Influence of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) in polymer LEDs

P. J. Brewer, J. Huang, P. A. Lane, A. J. DeMello, D. D.C. Bradley, J. C. DeMello

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Abstract

We investigate the influence of poly(3,4-ethylenedioxythiophene)- poly(styrenesulfonate) (PEDOT:PSS) on the optoelectronic properties of polymer light-emitting diodes containing poly(9,9-dioctylfluorene) (PFO). Electromodulation and IV luminance measurements are reported for a series of devices with bare indium tin oxide (ITO) or PEDOT:PSS-coated ITO anodes and Ba or Al cathodes. The ITO/PFO/Al, ITO/PFO/Ba, and ITO/PEDOT:PSS/PFO/Al devices all exhibit conventional field-induced electromodulation behavior, in both forward and reverse bias, consistent with the Stark effect (SE). The ITO/PEDOT:PSS/PFO/Ba devices by contrast exhibit conventional behavior only for applied biases below the flat-band voltage; at higher biases, the field-induced SE features vanish and are replaced by anomalous charge-induced electromodulation features. This anomalous behavior is observed only when PEDOT:PSS is used in conjunction with a strongly electron-injecting cathode such as Ba, and is attributed to the presence of trapped electrons at the PEDOT:PSS-emitter interface, which screen the electric field from the bulk of the device. The enhanced field at the interface increases the rate of field-assisted hole injection into the highest occupied molecular orbital (HOMO) of the PFO, resulting in lower drive voltages and increased electroluminescence efficiencies. © 2006 The American Physical Society.
Original languageEnglish (US)
JournalPhysical Review B - Condensed Matter and Materials Physics
Volume74
Issue number11
DOIs
StatePublished - Sep 15 2006
Externally publishedYes

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