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].

Electronic structures of donor-acceptor hetero-interfaces are crucial for performance of organic solar cell devices. In the present study, a well-defined organic molecular interface of pentacene single crystal and C60 overlayer is elucidated by x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and photoelectron yield spectroscopy (PYS). An energy level diagram at this hetero-molecular contact in the "head-on" orientation is derived, which reveals the vacuum level alignment and flat-band conditions.

The adsorption of atomic hydrogen on hexagonal boron nitride (h-BN) is studied using two element-specific spectroscopies, i.e., near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and x-ray photoelectron spectroscopy (XPS). B K-edge NEXAFS spectra show a clear change in the energy region of the π* band before and after reaction with atomic deuterium. On the other hand, N K-edge NEXAFS spectra show only a little change. B 1s XPS spectra show a distinct component at the low binding energy side of a main component, while N 1s XPS spectra show peak broadening at the high binding energy side. These experimental results are analyzed by the discrete variational Xα method with a core-hole effect and are explained by a model in which hydrogen atoms are preferentially adsorbed on the B sites of h-BN. Based on the experimental and theoretical results, we propose a site-selective property of BN material on adsorption of atomic hydrogen.

In this study, we investigated the spontaneous orientation polarization (SOP) in coevaporated films of polar and nonpolar molecules. SOP has been commonly observed in the evaporated films of organic light‐emitting diode materials, and it influences the device performances. However, the mechanism of the SOP formation is yet to be clarified. Herein, we characterized the molecular orientation of polar molecules of 1,3,5‐tris (1phenyl‐1H‐benzimidazol‐2‐yl) benzene (TPBi) in coevaporated films with nonpolar molecules of 1,4‐bis‐(triphenylsilyl) benzene (UGH‐2) or 4,4′‐bis (N‐carbazolyl)‐1,1′‐biphenyl (CBP). We measured the surface potential characteristics of the coevaporated films. We found that the molecular orientation of TPBi in both UGH‐2 and CBP hosts was enhanced, though the apparent enhancement factor was small in the UGH‐2 host. The enhancement of the molecular orientation is attributed to the reduction of the electrostatic interaction between polar molecules (TPBi), which deteriorates SOP as previously reported. In addition, our results suggest that the SOP of TPBi is robust even in the UGH‐2 host, in contrast to the random orientation of Ir complexes in the UGH‐2 host. Considering a polyhedral shape of Ir complexes, the robust SOP of TPBi in the host molecules with a weak van der Waals interaction, such as UGH‐2, is possibly due to its disk‐like molecular shape.