Understanding electron-withdrawing substituent effect on structural, electronic and charge transport properties of perylene bisimide derivatives

A series of n-type perylene bisimide (PBI) derivatives with electron-withdrawing substituents at both bay and imide nitrogen positions were investigated. The effects of these substituents on internal energy relaxation, molecular orbitals, air stability, electronic properties and charge transport behaviors were systematically discussed with density functional theory (DFT) which has been demonstrated reliable for organic semiconductor study. The investigated derivatives with electron-withdrawing substituents show favorable performances in terms of these properties. The LUMO levels are greatly stabilized by at least 0.3 eV and these derivatives show the strong absorption from 400 to 700 nm which match with the solar spectra very well. The charge transport mainly happens between molecules in the same organic molecular layer and electronic couplings between layer-to-layer molecules are very weak, thus the mobility along layer-to-layer direction is less efficient. The variation of molecular packings and intermolecular interactions produce the highly anisotropic mobilities. The derivative with two fluorine atoms at bay positions and –CH₂C₃F₇ at imide group has broad and strong absorption in the UV-Visible region and the electron mobility could get to 0.514 cm² V⁻¹ s⁻¹ which is greatly encouraging for molecular and material design in organic solar-cell devices. These calculated results are in good agreement with the experimental data. However, not all the functionalization with halogen substituents would induce the increase of the electronic coupling and electron mobility. The derivatives with four halogen substituents at the bay positions could not show advantages in terms of electron mobility which is induced by the distorted conjugated structures. The theoretical understanding of these n-type organic semiconductors figures out the importance of tuning the molecular geometry to get high performance semiconductor materials.     

Modeling of charge transport across disordered organic heterojunctions

Organic electronic devices often consist of a sandwich structure containing several layers of disordered organic semiconductors. In the modeling of such devices it is essential that the charge transport across the organic heterojunctions is properly described. The presence of energetic disorder and of strong gradients in both the charge density and the electric field at the heterojunction complicates the use of continuum drift-diffusion approaches to calculate the electrical current, because of the discrete positions of the sites involved in the hopping transport of charges. We use the results of three-dimensional Monte Carlo simulations to construct boundary conditions in a one-dimensional continuum drift-diffusion approach that accurately describe the charge transport across the junction. The important effects of both short- and long-range Coulomb interactions at the junction are fully accounted for. The developed approach is expected to have a general validity.     

INVESTIGATION OF CHARGE INTENSIFICATION EFFECT IN a-Si:H BY MEANS OF PHOTOELECTRIC SENSITIVITY METHOD

The photocurrent-voltage characteristics and photoelectric sensitivity of a-Si:H samples with slit and comb electrodes are measured. A method for calculating the charge intensifying gain from the photoelectric sensitivity is proposed. The obtained charge intensifying gain of a-Si:H under an electric field of 105 V/cm through this method is as high as 4.3×103. The generation process of the charge intensification effect in a-Si:H is discussed on the basis of the energy level diagram. And the product of electron's mobility and its lifetime is calculated from the measured values of the gains.     

Influence of equilibrium charge reservoir formation on photo generated charge transport in TiO2/organic devices

Charge transport measurements have been performed using the photo induced charge extraction by linearly increasing voltage (photo-CELIV) technique on indium tin oxide/titanium dioxide/poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester/copper (ITO/TiO₂/P3HT:PCBM/Cu) devices. By adjusting the offset voltage such that holes are accumulated at the ITO/TiO₂ contact we obtain space charge limited current (SCLC) extraction in the dark. Using photo-generation the current response is limited by SCLC extraction at low carrier concentrations but becomes purely recombination limited at high photo-generated carrier concentration. A 1-D drift diffusion model has been developed to simulate our results and we show that the hole blocking ITO/TiO₂ contact is responsible for the SCLC behavior. The highly reduced recombination of charges seen in P3HT:PCBM devices is necessary to obtain the large extraction current transients that are seen in the experimental measurements. By comparing the simulated dark CELIV and photo-CELIV we show that photo-generated extraction is more sensitive towards changes in the surface recombination velocity.     

Investigation of charge transport of monolayer polymeric films with field effect tuning and molecular doping for chemiresistive sensing application

Semiconducting and conducting 2D conjugated polymers have been widely investigated due to their merits such as mechanical flexibility, solution processability, and large-area fabrication. Nevertheless, the transport characteristics of field-effect tuning and molecule doping ultra-thin even monolayer conjugated polymers haven't been understood thoroughly. Moreover, a general and facile method for the direct synthesis of large-area conjugated polymer monolayer films with good uniformity and high coverage on the SiO₂/Si substrates is highly desirable and still remains challenging. Here, we have used the simple drop-casting method to successfully fabricate high-coverage and homogeneous poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno [3], [2-b]thiophene) (PBTTT) monolayer films directly on the surface of SiO₂/Si substrate. The electrical transport behaviors were investigated systematically in both field effect doping and molecular doping the systems. 2D hopping transport was observed dominating the conductance of the field effect tuned monolayer PBTTT. While Efros-Shklovskii variable range hopping (ES-VRH) contributed the main transport at higher carrier concentration system doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄-TCNQ). And the onset interlayer transport should effectively reduce the energetic disorder revealed in bi- or tri-layer films. In addition, the F₄-TCNQ doping conductive PBTTT monolayer films displayed superior chemiresistive sensing toward ammonia. The proposed semiconducting and conducting conjugated polymer monolayer-based ideal platform can open new avenues for ultra-thin film organic electronics.     

Effects of disorder on charge transport in semico++ nducting polymers

Polaron transport process in semiconducting polymers with disordered structure and morphology is simulated using a nonadiabatic evolution method. The simulations are performed within the framework of an extended version of the Su–Schrieffer–Heeger model modified to include diagonal disorder, off-diagonal disorder and an external electric field. It is found that the polaron transport mechanism is determined by the competition between the disorder and the electric field. For the case of pure diagonal disorder, the polaron transport undergoes a crossover from adiabatic drift to nonadiabatic hopping under a large scope of electric field (E < 2.0 mV/Å), whereas the field-assisted tunneling dominates charge transport for a higher electric field. For the combined disorder, it is demonstrated that the competition effects and the synergetic effects between the diagonal and off-diagonal disorder are equally important. The effects of diagonal disorder are negative in most cases, and the energetically easier pathways for charge transport are mainly opened by the off-diagonal disorder.     

The role of local potential minima on charge transport in thin organic semiconductor layers

We have performed a systematic study of dependence of time-resolved photocurrent on the point of charge excitation within the organic semiconductor channel formed by two coplanar metal electrodes. The results confirm that spatial variation of electric field between the electrodes crucially determines transport of photogenerated charge carriers through the organic layer. Time-of-flight measurements of photocurrent demonstrate that the transit time of photogenerated charge carrier packets drifting between the two electrodes decreases with increasing travelling distance. Such counterintuitive result cannot be reconciled with the spatial distribution of electric field between coplanar electrodes, alone. It is also in contrast to expected role of space-charge screening of external electric field. Supported by Monte Carlo simulations of hopping transport in disordered organic semiconductor layer, we submit that the space-charge screens the external electric field and captures slower charge carriers from the photogenerated charge carrier packet. The remaining faster carriers, exhibit velocity distribution with significantly higher mean value and shorter transit time.     

A theoretical study on the charge transport properties of DNA

The charge transport properties of DNA are studied by the first-principle simulation to discuss the possibility of applying DNA to molecular wire. Both the hopping model and band-like model are introduced. By using hopping model, the theoretical hole mobilities calculated by Marcus theory between the same bases in DNA are 5.6 × 10⁻³, 4.1 × 10⁻², 2.0 × 10⁻² and 1.2 × 10⁻⁴ cm²V⁻¹s⁻¹ for T-T, A-A, C-C and G-G; and the calculated electron mobilities are 5.3 × 10⁻⁸, 1.5 × 10⁻⁴, 8.1 × 10⁻⁷ and 7.5 × 10⁻¹⁰ cm²V⁻¹s⁻¹ for T-T, A-A, C-C and G-G, respectively. And the charge transport for both holes and electrons between different bases exhibits directivity. By using band-like model, we calculated the band width of DNA with double helix structure and bilinear structure to investigate which structure will facilitate to the charge transport. We found that the band width of DNA increased when DNA transforming from the double helix structure to the bilinear structure, which means DNA with the bilinear structure possesses better charge transport properties. This research sheds a light on the molecular design for the molecule serving as the molecular wire.     

Highly efficient all-solution processed blue quantum dot light-emitting diodes based on balanced charge injection achieved by double hole transport layers

Solution-processed blue quantum dot light-emitting diodes (QLEDs) suffer from low device efficiency, whereas the balance of electron and hole injection is critical for obtaining high efficiency. Herein, synergistical double hole transport layers (D-HTLs) are employed, which use poly(9-vinylcarbazole) (PVK) stacked on poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB). The fabrication of D-HTLs is achieved by using dimethyl formamide (DMF) as the solvent for PVK, with which the underlying TFB layer almost remains unwashed and undamaged during the spin-coating process of PVK layer. TFB/PVK D-HTLs form the stepwise energy level for hole injection, which reduces the hole injection barrier and favors the carrier balance in the emission layer (EML). The optimized blue QLED with TFB/PVK D-HTLs shows a maximum external quantum efficiency (EQE) of 13.7%, which is 3-fold enhancement compared to that of the control device with single TFB HTL. The enhancement of the QLED performance can be attributed to the improvement of surface morphology and charge injection balance for the stepwise D-HTLs based QLEDs. This work manifests the positive effect on performance boost by selecting appropriate solvents towards stepwise D-HTLs formation and paves the way to fabricate highly efficient all-solution processed light emitting diodes.     

Analysis of trap effect on reliability using the charge pumping technology in La-incorporated high-k dielectrics

In this paper, reliability characteristics of nMOSFETs with La-incorporated HfSiON and HfON and metal gate have been studied. HfLaSiON shows greater device degradation by hot carrier (HC) stress than by positive bias temperature (PBT) stress, while HfLaON exhibits similar degradation during HC stress and PBT stress. To evaluate the contribution of bulk trap during PBT stress, a novel charge pumping (CP) technique is applied to extract the distribution of bulk trap (N_bt) before and after PBT stress. To evaluate permanent damage during HC stress, an appropriate selection of frequency range in CP method is considered. The initial interface trap density of HfLaSiON and HfLaON is similar, while the near-interface trap (N_IT) density of HfLaSiON after HC stress is equal or greater than that of HfLaON.     

The relationship between intermolecular interactions and charge transport properties of trifluoromethylated polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) with the electron-withdrawing groups such as halogen atom, cyanide, perfluoroalkyl (PFA), or perfluoroary, etc. exhibit good air stability and better solid-state charge carrier mobility. To obtain a better understanding of structure property relationships of this kind of compounds, a series PAH(CF₃)_n derivatives a1, a2, b1, b2, c1, and c2, which contain different numbers of trifluoromethyls and benzene rings, were chosen and studied by both band-like model and hopping model. Their crystals contain different intermolecular interactions. It turns out that intermolecular hydrogen bonding interactions are mainly responsible for electron transport, while π-stacking interactions dominate hole transport. When the π-stacking and intermolecular hydrogen bonding interactions coexist in the same direction, a competitive relationship occurs between hole and electron transport, which tend to cause enhancement of electron transport, and restrain hole transport.     

Effect of dihydropyrazine on structures and charge transport properties of N-heteropentacenes matters: A theoretical investigation

A family of N-heteropentacenes acted as promising candidates for organic semiconductor materials is of immense interest. It should be attributed to the following reasons that (1) the positions, numbers and valence-states of N atom in N-heteropentacenes can effectively tune their electronic structure, stability, solubility, and molecular stacking; (2) diverse intermolecular interaction and π-stacking motifs appear in their crystals. The effect of the position and number of the 6-π-pyrazine on their structures and charge-transport properties has been systematically investigated in our previous work (J. Phys. Chem. C 115 (2011) 21416). Therefore, in this work, the study on the role of 8-π-dihydropyrazine with another valence-state N atoms is our focus. Density functional theory, Marcus electron transfer theory and Brownian diffusion assumption coupled with kinetic Monte-Carlo simulation are applied into this investigation. Our theoretical results indicate that in contrast with pyrazine, dihydropyrazine introduced is more helpful for promoting p-type organic semiconductor materials. For molecule 4, hole mobility of its single crystal theoretically reach 0.71 cm² V⁻¹ s⁻¹, and coupled with its fine hole-injection ability, it should be a promising candidate for p-type organic semiconductor materials. Although the lowest triplet-state energies of the molecules studied are very small, introduction of dihydropyrazine is very helpful for increasing the energies.     

Characterization of ultra-thin SiO2 by capacitance-voltage and charge pumping measurements

In this paper, we present results on electrical measurements of ultra thin SiO₂ layers (from 3.5 nm down to 1.7 nm), used as gate dielectric in metal-oxide-semiconductors (MOS) devices. Capacitance-voltage (C-V) measurements and simulations on MOS capacitors have been used for extracting the electrical oxide thickness. The SiO₂/Si interface and oxide quality have been analyzed by charge pumping (CP) measurements. The mean interface traps density is measured by 2-level CP, and the energy distribution within the semiconductor bandgap of these traps are investigated by 3-level charge pumping measurements. A comparison of the energy distribution of the SiO₂/Si interface traps is made using classical and quantum simulations to extract the surface potential as a function of the gate signal. When the gate oxide thickness <3.5 nm, we prove that it is mandatory to take into account the quantum effects to obtain a more accurate energy distribution of the SiO₂/Si interface traps. We also explain the increase of the apparent interface traps density measured by 2-levels CP with the increase of the oxide thickness, for transistors made from the same technological process.     

Characterization of various alloying metal oxide nanoparticles embedded in HfOxNy as charge trapping nodes in nonvolatile memory devices

This work compares Co_xMo_yO, Co_xFe_yO and Fe_xMo_yO alloying metal oxide nanoparticles (AMONs) that were individually embedded in HfO_xN_y high-k dielectric as charge trapping nodes. They were formed by chemical vapor deposition using Co/Mo, Co/Fe and Fe/Mo acetate, respectively, calcined and reduced in Ar/NH₃ ambient. The effects of various pre-treatments on Co_xMo_yO, Co_xFe_yO and Fe_xMo_yO AMONs preparation were investigated. The results indicate that the larger charge trap density, larger memory window and better programming characteristics of Co_xMo_yO AMONs are attributable to their higher surface density and smaller diameter. The average collected charge in each Co_xMo_yO AMON is the smallest among three AMONs, revealing that a local leakage path is associated with the least charge loss. The main mechanism that governs the programming characteristics involves the trapping of holes.     

Electrical transport properties in electroless-etched Si nanowire field-effect transistors

We report the electrical transport of the Si nanowires in a field-effect transistor (FET) configuration, which were synthesized from B-doped p-type Si(1 1 1) wafer by an aqueous electroless etching method based on the galvanic displacement of Si by the reduction of Ag⁺ ions on the wafer surface. The FET performance of the as-synthesized Si nanowires was investigated and compared with Ag-nanoparticles-removed Si nanowires. In addition, high-k HfO₂ gate dielectric was applied to the Si nanowires FETs, leading to the enhanced performance such as higher drain current and lower subthreshold swing.     

基于量子模型的碳纳米管场效应晶体管电子输运特性

采用量子输运模型和NEGF理论,自洽求解薛定谔方程和泊松方程,对类MOS-碳纳米管场效应晶体管的电子输运特性建模。考察了沟道长度Lg为5~25 nm时,其对器件的导通电流、阈值电压、关态泄漏电流、电流开关比、亚阈值摆幅等性质的影响。结果表明:当Lg≥15 nm时,MOS-CNTFET没有量子尺寸效应;当Lg<15 nm时,器件出现短沟道效应;Lg<10 nm时短沟道效应更加明显。     

The effect of oxygen on the epitaxial growth of diamond

We studied the effect of oxygen on the growth quality of diamond epitaxial layers. After oxygen is added during the growth of the diamond epitaxial layer, as the thickness of the epitaxial layer increases, the full width at half maximum of the rocking curve of the (004) plane of diamond epitaxial layer increases continuously, and, in addition, the intensities of both the Raman peaks and the free exciton emission peaks of the diamond epitaxial layer decrease continuously. These experimental results demonstrate that as the thickness of the diamond epitaxial layer increases, the quality of the diamond epitaxial layer degrades. The strong etching effect of the OH radical groups in the plasma on the diamond epilayers leads to the degradation of their crystallinity.     

The role of the electrode configuration on the electrical properties of small-molecule semiconductor thin-films

This paper presents a systematic analysis of the electrode configuration influence on the electrical properties of organic semiconductor (OSC) thin-film devices. We have fabricated and electrically characterized a set of planar two-terminal devices. The differences in I-V characteristics between the top and bottom contact structures are presented and analyzed. Top-contact configurations have a linear current vs. electric field behavior, while the bottom-electrode devices display a transition from ohmic to space-charge-limited conduction regime. The transition is temperature- and thickness-dependent. Finite-element calculations show that when the OSC film is connected using top electrodes, the current flows through the OSC bulk region. On the other hand, the bottom-electrode configuration allows most of the current to flow near the OSC/substrate interface. The current probes interfacial states resulting in a space-charge conduction regime. The results shed some light on the so-called “contact effects” commonly observed in organic thin-film transistors. The findings presented here have implications for both the understanding of the charge transport in OSC films and the design of organic semiconductor devices.     

一种半导体激光自混合效应模型参数的测量方法

从半导体激光自混合干涉系统的一般模型公式出发,建立了一个非线性函数模型,利用非线性最小二乘法的曲线拟合,来估计半导体激光自混合效应的模型参数——线宽展宽因数和光反馈因数。经计算仿真分析表明,该算法有较好的收敛性,迭代次数少且参数估计精度高,是半导体激光自混合效应参数测量的一种有效方法。