Zero-field Mobility Research Articles

Preliminary investigation on the method of determining electron mobility of tris (8-hydroxyquinolinato) aluminum by space charge limited current

The charge-carrier mobility of an organic semiconducting material determines the material potential applications in devices. The investigation on mobility of organic material plays a significant role in improving the performance of organic device, such as organic light emitting diode, organic solar cell and organic thin film transistor. In this paper, we employ the space charge limited current (SCLC) method to evaluate the electron mobility of the controlled device based on tris (8-hydroxyquinolinato) aluminum (Alq3). The zero-field mobilities and field-dependent factors of the four devices are fitted respectively. The results show that depositing Al as top-electrode onto buffer layer LiF (1 nm) and Alq3 (100 nm) can significantly improve the the zero-field mobility and field-dependent factor of Alq3. The reason for that is that LiF could strengthen the complex reaction between Al and Alq3 to form Li+1Alq-1 particles, which leads to the enhanced ohmic injection and electron injection.

Zero-field mobilities in helium: highly accurate values for use in ion mobility spectrometry

The zero-field mobilities of 46 atomic ions in helium are calculated as functions of the gas temperature in an ion mobility spectrometer. The calculations are based on highly accurate, ab initio potential energy curves obtained in the last few years. In general, they start from a small value at low temperature, rise steadily to a maximum at some specific temperature, T max , and then decline at higher temperatures. The ratio of T max to the dissociation energy (well depth) of the ion-neutral interaction potential is shown to be approximation the same for all singly-charged ions and a few multiply-charged ions.

Field-effect mobility extraction in nanowire field-effect transistors by combination of transfer characteristics and random telegraph noise measurements

A technique based on the combined measurements of random telegraph-signal noise amplitude and drain current vs. gate voltage characteristics is proposed to extract the channel mobility in inversion-mode and accumulation-mode nanowire transistors. This method does not require the preliminary knowledge of the gate oxide capacitance or that of the channel width. The method accounts for the presence of parasitic source and drain resistance effect. It has been used to extract the zero-field mobility and the field mobility reduction factor in inversion-mode and junctionless transistors operating in accumulation mode.

Ion collision cross sections with transport and reaction coefficients in Ar, Cl2 and N2 and their mixtures for photonic crystal applications

The ion swarm transport coefficients such as reduced mobility, diffusion coefficients and reaction rates of the following systems Ar+/Cl2, Ar+/N2, and have been determined from a Monte Carlo simulation using calculated elastic and experimentally estimated inelastic collision cross sections. The elastic momentum transfer cross sections have been determined from a semi-classical JWKB approximation based on a rigid core interaction potential model. The inelastic cross sections have been fitted using the measured reaction coefficients as for instance ion conversion reaction coefficients. Then, the cross section sets are fitted using either the measured reduced mobility when available in the literature or the zero-field mobility calculated from Satoh's relation. From the sets of elastic and inelastic collision cross sections thus obtained in Ar+/Cl2, Ar+/N2, and systems, the ion transport and reaction coefficients are then calculated in pure gases and also in binary and ternary mixtures involving Cl2, Ar and N2 over a wide range of reduced electric field. These ion data are very useful for modelling and simulation of non-equilibrium low pressure electrical discharges used more particularly for etching of the III–V compounds in the case of crystal photonic applications.

Collision cross sections and swarm coefficients of water vapour ion clusters (H2O) n H+ with n = 1, 2 and 3 in N2, O2 and air

The ion swarm transport coefficients such as reduced mobility, diffusion coefficients and reaction rates of three water vapour ion clusters (H2O) n H+ (with n = 1, 2 and 3) in N2 and O2 have been determined from a Monte Carlo simulation using calculated and measured elastic and inelastic collision cross sections. The elastic momentum transfer cross sections have been determined from a semi-classical JWKB approximation based on a rigid core interaction potential model. The inelastic cross sections have been deduced from the measured ones in the case of similar ion cluster. Then, the cross sections sets are fitted using either the measured reduced mobility at low electric field in the case of (H2O) n H+ in N2 or the zero-field mobility calculated from the Satoh's relation and the measured ones in N2. From the sets of elastic and inelastic collision cross sections thus obtained in pure N2 and O2, the ion transport and reaction coefficients for (H2O) n H+ are then calculated in dry air and also extended over a wide range of reduced electric field in N2 and O2. These ion data are very useful for modelling and simulation of non-equilibrium electrical discharges more particularly in humid gases at atmospheric pressure.

Role of single walled carbon nanotubes in improving the efficiency of poly-(3-hexylthiophene) based organic solar cells

Poly-(3-hexylthiophene) (P3HT)—single walled carbon nanotube (SWNT) solar cells were fabricated and compared with single layer P3HT devices. P3HT:SWNT devices were found to have higher efficiencies than P3HT only devices by at least a factor of two. Zero field mobility values of 1.20×10−7 cm2 V−1 s−1 and 5.97×10−7 cm2 V−1 s−1 were calculated from space charge regime for P3HT and P3HT:SWNT, respectively. The SWNTs were predominantly of metallic nature, as revealed by Raman spectroscopy. Morphology studies show that the SWNTs increase local ordering of P3HT nanocrystals which can improve hole transport. They also show that the cathode-organic surface roughness and volume increase in the presence of SWNTs which can have significant positive effect on charge generation and collection at this interface. Variation in short circuit current with incident light intensity shows higher superlinear slope with SWNTs which also indicates that SWNTs aid in charge extraction from the device.

Trap-free space-charge-limited electron transport in amorphous tin(IV) phthalocyanine dichloride thin film

The morphology and electrical properties of thin films of an n-type organic semiconductor tin(IV) phthalocyanine dichloride (SnCl2Pc) are investigated. An investigation of atomic force microscopy and x-ray diffraction shows that the vacuum-deposited SnCl2Pc films on room-temperature glass substrates exhibit a homogeneous amorphous state. The temperature-dependent current–voltage characteristics of the electron-only devices show that the electron transport in SnCl2Pc thin films is bulk-limited, from the Ohmic region (J ∼ V) at low voltages to the trap-free space-charge-limited current (TFSCLC) region (J ∼ V2) at high voltages. The linear dependence of current on voltage at low voltages yields the temperature-dependent conductivity, while the TFSCLC relationship provides a direct measurement of the free-electron mobility as a function of electric field and temperature. The zero-field electron mobility at 293 K is as high as (1.8 ± 0.3) × 10−4 cm2 V−1 s−1. It can be seen that the obtained mobility and conductivity obey well the Gaussian disorder model and the three-dimensional variable range hopping model, respectively.

Enhanced Performance of Bulk Heterojunction Solar Cells Using Block Copoly(3-alkylthiophene)s

The photovoltaic properties, charge transport, and morphology of a series of diblock conjugated copolymers, poly(3-butylthiophene)-b-poly(3-octylthiophene) (P3BT-b-P3OT), were investigated as a function of block composition. Bulk heterojunction solar cells comprising blends of P3BT-b-P3OT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were found to have power conversion efficiencies as high as 3.0%, which represents factors of 1.6−9 enhancements compared to those of the homopolymers made under similar conditions. Imaging of P3BT-b-P3OT/PC71BM blends by atomic force microscopy and transmission electron microscopy revealed interpenetrating network with 11−18 nm crystalline polymer domains. The zero-field space charge limited current mobility of holes (∼(1−3) × 10−4 cm2 V−1 s−1) was similarly enhanced in the diblock copolymer solar cells compared to the homopolymers. These results demonstrate that block conjugated copolymers offer a promising approach to advanced materials for polymer solar cells and that the block composition is an attractive means to optimize the materials.

Charge transport in disordered organic host–guest systems: Effects of carrier density and electric field

We investigate charge transport in disordered organic host–guest systems with a bimodal Gaussian density of states. The energy difference between the peaks of the two Gaussians defines the trap depth. By solving the Pauli master equation for the hopping of charge carriers on a regular lattice we obtain the dependence of the charge-carrier mobility on the relative guest concentration, the trap depth, the energetic disorder, the charge-carrier density, and the electric field. Our results for the zero-field mobility are generally in a good agreement with recent semi-analytical model results. However, in the regime where the mobility attains a minimum our results can be almost one order of magnitude larger than as predicted semi-analytically. Furthermore, it is shown that field-induced detrapping can contribute strongly to the electric-field dependence of the mobility.

Piezoelectric scattering limited mobility characteristics of a degenerate surface layer in compound semiconductors at low lattice temperatures

The theory is developed for piezoelectric scattering rate of carriers in a degenerate surface layer under the condition of low temperature when the approximations of the well-known traditional theory are not valid. The scattering rates thus obtained are then used to estimate the zero-field mobility characteristics for the surface layers under similar condition of low temperature. The results for the surface layers in GaAs and ZnO show that when one takes into account either the degeneracy of the carrier ensemble or the finite energy of the phonons or both, the energy dependence of the scattering rates changes significantly from what follows for a non-degenerate ensemble or from the traditional theory, where one makes use of the high-temperature approximation and thus assumes equipartition law for the phonon distribution, and neglects the phonon energy in the energy balance equation of the electron–phonon system. It is observed that the zero-field mobility characteristics that follow from these scattering rates are interesting in that they are quite different from what turns out either for a non-degenerate ensemble or in the high-temperature approximations.

High temperature carrier mobility as an intrinsic transport parameter of an organic semiconductor

The high temperature limit of hole mobility ( μ ∞) in N, N′-diphenyl- N, N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′diamine (NPB) has been studied by time-of-flight technique. The effect of dopants on μ ∞ was also investigated. It was found that the μ ∞ is independent of the nature of dopants. The common μ ∞ can be applied to estimate the full temperature dependence of zero-field mobility ( μ 0 ), if μ 0 at one temperature is known. We demonstrate this concept by predicting the room temperature μ 0 of NPB-doped with copper phthalocyanine (CuPc). The mobility prediction of CuPc-doped-NPB was then verified by the classic work of Hoesterey and Letson [D.C. Hoesterey, G.M. Letson, J. Phys. Chem. Solids 24 (1963) 1609].

Effects of carrier mobility, energy gap, and excitation size on the performance of single layer organic solar cells

A model of universal single layer organic solar cells in metal-insulator-metal (MIM) representation involving field-dependent carrier mobility is set up. The current-voltage characteristics as well as the distribution of electron density, hole density and recombination rate on a set of parameters are simulated. Subsequently, the dependences of the short-circuit current density (Jsc) and open-circuit voltage (Voc) on the electron and hole zero-field mobility, excitation generation rate, energy gap, as well as electron-hole pair distance in an excitation are investigated. It is demonstrated that the enhancement of either the electron mobility or the hole mobility can contribute to the increase of Jsc in the devices. The increase of the hole mobility can lead to the improvement of both Jsc and Voc, and the simultaneous increase of the electron mobility and hole mobility will greatly elevate Jsc but maintain a steady Voc. Additionally, all the increases of the excitation generation rate, energy gap and electron-hole pair distance are beneficial to both the remarkable increases of Jsc and Voc of the devices.

Hybrid Polymer Solar Cells from Highly Reactive Diethylzinc: MDMO–PPV versus P3HT

The degradation of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-p-phenylene vinylene] (MDMO–PPV) during the processing of hybrid organic/inorganic bulk-heterojunction solar cells with zinc oxide (ZnO) from a molecular precursor as acceptor is reported. Upon addition of diethylzinc, the absorption spectrum of MDMO–PPV shifts to the blue, and hole transport through the polymer deteriorates dramatically, indicating a reduction of the conjugation length of the polymer backbone. To prevent polymer degradation through the breaking of trans vinyl bonds, regioregular poly(3-hexylthiophene) (P3HT) is introduced as the electron donor. This system of P3HT and precursor ZnO reveals an unchanged UV–vis absorption profile and zero-field hole mobility with respect to the pristine polymer as well as an improved photovoltaic performance with an estimated power conversion efficiency of 1.4% (AM1.5 global reference spectrum, 1 kW/m2).

Interaction potentials and transport properties of coinage metal cations in rare gases

High-level ab initio calculations are performed on the coinage metal cations (Cu+, Ag+, and Au+) interacting with each of the rare gases [Rg (Rg=He to Rn)]. The RCCSD(T) procedure is employed, with basis sets being of approximately quintuple-zeta quality, but with the heavier species using relativistic effective core potentials. The interaction potentials are compared to experimental and theoretical data where they exist. In addition, transport coefficients for the mobility and diffusion of the cations in the rare gases are calculated. The latter have involved a rewriting of some of the programs used, and the required modifications are discussed. The mobility results indicate that, rather than being a rare occurrence, mobility minima may be common phenomena. Finally, a new estimate is put forward for the validity of zero-field mobilities in ion mobility spectrometry.

Emergence of a reentrant insulating phase around [formula omitted] in a series of GaAs/AlGaAs quantum wells with varying well widths

We report on magneto-transport measurements of a systematic set of five GaAs/AlGaAs quantum wells with well widths ranging from 7.9 to 33.0 nm. We find that as L is decreased into the regime where the zero-field mobility is limited due to surface roughness, a reentrant insulating phase around filling fraction ν = 1 3 emerges. As L is decreased further, only a single insulating phase is observed for ν < 1 .

Simultaneous measurement of the de Haas-van Alphen and the Shubnikov-de Haas effect in a two-dimensional electron system

In a simultaneous experiment we studied the de Haas--van Alphen (dHvA) and the Shubnikov--de Haas (SdH) effects in a two-dimensional electron system (2DES) in a modulation-doped $\mathrm{Ga}\mathrm{As}∕{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterostructure. For this, a gated 2DES mesa was monolithically integrated with a micromechnical cantilever with an interferometric fiber-optics readout. In situ measurement of the dHvA and SdH oscillations at $300\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ in a magnetic field $B$ allowed us to directly compare the variation of the ground state energy and the nonequilibrium transport behavior, respectively. This was done on a 2DES of a small carrier density ${n}_{s}$ ranging from $5\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}33\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. The wave forms of the dHvA oscillations were nonsinusoidal down to a magnetic field as small as $1.45\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. At the same time the zero-field mobility was as low as ${\ensuremath{\mu}}_{e}={10}^{5}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}∕\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{s}$. We found that at fixed $B$ the observed dHvA wave form and amplitude were independent of ${n}_{s}$ and ${\ensuremath{\mu}}_{e}$. This was unexpected and in contrast to the established picture in the literature. To understand the dHvA effect quantitatively in a disordered 2DES our data suggest that energetic details of the disorder potentials have to be considered.

Nondispersive hole transport in carbazole- and anthracene-containing polyspirobifluorene copolymers studied by the charge-generation layer time-of-flight technique

Nondispersive hole transport in two polyspirobifluorene copolymers containing either 10% anthracene or 10% carbazole was studied in detail by the charge-generation layer time-of-flight (TOF) technique over a wide range of electric fields and temperatures. The TOF transients of both polymers showed a clear plateau indicating nondispersive transport of charge carriers. Zero-field mobilities were found to be in the order of 10−6cm2∕Vs at room temperature. Results were analyzed within the framework of the Gaussian disorder model to extract the parameters of the charge-carrier transport. The width of the transport density of states was determined to be 83meV for the polyspirobifluorene-anthracene copolymer and 89meV for the polyspirobifluorene-carbazole copolymer. At lower temperatures a change of slope in the temperature dependence of the zero-field mobility was observed. At higher temperatures the TOF transients were modified by a cusp. Both phenomena can be explained within the framework of the Gaussian disorder model.

The quasi-ballistic model of electron mobility in liquid hydrocarbons : Effect of an external electric field

The quasi-ballistic model of electron transport in liquid hydrocarbons, introduced earlier by the author [A. Mozumder, Chem. Phys. Lett. 207 (1993) 245], has been extended to the field dependence of mobility, when the zero-field mobility is low. A sinh-type dependence is obtained, due mainly to the random direction of detrapping relative to the field. The length parameter is interpreted as the trap radius 1 In this letter, the trap radius is taken to mean the extent of the trapped electron wave function. 1 , in contradistinction to the jump length of the hopping model. Experimental data have been examined for six liquid hydrocarbons and a speculation is offered regarding the nature of the electron trap.

Comparative study of hole transport in polyspirobifluorene polymers measured by the charge-generation layer time-of-flight technique

Hole transport in a polyspirobifluorene homopolymer and a statistical polyspirobifluorene-triarylamine copolymer has been studied in detail employing the charge-generation layer time-of-flight (TOF) technique over a wide range of electric fields and temperatures. Both materials exhibit nondispersive TOF signals after injection of a sheet of charge carriers from a thin (10nm) perylene-diimide charge-carrier generation layer into a relatively thick (d&amp;gt;1μm) polymer film. Results were analyzed within the framework of the Gaussian disorder model and the charge transport parameters were extracted for both polymers. The zero-field hole mobility of the spirohomopolymer was found to be on the order of 10−6cm2∕Vs, whereas the copolymer showed a considerably lower hole mobility of 6×10−8cm2∕Vs. The width of the density of states σ was determined to be 86meV for the homopolymer and 107meV for the copolymer. The latter polymer also showed an increased positional disorder due to the statistically incorporated triarylamine units.

Impact of Structure and Morphology on Charge Transport in Semiconducting Oligomeric Thin‐Film Devices

We investigated various thin-film morphologies of vacuum-deposited highly luminescent 2,5-di-n-octyloxy-1,4-bis[4'-(styryl)styryl]benzene (Ooct-OPV5) in a typical light-emitting-diode device structure. Important modifications in the thin-film morphology and structure were obtained by changing the substrate temperature in the range 23-90 degrees C. Structural analysis by X-ray and electron diffraction provided clear evidence for polymorphism in evaporated thin films of Ooct-OPV5. Concomitantly, the hole mobility in the corresponding devices was determined by transient electroluminescence measurements. We demonstrate that the substrate temperature T(sub) is a key parameter that controls the hole mobility of the devices. Increasing T(sub) between 23 and 84 degrees C results in a progressive increase of the zero-field hole mobility from 10(-6) to 10(-4) cm(2) V(-1) s(-1). The increase in hole mobility is correlated to the average grain size in the thin films. In addition, we give evidence for the existence of a peculiar growth mode in the bulk crystal structure of Ooct-OPV5, whereby the (a,b) and (b,c) planes can grow in a homoepitaxial manner.