对硅片上自组装生长的Pentacene薄膜生长机制及其结晶相态的研究

采用热蒸发的方法在硅片衬底上自组装生长的Pentacene薄膜,薄膜在80℃温度下经2h恒温真空热处理,通过原子力显微镜(AFM)对Pentacene薄膜表面形貌及其生长机制进行研究。结果得到,在硅片上生长的Pentacene薄膜是以台阶岛状结构生长,其岛状直径约为100nm。且Pentacene分子以垂直于衬底的方向生长,台阶岛状结构中每个台阶的平均高度约为1.54nm.s-1,与Pentacene分子的沿长轴方向的长度相近。从Pentacene薄膜的XRD图谱中可以看出,薄膜在形成的过程中会因条件的不同而形成不同的结晶相,分别为薄膜相和三斜体相,且薄膜的结晶相将随着薄膜厚度的增加向三斜体相转变,其临界厚度为80和150nm,当薄膜大于150nm时,薄膜的三斜体相占主导地位,而当Pentacene薄膜的厚度小于80nm时,Pentacene薄膜呈薄膜相存在。

Density Functional Theory Calculations of Charge-Induced Spin Polarization in Pentacene

Based on density functional theory （DFT） calculations, we investigate the spin-related properties of spinless-hole injected organic molecule pentacene （Pc）. DFT calculations reveal that there is spontaneous spin polarization in Pc when spinless-hole is injected. The chargeinduced magnetic moment of Pc increases linearly with the increasing of the extra hole charge amount and its maximum can be up to 1 μB per injected spinless-hole per Pc molecule. The magnetic moment is expected due to the injected unpaired charge. The injected hole will preferably ll the spin-splitted carbon pz orbitals, which makes the Pc molecule spin polarize.

Pentacene分子的Ballistic传导特性(英文)

在考虑到Pentacene分子的碳原子中的唯一个活跃的π电子的情况下,从理论上研究了两端接有金属电极的Pentacene分子带的Ballistic传导特性.利用相邻原子的HOMO和LUMO之间的小能量级差和相对低的LUMO值,得出了由金属电极桥接的Pentacene分子带的量子传导方程,讨论了金属电极和Pen-tacene分子间界面阻抗对传导性的影响,得出了通过Pentacene分子系统的传导流.

Synthesis,characterization of the pentacene and fabrication of pentacene field-effect transistors

A comprehensive understanding of the organic semiconductor material pentacene is meaningful for organic fieldeffect transistors （OFETs）. Thin films of pentacene are the most mobile molecular films known to date. This paper reported that the pentacene sample was successfully synthesized. The purity of pentacene is up to 95%. The results of a joint experimental investigation based on a combination of infrared absorption spectra, mass spectra （MS）, element analysis, x-ray diffraction （XRD） and atom force microscopy （AFM）. The authors fabricated OFET with the synthesized pentacene. Its field effect mobility is about 1.23 cm^2/（V·s） and on-off ratio is above 10^6.

The effect of annealing temperature and film thicknesson the phase of pentacene on the p^+-Si substrate

This paper investigates the morphology and crystallization properties of the two crystalline phases of pentacene grown by thermal evaporation on p^＋-Si substrates at room temperature by the methods of atomic force microscopy and x-ray diffraction. This kind of substrate induces a thin film phase and a triclinic phase which are formed directly onto p^＋-Si substrates and constitute a layer consisting of faceted grains with a step height between terraces of 15.8A（1A=0.1 nm） and 14.9A, respectively. Above the critical thickness of the thin film phase, lamellar structures are found with an increasing fraction with the increase of the film thickness. When the film thickness is fixed, the fraction of lamellar structures increases with the increase of annealing temperature. These lamellar structures are identified as the second phase with a interplanar distance of 14.9A corresponding to the pentacene triclinic phase. Furthermore, the thin film phase consisting of several micrometre sized uniformly oriented grains at an annealing temperature of less than 80℃ and a deposition rate of 0.6A/s is observed.

Introduction of F_4-TCNQ/MoO_3 layers for thermoelectric devices based on pentacene

We introduced a dual electron accepting layer composed of tetrafluoro-tetracyanoquinodimethane （F4-TCNQ） and MoO3 for thermoelectric devices based on a pentacene layer. We found that the power factor is enhanced by placing an F4-TCNQ layer directly in contact with the pentacene layer and it is also enhanced by placing a MoO3 layer between the F4-TCNQ layer and the Au electrode. By examining the contact resistance using a field effect transistor and a hole-only diode, we confirmed that the hole injection is improved due to the reduction of contact resistance at the interface between the MoO3 layer and the Au electrode.

Characteristics of pentacene organic thin film transistor with top gate and bottom contact

High performance pentacene organic thin film transistors （OTFT） were designed and fabricated using SiO2 deposited by electron beam evaporation as gate dielectric material. Pentacene thin films were prepared on glass substrate with S-D electrode pattern made from ITO by means of thermal evaporation through self-organized process. The threshold voltage VTH was -2.75±0.1V in 0-50V range, and that subthreshold slopes were 0.42±0.05V/dec. The field-effect mobility （μEF） of OTFT device increased with the increase of VDS, but the μEF of OTFT device increased and then decreased with increased VGS when VDS was kept constant. When VDS was -50V, on/off current ratio was 0.48×10^5 and subthreshold slope was 0.44V/dec. The μEF was 1.10cm^2/（V.s）, threshold voltage was -2.71V for the OTFT device.

Top Contact Pentacene Organic Thin Film Field Effect Transistors

Using pentacene as an active material, the organic thin film transistors were fabricated on Si3N4/p-Si substrates by using RF-magnetron sputtered amorphous aluminium as the gate electrode contact, and using highly doped Si as the gate electrode and substrate with plasma-enhanced chemical vapor deposited （PECVD） silicon nitride as gate dielectric. Pentacene thin films were deposited by thermal evaporation on dielectrics as the active layer, then RF-magnetron sputtered amorphous aluminium was used as the source and drain contacts. Measurement results show that field effect mobility and threshold voltage are 0.043 cm2/（V·s） and 12.6 V, respectively, and on-off current ratio is nearly 1×103.

Lowering the driving voltage and improving the luminance of blue fluorescent organic light-emitting devices by thermal annealing a hole injection layer of pentacene

We chose pentacene as a hole injection layer（HIL） to fabricate the high performance blue fluorescent organic lightemitting devices（OLEDs）. We found that the carrier mobility of the pentacene thin films could be efficiently improved after a critical annealing at temperature 120℃. Then we performed the tests of scanning electron microscopy, atomic force microscopy, and Kelvin probe to explore the effect of annealing on the pentacene films. The pentacene film exhibited a more crystalline form with better continuities and smoothness after annealing. The optimal device with 120℃ annealed pentacene film and n-doped electron transport layer（ETL） presents a low turn-on voltage of 2.6 V and a highest luminance of 134800 cd/m^2 at 12 V, which are reduced by 26% and improved by 50% compared with those of the control device.

Remote-Plasma-Assisted Deposition of Pentacene Layer Using Atomic-Hydrogen

Pentacene thin layers were deposited on Si with the native oxide at 80°C by remote-plasma-assisted deposition (RPAD) using hydrogen-plasma cell to supply atomic hydrogen radicals. The deposition rate was increased by RPAD comparing to that by non-excited hydrogen gas supply whereas thermal evaporation rate of pentacene from crucible was same in the both process. DFM and XRD studies showed the grain laterally grew in the thin film phase with the size above 10 μm by RPAD. First-principles molecular orbital calculations suggested pentacene is evaporated from crucible as the trimer or larger cluster but atomic hydrogen penetrated into the cluster enhances cracking of pentacene clusters to the monomer.

Influence of Temperature and Pentacene Thickness on the Electrical Parameters in Top Gate Organic Thin Film Transistor

In this contribution, we report on the effect of pentacene thickness and temperature on the performance of top gate transistors. We first investigated the temperature dependence of the transport properties in the temperature range of 258 K - 353 K. The electrical characteristics showed that the threshold voltage (VT) and the onset voltage (Von) remain unchanged. However, the subthreshold current (Ioff), the on-current (Ion) and the field effect mobility (μ) are highly affected with a slight deterioration of subthreshold slope. We observed Arrhenius-like behavior suggesting a thermally activated mobility with an activation energy EA = 68 meV. Moreover the dependence of the charge carrier mobility on the organic semiconductor thickness has also been studied. The mobility decreased as the pentacene thickness increases whereas the threshold voltage and Ioff current remain minimally affected. In order to understand the transport properties and in view to put in light morphology peculiarities of pentacene, AFM images were performed. It turns out that the pentacene grain sizes are smaller and disorganized as the film thickness increases, and charge carriers are more prone to be trapped, leading to decrease the field effect mobility and the Ion current. The devices were also tested under bias stress and the transistors with low thicknesses exhibited a relatively good electrical stability compared to those with high pentacene thicknesses. This work points out the influence of temperature, semiconductor thickness and bias stress effect on the device performance and stability of transistor using top gate configuration.

Electrical Instability in Pentacene Transistors with Mylar and PMMA/Mylar Gate Dielectrics Transferred by Lamination Process

This study deals with electrical instability under bias stress in pentacene-based transistors with gate dielectrics deposited by a lamination process. Mylar film is laminated onto a polyethylene terephthalate (PET) substrate, on which aluminum (Al) gate is deposited, followed by evaporation of organic semiconductor and gold (Au) source/drain contacts in bottom gate top contact configuration (Device 1). In order to compare the influence of the semiconductor/dielectric interface, a second organic transistor (Device 2) which is different from the Device 1 by the deposition of an intermediate layer of polymethyl methacrylate (PMMA) onto the laminated Mylar dielectric and before evaporating pentacene layer is fabricated. The critical device parameters such as threshold voltage (VT), subthreshold slope (S), mobility (μ), onset voltage (Von) and Ion/Ioff ratio have been studied. The results showed that the recorded hysteresis depend on the pentacene morphology. Moreover, after bias stress application, the electrical parameters are highly modified for both devices according to the regimes in which the transistors are operating. In ON state regime, Device 1 showed a pronounced threshold voltage shift associated to charge trapping, while keeping the μ, Ioff current and S minimally affected. Regardless of whether Device 2 exhibited better electrical performances and stability in ON state, we observed a bias stress-induced increase of depletion current and subthreshold slope in subthreshold region, a sign of defect creation. Both devices showed onset voltage shift in opposite direction.

Effects of gate dielectric thickness and semiconductor thickness on device performance of organic field-effect transistors based on pentacene

In this paper,the pentacene-based organic field-effect transistors(OFETs)with poly(methyl methacrylate)(PMMA)as gate dielectrics were fabricated,and the effects of gate dielectric thickness and semiconductor thickness on the device performance were investigated.The optimal PMMA thickness is in the range of 350–400 nm to sustain a considerable current density and stable performance.The device performance depends on the thicknesses of the active layer non-monotonically,which can be explained by the morphology of the pentacene film and the position of the conducting channel in the active layer.The device with a pentacene thickness of 50 nm shows the best performance,which has a maximum hole mobility of 1.12 cm2/V·s.In addition,the introduction of a thin layer of tris-(8-hydroxyquinolinato)aluminum(Alq3)to the OFETs as a light-emitting material greatly decreases the device performance.

利用不同阴极缓冲层来改善Pentacene/C_(60)太阳能电池的性能

制备了结构为ITO/Pentacene/C60/Al的双层光伏电池器件,在C60/Al界面插入了常用的缓冲层材料bathocuproine(BCP)作为阴极缓冲层,通过优化BCP层的厚度来提高电池的性能并研究了阴极缓冲层的作用机理.实验发现,BCP厚度为10nm时器件的效率最高,为0.46.在此基础上,利用bathophenanthroline(Bphen)和3,4,9,10-Perylenetetracarb-oxylicdianhydride(PTCDA)材料取代BCP,分别研究了缓冲层材料电子迁移率以及光吸收特性对器件性能的影响.在使用电子迁移率比BCP高两个数量级的Bphen材料作为缓冲层后,电池效率提高到了0.56.而当使用在可见光区有较强光吸收的PTCDA材料作为缓冲层时,可以起到增加电池光吸收的作用,电池短路电流提高至5.97mA/cm2,效率达0.87.

Template-directed assembly of pentacene molecules on epitaxial graphene on Ru（0001）

The template-directed assembly of planar pentacene molecules on epitaxial graphene grown on Ru（0001） （G/Ru） has been investigated by means of low-temperature scanning tunneling microscopy （STM） and density functional theory （DFT） calculations. STM experiments find that pentacene adopts a highly selective and dispersed growth mode in the initial stage. By using DFT calculations including van der Waals interactions, we find that the configuration with pentacene adsorbed on face-centered cubic （fcc） regions of G/Ru is the most stable one, which accounts for the selective adsorption at low coverage. Moreover, at high coverage, we have successfully controlled the molecular assembly from amorphous, local ordering, to long-range order by optimizing the deposition rate and substrate temperature.

Self‐assembled fullerene(C60)‐pentacene superstructures for photodetectors

Fullerene assembling with specific donor molecules would yield multi-functional metamaterials via the collective behavior,wherein linear acenes are widely used as donor molecules to construct the charge‐transfer heterojunction structure with fullerene.However,they are generally prepared by vacuum deposition due to the insoluble property of high‐performance linear acenes molecules in common solvents,which makes the construction of fullerene with insoluble donor molecules still be a big challenge in the solution‐processed method.To this end,chemical modification provides an effective solution‐processed strategy to construct donor and acceptor systems.Here,the C60‐pentacene is assembled into controllable flower‐like superstructures by the surface grafting method.It is found that the nanofeatures of the microflowers could be regulated by temperature,resulting in dense‐flakes morphology at room temperature and loose flakes at high temperatures.Furthermore,the dense‐flakes microflowers structures with less mass but better crystalline structure exhibit better optoelectronic properties.Our results reveal an effective control on the nanofeatures of the self‐assembled fullerenes complex super-structures and their role for the optoelectronic performance,which may promote the exploring of fullerene superstructures as photodetectors.

Enhanced performance of C_(60) N-type organic field-effect transistors using a pentacene passivation layer

We investigated the properties of C_（60）-based organic field-enect transistors（OFETs）（？） a pentacene passivation layer inserted between the C_（60） active layer and the gate dielectric.After modification of the pentacene passivation layer,the performance of the devices was considerably improved compared to C_（60）-based OFETs with only a PMMA dielectric.The peak field-effect mobility was up to 1.01 cm^2/（V·s） and the on/off ratio shifted to 10~4.This result indicates that using a pentacene passivation layer is an effective way to improve the performance of N-type OFETs.

Transient photovoltage and photoluminescence study of exciton dissociation at indium tin oxide/pentacene interface

The exciton dissociation at ITO/pentacene interface is studied by means of transient photovoltage measurement.Opposite to ITO/NPB,ITO/CuPc or ITO/C60 interface where polarity change of transient photovoltage is observed,no interfacial dissociation is found at room temperature,which indicates a lack of Frenkel excitons in pentacene.Temperature-dependent photoluminescence (PL) is investigated.More like the behavior of inorganic semiconductors,the integrated PL intensity exhibits monotonic decrease with increasing temperature.A nonradiative path with characteristic activation energy of 8 meV is found to dominate at room temperature.The PL measurement also indicates that like in inorganic semiconductors,other types of excitation,for example,free carriers,could be responsible for the photoelectric processes.

Structures and Nonlinear Optical Properties of Lithium-adsorbed Polycyclic π-Conjugated Pentacene Systems

Structures and nonlinear optical（NLO） properties of eleven new Lin-Pm（n=1-5） species were investigated in detail with the help of ab initio computation, in which one to the maximum five Li atoms are doped over the polycyclic π-conjugated pentacene. These Li-doped pentacene systems exhibit large adsorption energies（ca. 107.0--141.3 kJ/mol） and considerable first hyperpolarizabilities（even up to 4.1×10^4 a.u.）, where the number of Li atoms, the doping site, and the distance between the neighboring Li atoms have important impacts on the flo value. In the doped pentacene systems with less Li atoms（one or two）, the improvement of β10 value can be attributed to the simple transfer of the charge from Li atom to pentacene. Differently, doped more Li atoms（three to five） can cause not only charge transfer but also excess electron, and this cooperation can endow the doped systems with the much larger first hyperpolarizabilities. These fascinating findings are advantageous for the design of new NLO materials based on the intriguing polycyclic π-conjugated systems.