Horizontal orientation of linear-shaped organic molecules having bulky substituents in neat and doped vacuum-deposited amorphous films

Organic amorphous films fabricated by vacuum deposition have been widely used in organic light-emitting devices, making use of their high-performance optical and electrical characteristics and taking advantage of the easy fabrication of pinhole-free thin smooth layers of a desired thickness. However, random orientation in amorphous films often makes it difficult to utilize their best optical and electrical potential. Here the authors demonstrate that the linear-shaped molecules of fluorescent styrylbenzene derivatives are horizontally oriented in organic amorphous films fabricated by conventional vacuum deposition even when the molecules are doped in an isotropic host matrix film. The longer the molecular length is, the larger the anisotropy of the molecular orientation becomes. The weak interaction between adjacent molecules and the linear-shaped molecular structure probably cause the horizontal orientation. The fact that the horizontal molecular orientation occurs on any underlying layers shows the high versatility of the horizontal orientation for various applications. Their findings will provide a new guideline for molecular designs that can be used to improve optical and electrical characteristics of organic optoelectronic devices, such as organic light-emitting diodes and organic laser devices.     

Orientation Control of Linear‐Shaped Molecules in Vacuum‐Deposited Organic Amorphous Films and Its Effect on Carrier Mobilities

The molecular orientation of linear‐shaped molecules in organic amorphous films is demonstrated to be controllable by the substrate temperature. It is also shown that the molecular orientation affects the charge‐transport characteristics of the films. Although linear‐shaped 4,4′‐bis[(N‐carbazole)styryl]biphenyl molecules deposited on substrates at room temperature are horizontally oriented in amorphous films, their orientation when deposited on heated substrates with smooth surfaces becomes more random as the substrate temperature increases, even at temperatures under the glass transition temperature. Another factor dominating the orientation of the molecules deposited on heated substrates is the surface roughness of the substrate. Lower carrier mobilities are observed in films composed of randomly oriented molecules, demonstrating the significant effect of a horizontal molecular orientation on the charge‐transport characteristics of organic amorphous films.     

Structure and electrical properties of polycrystalline SiGe films grown by molecular beam deposition

The structural and electrical properties of polycrystalline Si_0.5Ge_0.5 films 150 nm thick grown by molecular beam deposition at temperatures of 200–550°C on silicon substrates coated with amorphous layers of silicon oxynitride were studied. It is shown that the films consist of a mixture of amorphous and polycrystalline phases. The amorphous phase fraction decreases from ～50% in films deposited at 200°C to zero in films grown at 550°C. Subsequent 1-h annealing at a temperature of 550°C results in complete solid-phase crystallization of all films. The electron transport of charge carriers in polycrystalline films occurs by the thermally activated mechanism associated with the energy barrier of ～0.2 eV at grain boundaries. Barrier lowering upon additional annealing of SiGe films correlates with an increase in the average grain size.     

Simultaneous Edge‐on to Face‐on Reorientation and 1D Alignment of Small π‐Conjugated Molecules Using Room‐Temperature Mechanical Rubbing

In this study, room‐temperature mechanical rubbing is used to control the 3D orientation of small π‐conjugated molecular systems in solution‐processed polycrystalline thin films without using any alignment substrate. High absorption dichroic ratio and significant anisotropy in charge carrier mobilities (up to 130) measured in transistor configuration are obtained in rubbed organic films based on the ambipolar quinoidal quaterthiophene (QQT(CN)4). Moreover, a solvent vapor annealing treatment of the rubbed film is found to improve the optical and charge transport anisotropy due to an increased crystallinity. X‐ray diffraction and atomic force microscopy measurements demonstrate that rubbing does not only lead to an excellent 1D orientation of the QQT(CN)4 molecules over large areas but also modifies the orientation of the crystals, moving molecules from an edge‐on to a face‐on configuration. The reasons why a mechanical alignment technique can be used at room temperature for such a polycrystalline film are rationalized, by the plastic characteristics of the QQT(CN)4 layer and the role of the flexible alkyl side chains in the molecular packing. This nearly complete conversion from edge‐on to face‐on orientation by mechanical treatment in polycrystalline small‐molecule‐based thin films opens perspectives in terms of fundamental research and practical applications in organic optoelectronics.     

Time–Temperature–Transformation (TTT) Diagrams for Crystallization of Metal Oxide Thin Films

Time–temperature–transformation (TTT) diagrams are proposed for the crystallization of amorphous metal oxide thin films and their specific characteristics are discussed in comparison to glass‐based materials, such as glass‐ceramics and metallic glasses. The films crystallize from amorphous to full crystallinity in the solid state. As an example the crystallization kinetics for a single‐phase metal oxide, ceria, and its gadolinia solid solutions are reported made by the precipitation thin‐film method spray pyrolysis. The crystallization of an amorphous metal oxide thin film generally follows the Lijschitz–Sletow–Wagner (LSW) Ostwald ripening theory: Below the percolation threshold of 20 vol% single grains crystallize in the amorphous phase and low crystallization rates are measured. In this state no impact of solute on crystallization is measurable. Once the grains form primary clusters above the threshold the solute slows down crystallization (and grain growth) thus shifting the TTT curves of the doped ceria films to longer times and higher temperatures in comparison to undoped ceria. Current views on crystallization of metal oxide thin films, the impact of solute dragging, and primary TTT diagrams are discussed. Finally, examples on how to use these TTT diagrams for better thermokinetic engineering of metal oxide thin films for MEMS are given, for example, for micro‐Solid Oxide Fuel Cells and resistive sensors. In these examples the electrical properties depend on the degree of crystallinity and, thereby, on the TTT conditions.     

Single‐Crystal Germanium Growth on Amorphous Silicon

Growing single‐crystal semiconductors directly on an amorphous substrate without epitaxy or wafer bonding has long been a significant fundamental challenge in materials science. Such technology is especially important for semiconductor devices that require cost‐effective, high‐throughput fabrication, including thin‐film solar cells and transistors on glass substrates as well as large‐scale active photonic circuits on Si using back‐end‐of‐line CMOS technology. This work demonstrates a CMOS‐compatible method of fabricating high‐quality germanium single crystals on amorphous silicon at low temperatures of <450 °C. Grain orientation selection by geometric confinement of polycrystalline germanium films selectively grown on amorphous silicon by chemical vapor deposition is presented, where the confinement selects the fast‐growing grains for extended growth and eventually leads to single crystalline material. Germanium crystals grown using this method exhibit (110) texture and twin‐mediated growth. A model of confined growth is developed to predict the optimal confining channel dimensions for consistent, single‐crystal growth. Germanium films grown from one‐dimensional confinement exhibit a 200% grain size increase at 1 μm film thickness compared to unconfined films, while 2D confinement growth achieved single crystal Ge. The area of single crystalline Ge on amorphous layers is only limited by the growth time. Significant enhancement in room temperature photoluminescence and reduction in residual carrier density have been achieved using confined growth, demonstrating excellent optoelectronic properties. This growth method is readily extensible to any materials system capable of selective non‐epitaxial deposition, thus allowing for the fabrication of devices from high‐quality single crystal material when only an amorphous substrate is available.     

Co基磁性薄膜的快速循环纳米晶化

采用独特的快速循环纳米晶化技术(RRTA)对直流磁控溅射制备的非晶CoNbZr软磁膜进行纳米晶化。研究了不同的纳米晶化工艺条件下,薄膜的微观结构和软磁性能。结果表明,CoNbZr软磁薄膜晶粒细化到30 nm,RRTA晶化方法可有效地控制CoNbZr薄膜的软磁性能。     

PECVD制备纳米晶多晶硅薄膜

以Si(100)为衬底,采用磁控溅射和射频等离子体增强化学气相沉积系统制备了Si(100)/Al膜/非晶Si膜结构的样品。对该样品进行Al诱导真空退火以制备多晶硅薄膜,采用X射线衍射仪(XRD)和AFM分析薄膜微结构及表面形貌。实验结果表明,在经过500℃、550℃Al诱导退火后,形成了择优取向为〈111〉晶向的多晶硅薄膜。AFM给出了550℃退火后薄膜表面形貌,为100～200nm大小的圆丘状硅晶粒,密集排列在薄膜表面;并对Al诱导真空退火晶化的机理进行了分析。     

Infrared spectroscopy and transmission electron microscopy of polycrystalline silicon carbide

The polycrystalline structure of silicon carbide was investigated by infrared spectroscopy and transmission electron microscopy (TEM). The films were obtained by annealing in the temperature range 950–1250°C of amorphous silicon carbide films deposited on a silicon substrate by PECVD. The broad absorption band at around 750 cm⁻¹ in the infrared spectrum of amorphous material after annealing at high temperature changes from a Gaussian to a Lorentzian shape, corresponding to the transition from an amorphous to a polycrystalline phase. The SiC peak becomes sharper with increasing the annealing temperature, this effect being related to the growth of crystalline grains. TEM microscopy indicates that the crystallisation occurs homogeneously in the films and the diffraction pattern shows that the film crystallises into cubic 3C–SiC. The distribution of polycrystalline grains as determined by TEM evidences an increase of the grain size with increasing the annealing temperature. A correlation between infrared peak width and mean grain radius has been found.     

Influence of rapid thermal and low temperature processing on theelectrical properties of polysilicon thin film transistors

In this paper rapid thermal processing (RTP) is studied for the crystallization and oxidation of deposited silicon layers. The purpose is to present and compare the results obtained by RTP, low temperature processing (LTP), or a combination of both, for the fabrication of polycrystalline silicon thin film transistors (poly-TFT's). The polysilicon and polyoxide are obtained by low thermal annealing, oxidation (LTA, O) and/or rapid thermal annealing, oxidation (RTA, O) of amorphous silicon films deposited from disilane at a temperature of 465°C. For the Si films annealed at 750°C or higher, using RTA, the grain average sizes are reduced whereas the electron/hole mobilities are increased. We suggest that there is a correlation between the optical extinction coefficient k (at λ=405 nm), the potential barrier height Φ_B due to the grains, and the field-effect mobility, μ_n,p, of the polysilicon film. This correlation indicates that the polysilicon film electrical properties depend not only on the grain size, but also on the crystalline quality of the grains. Moreover, it appears that the large amount of crystalline defects remaining in the so-called “grains” of the films annealed at 600°C (LTA) are partially annihilated when the films are annealed at higher temperatures. With regards to the TFT's electrical characteristics, the work suggests combining RT and LT steps to obtain TFT's with improved electrical performance     

Microstructure evolution of hydrogenated silicon thin films

This paper describes the microstructure evolution of hydrogenated silicon films containing various amounts of hydrogen. Microcrystalline silicon films were produced when the hydrogen content of the films was adjusted by using the diluted hydrogen and hydrogen atom treatment methods. Polycrystalline silicon films having grain sizes in the micrometre range were deposited at low temperatures (250°C) by electron cyclotron resonance chemical vapour deposition with the hydrogen dilution method. The micro crystalline and polycrystalline films were characterized by NMR, FTIR, Raman, X-ray and optical spectroscopy and electrical measurements. The results suggest the possibility of even larger grain silicon films suitable for high-performance solar cells which avoid the fundamental difficulties of amorphous Si:H solar cells.     

Characteristics of the electrochromic materials Au-Wo3 and Pt-Wo3

The electrochromic cermet Au-WO₃ is composed of grains of Au, approximately 20-120å in diameter, embedded in a matrix of amorphous WO₃. As prepared, the cermet is blue and when electrochemically colored it is red or pink. The matrix must be amorphous in order for the red color to develop. In polycrystalline Au-WO₃ films, the colored state is dark blue. We find that the properties of the amorphous WO₃ matrix in the presence of Au are different from pure amorphous WO₃. In particular, the presence of the Au grains suppresses the high electrical conductivity characteristic of colored WO₃. Pt-WO₃ cermets are compared and contrasted with Au-WO₃. We discuss the optical and electrical properties of the Au-W0₃ cermets and their possible use in display devices.     

The Influence of Solubilizing Chain Stereochemistry on Small Molecule Photovoltaics

Three stereochemically pure isomers and two isomeric mixtures of a solution‐processable diketopyrrolopyrrole‐containing oligothiophene ( SMDPPEH ) have been used to study the effect of 2‐ethylhexyl solubilizing group stereochemistry on the film morphology and bulk heterojunction (BHJ) solar cell characteristics of small molecule organic photovoltaics. The different SMDPPEH stereoisomer compositions exhibit nearly identical optoelectronic properties in the molecularly dissolved state, as well as in amorphous films blended with PCBM. However, for films in which SMDPPEH crystallization is induced by thermal annealing, significant differences in molecular packing between the different stereoisomer formulations are observed. These differences are borne out in photovoltaic device characteristics for which unannealed devices show very similar behavior, while after annealing the RR‐ and SS‐SMDPPEH enantiomers show blue‐shifted peak EQE relative to the SMDPPEH isomer mixtures. Unannealed devices made from the most crystalline stereoisomer, meso RS‐SMDPPEH , are not completely amorphous, and show improved photocurrent generation as a result. Unlike the other compounds, after thermal annealing the RS‐SMDPPEH devices show reduced device performance. The results reveal that the chirality of commonly used 2‐ethylhexyl solubilizing chains can have a significant effect on the morphology, absorption, and optimum processing conditions of small molecule organic thin films used as photovoltaic device active layers.     

Molecular Stacking Induced by Intermolecular C–H···N Hydrogen Bonds Leading to High Carrier Mobility in Vacuum‐Deposited Organic Films

Simple bottom‐up fabrication processes for molecular self‐assembly have been developed for the construction of higher‐order structures using organic materials, and have contributed to maximization of the potential of organic materials in chemical and bioengineering. However, their application to organic thin‐film devices such as organic light‐emitting diodes have not been widely considered because simple fabrication of a solid film containing an internal self‐assembly structure has been regarded as difficult. Here it is shown that the intermolecular C–H···N hydrogen bonds can be simply formed even in vacuum‐deposited organic films having flat interfaces. By designing the molecules containing pyridine rings properly for the intermolecular interaction, one can control the molecular stacking induced by the intermolecular hydrogen bonds. It is also demonstrated that the molecular stacking contributes to the high carrier mobility of the film. These findings provide new guidelines to improve the performance of organic optoelectronic devices and open up the possibilities for further development of organic devices with higher‐order structures.     

Nano-morphology characterization of organic bulk heterojunctions based on mono and bis-adduct fullerenes

We have studied organic bulk heterojunction photovoltaic devices based on a bridged-bithiophene donor–acceptor type low-band gap polymer blended with PCBM and bis-PCBM. The impact of the molecular arrangement is discussed in terms of the correlation between the solar-cell performance and the degree of crystallization. Differential scanning calorimetry (DSC) and grazing-incidence X-ray diffraction (GIXRD) prove that films with bis-PCBM typically result in more amorphous blends than comparable films with PCBM. Electron tomography (ET) is used to visualize the three dimensional morphology of photoactive layers, confirming the presence of nanofibers, formed in different scales through the thickness in the blended films with mono and bis-fullerenes.     

Efficient Perovskite Light‐Emitting Diodes Using Polycrystalline Core–Shell‐Mimicked Nanograins

Making small nanograins in polycrystalline organic–inorganic halide perovskite (OIHP) films is critical to improving the luminescent efficiency in perovskite light‐emitting diodes (PeLEDs). 3D polycrystalline OIHPs have fundamental limitations related to exciton binding energy and exciton diffusion length. At the same time, passivating the defects at the grain boundaries is also critical when the grain size becomes smaller. Molecular additives can be incorporated to shield the nanograins to suppress defects at grain boundaries; however, unevenly distributed molecular additives can cause imbalanced charge distribution and inefficient local defect passivation in polycrystalline OIHP films. Here, a kinetically controlled polycrystalline organic‐shielded nanograin (OSN) film with a uniformly distributed organic semiconducting additive (2,2′,2′′‐(1,3,5‐benzinetriyl)‐tris(1‐phenyl‐1‐H‐benzimidazole), TPBI) is developed mimicking core–shell nanoparticles. The OSN film causes improved photophysical and electroluminescent properties with improved light out‐coupling by possessing a low refractive index. Finally, highly improved electroluminescent efficiencies of 21.81% ph el^?1 and 87.35 cd A^?1 are achieved with a half‐sphere lens and four‐time increased half‐lifetime in polycrystalline PeLEDs. This strategy to make homogeneous, defect‐healed polycrystalline core–shell‐mimicked nanograin film with better optical out‐coupling will provide a simple and efficient way to make highly efficient perovskite polycrystal films and their optoelectronics devices.     

Interaction of heterogeneous thin films and phase formation in the Tl-Fe-S system

The processes of phase formation and phase transitions in Tl-Fe-S thin layers are investigated by the methods of high-energy electron diffraction and kinematic electronography using an EMR-102 electron diffractometer. It is established that compounds of the Tl-S and Fe-S systems are formed in amorphous and highly dispersed polycrystalline states, respectively. A ternary compound with the composition TlFeS₂ formed in the amorphous state crystallizes in the monoclinic lattice under heat treatment. Upon recrystallization of the polycrystalline films of monoclinic TlFeS₂, the monoclinic lattice of the TlFeS₂ secondary phase is formed.     

Room‐Temperature Self‐Organizing Characteristics of Soluble Acene Field‐Effect Transistors

We report on the room‐temperature self‐organizing characteristics of thin films of the organic small‐molecule semiconductor triethylsilylethynyl‐anthradithiophene (TES‐ADT) and its effect on the electrical properties of TES‐ADT‐based field‐effect transistors (FETs). The morphology of TES‐ADT films changed dramatically with time, and the field‐effect mobility of FETs based on these films increased about 100‐fold after seven days as a result of the change in molecular orientation from a tilted structure in the as‐prepared film to a well‐oriented structure in the final film. We found that the molecular movement is large enough to induce a conformational change to an energetically stable state in spin‐coated TES‐ADT films, because TES‐ADT has a low glass‐transition temperature (around room temperature). Our findings demonstrate that organic small‐molecule semiconductors that exhibit a low crystallinity immediately after spin‐coating can be changed into highly crystalline structures by spontaneous self‐organization of the molecules at room temperature, which results in improved electrical properties of FETs based on these semiconductors.     

Doping Molecular Monolayers: Effects on Electrical Transport Through Alkyl Chains on Silicon

n‐Si/C_nH_{2n + 1}/Hg junctions (n = 12, 14, 16 and 18) can be prepared with sufficient quality to assure that the transport characteristics are not anymore dominated by defects in the molecular monolayers. With such organic monolayers we can, using electron, UV and X‐ray irradiation, alter the charge transport through the molecular junctions on n‐ as well as on p‐type Si. Remarkably, the quality of the self‐assembled molecular monolayers following irradiation remains sufficiently high to provide the same very good protection of Si from oxidation in ambient atmosphere as provided by the pristine films. Combining spectroscopic (UV photoemission spectroscopy (UPS), X‐ray photoelectron spectroscopy (XPS), Auger, near edge‐X‐ray absorption fine structure (NEXAFS)) and electrical transport measurements, we show that irradiation induces defects in the alkyl films, most likely C?C bonds and C? C crosslinks, and that the density of defects can be controlled by irradiation dose. These altered intra‐ and intermolecular bonds introduce new electronic states in the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap of the alkyl chains and, in the process, dope the organic film. We demonstrate an enhancement of 1–2 orders of magnitude in current. This change is clearly distinguishable from the previous observed difference between transport through high quality and defective monolayers. A detailed analysis of the electrical transport at different temperatures shows that the dopants modify the transport mechanism from tunnelling to hopping. This study suggests a way to extend significantly the use of monolayers in molecular electronics.     

Structural phase transition in pentacene caused by molecular doping and its effect on charge carrier mobility

The structural properties and charge carrier mobility of pentacene doped by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) and 2,2-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6-TCNNQ) are studied by X-ray diffraction, scanning electron microscopy, field effect transistor measurements, and space charge limited currents (SCLC). We observe the presence of polycrystalline and amorphous domains within the doped pentacene film grown under co-deposition conditions. The appearance of the amorphous phase is induced by the molecular dopants F4-TCNQ and F6-TCNNQ. A strong drop of crystallite size is obtained at a doping concentration of around 7 and 4 wt.%, respectively. The loss of the polycrystalline structure is correlated to a strong decrease of the charge carrier mobility in pentacene in horizontal and vertical film structures. We discuss typical scenarios of charge transport for polycrystalline and amorphous thin films in order to explain the observed loss of mobility originated by the doping induced structural phase transition. In this way an optimum doping concentration for highest conductivity with acceptable mobility is determined which can help to improve the performance of organic solar cells and organic high-frequency rectification diodes.