Interfacial electronic structure for Ca and an electroluminescent polymer: Poly (2,5-diheptyl-1,4-phenylene-alt-2,5-thienylene)

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X-ray photoemission spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) have been used to investigate the evolution of the interfacial electronic structure of a blue light-emitting polymer: poly (2,5-diheptyl-1,4-phenylene-alt-2,5-thienylene) (PDHPT) as a function of Ca metal coverage. UPS results show a decrease in intensity of the highest molecular orbital-derived feature of the polymer upon Ca deposition and a decrease in work function. XPS data suggest that Ca interacts preferentially with S atoms in the thienylene functional group, and with the aromatic carbon atoms of the π-conjugated backbone. Both sets of experiments are compatible with a significant perturbation of electronic structure of the polymer at the Ca–PDHPT interface. Widths and energy positions of the C 1s, S 2p, and Ca 2p spectra suggest cluster formation and island growth of Ca on PDHPT. There is diffusion of Ca into PDHPT with time, and heating the Ca-covered PDHPT to 100 °C in ultrahigh vacuum leads to improved wetting of the surface. These results provide further insight into the performance of an organic light-emitting device consisting of a Ca–PDHPT interface.