Influence
of Chemical Structure on the Charge Transfer
State Spectrum of a Polymer:Fullerene Complex
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Abstract
Charge transfer (CT) state properties
at the donor:acceptor interface
in organic photovoltaic materials are widely regarded as crucial in
determining the charge separation efficiency of organic photovoltaic
devices. In this work, we use time dependent density functional theory
(TDDFT) with B3LYP/6-31g* to study the influence of chemical structure
on excitation energies, oscillator strengths, and electronic structure.
We vertically excite states in a series of polymer:fullerene blends,
modeling each blend as an oligomer:fullerene pair in vacuo. Our method
reproduces experimentally observed trends in CT state energy with
chemical structure as measured by electroluminescence. For oligothiophene:PCBM
(PCBM = [6,6]-phenyl C61-butyric acid methyl ester), we find that
Coulomb binding tends to reduce in higher excited CT states. In the
case of several isoindigo and diketopyrrolopyrrole (DPP) based donors,
we find that the first excited state of the pair lies close in energy
to the first singlet of the oligomers, and low excited states have
hybrid, or incomplete charge transfer, character. The natures and
energies of these states are dependent on the fullerene position.
We discuss the effect of thiophene substitution in a DPP polymer on
charge generation in terms of the calculated CT state properties and
rationalize the observed charge separation efficiency of corresponding
experimental systems in terms of these calculations