Using Self-Assembling Dipole Molecules to Improve Hole Injection in Conjugated Polymers

• Published in: Advanced functional materials 2004-12, Vol.14 (12), p.1205-1210
• Main Authors: Khodabakhsh, S., Poplavskyy, D., Heutz, S., Nelson, J., Bradley, D. D. C., Murata, H., Jones, T. S.
• Format: Article
• Language: English
• Subjects: Light-emitting diodes; Light‐emitting diodes, organic; organic; Photoluminescence (PL); Self assembled monolayers (SAMs)
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.200400035

Surface modification of indium‐tin‐oxide (ITO)‐coated substrates through the use of self‐assembled monolayers (SAMs) of molecules with permanent dipole moments has been used to control the ITO work function and device performance in polymer light‐emitting diodes based on a polyfluorene hole transporting copolymer. Measured current–voltage characteristics of the devices reveal greatly increased hole injection currents from the SAM‐altered electrodes with higher work function, in agreement with an expected reduction in the barrier for hole injection. In particular, it is shown that the SAM‐modified electrode with the highest work function provides an ohmic contact for hole injection into the studied polymer. Injection from the widely used poly(2,3‐ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT:PSS)‐coated ITO anode system, is less efficient compared with some of the studied SAM‐coated ITO anodes despite the significantly higher work function measured by a Kelvin probe. This apparently anomalous situation is attributed to the inhomogenities in the injection processes that occur over the area of the device when the PEDOT:PSS‐coated ITO electrode is used. Self‐assembled monolayers (SAMs) are used in order to control the work function of indium tin oxide (ITO) electrodes and the device performance in polyfluorene based polymer light‐emitting diodes (see Figure).