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Title:
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Mobility and decay kinetics of charge barriers in photoexcited PCBM/PPV blends
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Author(s):
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Kroon, J.M.; Wienk, M.M.; Savenije, T.J.; Kroeze, J.E.; Warman, J.M.
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Published by:
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Publication date:
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ECN
Solar Energy
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1-4-2004
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ECN report number:
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Document type:
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ECN-RX--04-117
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Article (scientific)
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Number of pages:
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11
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Published in: Physical Review B (), , 2004, Vol.69, p.155205-.
Abstract:
The transient photoconductivity of blends of a highly soluble C60 derivative
~PCBM! and a dialkoxyphenylene-
vinylene polymer
~MDMO-PPV! has been studied
using the electrodeless flash-photolysis timeresolved
microwave conductivity technique
~FP-TRMC!. Films approximately 100 nm thick on a quartz
substrate were prepared by spin-coating PCBM/PPV solutions with PCBM
weight fractions (
WPCBM) from 0.2
to 0.95. For all blends, the wavelength dependence of the photoconductivity
in the range 420?700 nm closely
resembled the photon attenuation spectrum, indicating that photoexcitation
of both components contributes to
mobile charge carrier formation. The product of the quantum yield for
charge separation
fand the sum
of the
charge carrier mobilities
S mwas determined from the maximum ~end-of-pulse! value of the
transient photoconductivity.
On excitation at 500 nm,
fS mremained almost constant in going from WPCBM50.2 to 0.6 with
an average value of 0.6
31023 cm2/V s. Above
WPCBM50.6, fS mincreased dramatically, reaching a maximum
value of 83
31023 cm2/V s for WPCBM50.85. This effect is attributed to the
occurrence of phase separation
above
WPCBM50.6 and to
the resulting formation of highly mobile electrons within PCBM-rich
aggregates.
The much lower value of
fS mobserved below WPCBM50.6 is assigned mainly to mobile holes within
the polymer component of the blend. Possible explanations for the decrease
in
fS m with increasing
light
intensity, found for all blend compositions, are discussed.
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