Displacement Current Measurement Research Articles

Sequential Measurement of Displacement and Conduction Currents in Electronic Devices

The extension of the Ramo–Schockley–Pellegrini theorem for quantum systems allows to define a positive-operator valued measure (POVM) for the total conduction plus displacement electrical current. The resulting current operator is characterized by two parameters, viz. the width of the associated Gaussian functions and the lapse of time between consecutive measurements. For large Gaussian dispersions and small time intervals, the operator obeys to a continuous weak-measurement scheme. Contrarily, in the limit of very narrow Gaussian widths and a single-shot measurement, the operator corresponds to a standard von Neumann (projective) measurement. We have implemented the resulting measurement protocol into a quantum electron transport simulator. Numerical results for a resonant tunneling diode show the great sensibility of current–voltage characteristics to different parameter configurations of the total current operator.

Observation of turnover of spontaneous polarization in ferroelectric layer of pentacene/poly-(vinylidene-trifluoroethylene) double-layer capacitor under photo illumination by optical second-harmonic generation measurement

The details of turnover process of spontaneous polarization and associated carrier motions in indium-tin oxide/poly-(vinylidene-trifluoroethylene)/pentacene/Au capacitor were analyzed by coupling displacement current measurement (DCM) and electric-field-induced optical second-harmonic generation (EFISHG) measurement. A model was set up from DCM results to depict the relationship between electric field in semiconductor layer and applied external voltage, proving that photo illumination effect on the spontaneous polarization process lied in variation of semiconductor conductivity. The EFISHG measurement directly and selectively probed the electric field distribution in semiconductor layer, modifying the model and revealing detailed carrier behaviors involving photo illumination effect, dipole reversal, and interfacial charging in the device. A further decrease of DCM current in the low voltage region under illumination was found as the result of illumination effect, and the result was argued based on the changing of the total capacitance of the double-layer capacitors.

Determination of charge injection barrier using the displacement current measurement technique

A new technique to determine the injection barrier using displacement current measurement (DCM) is reported. This technique was applied to junctions of metal-free phthalocyanine (H2Pc)/Ag and H2Pc/MoO3/Ag. The injection barriers from Ag to the highest occupied molecular orbital of H2Pc and to the lowest unoccupied molecular orbital of H2Pc were evaluated and found to be 0.4 V and 1.8 V, respectively. No barrier was formed for the hole injection from the MoO3/Ag electrode into H2Pc. These results are relatively consistent with an energy diagram built on the basis of the ionization energy for H2Pc, Ag, and MoO3, in addition to the electron affinity for H2Pc.

Influence of Pentacene Interface Layer in ITO/α-NPD/Alq3/Al Organic Light Emitting Diodes by Time-Resolved Electric-Field-Induced Optical Second-Harmonic Generation Measurement.

By using I-V, EL-V, displacement current measurement (DCM) and time-resolved electric-field-induced optical second-harmonic generation (TR-EFISHG) measurement, we studied the influence of interface pentacene layer inserted between ITO and a-NPD layers in ITO/α-NPD/Alq3/Al OLEDs. All experiments were carried out for the OLEDs with and without a pentacene interface layer. The I-V and EL-V measurements showed the decrease of operating voltage of EL, the DCM showed the lowering of inception voltage of carrier injection by inserting a pentacene interface layer. The TR-EFISHG measurement showed the faster accumulation of holes at the interface between the a-NPD and Alq3 layers, which resulted in the relaxation of electric field of a-NPD layer accomplished by the increase of the conductivity and the increase of the electric field in the Alq3 layer. We conclude that TR-EFISHG measurement is helpful for understanding I-V and EL-V characteristics, and can be combined with other methods to give significant information which are impacted by the interface layer.

A comprehensive study of charge trapping in organic field-effect devices with promising semiconductors and different contact metals by displacement current measurements

A systematic and comprehensive study on the charge-carrier injection and trapping behavior was performed using displacement current measurements in long-channel capacitors based on four promising small-molecule organic semiconductors (pentacene, DNTT, C10-DNTT and DPh-DNTT). In thin-film transistors, these semiconductors showed charge-carrier mobilities ranging from 1.0 to 7.8 cm2 V−1 s−1. The number of charges injected into and extracted from the semiconductor and the density of charges trapped in the device during each measurement were calculated from the displacement current characteristics and it was found that the density of trapped charges is very similar in all devices and of the order 1012 cm−2, despite the fact that the four semiconductors show significantly different charge-carrier mobilities. The choice of the contact metal (Au, Ag, Cu, Pd) was also found to have no significant effect on the trapping behavior.

Measurement of Maxwell's displacement current

The measurement of Maxwell's displacement current has remained a veritable challenge. Here we present a novel approach that allows a detailed verification in a lecture hall-type experiment.

Analyzing degradation effects of organic light-emitting diodes via transient optical and electrical measurements

Although the long-term stability of organic light-emitting diodes (OLEDs) under electrical operation made significant progress in recent years, the fundamental underlying mechanisms of the efficiency decrease during operation are not well understood. Hence, we present a comprehensive degradation study of an OLED structure comprising the well-known green phosphorescent emitter Ir(ppy)3. We use transient methods to analyze both electrical and optical changes during an accelerated aging protocol. Combining the results of displacement current measurements with time-resolved investigation of the excited states lifetimes of the emitter allows for a correlation of electrical (e.g., increase of the driving voltage due to trap formation) and optical (e.g., decrease of light-output) changes induced by degradation. Therewith, it is possible to identify two mechanisms resulting in the drop of the luminance: a decrease of the radiative quantum efficiency of the emitting system due to triplet-polaron-quenching at trapped charge carriers and a modified charge carrier injection and transport, as well as trap-assisted non-radiative recombination resulting in a deterioration of the charge carrier balance of the device.

Analysis of Carrier Behaviors in Double-layer Organic Devices by Displacement Current Measurement and Electric-field-induced Optical Second-harmonic Generation Measurement

Analysis of Carrier Behaviors in Double-layer Organic Devices by Displacement Current Measurement and Electric-field-induced Optical Second-harmonic Generation Measurement

Charge carrier dynamics and degradation phenomena in organic light-emitting diodes doped by a thermally activated delayed fluorescence emitter

We report on the charge carrier dynamics and their degradation phenomena in an organic light-emitting diode (OLED) doped by a thermally activated delayed fluorescence emitter; (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN). Displacement current measurement (DCM) data revealed the presence of negative interface charge originating from the spontaneous orientation polarization of the electron transport layer (ETL), 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl) benzene. The negative interface charge acts as a hole reservoir and thus confines the recombination zone near the emission layer (EML); 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP):4CzIPN (5wt%)/ETL interface. By keeping the recombination zone far from the hole transport layer, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (α-NPD), a better electroluminescence efficiency is expected because the Dexter energy transfer from the triplet state of 4CzIPN to that of α-NPD is suppressed. Moreover, we found an excellent linear relation between the accumulated hole amount and luminance losses owing to device aging. The results are consistent with hole traps originating at the degradation products of CBP as the main factor of device degradation. We suggest device architectures based on the charge carrier dynamics for better efficiency and lifetime.

Self‐Powered Trace Memorization by Conjunction of Contact‐Electrification and Ferroelectricity

Triboelectric nanogenerator (TENG) is a newly invented technology that can effectively harvest ambient mechanical energy from various motions with promising applications in portable electronics, self‐powered sensor networks, etc. Here, by coupling TENG and a thin film of ferroelectric polymer, a new application is designed for TENG as a self‐powered memory system for recording a mechanical displacement/trace. The output voltage produced by the TENG during motion can polarize the dipole moments in the ferroelectric thin film. Later, by applying a displacement current measurement to detect the polarization density in the ferroelectric film, the motion information of the TENG can be directly read. The sliding TENG and the single‐electrode TENG matrix are both utilized for realizing the memorization of the motion trace in one‐dimensional and two‐dimensional space, respectively. Currently, the ferroelectric thin film with a size of 3.1 mm2 can record a minimum area changing of 30 mm2 and such resolution can still be possibly improved. These results prove that the ferroelectric polymer is an effective memory material to work together with TENG and thereby the fabricated memory system can potentially be used as a self‐powered displacement monitor.

Analysis of carrier behavior in α-NPD/P(VDF–TrFE) double-layer capacitor by using electric-field-induced optical second-harmonic generation

The effect of ferroelectric polymer on carrier behaviors in indium–tin oxide (ITO)/poly(vinylidene fluoride–trifluoroethylene) [P(VDF–TrFE)]/N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD)/Au structure was investigated. The internal electric field in α-NPD introduced by the dipole moments from ferroelectric P(VDF–TrFE) together with the α-NPD/P(VDF–TrFE) interface accumulated charges plays an important role in the carrier motion in α-NPD. In displacement current measurement (DCM), two asymmetric peaks and reduced polarization resulting from a large injection barrier from Au electrode to α-NPD layer were observed. Coupled with the electric-field-induced optical second-harmonic generation (EFISHG) measurement, the electric field in α-NPD layer was directly probed and gave a support that only part of the dipoles in P(VDF–TrFE) was polarized. We proposed that the formation of an amorphous insulating layer between amorphous α-NPD layer and partially crystallized P(VDF–TrFE) layer markedly decreased the voltage directly applied on the P(VDF–TrFE) film and caused a reduced polarization. Our findings here will be helpful in analyzing the carrier behavior in organic electronic devices using ferroelectric layers.

Analysis of carrier behavior in C60/P(VDF-TrFE) double-layer capacitor by using electric-field-induced optical second-harmonic generation measurement

By using displacement current measurement (DCM) and electric-field-induced optical second-harmonic generation (EFISHG) measurement, we studied the carrier behavior in the indium-tin oxide (ITO)/Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE))/C60/Au(or Al) capacitors. Two DCM peaks appeared asymmetrically at around −35.5 V and +30.0 V in the dark. Correspondingly, the EFISHG response from the C60 layer was observed, but the peak positions were different with respect to DCM ones. The results show that the spontaneous polarization of the ferroelectric P(VDF-TrFE) polymeric layer directly affects the electric field in the C60 layer, and thus governs the carrier motion in this layer. As a result, the C60 layer serves like an insulator in the dark, while electrons and holes are captured and released at the interface in response to the turn-over of spontaneous polarization of ferroelectric layer. On the other hand, under white light illumination, C60 layer serves like a conductor due to the increase of photogenerated mobile carriers, and these carriers dominate the carrier motions therein. Our findings here will be helpful for analyzing carrier behaviors in organic electronic devices using ferroelectric polymers.

Study of effect of inserted pentacene layer in ITO/P(VDF-TrFE)/α-NPD/Au capacitor using electric-field-induced optical second-harmonic generation and displacement current

The effect of inserted pentacene layers on the interface optimization was investigated in the indium-tin oxide (ITO)/Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE))/N, N′-bis(1-naphthyl)-N,N′-diphenyl-1, 1′-biphenyl-4,4′-diamine (α-NPD)/Au capacitors, by using displacement current measurement (DCM) and electric-field-induced optical second-harmonic generation (EFISHG) measurement. Results show that the inserted pentacene layer between the α-NPD and P(VDF-TrFE) layers effectively improves the contact between the semiconductor and the ferroelectric P(VDF-TrFE) and it further enhances dipolar switching, resulting in three peaks in the DCM, while the insertion of a pentacene layer between the α-NPD and the Au electrode can effectively acquire holes from the Au electrode to be conductive, only leading to peak shift in the DCM with only two peaks. It is concluded that pentacene is a good candidate available for the interface optimization of organic devices.

Relaxation Electron Surface Emission From Space Used Polymers

Dedicated experiments have been developed at ONERA to characterize the charging and relaxation behavior of irradiated space polymers. Postirradiation analyses have been performed through potential kelvin probe and leakage or displacement current measurements. We have been able to demonstrate that polymers are submitted, after irradiation, to electron emission from their surface during the relaxation phase. This electron emission presents very low kinetics and can be intense enough to contribute at significant level to the surface potential drop of these materials. A parametric study has been performed to confirm electron relaxation emission and get a better understanding of the underlying steering mechanisms. This emission process is strongly dependent on the energy of the incident electrons used during irradiation and nature of the irradiated material. We present here the experimental results on electron emission from Kapton and Teflon and discuss the different physical mechanisms that could account for this process. The influence of charge double layer and injected radiation dose could especially explain electron emission from the surface through bulk diffusion and energy release by electron-hole recombination process.

Device intended for measurement of induced trapped charge in insulating materials under electron irradiation in a scanning electron microscope

A device for simultaneously measuring two currents (i.e. leakage and displacement currents) induced in insulating materials under electron irradiation has been built. The device, suitably mounted on the sample holder of a scanning electron microscope (SEM), allows a wider investigation of charging and discharging phenomena that take place in any type of insulator during its electron irradiation and to determine accurately the corresponding time constants. The measurement of displacement current is based on the principle of the image charge due to the electrostatic influence phenomena. We are reporting the basic concept and test results of the device that we have built using, among others, the finite element method for its calibration. This last method takes into account the specimen chamber geometry, the geometry of the device and the physical properties of the sample. In order to show the possibilities of the designed device, various applications under different experimental conditions are explored.

Efficient electron transport in 4,4′-bis[N-(1-napthyl)-N-phenyl-amino] biphenyl and the applications in white organic light emitting devices

The electron transport capability of 4,4′-bis[N-(1-napthyl)-N-phenyl-amino] biphenyl (α-NPD) was investigated by fundamental physical measurements named as current–voltage (I–V) electrical property evaluation and displacement current measurement (DCM). In electron-dominated devices, the I–V characteristics of α-NPD were similar as that of (8-hydroxyquinolino) aluminum (Alq3) owing to their same order of electron mobilities. The interface of Al/LiF and α-NPD was proven to be an Ohmic contact through the evaluation of I–V characteristics at low bias regime (<3V). And an electron injection barrier, 0.21eV, at Al/LiF/α-NPD was obtained by extrapolating the temperature dependent I–V curves. The electron transport behavior in α-NPD film was further confirmed by DCM evaluations. Furthermore, an efficient white organic light emission device was successfully fabricated by using α-NPD as hole transport layer and electron transport layer, respectively.

Reduction of Power Consumption of Counter Electrode Structure in AC-PDP

The low capacitance in the panel contributes to a reduction of power consumption in a Plasma Display Panel system. In this paper, the capacitances of the counter and coplanar electrode structures are compared on the basis of the measurement of discharge and displacement current. The sustain gap of the counter structure is wider than that of the coplanar structure under the same discharge volume. Furthermore, the sustain electrode area of the counter structure is smaller than that of the coplanar structure. Accordingly, the capacitance of the counter structure is lower than that of the coplanar structure for the same discharge volume and sustain gap.

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Displacement current measurement (DCM) is a simple but powerful tool for exploring charge carrier dynamics in organic semiconductor devices. In the first section, we review the basic concept of DCM and how it detects the charge injection, extraction, accumulation, and trapping behaviors in organic semiconductor devices within a quasistatic regime. Subsequently, we present applications of this technique to investigate the device properties of tris-(8-hydroxyquinolate) aluminum (Alq3)-based organic light-emitting diodes. We observed that light irradiation during device fabrication induces additional negative space charges and charge traps in the Alq3 layer. In addition, the device containing the illuminated Alq3 film exhibits a lower luminous efficiency and shorter lifetime compared to the device fabricated in dark conditions, possibly because of the additional hole accumulation in the illuminated Alq3 film. DCM detects the formation of charge traps in the aged devices, decay of the negative space charge, and increase in hole injection voltage with device aging. The origins of these behaviors can be attributed to orientation polarization and charge traps in Alq3 film.

Charge accumulation at organic semiconductor interfaces due to a permanent dipole moment and its orientational order in bilayer devices

Charge accumulation at the organic heterointerfaces in multilayer organic light-emitting diodes (OLEDs) is an important process for understanding their device operation, efficiency, and degradation properties. Charge accumulation behavior has typically been analyzed in terms of the energy barrier and difference of the charge carrier mobility across heterointerfaces. In this study, we demonstrate that permanent dipole moments and their orientational order also play a significant role in the charge behavior at organic semiconductor interfaces. The charge accumulation properties of bilayer devices composed of polar or nonpolar molecules deposited on a 4,4’-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl layer between the anode and cathode were examined by displacement current measurement and impedance spectroscopy. In addition, Kelvin probe measurements for the corresponding bilayer structures excluding the cathode were performed to analyze the relationship between the potential profile and charge accumulation properties of the bilayer devices. We found that several polar molecules including tris-(8-hydroxyquinolate) aluminum, 1,3,5-tris(1-phenyl-1 H-benzimidazol-2-yl)benzene, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7) are spontaneously ordered in evaporated films, and orientation polarization remains in bilayer devices. Orientation polarization leads to interface charge which determines the least amount of accumulated charge in the device under operation. The estimated interface charge density for these molecules ranged from −2.3 (OXD-7) to −0.5 (BCP) mC/m2. Furthermore, impedance spectroscopy revealed that the presence of a permanent dipole moment can suppress the charge dispersion along the interface probably owing to the microscopic potential fluctuation formed in the vicinity of the interface. These results indicate that the permanent dipole moment and orientation polarization contribute to the efficient charge accumulation at organic heterointerfaces and are important parameters for understanding and controlling the charge carrier dynamics in multilayer OLEDs.

Synthesis, Structure, Physical Properties, and Displacement Current Measurement of an n-Type Organic Semiconductor: 2:3,5:6-Bis(1,1-dicyanoethylene-2,2-dithiolate)-quinone

An n-type organic semiconductor 2:3,5:6-bis(1,1-dicyanoethylene-2,2-dithiolate)-quinone (BDQ) has been successfully synthesised and characterised. The single crystal structure of BDQ, determined by single-crystal X-ray crystallography, indicated that BDQ is a planar molecule with strong face-to-face π-π stacking (3.40 Å) and S···S interactions (3.66 Å) between neighbouring molecules, which might be in favour of the effective charge transport. The UV-vis spectrum shows that BDQ has shoulder absorption up to 550 nm (2.25 eV). The electrochemical property displays that the HOMO-LUMO gap of BDQ obtained from the half-wave redox potential is 1.92 eV. The displacement current measurement of BDQ-based devices is demonstrated and discussed.