Conjugated polymers films

The apphcation of a high electric field across a thin conjugated polymer film has shown the materials to be electroluminescent (216—218). Until recentiy the development of electroluminescent displays has been confined to the use of inorganic semiconductors and a limited number of small molecule dyes as the emitter materials. Expansion to the broad array of conjugated polymers available gives advantages in control of emission frequency (color) and facihty in device fabrication as a result of the ease of processibiUty of soluble polymers (see Chromogenic materials,electrochromic). 

The proposed scenario is mainly based on the molecular approach, which considers conjugated polymer films as an ensemble of short (molecular) segments. The main point in the model is that the nature of the electronic state is molecular, i.e. described by localized wavefunctions and discrete energy levels. In spite of the success of this model, in which disorder plays a fundamental role, the description of the basic intrachain properties remains unsatisfactory. The nature of the lowest excited state in m-LPPP is still elusive. Extrinsic dissociation mechanisms (such as charge transfer at accepting impurities) are not clearly distinguished from intrinsic ones, and the question of intrachain versus interchain charge separation is not yet answered. 

Milani M, Grave C, Ferri V, Samori P, Rampi MA (2007) Ultrathin -conjugated polymer films for simple fabrication of large-area molecular junctions. ChemPhysChem 8 515-518... 

The unique simplicity of TNT detection by this method renders the fabrication of an operable device built around amplified fluorescent polymer technology a relatively simple proposition in comparison to other technological platforms. The method relies purely on changes in fluorescence intensity. AH that is theoretically required is a means of introducing vapor samples to a conjugated polymer film, a light source to excite the polymer film, and a photodetector to measure the emission intensity as a function of time. 

This concept is in essence a chromatographic effect similar to that observed in gas chromatography (GC), with the conjugated polymer film acting as the stationary phase. It is possible that like in GC and other candidate technologies for explosive detection, these responses could be empirically standardized for expected analytes of interest and sensory devices caHbrated to deconvolute temporal quenching signals to determine which analytes are present. This would further enhance the selectivity of what is already a very selective sensor for TNT and related compounds. 

A word of caution the case of conjugated polymer films is generally identical to the case of condensed molecular solid films, as described above, but with occasional small differences. In general, the Fermi level is found to lie very near the centre of the energy gap, Eg. Small amounts of impurities, defects, or other charge donation (acceptor) species, however, can move the position of the Fermi... 

Tatsuura et al. employed the earlier mentioned E-CVD process to deposit polyazomethine conjugated polymer films with second order nonlinear optical properties. The monomers used were 4-methoxy-o-phenylenediamine (MPDA) ando-phtalaldehyde (o-PA). Ortho-type monomers were used to ensure a total dipole moment for the polymer in the direction perpendicular to the chain. Figure 22 shows the structure of the monomers and the resultant polymer. 

ELECTROCONDUCTIVE MATERIALS BASED ON -CONJUGATED POLYMER FILMS WITH METAL NANOPARTICLES... 

Martini IB, Smith AD, Schwartz BJ (2004) Exciton-exciton annihilation and the production of interchain species in conjugated polymer films Comparing the ultrafast stimulated emission and photoluminescence dynamics of MEH-PPV. Phys Rev B 69(3)... 

G. H. GeUnck, J.M. Warman, M. Remmers, D. Neher, Narrow-band emissions from conjugated-polymer films, Chem. Phys. Lett. 1997, 265, 320. 

Kim Y, Cook S, Tuladhar SM, Choulis SA, Nelson J, Durrant JR, Bradley DDC, Giles M, McCulloch 1, Ha C-S, Ree M (2006) A strong regioregularity effect in selforganizing conjugated polymer films and high-efficiency polythiophene fullerene solar cells. Nat Mater 5 197... 

Comparing with conventional amorphous organic semiconductors [16], it is characteristic that conjugated polymer films consist of crystal-like domains, in which jt-conjugated chains are stacked closely, and an amorphous domain, in which polymer chains are disordered [115]. In crystal-like domains, band-like carrier transport that is similar to that of molecular crystals should be possible. However, hopping transport influenced by structural disorder is the dominant process in the amorphous domains. The total carrier transport process is dominated mainly by that in amorphous domains, resulting in bulk carrier transport characteristics similar to those of amorphous organic semiconductors. 

The energy barrier for hole injection at the metal-poymer interface is determined by the vacuum work function of the metal contact and the ionization potential Ip of the polymer. For conjugated polymer films spin-coated onto hole injecting metal electrodes, it has been reported that as long as is smaller than a critical value characteristic of the polymer, no interface dipole is formed [104]. In this case, the barrier for hole injection can be estimated simply by aligning the vacuum levels of the metal and the polymer (Mott-Schottky limit) the measured work function of the metal with the polymer deposited on top increases linearly with with a slope of one (see Figure 2.3.11). 

Optically Detected Magnetic Resonance (ODMR) Studies of Ti-conjugated Polymer Films, Solutions, Blends, and Devices... 

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