Negative Capacitance in Organic Light-Emitting Diodes: Implications for Display Applications

摘要

As Organic Light Emitting Diodes (OLEDs) enter the consumer market, their frequency dependent charge transport characteristics are becoming more important. When these devices are rapidly cycled on and off, as in display applications, any inductive nature in the transport characteristics will lead to non-optimal device performance. While this inductive-type transport, or negative capacitance (NC), was reported in the literature only recently [1,2], similar characteristics have been seen in the electroluminescence transients of OLEDs for some time [3]. In our initial work [1], it was demonstrated that dangling bonds, surface dipoles, and defects at the anode/hole transport layer (HTL) interface create a series of charge traps that dominate the low-frequency response, thus masking the inductive nature of OLEDs. Once this interface was modified through the addition of a thin adhesion promoting layer, charge trapping did not occur, and the underlying behavior could be monitored using impedance spectroscopy. An equivalent circuit for hetero-layered OLEDs was developed, consisting of a parallel resistor and capacitor in series with a parallel resistor and inductor that models the charge transport through the electron transport layer (ETL) and HTL, respectively. In this work the robustness of the model is tested through impedance spectroscopy characterization as a function of applied bias 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 intentionally tuned. In particular, increasing the thickness of the electron transport layer increases the NC magnitude from 2.5% to 11%, 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. The ability to modify time constants and magnitude of NC may provide an avenue to increase display efficiency.