ECN publication
Mobility and decay kinetics of charge barriers in photoexcited PCBM/PPV blends
Kroon, J.M.; Wienk, M.M.; Savenije, T.J.; Kroeze, J.E.; Warman, J.M.
Published by: Publication date:
ECN Solar Energy 1-4-2004
ECN report number: Document type:
ECN-RX--04-117 Article (scientific)
Number of pages:

Published in: Physical Review B (), , 2004, Vol.69, p.155205-.


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 0.631023 cm2/V s. Above WPCBM50.6, fS mincreased dramatically, reaching a maximum

value of 8331023 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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