Spatial electron tunneling leads to space-charge-limited current in organic hole transport materials

The injection of electrical charge from an electrode into organic semiconductors directly influences the performance of organic optoelectronic devices. However, our understanding of the mechanisms behind charge injection remains incomplete. In this study, we explored the hole injection from an indium tin oxide (ITO) electrode into a hole transport layer (HTL) by employing various organic interlayers (ILs) with different ionization potentials (IPs). It was demonstrated that using O₂ plasma treatment onto an ITO surface and incorporating an interlayer (IL) with a higher IP between the ITO electrode and the HTL can substantially increase the hole current density. This improvement leads to the achievement of barrier-free injection and the establishment of space-charge-limited current. We propose two synergistic mechanisms of spatial electron tunneling that govern the injection characteristics: electron tunneling from the HTL across the IL to the electrode that establishes an electrostatic equilibrium with a zero-injection barrier and an electric-field-induced spatial tunneling effect that occurs during device operation with applying bias. This research offers a strategy to achieve space-charge-limited hole current and provides an explanatory framework for understanding the underlying physics of charge injection.