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Spin cast thin polymer interlayers in polymer light-emitting diodes: Thickness control through use of cross-linkable polymers

Bernardo , G. ; Charas, A. ; Alcácer, L. ; Morgado, J.

Journal of Appl. Physics Vol. 103, Nº 8, pp. 084510 - 7 pages, April, 2008.

ISSN (print): 0021-8979
ISSN (online): 0021-8979

Journal Impact Factor: 2,201 (in 2008)

Digital Object Identifier: 10.1063/1.2896452

Abstract
The effect of very thin polymer interlayers made of either acid-initiated cross-linkable polyfluorenes or the parent non-cross-linkable polyfluorenes, on top of poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid, on the efficiency of light-emitting diodes based on three different electroluminescent (EL) polymers is studied. We find that the use of non-cross-linkable polymers leads to the formation of interlayers with a thickness not exceeding 1 nm and their effect on the light-emitting diode characteristics is not directly correlated with the energetic position of the frontier levels, indicating that other effects, such as interfacial morphology and contact area, which may depend on the specific interlayer/EL polymer combination, play a significant role. When cross-linkable polymers are used, thicker interlayers are formed (up to about 7 nm), and their effect is better related to their frontier level energy. In particular, we found that with these interlayers, a quite significant EL efficiency increase can be obtained.