The semiconducting electroluminescent oligomer para-sexiphenyl (6P) is a promising candidate for application as the active layer in organic light emitting devices. The interaction of Cs atoms evaporated (in a step-wise manner) in ultrahigh-vacuum on thin 6P films has been studied with synchrotron ultraviolet photoelectron spectroscopy. No formation of metallic Cs or of new chemical compounds has been evidenced, but it appears that a doping of the oligomer occurs for the whole range of Cs concentrations we studied. Our data show a progressive growth of new density of states in the (former empty) band gap. Upon progressively increased doping, the material work function decreases, its Fermi-energy remains well below the newly occupied electronic states measured at 5.8 and 3.6 eV (energies relative to the vacuum level). These observations are interpreted as bipolaron formation in the charged 6P material.