Modification of PEDOT:PSS as hole injection layer in polymer LEDs

M.M. de Kok, M. Buechel, S. I.E. Vulto, P. van den Weijer, E. A. Meulenkamp, S.H.P.M. de Winter, A. J.G. Mank, H.J.M. Vorstenbosch, C.H.L. Weijtens, V. van Elsbergen

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Abstract

Poly(3,4-ethylenedioxythiophene):poly(styrenesulphonic acid) (PEDOT:PSS) is commonly used as an anode in polymer light-emitting diodes (PLED). We have studied the effect of the pH and Na+ ion concentration of the aqueous PEDOT:PSS dispersion on the bulk and surface properties of spincoated films by various techniques, including UV-vis-NIR optical absorbance spectrometry, Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoemission Spectroscopy (UPS). A pH increase by addition of NaOH modifies the PEDOT : PSS properties in a similar way as electrochemical dedoping: the IR absorbance decreases, the Raman peaks shift, sharpen and increase in intensity, and the work function decreases. Consequently, a barrier for hole injection is introduced for several classes of light-emitting polymers. We argue that the mechanism of the pH-effect is different from electrochemical dedoping, and originates from a change in the relative stability of polarons and bipolarons on the doped thiophene. The changes in the electronic properties of PEDOT:PSS point to the determining role of the counter-ion in the stabilisation of oxidised thiophene units. Polymer LEDs comprising Na+-rich, proton poor PEDOT:PSS can show lower lifetime and efficiency than the corresponding Na+-free, proton-rich devices. For light emitting polymers which suffer from the addition of sodium to the hole injecting PEDOT:PSS, the decreased lifetime hints at hole injection as limiting factor in the degradation of these PLEDs.

Original languageEnglish
Pages (from-to)1342-1359
Number of pages18
JournalPhysica Status Solidi A : Applied Research
Volume201
Issue number6
DOIs
Publication statusPublished - 1 May 2004
Externally publishedYes

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de Kok, M. M., Buechel, M., Vulto, S. I. E., van den Weijer, P., Meulenkamp, E. A., de Winter, S. H. P. M., ... van Elsbergen, V. (2004). Modification of PEDOT:PSS as hole injection layer in polymer LEDs. Physica Status Solidi A : Applied Research, 201(6), 1342-1359. https://doi.org/10.1002/pssa.200404338