Using self-assembling dipole molecules to improve hole injection in conjugated polymers

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.