Under these conditions, k ET for an exciton formed 23 Å from the interface
(corresponding to six chains in the π-stacking direction) is on the order of
10
12 s
À1 , which leads to polaron formation on a time scale comparable with that
found experimentally in annealed P3HT:PCBM.
In P3HT:PCBM, both mechanisms described above may contribute to free
charge photogeneration. Tunneling, as proposed by Troisi and coworkers, is important when exciting the blend at 500–650 nm, where aggregated P3HT absorbs. On
the other hand, sub-bandgap absorption excites CT states predominately in the
intermixed region, and the split-up of these coulombically interacting electron–hole
pairs benefits from the morphology-related driving force. One might, therefore,
expect the IQE to depend on the excitation energy, which is apparently not the case
in P3HT:PCBM. In a very recent work, Vandewal et al. demonstrated the IQE to be
insensitive to excitation energy for various organic donor–acceptor BHJ systems
[183]. It is proposed that photogeneration proceeds via the split-up of thermalized
(electronically and vibronically relaxed) CT states at all illumination conditions,
even if illumination primarily excites the donor or acceptor component. This
situation was encountered in an inefficient blend with a pronounced field dependence of generation, but also for a highly efficient blend with field-independent
generation. Accordingly, we propose that photogeneration in P3HT:PCBM is
entirely governed by the efficient split-up of low-energy CT states.
We note that activationless free charge generation in P3HT:PCBM was unambiguously proven with ultrafast vibrational spectroscopy [180, 181]. It had previously been shown that the frequency of the carbonyl group in PCBM depends on the
local environment: the vibration frequency of a PCBM molecule located in the
interior of a fullerene cluster is lower than that of a PCBM molecule at the interface
to the donor polymer [184] (see Fig. 19c). Therefore, the motion of the electron
away from the heterojunction during charge separation can be monitored through a
decrease in the carbonyl frequency. For P3HT mixed with PCBM, the timedependence of the carbonyl center frequency was independent of the temperature,
meaning that free charge formation via CT split-up does not require thermal
activation (Fig. 19d) [181]. In agreement with this, TAS experiments presented
by Mauer et al. showed free carrier generation in annealed regioregular (rr)-P3HT:
PCBM blends to be independent of temperature [185], indicating again
activationless geminate pair separation.
In conclusion, free charge generation in blends of regioregular P3HT with
PCBM is independent of electric field and possibly temperature for a wide range
of preparation conditions and efficiencies. Carrier formation seems to be equally
efficient when CT states are formed via the split-up of singlet excitons at the BHJ or
when they are generated directly via sub-bandgap excitation. The findings are
explained by the complex morphology of these layers, which comprise pure and
intermixed regions. The answer to the question of why the exciton-to-polaron
conversion in rr-P3HT:PCBM blends is rather insensitive to the preparation conditions might lie in the fact that even samples as-cast from chloroform with very
216
A.J. Moule ´ et al.
(corresponding to six chains in the π-stacking direction) is on the order of
10
12 s
À1 , which leads to polaron formation on a time scale comparable with that
found experimentally in annealed P3HT:PCBM.
In P3HT:PCBM, both mechanisms described above may contribute to free
charge photogeneration. Tunneling, as proposed by Troisi and coworkers, is important when exciting the blend at 500–650 nm, where aggregated P3HT absorbs. On
the other hand, sub-bandgap absorption excites CT states predominately in the
intermixed region, and the split-up of these coulombically interacting electron–hole
pairs benefits from the morphology-related driving force. One might, therefore,
expect the IQE to depend on the excitation energy, which is apparently not the case
in P3HT:PCBM. In a very recent work, Vandewal et al. demonstrated the IQE to be
insensitive to excitation energy for various organic donor–acceptor BHJ systems
[183]. It is proposed that photogeneration proceeds via the split-up of thermalized
(electronically and vibronically relaxed) CT states at all illumination conditions,
even if illumination primarily excites the donor or acceptor component. This
situation was encountered in an inefficient blend with a pronounced field dependence of generation, but also for a highly efficient blend with field-independent
generation. Accordingly, we propose that photogeneration in P3HT:PCBM is
entirely governed by the efficient split-up of low-energy CT states.
We note that activationless free charge generation in P3HT:PCBM was unambiguously proven with ultrafast vibrational spectroscopy [180, 181]. It had previously been shown that the frequency of the carbonyl group in PCBM depends on the
local environment: the vibration frequency of a PCBM molecule located in the
interior of a fullerene cluster is lower than that of a PCBM molecule at the interface
to the donor polymer [184] (see Fig. 19c). Therefore, the motion of the electron
away from the heterojunction during charge separation can be monitored through a
decrease in the carbonyl frequency. For P3HT mixed with PCBM, the timedependence of the carbonyl center frequency was independent of the temperature,
meaning that free charge formation via CT split-up does not require thermal
activation (Fig. 19d) [181]. In agreement with this, TAS experiments presented
by Mauer et al. showed free carrier generation in annealed regioregular (rr)-P3HT:
PCBM blends to be independent of temperature [185], indicating again
activationless geminate pair separation.
In conclusion, free charge generation in blends of regioregular P3HT with
PCBM is independent of electric field and possibly temperature for a wide range
of preparation conditions and efficiencies. Carrier formation seems to be equally
efficient when CT states are formed via the split-up of singlet excitons at the BHJ or
when they are generated directly via sub-bandgap excitation. The findings are
explained by the complex morphology of these layers, which comprise pure and
intermixed regions. The answer to the question of why the exciton-to-polaron
conversion in rr-P3HT:PCBM blends is rather insensitive to the preparation conditions might lie in the fact that even samples as-cast from chloroform with very
216
A.J. Moule ´ et al.