Excitons in conjugated polymers

The study of excitons in conjugated polymers has often been inspired by the treatment of excitons in bulk three-dimensional semiconductors (as described in Knox (1963)). A particle-hole excitation from the valence band to the conduction band in a semiconductor leaves a positively charged hole in the valence band and a negatively charged electron in the conduction band. The Coulomb attraction between these particles results in bound states, or excitons. In three-dimensional semiconductors the excitons are usually weakly bound, with large particle-hole separations, and are well described by a hydrogenic model. Excitons in this limit are known as Mott- Wannier excitons. 

For simplicity, however, we prefer to denote all excitons formed from bound states of conduction band electrons and valence band holes as Mott-Wannier excitons, recognizing that this term includes both small and large radius excitons. We call this limit the weak-coupling limit, as the starting point in the construction of the exciton basis is the noninteracting band limit. As we will see, a real space description of a Mott-Wannier exciton is of a hole in a valence band Wannier orbital bound to an electron in a conduction band Wannier orbital. 

An opposite, strong-coupling limit has also been used to describe excitons in conjugated polymers (Gallagher and Mazumdar 1997 Gebhard et al. 1997 Essler et al. 2001 Harford 2002). As described in the previous chapter, in this limit a correlation gap separates the electron removal spectral weight (the lower Hubbard band) from the electron addition spectral weight (the upper Hubbard band). Now the bound particle-hole excitations are Mott-Huhhard excitons. That is, a particle excited from the lower Hubbard band to the upper Hubbard band 

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R. Kersting, et al., Ultrafast field-induced dissociation of excitons in conjugated polymers, Phys. Rev. Lett. 73 (1994) 1440-1443 ... 

Kersting R, Lemmer U, Deussen M, Bakker HJ, Mahrt RF, Kurz H, Arkhipov VI, Bassler H, Gobel EO (1994) Ultrafast Field-Induced Dissociation of Excitons in Conjugated Polymers. Phys Rev Lett 73(10) 1440—1443... 

From the obtained results it is known that the deposited composites film showed a higher photoactivity when compared to the single components due to the availability of numerous interfaces for enhanced charge transfer at the hetero-junction. Effective transport of excitons in conjugated polymers is extremely important for performances of organic light emitting... 

To understand the kinetics of triplet excitons in conjugated polymers it is of advantage to recall the scenario of processes involving triplets in molecular crystals such as anthracene. If triplets are generated via ISC from the optical excited singlet state they can decay in a monomolecular or bimolecular fashion. The rate equation for the triplet concentration [7] can be expressed as [47] ... 

Based on these direct experimental measurements, we conclude that the binding energy of the singlet polaron-excitons in conjugated polymers might be relatively small... 

Excitons in conjugated polymers can move by transfer from their current location (the donor site) to an acceptor location on the same polymer chain or on another nearby polymer backbone. This motion of energy plays important roles in the photophysics of the material, and is critical to the function of optoelectronic devices such as solar cells. Singlet motion is usually considered in terms of Forster... 

Schweitzer B, Bassler H (2000) Excitons in conjugated polymers. Synth Met 109 1-6... 

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