Ultrafast Long-Range Charge
Separation in Organic Semiconductor
Simon Gélinas,1 Akshay Rao,1 Abhishek Kumar,1 Samuel L. Smith,1 Alex W. Chin,1 Jenny Clark,1
Tom S. van der Poll,2 Guillermo C. Bazan,2 Richard H. Friend1*
Understanding the charge-separation mechanism in organic photovoltaic cells (OPVs) could facilitate
optimization of their overall efficiency. Here we report the time dependence of the separation of
photogenerated electron hole pairs across the donor-acceptor heterojunction in OPV model systems.
By tracking the modulation of the optical absorption due to the electric field generated between the
charges, we measure ~200 millielectron volts of electrostatic energy arising from electron-hole separation
within 40 femtoseconds of excitation, corresponding to a charge separation distance of at least 4 nanometers.
At this separation, the residual Coulomb attraction between charges is at or below thermal energies, so
that electron and hole separate freely. This early time behavior is consistent with charge separation through
access to delocalized p-electron states in ordered regions of the fullerene acceptor material.
Organic photovoltaic cells (OPVs) consist of a nanostructured blend of donor (D) and acceptor (A) semiconductors (1, 2).
Photons absorbed in either material create mo-
lecular excitons, which can dissociate at the D-A
heterojunction into holes on D and electrons on A
(3, 4). The hole and electron are still subject to
their mutual Coulomb interaction and can self-trap
at the heterojunction (5–8), giving rise to charge
transfer (CT) excitons. However, in efficient OPV
blends that use fullerenes as the acceptor, elec-
tron and hole escape from the heterojunction and
long-range charge separation is efficient (9, 10)
(Fig. 1A). The motion of charges away from the
heterojunction had been generally attributed to diffusion (11, 12); however, recent results have
suggested that delocalized states may play a role in
this process (9). Here we directly measure the electron-hole separation process at the heterojunction and
find that the Coulomb barrier is surmounted at times
as short as 40 fs, suggesting that rapid charge motion
away from the interface through delocalized band
states is necessary for long-range charge separation.
To temporally resolve the electron-hole sep-
aration process, we require a probe that is sen-
sitive to the distance between these charges. The
electric field generated as the charges separate
(Fig. 1A) (13–15) serves this purpose by shifting
the energy levels of neighboring molecules,
causing a change in their electronic transition
energies and an associated optical electro-absorption
(EA) (16) signature, represented schematically in
Fig. 1B (17). We measured these EA signals with
sub–30 fs precision, using transient absorption (TA)
spectroscopy. This allows us to calculate the energy
stored in the electric field as the charges separate
and hence the mean electron-hole distance as a
function of time, as we can calibrate the time-
resolved data against steady-state EA measurements.
We studied two high-efficiency model sys-
tems. The first consists of blends of a solution-
processable small molecule (18), p-DTS(FBTTh2)2,
as electron donor with PC71BM, (phenyl-C71-
butyric acid methyl ester), as acceptor (19). The
second system consists of blends of the polymer
PCDTBT (20), (poly[N-11W–henicosanyl-2,7-carbazole-
alt-5,5-(40, 70- di-2-thienyl-20, 10, 30-benzothiadiazole)]),
as electron donor with PC61BM, (phenyl-C61-butyric
acid methyl ester), as acceptor. Figure 1, C to E,
shows their molecular structure and absorption
The small-molecule fullerene blend system
[p-DTS(FBTTh2)2:PC71BM] was chosen as it
exhibits sharper optical transitions than literature-standard polymer-fullerene blends. As we develop below, this property leads to a strong EA
response that enables us to separate this feature from the other excited-state absorption
features. We investigated two blends with different donor/acceptor composition, 60:40 and
90:10. Blends containing a 60:40 weight ratio
of p-DTS(FBTTh2)2:PC71BM, processed from
chlorobenzene with 0.4% diiodooctane (DIO)
as a solvent additive, achieve very high internal
quantum efficiency (IQE) and power-conversion
efficiencies (PCEs) above 7% (19, 21). They
1Cavendish Laboratory, University of Cambridge, Cambridge,
UK. 2Center for Polymers and Organic Solids, University of
California, Santa Barbara, CA, USA.
*Corresponding author. E-mail: email@example.com
Fig. 1. Schematics of interfacial photophysical processes
in OPVs (and molecules studied).
(A) Overview of charge photo-generation at a heterojunction.
Light absorption generates excitons in the bulk (1a) and at
interfaces (1b). When next to
an interface, excitons undergo
rapid charge transfer into an
electron-hole pair (2), generating a dipole-like electric field
in its surroundings (E
). The electron and hole then separate further and form free charges (3).
(B) Stark shift of the absorption
spectrum (S) due to an electric
(dark blue) and the resulting electro-absorption signature (red) calculated from the
difference between the shifted
and unshifted spectra. The EA
amplitude is proportional to jE→j2. (C) Chemical structure of PC61BM (gray) and PCDTBT (red). (D) Chemical structure of PC71BM (gray) and p-DTS(FBTTh2)2 (blue).
(E) Absorption spectra of the molecules presented in (D).