No abstract

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.

We have characterized amorphous to crystalline transformation of hydrogen (H)-doped In2O3 (In2O3:H) films by transmission electron microscopy, thermal desorption spectroscopy, spectroscopic ellipsometry, and Hall measurements. The In2O3:H films that show a mixed-phase structure embedded with small density of crystalline grains in a large volume fraction of amorphous phase have been fabricated at room temperature by the sputtering of an In2O3 ceramic target with introduction of H2O vapor, and the films have been postannealed in vacuum to crystallize the amorphous phase. With increasing annealing temperature up to 200 °C, the film shows a large increase in Hall mobility (μHall) from 42 to 110 cm2/V s and a decrease in carrier density (NHall) from 4.6×1020 to 2.1×1020 cm−3 with slight decrease in resistivity. The change in μHall and NHall with annealing temperature is strongly correlated with the volume fractions of the amorphous and crystalline phases in the films. Analyses of dielectric functions of the films using the Drude model revealed that the high electron mobility in the crystallized films is attributed mainly to longer relaxation time rather than smaller effective mass, as compared with as-deposited films. Temperature-dependent Hall analysis, relationship between NHall and μHall, and comparison between μHall and optical mobility showed that (i) scattering processes inside amorphous and/or crystalline matrices limit the mobility, (ii) doubly charged ionized impurity scattering is reduced by crystallization, and (iii) phonon scattering becomes dominant after crystallization in the In2O3:H films. The above results suggest that H-doping reduces carrier scattering in the crystallized In2O3:H and structural rearrangements during crystallization eliminate oxygen deficiency and generate H+ that acts as a singly charged donor. In this article, we discuss the transport properties with the variation in microscopic and chemical structures in the In2O3:H films.

Using wide-range variable angle spectroscopic ellipsometry, the authors demonstrate large optical uniaxial anisotropy of vacuum-deposited organic amorphous films of hole and electron transport materials having long or planar molecular structures. The ordinary refractive indices and extinction coefficients were higher than the extraordinary ones, revealing that the molecules in the amorphous films are horizontally oriented. The horizontal orientation requires significant modifications in the understanding of both the electrical and optical characteristics of amorphous films when we use materials having long or planar molecular structures.

To demonstrate the effect of molecular orientation on charge transport characteristics in vacuum-deposited organic amorphous films, the electrical and optical properties of films of two oxadiazole derivatives were investigated. One of them has bulky tert-butyl terminals and forms an isotropic film, whereas the other has planar bipyridyl terminals and forms a highly anisotropic film. The very large optical anisotropy of the latter means that the planar molecules stack horizontally, leading to large overlaps of π-orbitals and more than 30 times higher electron mobility though the geometric and electronic structures of the main conformers of the two derivatives are quite similar.