Electronic structure of pristine and sodium-doped cyano-substituted poly(2,5-dihexyloxy-p-phenylenevinylene): A combined experimental and theoretical study

The electronic structure of cyano-substituted poly(2,5-dihexyloxy-p- phenylene-vinylene), or CN-PPV, has been studied in both pristine and doped states. Ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS), as well as optical absorption spectroscopy have been carried out under ultrahigh vacuum (UHV) conditions, and the results have been interpreted with the help of quantum-chemical calculations. For the pristine polymer, the addition of cyano groups to the vinylene units does not affect the width of the π-bands closest to the Fermi level; however, the positions of the flat parts of the upper π-bands are shifted by approximately 0.4 eV towards higher binding energies relative to the Fermi energy, as compared with the corresponding bands of other alkoxy-substituted poly(p-phenylenevinylene)s. On the other hand, there are only marginal differences in the optical absorption spectra; the interband absorption onset is comparable to the values for alkoxy-substituted poly(p-phenylenevinylene)s. In the case of sodium doping, it is found experimentally that at saturation doping, there is about one sodium ion per phenylene vinylene unit; in that situation, two new states appear in the previously forbidden energy bandgap, which are consistent with the formation of bipolaron bands. These results are similar to those obtained for sodium-doping of poly(p-phenylenevinylene) (PPV). The peak-to-peak splitting of the bipolaron peaks in CN-PPV is 1.05 eV, compared with about 2.0 eV for sodium-doped PPV at saturation doping; this difference is related to the pinning of some of the transferred charges to the cyano vinylene groups and the phenylene rings that they are conjugated to in CN-PPV, causing a stronger confinement of the bipolaron charge carriers.