ABSTRACT
Poly(2,5-diheptyl-1,4-phenylene-alt-2,5-thienylene) (PDHPT) is a blue light emitting conjugated polymer with a band gap of ca. 3 eV, and is a promising candidate as an active material in organic light emitting diodes. The interfaces of this polymer with a) a blue light emitting oligomer (p-sexiphenyl-6P), and with b) a low work-function cathode material, calcium (Ca) have been investigated using both ultraviolet and x-ray photoelectron spectroscopies (UPS and XPS). Both 6P and PDHPT have a band gap of ca. 3 eV, and slightly different ionization potentials, and this similarity makes the interface an interesting one. A systematic study of the in-situ growth of 6P on PDHPT allows the estimation of the relative positions of the HOMOs and the vacuum levels. We find that the HOMO of PDHPT is almost at the same level as that of 6P, however, there is a vacuum level offset of about 0.2 eV. In case of the metal-PDHPT interface study, Ca was deposited in-situ in extremely small steps, and both XPS and UPS data were recorded after each subsequent deposition. Contrary to what can be predicted based on calculations and experiments on similar systems, namely phenylenes and thiophenes, we propose that Ca forms covalent bonds with the polymer. The consequences of these experimental findings will be discussed within the context of LEDs.
