Dopant molecule

The picture presented above for confinement of the excitons within the device is for the EM layer sandwiched between the HTL and ETL. The EM need not be a discrete layer in the OLED, however, for exciton confinement to occur. Alternatively, the EM can consist of a luminescent molecule doped (- 1%) into a polymeric or molecular host material (40,41,54,55). So long as the energy gap (or band gap) of the host is higher than that of the EM dopant, excitons will be effectively trapped or confined on the dopant molecules leading to improved EL efficiency. An example of such a dopant-based device... 

At very high dopant concentrations, transport occurs direcdy between the dopant molecules. The polymer acts only as a binder in most cases. Taking TPD-doped PVK as an example, at low TPD concentrations the hole mobihty first decreases from 3 x 10 cm /Vs to 10 cm /Vs with increasing TPD concentration, because TPD molecules act as hole traps (48,49). At higher TPD concentrations, new direct transport channels between the TPD molecules open up and the hole mobihty increases to lO " cm /Vs for ca 60% TPD doping (Table 1, entries 9—11) (48,49). In this case, there is no evidence for unusual interaction between TPD and PVK that affects the hole transport process. 

The hydrolytic polycondensation of silicon alkoxides of general formula Si(OR)4 or R/ Si(OR)4 , where the non-reactive organofunc-tional group R acts as a network modifier, is carried out in the presence of dopant molecules resulting in the formation of highly porous, reactive organosilicates whose applications span many traditional domains of chemistry. 

These and several other researchers extended the methodology with the aim to widen functionality, using dopant molecules and silicon precursors derivatized with organic moities giving place to a vast class of hybrid organic-inorganic organosilica nanocomposites capable to meet numerous, advanced requirements in fields as diverse as catalysis,... 

Indeed, the mobility of the entrapped dopant is crucial in promoting the reactivity of the final materials. Thus, provided that the dopant molecules are at the surface and enjoy enough freedom, high porosity will certainly promote reactivity by limiting intraparticle diffusion but that will not be the case if microporous xerogels of different HLB are compared (c/. entrapped lipase and tetra-//-propy 1 am monium perruthe-nate (TPAP) where ORMOSIL with the smaller pores are more reactive). 

Figure 3. The harmonic oscillator in the idealized picture is one of the vibrational modes of a dopant molecule in an actual junction. Each vibrational mode is revealed as a peak in d2V/dI2 at a voltage of V = hv/e. The tunneling spectrum can be compared to infrared and Raman spectra 0.1 V corresponds to 806.5 cm"1. Reproduced with permission from Catal Rev. 23. 553 (1981)(Marcel Dekker, Inc.).

Phenylquinoxalines (30) turned out to be both fluorescent and phosphorescent emitters [167, 168]. By using these molecules as host, singlet excitons as well as triplet excitons can be transfered by long-range interactions to phosphorescent dopant molecules such as 66. 

L. Kador, D. E. Home, and W. E. Moemer, Optical detection and probing of single dopant molecules of pentacene in a p-terphenyl host crystal by means of absorption spectroscopy, J. Phys. Chem. 94, 1237-1248 (1990). 

Through reduction or oxidation of the molecule by a dopant molecule. Atoms or molecules with high electron affinity, such as iodine, antimony pentafluoride (SbCls), or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), may oxidize a typical organic semiconductor such as poly(p-phenylene) derivatives, leaving them positively charged. Reduction, i.e., addition of an electron, may be obtained by doping with alkali metals. 

Attempts to dope organic semiconductors have been made very early in the field, motivated by the prospect of possibly reaching metallic conductivities [108, 109]. These synthetic metals, however, have not been realized. While p-type doping could be obtained, for example, with iodine gases for poly-p-phenylene vinylene (PPV) derivatives, and n-type doping was demonstrated with sodium for a cyano-derivative of PPV, the doping levels obtained were not stable with time. The dopant molecules readily diffused into the organic semiconductor, yet also out of it. Due to the lack of stability, these approaches were not suitable for commercial applications. 

Photoconductivity of polymers will be reviewed within the framework of semiconductor physics. The focus of attention will be the photogeneration and transport of the charge carriers, the relation between chemical structure and photoelectrical properties, and sensitization processes involving dyes and dopant molecules. 

Carrier photogeneration processes versus polymer structure and various types of the dopant molecules were investigated by many authors [50-53],... 

The dependence of the drift mobility p on the electric field is represented by formula p (p-E1/2/kTcf) which corresponds to the Pool-Frenkel effect. The good correspondence between experimental and theoretical quantity for Pool-Frenkel coefficient 3 was obtained. But in spite of this the interpretation of the drift mobility in the frame of the Coulombic traps may be wrong. The origin of the equal density of the positive and negative traps is not clear. The relative contribution of the intrinsic traps defined by the sample morphology is also not clear [17,18]. This is very important in the case of dispersive transport. A detailed analysis of the polymer polarity morphology and nature of the dopant molecules on mobility was made by many authors [55-58]. 

The photoconductivity and absorption spectra of PVC with pynacyanol as a sensitizer are given in Fig. 11 [59]. It can be seen that a photo response appears in the range of the absorption maximum of the monomolecular form of the dye. The results are typical for all sensitization data [60-62]. The addition of the dopant molecules leads to the change of photogeneration, transfer and recombination efficiency. The effectiveness of the sensitization increases with lowering of the first excited state of the dye molecule. 

Photovoltaic and photoconductive phenomena for various types of CT complexes between saturated polymers and dopant molecules, heterojunctions between polymers and organic and inorganic photoconductors were also investigated in the last few years [86-92]. The quantum efficiency of the energy conversion of 10-3% was obtained for such systems and output power density of 3 x 102 mV cm-2. The mobilities of the heterogeneous polymer systems with despersed inorganic photoconductors reach the value — 10 3-10-4 m2 V 1s 1. 

The soliton conductivity model for rrans-(CH) was put forward by Kivelson [115]. It was shown that at low temperature phonon assisted electron hopping between soliton-bound states may be the dominant conduction process in a lightly doped one - dimensional Peierls system such as polyacetylene. The presence of disorder, as represented by a spatially random distribution of charged dopant molecules causes the hopping conduction pathway to be essentially three dimensional. At the photoexitation stage, mainly neutral solitons have to be formed. These solitons maintain the soliton bands. The transport processes have to be hopping ones with a highly expressed dispersive... 

The doping procedure is simple and straightforward the host molecule is added to the polymerizing mixture. When the polycondensation is completed, the dopant molecules are entangled in the inorganic polymeric network (Figure 1). 

Fig. 4 Selective reflection wavelength (optical pitch) as a function of temperature for three samples host material showing SmC and SmCA without dopant, with 1% and 3% dopant The chemical structures of the rod-shaped host and the bent-shaped dopant molecules are also shown. As shown in inset, inverse pitch increases almost linearly with dopant content [4]...

Perhaps the simplest optically controlled switches are single molecules embedded in a solid host matrix. These systems consist of an amorphous, polycrystalline, or crystalline film doped with dilute concentrations of impurity molecules. The most commonly used dopant molecules are fused polycyclic aromatic hydrocarbons and porphyrins. In addition to facile sample preparation, these planar molecules absorb in the visible to near IR regions of the spectrum, possess large extinction coefficients in both the ground and excited states, and have high fluorescence quantum yields. 

For the 6 mol % doped device the number of dopant molecules inside the doped volume can be calculated with the hypothesis of homogeneous distribution of the molecules. With 12 ts of the averaged lifetime of the excited state of complex 53 and the additional assumption of homogeneous distribution of excited molecules, the upper level of the effective distance between excited molecules as a function of the current density can be estimated. Thereby, the charge density distribution was admitted to half of the doped zone, which is a reasonable assumption as demonstrated by model calculations [ 127]. This effective distance is directly related to the current-dependent quantum efficiency r)( ) of the device. 

In the presence of a dopant in the positive ion mode, the first step of the ionization process is the production of a radical ion from the dopant molecule by direct photoionization ... 

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