Browsing by Author "de Leeuw, D. M."
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- Dynamics of threshold voltage shifts in organic and amorphous silicon field-effect transistorsPublication . Mathijssen, S. G. J.; Colle, M.; Gomes, Henrique L.; Smits, E. C. P.; de Boer, B.; McCulloch, I.; Bobbert, P. A.; de Leeuw, D. M.The electrical instability of organic field-effect transistors is investigated. We observe that the threshold-voltage shift (see figure) shows a stretched-exponential time dependence under an applied gate bias. The activation energy of 0.6 eV is common for our and all other organic transistors reported so far. The constant activation energy supports charge trapping by residual water as the common origin.
- Lithium fluoride injection layers can form quasi-Ohmic contacts for both holes and electronsPublication . Bory, B. F.; Rocha, Paulo R. F.; Janssen, R. A. J.; Gomes, Henrique L.; de Leeuw, D. M.; Meskers, S. C. J.Thin LiF interlayers are typically used in organic light-emitting diodes to enhance the electron injection. Here, we show that the effective work function of a contact with a LiF interlayer can be either raised or lowered depending on the history of the applied bias. Formation of quasi-Ohmic contacts for both electrons and holes is demonstrated by electroluminescence from symmetric LiF/polymer/LiF diodes in both bias polarities. The origin of the dynamic switching is charging of electrically induced Frenkel defects. The current density-electroluminescence-voltage characteristics can qualitatively be explained. The interpretation is corroborated by unipolar memristive switching and by bias dependent reflection measurements. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.
- Relation between the electroforming voltage in alkali halide-polymer diodes and the bandgap of the alkali halidePublication . Bory, B. F.; Wang, J. X.; Gomes, Henrique L.; Janssen, R. A. J.; de Leeuw, D. M.; Meskers, S. C. J.Electroforming of indium-tin-oxide/alkali halide/poly(spirofluorene)/Ba/Al diodes has been investigated by bias dependent reflectivity measurements. The threshold voltages for electrocoloration and electroforming are independent of layer thickness and correlate with the bandgap of the alkali halide. We argue that the origin is voltage induced defect formation. Frenkel defect pairs are formed by electron-hole recombination in the alkali halide. This self-accelerating process mitigates injection barriers. The dynamic junction formation is compared to that of a light emitting electrochemical cell. A critical defect density for electroforming is 10(25)/m(3). The electroformed alkali halide layer can be considered as a highly doped semiconductor with metallic transport characteristics. (C) 2014 Author(s).
- Reproducible resistive switching in nonvolatile organic memoriesPublication . Verbakel, F.; Meskers, S. C. J.; Janssen, R. A. J.; Gomes, Henrique L.; Coelle, M.; Buechel, M.; de Leeuw, D. M.Resistive switching in nonvolatile, two terminal organic memories can be due to the presence of a native oxide layer at an aluminum electrode. Reproducible solid state memories can be realized by deliberately adding a thin sputtered Al2O3 layer to nominal electron-only, hole-only, and bipolar organic diodes. Before memory operation, the devices have to be formed at an electric field of 10(9) V/m, corresponding to soft breakdown of Al2O3. After forming, the structures show pronounced negative differential resistance and the local maximum in the current scales with the thickness of the oxide layer. The polymer acts as a current limiting series resistance.
- Reversible post-breakdown conduction in aluminum oxide-polymer capacitorsPublication . Chen, Q.; Gomes, Henrique L.; Rocha, Paulo R. F.; de Leeuw, D. M.; Meskers, S. C. J.Aluminum/Al2O3/polymer/metal capacitors submitted to a low-power constant current stress undergo dielectric breakdown. The post-breakdown conduction is metastable, and over time the capacitors recover their original insulating properties. The decay of the conduction with time follows a power law (1/t)(alpha). The magnitude of the exponent alpha can be raised by application of an electric field and lowered to practically zero by optical excitation of the polyspirofluorene polymer. The metastable conduction is attributed to formation of metastable pairs of oppositely charged defects across the oxide-polymer interface, and the self-healing is related to resistive switching. (C) 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4802485]
- Role of Hole Injection in Electroforming of LiF-Polymer Memory DiodesPublication . Bory, B. F.; Gomes, Henrique L.; Janssen, R. A. J.; de Leeuw, D. M.; Meskers, S. C. J.Al/LiF/poly(spirofluorene)/Ba/Al diodes submitted to bias voltages near 15 V undergo a change to a nonvolatile memory known as electroforming. Prior to electroforming, electron trapping occurs, followed by a tunneling current due to electrons from the polymer into LiF. At the onset voltage for electroforming, a sudden electroluminescence burst originating from electronic excitations in the polymer is detected. This confirms that hole injection into the polymer through LiF occurs leading to nonvolatile resistive switching.
- Sudden death of organic light-emitting diodesPublication . Rocha, P. R. F.; Gomes, Henrique L.; Asadi, K.; Katsouras, I.; Bory, B.; Verbakel, F.; van de Weijer, P.; de Leeuw, D. M.; Meskers, S. C. J.The degradation in light output of an Organic Light Emitting Diode (OLED) has been studied extensively and has been explained by different mechanisms, such as formation of chemical defects or electrical traps and by thermally induced inter-diffusion of dopants. However, there is an overlooked type of degradation, where the light output decreases rapidly with time. This catastrophic failure can often be attributed to a hard electrical short due to local defects. Here, we show that this "sudden death" can also occur in the absence of a hard electrical short. We investigate this phenomenon by current-voltage characteristics and small-signal impedance measurements on typical OLEDs with a LiF cathode interlayer. We show that in a short period of time the built-in voltage of the diode vanishes; the J-V characteristics become symmetric. The origin is a dramatic increase in the work-function of the LiF interlayer. The interlayer changes from an electron-injecting contact to a quasi-Ohmic hole-injecting contact. The pristine bipolar diode does not become electrically shorted, but suddenly transforms into a unipolar hole-only diode. Upon applying a high voltage the original diode is restored, manifesting the dynamic switching of the LiF contact. (C) 2015 Elsevier B.V. All rights reserved.
- Ultralow Power Microfuses for Write-Once Read-Many Organic Memory ElementsPublication . de Brito, B. C.; Smits, E. C. P.; van Hal, P. A.; Geuns, T. C. T.; de Boer, B.; Lasance, C. J. M.; Gomes, Henrique L.; de Leeuw, D. M.Ultralow power microfuses for write-once read-many organic memory elements that rely on electrolysis of water rather than conventional Joule heating are presented. Gasses formed upon electrolysis cannot escape, the pressure rises, the top electrode delaminates and the electrical contact is lost. The voltage-driven fuses are ideally suited for low-end applications such as in electronic bar codes.