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

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Abstract
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.