Interface formation between poly (9,9-dioctylfluorene) and alkali metals investigated by photoemission spectroscopy

摘要

We study the interface between poly (9,9-dioctylfluorene) (PFO) and alkali metals (Cs, K, Na and Li) by photoemission spectroscopy. These low work-function metals led to low or no electron injection barrier at the PFO interface. Two new gap states known as bipolarons were found above the highest-occupied molecular orbital of PFO. From the deduced energy level diagram, 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 hours 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.