Advanced Characterization of the Electronic Structure of MEH-PPV

Abstract

In this paper, we present research results that explore the basic molecular structure, orientation, and electrical properties of the conjugated polymer poly(2-methoxy-5- (2,9-ethyl-hexyloxy)-1,4-phenylenevinylene) (MEH-PPV). The bandgap structure of MEH-PPV was investigated through optical absorption and emission spectra, and ultraviolet photoemission spectroscopy (UPS). Based on the optical absorption of MEH-PPV, the π-π* bandgap energy is 2.14eV. The emission spectrum of ITO/MEH-PPV/Al light emitting diode has a peak at 590nm in wavelength which corresponds to photon energies of 2.1eV. By using ultraviolet synchrotron radiation to investigate the highest occupied molecular orbitals (HOMO) of the MEH-PPV, the relative change in photoemission cross-section for various electron states was measured. The HOMO to Fermi level gap obtained from UPS spectra is about 3eV. The UPS measurements include angle-resolved and incident energy dependent spectra. Some indication of bulk film orientation was inferred by dispersion of incident energy dependent UPS. Disparities in these spectra imply some role for defect states, extrinsic carrier involvement, or Fermi-level pinning.

Determination of the optical constants of polymer light-emitting diode films from single reflection measurements

“…By extrapolating the linear portions to (αhv) 2 = 0 in the plots of (αhv) 2 versus hv, the optical gap could be obtained. As shown in figure 3, the approximate values of E 0 g are 2.36 eV and 3.01 eV for MEH-PPV and PFO, respectively, which are close to the values reported in the literature (2.2 and 3.0 eV for MEH-PPV and PFO) [18,20]. The FB gap (E g ) of both films (2.069 and 2.469 eV listed in table 1) is found to be slightly less than the values E 0 g determined from figure 3, which are attributed to the different definitions.…”

“…The refractive indices of PVK and PEDOT : PSS also agree well with the literature value [16,17]. The little deviation between them may be caused by the molecular weight dependence of the refractive index of polymer material as well as the deposition process [18].…”