Reducing
Exciton Binding Energy by Increasing Thin
Film Permittivity: An Effective Approach To Enhance Exciton Separation
Efficiency in Organic Solar Cells
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Abstract
Photocurrent generation in organic
solar cells requires that excitons,
which are formed upon light absorption, dissociate into free carriers
at the interface of electron acceptor and donor materials. The high
exciton binding energy, arising from the low permittivity of organic
semiconductor films, generally causes low exciton separation efficiency
and subsequently low power conversion efficiency. We demonstrate here,
for the first time, that the exciton binding energy in B,O-chelated
azadipyrromethene (BO-ADPM) donor films is reduced by increasing the
film permittivity by blending the BO-ADPM donor with a high dielectric
constant small molecule, camphoric anhydride (CA). Various spectroscopic
techniques, including impedance spectroscopy, photon absorption and
emission spectroscopies, as well as X-ray spectroscopies, are applied
to characterize the thin film electronic and photophysical properties.
Planar heterojunction solar cells are fabricated with a BO-ADPM:CA
film as the electron donor and C<sub>60</sub> as the acceptor. With
an increase in the dielectric constant of the donor film from ∼4.5
to ∼11, the exciton binding energy is reduced and the internal
quantum efficiency of the photovoltaic cells improves across the entire
spectrum, with an ∼30% improvement in the BO-ADPM photoactive
region