Impedance Spectroscopy on Poly-p-Phenylene-Vinylene Light-Emitting Diodes

We report on electrical impedance measurements on poly-p-phenylene-vinylene (PPV) light-emitting diodes in the frequency range between 100 Hz and 10 MHz. A significant difference can be revealed between the device characteristics of light-emitting diodes eliminated on indium-tin-oxide (ITO) and those of other high-work-function metals (e.g. Au). Thermal conversion of the precursor polymer on ITO substrates results in a p-type doping of the conjugated polymer PPV. Hence, devices in the configuration ITO/PPV/Al display Schottky behavior, which can be modeled by a simple equivalent circuit of two RC elements in series, representing a bulk and a junction region. The low-frequency device capacitance displays a pronounced voltage dependence and, from a detailed analysis, the ionized acceptor concentration N(A), the diffusion potential V(D), and the width of the space charge region w can be obtained. Typical values for N(A) are10**(16)-10**(17) cm**(-3), and for V(D) within the range 1-1.5 V, resulting in a width w of the space charge region at zero bias of about 50-150 nm. Via temperature-dependent investigations we reveal a transition from a p-type semiconductor Schottky diode at room temperature to a metal/insulator(polymer)/metal structure at lower temperatures.