TITLE

Improving efficiency by balancing carrier transport in poly(9,9-dioctylfluorene) light-emitting diodes using tetraphenylporphyrin as a hole-trapping, emissive dopant

AUTHOR(S)
Campbell, Alasdair J.; Bradley, Donal D. C.; Virgili, Tersilla; Lidzey, David G.; Antoniadis, Homer
PUB. DATE
December 2001
SOURCE
Applied Physics Letters;12/3/2001, Vol. 79 Issue 23, p3872
SOURCE TYPE
Academic Journal
DOC. TYPE
Article
ABSTRACT
Unbalanced carrier transport is known to strongly affect the efficiency of polymer light-emitting diodes. Here, we report the results of time-of-flight (TOF), current density–voltage, and electroluminescence (EL) quantum efficiency measurements on single-layer poly(9,9-dioctylfluorene) (PFO) devices doped with the red-emitter tetraphenylporphyrin (TPP). TOF shows that PFO is a unipolar conductor, with hole transport much better than electron transport. At a field of 5×10[sup 5] V/cm, a nondispersive hole mobility of 4×10[sup -5]–5×10[sup -4] cm[sup 2]/V s, dependent on sample morphology, is obtained. Upon the addition of 5% by weight TPP, hole transport becomes as highly dispersive as electron transport, having no measurable average mobility. This results in a decrease in the current for a given applied bias but an increase in the external EL quantum efficiency. TPP acts as a strong hole trap, reducing the dominant hole current and producing more balanced carrier transport. At TPP concentrations above 6%, the device characteristics start to revert to those found at lower TPP concentrations. This is due to the onset of efficient hole transport between the dopant molecules that reestablishes a transport imbalance. © 2001 American Institute of Physics.
ACCESSION #
5562332

 

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