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

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.

Conductivity Degradation Study of PEDOT: PSS Films under Heat Treatment in Helium and Atmospheric Air

“…PEDOT:PSS has a heterogeneous structure, in which small conductive grains 20 -50 nm in diameter, are separated by regions rich in insulating PSS [20,21]. In the conductivity mechanisms of such a heterogeneous material the carrier conduction into the grains and the tunneling of the charge carriers between them are contributing [22].…”

Anisotropic hopping conduction in spin-coated PEDOT:PSS thin films

“…2 The combination of a relatively high conductivity 3 with optical transparency in the doped state mates PEDOT:PSS suitable for applications such as antistatic coatings, 4 electrode in light-emitting diodes, 5 photovoltaics, 6 memories, 7 sensors, 8 and as active material for electrochromic devices. 9 As such, PEDOT:PSS has found application in basically all organic and/or polymeric devices.…”

Effect of channel thickness, electrolyte ions, and dissolved oxygen on the performance of organic electrochemical transistors

“…[1][2][3][4][5][6][7] Poly(3,4-ethylenedioxythiophene) doped with poly(styrene-sulfonate) (PEDOT:PSS) is a well known highly conducting and optically transparent polymer. 8,9 Organic electrochemical transistors (OECTs) based on PEDOT:PSS are exploited in bioelectronics and sensing. 3,[10][11][12] OECTs consist of a conducting polymer channel, connected to source and drain electrodes that are in ionic contact with a gate electrode via an electrolyte solution.…”