Polymer heterojunctions

X. Zhang and S.A. Jenekhe, Electroluminescence of multicomponent conjugated polymers. 1. Roles of polymer/polymer interfaces in emission enhancement and voltage-tunable multicolor emission in semiconducting polymer/polymer heterojunction, Macromolecules, 33 2069-2082, 2000. 

Fig. 2 Molecular structure of a model F8BT(top) TFB(bottom) polymer heterojunction in the eclipsed stacking configuration, see also Fig. 3. In the actual polymer, the residues are R = C8Hi2 in the calculations reported here, R = H was used (Adapted from Ref. [43]).

In the examples addressed below, for a two-band phonon distribution, the HEP scheme willl be shown to yield essential insight into the interplay between high-frequency (C=C stretch) and low-frequency (ring-torsional) modes of the polymer heterojunction model of Eq. (7). 

Quantum dynamics of exciton dissociation at a polymer heterojunction... 

The dynamics associated with the Hamiltonian Eq. (8) or its variants Eq. (11) and Eq. (14) can be treated at different levels, ranging from the explicit quantum dynamics to non-Markovian master equations and kinetic equations. In the present context, we will focus on the first aspect - an explicit quantum dynamical treatment - which is especially suited for the earliest, ultrafast events at the polymer heterojunction. Here, the coherent vibronic coupling dynamics dominates over thermally activated events. On longer time scales, the latter aspect becomes important, and kinetic approaches could be more appropriate. 

Morteani et al. demonstrated that after photoexcitation and subsequent dissociation of an exciton at the polymer-polymer heterojunction, an intermediate bound geminate polaron pair is formed across the interface [56,57]. These geminate pairs may either dissociate into free charge carriers or collapse into an exciplex state, and either contribute to red-shifted photoliuni-nescence or may be endothermically back-transferred to form a bulk exciton again [57]. In photovoltaic operation the first route is desired, whereas the second route is an imwanted loss channel. Figure 54 displays the potential energy ciu ves for the different states. 

Fig. 54 Potential energy diagram describing the energetics and kinetics at type II polymer heterojunctions. The energetic order of A D")r = oo and A D)r = oo may be reversed for PFB F8BT vs TFB F8BT. The inset shows the band offsets at a type II heterojunction. (Reprinted with permission from [57], 2004, American Physical Society)...

Jenekhe SA, Yi S (2000) Efficient photovoltaic cells from semiconducting polymer heterojunctions. Appl Phys Lett 77 2635... 

Romero DB, Garrard M, De Heer W, Zuppiroli L (1996) A carbon nanotube/organic semiconducting polymer heterojunction. Adv Mater 8 899... 

Electronic Processes at Semiconductor Polymer Heterojunctions 2.1.4.2 The PFB F8BT Exciplex... 

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