41 research outputs found
Theory of interfacial charge-transfer complex photophysics in -conjugated polymer-fullerene blends
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 -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 -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.Comment: 9 pages, 8 figure
Correlated-electron description of the photophysics of thin films of -conjugated polymers
We extend Mulliken's theory of ground state charge transfer in a
donor-acceptor complex to excited state charge transfer between pairs of
identical -conjugated oligomers, one of which is in the optically excited
state and the other in the ground state, leading to the formation of a
charge-transfer exciton. Within our theory, optical absorptions from the
charge-transfer exciton should include a low energy intermolecular
charge-transfer excitation, as well as distinct intramolecular excitations from
both the neutral delocalized exciton component and the Coulombically bound
polaron-pair component of the charge-transfer exciton. We report high order
configuration-interaction calculations for pairs of oligomers of
poly-paraphenylenevinylene (PPV) that go beyond our previous single
configuration-interaction calculation and find all five excited state
absorptions predicted using heuristic arguments based on the Mulliken concept.
Our calculated excited state absorption spectrum exhibits strong qualitative
agreement with the complete wavelength-dependent ultrafast photoinduced
absorption in films of PPV derivatives, suggesting that a significant fraction
of the photoinduced absorption here is from the charge-transfer exciton. We
make detailed comparisons to experiments, and a testable experimental
prediction
Symmetric Dimers with Time-Periodic Gain/Loss Function
symmetric dimers with a time-periodic gain/loss function in a
balanced configuration where the amount of gain equals that of loss are
investigated analytically and numerically. Two prototypical dimers in the
linear regime are investigated: a system of coupled classical oscillators, and
a Schr\"{o}dinger dimer representing the coupling of field amplitudes; each
system representing a wide class of physical models. Through a thorough
analysis of their stability behaviour, we find that turning on the coupling
parameter in the classical dimer system, leads initially to decreased stability
but then to re-entrant transitions from the exact to the broken phase and vice versa, as it is increased beyond a critical value. On the
other hand, the Schr\"{o}dinger dimer behaves more like a single oscillator
with time-periodic gain/loss. In addition, we are able to identify the
conditions under which the behaviour of the two dimer systems coincides and/or
reduces to that of a single oscillator.Comment: 9 pages, 9 figures, META14 Conference, subm. Special Issue Appl.
Phys.