Derivatives of PPV

5-Disubstituted PPVs have also been explored by Martens et al. [284, 285], using x-ray and electron methods and iinstrctchcd films as well as oriented filnis. Both are obtained from spin-coated precursor films. Stretch alignment is carried out in vacuum during the initial phase of the conversion procedure above 80°C, after which conversion completes above 200°C. In the mote recent of these papers, the results for ethoxy substituents are also presented stretch-orienting has not been possible for this polymer [285], 

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F. Cacialli, R.H. Friend, W.J. Feast, and P.W. Lovenich, Poly(distyrylbenzene- WocA>sexi(ethylene oxide)), a highly luminescent processible derivative of PPV, Chem. Common., 1778-1779, 2001. 

L. Duan, Y. Qiu, and H. Wang, Blue electroluminescence from a processible derivative of PPV based copolymer with tri(ethylene oxide) segments in the backbone, Synth. Met., 137 1133-1135, 2003. 

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...

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. 

Figure 167 Time for the derivative of PPV (OCiCio)-based device, ITO/polyaniline(50nm)/OCiCi0(100nm)/Ca/Al, to drop to half-luminescence of its initial value plotted as a function of temperature. The circles indicate 3 cm2 single pixel devices driven in DC. Squares indicate data for pixellated displays driven in pulsed mode (1/16 duty cycle, 200 Hz, same average luminance as DC devices). Current density for DC driven devices is 8.3mA/cm2. After Ref. 587. Copyright 1999 American Institute of Physics, with permission.

Recent experiments [158] on polymer light emitting diodes using alkoxy derivatives of PPV as the semiconducting and luminescent material have demonstrated that the ratio of the EL quantum efficiency (ihel) to the PL quantum efficiency (t]pl) can be increased to t1el/ 9pl>0 5, well beyond the theoretical limit for singlet and triplet excitons as the low energy excited states. 

The devices used for the EL and PL measurements were fabricated in the thin film sandwich configuration anode/polymer/cathode. The electronic structure of alkoxy derivatives of PPV has been studied via internal field emission [42], internal photoemission [76], cyclovoltammetric spectroscopy [159]) and photoelectron spectroscopy [160]. The data indicate that the bottom of the Tr -band and the top of the ir-band are at 3 eV and 5 eV respectively, with respect to the vacuum. Thus, relatively good hole and electron injection can be achieved by using transparent indium/tin-oxide (ITO) as the anode and calcium (Ca) or barium (Ba) as the cathode [42,161]. In practice, however, hole injection is very sensitive to the quality of the ITO. A thin layer (= 300 A) of conducting polymer provides excellent, reproducible hole injecting contacts [70,71,162]. 

Subsequent work has developed several derivatives of PPV that are soluble in organic solvents and can be readily cast as thin films. These are produeed by substituting in the various positions Rj to R5 thus. 

The preparation of poly(2,5 dimethoxy phenylene vinylene)(DM-PPV) utilized the xanthate precursor route. Many similarities exist between the PPV xanthate thermal elimination and this PPV derivative. Previous studies had attempted to synthesis DM-PPV using the SPR method. The SPR resulted in gel formation during dialysis so that film casting was not possible. The XPR was successful in preparing precursor films that can be thermally converted to DM-PPV. The derivatives of PPV are useful to prepare a polymer with a shift in the absorbance and photoluminescence maximum. DM-PPV exhibits an 100 nm red shift as compared with PPV synthesized via the XPR. 

Cyano derivatives of PPV with high oxidation potential are commonly synthesized by Knoevenagel condensation of substituted terephthaldehyde with... 

The process used for the photovoltaic device can be reversed to produce a light emitting diode (LED). When an electric field is applied to the two electrodes shown in Figure 1.7, electrons are injected into the conduction band of the polymer layer from the cathode (usually a low-work-function metal such as aluminum or calcium). At the ITO anode, electrons are removed from the valence band of the polymer to produce vacancies, or holes. The free electrons and holes move in opposite directions under the influence of the electric field and, when they combine, a photon of light is emitted. The color of the light emitted depends on the band gap between the valence and conduction bands in the polymer. Appropriate derivation of PPV polymers has produced PLEDs that emit the three colors red, blue and green. 

We have first reported organic solar cells made of electrospun conducting nanofibers from a derivative of PPV (i.e., MEH-PPV) in 2008 [59]. MEH-PPV... 

In 2009, the fabrication of nanofibers from another conducting polymer (a derivative of PPV, that is, poly[2,5-(20-ethylhexyloxy)]-l,4-phenylenevinylene (BFH-PPV)) by electrospinning with PVP blend was reported by Liu et al. [60]. A composite of BFH-PPV/PCBM nanofibers was also successfully fabricated in that report. This study showed the feasibility of fabrication of donor-acceptor composite nanofibers by electrospinning. 

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