Charge transport properties and microstructure of polythiophene/polyfluorene blends

We present a combined charge transport and X-ray diffraction study of blends based on regioregular poly(3-hexylthiophene) (P3HT) and the polyfluorene co-polymer poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2′′-diyl) (F8TBT) that are used in efficient all-polymer solar cells. Hole mobility is observed to increase by nearly two orders of magnitude from less than 10^?7 cm² V^?1 s^?1 for as spin-coated blends to 6 × 10^?6 cm² V^?1 s^?1 for blends annealed at 453 K at a field of 2.7 × 10⁵ V/cm, but still significantly below the time-of-flight mobility of unblended P3HT of 1.7 × 10^?4 cm² V^?1 s^?1. The hole mobility of the blends also show a strong negative electric-field dependence, compared with a relatively flat electric-field dependence of unblended P3HT, suggestive of increased spatial disorder in the blends. X-ray diffraction measurements reveal that P3HT/F8TBT blends show a phase separation of the two components with a crystalline part attributed to P3HT and an amorphous part attributed to F8TBT. In as-spun and mildly annealed blends, the measured d-values and relative intensities of the 100, 200 and 300 P3HT peaks are noticeably different to unblended P3HT indicating an incorporation of F8TBT in P3HT crystallites that distorts the crystal structure. At higher anneal temperatures the blend d-values approach that of unblended P3HT suggesting a well separated blend with pure P3HT crystallites. P3HT crystallite size in the blend is also observed to increase with annealing from 3.3 to 6.1 nm, however similar changes in crystallite size are observed in unblended P3HT films with annealing. The lower mobility of P3HT/F8TBT blends is attributed not only to increased P3HT structural disorder in the blend, but also due to the blend morphology (increased spatial disorder). Changes in hole mobility with annealing are interpreted in terms of the need to form percolation networks of P3HT crystallites within an F8TBT matrix, with a possible contribution due to the intercalation of F8TBT in P3HT crystallites acting as defects in the as-prepared state.     

Charge transport mechanism in metal-nitride-oxide-siliconstructures

A charge transport mechanism in double oxide-nitride dielectric was studied experimentally and theoretically. We have found that widely accepted Frenkel effect or thermally assisted tunneling could not explain experimental current-field-temperature dependences. For the first time we demonstrate that ionization mechanism of deep traps, which control charge transport in silicon nitride, is due to multiphonon process     

Charge transport and optical properties of cross-conjugated organic molecules: A theoretical study

The structure, charge transport and optical properties of two-dimensional cruciform molecules 1,4-distyryl-2,5-bis(arylethynyl)benzenes, 1,4-distyryl-2,5-bis(arylethynyl)pyridines and 1,4-bis(ethylenediyl)dipyridine-2,5-bis(benzene) have been studied using density functional theory methods. The effective charge transfer integral and site energy corresponding to hole and electron transports in the above molecules were calculated directly from the matrix elements of Kohn–Sham Hamiltonian. The charge carrier mobility on the cruciform molecules was calculated using Monte Carlo simulation based on the Marcus rate theory and the results show that these cruciform molecules are the p-type organic semiconductors. The excited state calculations were performed using the time-dependent density functional theory method in gas phase, hexane and dichloromethane mediums. The trifluoromethyl substituted 1,4-distyryl-2,5-bis(arylethynyl)benzenes molecule possesses the maximum absorption and emission wavelength of 443 and 504 nm, respectively in dichloromethane medium.     

Charge transport in an AC plasma panel

A transverse voltage applied between a display cellDand a transfer cellTon a standard 60 lines/inch ac plasma panel can simultaneously cause a previously ON cellDto fire and transport a large amount of electrons from cellDtoward cellT. The same transverse voltage combines with the voltage due to transported electrons to produce subsequent discharges which, initiated near cellT, grow rapidly as they propagate along the row toward cellD. A voltage pulse (≃sustain pulse), applied to cellT, will combine with the local row wall voltage to produce another sequence of discharges along the row. These discharges, initiated near cellD, gain intensity as they spread toward cellT. This dynamic process results in a large and controllable charge transfer between the display cellDand the transfer cellT, a key mechanism for shift address display. By reversing the polarity of the transverse voltage, ions also can be transported, but ion transport produces smaller charge transfer.     

Charge transport properties of bulk-heterojunction organic solar cells investigated by displacement current measurement technique

We have investigated charge transport properties of bulk-heterojunction (BH) solar cells in which P3HT (Poly(3-hexylthiophene)) and PCBM ([6,6]-phenyl-C₆₁-butyric acid methyl ester) are used as the active layer, by using the displacement current measurement (DCM) method. In order to investigate the charge transport properties of the BH solar cells, we fabricated a dedicated device that consists of P3HT and PCBM, and used the DCM method to measure the charge distribution of the devices with different composition ratios of P3HT and PCBM. DCM data suggested that a BH film with 50 wt% of PCBM exhibits a preferable charge transport property suited for BH solar cells. We confirmed that the DCM results are consistent with the measured performance of the BH solar cells, indicating that the DCM method is a simple and effective method for optimizing the structure of BH solar cells as well as other electronic devices composed of binary materials.     

Charge transport in HgCdTe-based n +-p photodiodes

It is shown that dark currents measured at 77 K in Hg_1−x Cd_xTe (x⋍0.21) homojunctions can be adequately described by the balance equations with allowance made for the two main charge-transport mechanisms, i.e., tunneling assisted by traps in the band gap and recombination via these traps; the above homojunction may find application in microphotoelectronics in the infrared spectral range of 8–12 μm. Other charge-transport mechanisms are included in the consideration as additive terms. A comparison between the experimental current-voltage characteristics and dynamic resistance of HgCdTe diodes with the results of calculations was carried out. A good agreement was obtained between experimental data and the results of calculations, in which the donor and acceptor concentrations in the n and p regions of diodes, the concentration of traps and the position of their levels in the band gap, and the lifetimes of charge carriers for recombination via these traps were used as adjustable parameters. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 7, 2001, pp. 835–840. Original Russian Text Copyright ? 2001 by Gumenjuk-Sichevskaja, Sizov, Ovsyuk, Vasil’ev, Esaev.     

Charge transport properties in a series of five-ring-fused thienoacenes: A quantum chemistry and molecular mechanic study

The charge transport properties in a series of five-ring-fused thienoacenes are detailedly investigated here. With both high 3-D hole mobilities and good oxidative stability, the thiophene–benzene alternate structures in series B should be better than other two series as p-type organic semiconductors. In addition, a theoretical explanation for the difference between experimental hole mobilities in B-3 and B-3^* is provided based on crystal structure prediction. The role of dynamic disorder in C-3 is also investigated here and the results indicate that dynamic disorder plays a crucial role both in the 1-D and 2-D carrier transporting process.     

Charge transport and trapping characteristics in thin nitride-oxidestacked films

A charge transport and trapping model for thin nitride-oxide stacked films between silicon substrates and polysilicon gates is proposed. Nitride-oxide stacked films can be thought of as an oxide film with electron trapping at the nitride/oxide interface. The density of electron trapping is determined by the current-continuity requirement. The electron trapping reduces the leakage current and helps to lower the incidence of early failures for nitride-oxide stacked films     

Charge transport mechanisms in anisotype n-TiO2/p-Si heterostructures

Anisotype n-TiO₂/p-Si heterojunctions are fabricated by the deposition of a TiO₂ film on a polished poly-Si substrate using magnetron sputtering. The electrical properties of the heterojunctions are investigated and the dominant charge transport mechanisms are established; these are multi-step tunneling recombination via surface states at the metallurgical TiO₂/Si interface at low forward biases V and tunneling at V > 0.6 V. The reverse current through the heterojunctions under study is analyzed within the tunneling mechanism.     

Charge transport and recombination in heterostructure organic light emitting transistors

Light-emitting field effect transistors (LEFETs) are a class of organic optoelectronic device capable of simultaneously delivering the electrical switching characteristics of a transistor and the light emission of a diode. We report on the temperature dependence of the charge transport and emissive properties in a model organic heterostructure LEFET system from 300 K to 135 K. We study parameters such as carrier mobility, brightness, and external quantum efficiency (EQE), and observe clear thermally activated behaviour for transport and injection. Overall, the EQE increases with decreasing temperature and conversely the brightness decreases. These contrary effects can be explained by a higher recombination efficiency occurring at lower temperatures, and this insight delivers new knowledge concerning the optimisation of both the transport and emissive properties in LEFETs.     

Charge transport in Si-SiO2 and Si-TiO2 nanocomposite structures

The experimental data on dispersed Si-SiO₂ and Si-TiO₂ nanocomposite structures different in terms of physical, chemical, and insulator properties of oxide components are reported. The parameters of the nanocomposite structures are studied by FTIR spectroscopy and impedance spectroscopy. It is shown that, in such structures, the mechanisms of charge-carrier transport are defined by the properties of Si nanocrystallites and the corresponding oxide as well as by interaction processes at interfaces between grains.     

Charge transport in silver chalcogenides in the region of phase transition

Data on the σ(T), R(T), and U(T) dependences in Ag₂Te, Ag₂Se, and Ag₂S in the region of the phase transition are analyzed. It is found that the phase transition in Ag₂Te is accompanied by a decrease in the electron concentration and this transition in Ag₂Se is accompanied by an increase in this concentration. The concentration of intrinsic charge carriers in Ag₂Te decreases by a factor of 4 as a result of the phase transition and increases by a factor of 2 in Ag₂Se. The effect of variation in the energy-band parameters in the region of phase transition on the electron mobility is considered. It is established that, in Ag₂Te and Ag₂S, electrons are scattered by optical phonons in the region of the phase transition, while electrons are scattered by acoustic phonons in the α and β phases. It is assumed that the anomalously large increase in σ and U in Ag₂S as a result of the phase transition is caused by an increase in the concentration n and a simultaneous decrease in σ _g and m _n ^* by a factor of about 2.     

Charge transport in thick SiO2-based dielectric layers

Charge transport in thick (≥ 1 μm) SiO₂-based dielectric layers was investigated by means of I(U) measurements. Investigations were carried out on thermally grown field oxide (FOX) as well as on TEOS and BPSG. Gate oxide layers (GOX) were measured as reference. C(U) measurements were performed for the determination of charges in the oxide. To determine the electrical parameters of the layers, the model from Chen and Wu[1] was developed further. The model takes into account tunnelling, capture and emission processes, impact ionization, recombination, interface states and ohmic currents.

The I(U) characteristics for all dielectrics examined can be described with the aid of the model. The FOX parameters correspond to those of GOX. The parameters of TEOS and BPSG fluctuate strongly with the process parameters. After high-temperature annealing and from measurement of the examined parameters, the insulation properties of TEOS and BPSG were found to be at least as good as those of FOX.     

Charge transport in oligo phenylene and phenylene–thiophene nanofibers

Charge carrier mobilities have been measured in elongated crystalline aggregates with nanoscale cross-sectional dimensions. These molecular crystals are grown via dedicated epitaxial surface growth. They consist of well-ordered phenylene–thiophene (co-) oligomers that have been systematically tailored to study mobility of oligomers containing six rings and a variable number of thiophene rings. These organic nanoscale one-dimensional systems demonstrate charge mobilities as high as 1 cm² V⁻¹ s⁻¹. Density functional theory calculations have been performed in order to correlate the experimentally observed trends in mobility with molecular charge transport properties.     

Charge transport in lead sulfide quantum dots/phthalocyanines hybrid nanocomposites

A hybrid composite of non-aggregated lead sulfide (PbS) nanoparticles of average size 5.8 ± 1 nm embedded within a film of an octaalkyl substituted metal-free phthalocyanine (Compound 2) was prepared on interdigitated gold electrodes by mild acidic treatment of newly synthesised octasubstituted lead phthalocyanine analogue (Compound 1) in solid state phase. This nanocomposite film shows an enhancement of in-plane electrical conductivity over that of a film of octaalkyl substituted metal-free phthalocyanine alone by nearly 65%. This observation is consistent with the formation of charge complex compound as indicated by Raman and XPS data. The presence of PbS in the composite was examined on the basis of XRD peak positions which are comparable with those of bulk PbS. A band gap of 2.22 eV was calculated from optical absorption data using Tauc's law, implying quantum confinement. The mono dispersal behaviour of PbS nanoparticles was established from TEM and XRD studies. The hopping conduction mechanism is found to be primarily responsible for charge transport in the hybrid nanocomposite film with the hopping distance larger than PbS diameter.     

Photophysical and electrical properties of polyphenylquinolines containing carbazole or indolo[3,2-b]carbazole fragments as new optoelectronic materials

Photophysical and electrical properties of new synthesized 2,6-polyphenylquinolines (PPQs) containing an oxygen or phenylamine bridging group between quinoline cycles and, as an arylene radical, alkylated derivatives of carbazole or indolo[3,2-b]carbazole are studied. It is shown that the photosensitivity for new PPQs is 10⁴–10⁵ cm²/J and the photogeneration quantum yield of free carriers is as high as 0.15. Photophysical parameters increase with the phenylamine bridging group in place of the oxygen one and when using indolocarbazole instead of carbazole. It is found that a film of polyphenylquinoline containing an oxygen bridging group and an alkylcarbazole fragment in the polymer repeat unit exhibits “white” luminescence. Both electron and hole transport with a mobility of ∼10⁻⁶ cm²/(V s) are detected in films of all studied polymers. The conductivity value and type can be controlled by varying the chemical structure of the (oxygen or phenylamine) bridging group between PPQ cycles and by choosing carbazole or indolo[3,2-b]carbazole derivatives as an arylene radical.     

Charge transport mechanisms in Schottky diodes based on low-resistance CdTe:Mn

CdTe:Mn crystals with a resistivity of ～1 Ω cm at 300 K and Schottky diodes based on them are investigated. The electrical conductivity of the material and its temperature variations are explained in terms of the statistics of electrons and holes in semiconductors with allowance for the compensation processes. The ionization energy and the degree of compensation of the donors responsible for the conductivity are determined. It is shown that, in the case of forward connection and low reverse biases, the currents in Au/CdTe:Mn Schottky diode are determined by generation-recombination processes in the space-charge region. At higher reverse biases (above 1.5–2 V) the excess current is caused by electron tunneling from the metal to the semiconductor, and at even higher voltages (>6–7 V) an additional increase in the reverse current due to avalanche processes is observed.     

Charge transport through conjugated azomethine-based single molecules for optoelectronic applications

The single-molecule conductance of a 3-ring, conjugated azomethine was studied using the mechanically controlled breakjunction technique. Charge transport properties are found to be comparable to vinyl-based analogues; findings are supported with density functional calculations. The simple preparation and good transport properties make azomethine-based molecules an attractive class for use in polymer and single-molecule organic electronics.     

Synthesis and device properties of mCP analogues based on fused-ring carbazole moiety

Novel mCP analogues consisting of blue phosphorescent host materials with fused-ring, 1,3-bis(5H-benzofuro[3,2-c]carbazol-5-yl)benzene (BFCz) and 1,3-bis(5H-benzo[4,5]thieno[3,2-c]carbazol-5-yl)benzene (BTCz) were designed and synthesized using benzofurocarbazole and benzothienocarbazole donor moieties. BFCz and BTCz exhibit high glass transition temperatures of 147 and 157 °C, respectively, and high triplet bandgaps of 2.94 and 2.93 eV, respectively. To explore the electroluminescence properties of these materials, multilayer blue phosphorescent organic light-emitting diodes (PHOLEDs) were fabricated in the following device structure: indium–tin-oxide (ITO)/PEDOT:PSS/4,4’-cyclohexylidene bis[N,N-bis(4-methylphenyl)aniline] (TAPC)/1,3-bis(N-carbazolyl) benzene (mCP)/host:FIrpic/diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1)/LiF)/Al. The PHOLEDs with BTCz exhibited efficient blue emission with luminous and quantum efficiencies of 30.9 cd/A and 15.5% at 1000 cd/m², respectively.     

Electronic transport properties in copper nanowire

In present work, we study the electronic transport properties of copper nanowire using the non-equilibrium Green’s function technique on the density functional tight binding method for modelling the conductance under external bias voltage. We also carried out the calculations for the Au nanowire for comparison.