EHO-OPPE

FIGURE 5.4 Chemical structures of photo- and electroluminescent polymers employed for polarized LEDs poly(2-methoxy-5-(2 -ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) poly[2,5-dioctyloxy-l, 4-diethynyl-phenylene-a/t-2,5,-bis(2 -ethylhexyloxy)-l,4-phenylene] (EHO-OPPE) poly(p-phenylene), PPP poly(3-(4-octylphenyl)-2,2 -bithiophene), PTOPT poly(p-phenylene vinylene), PPV poly(3-alkylthio-phene vinylene), P3AT Acetoxy-PPY PPV-polyester, poly(9,9-dialkyl fluorene), PF. 

FIGURE 5.5 Polarized PL from a gel-processed, uniaxially drawn film of EHO-OPPE (cf. Figure 5.4) in UHMW-PE. Twisted tapes (drawn to a draw ratio A = 80) are shown under excitation with UV light (365 nm) and the pictures were taken through a linear polarizer with its polarization axis oriented horizontally (a) and vertically (b). (After Weder, C., Sarwa, C., Bastiaansen, C., and Smith, P., Adv. 

EHO-OPPE Poly[2,5-dioctyloxy-l,4-diethynyl-phenylene-fl/t-2,5-bis(2 -ethylhexyloxy)-1,4-phenylene]... 

Fig. 5 Chemical structure of poly[2,5-dioctyloxy-l,4-diethynylphenylene-fl/t-2,5-bis(2 -ethylhexyloxy)-l,4-phenylene] (EHO-OPPE), the alkyloxy-PPE employed in the charge-transport studies by Weder and Singer et al. [61,62]...

Fig. 6 Electron time-of-flight photocurrent transients of solution-cast film of EHO-OPPE (L=8 pm), measured at 295 K and an electric field of 2.5-10 V cm in (a) linear and (b) double logarithmic plots. Reproduced with permission from [61]...

Typical photocurrent transients are shown in Fig. 6 for electrons and in Fig. 7 for holes. The shape of these curves is representative for all transients observed in the study and is characteristic of dispersive transport [64-68]. The carrier mobility p was determined from the inflection point in the double logarithmic plots (cf. Fig. 6b and Fig. 7b) [74]. TOF measurements were performed as a function of carrier type, applied field, and film thickness (Fig. 8). As can be seen from Fig. 8, the drift mobility is independent of L, demonstrating that the photocurrents are not range-limited but indeed reflect the drift of the carrier sheet across the entire sample. Both the independence of the mobility from L, and the fact that the slopes of the tangents used to determine the mobility (Fig. 6 and Fig. 7) do not add to -2 as predicted by the Scher-Montroll theory, indicate that the Scher-Montroll picture of dispersive transients does not adequately describe the transport in amorphous EHO-OPPE [69]. The dispersive nature of the transient is due to the high degree of disorder in the sample and its impact on car-... 

Fig. 8 Electron (a) and hole (b) mobilities of EHO-OPPE films as a function of electric field at various film thicknesses open triangles L=6.5 pm, filled circles L=8 ]im, open squares L=12 p.m). Reproduced with permission from [61]...

Fig. 9 Temperature dependence of (a) electron and (b) hole mobilities of an EHO-OPPE film (1=8 pm) measured at =2.0-10 (squares), 3.0-10 (circles), and 4.0-10 Vcm" (triangles). Reproduced with permission from [61]...

To analyze the negative field dependence of the mobihty in EHO-OPPE within the Gaussian disorder transport formahsm and to determine the diagonal (energetic) disorder parameter a and the off-diagonal (positional) disorder parameter d, the following relation between the charge mobihty p and the disorder parameters was employed [75] ... 

Scheme 2 Ligand-exchange reaction between EHO-OPPE and [Pt(PhCH=CH2)3] leading to EHO-OPPE-Pt networks...

As expected, the coordination of Pt markedly influences the photophysical characteristics of the PPE. The photoluminescence is efficiently quenched, and the absorption maximum in the visible regime experiences a hypsochromic shift. The charge-carrier mobility of different EHO-OPPE-Pt samples was determined by TOE measurements as described above for the neat EHO-OPPE. The shape of the photocurrent transients of all EHO-OPPE-Pt samples was similar to those shown in Figs. 6 and 7 for the neat EHO-OPPE. This indicates that these organometallic conjugated polymers networks are also characterized... 

Fig. 16 ITO/EHO-OPPE/Al LED in operation. The size of the device was 9 mm the picture was taken under ambient illumination. Reproduced with permission from [ 107]...

In order to overcome this problem, a subsequent study focused on devices in which EHO-OPPE was used in combination with a hole-conducting poly-... 

Fig. 17 Schematic representation of the device structures described in Refs. 107 and III a single-layer EHO-OPPE, b two-layer EHO-OPPE/poly-TPD, c single-layer EHO-OPPE poly-TPD blend, and d two-layer EHO-OPPE poly-TPDblend with additional spiro-Qux holeblocking layer, and their corresponding energy-level diagrams. The working functions of Ca (2.9 eV) and Cr (4.5 eV) were omitted. Reproduced with permission from [111]...

Fig. 21. Molecular structure and measured optical properties of EHO-OPPE. From [22]...

Fig. 1 (a) Schematic representation of the mechanically induced transformation of a disordered semicrystalline polymer comprising small aggregates of self-assembled ehromophores (small-molecule or polymeric) into an oriented structure in which the guest moleeules are well dispersed, (b) Stmcture of EHO-OPPE, a poly(p-phenylene ethynylene) derivative... 

Fig. 2 Scanning confocal microscopy images of blend films of ultra-high-molecular-weight polyethylene and 10 wt% EHO-OPPE (see Fig. 1). (a) As prepared film, (b) Uniaxially deformed sample drawn to a draw ratio of 80. Both images were acquired by detecting the polarization direction oriented parallel to the (eventual) deformation direction. Inset shows images acquired by detecting the orthogonal polarization direction. Adapted with permission from [35]. Copyright 2000 American Chemical Society...

Figure 1. Chemical structures of the poly(2,5-dialkoxy-p-phenylene-ethynylene) derivatives EHO-OPPE and co-PPE blue.

Figure 2. Polarized absorption (a) and photoluminescence (b) spectra of a PL polarizer (X = 80) based on a 2 % w/w EHO-OPPE / UHMW-PE blend, recorded for absorption and emission parallel (solid line) and perpendicular (dashed line) to the drawing direction. (Adapted with permission from ref. 31.) Copyri t 1998 American Association for the Advancement of Science.

Polarized absorption spectra, acquired with p- and 5-polarized incident light (Figures 2a and 6) show that the characteristics of the ternary blend are a combination of those of the two respective binary blends. The ternary blend exhibits high absoiption dichroic ratios DR of up to 13 at 440 nm, resulting from a high degree of orientation of EHO-OPPE. By contrast, the absorption at 365 nm is essentially isotropic (DR = 1.5) and reflects the nearly random orientation of the sensitizer within the oriented UHMW-PE matrix. 

Polarized emission spectra, obtained under isotropic excitation at 365 nm and polarized detection in either p- or j-mode are shown in Figures 2b and 7. In binary UHMW-PE / DMC films, the emission from DMC, centred around 400 nm, exhibits only minor polarization, expressed by an emission dichroic ratio, of 2.3, consistent with the low degree of orientation of die sensitizer. In the ternary blend, importantly, the DMC emission is almost fully suppressed, while the emission from EHO-OPPE is highly polarized (DRe= 16). The fact that DR is somewhat lower in the ternary than in a comparable binary UHMW-PE / EHO-OPPE blend (DRe= 27) is explained with a plastisizing effect of DMC on EHO-OPPE that reduces the efficiency of the orientation process. 

Figure 6. Polarized absorption spectra of oriented films obtained with p-(solid line) and s- (dashed line) polarized light (a) Binary UHMW-PE / DMC blend (b) Ternary UHMW-PE / EHO-OPPE / DMC blend. (Adapted with permission from ref. 34,) Copyright 1998 Macmillan Magazines, Ltd.

---