Capacitance-voltage characteristics of organic light-emitting diodes varying the cathode metal: Implications for interfacial states

Abstract

Capacitance-voltage $(C\text{\ensuremath{-}}V)$ characteristics of organic light-emitting diodes based on a polyphenylenevinylene have been measured by means of impedance spectroscopy. The effect of the metallic cathode (Au, Ag, Al, Mg, and Ba) was analyzed in the low-frequency region $(2\phantom{\rule{0.3em}{0ex}}\mathrm{Hz})$ of the capacitive response. The $C\text{\ensuremath{-}}V$ curves collapse into a single pattern in the low bias region, and exhibit a dependence on the cathode work function showing a crossover from positive to negative (inductive) values. The voltage corresponding to the onset of the inductive behavior shifts toward higher bias as the cathode work function increases. Simulations based on an electron-injection model result in agreement with the observed $C\text{\ensuremath{-}}V$ characteristics. From the comparison between the predicted and the experimentally observed data an estimation of the energetics of the metal-organic interface could be given. First, an interfacial state located $\ensuremath{\sim}0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the metal Fermi level appears independently of the metallic cathode. Second, the existence of a dipole layer with interface slope $S=\ensuremath{-}0.43$ is deduced which enhances relatively the electron injection for large work function metals. In the case of a Ba cathode, the electron barrier determining carrier injection is due to the interface states.