Charge Injection Mechanism Research Articles

Efficient tandem organic light-emitting diodes with non-doped structures.

Highly efficient tandem organic light-emitting diodes (TOLEDs) were achieved based on a non-doped charge generation unit (CGU) consisting of LiF/Al/C60/4,4',4"-tris(N-3-methylphenyl-N-phenyl-amino) triphenylamine (m-MTDATA) and ultrathin emitting layers. The current-voltage characteristics of the CGU devices and electron-only devices and the capacitance-voltage characteristics of the CGU-based capacitance devices were characterized to explore the charge generation and injection mechanisms. The charge generation process occurs at the interface of C60/m-MTDATA through electron transferring from the highest occupied molecular orbital of m-MTDATA to the lowest unoccupied molecular orbital of C60. It is found that the thinner C60 layer contributes to efficient electron injection. Under the optimal structure, the blue TOLEDs exhibit a maximum current efficiency (CEmax) of 43.3 cd/A. The CEmax and maximum external quantum efficiency (EQEmax) of the white TOLEDs reach 84.6 cd/A and 26.7%, respectively.

Electrochemical performance of polymer-derived SiOC and SiTiOC ceramic electrodes for artificial cardiac pacemaker applications

In an implantable electrode, such as a pacemaker electrode, fibrotic tissue formation due to a foreign body reaction is an important challenge affecting the efficiency to transmit the electrical signal of the device. The chemical inertness, biocompatibility, and electrical conductivity of polymer-derived ceramics (PDCs) are promising features in terms of overcoming this challenge. Here, the electrochemical behavior of polymer-derived silicon oxycarbide (SiOC) and titanium-doped SiOC (SiTiOC) ceramic electrodes for use as pacemaker electrodes is investigated by measuring impedance spectroscopy and cyclic voltammetry. In addition, typical stimulation electrodes such as iridium oxide, titanium nitride, platinum, and glassy carbon were prepared and loaded simultaneously into a custom-made electrochemical testing platform for comparison with SiOC and SiTiOC electrodes under identical conditions. The SiOC and SiTiOC electrodes shows a wide electrochemical stability window in the range of −0.9 to 1.2 V with a double layer capacitance as the charge injection mechanism at the electrode/phosphate-buffered saline interface. Also, analyzing the voltage transient shows that the maximum charge injection of the SiTiOC electrode was about 28 μC/cm2. The results of the electrochemical evaluation and comparison of SiOC and SiTiOC stimulating electrodes will be helpful to understand fundamental characteristics for the potential of this material as candidate for next-generation pacemaker electrodes.

Effects of fullerene nanoparticles and fullerite nanoparticles on the charge injection mechanism of methyl red dye based organic device

The present work studies the injection of current at the metal–organic contact of a methyl red organic device. The poor charge injection mechanism of these organic devices is due to the high energy barrier at the contact. Our main aim is energy barrier lowering to ameliorate the current flow of the device. The estimation of the energy barrier and the influence of both fullerene and fullerite nanoparticles on it have been observed. We have sandwiched the blend of the organic dye and nanoparticles by the spin coating method in between two electrodes. We have analyzed the dark current–voltage plots of the organic device to calculate the energy barrier and turn-on voltage. Lowering of the energy barrier from 0.87 eV to 0.39 eV with fullerene nanoparticles and to 0.37 eV with fullerite nanoparticles has been observed. The turn-on voltage also reduces from 2.52 V to 2.32 V in the presence of fullerene nanoparticles and to 2.00 V in the presence of fullerite nanoparticles. To check the consistency of the obtained results from I–V plots of the device, the energy barrier has been estimated by the Norde function. This function also shows a decrease in the value of the energy barrier from 0.85 eV to 0.35 eV with fullerene nanoparticles and to 0.32 eV with fullerite nanoparticles. It has been found out that the fullerite nanoparticles show better performance in terms of the charge injection process in these organic dye based devices than the fullerene nanoparticles.

The study of charge injection mechanism of C60/CuPc organic heterojunction connector layer

The charge injection mechanism of a connector layer is one of the critical issues influencing the performance of tandem organic light-emitting diodes (OLEDs). In this paper, to explore the charge injection mechanism of an organic heterojunction connector (OHJC) layer, we studied the device current density (J) characteristics generated by the C60/CuPc OHJC layer under different applied voltages (V). By analyzing the log(J)–V1/2 and ln(J/E)–1/E2 characteristics of devices, we found that the charge injection mechanism of the C60/CuPc OHJC layer is in accordance with the Richardson–Schottky model. This study can be a theoretical basis to design high-efficiency OLEDs.

Dependence of mobility and charge injection on active layer thickness of bulk heterojunction organic solar cells: PCBM:P3HT

The asymmetric behavior in the dark current of any solar cell is essential for decoupling recombination and charge extraction for efficient charge collection. Therefore current density dependence on applied voltage can be used to investigate the complex interplay between bulk charge transport, interface exchange effects and recombination mechanisms. In the present work, we investigate from dark current–density [J(V)] of bulk heterojunction solar cells, the dependence of mobility and charge injection mechanisms on different blend layer thicknesses. The active layer thickness has been established through varying the spin-coating speeds between 1000, 2000, 3000 and 4000 revolutions per minute (rpm) and confirmed by a dektak surface profilometer. The carrier mobility (μ) as a factor limiting the efficiency of organic solar cells was investigated from dark space charge limited current and trap free space charge limited current conduction mechanisms to distinguish between charge extraction and recombination. This approach allows the determination of the effects of threshold field through variation of the active layer thickness (ALT) on the potential barrier height $$(\phi_{B} )$$ at the electrode contacts. Low values of charge carrier mobilities (10–6 cm2 V−1 s−1) in the trap free space charge limited current conduction region have been correlated to the Langevin recombination constants. In the ohmic region, the highest dark carrier mobility corresponded to the 77.1 nm ALT. Further we observe a shift in the transition voltage at the inflection point of J–V curves with increasing film thickness in the forward bias.

First-Principles Approach for Assessing Cold Electron Injection Efficiency of Dye-Sensitized Solar Cell: Elucidation of Mechanism of Charge Injection and Recombination

The adequacy of the inclusion of spacer units in the metal-free D−π–A organic dyes concerning the augmentation of dye-sensitized solar cell (DSSC) efficiency has been examined through the excited-s...

Barrier Formation at the Contacts of Vanadium Dioxide and Transition-Metal Dichalcogenides.

Phase-transition field-effect transistors (FETs) are a class of steep-slope devices that show abrupt on/off switching owing to the metal-insulator transition (MIT) induced in the contacting materials. An important avenue to develop phase-transition FETs is to understand the charge injection mechanism at the junction of the contacting MIT materials and semiconductor channels. Here, toward the realization of high-performance phase-transition FETs, we investigate the contact properties of heterojunctions between semiconducting transition-metal dichalcogenides (TMDCs) and vanadium dioxide (VO2) that undergoes a MIT at a critical temperature (Tc) of approximately 340 K. We fabricated transistors based on molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) in contact with the VO2 source/drain electrodes. The VO2-contacted MoS2 transistor exhibited n-type transport both below and above Tc. Across the MIT, the on-current was observed to increase only by a factor of 5, in contrast to the order-of-magnitude change in the resistance of the VO2 electrodes, suggesting the existence of high contact resistance. The Arrhenius analyses of the gate-dependent drain current confirmed the formation of the interfacial barrier at the VO2/MoS2 contacts, irrespective of the phase state of VO2. The VO2-contacted WSe2 transistor showed ambipolar transport, indicating that the Fermi level lies near the mid gap of WSe2. These observations provide insights into the contact properties of phase-transition FETs based on VO2 and TMDCs and suggest the need for contact engineering for high-performance operations.

First-principle investigation of the charge injection barriers of polyethylene and polytetrafluoroethylene oligomers

Experimental research has shown that much less charge injection occurs in polytetrafluoroethylene (PTFE) compared to polyethylene (PE). To clarify the mechanisms of charge injection from metals into polymer insulators, we comparatively studied charge injection in PE and PTFE oligomers using first-principles calculations. Two different models were studied: chemisorption (bonding) and physisorption (nonbonding). The results show that the electron injection barrier of the metal/PTFE interface is larger than that of the metal/PE interface only in the case of chemisorption. The larger electron injection barrier of the metal/PTFE oligomer interface is mainly affected by the positive vacuum level shift of the metal/PTFE interface induced by electron transfer from the metal to PTFE along the chemical bonds. In the case of physisorption, the hole injection barrier of the metal/PTFE interface is larger than that of the metal/PE interface. This is attributed to the larger ionization potential of PTFE compared to PE. The calculated results reasonably explain the experimental phenomena. The agreement between the experimental and calculated results verifies the rationality of our calculation models. The models used herein can likely be applied in other metal/polymer interfacial systems with acceptable accuracy.

Interface Electrostatics of Solid-State Dye-Sensitized Solar Cells: A Joint Drift-Diffusion and Density Functional Theory Study

Dye-sensitized solar cells (DSCs) have gained great attention in recent years due to their low-cost fabrication, flexibility and high power conversion efficiency. In a DSC, due to interfaces between the dye and the charge transport materials, the interface electrostatics becomes a key factor determining the overall performance of the cell. Liquid electrolyte based DSCs suffer low stability, electrolyte leakage and in some cases electrode corrosion. Replacing liquid electrolyte with a solid semiconducting material leads to poor interfacial contacts, hence the interface electrostatics becomes one of the limiting factors. In this work, we present a drift-diffusion (DD) and density functional theory (DFT) study of solid-state DSCs to investigate the electrostatics at the TiO2/organic dye/Spiro OMeTAD interface, and its impact to the adsorbed dye energy levels, its absorption spectrum and the related charge injection. In our 3D drift-diffusion model, we solve a set of drift-diffusion equations coupled to Poisson equation for electrons, holes, doping impurities and the interface traps simultaneously. After that, we use first principles DFT modeling of dye-sensitized interfaces in the presence of the calculated electric fields. We find that interface traps located below the conduction band edge of mesoporous TiO2 influence the accumulation of photogenerated holes and built-in electric field near the interface. The built-in electric field leads to change the energetics at the dye/TiO2 interface leading to poor charge injection from excited dye into the TiO2. The simulations were carried out for different electronic trap density in TiO2 and different doping levels in the Spiro OMeTAD hole transport layer. This study helps to a better understanding of interface electrostatics and its role in the charge injection mechanism of solid-state DSCs.

(Invited) Nanoporous Titaninum Nitride Electrodes for Neural Probes

We report corrosion-resistant and high-capacity implantable nanoporous titanium nitride (TiN) electrodes for neural probes applications Traditionally, the TiN electrodes are prepared using reactive sputtering techniques and have limited surface areas. To research smaller electrodes to minimize tissue damage, high aspect ratio TiN nanotube structures were fabricated using electrochemical anodization of Ti wires followed by high-temperature nitration. The specific charge capacity of nanoporous TiN correlates proportional with the surface area and pore size. According to Shannon criteria, an empirical rule in neural engineering for possibility of tissue damage from electrical stimulation, the recommended limit density of a stimulation pulse is 30 μCcm-2 for a geometric surface area of 0.06 cm2. Nevertheless, the charge injection capacity of chronically implanted electrodes has shown degraded over the time (within one month to a year), including sputtered iridium oxide (SIROF), porous platinum and tungsten. One of the challenges for these implantable electrodes involves irreversible reduction and oxidation reactions occurring at the electrode surface through faradic or pseudocapacitive charge transfer. On the other hand, TiN has different mechanism of charge injection (through capacitive double layers). TiN has been shown promising electrode material in neural implants thanks to its super electrical conductivity, biocompatibility and chemical stability. TiN is also known for physiologically inert and corrosion resistant. To increase the spatial resolution of neural stimulation, small electrodes with high surface areas are more desirable. To the best of our knowledge, TiN nanotubes electrodes have not been studied in the literature for neural implants. Compared to tungsten electrode, the impedance and morphology of nanoporous TiN was found stable over a long-term in stress tests (at an elevated temperatures in phosphorous buffered solution).

Plasma charge injection technology and its application to c-Si solar cells for field-effect passivation

A plasma charge injection technology applicable for field-effect passivation in crystalline silicon solar cells is discussed. The technology uses an inert-gas plasma (helium, argon, N2, etc.) as a charge source and a DC bias to extract desired charges from a remote plasma source. A charging model is proposed, and it gives a good guideline to determine a proper charging operation condition for a desired injection charge density. The technology can introduce electric charges into a nitride–oxide or oxide–nitride–oxide passivation stack in a few hundred milliseconds with a charge density equivalent to or higher than that of Al2O3 in a range of 5 × 1012 to 8 × 1012 cm−2. Most-like charge injection mechanisms are discussed for both negative and positive charging. This technology uses a cheap inert-gas plasma which does not cause any parasitic film deposition nor any corrosion inside the chamber during the charging operation and, thus, does not require regular maintenance for chamber cleaning, which leads to a very low cost of ownership. This charging technology is promising for a low-cost alternative to the complex Al2O3 technology. This plasma charge injection can be another important application of the plasma technology among other well-known applications such as dry etching, thin-film deposition, sputtering, etc.

Using Chronopotentiometry to Better Characterize the Charge Injection Mechanisms of Platinum Electrodes Used in Bionic Devices.

The safe charge injection capacity and charge density of neural stimulating electrodes is based on empirical evidence obtained from stimulating feline cortices. Stimulation induced tissue damage may be caused by electrochemical or biological mechanisms. Separating these mechanisms requires greater understanding of charge transfer at the electrode-tissue interface. Clinical devices typically use a biphasic waveform with controlled current. Therefore, the charge injection mechanism and charge injection capacity of platinum was assessed on a commercial potentiostat by chronopotentiometry (controlled current stimulation). Platinum is a non-ideal electrode, charge injection by chronopotentiometry can be passed via capacitive and Faradaic mechanisms. Electrodes were tested under a variety of conditions to assess the impact on charge injection capacity. The change in electrode potential (charge injection capacity) was affected by applied charge density, pulse length, pulse polarity, electrode size, polishing method, electrolyte composition, and oxygen concentration. The safe charge injection capacity and charge density could be increased by changing the electrode-solution composition and stimulation parameters. However, certain conditions (e.g., acid polished electrodes) allowed the electrode to exceed the water electrolysis potential despite the stimulation protocol being deemed safe according to the Shannon plot. Multiple current pulses led to a shift or ratcheting in electrode potential due to changes in the electrode-solution composition. An accurate measure of safe charge injection capacity and charge density of an implantable electrode can only be obtained from suitable conditions (an appropriately degassed electrolyte and clinically relevant electrode structure). Cyclic voltammetric measurement of charge storage capacity can be performed on implantable electrodes, but will not provide information on electrode stability to multiple chronopotentiometric pulses. In contrast, chronopotentiometry will provide details on electrode stability, but the minimum time resolution of typical commercial potentiostats (ms range) is greater than used in a clinical stimulator (μs range) so that extrapolation to short stimulation pulses is required. Finally, an impedance test is typically used to assess clinical electrode performance. The impedance test is also based on a biphasic chronopotentiometic waveform where the measured potential is used to calculate an impedance value. Here it is shown that the measured potential is a function of many parameters (solution composition, electrode area, and surface composition). Subsequently, impedance test results allow electrode comparison and to indicate electrode failure, but use of Ohm’s law to calculate an impedance value is not valid.

Photosensitivity and charge injection dynamics of pentacene based thin-film transistors: influence of substrate temperature

In this work, we have performed an in-depth analysis to investigate the effect of substrate temperature on the molecular packing arrangement and energy levels of pentacene films. We have also explored their influence on the charge injection mechanism and photosensing behaviour of pentacene-based organic field-effect transistors (OFETs). In this study, we find the contact resistance and photosensitivity of the devices to be severely influenced by the active layer processing condition owing to the aforementioned structural and energy level modifications. Contact resistance of the devices at metal-semiconductor interfaces was observed to be reduced significantly upon increase in the substrate temperature; however, above a certain temperature, formation of pentacene thin-films was severely affected and no transistor characteristics were obtained afterwards. Detailed experimental analysis and theoretical investigations revealed that the processing temperature could strongly influence the grain structure and unit cell volume of the pentacene molecules, which consequently enhanced the carrier injection across the interface through a control over carrier mobility and the distribution of electronic states in the proximity of Fermi energy. Furthermore, our study demonstrates the role of substrate temperature in effectively enhancing the photosensitivity of the transistors. This report thus represents a step forward towards understanding a correlation between the processing temperature and the dynamics of charge injection in pentacene based organic transistors. The results also illustrate the viability of using proper substrate temperature to achieve an efficient photosensitivity from OFET devices. • Effect of substrate temperature on the charge injection dynamics is investigated. • Substrate temperature induced structural and energy level modification of pentacene is explored. • Delocalization of electronic states strongly influenced the interfacial charge injection. • High substrate temperature processed devices exhibited excellent photo sensitivity.

Current Transport Mechanism in p-InSb–n-CdTe Heterojunction

Based on the analysis of the forward current-voltage characteristics of the p-InSb–n-CdTe heterojunction fabricated by a method of pulsed laser deposition, the existence of two charge injection mechanisms was experimentally confirmed. At relatively small external bias voltages (0.03 V < U < 0.15 V), the current is in a satisfactory agreement with the expression I ~ exp(qU/ηkT) with the ideality factor η = 1. Above 0.18 V, the current-voltage characteristic obeys the law I ~ U3/2 followed by an attainment to the linear section (the cut-off voltage of current is 0.47 V). A theoretical model of current transport is given taking into account the peculiarities of the heterojunction band discontinuity, leading to the origination of inversion layer near the interface.

On the injection and generation of charge carriers in mineral oil under high electric fields

Charge injection and generation mechanisms under intense electric fields (up to 109 V m−1 ) in mineral oil are assessed experimentally and numerically. For this, current-voltage characteristics under positive and negative polarities are measured in a needle–plane configuration using sharp needles (with tip radius Rtip ≤ 1.1 μm). In addition, a state of the art electro-hydrodynamic (EHD) model is implemented to calculate the contribution of the different mechanisms on the high-field conduction currents in the liquid. In order to evaluate exclusively the contribution of field emission, experiments are also performed in vacuum. It is found that neither field emission nor field ionisation can explain the conduction currents measured in mineral oil. It is proposed that field molecular ionisation, as described by Zener tunnelling model for solids, and electron impact ionisation are the processes dominating the generation of excess electron-ion pairs in mineral oil under positive and negative polarity, respectively. It is also shown that Zener molecular ionisation alone grossly overestimates the measured currents when parameters previously suggested in the literature for mineral oil are used. Preliminary model parameters for these mechanisms that best fit the conduction currents measured in mineral oil are presented and discussed.

Negative contribution to the reverse bias capacitance of organic diodes due to field dependent mobility: Determination of barrier height and transport parameters

We analyse the capacitance voltage characteristics of a small molecule-based single carrier diode consisting of a p-doped/intrinsic layer interface using m-MTDATA. In such homojunction diodes, in which the intrinsic layer thickness varies between 10 and 50 nm, the capacitance in deep reverse bias falls nearly exponentially and goes below the geometrical capacitance (Cg) beyond a critical electric field. We mainly focus on this reduction of the capacitance, which is interpreted based on charge injection mechanism in reverse bias. The small signal capacitance in such cases is shown to have negative contribution which is directly related to the delay time introduced by the charge transport. Thus, the capacitance characteristics have been modelled in deep reverse bias using the charge transport mechanisms, and the barrier height so estimated is in excellent agreement with the values calculated from current density-voltage (J-V) characteristics. The technique also allows the determination of mobility, which in turn yields the disorder parameters through its temperature and field dependence.

Organic–Inorganic Heterojunctions toward High‐Performance Ambipolar Field‐Effect Transistor Applications

AbstractThis work reports on combination of organic–inorganic heterojunctions between amorphous indium–gallium–zinc oxide (a‐IGZO) and organic semiconductors for design of high‐performance ambipolar transistors. A vertically sequential layer device configuration that the organic small molecule dinaphtho‐thieno‐thiophene (DNTT) and dioctylbenzothieno[2,3‐b]benzothiophene (C8‐BTBT) are directly vacuum deposited on the surface of a‐IGZO without any interface modification is employed in ambipolar transistors. The ambipolar transistors based on C8‐BTBT/a‐IGZO featured with V‐shaped transfer curves exhibit an outstanding electrical performance with mobilities as high as 5.1 and 4.5 cm2 V−1 s−1 for electrons and holes, respectively. The formation of N‐type channel even if covered with several tens of nanometers thick small molecule film is clarified with the charge injection mechanisms based on both thermionic injection and/or tunneling transport processes. High‐performance ambipolar inverter with extremely large gain of 124 V/V is fabricated based on the C8‐BTBT/a‐IGZO ambipolar transistors. Moreover, a single‐transistor frequency doubler shows high spectral purity with 70% of the output energy at the doubling frequency of 2 kHz. The present work provides a strategy for manufacturing high‐performance ambipolar transistor with straightforward processing approaches, which may help deepen the understanding of ambipolar channel's working mechanisms and optimize the design procedures of logic electrical components.

Low-Voltage Solution-Processed Hybrid Light-Emitting Transistors.

We report the development of low operating voltages in inorganic-organic hybrid light-emitting transistors (HLETs) based on a solution-processed ZrO x gate dielectric and a hybrid multilayer channel consisting of the heterojunction In2O3/ZnO and the organic polymer "Super Yellow" acting as n- and p-channel/emissive layers, respectively. Resulting HLETs operate at the lowest voltages reported to-date (<10 V) and combine high electron mobility (22 cm2/(V s)) with appreciable current on/off ratios (≈103) and an external quantum efficiency of 2 × 10-2% at 700 cd/m2. The charge injection, transport, and recombination mechanisms within this HLET architecture are discussed, and prospects for further performance enhancement are considered.

Measuring the effective area and charge density of platinum electrodes for bionic devices

Objective. Neural stimulation is usually performed with fairly large platinum electrodes. Smaller electrodes increase the applied charge density, potentially damaging the electrode. Greater understanding of the charge injection mechanism is required for safe neural stimulation. Approach. The charge injection mechanism and charge injection capacity were measured by cyclic voltammetry. Electrodes were cleaned mechanically or by potential cycling in acidic solutions. The effective electrode area was measured by hydrogen adsorption or reduction of . Main results. The water window and safe potential window were affected by changes to electrolyte, electrode size, polishing method and oxygen concentration. Capacitance and Faradaic current contribute to the charge injection capacity. Varying voltammetric scan rate (measurement time), electrode size, polishing method, potential window, electrolyte and oxygen concentration affected the charge injection capacity and ratio of oxidation to reduction charge. Hydrogen adsorption in acidic solutions provided an inaccurate effective electrode area. Reduction of a solution phase redox species with a linear or radial diffusion profile could provide an effective electrode area. The charge density (charge injection capacity divided by electrode area) of a platinum electrode is dependent on the charge injection capacity and electrode area measurement technique. By varying cyclic voltammetric conditions, the charge density of platinum ranged from 0.15 to 5.57 mC cm−2. Significance. The safe potential window, charge injection mechanism, charge injection capacity and charge density of platinum depends on electrolyte, size of the electrode, oxygen concentration and differences in electrode polishing method. The oxidation and reduction charge injection capacities are not equal. Careful control of a platinum electrodes surface may allow larger charge densities and safe use of smaller electrodes. New electrode materials and geometries should be tested in a consistent manner to allow comparison of potential suitability for neural stimulation.

Multifunctional hybrid diode: Study of photoresponse, high responsivity, and charge injection mechanisms

A multifunctional hybrid heterojunction diode is developed on porous silicon and its current density-voltage characteristics reveal a good rectification ratio along with other superior parameters such as ideality factor, barrier height and series resistance. The diode also functions as an efficient photodiode to manifest high photosensitivity with high responsivity under illumination with broadband solar light, UV light, and green light. The diode is also carefully scrutinized for its sensitivity and repeatability over many cycles under UV and green light and is found to have a quick response and extremely fast recovery times. The notable responsivity is attributed to the generation of high density of excitons in the depletion region by the absorption of incident photons and their separation by an internal electric field besides an additional photocurrent due to the charging of polymer chains. The mechanisms of generation, injection and transport of charge carriers are explained by developing a schematic energy band diagram. The transport phenomenon of carriers is further investigated from room temperature down to a very low temperature of 10 K. An Arrhenius plot is made to determine the Richardson constant. Various diode parameters as mentioned above are also determined and the dominance of the transport mechanism of charge carriers in different temperature regimes such as diffusion across the junction and/or quantum tunneling through the barriers are explained. The developed multifunction heterojunction hybrid diodes have implications for highly sensitive photodiodes in the UV and visible range of electromagnetic spectrum that can be very promising for efficient optoelectronic devices.