f_（max）─载流子迁移率关系在高频HBT设计中的意义

在导出ＨＢＴ的最高振荡频率与载流子迁移率关系的基础上［１］，指出了多子在影响频率性能方面与少子同样重要，呈现的几何平均关系清楚地揭示出晶体管的“双极”工作原理。在比较Ｎｐｎ和ＰｎｐＨＢＴ的频率性能中给出了目前少子迁移率的选择方法。为了提高计算的正确性，提出应进行深入的少子迁移率测量研究，以能为计算提供系统的、可靠的实验数据。初步估算得出ＰｕｐＨＢＴ的ｆ（ｍａｘ）稍高于或等于Ｎｐｎ的ｆ（ｍａｘ），最后，根据最高振荡频率与载流子迁移率的关系，对高频ＨＢＴ的最佳设计进行了比较系统的分析。

应变调控单层2H-MoS_(2)的能带结构和光学性质

为了从电子层面分析应变对能带结构以及光学性质产生影响的机理,采用密度泛函理论研究了应变对单层2H-MoS_(2)能带结构、光学性质、载流子迁移率和光催化分解水的影响.结果表明:晶格拉伸后带隙由2.15 eV减小到了1.65 eV,而晶格压缩后带隙由2.15 eV增大到2.66 eV.随着拉应变的增大,吸收曲线产生了蓝移,压应变对光学吸收系数的影响刚好相反.电子的载流子迁移率比空穴的大10倍左右,所以光照下电子和空穴能够有效的分离.综合光的吸收系数和光催化制氢条件这两个方面的因素可知,应变在2%、-2%、-6%这三种情况下能够得到最好的光催化制氢效果.

Numerical Study of Optimization of Layer Thickness in Bilayer Organic Light-Emitting Diodes

A numerical model for bilayer organic light-emitting diodes (OLEDs) is developed under the basis of trapped charge limited conduction.The dependences of the current density on the layer thickness,trap properties and carrier mobility of the hole transport layer (HTL) and emission layer (EML) in bilayer OLEDs of the structure anode/HTL/EML/cathode are numerically investigated.It is found that,for given values of the total thickness of organic layers,reduced depth of trap,total density of trap,and carrier mobility of HTL as well as EML,there exists an optimal thickness ratio of HTL to EML,by which a maximal quantum efficiency can be achieved.Through optimization of the thickness ratio,an enhancement of current density and quantum efficiency of as much as two orders of magnitude can be obtained.The dependences of the optimal thickness ratio to the characteristic trap energy,total density of trap and carrier mobility are numerically analyzed.

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-depen-dent 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.

Properties of multilayer gallium and aluminum doped ZnO(GZO/AZO) transparent thin films deposited by pulsed laser deposition process

Multilayer gallium and aluminum doped ZnO （GZO/AZO） films were fabricated by alternative deposition of Ga-doped zinc oxide（GZO） and Al-doped zinc oxide（AZO） thin film by using pulsed laser deposition（PLD） process. The electrical and optical properties of these GZO/AZO thin films were investigated and compared with those of GZO and AZO thin films. The GZO/AZO （1：1） thin film deposited at 400 ~C shows the electrical resistivity of 4.18 x 10 4 ~.cm, an electron concentration of 7.5 x 1020/cm3, and carrier mobility of 25.4 cm2/（V.s）. The optical transmittances of GZO/AZO thin films are over 85%. The optical band gap energy of GZO/AZO thin films linearly decreases with increasing the AI ratio.

Advances of the Vertical Directional Solidification Technique for the Growth of High Quality InSb Bulk Crystals

Since 1994, the vertical directional solidification （VDS） technique is employed for the growths of bulk crystals-without the seed, without contact to the ampoule wall, without coating and without external pressure, which leads to the detached growth. Growth velocities ranged from 3 mm/h to 10 mm/h, and rotation rates 10-20 rpm have been used. Ingots, 10-20 mm diameter and 60-65 mm length, have been grown with the conical ampoule geometry and these ingots have shown symmetric detachment. Crystals grown under such conditions showed the relatively low dislocation density and the highest carrier mobility,/tn = 5.9 x 104 cm2＂Vl-sl than the crystal grown ever. For the detached crystals, the dislocation density is 104 cm＂2 in conical region, and reached less than 103 cm-2 in the direction of the growth, when the ingots are not in contact with the ampoule wall. Experiments for indium-antimonide （InSb） growth have shown that the 80% growth environments have detachment, 15% entrapped in conical region and 5% attached.

Oxygen vacancy induced carrier mobility enhancement in nano-multilayered ZrO_(2):Y_(2)O_(3)/SrTiO_(3)thin films for non-volatile memory devices

The influence of oxygen vacancy-dominated carrier mobility on the performance of memristors has attractedconsiderable attention.The device’s carrier mobility can be significantly improved by forming a nano-multilayeredheterostructure when the individual layer thickness is below a critical value.In this work,Pt/[ZrO_(2):Y_(2)O_(3)(YSZ)/SrTiO_(3)(STO)]n/Nb:SrTiO_(3)(NSTO)memristive devices were configurated through laser pulse deposited YSZ/STO nanomultilayeredactive layer with both Pt and NSTO acting as top and counter electrodes.Specifically,the Pt/[YSZ/STO]5/NSTO device with five consecutive layers of YSZ/STO thin film shows superior memristor performance,and itscorresponding carrier mobility presents a significantly enhanced value compared to that of other periodic numbers ofYSZ/STO composed memristive devices.This can be attributed to the increase of oxygen vacancy concentration in thedevice,as evidenced by both experimental results and theoretical analysis.This work provides a significant approach inimproving the performance of memristor dominated by oxygen vacancy transporting mechanism.

Length-Sorted Semiconducting Carbon Nanotubes for High-Mobility Thin Film Transistors

We have developed a process for chemical purification of carbon nanotubes for solution-processable thin-film transistors （TFTs） having high mobility. Films of the purified carbon nanotubes fabricated by simple drop coating showed carrier mobilities as high as 164 cm2V-1s -1 normalized transconductances of 0.78 Sm-1 and on/off current ratios of 10^6. Such high performance requires the preparation of a suspension of micrometer-long and highly purified semiconducting single-walled carbon nanotubes （SWCNTs）. Our purification process includes length and electronic-type selective trapping of SWCNTs using recycling gel filtration with a mixture of surfactants. The results provide an important milestone toward printed high-speed and large-area electronics with roll-to-roll and ink-jet device fabrication.

Low carrier concentration leads to high in-plane thermoelectric performance in n-type SnS crystals

As a simple binary compound, p-type SnS shows great competitiveness in thermoelectrics due to the certain appealing carrier and phonon transport behaviors, coupled with its cost-effectiveness, earth-abundance and environmental compatibility. To promote the application of low-cost thermoelectric devices, we synthesized n-type SnS crystals through bromine doping. Herein, we report a high in-plane power factor of ~28 μW cm^(-1)K^(-2), and attribute it to an outstanding in-plane carrier mobility in the crystal form and the large Seebeck coefficient benefitting from the low carrier concentration. The calculations of elastic properties show that the low lattice thermal conductivity in SnS is closely related to its strong anharmonicity. Combining the excellent electrical transport properties with low thermal conductivity, a final ZT of ~0.4 is attained at 300 K, projecting a conversion efficiency of ~5% at 873 K along the in-plane direction.

A carbon-oxygen-bridged ladder-type building block for efficient donor and acceptor materials used in organic solar cells

A carbon-oxygen-bridged ladder-type donor unit （CO5） was invented and prepared via an ＂intramolecu- lar demethanolization cyclization＂ approach. Its single crystal structure indicates enhanced planarity compared with the carbon-bridged analogue indacenodithiophene （IDT）. Owing to the stronger electron-donating capability of CO5 than IDT, CO5-based donor and acceptor materials show narrower bandgaps. A donor-acceptor （D-A） copolymer donor （PCO5TPD） and an A-D-A nonfullerene acceptor （COSIC） demonstrated higher performance than IDT-based counterparts, PIDTTPD and IDTIC, respec-tively. The better performance of CO5-based materials results from their stronger light-harvesting capability and higher charge-carrier mobilities.

Penetration depth at various Raman excitation wavelengths and stress model for Raman spectrum in biaxially-strained Si

The carrier mobility of Si material can be enhanced under strain,and the stress magnitude can be measured by the Raman spectrum.In this paper,we aim to study the penetration depths into biaxially-strained Si materials at various Raman excitation wavelengths and the stress model corresponding to Raman spectrum in biaxially-strained Si.The experimental results show that it is best to use 325 nm excitation to measure the material stress in the top strained Si layer,and that one must pay attention to the distortion of the buffer layers on measuring results while 514 nm excitation is also measurable.Moreover,we established the stress model for Raman spectrum of biaxially-strained Si based on the Secular equation.One can obtain the stress magnitude in biaxially-strained Si by the model,as long as the results of the Raman spectrum are given.Our quantitative results can provide valuable references for stress analysis on strained materials.

Progress in organic-inorganic hybrid halide perovskite single crystal:growth techniques and applications

As a new generation of solution-processable optoelectronic materials, organic-inorganic hybrid halide perovskites have attracted a great deal of interest due to their high and balanced carrier mobility, long carrier dif- fusion length and large light absorption coefficient. These materials have demonstrated wide applications in solar cell, light-emitting diode, laser, photodetector, catalysis and other fields. Comparing with their polycrystalline film counter- part, perovskite single crystals have low trap density and no grain boundaries and thus are anticipated to possess much better optoelectronic performances. Herein, we review the key progress in the development of organic-inorganic halide perovskite single crystals. Particularly, the crystal growth techniques and applications of these advanced materials are highlighted.

Large open-circuit voltage polymer solar cells by poly(3-hexylthiophene) with multi-adducts fullerenes

Polymer solar cells (PSCs) made by poly(3-hexylthiophene) (P3HT) with multi-adducts fullerenes, [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), PC61BM-bisadduct (bisPC61BM) and PC61BM-trisadduct (trisPC61BM), were reported. Electrochemistry studies indicated that PC61BM, bisPC61BM and trisPC61BM had step-up distributional lowest unoccupied molecular orbital (LUMO) energy. PSCs made by P3HT with above PC61BMs show a trend of enlarged open-circuit voltages, which is in good agreement with the energy difference between the LUMO of PC61BMs and the HOMO of P3HT. On the contrary, reduced short-circuit currents (Jsc) were observed. The investigation of photo responsibility, dynamics analysis based on photo-induced absorption of composite films, P3HT:PC61BMs and n-channel thin film field-effect transistors of PC61BMs suggested that the short polaron lifetimes and low carrier mobilities were response for reduced Jsc. All these results demonstrated that it was important to develop an electron acceptor which has both high carrier mobility, and good compatibility with the electron donor conjugated polymer for approaching high performance PSCs.

Organic-inorganic hybrid Sn-based perovskite photodetectors with high external quantum efficiencies and wide spectral responses from 300 to 1000 nm

Organic-inorganic hybrid perovskites are ideal materials for photodetection owing to their high charge carrier mobility, long charge carrier diffusion length, low dark current density and sharp absorption edge. However, a relatively small band gap(1.6 e V) limits their photonharvesting efficiency in the near-infrared region. In the present work, we demonstrate a hybrid methylamine iodide and Pb-Sn binary perovskite as the light absorption layer in photodetectors. Experimentally, the wavelength of photoresponse onset for the photodetectors can be extended to as great as 1,000 nm when the Sn content of the hybrid perovskite is increased to 30 mol%. In addition, the photodetectors exhibit a photoresponsivity of 0.39 A W^-1, a specific detectivity of 7×10^12 Jones, a fast photoresponse with rise and decay time constants and an external quantum efficiency greater than 50% in the wavelength range of350–900 nm, with a maximum value of about 80% at 550 nm.

Full-solution-processed high mobility zinc-tin-oxide thin-film-transistors

The full solution-processed oxide thin-film-transistors(TFTs) have the advantages of transparency, ease of large-area fabrication, and low cost, offering great potential applications in switching and driving fields, and attracting extensive research interest. However, the performance of the solution-processed TFTs is generally lower than that of the vacuum-deposited ones. In this article, the full-solution processed TFTs with zinc-tin-oxide(ZTO) semiconductor and aluminium(Al_2O_3) dielectrics were fabricated, and their mobilities in the saturation region are high. Besides, the effect of the Al_2O_3 dielectrics' preparation technology on ZTO TFTs' performance was studied. Comparing the ZTO TFTs using the spin-coated Al_2O_3 dielectrics of 1–4 layers, the ZTO TFT with 3-layer Al_2O_3 dielectrics achieved the optimal performance as its field-effect carrier mobility in the saturation region is 112 cm^2/V s, its threshold voltage is 2.4 V, and its on-to-off current ratio is 2.8×105. This is also the highest reported carrier mobility of the solution-processed ZTO TFTs.

Synergistic high efficiency and low energy loss of all-small-molecule organic solar cells based on benzotriazole-basedπ-bridge unit

Reducing energy loss(V_(loss))is one of the most crucial challenges in organic photovoltaic cells.The V_(loss),determined by the differences between the optical band gap(E_(g))of the active layer material and the open-circuit voltage(V_(oc))of the device,is generally alleviated by lowering the energy difference between the lowest unoccupied molecular orbital(LUMO)and highest occupied molecular orbital(HOMO)level of the donor(D)and acceptor(A).In this work,we synthesized two A-π-D-π-A-type small-molecule donors(SMDs)SM-benzotriazole(BTz)-1 and SM-BTz-2 by introducing a BTzπ-bridge unit and terminal regulation.The BTzπ-bridge unit significantly lowers the HOMO energy level of SMDs,resulting in high V_(oc)and high mobility,achieving a balance of low energy loss(<0.5 eV)and high efficiency.Ultimately,the organic solar cells based on SM-BTz-2 as the donor and Y6 as the acceptor obtain a high V_(oc)of 0.91 V,J_(sc) of 22.8 mA cm^(−2),fill factor of 68%,and power conversion efficiency(PCE)of 14.12%,which is one of the highest efficiencies based on the SMDs with triazoleπ-bridges to date.What’s more,the BTzπ-bridge unit is a potential unit that can improve mobility and reduce energy loss.

Excellent thermoelectric performance of boron-doped n-type Mg_(3)Sb_(2)-based materials via the manipulation of grain boundary scattering and control of Mg content

Thermoelectric devices require thermoelectric materials with high figure-of-merit(ZT)values in the operating temperature range.In recent years,the Zintl phase compound,n-Mg_(3)Sb_(2),has received much attention owing to its rich chemistry and structural complexity.However,it hardly achieves high ZT values throughout the medium temperature range.Herein,by increasing the sintering temperature as much as possible,we successfully increased the average grain size of the compound by 15 times,and the grain boundary scattering was manipulated to obtain high carrier mobility of up to 180 cm^(2)V^(-1)s^(-1).Simultaneously,we optimized the Mg content for ultralow lattice thermal conductivity.We first doped the Mg_(3)Sb_(2)-based materials with boron for higher sintering temperature,good thermal stability,and higher hardness.The synergistic optimization of electrical and thermal transport resulted in excellent ZT values(0.62 at 300 K,1.81 at 773 K)and an average ZT of 1.4(from300 to 773 K),which are higher than the state-of-the-art values for n-type thermoelectric materials,demonstrating a high potential in device applications.

CVD synthesis of nitrogen-doped graphene using urea

This work provides an effective low-cost synthesis and in-depth mechanistic study of high quality large-area nitrogen-doped graphene(NG) films. These films were synthesized using urea as nitrogen source and methane as carbon source, and were characterized by scanning electron microscopy(SEM), Raman spectroscopy and X-ray photoelectron spectroscopy(XPS). The N doping level was determined to be 3.72 at.%, and N atoms were suggested to mainly incorporated in a pyrrolic N configuration. All distinct Raman peaks display a shift due to the nitrogen-doping and compressive strain. The increase in urea concentration broadens the D and 2D peak's Full Width at Half Maximum(FWHM), due to the decrease of mean free path of phonons. The N-doped graphene exhibited an n-type doping behavior with a considerably high carrier mobility of about 74.1 cm2/(V s), confirmed by electrical transport measurements.

Graphdiyne oxide-accelerated charge carrier transfer and separation at the interface for efficient binary organic solar cells

Interfacial engineering for the regulation of the charge carrier dynamics in solar cells is a critical factor in the fabrication of high-efficiency devices.Based on the successful preparation of highly dispersible graphdiyne oxide(GDYO)with a large number of functional groups,we fabricated organic solar cells employing GDYO-modified poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate)(PEDOT:PSS)as hole transport materials.Results show that theπ±πinteraction between GDYO and PEDOT:PSS is beneficial to the formation of an optimized charge carrier transfer channel and improves the conductivity and charge carrier mobility in the hole transport layer.Moreover,the improved interfacial contact contributes to the suppression of charge carrier recombination and the elevation of charge carrier extraction between the hole transport layer and the active layer.More importantly,the occurrence of charge carrier separation benefits from the optimized morphology of the active layer,which efficiently improves the performance,as proven by the results of transient absorption measurements.Therefore,with the holistic management approach to the multiobjective optimization of the charge carrier dynamics,a photoelectric conversion efficiency of 17.5%(with the certified value of 17.2%)is obtained for binary organic solar cells.All of these results indicate the potential application of the functionalized graphdiyne in the field of organic optoelectronic devices.

Theoretical study of the charge carrier mobilities of the molecular materials tetrathiafulvalene (TTF) and 2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene (BDH-TTP)

Tetrathiafulvalene (TTF) is a kind of fused ring aromatic compound containing four sulfur atoms in one molecule, which is well known as a charge transport material. In order to calculate the charge mobility of this semiconductor, Marcus electron transfer theory and the embedded model, which can give small intramolecular reorganization energies, were employed. The calculated results were in good agreement with the experimental values, so the above computing model is appropriate to assess the electrical property of TTF. On this basis, we predicted the charge mobility of 2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene (BDH-TTP) crystals, for which the molecular structure is similar to TTF. The calculated results indicated that BDH-TTP is a p-type material, which has a better performance than TTF in hole transfer due to larger hole coupling and the smaller hole injection barrier. In addition, the direct coupling (DC) and the site energy correction (SEC) methods were used to calculate the charge transfer integrals. Although the results were slightly different, the qualitative trends were the same. Furthermore we took into account the anisotropic transfer properties of TTF and BDH-TTF, since obviously the mobilities along one dimension are larger than those along three dimensions. Finally, natural bond orbital analysis was used to study the interactions in all of the dimers.