MEH-PPV films

Nguyen TQ, Martini IB, Liu J, Schwartz BJ (2000) Controlling interchain interactions in conjugated polymers the effects of chain morphology on exciton-exciton annihilation and aggregation in MEH-PPV films. J Phys Chem B 104 237... 

Schaller, R.D., L.F. Lee, J.C. Johnson, L.H. Haber, R.J. Saykally, J. Vieceli, I. Benjamin, T.-O. Nguyen, and B.J. Schwartz. 2002. The nature of interchain excitations in conjugated polymers Spatially varying interfacial solvatochromism of annealed MEH-PPV films studied by near-field scanning optical microscopy (NSOM). J. Phys. Chem. B 106 9496-9506. 

FIGURE 6.6. The normalized absorption spectra for MEH-PPV films spin-cast on glass plates using 0.3-wt% and l-wt% MEH-PPV solutions. The spin speed used was 8000 rpm. 

As discussed earlier, different solvents may result in distinct conformations and aggregation styles of the polymer chains in the solution. It is expected that such differences will be carried on into the spun film. The direct evidence for this is the solvent dependence of the surface energy for spin-coated polymer films. Shi et al.17 have observed that the contact angles (86°-87°) between H2O and the polymer films spun from THF and CHCI3 are significantly smaller than those (average 95°) spun from aromatic solvents (Table 6.1). This indicates that these films have different surface energies. On the other hand, an MEH-PPV film spun... 

As discussed earlier, the solvation effect is expected to become less significant at lower concentrations. Thus, the above solvent dependence of the contact angle should become less significant when more dilute solutions are used. This is indeed observed experimentally. For example, the contact angle between water and an MEH-PPV film spun with a 0.4% solution in THF has the same value (95°) as that spun from p-xylene under the same concentration (0.4%) and the same spin speed (4800 rpm). 

TABLE 6.1. Contact Angles (0) and Surface Tension of MEH-PPV Films... 

For devices using ITO/PEDOT anodes, because the PEDOT layer is much more conductive than the MEH-PPV film, the major energy barrier for hole injection depends on the PEDOT/MEH-PPV interface. If the organic solvent used to dissolve the MEH-PPV does not dissolve the PEDOT layer, it is expected that the resulting PEDOT/MEH-PPV interface will be similar to those obtained by spin-coating the polymer on top of a metal electrode. This is true (or nearly true) in most cases, because PEDOT has a very limited solubility in many commonly used organic solvents. In fact, it was found that the MEH-PPV film spun on top of the PEDOT layer could be easily pealed off from the PEDOT surface by a piece of Scotch tape,... 

FIGURE 6.13. The I-V curves for a series of PLED devices, in which the MEH-PPV films were spun at different spin speeds (solvent xylenes). 

Figs. 6.13 and 6.14. The Vi on determined from these Figures are independent of spin-speed, solvent, and thickness of the MEH-PPV film. The Vi on value is approximately equal to AO ( 1.6 V) This suggests that the electron injection from the Ca/polymer interface is very small or even barrierless at room temperature, consistent with the findings of Campbell et al.41... 

The electroluminescence (EL) and photoluminescence (PL) spectra of spin-cast films are also morphologically dependent. It is found that within the CLA region, the EL and PL spectra of spin-cast MEH-PPV films are strongly dependent on spin speed (Fig. 6.19). It is consistently observed that when the polymer solution is coated at high speeds (e.g., 4000-8000 rpm, 0.7 wt%), the resulting devices have... 

It is generally observed for many polymers that the aggregation of the polymer chains leads to a spectral red-shift in their absorption spectra.11 Quantum mechanics calculations also suggest that a tt-tt stacking of the polymer backbones can red-shift the absorption spectrum.47 If this explanation also applies to MEH-PPV films, a spectral red-shift in the absorption spectrum is expected as the aggregation... 

The emission peak of MEH-PPV films at A,max 580 nm is predominately due to the single-chain exciton emission, whereas that at 630 nm is most likely due to an interchain species (the Ex-I species). The devices with a stronger red (630 nm peak) emission usually have higher quantum efficiency. 

Fig. 2. PL intensity as a function of time in logarithmic scale. The symbols are experimental data for MEH-PPV film deposited on quartz (1) and Ti02 (2) substrates, respectively. The dashed curve corresponds to monoexponential decay enabling determination of exciton lifetime T. The solid curve is theoretically calculated (Burlakov, et al., 2005)...

Fig. 8. Current-voltage and radiance-voltage curves for 1 1 Ti02 (anatase)/MEH-PPV(circles), 1 1 Ti02 (rutile)/MEH-PPV (diamonds), 1 1 S102/MEH-PPV (triangles), and for MEH-PPV film with no nanoparticles (squares). Close symbols are for current. Open symbols are for radiance. IW/mm = 7.3 xlO cds/ m (Carter et al., 1997)...

The effect of HTL, ETL and/or SCL on the enhancement of the I-V characteristics was well demonstrated, associated with the equalization process of injection rates of holes and electrons. But the reason why the nanoparticles can improve the device performance is still open for discussion. For instance, in (Scott et al., 1996) the authors attributed this enhancement to the stimulated emission of optically-pumped MEH-PPV films when Ti02 particles were embedded in. Whereas, in (Carter et al., 1997) the authors indicated that no evidence of line narrowing or changes in the line shape was observed at different voltages, implying that the mechanism for improved performance was distinctly different from that... 

Schaller et al. [115] used NSOM to collect local light emission from the surface of MEH-PPV polymer samples in contact with various solvents. The goal of this study was to identify various emissive interchain species present in annealed MEH-PPV films. The... 

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