Pristine conjugated polymers

The pristine conjugated polymers have been reported to contain electronic spins, presumably originating from inter-chain cross-linking in polyacetylene [25,26], formation of polynuclear structures in polypara-phenylene [27] and so on. The inter- and intra-chain reactions between these reactive sites can alter the chemical structure of conductive polymers even when they are pure, affecting their dopability and hence the electroactivity. 

Nanowhiskers with different thicknesses of PANI were obtained. One ofthe advantages of using these coated whiskers instead of pristine conjugated polymers is the inherent strong nature of cellulose allied with the conductive nature of PANI. 

The results of photoinduced absorption in polymer solutions and oligothiophene films and solutions (see Sections 3 and 4) show a profound effect of the surrounding medium on the character of the photo-excited species. Based on these results, we inferred that for pristine conjugated polymers embedded in a nonpolar medium, neutral photoexcitations are favored, whereas in polar media, charged and neutral photoexcitations co-exist. In conjugated polymer/C6o mixtures, embedded in a non-polar medium, energy transfer and triplet sensitization are favored, whereas in polar media, photoinduced electron transfer phenomena clearly dominate. 

An alternative explanation for the photoinduced absorption in the bulk polymer has been discussed by the Cambridge group [28]. It was shown that the amount of stimulated emission depends critically on the degree of photooxidation of the conjugated polymer. Figure 10-5 compares the stimulated emission of pristine PPV (see Fig. 10-5 a) and its heavily photooxidized counterpart (sec Fig. 10-5b). 

The aim of this chapter is to give a state-of-the-art report on the plastic solar cells based on conjugated polymers. Results from other organic solar cells like pristine fullerene cells [7, 8], dye-sensitized liquid electrolyte [9], or solid state polymer electrolyte cells [10], pure dye cells [11, 12], or small molecule cells [13], mostly based on heterojunctions between phthaocyanines and perylenes [14], will not be discussed. Extensive literature exists on the fabrication of solar cells based on small molecular dyes with donor-acceptor systems (see for example [2, 3] and references therein). 

The chapter is organized as follows the second section will discuss the photophysics of conjugated polymer/fullerene composites as a standard model for a charge-generating layer in plastic solar cells. Pristine polymer devices will be discussed in the third section while bilayer and interpenetrating network devices are presented in Sections 4 and 5. Section 6 contains some remarks on large area plastic solar cells and Section 7 conclusions. 

Heterogeneous mixing of fullerenes and fullerene derivatives with Ji-conjugated polymers has been used to produce excellent materials for photovoltaic devices [141], Upon irradiation of fullerene/polymer blends, charge transfer from the polymer to occurs, resulting in efficient photoconductivities. Better behavior of fullerene derivatives than with pristine Cgg has been observed, and attributed to the improved miscibility of the functionalized species. 

It is the purpose of this chapter to introduce photoinduced charge transfer phenomena in bulk heterojunction composites, i.e., blends of conjugated polymers and fullerenes. Phenomena found in other organic solar cells such as pristine fullerene cells [11,12], dye sensitised liquid electrolyte [13] or solid state polymer electrolyte cells [14], pure dye cells [15,16] or small molecule cells [17], mostly based on heterojunctions between phthalocyanines and perylenes [18] or other bilayer systems will not be discussed here, but in the corresponding chapters of this book. 

Several organics, e.g. pristine poly(3-octylthiophene), polyfluorene, bifunctional spiro compounds and polyphenyleneethynylene derivative, have been used for fabricating photOFETs. Responsivity as high as 0.5-1 A/W has been achieved in some of these transistors. We have already discussed the bulk heterojunction concept in Chapter 5. The bulk heterojunctions are fabricated using acceptor materials with high electron affinity (such as C<5o or soluble derivatives of C6o) mixed with conjugated polymers as electron donors. PhotOFETs based on conjugated polymer/fullerene blends are expected to show... 

Furthermore, the positions of the absorption maxima of the single components in the composite are not shifted compared to the pristine materials. There is no indication of states below the ti-ti- gap of the conjugated polymer that might arise from grormd state interactions such as doping. 

B. Francois (1997). Synth. Met., 84, 941-942. Dynamics of pristine and doped conjugated polymers A combined inelastic neutron scattering and computer simulation analysis. 

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