Space Charge Limited Current Regime Research Articles

Charge transport and trap characterization in individual GaSb nanowires

Charge transport of unintentionally doped GaSb nanowires was studied through the fabrication and analysis of nanowire field effect transistors (FETs). In this work, both temperature dependent and voltage dependent measurements demonstrate various operating regimes, including a transition from linear current-voltage behavior at low bias to a space-charge limited current (SCLC) at large bias. Analysis of the voltage and temperature variation in the SCLC regime provided quantitative information about the trap energy distribution in the nanowires, which, after thermal annealing, has been shown to reduce from 0.26 eV to 0.12 eV. The measurements also indicate that the GaSb nanowire FETs exhibit n-type behavior, which is likely due to oxygen impurities in the nanowires.

Capacitance–voltage characteristics of P3HT:PCBM bulk heterojunction solar cells with ohmic contacts and the impact of single walled carbon nanotubes on them

Capacitance–voltage (C–V) characteristics of P3HT:PCBM devices of two different thicknesses are correlated with current density–voltage (J–V) characteristics. The rising portion of the C–V characteristics coincides with the exponential current density below the built-in voltage. The negative capacitance (NC) of these devices is a low frequency phenomenon and it occurs in trap-free space charge limited current (SCLC) regime. The onset frequencies of NC for devices with and without SWNTs also do not follow direct relation with effective mobility. The NC in thin devices has non-monotonic change with voltage for thin devices showing that interface state kinetics can be the reason for its occurrence. The NC of thick devices, on the other hand, increases monotonically with voltage showing that bulk properties dominate in these. Addition of SWNTs to these devices for efficiency improvement does not modify their built-in voltage. Also, the SWNTs do not affect the forward NC behaviour. However, the devices containing SWNTs show NC in reverse bias also which has different frequency dependence with voltage. The reverse bias NC is attributed to the large non-linear reverse current by charge injection into the additional energy levels introduced by SWNTs.

Reformulated space-charge-limited current model and its application to disordered organic systems

We have reformulated a traditional model used to describe the current-voltage dependence of low mobility materials sandwiched between planar electrodes by using the quasi-electrochemical potential as the fundamental variable instead of the local electric field or the local charge carrier density. This allows the material density-of-states to enter explicitly in the equations and dispenses with the need to assume a particular type of contact. The diffusion current is included and as a consequence the current-voltage dependence obtained covers, with increasing bias, the diffusion limited current, the space-charge limited current, and the injection limited current regimes. The generalized Einstein relation and the field and density dependent mobility are naturally incorporated into the formalism; these two points being of particular relevance for disordered organic semiconductors. The reformulated model can be applied to any material where the carrier density and the mobility may be written as a function of the quasi-electrochemical potential. We applied it to the textbook example of a nondegenerate, constant mobility material and showed how a single dimensionless parameter determines the form of the I(V) curve. We obtained integral expressions for the carrier density and for the mobility as a function of the quasi-electrochemical potential for a Gaussianly disordered organic material and found the general form of the I(V) curve for such materials over the full range of bias, showing how the energetic disorder alone can give rise, in the space-charge limited current regime, to an I∝V(n) dependence with an exponent n larger than 2.

Temperature and electric field dependent mobility in poly(3-hexylthiophene) diodes

Current-voltage (I-V) curves of poly(3-hexylthiophene) (P3HT) diodes have been collected to investigate the polymer hole-dominated charge transport. At room temperature and at low electric fields the I-V characteristic is purely Ohmic whereas at medium-high electric fields, experimental data shows that the hole transport is trap dominated in the space charge limited current (SCLC) regime. In this regime, it is possible to extract the I-V characteristic of the P3HT/Al junction showing the ideal Schottky diode behavior over five orders of magnitude. At high-applied electric fields, holes’ transport is found to be in the trap free SCLC regime. We have measured and modeled in this regime the holes’ mobility to evaluate its dependence from the electric field applied and the temperature of the device.

Transport and excess noise in polyacenes under trap filling transition

Recent measurements in organic semiconductors (polyacenes) have evidenced a strong superlinear increase of the current-voltage characteristics associated with a sharp peak of the relative spectral density of current noise. Both features occur at the trap filling transition (TFT), i.e. at voltages corresponding to the transition between the Ohmic regime and the space charge limited current (SCLC) regime. Here we discuss the interpretation of this behavior in terms of a contribution from trapping and detrapping (TD) processes of the injected carriers. We found an excellent agreement between the predictions of our model and the experimental results in tetracene, which allows us to conclude that TD processes are responsible for the significant enhancement of the relative current noise observed in polyacenes in the TFT region.

Study of molecular spin-crossover complex Fe(phen)2(NCS)2 thin films

We report on the growth by evaporation under high vacuum of high-quality thin films of Fe(phen)2(NCS)2 (phen=1,10-phenanthroline) that maintain the expected electronic structure down to a thickness of 10 nm and that exhibit a temperature-driven spin transition. We have investigated the current-voltage characteristics of a device based on such films. From the space charge-limited current regime, we deduce a mobility of 6.5×10−6 cm2/V s that is similar to the low-range mobility measured on the widely studied tris(8-hydroxyquinoline)aluminum organic semiconductor. This work paves the way for multifunctional molecular devices based on spin-crossover complexes.

Theory of space charge limited regime of thermionic energy converter with negative electron affinity emitter

A theory of the space charge limited output current regime of a vacuum thermionic energy conversion device (TEC) employing a negative electron affinity (NEA) diamond emitter electrode is derived. The theory is developed by assuming that the electrons behave as a collisionless gas and self-consistently solving Vlaslov’s equation and Poisson’s equation. Special cases of the theory are addressed. Calculations are performed to model a TEC with a nitrogen doped diamond emitter material under various conditions. It is shown that the NEA material outperforms a similar positive electron affinity material in terms of output power and efficiency because the NEA lowers the electrostatic boundary condition at the emitter and therefore mitigates the negative space charge effect.

Crystal structure and charge-transport properties of N-trimethyltriindole: Novel p-type organic semiconductor single crystals

We report on a new p-type organic semiconductor single crystal, 5,10,15-trimethyl-10,15-dihydro-5 H-diindolo[3,2- a:3’,2’- c]carbazole ( N-trimethyltriindole). This molecule crystallizes forming a highly ordered columnar structure in which stacked molecules are situated at two alternating distances (3.53 Å and 3.68 Å) along the column as determined by single crystal X-ray diffraction analysis. These short intermolecular distances between adjacent units, make this system an ideal candidate for charge-transport processes along the stacks. Relevant parameters for transport (i.e. internal reorganization energies, transfer integral) have been estimated by DFT calculations at a 6-311G(d,p)/B3LYP level of theory. As a double check for the transfer integral, the electronic band structure of a one-dimensional stack of molecules has been computed. The electronic properties of this material have been studied both theoretically and experimentally. Its HOMO value is found to coincide with Au work function ( Φ Au = 5.1 eV), thus low barriers are expected for hole injection from gold electrodes. The hole mobility of this material has been predicted theoretically considering a hopping-type mechanism for the charge-transport and determined experimentally at the space charge limited current (SCLC) regime of the current–voltage measurements. Both theoretical and experimental values are in good agreement. The high hole mobility ( μ min = 0.4 cm 2 V −1 s −1) of this material points towards its useful application in the organic electronics arena. N-Trimethyltriindole single crystals constitute an essential model to study transport properties of triindole-based materials and to design new derivatives with improved electronic performance.

Current-voltage characteristics of poly(3-hexylthiophene) diodes at room temperature

We report on the analysis of current-voltage characteristics of regioregular poly(3-hexylthiophene) diodes. Experimental curves were fitted to two models, to take into account at low-moderate electric fields Schottky behavior mixed with space charge limited current (SCLC) regime and, at higher fields, trap-free SCLC. The results provide a description of IV curves over five decades, along with the determination of zero field and effective mobility and the field dependence prefactor. Forward and reverse IV measurements highlighted the presence of shallow and deep localized states inside the band gap. The latter enhance the current over time and have been modeled as an inductorlike element.

The effect of adsorbates on the space–charge-limited current in single ZnO nanowires

We studied the influence of adsorbates on the space–charge-limited current (SCLC) inindividual ZnO nanowires through varying the bias voltage, laser illumination, and ambientpressure. In dark and air conditions, the free carriers were depleted by the surfaceadsorbates, and electrons injected from the electrode to the nanowire dominated theelectron transport properties. Under laser illumination, the current–voltage characteristicwas linear at low voltage and superlinear at high voltage, and the SCLC regime occurred athigh voltages due to the surface desorption. The time response of photoconductivity furtherrevealed the dynamic process of elimination of SCLC by desorption of oxygen molecules atthe ZnO nanowire surface.

Interpretation of capacitance spectra and transit times of single carrier space‐charge limited transport in organic layers with field‐dependent mobility

AbstractThe ac impedance characteristics of a single carrier with space‐charge limited current (SCLC) transport in organic layers with field‐dependent mobility is analyzed, indicating the similarities as well as the differences to the constant mobility case. The model provides capacitance spectra and transit times from different calculation methods, in relation to the electric field distribution in the SCLC regime. It is found that the low frequency capacitance lies in the range 3Cg/4 < Cif < Cg, with respect to the geometric capacitance Cg. An approximated expression for the variation of the transit time with applied bias is derived, in good agreement with exact calculations. Experimental results of capacitance spectra of hole‐only PPV devices indicate the validity of the SCLC model but also show a strong contribution of trapping at low frequencies that overlap the low‐frequency value of the SCLC capacitance. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Deep levels in silicon Schottky junctions with embedded arrays of β-FeSi2 nanocrystallites

Schottky contacts on p-type silicon, with embedded arrays of β-FeSi2 nanocrystallites, were studied by current-voltage (I-V), deep level transient spectroscopy (DLTS), and low-frequency noise measurements. Forward I-V characteristics on logarithmic scale indicate that space-charge limited current (SCLC) dominates the carrier transport in these diodes. From an analysis of the SCLC characteristics, we found that two arrays of β-FeSi2 nanocrystallites induce a trap level at 320meV above the valence band edge with concentration of 5×1014cm−3, which is in relatively good agreement with the trap detected by DLTS. By inserting ten arrays of β-FeSi2 nanocrystallites, two trap levels at 300 and 340meV with concentrations of 2.6×1014 and 1.1×1015cm−3, respectively, were found from the SCLC analysis. By increasing the number of the inserted arrays of β-FeSi2 nanocrystallites from 2 to 10, a trap at 402meV with concentration of 6.5×1015cm−3 has been detected by DLTS, failing to detect the second trap observed by SCLC measurements. The normalized power spectral density SI∕I2 vs V exhibits a peak at the transition voltage from the trap-filling regime to the SCLC regime, corresponding to a deep trap level. It is demonstrated that the noise technique is more sensitive than the SCLC and DLTS techniques in extracting information about the deep trap parameters.

DC and AC electrical characteristics of porous GaAs/p +–GaAs heterostructure

This paper reports on studying Au/porous GaAs/p +–GaAs heterostructure through current–voltage I ( V), conductance– and capacitance–frequency dependencies ( G ( f) and C ( f)). Diode parameters such as ideality factor and zero bias barrier energy have been calculated by using Schottky model corrected from the series resistance. The obtained ideality factor is generally high and it was explained by space charge limited current (SCLC) regime, characterised by the presence of single trapping level. A model based upon TFE tunnelling of carriers at reverse current was used to explain the non-saturation of reverse current after series resistance correction. The G ( f) characteristics exhibit the presence of three frequency regions. In the first region the conductance is independent of frequency. Moreover, in the second region the AC conductance increases with increasing frequency and approximately follows ω s dependence. These results were interpreted as the result of the transport of injected carrier via hopping mechanism involving defect. In the third region a peak conductance appeared after which the conductance decreased with increasing frequency. C ( f) measurements exhibit that the capacitance behaviour is typical of material with traps. From G ( f) and C ( f) measurement trap density and relaxation time were calculated.

Impedance spectroscopy study on transport properties of N, N′-diphenyl- N, N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine

The transport properties of N, N′-diphenyl- N, N′-bis(1-naphthyl)-1,1′-biphenhyl-4,4′-diamine (NPB) have been investigated by using impedance spectroscopy (IS) and J– V measurements based on a single-layer structure of ITO/NPB/Al. The J– V characteristic of the device obeys Ohmic law below 0.6 V, and is in the space-charge limited current (SCLC) regime above 0.6 V. The density of the thermally generated charge carrier is determined to be about 3.1×10 16 cm −3. Impedance spectroscopy measurements showed that the device can be simulated by an equivalent parallel RC network with a contact resistance R S in series, in which the bulk capacitance C of NPB remains unchanged while the resistance R decreases along with bias voltage. The variation of the whole conductance and capacitance of the device with frequency and bias can be understood by the carrier response and the transit time dependence on the bias in the NPB film.

Anisotropies in the electrical properties of rod-like aggregates of liquid crystalline phthalocyanines: Direct current conductivities and field-effect mobilities

The direct current (dc) conductivities and organic field-effect transistor (OFET) characteristics of a class of octa-substituted liquid crystalline (discotic mesophase) phthalocyanines (Pcs) are discussed. These molecules self-organize into columnar aggregates with large coherence lengths (up to approximately 300 nm). Langmuir–Blodgett films of these molecules were horizontally transferred to either interdigitated microelectrodes (IME) or OFET substrates, so that current flow could be measured either parallel or perpendicular to the column axis. Twenty-eight bilayer films of these Pcs on the IME substrates showed anisotropies in dc conductivity up to 50:1, whereas similar Pc films showed anisotropies in field effect mobilities of approximately 10:1, for a variety of W/L ratios (source/drain dimensions and spacing). Field-effect mobilities of 1 to 5 × 10-6 cm2·V-1·s-1 were determined from OFET measurements, along the Pc column axis, whereas charge mobilities measured from the space charge limited current regime on the IME substrates were in the range of 10-4 cm2·V-1·s-1. Conductive tip atomic force microscopy measurements on the apprximately 500-nm length scale showed that the conductivity anisotropy can be as high as 1000:1 when the Pc columns are intimately contacted to an adjacent Au bond pad.

Gated nonlinear transport in organic polymer field effect transistors

We measure hole transport in poly(3-hexylthiophene) field effect transistors with channel lengths from 3 μm down to 200 nm, from room temperature down to 10 K. Near room temperature effective mobilities inferred from linear regime transconductance are strongly dependent on temperature, gate voltage, and source–drain voltage. As T is reduced below 200 K and at high source–drain bias, we find transport becomes highly nonlinear and is very strongly modulated by the gate. We consider whether this nonlinear transport is contact limited or a bulk process by examining the length dependence of linear conduction to extract contact and channel contributions to the source–drain resistance. The results indicate that these devices are bulk limited at room temperature, and remain so as the temperature is lowered. The nonlinear conduction is consistent with a model of Poole–Frenkel-like hopping mechanism in the space-charge limited current regime. Further analysis within this model reveals consistency with a strongly energy dependent density of (localized) valence band states, and a crossover from thermally activated to nonthermal hopping below 30 K.

A comparative study of the I– V characteristics of diodes fabricated on as-grown and thermochemically polished CVD diamond films

Energies ranging between 24 and 50 keV, inclusively, are used for boron ion implantation of as-grown and thermochemically polished CVD diamond films at a total dose of 10 16 cm −2 in order to achieve p-type conductivity. N-type conductivity is acquired by lithium ion implantation of the films by a single energy of 50 keV at a dose of 2×10 16 cm −2. Current–voltage ( I– V) characteristics reveal that leakage currents of diodes fabricated on as-grown films are approximately 2 orders of magnitude higher that those on polished films. Moreover, currents at higher voltages as less in the diodes of as-grown films in comparison to identical diodes on the polished films. The I– V characteristics of diodes on the polished films contain the trap filling, Frenkel-pole emission, thermionic emission and the space charge limited current regimes but those on the as-grown films manifest a quasi-linear behavior probably due to lateral penetration of implants in the i-regions. A reduction in current of approximately 2 orders of magnitude is observed on n–i–n structures after annealing the films at 300 °C. Onset voltages of <10 V are due to the low concentration (∼10 14 cm −3) of nitrogen centers in the diamond films.

Gated Nonlinear Transport in Organic Thin Film Transistors

AbstractCharge transport in poly(3-hexylthiophene) field effect transistors has been studied in a series of devices with channel lengths ranging from 3 μm down to 200 nm over a broad range of temperatures and gate voltages. We report gate-modulated highly nonlinear transport at temperatures below ∼200 K that is consistent in form with a Poole-Frenkel-like hopping mechanism in the space charge limited current regime. There is also consistency between this behavior and the hypothesis that density of localized states is strongly energy dependent. We also observe what appears to be a crossover from thermally activated to nonthermal hopping below 30 K.

Double injection of charge carriers in chemical vapor deposited diamond-based diodes

Boron ion implantation of thermochemically polished chemical vapor deposited diamond films with multienergies ranging between 24 and 150 keV and a total dose of about 2×1016 cm−2 is used to get p-type conductivity. n-type conductivity is achieved by lithium ion implantation of the diamond films with a single energy of 50 keV and a dose of 2×1016 cm−2. The intrinsic areas separating the p- and n-doped regions form the active areas of the diodes into which charge carriers are simultaneously injected upon bias. Onset voltages of &amp;lt;10 V are due to the low concentration (∼1014 cm−3) of nitrogen related centers in the diamond films. The current–voltage (I–V) characteristics of the p-i-n diodes manifest trap filling, Frenkel–Poole emission, thermionic emission, and space charge limited current regimes. At 300 °C, currents as high as 12 mA are measured at a forward bias of 100 V. Rectification ratios of up to about four orders of magnitude are achieved. They increase with increasing width of the i region within the measured i region of 3–10 μm, as a result of the considerable decrease of the reverse bias current relative to the forward bias current.

Photoelectric properties of printed thin films of silicon nanocrystals dispersed in polymer binder

Photoelectric properties of thin films of Si nanocrystals (nc-Si) dispersed in an electrically inactive polymer (polyester) binder prepared by printing have been studied by means of dark current–voltage ( I– V) and steady-state photoconductivity (SSPC) measurements. The I– V characteristics exhibit a varistor-like behavior. Space-charge-limited current (SCLC) theory for exponential localized-state distributions can be successfully applied to the interpretation of such a behavior. In the SCLC regime, a mobility of 4 cm 2 /V s in 30 wt% nc-Si dispersed polyester thin films is found, and the characteristic temperatures of the exponentially distributed localized states increase with increasing nc-Si concentration, which is consistent with the results obtained from the light intensity dependence of SSPC of the same materials. The results of the photoelectric measurements for printed thin films of nc-Si dispersed in an electrically active polymer binder (polymethylphenylsilane) are also shown.