Theory of interfacial charge-transfer complex photophysics in $\pi$-conjugated polymer-fullerene blends

TL;DR

This paper develops a theoretical framework for understanding the electronic structure and photophysics of polymer-fullerene blends, revealing the role of exciplex states and explaining transient absorption phenomena without free polaron formation.

Contribution

It extends Mulliken's ground-state charge transfer theory to excited states, providing a unified explanation for photophysical behaviors in polymer-fullerene blends.

Findings

Exciplex states occur below the polymer's optical exciton.

Transient absorptions are explained by the theory for both above-gap and below-gap excitations.

No free polarons are generated within the experimental timescale.

Abstract

We present a theory of the electronic structure and photophysics of 1:1 blends of derivatives of polyparaphenylenevinylene and fullerenes. Within the same Coulomb-correlated Hamiltonian applied previously to interacting chains of single-component $\pi$-conjugated polymers, we find an exciplex state that occurs below the polymer's optical exciton. Weak absorption from the ground state occurs to the exciplex. We explain transient photoinduced absorptions in the blend, observed for both above-gap and below-gap photoexcitations, within our theory. Photoinduced absorptions for above-gap photoexcitation are from the optical exciton as well as the exciplex, while for below-gap photoexcitation induced absorptions are from the exciplex alone. In neither case are free polarons generated in the time scale of the experiment. Importantly, the photophysics of films of single-component $\pi$-conjugated polymers and blends can both be understood by extending Mulliken's theory of ground-state charge transfer to the case of excited-state charge transfer.