Distance
Distributions of Photogenerated Charge Pairs
in Organic Photovoltaic Cells
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Abstract
Strong
Coulomb interactions in organic photovoltaic cells dictate
that charges must separate over relatively long distances in order
to circumvent geminate recombination and produce photocurrent. In
this article, we measure the distance distributions of thermalized
charge pairs by accessing a regime at low temperature where charge
pairs are frozen out following the primary charge separation step
and recombine monomolecularly via tunneling. The exponential attenuation
of tunneling rate with distance provides a sensitive probe of the
distance distribution of primary charge pairs, reminiscent of electron
transfer studies in proteins. By fitting recombination dynamics to
distributions of recombination rates, we identified populations of
charge-transfer states and well-separated charge pairs. For the wide
range of materials we studied, the yield of separated charges in the
tunneling regime is strongly correlated with the yield of free charges
measured via their intensity-dependent bimolecular recombination dynamics
at room temperature. We therefore conclude that populations of free
charges are established via long-range charge separation within the
thermalization time scale, thus invoking early branching between free
and bound charges across an energetic barrier. Subject to assumed
values of the electron tunneling attenuation constant, we estimate
critical charge separation distances of ∼3–4 nm in all
materials. In some blends, large fullerene crystals can enhance charge
separation yields; however, the important role of the polymers is
also highlighted in blends that achieved significant charge separation
with minimal fullerene concentration. We expect that our approach
of isolating the intrinsic properties of primary charge pairs will
be of considerable value in guiding new material development and testing
the validity of proposed mechanisms for long-range charge separation