By using ultraviolet photoelectron spectroscopy, the hole-injection barrier of poly-(9,9-dioctylfluorene) (PFO) cast on top of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) electrode is found to vary with thickness of both films, as confirmed by current profiles of hole-dominated and bipolar devices. The variation can be attributed to a migration of PSS chain as a dopant into PFO layer, as indicated by the broadening and shifting (towards higher binding energy) of highest occupied molecular orbital edge peak of PFO film. The barrier at the thinnest PEDOT:PSS film is found to be the smallest due to the least extent of doping by PSS chains.
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