Vacuum-deposited Amorphous Films Research Articles

Effect of the conformer distribution on the properties of amorphous organic semiconductor films for organic light-emitting diodes.

With the remarkable improvement in the electrical and optical properties of organic light-emitting diodes (OLEDs) in recent years, the details of the higher-order structure of vacuum-deposited amorphous organic films and its formation mechanism need to be understood. In particular, to clarify the effect of the higher-order structure on the film properties, it is necessary to analyze the molecular aggregation states in the vacuum-deposited amorphous films. Toward their deep understanding, the higher-order structure and film properties have often been discussed with relation to the surface diffusion and structural relaxation of the molecules immediately after deposition on the film surface. However, the effect of the variety of conformers, which is specific to amorphous organic materials, on the thermal and electrical properties of the films has not been deeply discussed. In this study, we focused on three structural isomers of OLED materials and discuss the effect of the conformer distribution on the molecular aggregation states and thermal and electrical properties of the vacuum-deposited films. From their comparison, we found that the properties of the film composed of a relatively small number of stable conformers are superior to those of the other two films composed of relatively large numbers of stable conformers. This superiority originates from formation of aggregates of the same conformer, which become the starting points for crystallization when the film is heated. Our detailed comparison and discussion focusing on the variety of conformers will lead to a deeper understanding of the molecular aggregation states and physical properties of amorphous organic films.

On-top π-stacking of quasiplanar molecules in hole-transporting materials: inducing anisotropic carrier mobility in amorphous films.

Dimers of partially oxygen-bridged triarylamines were designed and synthesized as hole-transporting materials. X-ray structural analyses revealed that these compounds form on-top π-stacking aggregates in the crystalline state. TRMC measurements showed that high levels of anisotropic charge transport were induced in the direction of the π-stacking. Surprisingly, even in vacuum-deposited amorphous films, these compounds retained some of the face-on π-stacking, thus facilitating an out-of-plane carrier mobility.

Molecular orientation in small-molecule organic light-emitting diodes

In research on small-molecule organic light-emitting diodes (OLEDs), the molecular orientation in vacuum-deposited amorphous films has been disregarded for around 20 years, and its effects on device performance have not been sufficiently discussed at the microscopic level. Only recently have the intermolecular interaction and subsequent horizontal molecular orientation in OLEDs been investigated and reported. In this article, an overview of the recent studies on molecular orientation in OLEDs is presented. The general properties of molecular orientation of OLED materials are shown, and its significant effects on the electrical and optical properties of devices are discussed to understand device physics and improve the future performance and reliability of OLEDs.

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.

Forming oriented organic crystals from amorphous thin films on patterned substrates via solvent-vapor annealing

Abstract The challenge of generating crystals of organic materials has been pursued by many research groups who aim to develop materials sets for active electronic and optoelectronic devices, including field-effect transistors [C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99; H.E. Katz, Z. Bao, J. Phys. Chem. B 104 (2000) 671; Y.Y. Lin, D.J. Gundlach, S.F. Nelson, T.N. Jackson, IEEE Trans. Electron Dev. 44 (1997) 1325], photodetectors [P. Peumans, V. Bulovic, S.R. Forrest, Appl. Phys. Lett. 76 (2000) 2650] and optical modulators [S.R. Marder, J.W. Perry, C.P. Yakymyshyn, Chem. Mater. 6 (1994) 1137]. Here we describe a method for in-plane growth of millimeter-scale crystalline organic needles from initially amorphous thin films of the molecular organic semiconductor tris(8-hydroxyquinoline)aluminum (Alq 3 ). The needles form when the vacuum-deposited amorphous films are exposed to chloroform vapor at room temperature and pressure, and can be as large as several microns thick, several microns wide, and one centimeter long (limited in length by the substrate dimensions). As such, the Alq 3 needles are more than 1000 times longer than any previously reported organic crystals formed via a similar solvent-vapor annealing process [F. Iwatsu, T. Kobayashi, N. Uyeda, J. Phys. Chem. 84 (1980) 3223; F. Iwatsu, J. Cryst. Growth 71 (1985) 629; A.M. Hor, R.O. Loutfy, Thin Solid Films 106 (1983) 291; B.A. Gregg, J. Phys. Chem. 100 (1996) 852; J.C. Conboy, E.J.C. Olson, D.M. Adams, J. Kerimo, A. Zaban, B.A. Gregg, P.F. Barbara, J. Phys. Chem. B 102 (1998) 4516; M. Brinkmann, J.C. Wittmann, C. Chaumont, J.J. Andre, Thin Solid Films 292 (1997) 192; F. Toffolo, M. Brinkmann, O. Greco, F. Biscarini, C. Taliani, H.L. Gomes, I. Aiello, M. Ghedini, Synth. Met. 101 (1999) 140]. Moreover, the Alq 3 needles are spatially separated from one another and oriented with their long axes parallel to lithographically pre-defined periodic submicron grooves in the substrate surface.

Growth of hexagonal plate crystal in vacuum-deposited amorphous films of antimony

Hexagonal plate crystals (HPC) grow at the expense of the surrounding amorphous regions during isothermal annealing at an arbitrary temperature, Ta, for Sb films deposited onto glass or SiOx coated glass substrates held at -130°C in a vacuum of 10-5 Torr. The growth rate of HPC, Rh, was determined from the measurement of the diagonal of HPC with an optical microscope. A ln Rh versus 1/Ta relationship leads to the conclusion that the growth of HPC is under the control of some activation process whose activation energy changes at a certain temperature, Tc: 0.5 eV at TaTc and 1.8 eV at Ta < Tc. It is proposed that growth is due to a successive or series type reaction in the amorphous phase: One process is similar to self-diffusion in a crystalline-like amorphous phase; the other is associated with some re-ordering process to form a crystalline-like amorphous phase in the as-deposited amorphous phase.

Crystallization during iso-chronal annealing of vacuum-deposited amorphous films of antimony

Growth of crystallized regions during iso-chronal annealing of amorphous Sb films deposited at -130°C was observed. A disk-shaped crystallized region grows at thicknesses d below 400Å. At 400 <d < 650Å, the initial hexagonal plate crystals grow changing gradually to a shape with threefold symmetry. The relation between the crystallization temperature and d is also reported.

High-Resolution Electron Microscopic Studies of CS Structure in Reduced WO3 Thin Crystals

The crystallographic shear (CS) structure in reduced WO3 crystals grown in vacuum-deposited amorphous films by electron heating, is investigated with a 200 kV electron microscope and by computer simulation. A new type of ordered (100) CS structure with the shear vector [101]a/4 is found in the reduced WnO3n-1 crystals with the (001) surface. Images which may be associated with ordered (100) oxygen vacancy walls and imperfect (100) CS planes are observed. The most reduced crystal observed is W2O5(n=2).

A (110) CS structure in reduced rutile crystals

Abstract Abstract Rutile crystals, grown in vacuum-deposited amorphous films by electron beam heating, have been investigated by high-resolution transmission electron micro scopy with the aid of computer image simulation. A new type of ordered (110) CS structure with 1/2 [0I1] shear has been found in reduced TinO2n−1 crystals with n = 2.

Estimation of density changes associated with growth in vacuum-deposited amorphous films of antimony

Amorphous antimony films consisting of separate islands were deposited onto SiO x and GeO x film substrates at room temperature in a vacuum of 10 -5 Torr. By comparison of the interferometrically determined thickness and the total mass of incident vapour determined with a quartz oscillator, it was found that a sudden change in film density occurs at a certain film thickness d 0. Another change in film density, which is gradual, occurs in the vicinity of a certain thickness d c (d c > d 0 ) . The change at d 0 is caused by the coalescence of neighbouring islands. The change at d c, at which the coalescence of amorphous islands is almost completed, is associated with crystallization.