Numerical simulation of impedance and admittance of OLEDs

The electrical characteristics of organic light-emitting devices are calculated for the dc and ac regimes by numerically solving the basic semiconductor equations under steady-state and small-signal conditions. For a given structure, the dc and ac electric potential and electric field, the electron and hole concentrations, as well as the different components of the current density are obtained as function of the one-dimensional spatial coordinate. This approach allows a detailed microscopic description of the dependencies of these quantities on the applied steady-state voltage V0 and the frequency of the modulating voltage. The final output consists in the frequency-dependent complex admittance and impedance of the device, the real and imaginary parts of which are the experimentally-available data. As a typical example, we show the results for a two-layer structure where α-NPD is the hole-transporting material and Alq3 the electron-transporting material. The anode is made of ITO and Al/LiF composes the cathode. The admittance and impedance curves, yielded by the numerical simulation as functions of the modulation frequency, are fitted by an equivalent electrical circuit, the elements of which are resistances and capacitances. The number of com- ponents depends on the structure composition and on the applied steady-state voltage. We show that each element can be associated with a particular region of the device. This allows to correlate the dependence of each feature of the admittance and impedance curves with one or several parameters describing the ma- terial system. Such an analysis can be useful for the inverse approach, where, starting from measurements of the electrical ac characteristics, the aim is to get information on the microscopic mechanisms which contribute to the electrical conduction of the device.