Nanoparticles assembled at the semiconducting polymer/electrode interface affect charge injection into polymer light-emitting diodes (LEDs), as reported here. It is demonstrated that modification of the ITO anode using self-assembled monolayers and electrostatically assembled SiO2 nanoparticles can dramatically improve the electroluminescence properties of LEDs-the charged nanoparticle surface induces a dipole moment across the electrode interface, effectively increasing the local electric field and promoting carrier injection.