Journal
APPLIED PHYSICS LETTERS
Volume 79, Issue 2, Pages 174-176Publisher
AMER INST PHYSICS
DOI: 10.1063/1.1383800
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We report electroabsorption and electroluminescence investigations of polymer light-emitting diodes featuring a LiF/Ca/Al cathode, for efficient electron injection into the electroluminescent polymer layer. Our measurement of the built-in potential gives direct evidence of a sizeable reduction of the cathodic barrier height not only with respect to Ca, but also versus LiF/Al or CsF/Al bilayer cathodes, currently amongst the most efficient electron injectors for low electron affinity polymers. In blue-emitting (similar to2.7 at peak) polyfluorene-based LEDs, with poly(ethylenedioxythiophene)/poly(styrene sulphonic acid) anodes and LiF/Ca/Al cathodes, we measure a built-in potential of 2.7 V, a luminance of similar to 1600 cd/m(2) (the highest among the devices studied here) at a driving voltage of 5 V, and efficiencies as high as similar to3 lm/W. We also find that the turn-on voltage essentially coincides with the built-in potential within the experimental error. (C) 2001 American Institute of Physics.
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