Trap-charge Limited Conduction Research Articles

Electrical charge transport properties of caesium tin chloride perovskite microrods: An analysis of microstructure conductivity and charge trapping

This study presents the electrical charge transport properties of caesium tin chloride (CsSnCl3) perovskite microrods. The as-synthesized microstructure rods were characterized using XRD, FESEM, EDS, and elemental mapping. The current–voltage (I-V) characteristics of microstructures were recorded. The I-V analysis demonstrates that as the side length of a microrod varies from 356 µm to 136 µm, the conductivity, barrier height, and ideality factor changes from 3.08 × 10−4 to 1.32 × 10−4 S/m, 0.84 eV to 0.78 eV, and 2.09 to 2.86, respectively. The charge transport investigation revealed that the sample exhibited ohmic conduction at a lower voltage and trap charge-limited conduction (TCL) after a transition voltage. The discrete trap charge concentration was found to be in the order of 1014 /m3 in the TCL region. The excellent conductivity of microstructure rods and the low trap charge concentration suggest that microrods have the potential for use in optoelectronic device applications.

Effect of graphene concentration on performance of MEH:PPV/graphene nanocomposite based devices

Herein, we explore the effect of graphene concentration on performance of poly [2-methoxy-5-2-ethylhexoxy-p-phenylene vinylene] (MEH:PPV)/graphene nanocomposite based devices. The surface morphology of the nanocomposites analyzed through emission scanning electron microscopy suggest that increase in graphene concentration results in the formation of aggregation. Optical and structural properties of the nanocomposites examined through UV–Vis absorption and Raman spectra revealed that the addition of graphene has no affect on the conjugation length and structure of the MEH:PPV. The electrical characteristics of devices have been investigated by I–V measurement under dark and illumination at room temperature. The devices show increase in the current value and reduction on turn-on voltage with increase in the graphene concentration up to 5 wt%. However, increase in graphene concentration above 5 wt% leads to the performance degradation of the devices. Moreover, charge transport mechanisms of the devices have been explained through Ohmic behavior at lower voltages and trap charge limited conduction at higher voltages.

Investigation of the optical and electrical characteristics of solution-processed poly (3 hexylthiophene) (P3HT): multiwall carbon nanotube (MWCNT) composite-based devices

Devices comprised of solution-processed poly (3-hexylthiophene) (P3HT)/multiwall carbon nanotubes (MWCNTs), with various concentrations of MWCNTs, were fabricated and characterized. The morphology of the P3HT: MWCNT nanocomposite was characterized by using field emission scanning electron microscopy (FESEM). The optical characteristics of the nanocomposite were studied by UV/VIS/NIR spectroscopy and Raman spectroscopy. The electrical properties of the fabricated devices were characterized by measuring the current density–voltage (J–V) characteristics. While the J–V characteristics of a pristine P3HT device reveal thermal injection limited charge transport, the P3HT: MWCNT nanocomposite-based devices exhibit three distinct voltage-dependent conduction regimes. The fitting curve with measured data reveals Ohmic conduction for a low voltage range, a trap-charge limited conduction (TCLC) process at an intermediate voltage range followed by a trap free space-charge limited conduction (SCLC) process at much higher voltages. A fundamental understanding of this work can assist in creating new charge transport pathways which will provide new avenues for the development of highly efficient polymer-based optoelectronic devices.

Fabrication and electrical characterizations of graphene nanocomposite thin film based heterojunction diode

The use of graphene in electronic devices is becoming attractive due to its inherent scalability and is thus well suited for flexible electronic devices. Here we present the electrical characterization of heterojunction diode, based on the nanocomposite of graphene (G) with silver nanoparticles (Ag NPs), at room temperature. The diode was fabricated by depositing nanocomposite on the n-Si substrate. The current – voltage (I – V) characteristic of the fabricated junction shows rectifying behavior similar to a Schottky junction. The junction parameters such as ideality factor (n), series resistance (Rs), and barrier height (ϕb) has been extracted, using various methods, from the experimentally obtained I – V data. The measured values of n, Rs and ϕb are 3.86, 45Ω and 0.367eV, respectively, as calculated from the I – V curve. The numerical values of these parameters calculated by different methods are in good agreement with each other showing the consistency of the applied calculating techniques. The conduction mechanism of the fabricated diode seems to have been dominated by the Trap Charge Limited Conduction (TCLC) behavior. The energy distribution of interface states density determined from forward bias I – V characteristic shows an exponential decrease with bias from 27 × 1013cm−2eV−1 at (Ec – 0.345)eV to 3 × 1013cm−2eV−1at (Ec – 0.398)eV.

Low voltage resistive memory devices based on graphene oxide–iron oxide hybrid

Low cost resistive switching memory devices using graphene oxide–iron oxide (GF) hybrid thin films, sandwiched between platinum (Pt) and indium-tin-oxide (ITO) electrodes, were demonstrated. The fabricated devices with Pt/GF/ITO structure exhibited reliable and reproducible bipolar resistive switching performance, with an ON/OFF current ratio of 5×103, excellent retention time longer than 105s, SET voltage of 0.9V, and good endurance properties. In all aspects of the device characteristics, the GF based devices outperformed graphene oxide (GO) based devices. Ohmic conduction was found to be dominant current conduction mechanism in all switching regions except for the high voltage regime where space charge limited conduction and trap charge limited conduction were found to be the main current conduction mechanism. X-ray photoelectron spectroscopy and transmission electron microscopy/selected area diffraction analysis revealed γ-Fe2O3 and Fe3O4 iron oxide phases coexist in the hybrid films. While the desorption/adsorption of oxygen-related functional groups on the GO sheets is the dominant resistive switching mechanism in Pt/GO/ITO devices, the formation/rupture of multiple highly conducting Fe3O4 filaments at the iron oxide/GO interface additionally facilitate the switching in the present Pt/GF/ITO devices. Thereby, excellent electrical switching performance was achieved.

Effect of Cu doping on the resistive switching of NiO thin films

Bipolar resistive switching is observed in the GaIn/Cu:NiO film/ITO device with active layer deposited by sol-gel spin-coating. The first-principles calculations indicate that Cu dopants with valence of +1 are located at the substitutional Ni sites rather than the interstitial ones. Cu doping introduces more oxygen vacancies in the film and increases the carrier mobility, however, excessive Cu dopants may assemble at the grain boundary resulting in larger set voltage. Current–voltage measurements indicate that the trap charge limited and space charge limited conduction dominate the high resistance state, while the low resistance state follows the Ohmic mechanism. The switching is attributed to the formation/rupture of oxygen vacancy filaments.

Analysis of device parameters of Al/In2O3/p-Si Schottky diode

Thin film of indium oxide (In2O3) is deposited on p-silicon substrate using sol-gel spin technique. The sol-gel spin deposited film is very smooth with grain size and root mean square surface roughness of ~40nm and ~7.9nm, respectively. The device parameters of Al/In2O3/p-Si Schottky diode were investigated using direct current current-voltage (I-V) and impedance spectroscopy. The ideality factor and barrier height of the diode were determined to be 1.07+/-0.03 and 0.72+/-0.02eV, respectively. At higher voltages, the charge transport mechanism of the diode is controlled by a trap-charge limited conduction mechanism (TCLC). The series resistance profile of the diode was extracted to show the presence of the interface states changing with frequency.

Thickness dependent trap states and mobility in 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole

Electron transport in 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole is investigated as a function of temperature and organic layer thickness. Conduction mechanism has been found to be affected by trap states. The current density-voltage characteristics have been found to follow the trap charge limited conduction model. The density of trap states and trap energy has been found to be dependent on sample thickness. As the thickness has increased from 100 nm to 150 nm, trap energy has correspondingly increased from 80 meV to 135 meV and density of trap states has been found to decrease from 1.6 × 1018 to 3.5 × 1017 per cm3. Mobility has been found to be increasing with the decrease in thickness.

Spectral–optical–electrical–thermal properties of deposited thin films of nano-sized calcium(II)-8-hydroxy-5,7-dinitroquinolate complex

Spectral–optical–electrical–thermal properties of deposited thin films of nano-sized calcium(II)-8-hydroxy-5,7-dinitroquinolate complex, Ca[((NO 2) 2-8HQ) 2], were explored, studied and evaluated in this work. Thin films of Ca[((NO 2) 2-8HQ) 2] were assembled by using a direct, simple and efficient layer-by-layer (LBL) chemical deposition technique. The optical properties of thin films were investigated by using spectrophotometric measurements of transmittance and reflectance at normal incidence in the wavelength range 200–2500 nm. The refractive index, n, and the absorption index, k, of Ca[((NO 2) 2-8HQ) 2] films were determined from the measured transmittance and reflectance. The real and imaginary dielectric constants were also determined. The analysis of the spectral behavior of the absorption coefficient in the intrinsic absorption region reveals a direct allowed transition with band gaps of 1.1 eV and 2.4 eV for the optical and transport energy gaps, respectively. The current–voltage characteristics of Ca[((NO 2) 2-8HQ) 2] showed a trap-charge limited conduction in determining the current at the intermediate and high bias regimes. Graphical representation of the current–voltage characteristics yields three distinct linear parts indicating the existence of three conduction mechanisms. Structural characterization and identification were confirmed by using Fourier transform infrared spectroscopy (FT-IR). Scanning electron microscopy (SEM) was also used to image the surface morphology of the deposited nano-sized metal complex and such study revealed a high homogeneity in surface spherical particle distribution with average particles size in the range 20–40 nm. Thermal gravimetric analysis (TGA) was also studied for [(NO 2) 2-8HQ] and Ca[((NO 2) 2-8HQ) 2] to evaluate and confirm the thermal stability characteristics incorporated into the synthesized nano-sized Ca[((NO 2) 2-8HQ) 2] complex.

Effect of doping of 8-hydroxyquinolinatolithium on electron transport in tris(8-hydroxyquinolinato)aluminum

Effect of doping of 8-hydroxyquinolinatolithium (Liq) on the electron transport properties of tris(8-hydroxyquinolinato)aluminum (Alq3) has been investigated as a function of temperature and doping concentration by fabricating electron only devices. It has been observed that current density in the devices increases with the doping of Liq up to a doping concentration of 33 wt. % and then decreases. Current density-voltage (J-V) characteristics of 0, 15, and 33 wt. % Liq doped Alq3 devices were found to be bulk limited and analyzed on the basis of trap charge limited conduction model. The J-V characteristics of 50 and 100 wt. % Liq doped Alq3 devices were found to be injection limited and were analyzed using the Fowler-Nordheim model. The increase in current density with doping up to 33 wt. % was found to be due to an increase in electron mobility upon doping, whereas the decrease in current density above 33 wt. % was due to the switching of transport mechanism from bulk limited to injection limited type due to an increase in barrier height. Electron mobility and variance of energy distribution have been measured by using transient electroluminescence technique to support our analysis. Electron mobility for pure Alq3 was found to be 1 × 10−6 cm2/V s, which increased to 3 × 10−5 cm2/V s upon doping with 33 wt. % Liq. The measured values of variance were 95, 87.5, 80, 72, and 65 meV for 0, 15, 33, 50, and 100 wt. % Liq doped Alq3 respectively. The increase in electron mobility upon doping has been attributed to a decrease in energetic disorder upon doping as evidenced by the decrease in variance. The increase in barrier height for the higher doping concentration was due to the disorder related correction σ2/2kT in the barrier height, which decreases with the increase in doping concentration.

Field, temperature and thickness dependent electron transport in 5,5′-(2,6-di-tert-butylanthracene-9,10-diyl)bis(2-p-tolyl-1,3,4-oxadiazole)

Charge transport in 5,5′-(2,6-di-tert-butylanthracene-9,10-diyl)bis(2-p-tolyl-1,3,4-oxadiazole) is investigated as a function of temperature and organic layer thickness. The thickness dependence of the current indicates towards the trap charge limited conduction (TCLC) with a field and temperature dependent mobility. The density of trap states has been found to be dependent on sample thickness. As the thickness has increased from 80 nm to 120 nm, trap energy has correspondingly increased from 78 meV to 130 meV. TCLC model with Poole Frenkel type field dependent mobility has been fitted into the data and has been found in excellent agreement. Temperature dependency of zero field mobility ( μ 0) and β has been estimated from the model.

Current–voltage characteristics of Al/Rhodamine-101/ n-GaAs structures in the wide temperature range

The forward bias current–voltage ( I–V) characteristics of Al/Rhodamine-101/ n-GaAs structure have been investigated in the temperature range of 80–350 K. It has been seen a decrease in ideality factor ( n) and an increase in the zero-bias barrier height (BH) with an increase in temperature. It has been seen that such a behavior of the BH and n obey Gaussian distribution of the BHs due to the BH inhomogeneities at the metal/semiconductor (MS) interface. The very strong temperature dependence of ideality factor of the structure has shown that the current processes occurring in the organic layer at the MS interface would be a possible candidate such as trap-charge limited conduction in determining the current at the intermediate and high bias regimes. Furthermore, it has been show that the Rh101 can be used to vary effective BHs for the metal/GaAs Schottky diodes. As a result, it has been determined that the BH value for conventional Al/ n-GaAs SBD is remarkably higher than our own values of 0.68 eV obtained for the Al/Rh101/ n-GaAs at 290 K.

Electron injection and transport mechanism in organic devices based on electron transport materials

Electron injection and transport in organic devices based on electron transport (ET) materials, such as 4,7- diphyenyl-1,10-phenanthroline (Bathophenanthroline BPhen), 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (Bathocuproine BCP) and bipyridyl oxadiazole compound 1,3-bis [2-(2,2′-bipyridin-6-yl)-1,3,4-oxadiazol-5-yl]benzene (Bpy-OXD), have been reported. The devices are composed of ITO/ET materials (BPhen, BCP Bpy-OXD)/cathodes, where cathodes = Au, Al and Ca. Current–voltage characteristics of each ET material are performed as a function of cathodes. We have found that Ca and Al exhibit quite different J–V characteristics compared with the gold (Au) cathode. The current is more than one order of magnitude higher for the Al cathode and more than three orders of magnitude higher for Ca compared with that of the Au cathode at ∼8 V for all ET materials. This is because of the relatively low energy barrier at the organic/metal interface for Ca and Al cathodes. Electron-only devices with the Au cathode show that the electron transfer limitation is located at the organic/cathode interface and the Fowler–Nordheim mechanism is qualitatively consistent with experimental data at high voltages. With Ca and Al cathodes, electron conduction is preponderant and is bulk limited. A power law dependence J ∼ Vm with m > 2 is consistent with the model of trap-charge limited conduction. The total electron trap density is estimated to be ∼5 × 1018 cm−3. The critical voltage (Vc) is found to be ∼45 V and is almost independent of the materials.

Injection and transport processes in organic light emitting diodes based on a silole derivative

This paper reports on charge injection and transport in electroluminescent devices based on a silole derivative 1,1-dimethyl-2,5-bis(p-2,2′-dipyridylaminophenyl)-3,4-diphenylsilole (DMPPS). The devices are composed of tin doped indium oxide (In2O3:Sn or I TO)/poly(3,4-ethylene dioxythiophene doped with poly(styrene sulfonate)/DMPPS/metal. Current-voltage and luminance-voltage characteristics are first performed as a function of the electron injection barrier height and of the organic layer thickness. The voltage dependence of current and luminance varies with the metal cathode, i.e., Ca, Al, Cu, and Au. The effect of the DMPPS thickness in a double carrier device shows that electrons predominate and are bulk limited. An accurate investigation is carried out as a function of temperature for hole-only and bipolar devices, i.e., with gold and calcium cathodes. Hole-only devices (with Au cathode) exhibit an Ohmic behavior for low voltages. A hopping mechanism (thermally assisted tunnel transfer between localized states) agrees with experimental data, since activation energy is found close to 50meV. The electron transfer limitation is located at the DMPPS/cathode interface and the Fowler-Nordheim mechanism is qualitatively consistent with experimental data at high voltages. With a Ca cathode, electron conduction is preponderant and is bulk limited. A power dependence J∝Vm with m>2 is consistent with the model of trap charge limited conduction. The total electron trap density was estimated to be 2×1018cm−3.

Green OLED's: Electroluminescence and the electrical carrier transport

AbstractOrganic Light Emitting Diodes (OLED) based on metal chelate complexes (Alq3) as the emmiting layer has been prepareded and studied in order to characterise their optical and electrical properties. Electrical measurements have been performed in order to establish the nature of carrier transport. The results point to a trap charge-limited conduction (TCL) with characteristic trap energy near 0.13 eV. In electroluminescence a large green band with a maximum at 2.4 eV was found as a result of three overlapping bands that are correlated with the organic layers. The origin of this luminescence spectrum was analysed.

Device physics of organic light-emitting diodes based on molecular materials

Abstract Electrical transport in single- and hetero-layer organic light-emitting diodes based on aromatic amines like N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) or N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) and the aluminium chelate complex Alq (tris(8-hydroxyquinolato)aluminium) has been investigated as a function of temperature and organic layer thickness. It is shown that the thickness dependence of the current–voltage (I–V) characteristics provides a unique criterion to discriminate between (1) injection limited behaviour, (2) trap-charge limited conduction with an exponential trap distribution and a field-independent mobility, and (3) trap-free space-charge limited conduction (SCLC) with a field and temperature dependent mobility. The I–V characteristics of NPB-based hole-only devices with indium–tin oxide anodes are neither purely injection nor purely space-charge limited, although the current shows a square-law dependence on the applied voltage. In Al/Alq/Ca electron-only devices with Alq thickness in the range 100–350 nm the observed thickness and temperature dependent I–V characteristics can be described by SCLC with a hopping-type charge carrier mobility. Additionally, trapping in energetically distributed trap states is involved at low voltages and for thick layers. The electric field and temperature dependence of the charge carrier mobility in Alq has been independently determined from transient electroluminescence. The obtained values of the electron mobility are consistent with temperature dependent I–V characteristics and can be described by both the phenomenological Poole–Frenkel model with a zero-field activation energy ΔE=0.4–0.5 eV and the Gaussian disorder model with a disorder parameter σ=100 meV. Measurements of the bias-dependent capacitance in NPB/Alq hetero-layer devices give clear evidence for the presence of negative charges with a density of about 6.8×10 11 cm −2 at the organic–organic interface under large reverse bias. This leads to a non-uniform electric field distribution in the hetero-layer device, which has to be considered in device description.

Space-charge limited conduction with a field and temperature dependent mobility in Alq light-emitting devices

Electrical transport in organic light-emitting devices (OLEDs) based on tris(8-hydroxyquinolato)aluminium (Alq) is investigated as a function of temperature and organic layer thickness. It is shown that the thickness dependence of the current provides a unique criterion to discriminate between (1) injection limited behavior, (2) trap-charge limited conduction with an exponential trap distribution and a field independent mobility, and (3) trap-free space charge limited conduction with a field and temperature dependent mobility. The observed thickness and temperature dependent current–voltage characteristics are found to be in excellent agreement with trap-free SCLC with a hopping type charge carrier mobility.

Chemical Aging, Charge Transport, and Electroluminescence in Alq3-based Organic Light-Emitting Diodes

Conduction in aluminum(III) 8-hydroxyquinoline (Alq3) was modeled based on trap-charge limited conduction of electrons in the bulk. The evolution of a narrow Gaussian distribution of localized trap states below the lowest unoccupied molecular orbital (LUMO) of Alq3, lying against a natural exponential background, was used to explain changes in the current-voltage characteristic and external quantum efficiency with time observed by many researchers for organic light-emitting diodes. Based on the change of the shape of the J-V curve, the depth of the electron trap states that were formed during aging was about 0.25 eV below the LUMO of Alq3. An increase in drive voltage and decrease in efficiency is predicted with aging by this model for current densities in a reasonable range, assuming that the evolved trap states are non-emissive and also non-quenching. The products of chemical aging can account for the generation of traps at the observed depth.

Doping effects in organic electroluminescent devices

Several possible doping effects in organic electroluminescent devices are studied within the framework of the trap-charge limited conduction models. A significant increase in the trapped charges in the doped layer can cause an overall trap charge redistribution and a shift of the location of the recombination zone. Some experimental data are analyzed and their possible relevance to our predictions is discussed. An experimental verification scheme is also proposed.