Two-Dimensional Transition Metal Dichalcogenides and Their Charge Carrier Mobilities in Field-Effect Transistors

Two-dimensional(2D) materials have attracted extensive interest due to their excellent electrical, thermal,mechanical, and optical properties. Graphene has been one of the most explored 2D materials. However, its zero band gap has limited its applications in electronic devices. Transition metal dichalcogenide(TMDC), another kind of 2D material,has a nonzero direct band gap(same charge carrier momentum in valence and conduction band) at monolayer state,promising for the efficient switching devices(e.g., field-effect transistors). This review mainly focuses on the recent advances in charge carrier mobility and the challenges to achieve high mobility in the electronic devices based on 2DTMDC materials and also includes an introduction of 2D materials along with the synthesis techniques. Finally, this review describes the possible methodology and future prospective to enhance the charge carrier mobility for electronic devices.

First-principles hybrid functional study of the electronic structure and charge carrier mobility in perovskite CH_3NH_3SnI_3

We calculate the electronic properties and carrier mobility of perovskite CH3NH3SnI3as a solar cell absorber by using the hybrid functional method. The calculated result shows that the electron and hole mobilities have anisotropies with a large magnitude of 1.4 × 104cm2·V-1·s-1along the y direction. In view of the huge difference between hole and electron mobilities, the perovskite CH3NH3 Sn I3can be considered as a p-type semiconductor. We also discover a relationship between the effective mass anisotropy and electronic occupation anisotropy. The above results can provide reliable guidance for its experimental applications in electronics and optoelectronics.

Carrier diffusion coefficient is independent of defects in CH_(3)NH_(3)PbBr_(3) single crystals:Direct evidence

Organolead halide perovskite solar cells have attracted extensive interests in recent years. Thanks to innovations in materials process and technology, the power conversion efficiency (PCE) of perovskites-based solar cells increases from 3.8% to 25.2%[1–4].In evaluating the quality of perovskite materials, a few key photophysical properties such as the lifetime, mobility and diffusion length of photogenerated carriers are usually measured.

First-principles analysis of the structural, electronic, and elastic properties of cubic organic-inorganic perovskite HC(NH_2)_2PbI_3

The structural, electronic, and elastic properties of cubic HC（NH2）2PbI3 perovskite are investigated by density functional theory using the Tkatchenko-Scheffler pairwise dispersion scheme. Our relaxed lattice parameters are in agreement with experimental data. The hydrogen bonding between NH2 and I ions is found to have a crucial role in FAPbI3 stability. The first calculated band structure shows that HC（NH2）2PbI3 has a direct bandgap （1.02 eV） at R-point, lower than the bandgap （1.53 eV） of CH3NH3PbI3. The calculated density of states reveals that the strong hybridization of s（Pb）-p（I） orbital in valence band maximum plays an important role in the structural stability. The photo-generated effective electron mass and hole mass at R-point along the R-Γ and R-M directions are estimated to be smaller：me^＊=0.06m0 and mh^＊=0.08m0 respectively, which are consistent with the values experimentally observed from long range photocarrier transport. The elastic properties are also investigated for the first time, which shows that HC（NH2）2PbI3 is mechanically stable and ductile and has weaker strength of the average chemical bond. This work sheds light on the understanding of applications of HC（NH2）2PbI3 as the perovskite in a planar-heterojunction solar cell light absorber fabricated on flexible polymer substrates.

单层MoS_(2)全包覆晶体管的电学性能极限

基于二维过渡金属硫化物的全包覆晶体管是后硅时代的理想电子器件,从玻尔兹曼输运理论出发,在考虑载流子的主要外秉散射机制后,针对三种典型的栅介质(Al_(2)O_(3),HfO_(2)和BN)构筑的单层MoS_(2)晶体管的电学性能进行了系统的理论研究。为解决介质中表面光学声子散射被高估这一常见问题,使计算更为准确,在电介质和沟道间的建模中引入了“死区”概念。重点针对微电子1nm及以下技术节点用途晶体管的电荷迁移率和电流密度等指标进行了探讨.研究表明,在沟道长度小于10nm的情况下,晶体管开态电流均可超2mAμm。上述结果阐明了基于单层半导体构筑晶体管在终极微缩条件下的性能潜力,对深度摩尔器件设计具有一定的参考价值。

Meniscus-Assisted Solution Printing Enables Cocrystallization in Poly(3-alkylthiophene)-based Blends for Field-Effect Transistors

The formation of cocrystallization in two various conjugated components may endow the newly formed conjugated cocrystals with multiple functionalities and improved charge transport properties.However,compared to conjugated small molecules,this strategy is rather limitedly realized in conjugated polymers.Herein,a simple meniscus-assisted solution printing(MASP)strategy is utilized to achieve the cocrystallization in the blends of two conjugated polymers,i.e.,poly(3-hexylthiophene)(P3HT)and poly(3-octylthiophene)(P3OT),and the cocrystalline structures are correlated closely to their charge mobilities.The P3HT/P3OT blends phase separate and crystallize individually in their drop-cast thin films.When subjecting the P3HT/P3OT blended solution to MASP,the confined solvent evaporation between two nearly parallel plates triggers them to cocrystallize progressively when accelerating the moving lower plate.The cocrystallization kinetics and the changes in P3HT/P3OT molecular structures are elucidated.Finally,these different crystalline structures of P3HT/P3OT blends are applied in organic fieldeffect transistors,imparting the cocrystallization-enhanced charge transport than respective P3HT and P3OT crystal domains.Such MASP method can be extended to craft cocrystals of other conjugated polymer blends for their diverse optoelectronic applications.

Trap Characteristics and Their Temperature-dependence of Silicone Gel for Encapsulation in IGBT Power Modules

Silicone gel is a prevailing material for encapsulation in insulated gate bipolar transistor(IGBT)power modules.The space charge transport behavior in silicone gel is significant to evaluate the electrical insulation characteristics.This paper focuses on the trap characteristics and electrical properties of the silicone gel,which were rarely studied before.The experiments are performed on the surface potential decay of silicone gel after the charge injection.Then,the energy distributions of electron or hole traps are determined by a double-trap energy level model,which can be fitted by the Gaussian distribution.In addition,the mobilities of positive and negative charges are determined,which are 1.38×10^(-12) m^(2)·V^(-1)·s^(-1) and 1.74×10^(12) m^(2)·V^(-1)·s^(-1),respectively.Furthermore,considering the heat as a byproduct resulting in thermal issues,the temperature-dependence of surface potential decay characteristics are also studied in this paper.When temperature rises,the decay rate of surface potential increases,especially when the temperature is higher than 80℃.Finally,the contrastive analysis illustrates that the trap characteristics of silicone gel are between the trap characteristics in liquid-state material and solid-state material,which supports the phenomenon that silicone gel is more resistive to the sharp edges in power modules.This work can provide a useful reference for the design of encapsulation in high-voltage IGBT power modules.

通过晶相调控实现高性能十字形TTF分子场效应晶体管器件构筑

有机分子溶剂的选择对于调控不同晶相来说有着关键性作用.本文通过两种溶剂的调控,成功得到茚并芴四硫富瓦烯(IF-TTF)两种不同的晶相—α相带状晶体和β相片状晶体,并对两种晶相的内部分子排列堆积情况进行了一系列的对比分析.结果表明场效应电荷传输能力随着晶相的不同而有所差异,直接证明了分子堆积的合理调控对实现有机半导体材料高性能电荷传输性能的重要性.

Fine-tuning of Polymer Photovoltaic Properties by the Length of Alkyl Side Chains

This paper reports the synthesis and characteristics of a series of alkyl-substituted planar polymers. The physical properties are carefully tuned to optimize their photovoltaic performance. Depending on the length of soluble alkyl side chains which modify the structural order and orientation substantially in polymer backbones, the device performance can be improved significantly. The tuning of HOMO energy levels optimized polymers＇ spectral coverage of absorption and their hole mobility, as well as miscibility with fullerene; all these efforts enhanced polymer solar cell performances. The short- circuit current, Jsc for polymer solar cells was increased by adjusting polymer chain packing ability. It was found that films with well distributed polymer/fullerene interpenetrating network exhibit improved solar cell conversion efficiency. Enhanced efficiency up to 5.8% has been demonstrated. The results provide important insights about the roles of flexile chains in structure-property relationship for the design of new polymers to be used in high efficient solar cells.

Selenium-containing core-expanded naphthalene diimides for high performance n-type organic semiconductors

The incorporation of heavy atoms into molecular backbone is an extremely straightforward strategy for fine-tuning the optoelectronic properties of organic semiconductors.However,it is rarely studied in n-type small molecules.Herein,by selenium substitution of NDI3 HU-DTYM2,two Se-decorated core-expanded naphthalene diimides(NDI)derivatives DTYM-NDI3 HUDSYM(1)and NDI3 HU-DSYM2(2)were synthesized.In comparison with the reference S-containing compound NDI3 HUDTYM2,the highest occupied molecular orbital(HOMO)and lowest unoccupied molecular orbital(LUMO)energy levels of 1 and 2 were fine-tuned with?HOMO of about 0.2 e V,?LUMO of 0.1 e V and the narrowed HOMO-LUMO gaps.More surprisingly,the as-spun organic thin film transistors(OTFTs)based on 1 and 2 both showedμe,satvalues as high as1.0 cm2 V-1 s-1,which are 2-fold higher than that of NDI3 HU-DTYM2 with the same device structure and measurement conditions.In addition,the single crystal OFET devices based on Se-containing compound NDI2 BO-DSYM2 showed a highμe,satvalue of 1.30 cm2 V-1 s-1.The molecular packing of NDI2 BO-DSYM2 in single crystals(two-dimensional supramolecular structure formed by intermolecular Se···Se interactions)is quite different from that of a S-containing compound NDI-DTYM2(one dimensional supramolecular structure formed by intermolecularπ-πstacking).Therefore,the Se substitution can cause dramatic change about molecular stacking model,giving rise to high n-type OTFT performance.Our results demonstrated an effective strategy of the heavy atom effect for designing novel organic semiconductors.

Theoretical Study of Electronic Structures and Charge Transport Properties of 9,10-Bis（（E）-2-（pyrid-n-yl） vinyl） （n=2, 3, 4） Anthracene

Charge carrier mobility is one of the most significant properties for organic semiconductors. In this work, the electronic structures and charge transport properties of 9,10-bis（（E）-2-（pyrid-n-yl）vinyl） （n=2, 3, 4） anthracene （BP2VA, BP3VA and BP4VA） were investigated via the analysis of the molecular geometry, the reorganization en- ergy, the frontier orbital and density of state, as well as the electronic coupling and the charge mobility. The results indicated that the linkage between 9,10-divinyl anthracene unit and pyridine （ortho-, meta- and para-） influenced not only the intra-molecular conformation （i.e., the reorganization energies）, but also the intermolecular interaction （i.e., transfer integrals）, and finally the charge mobility of the molecules. It is also found that： （1） The calculated charge mobilties of holes are dozens of times higher than those of electrons for the three molecules. （2） The charge mobilities of hole and electron of the three molecules display the trend： μBP4VA〉μBPZVA〉μBP3VA, and the hole mobility of BP4VA is as high as as-cm^2/（V·s）.

Novel n-channel organic semiconductor based on pyrene-phenazine fused monoimide and bisimides

Large π-conjugated pyrene-phenazine monoimide and bisimides were synthesized by imine condensation reaction. These imides form well ordered 1D nanotapes upon self-assembly in solution. Electrochemical and electric conductivity measurement reveal it can be served as an n-channel semiconductor with large charge carrier mobility up to 4.1 cm^2 V^-1 s^-1. Both alkylated imides are highly luminescent, and can be quenched via protonization using trifluoroacetic acid, which could be served as potential colorimetric acid sensors.