Enhanced hole injection in organic light-emitting diodes using a SAM-derivatised ultra-thin gold anode supported on ITO glass

Abstract

In order to increase the performance of organic light-emitting diodes (OLEDs) we report the modification of the indium-tin oxide (ITO) anode invariably used in OLEDs, with a dipolar self-assembled monolayer-derivatised (4-nitrophenylthiolate) ultra-thin gold overlayer. This composite approach allows the work function of the anode to be tuned to the hole-transporting band of the adjacent semiconductor, while facilitating good mechanical adhesion at this interface. When this modification is incorporated into the model OLED system ITO/TPD/Alq₃/C₆H₅CO₂Li/Al [where TPD is N,N′-bis(3-methylphenyl)-N,N-diphenyl-1,1-biphenyl-4,4′-diamine, Alq₃ is tris(quinolin-8-olato)aluminium and C₆H₅CO₂Li is lithium benzoate], the power efficiency is dramatically enhanced. Furthermore, these devices exhibit a maximum external quantum efficiency of more than 5 cd A⁻¹ and a peak luminance of 36,000 cd m⁻². In combination with the current–voltage–luminance (LIV) characterisation of these devices, scanning Kelvin probe, polarisation modulation reflection absorption infrared spectroscopy and time of flight secondary ion mass spectroscopic techniques have been employed to probe the ITO–Au–SAM interface. This research builds on our earlier work with dipolar organosilanes, phosphonic acids and charge-transfer films at the ITO–organic interface.