In this article we study the interface between poly (9,9-dioctylfluorene) (PFO) and different alkali metals (Cs, K, Na, and Li) by photoelectron spectroscopy. The low work-function alkali metals led to low or no electron injection barrier at the PFO interface. From the ultraviolet photoelectron spectroscopy, alteration of electronic structures upon Cs, K, Na, or Li doping into PFO represented a charge transfer process among them. Two new gap states known as bipolaron states were found above the highest-occupied molecular orbital of PFO. Variations in the intensity and feature of these gap states with increasing coverage of the alkali metals were correlated with changes of C Is shakeup peaks acquired from x-ray photoelectron spectroscopy. From the deduced energy level diagram, it is suggested that the new gap states may reduce the radiative recombination of holes and electrons in the polymer light-emitting devices. Films exposed either to residual gases at a pressure of 2.0 x 10(-9) mbar for 3 h or to small amounts of oxygen or water vapor practically eliminated the gap states and restored the affected valence band structures. Deposition of a monolayer of Ag immediately onto the alkali metal/PFO interface protected the films from the influence of residual gases and hence retained the bipolaron states.