How the Size and Density of Charge-Transfer Excitons Depend on Heterojunction’s Architecture

We
have characterized the size, intensity, density, and distribution
of charge-transfer (CT) excitons as a function of the acceptor–donor
architecture of prototypical organic interfaces. This characterization
was done by computational analysis of 17 models of varying numbers,
positions, and orientations of the donor and acceptor molecules. The
models’ building blocks were phenyl-C61-butyric
acid methyl ester (PCBM) fullerene acceptors and dual-band donor polymers
composed of thiophene, benzothiadiazole, and benzotriazole subunits.
The electronic structure of the donor–acceptor complexes was
computed with the time-dependent long-range-corrected density-functional
tight-binding method and analyzed with the fragment-based one-electron
transition density matrix. In all models, the complexes with edge-on
orientation have denser spectra of low-energy CT states lying below
the absorption bands compared to the complexes with face-on orientation.
This CT-state distribution in edge-on complexes provides a gate to
efficiently populate cold CT excitons. Moreover, the cold CT excitons
have a higher degree of charge separation in the edge-on than in the
face-on complexes. The CT amount and the CT exciton size generally
increase with the energy of the CT states, although the electron remains
localized on a single molecule in cold CT states. Delocalization over
two PCBM molecules was observed for high-energy CT states. The exciton
size also depends on the orientation. Larger excitons are produced
by the delocalization of the electrons perpendicularly to the interface.
When the delocalization is parallel, the smaller electron–hole
distances yield moderately sized CT excitons.