Conjugated polymers are becoming interesting materials for a range of optoelectronic applications. However, their often complex electronic and structural properties prevent establishment of straightforward property-function relationships. In this paper, we summarize recent results on the photophysics and excited state dynamics of conjugated polymers, in order to paint a picture of exciton formation, quenching, and generation of charge carriers.
Time-resolved fluorescence spectra of three amino-substituted
coumarin dyes have been recorded in methanol
and dimethyl sulfoxide using the fluorescence upconversion technique
with an apparatus response function
of ≈200 fs fwhm. The three fluorinated coumarins are the
7-amino-4-trifluoromethylcoumarin (C151), the
7-diethylamino-4-trifluoromethylcoumarin (C35), and the rigidified
aminocoumarin with a julolidine structure
(C153). The dynamic Stokes shifts are found to be dominated by an
ultrafast component with a characteristic
time shorter than the present time resolution of ≈50 fs. The
dynamic Stokes shifts are compared to estimations
based on a “Kamlet and Taft” analysis of steady-state data in 20
solvents. It is found that the ultrafast
component can be assigned mainly to intramolecular relaxation. The
influences of photoinduced changes of
solute−solvent hydrogen bonds on the observed spectral shifts are
discussed. The breaking of hydrogen
bonds at the amino group is very fast in both solvents and embedded in
the ultrafast solvent inertial relaxation,
while the reformation of hydrogen bonds at the carbonyl group is
believed to occur on the 10−20 ps time
scale in the hydrogen bond donating (HBD) solvent methanol.
However, it is impossible to unambiguously
correlate a particular experimental time constant with the breaking or
the formation of a hydrogen bond.
Solar cells based on conjugated polymer and fullerene blends have been developed as a low-cost alternative to silicon. For efficient solar cells, electron-hole pairs must separate into free mobile charges that can be extracted in high yield. We still lack good understanding of how, why and when carriers separate against the Coulomb attraction. Here we visualize the charge separation process in bulk heterojunction solar cells by directly measuring charge carrier drift in a polymer:fullerene blend with ultrafast time resolution. We show that initially only closely separated (o1 nm) charge pairs are created and they separate by several nanometres during the first several picoseconds. Charge pairs overcome Coulomb attraction and form free carriers on a subnanosecond time scale. Numerical simulations complementing the experimental data show that fast three-dimensional charge diffusion within an energetically disordered medium, increasing the entropy of the system, is sufficient to drive the charge separation process.
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