Small-sized Al nanoparticles as electron injection hotspots in inverted organic light-emitting diodes

• Small-sized Al nanoparticles on ITO can act as electron injection hotspots in inverted OLEDs.
• Injection hotspots can reduce voltage and improve efficiency for both fluorescent and phosphorescent OLEDs.
• Injection hotspots have minor effects on surface roughness, transparency and work function of ITO.
• The hotspot effect arises from efficient electron injection due to enhanced local electric field.

Al nanoparticles, with small size and ultralow coverage on ITO, can play a key role as the electron injection hotspots in both the inverted fluorescent and phosphorescent organic light-emitting diodes. The presence of the hotspots greatly reduces the operational voltage and improves the current efficiency of the devices, which are strongly dependent on the hotspot size. The microscopic and spectroscopic characterization demonstrate that the small-sized hotspots have a minor influence on the surface roughness, transparency and work function of ITO. The hotspot effect is ascribed to the highly efficient electron injection at the Al nanoparticles enhanced by the local electric field, and a physical model is proposed to clarify this mechanism. The finding indicates a promising strategy by design and craft of the injection hotspots in nanoscale to facilitate carrier injection in organic thin film devices.

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