Doped PPV

Using a variety of transient and CW spectroscopies spanning the time domains from ps to ms, we have identified the dominant intrachain photoexcitations in C )-doped PPV films. These are spin-correlated polaron pairs, which are formed within picoseconds following exciton diffusion and subsequent dissociation at photoinduced PPV+/Cw> defect centers. We found that the higher-energy PA band of polaron pairs is blue-shifted by about 0.4 eV compared to that of isolated polarons in PPV. 

One can conclude from this data that although polaron states are present, they are present to the extent of 1% or less. Thus it is not surprising that the dominant optical absorption is bipolaronic. Since Han and Elsenbaumer (26) do not report a quantitative spin concentration for their protonically doped PPV,... 

For PPV doped with AsFg anion, a new high BE Cls component appeared at about 286.5 eV, in addition to the two Cls components at 284.6 eV and 285.5 eV for pristine PPV [60], It has been associated with carbon atoms in a highly oxidized state resulting from doping. Thus, the Cls core-level spectrum of doped PPV readily indicates that the positive charge is non-uniformly distributed over the polymeric matrix. Furthermore, the satellite structures of the doped polymer are found to have a substantially enhanced intensity at about 290 eV, which is... 

As mentioned in the introduction, the electrical conductivity upon doping is one of the most important physical properties of conjugated polymers. The conductivity ranges from lOOOOOS/cm for iodine-doped polyacetylene [41], 1000 S/cm for doped and stretched polypyrrole [42], to 500 S/cm for doped PPP [43], 150 S/cm for hydrochloric acid doped and stretched polyaniline [44], and 100 S/cm for sulfuric acid doped PPV [45] to 50 S/cm for iodine-doped poly thiophene [46]. The above listed conductivities refer to the unsubstituted polymers other substitution patterns can lead to different film morphologies and thus to a different electrical conductivity for the same class of conjugated polymer in the doped state. 

Dopants also influence the emission processes from PPVs. Improved red dopants have been based on pyran dyes" while Qo doping appears to be variable" "". Doping with electron transport materials such as oxadiazoles give polymers with balanced properties for hole transport". The avoidance of low molecular weight material in the synthesis of cyano based PPVs is important" as are head to head and tail to tail chain sequences in thiophene based polymers. Head to tail tetramer sequences were the most fluorescent. Metal ion doped PPV s are claimed to be good chemosensors and broad emission is observed from titania doped PPV . Electron rich dopants enhance the emission in the red region while electro and photoinduced infrared bands from PPV are similar . ... 

Figure 1.38. Projected equatorial structure (perpendicular to the c-axis) for (a) undoped PPV (b) the hexagonal phase of Na-doped PPV and (c) PPV-Na at saturation doping. (Reproduced from ref. 274 with kind permission. Copyright (1990) American Physical Society.)...

The structure found for cesium-doped PPV is quite different from that induced by smaller alkali ions (Winokur et al. [279]). It can be described by a 2D projected cell which is square, with a = 6.81 A. Each column of Cs ions is surrounded by four polymer chains in four-fold symmetry, while each polymer chain is now situated between two Cs columns (Figure... 

Figure 1,40, Room-temperature (hkQ) scans of alkali-doped PPV Experimental data is designated by dotted lines and calculated profiles by solid lines. Right, sketches of the projected equatorial structures for the three-fold hexagonal phase (top) and the fourfold PPV-Cs tetragonal phase (bottom). (Reproduced from ref. 279 with kind permission. Copyright (1991) Elsevier Science S.A.)...

The retention of the original morphology and crystallite orientation upon doping has been demonstrated by TEM studies [272], for the case of H2SO4 doping. The size of the diffracting regions in doped PPV films appears to be 40 A, about 10 A less than in the pristine material. 

Figure 3.29. Cls core-level spectra of (a) undoped (b) 5% AsFs-doped and (c) 7% AsF5-doped PPV Components Ca and Cb arc associated with pristine PPV while component Ct is associated with the oxidized stale. (Reprinted with permission from ref. 139, Am Chem. Soc.)...

In FeCl3-doped PPV, we also showed that the observed modifications could be interpreted in a way similar to doped PPP, changes of force constants between carbon-carbon bonds being again consistent with a quinoid charactra- along the polynoeric chain. 

Raman spectra of doped polythiophene exhibit also modifications provided excitation wavelengths are taken in the red range. The main Ag mode is shifted to 1411 cm. An analysis of these spectra in the frame of our method, i.e. by modifying the main force constants associated to the bonds of the polymer backbone, leads also to a good fit of the modified Raman spectra. In contrary to doped PPP or doped PPV, it is not obvious whether the quinoid structure appears clearly. Instead, we do need to modify the C-S force constant, putting in evidence that the S atom plays a certain role in the electronic structure modification. Further details on the calculations will also be published elsewhere. 

Dye doping consists of the addition of a fluorescent dye to the polymer. The symmetry of electrons and holes is not maintained in doped PPV. In this way, the difference between electron and hole intra-chain mobility in PPV can be explained. Dopants and doping methods are listed in Table 3.4. 

Since PPV is insoluble, it is not possible to dope PPV with dyes utilizing the solution method. Vapor transportation is a dye doping method, where the dye molecules are introduced into the polymers via the gas phase in vacuo.Besides fabrication of photoelectric devices, the method of vapor transportation has been used in the fields of optical memories and waveguides. 

Photovoltaic devices based on DCM-doped PPV have been fabricated. DCM absorbs light at a longer wavelength than PPV. Glass plates with ITO are used as substrates. An interfacial layer of PEDOT doped with poly(styrene sulfonic acid) is used, and then the DCM-doped PPV is applied. Eventually, aluminum electrodes are deposited by vacuum evaporation. 

The Raman spectra of doped PPP and doped PPV are well interpreted, if modifications in some force constants are made. They concern, more specially, the intra-ring and inter-ring carbon-carbon bonds. The changes are consistent with a quinoid-like structure along the chain. 

The electrical conductivity sensitivity of H2SO4-doped PPV can be improved by the addition of Zeolite. These composites show positive responses toward NH4NO3 [160]. 

Thus, doped PPV with H2SO4 can be utilized as a gas sensor for NH4NO3 due to its positive electrical conductivity response. 

The first direct measure of multiple, resolved gap states in a doped conjugated polymer was reported by Fahlman et al., in the case of sodium doped PPV [67]. The UPS spectra indicated a slight decrease in the work function as the first monolayers (equivalent) of sodium atoms were deposited on the surface. XPS data showed that the sodium atoms diffuse uniformly throughout the thin film (a few tens of nm in thickness). At 40% doping, defined as the Na/monomer ratio, a large change in the work function of... 

There are no reports in the hterature concerning the temperature dependence of the susceptibility of doped PPV. However, if the reported room temperature magnetic susceptibility measured of BF4 doped poly(2,5-diethoxy-p-phenylene-vinylene) [C2H50-PPV(BF4)] [143] is entirely due to a Pauli contribution, an upper estimate of A C f) is 0.03 states/eV-C. 

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