When Electrons Leave Holes
in Organic Solar Cells
Ultrafast spectroscopy reveals how the charge
carriers in organic solar cells separate at
interfaces and avoid substantial energy loss.
chemically if not physically, to fertilize their
eggs using the male’s sperm. In this scenario,
selection could well have led to the evolution
of male pheromones that alter female physiology in a way that enhances male fitness,
even if they shorten female life span. Given
that males are rare and do not know which
female in their environment they might mate
with, releasing pheromones that influence all
females in their vicinity might be the best way
to maximize fitness.
But why don’t female worms evolve ways
to avoid such costly consequences of perceiving males? This might be a case of the
“rare enemy” effect. If males are seldom
encountered by hermaphrodites, then selection might be too weak to favor what could be
costly measures for hermaphrodites to fight
off male attempts at manipulation.
What about the observation that male
mortality in flies increases merely by per-
ceiving the presence of females? Here, too,
sexual conflict might be the proximate expla-
nation. Interestingly, the detrimental effects
from exposure to female pheromones can be
largely alleviated by allowing the male fruit
fly to mate with multiple females (5). Is this
a cost of false expectations? An alternative
possibility is that the very traits that males
have evolved to manipulate females can
be costly to the male. Males produce toxic
proteins in their seminal fluid that promote
egg-laying in the female but also reduce
her life span (1). It may be that perceiving
females are sufficient to drive production of
these or other toxic molecules, and failure to
ejaculate them during mating could result in
deleterious consequences for the male flies—
the evolutionary equivalent of friendly fire.
Perhaps sexually antagonistic coevolution
has shaped sensory pathways—the signals
produced by one sex, and the receptors and
neural pathways that transduce these signals
in the other sex. Although additional studies
are needed to definitively test this idea, these
same pathways have evolved to modulate
longevity and fecundity in response to subtle
and complex changes in a multitude of envi-
ronmental parameters, including nutrients,
temperature, pathogens, and population den-
sity. How humans age might thus be a conse-
quence not only of the way we treat our body,
and the genetic lottery handed to us by our
parents; it might also be affected by the social
interactions—some cooperative, some con-
flictual—played out by males and females
over millions of years.
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Organic solar cells convert sunlight into electricity by exploiting the elec- tronic properties of electrically and
optically active organic materials. Since the
initial report of an organic solar cell reaching
a power conversion efficiency near 1% (1),
many efforts have brought the efficiency of
carefully optimized devices in the 10 to 12%
range (2). These efficiencies remain, however, well below the thermodynamic limit for
single-junction organic solar cells, estimated
to be >20% (3). Part of the failure in reaching the full potential of these devices is the
lack of a comprehensive mechanistic picture
of energy harvesting and carrier generation,
transport, and recombination, particularly as
a function of materials properties and active-layer morphology. On page 512 of this issue,
Gélinas et al. (4) present a major step forward
in the characterization of the charge-separation mechanism in organic solar cells.
An established view of organic π-
conjugated materials is that the primary
photoexcitations are excitonic in nature. The
photoexcited electron is not free to move on
its own; it remains bound to the hole (positive
charge carrier) that forms on the molecular
orbital from which the electron was excited.
The binding energies of these electron-hole
pairs, or excitons, are generally
more than one order of mag-
nitude greater than the ther-
mal energy at room tempera-
ture. The formation of strongly
bound excitons is in contrast to
the situation in inorganic mate-
rials, such as crystalline silicon,
where photoexcitations lead to
immediate separation of elec-
1School of Chemistry and Biochemistry, and Center for
Organic Photonics and Electronics, Georgia Institute of
Technology, Atlanta, GA 30332–0400, USA. 2Department of
Chemistry, King Abdulaziz University, 21589 Jeddah, Saudi
Arabia. E-mail: email@example.com
PC71BM p-DTS(FBTTh2)2 R1
SS F F
NN NN Si
Charges at the interface. In this
illustration of the donor-acceptor
interface and the ultrafast process of
charge separation of hole and electron probed by Gélinas et al., the
ovals represent the donor molecules
[p-DTS(FBTTh2)2] and the circles
the fullerene acceptor molecules
(PC71BM). The modeling carried out
by Gélinas et al. suggests that the
electron wave function is delocalized
over several fullerene molecules.