Uncoupling nanoparticle geometry from material properties for improved hole injection at submonolayer nanoparticle electrode interlayers in organic hole-only devices

We demonstrate enhanced hole injection into organic hole transport layers (HTL) from submonolayer NiO_x, SnO_x, and LiF nanoparticle hole injection layers (HIL) produced using reverse micelle templating. Decoration of ITO with nanoparticle HIL shifts the hole barrier height at the interface by modifying the electrical structure of the interfacial surface even though the submonolayers show different coverage rates with different sizes and spacing. The current density–voltage characteristics of hole-only devices using nanoparticles are higher than a non-decorated ITO anode, corresponding to the dynamic barrier height shift. Using reverse micelle templating allows the uncoupling of the effect of a non-uniform electric field from nanoscale objects from the impact of the material properties. Larger nanoparticles with greater coverage show less effective injection than smaller particles with low coverage. Using this assessment, a single monolayer of LiF with particles \(\sim\)6 nm, and 21.5% coverage shows the highest hole injection for a single layer of particles, but NiO as a material has greater capacity for hole injection per volume of material on the surface, as particles of \(\sim\)8 nm with only 5.6% coverage can still act to improve charge injection. As this optimal performance is highly dependent on the organic layer and the nature of the contact with respect to trap states, tuning the size and density of the nanoparticle arrays seems to be an effective route to optimizing HILs for novel organic materials in next generation devices.