Tuning the Interfacial
Electronic Structure at Organic
Heterojunctions by Chemical Design
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Abstract
Quantum-chemical techniques are applied to assess the
electronic
structure at donor/acceptor heterojunctions of interest for organic
solar cells. We show that electrostatic effects at the interface of
model 1D stacks profoundly modify the energy landscape explored by
charge carriers in the photoconversion process and that these can
be tuned by chemical design. When fullerene C<sub>60</sub> molecules
are used as acceptors and unsubstituted oligothiophenes or pentacene
are used as donors, the uncompensated quadrupolar electric field at
the interface provides the driving force for splitting of the charge-transfer
states into free charges. This quadrupolar field can be either enhanced
by switching from a C<sub>60</sub> to a perylene-tetracarboxylic-dianhydride
(PTCDA) acceptor or suppressed by grafting electron-withdrawing groups
on the donor