Organic semiconductors have great potential for the development
of large-scale, fl exible, and semitransparent solar panels.
The primary excitations in organic materials are strongly bound
excitons therefore for effi cient charge carrier generation it is
necessary to use a heterojunction of two materials, one an electron
donor and the other an electron acceptor. The free energy
difference between the initial (exciton) and fi nal (electron–hole
pair) states is known as the “driving force” for electron transfer
(ET). Photoinduced ET is a critical process for a wide range of
biological, chemical, and physical systems, including natural [ 1 ]
and artifi cial photosynthesis, [ 2 ] photocatalysis and excitonic
photovoltaic devices. [ 3 ] A key issue in solar cells is that to generate
a high power conversion effi ciency, it is desirable to have
the smallest driving force necessary to generate free charges,
as any excess will lead to increased thermalization losses and
consequently a reduced open circuit voltage. A recent study has
shown that the photocurrent generation effi ciency at short circuit
conditions is independent of the excess vibrational energy,
suggesting that it is possible to develop effi cient blends with a
minimal driving force and consequently with almost no energy
loss at the interface between donor and acceptor. [ 4 ] Another
recent study showed the existence of an optimal driving force
for the highest relative effi ciency of the mobile charge generation.
[ 5 ] Time-resolved spectroscopies have shown that ET