Numerical simulation of impedance and admittance of OLEDs

Abstract

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 V 0 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 Alq 3 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 components 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 material 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.

Impedance Characterization of Organic Light‐Emitting Structures with Thermally Activated Delayed Fluorescence

“…By analyzing the voltage dependences of the elements of the equivalent circuit, various CR chains are associated with different regions in the OLED structure. These regions are not uniquely associated with layers of various materials, but differ in electrical properties (e.g., these are depleted and conducting regions) . The following physical meaning of the elements of the equivalent circuit is possible.…”

“…To describe the elements of the equivalent circuit, we used for a capacitor the model of two flat plates. This approach is often used, but it is an approximation, as it assumes interface flatness and layer homogeneity. Within this approximation, the capacitance C 1 can be described by the expression: , where ε 0 is the dielectric constant of vacuum, ε is the relative dielectric constant of organic semiconductor, S is the structure area, and d 1 is the thickness of the depleted layer.…”

“…Within this approximation, the capacitance C 1 can be described by the expression: , where ε 0 is the dielectric constant of vacuum, ε is the relative dielectric constant of organic semiconductor, S is the structure area, and d 1 is the thickness of the depleted layer. If we assume that for organic semiconductors ε = 3, then the thickness d 1 is 20.9 and 48.9 nm at 0 and 6 V, respectively. The values of R 2 and C 2 characterize the properties of the bulk of the emission TADF layer.…”

Reducing exciton-polaron annihilation in organic planar heterojunction solar cells

“…Assuming a relative dielectric constant ε r = 3.8 for C 60 and a diode area A = 0.134 cm 2 , t = 32 nm can be calculated as the thickness of the layer which functions as a dielectric for this capacitance (t = ε r ε 0 A/C p ). Since the C 60 thickness is approximately 30 nm, this suggests that the C 60 region remains largely depleted, while the ac signal charges the diode at the PEDOT:PSS anode and the C 60 /BCP heterojunction [22]. Since it appears that for frequencies f < 1 kHz, charges at the C 60 /BCP interface are modulated with the ac voltage, this implies that the BCP has relatively low resistance, extracted as 1 k , because it is filled with charge injected by the Ag cathode under static conditions.…”

Quantum efficiency harmonic analysis of exciton annihilation in organic light emitting diodes

“…While OLED impedance spectra are generally difficult to interpret due to complicating factors such as space charge accumulation, unknown trap distributions, and injection/heterojunction barriers, 12,13 the relationship between electroluminescence and current density is accurately described via a simple rate equation. Consider applying a voltage,…”