Schematic of the test setup. Four identical unipolar devices grown and packaged on the same substrate are each subjected to one of the following conditions: constant current density of , uniform illumination at , and the combination of each . The control device is subjected to neither nor . A computer controlled multiplexer switches between measurement instruments and current sources. All measurements are conducted in the dark, while an electronic shutter temporarily isolates devices exposed to and conditions.
Unipolar test and control device structures. Electron-only transport is obtained in devices and with the use of the hole blocking buffer material BAlq in the test and control devices shown in (a) and (b), respectively. Analogously, an NPD buffer leads to hole-only current in devices and . Different contact treatments (i.e., LiF with the Al cathode and UV-ozone treatment of the ITO anode) are used to ensure unipolar injection. Highest occupied molecular orbital and LUMO energies are shown for each material (from Ref. 25).
(a) Forward biased capacitance-voltage characteristics of as-grown electron test, control, and hole control devices obtained at a frequency of 100 Hz. (b) Current-voltage data obtained for the same devices as in (a). Solid lines in both plots indicate fits to the TCL model in the text using the parameters in Table I.
(a) Typical shift in capacitance over time for (black lines) and (gray lines) devices under conditions at an intensity of . The peak observed in the capacitance at is ascribed to the small electron energy barrier between the BAlq and mCBP layers. (b) Conductance data reflect the same shift over time as the capacitance in (a). Changes are only observed for the test device, indicating that degradation occurs in the layer.
(a) Capacitance change at 12 V obtained for devices under the four conditions described in Fig. 1. Only conditions (open squares) and (open circles) induce degradation, as marked by the nonlinear decrease in capacitance with time. Three sets of (open triangles) and data are shown, corresponding to illumination intensities of , , and (see arrow). Solid black lines are fits to Eq. (7) in the text using the parameters in Table I. (b) Analogous capacitance data for devices under the same test conditions. No changes are evident, indicating that BAlq does not degrade. Note that the legend applies to both plots.
(a) Drive voltage for devices at constant current density of under the test conditions in Fig. 1. As evidenced by the voltage increase over time, devices degrade under conditions (open squares) and (open circles). As in Fig. 5, data sets for (open triangles) and are given for illumination intensities of , , and , as shown by the arrow. Solid black lines indicate fits to Eq. (6) in the text using the parameters in Table I. The curve is only fit for since the data change behavior at this point most likely due to the onset of a new device failure mode unrelated to defect formation; this deviation is shown by the dotted line extension of the model prediction. (b) Drive voltage data for devices under the same test conditions. No changes in voltage are observed, signifying that BAlq is stable under all test conditions. The legend applies to both plots.
Changes in capacitance at 8 V and the drive voltage at constant current for are plotted in (a) and (b), respectively. The devices degrade under (open triangles) and (open circles) conditions, where the degradation scales with illumination intensity for , 19.5, and , as denoted by the arrow. The devices (not shown) yield very similar results; hence, degradation of individual layers cannot be identified unambiguously. In contrast, the trends confirm NPD degradation; solid black lines indicate fits to these databased on Eqs. (6) and (7) in the text using the parameters in Table I.
Degradation rate extracted from fits to the data in Figs. 5 and 6 and the data in Fig. 7. Solid black lines are linear fits, where the slope indicates instability of optical excitations, and the -intercept reflects instabilities of the charged molecules.
Parameters used in fits to degradation data.
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