New copolymer for polymeric light emitting diodes with
low threshold voltage
F. C. Correia1, L. O. Peres2, E. R. Santos1,3, F. J. Fonseca3, A. M. de Andrade4, S. H. Wang1
1
Departamento de Engenharia Metalúrgica e de Materiais, Escola Politécnica da
Universidade de São Paulo, SP, Brazil.
2
Laboratório de Materiais Híbridos, Universidade Federal de São Paulo, SP Brasil.
3
Departamento de Engenharia de Sistemas Eletrônicos, Escola Politécnica da Universidade
de São Paulo, SP, Brazil.
4
Instituto de Eletrotécnica e Energia da universidade de São Paulo, SP, Brazil
In recent years, conjugated polymers have been subject of intense studies in order to develop
polymeric light emitting diodes (PLEDs) with suitable commercial applications, thus high
quantum efficiency, low threshold voltage, longer lifetime and simplified manufacturing
process are among the main requirements for the devices. Polyfluorene and its derivatives are
preferred for this application due to its well-known high photoluminescence and thermal
stability [1].
In this work, a new copolymer of 9,9-dioctylfluorene having electron and hole carriers was
synthesized through the incorporation of a quinoline [2] derivative by the Suzuki crosscoupling polymerization [3]. The absorption spectra of the copolymer in chloroform solution
showed two maximum absorption peaks at 314 nm and 245 nm and the photoluminescence
(PL) spectra of the copolymer due to excitation at 314 nm and 245 nm showed a broad band
emission in the whole visible spectra with the emission peak maximum at 419 nm. The
quantum yield in solution was approximately 68 %.
Polymeric light emitting diodes (PLEDs) were prepared having a multilayer architecture,
comprised by a semiconductor polymer layer, poly(3,4-ethylenedioxy thiophene)/poly(4styrenesulfonate) (PEDOT:PSS), deposited on the anode ITO/glass substrate, previously
treated with UV-Ozone for 5 minutes, followed by the synthesized photo- and electroactive
copolymer and, finally, aluminum cathode. Another series of devices were prepared having an
additional layer of 8-hydroxyquinoline aluminum salt (Alq3) prior to the aluminum cathode
deposition was also prepared for comparison.
The IxV curves of the assembled PLEDs showed low threshold voltages. The PLED
containing only the copolymer showed the threshold voltage around 5.4 V, while those
prepared having Alq3 presented a threshold voltage around 5.2 V, indicating limited influence
of this additional electron transport layer.
The electroluminescence (EL) spectra of the PLEDs here assembled have shown emission in
the whole visible spectra with a peak maximum at 550nm, red shifted in comparison to the
correspondent solution photoluminescence, as expected.
Keywords: PLEDs, Copolymer, Quinoline, Fluorene.
Thanks to CNPq and CAPES for financial support.
[1] M. Leclerc, Journal of Polymer Science, 39, 2867-2873, (2001).
[2] X. Zhang, A. S. Shetty, S. A. Jenekhe, Macromolecules, 32, 7422–7429, (1999).
[3] N. Miyaura, A. Suzuki, Chem. Rev, 95, 2457-2483 (1995).
[email protected], Av. Prof. Mello Moraes nº 2463, Cidade Universitária, SP, Brasil.