Physica status solidi. A, Applied research, 2004-05, Vol.201 (6), p.1342-1359
2004
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Details
- Autor(en) / Beteiligte
- Titel
- Modification of PEDOT:PSS as hole injection layer in polymer LEDs
- Ist Teil von
- Physica status solidi. A, Applied research, 2004-05, Vol.201 (6), p.1342-1359
- Ort / Verlag
- Berlin: WILEY-VCH Verlag
- Erscheinungsjahr
- 2004
- Quelle
- Wiley Online Library All Journals
- Beschreibungen/Notizen
- 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. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)