Permanent polarization and charge distribution in organic light-emitting diodes (OLEDs): Insights from near-infrared charge-modulation spectroscopy of an operating OLED

摘要

Vapor-deposited Alq_3 layers typically possess a strong permanent electrical polarization, whereas NPB layers do not. (Alq_3 is tris(8-quinolinolato)aluminum(Ⅲ); NPB is 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl.) The cause is a net orientation of the Alq_3 molecules with their large dipole moments. Here we report on consequences for an organic light-emitting diode (OLED) with an NPB hole-transport layer and Alq_3 electron-transport layer. The discontinuous polarization at the NPB|Alq_3 interface has the same effect as a sheet of immobile negative charge there. It is more than compensated by a large concentration of injected holes (NPB~+) when the OLED is running. We discuss the implications and consequences for the quantum efficiency and the drive voltage of this OLED and others. We also speculate on possible consequences of permanent polarization in organic photovoltaic devices. The concentration of NPB~+ was measured by charge-modulation spectroscopy (CMS) in the near infrared, where the NPB~+ has a strong absorption band, supplemented by differential-capacitance and current-voltage measurements. Unlike CMS in the visible, this method avoids complications from modulation of the electroluminescence and electroabsorption.