Necessity of submonolayer LiF anode interlayers for improved device performance in blue phosphorescent OLEDs

Abstract

Lithium fluoride (LiF) is a widely used interlayer in organic optoelectronic devices, at both top and bottom electrodes, with various mechanisms proposed for its effectiveness at each interface. Here we examine the influence of LiF at the ITO electrode as a function of surface coverage. Both thermally evaporated LiF and LiF nanoparticles deposited from solution using di-block copolymer reverse micelles were used to probe the effect of island coverage on device characteristics. From hole-only devices (HOD) with a deep highest occupied molecular orbital (HOMO) level, capacitance-voltage and current-voltage characteristics show that LiF is effective only in the submonolayer range. Injection of holes is maximized at similar to 12% coverage, and decreases with increasing coverage of LiF. Above a critical surface coverage (similar to 50%), the barrier to injection becomes greater than that of bare ITO surface, but is mediated by dipole-induced interfacial trap states. Eventually, the barrier becomes so high that charge carriers cannot be effectively injected and the device does not operate. Using this insight, we fabricated blue phosphorescent organic light-emitting diode (PHOLED) using the optimal coverage of LiF nanoparticles, and observed maximum luminous and quantum efficiency that were improved by 30%. The effect of an array of LiF nanoparticles on emitter orientation was observed by angular-dependent photoluminescence spectroscopy. With this deeper understanding of how LiF operates at the ITO surface, it will be possible to further tune electrode interfaces to accommodate organic transport layers with deeper HOMO levels and larger bandgaps for next-generation devices.