Spontaneous orientation polarization (SOP) has been frequently observed in the evaporated films of organic light‐emitting diode materials. Because SOP modifies the charge injection and the accumulation properties of the device, understanding and controlling SOP is crucial in optimizing the performance of the device. In this study, we investigated the dominant factors for SOP formation by focusing on intermolecular interactions. We examined the giant surface potential characteristics of coevaporated films incorporating 1,3,5‐tris(1‐phenyl‐1H‐benzimidazol‐2‐yl)benzene (TPBi) that is a typical polar molecule exhibiting SOP. In the coevaporated films of TPBi and nonpolar molecules such as 4,4′‐bis(N‐carbazolyl)‐1,1′‐biphenyl and 4,4′,4″‐tris (carbazol‐9‐yl)triphenylamine, the orientation degree of the permanent dipole moment (PDM) of TPBi is significantly enhanced with diluted TPBi density, though the enhancement is weak on the film with N,N′‐bis(1‐naphthyl)‐N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′‐diamine. The results indicate that the PDM interaction between polar molecules results as a negative factor for SOP formation. Furthermore, we found that SOP formation is suppressed by the surface treatment of the self‐assembled monolayer on the gold substrate, indicating a positive effect of the van der Waals interaction between the molecule and the substrate surface.

The valence band structure of rubrene single crystals was experimentally determined by high-resolution angle-resolved and excitation-energy-dependent photoelectron spectroscopy at room temperature. The energy position of the peak derived from the highest occupied molecular orbital did not depend on the excitation energy, reflecting an absence of energy dispersion along the surface normal direction. A two-dimensional valence band dispersion relation over the surface Brillouin zone obtained by angle-resolved photoemission to three critical points was reproduced excellently by a two-dimensional tight binding approximation. Highly anisotropic values of intermolecular transfer integrals to four adjacent molecules were obtained from the present results.

Well-defined monolayers with single-crystalline-like molecular arrangements of dinaphtho­[2,3-b:2′,3′-f]­thieno­[3,2-b]-thiophene (DNTT) and picene, which are a new class of organic semiconductors with enhanced intermolecular interactions, were fabricated and characterized. Although both molecules initially form a loosely packed monolayer with a flat-lying molecule, they undergo phase transition into a densely packed monolayer with single-crystalline-like molecular arrangements with increasing molecular density. Upon the phase transition of the monolayer, the highest occupied molecular orbital (HOMO) level of these molecules splits into two features, as suggested from both the ultraviolet photoelectron spectroscopy and density functional theory calculations. The splitting of the HOMO level was observed to be similar to that expected for the molecular arrangement in the single crystal. This splitting, which has not been observed in the polycrystalline film, suggests a substantial overlap of the HOMO in the well-ordered monolayers.

Synchrotron radiation (SR), as a result of its high-intensity, brilliant, monochromatic, and collimated beams, is becoming one of the most crucial components of research in various fields of materials science such as nanomaterials, biomaterials, and energy materials. SR-based characterization methods can be employed to analyze different systems such as powders, thin films, and bulk forms having complex crystalline or amorphous structures. In this review, peculiarities of SR are briefly explained. Moreover, various techniques carried out utilizing this instrument for material characterization such as X-ray powder diffraction, grazing-incidence X-ray diffraction, small/ wide-angle X-ray scattering, X-ray absorption spectroscopy, different techniques of X-ray imaging, X-ray photoelectron spectroscopy, and X-ray microprobes/nanoprobes are presented. As a result, by shedding light on the advantages of SR and its superiority to the equivalent laboratory experiments, researchers are recommended to exploit the capabilities of this invaluable tool in their materials characterization.

The electronic structures of pentacene single crystals (SCs) were elucidated by ultraviolet photoelectron spectroscopy (UPS) and photoelectron yield spectroscopy (PYS). An asymmetric HOMO peak profile of the pentacene SCs obtained by UPS exhibits a close similarity to the k-projected density-of-states of the valence band that has been predicted by a theoretical calculation [H. Yoshida and N. Sato, Phys. Rev. B 77, 235205 (2008)]. The ionization energy of the pentacene SCs is successfully determined to be 4.95 (+ 0.03) eV which is evidently greater than that of the bulk films of pentacene [4.90 (+ 0.02) eV].