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

We present a theory of the electronic structure and photophysics of 1:1 blends of derivatives of polyparaphenylenevinylene and fullerenes [1]. Within the same Coulomb-correlated Hamiltonian applied previously to interacting chains of single-component $\pi$-conjugated polymers [2], 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 [3], 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. [1] K.~Aryanpour, D.~Psiachos, and S.~Mazumdar, arXiv:0908.0366 [2] D.~Psiachos and S.~Mazumdar, Phys. Rev. B. {\bf 79} 155106 (2009) [3] T.~Drori {\it et al}., Phys. Rev. Lett. {\bf 101}, 037402 (2008)