PPV Polymers and Solvent Effect

The effects of silicon substitution on the luminescence properties are of interest in the field of polymer LEDs. Poly(2-dimethyloctylsilyl-l,4-phenylenevi-nylene) (DMOS-PPV) is completely soluble in common organic solvents. Hwang et al. reported synthesis and LED properties of DMOS-PPV [85]. Scheme 12 shows the synthetic route to DMOS-PPV by dehydrohalogenation. 

FET mobility measurements thus constitute a sound method for investigating changes in the mobility of an organic semiconductor due to morphology variations. On the basis of the FET characteristics of MDMO-PPV films spin-cast from different solvents, we will discuss the influence of interchain polymer aggregates on the hole field-effect mobility and further consequences for the short-circuiting of solar cells. 

A spectral red-shift similar to that shown in Fig. 6.6 can also be demonstrated by varying the spin speed if the polymer concentration is within the CL A region. For example, a 0.7-wt% MEH-PPV solution in CHO (refer to Fig. 6.3) spun at 2000 rpm resulted in a film with an absorption peak at A,max = 499 nm (film thickness = 700A), whereas the same solution spun at 8000 rpm resulted in a film with A.max = 509 nm (film thickness = 300 X) (Fig. 6.7). At higher (> 1 wt%) or lower (< 0.4 wt%) concentrations, however, the ultraviolet-visible (UV-vis) spectra were not observed to shift with spin speed. These effects are also reproducible in other solvents such as THF, chloroform, p-xylene, and so forth, although the exact CLA regions for the individual solvents are slightly different. 

Solvent porogen effects for macroporous resins are often explained in terms of the degree of solvation imparted to the incipient polymer netwoik, the point at which phase separation takes place, and the resultant degree of in filling between primary particles [26]. This may play a role in some amorphous MOPs (for example, micro/ mesoporous PPV [13]) however other systems such as HCPs (Sect. 2.1) do not undergo phase separation in this way [21, 22]. This basic mechanistic difference also accounts for the apparent independence of surface area on monomer concentration for conjugated microporous PAE networks [ 19], for example, in comparison with macro-porous polymer resins where surface area may be strongly concentration dependent. 

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