Energy level line-up in polymer light-emitting diodes via electroabsorption spectroscopy

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Abstract
The influence of carrier-injecting layers on the built-in potential of polymer light-emitting diodes (PLEDs) is analysed by means of electroabsorption spectroscopy. Two promising materials on which research has recently focused are: first, poly(ethylene-dioxythiophene)/poly(styrene sulphonic acid) (PEDOT:PSS), a doped conducting polymer employed as hole injector; and secondly, lithium fluoride (LiF), which has been shown to enhance electron injection of Al-based cathodes. In the first samples LEDs with an indium tin oxide (ITO) anode were compared to those incorporating a PEDOT:PSS layer (in an ITO/PEDOT:PSS bilayer anode). In the second set of samples the comparison was between LEDs with an aluminium cathode and those incorporating a LiF layer (in an Al/LiF bilayer cathode) as a function of LiF thickness. Electroabsorption spectroscopy results supported by work function measurements clearly prove that both PEDOT:PSS and LiF introduce a large reduction in the barrier height at the anode and cathode interfaces, respectively. The ensuing enhancement in carrier injection is considered to be responsible for the marked improvement in device performance (lower operating voltages, higher luminous efficiencies and longer lifetimes).