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Using Self‐Assembling Dipole Molecules to Improve Hole Injection in Conjugated Polymers
Author(s) -
Khodabakhsh S.,
Poplavskyy D.,
Heutz S.,
Nelson J.,
Bradley D. D. C.,
Murata H.,
Jones T. S.
Publication year - 2004
Publication title -
advanced functional materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/adfm.200400035
Subject(s) - materials science , work function , pedot:pss , ohmic contact , indium tin oxide , anode , optoelectronics , oled , electrode , kelvin probe force microscope , dipole , polymer , diode , polyfluorene , monolayer , conjugated system , nanotechnology , thin film , composite material , layer (electronics) , organic chemistry , chemistry , atomic force microscopy
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.