PPV polymer films

The following experimental results were obtained from two groups of carefully prepared samples, PPV polymer films and molecular Alqs vapour-deposited films. They were chosen from the large number of molecular and polymeric films which have been studied, because the fundamental empirical results in these two groups of samples can be especially clearly presented. 

Figure 8.56 shows the hole current-voltage characteristic of a PPV polymer film (PPV = poly-(para-phenylene-vinylene), see Fig. 8.55) for three different film thicknesses at room temperature as a log-log plot. The hole current was attained through a suitable choice of the work functions of the electrode indium-tin oxide (ITO) injects holes, Au injects no electrons. All three characteristics are parallel. The current... 

In 1994, a polymer-based transistor [237] constructed in a geometry closely related to that of a vacuum tube triode was demonstrated [237]. The structure consists of a thin MEH-PPV polymer film sandwiched between two electrodes, with a third electrode (PANI network) [209, 238-241] embedded within the MEH-PPV film (Fig. 13). This third electrode plays a role similar to that of the grid in a vacuum tube, controlling the current between the two outermost electrodes. Using a generalized analysis based on field-assisted carrier injection by tunneling, the carrier injection and transport in the polymer grid triode could be modeled [242]. 

By 1988, a number of devices such as a MOSFET transistor had been developed by the use of poly(acetylene) (Burroughes et al. 1988), but further advances in the following decade led to field-effect transistors and, most notably, to the exploitation of electroluminescence in polymer devices, mentioned in Friend s 1994 survey but much more fully described in a later, particularly clear paper (Friend et al. 1999). The polymeric light-emitting diodes (LEDs) described here consist in essence of a polymer film between two electrodes, one of them transparent, with careful control of the interfaces between polymer and electrodes (which are coated with appropriate films). PPV is the polymer of choice. 

Since multiple electrical and optical functionality must be combined in the fabrication of an OLED, many workers have turned to the techniques of molecular self-assembly in order to optimize the microstructure of the materials used. In turn, such approaches necessitate the incorporation of additional chemical functionality into the molecules. For example, the successive dipping of a substrate into solutions of polyanion and polycation leads to the deposition of poly-ionic bilayers [59, 60]. Since the precursor form of PPV is cationic, this is a very appealing way to tailor its properties. Anionic polymers that have been studied include sulfonatcd polystyrene [59] and sulfonatcd polyanilinc 159, 60]. Thermal conversion of the precursor PPV then results in an electroluminescent blended polymer film. 

The rest of this chapter will focus primarily on results using the soluble PPV-based polymer MEH-PPV. The results obtained for MEH-PPV are typical of the conjugated polymers used in LEDs. The models and results presented are generally applicable. They describe the operation of a wide range of polymer LEDs if the appropriate polymer film properties arc used. 

For PPV-imine and PPV-ether the oxidation potential, measured by cyclic voltammetry using Ag/AgCl as a reference are ,M.=0.8 eV and 0.92 eV, respectively. By adopting the values 4.6 eV and 4.8 eV for the work functions of a Ag/AgCl and an 1TO electrode, respectively, one arrives at zero field injection barriers of 0.4 and 0.55 eV. These values represent lower bounds because cyclic voltammetry is carried out in polar solvents in which the stabilization cncigy of radical ions exceeds that in a polymer film, where only electronic polarization takes place. E x values for LPPP and PPPV are not available but in theory they should exceed those of PPV-imine and PPV-ether. 

LAJs incorporating polymer films on top of the organic SAMs showed that these junctions allow for studying the correlation between electrical properties and chemical structure. Both de Boer et al. [80] (Fig. 7) and Rampi et al. [79], by spin-coating respectively PEDOT PSS and PPV on top of alkane SAMs, extracted from the electrical measurement [i = 0.57 A-1 and [j = 0.90 A-1. Rampi et al. also showed that it is possible to measure electrical properties of polyphenyl chains with a [1 0.61 A, and that the PPV polymer layer is much more conductive that... 

Using the b-1 route, PPV LB films were successfully fabricated [35], The precursor polymer of PPV was prepared according to ref.27. Anionic amphiphile 2C12SUC was purchased from Sogo Pharmaceutical Co. and used without further purification. An aqueous solution of the precursor polymer was added dropwise to an aqueous 2C12SUC solution. Then, the precursor-anionic amphiphile polyion complex was precipitated. The precipitates were filtered and washed with deionized... 

The drawback of the CVD method is eliminated in ROMP, which is based on a catalytic (e.g., molybdenum carbene catalyst) reaction, occurring in rather mild conditions (Scheme 2.3). A living ROMP reaction ofp-cyclophanc 3 or bicyclooctadiene 5 results in soluble precursors of PPV, polymers 4 [31] and 6 [32], respectively, with rather low polydispersity. In spite of all cis (for 4) and cis and trans (for 6) configuration, these polymers can be converted into aW-trans PPV by moderate heating under acid-base catalysis. However, the film-forming properties of ROMP precursors are usually rather poor, resulting in poor uniformity of the PPV films. 

The basic poly(phenylene vinylene) (PPV) polymer is commonly prepared by the sufonium prepolymer route developed by WessUng and Zimmerman in 1968 but much modified by subsequent workers. The synthesis starts from 1,4-bis(chloromethyl)benzene, via the bis-sulfonium salt formed by reaction with tetrahydrothiophene, and then polymerisation is effected to give the prepolymer by reaction with lithium hydroxide (Figure 3.39). Because of the inherent insolubility of PPV it is this prepolymer that is used to form the film coating on the substrate, for example by using a doctor blade technique. The prepolymer is converted into PPV on the substrate by heating in an oven under vacuum at 200 °C for 8-10 h. 

Fig. 13 Experimental (symbols) and theoretical (lines) data for the current-density as a function of applied voltage for a polymer film of a derivative of PPV under the condition of space-charge-limited current flow. Full curves are the solution of a transport equation that includes DOS filling (see text), dashed lines show the prediction of Child s law for space-charge-limited current flow assuming a constant charge carrier mobility. From [96] with permission. Copyright (2005) by the American Institute of Physics...

The materials (metals and conjugated polymers) that are used in LED applications were introduced in the previous chapter. The polymer is a semiconductor with a band gap of 2-3 eV. The most commonly used polymers in LEDs today are derivatives of poly(p-phenylene-vinylene) (PPV), poly(p-phenylene) (PPP), and polythiophene (PT). These polymers are soluble and therefore relatively easy to process. The most common LED device layout is a three layer component consisting of a metallic contact, typically indium tin oxide (ITO), on a glass substrate, a polymer film r- 1000 A thick), and an evaporated metal contact4. Electric contact to an external voltage supply is made with the two metallic layers on either side of the polymer. 

Nonconjugated poly[l,3-propanedioxy-l,4-phenylene-l,2-ethenylene(2,5-bis(tri-methylsilyl)-l,4-phenylene)- l,2-ethenylene-l,4-phenylene] (DSiPV) and conjugated MEH-PPV polymers are mixed by changing their weight ratios in 1,2-dichloroethane. The films of the blend polymers could be spin-cast from 1,2-dichloroethane solution with excellent reproducibility. AFM (atomic force microscopy) and SEM images show no indication of phase separation or layer formation due to the immiscibility of two polymers. The structures of DSiPV and MEH-PPV are shown in Fig. 24. 

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