AVS 51st International Symposium
    Organic Films and Devices Thursday Sessions
       Session OF+EM-ThA

Paper OF+EM-ThA7
Negative Capacitance in Hetero-Layered Organic Light Emitting Diodes

Thursday, November 18, 2004, 4:00 pm, Room 304C

Session: Molecular and Organic Films and Devices - Optoelectronic
Presenter: L.S.C. Pingree, Northwestern University
Authors: L.S.C. Pingree, Northwestern University
B.J. Scott, Northwestern University
T.J. Marks, Northwestern University
M.C. Hersam, Northwestern University
Correspondent: Click to Email
Negative capacitance (NC) has been measured by impedance spectroscopy and correlated with time domain waveforms in ITO/NBP/Alq@sub 3@/Al OLED macroscopic devices. In addition, this behavior has been measured by Nanoscale Impedance Microscopy@footnote 1@ on 8 µm x 8 µm microscopic OLED devices. Beyond providing evidence of the scaling of NC, this AFM based technique provides spatially resolved capacitance variations in these structures. Due to the field dependent mobility of the charge carriers, and their subsequent dependence upon both bias and space charge, a slow rise time (t@sub R@) in the current response is detected when a step voltage is applied to the OLED. Such behavior is typical of a NC impedance response@footnote 2@. The typical t@sub R@ varies from 100 ms at 1 volt to 15 ms at 10 volts for electrons, and 100 ms at 1 volt to 1 ms at 10 volts for holes. The onset of NC in the frequency domain correlates strongly with the hole t@sub R@, and the corresponding frequencies are exponentially dependent upon the applied bias. These results agree with the exponential dependence of mobility upon applied field given by Poole-Frenkel theory. Also, the variation between the carrier rise times suggests that Richardson-Schottky injection dominates electron flow. Correlation of the data with device physics implies that NC behavior is hole dominated since the flow of electrons is modulated solely by holes trapped at the Alq@sub 3@/NPB interface, whereas holes respond to both bias and space charge. Additionally, temporal variations in the behavior of light emission as a function of frequency were acquired though the use of a dual lock-in technique. @FootnoteText@ @footnote 1@R. Shao et. al. Appl. Phys. Lett. 82 1869 (2003).@footnote 2@M. Ershov et. al. IEEE Trans. Elect. Dev. 45 2196 (1998).