Frequency dependent charge transport in organic light-emitting diodes, including marked negative capacitance (NC), is reproduced through an equivalent circuit model. The robustness of the model is tested through impedance spectroscopy characterization as a function of bias changes and layer thickness modifications. Correlations with current-voltage measurements reveal that the NC occurs once trap assisted space charge limited transport is reached. Through variation of the organic layer thicknesses, the magnitude of the NC response can be precisely tuned. In particular, increasing the thickness of the electron transport layer increases the NC magnitude, whereas hole transport layer thickness modifications have little effect on the magnitude of NC. Subsequent modeling indicates that alterations in the distribution of the electric field across the individual organic layers account for the observed variations in NC. In addition, it is found that the time constants for the inductive elements of the model increase with applied bias, unlike their capacitive counterparts, suggesting that an accumulation of charge at the organic/organic interface is responsible for both the increasing NC and redistribution of the applied field.
ASJC Scopus subject areas
- Physics and Astronomy(all)