Therefore, other factors should be
Table 3 Quenching abilities (k values) of 1a–f and reference fullerene
compounds derived from the Stern–Volmer plots shown in Figure 2.
Fullerene
k
Fullerene
k
1e
4.75
4.72
4.30
3.21
1c
PCBM
1a
2.87
2.56
2.36
2.13
1b
1d
C60
1f
Received: 25th December 2006; Com. 06/2849
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