Semiconducting polymers, exciton

More General Treatments of Electron Correlation in Polymers.—The introduction of excitonic states was just a simple example to show how one can go beyond the HF approximation to obtain correlated electron-hole pairs, whose energy level(s) may fall into the forbidden gaps in HF theory, and form the basis for interpretation of optical phenomena in semiconducting polymers. The schemes described until now for investigation of certain types of correlation effects (the DODS method for ground-state properties and the exciton-picture for excited states) are relatively simple from both the conceptual and computational points of view and they have been actually used at the ab initio level. It is evident, on the other hand, that further efforts are needed in polymer electronic structure calculations if we want to reach the level of sophistication in correlation studies on polymers which is quite general nowadays in molecular quantum mechanics. 

The band description ignores correlations between electrons that result from the electron-electron repulsive interaction. Alternatively stated, the band description ignores the attractive Coulomb interaction between electrons in the TT -band and holes in the ir-band. This attraction causes the formation of excitons i.e. neutral electron-hole bound states. One of the fundamental unresolved issues of the physics of semiconducting polymers is the magnitude of the exciton binding energy (see Section V-B). 

Poly(phenylene vinylene), PPV, and its soluble derivatives have emerged as the prototypical luminescent semiconducting polymers. Since PPV has a nondegenerate ground state, structural relaxation in the excited state leads to the formation of polarons, bipolarons, and neutral excitons. However, prior to treating the structural relaxation in the excited state, one needs to develop a satisfactory description of the electronic excited states. 

Concentration quenching is not a major problem in semiconducting polymers. Typically, the quantum efficiency for photoluminescence from thin films is comparable to that from dilute solutions. The absence of strong concentration quenching is the result of the spatial delocalization of the excited states. Because the weakly bound excitons are spread over many repeat units, the Davidov splitting that results from interchain interactions is small. As a result, the disorder that is present in films cast from solution is sufficiently large to mix the dark and emissive Davidov-split states. Thus, quantum efficiencies in excess of 60-70% can be obtained from thin solid films of luminescent semiconducting polymers. 

By making such controlled nanoscale morphology between the donor and the acceptor, and having the donor-acceptor interface vertically oriented to the cathode and the anode, the excitons can be fully dissociated to electrons and holes, and can be efficiently transported to the electrodes before recombination, maximizing both ed and j/cc- One of the major challenges to realize this type of structure is the difficulty to access those periodic pillar and hole structures of tens of nanometer pitch by using the semiconducting polymer and fullerene derivate as a donor and acceptor pair. 

The first reports on photovoltaic effects recorded for organic devices were based on single-layer devices discussed above, which were simply a semiconducting polymer sandwiched between two electrodes with different work functions (exciton dissociation takes place at the interface between the active layer and electrode) (e.g. [65-67]). Although, the devices could show fairly high Vqc of up to 1.7 V, the /sc values were very low, which resulted in very poor PCEs. A significant improvement was achieved by bringing... 

The origin of the EL emission is provided by the formation of excitons, or electron-hole pairs, which are formed at defect sites, i.e. donor/acceptor sites, in the material and at the interface of a heterojunction of two semiconducting polymer layers. Little is known at this time about the mobility of the electrons, holes, and excitons at the interface of heterojunctions. Limited exciton diffusion lengths in the materials and the interfacial nature of the photogeneration process could explain these exotic transport properties in terms of topological constraints. 

The stabilization of the charge separated state in these composites is assumed to result from the stability and delocalization of the radical cations (positive polarons) on the semiconducting polymer backbone and from structural relaxation of the fiillerene following the photoexcitation. Thus, the possible sensitivity of the photoinduced electron transfer process (from D to A) to the excited state dynamics of the semiconducting polymer (as D) is an important issue. Does electron transfer occur even faster than soliton, or exciton formation We analysed this question by comparative... 

Recent experiments [158] on polymer light emitting diodes using alkoxy derivatives of PPV as the semiconducting and luminescent material have demonstrated that the ratio of the EL quantum efficiency (ihel) to the PL quantum efficiency (t]pl) can be increased to t1el/ 9pl>0 5, well beyond the theoretical limit for singlet and triplet excitons as the low energy excited states. 

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