Spontaneous orientation polarization (SOP) of amorphous organic semiconducting films has attracted much attention because of its frequent observation in common organic light-emitting diodes (OLEDs) ...and potential influences on the device properties of OLEDs. On the other hand, the formation mechanism of SOP has been controversial for a long time, ever since its discovery in 2002. Recently, the formation mechanism of SOP was explained in terms of the surface equilibration mechanism of vapor-deposited glasses, and the understanding of SOP has progressed significantly. Based on the improved understanding, some active control methods of SOP have been demonstrated and further influences on the device performance of OLEDs were revealed, suggesting that higher efficiency can be achieved by managing SOP properly. Furthermore, some applications of SOP have also been proposed, such as a self-assembled electret and a tool for evaluating materials properties. In this paper, recent progress in the understanding of SOP and its applications to devices are reviewed.
•Recent progress in understanding of spontaneous orientation polarization (SOP) in organic semiconducting films is reviewed.•Formation mechanism and active control methods of SOP are discussed.•Influences of SOP on the device performance of organic light-emitting diodes are discussed.•Applications of SOP are introduced.
The molecular orientation in organic semiconductor films determines device performances. In particular, the spontaneous orientation of a permanent dipole moment (PDM) along the surface normal ...direction induces a polarization charge at the hetero-interfaces of stacked multilayer devices, and the interface charge dominates the charge accumulation and injection properties. Spontaneous orientation polarization (SOP) has been observed in the “randomly oriented” films of several organic semiconductor materials, and is potentially inherent in many common materials. Herein, we report that 11 additional molecules of organic light-emitting diode materials, including thermally activated delayed fluorescence emitters, and horizontally oriented emitters and electron transporters, exhibit SOP in their evaporated films. The experimental results clearly indicate that SOP frequently occurs in “horizontally oriented” films as well as “randomly oriented” films. The factors contributing to SOP formation are discussed in terms of the figure of merit per PDM. We found that strong intermolecular interactions tend to reduce the figure of merit. Moreover, we suggest the impact of SOP on device performances.
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•Spontaneous orientation polarization (SOP) is common in organic semiconductor films.•11 additional organic molecules exhibit SOP in evaporated films.•SOP occurs in both “randomly” and “horizontally oriented” films.•Strong intermolecular interactions tend to reduce SOP.
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•Oxidative vaporization controls a Mo tip formation within a second in air.•Generation and immediate vaporization of MoO3 is a driving force.•Temperature gradient in a flame ...determines a tip sharpness.•The Mo tips were used for STM, field emission, and conductance measurements.
Oxidative vaporization in air using a flame with a temperature range of 1950–2300 K was employed for controlling the tip shape made from a nipper-cut metal molybdenum (Mo) wire edge. An extremely high vapor pressure of MoO3 generated on the Mo surface in flame is a driving force behind the tip shape formation. Since the MoO3 vaporization rate follows the flame temperature gradient, we could control the tip apex shape by selecting the proper flame etching condition. The best condition to obtain a sharp tip apex based on statistical tests of dozens of Mo tips was obtained by Mo wire edge insertion into the 2100 K flame from the side for one second. This was repeated twice, which reproducibly provided a tip apex with a radius of 50–100 nm and a cone angle of 20–30 degrees. The present Mo tips, fabricated without aqueous solutions, were examined for their suitability as probe tips through air-scanning tunneling microscopy (STM), ultrahigh vacuum STM, field emission spectroscopy, and conductance measurements.
The electrical and optical properties of certain semiconducting polymers are improved in blend films with inert host materials. The improvements have been attributed to the dilution effect of ...electron traps and interchain species in semiconductor materials. In this paper, we report on anisotropy in the blend films of poly 2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) and its strong dependence on an inert host material, such as polystyrene (PS) or polyvinylcarbazole (PVK). We found that the orientation of the MEH-PPV backbone in the blend film changed from horizontal to random on dilution with PS. Moreover, the in-plane hole conductivity was enhanced in the MEH-PPV:PVK blend film, although the out-of-plane conductivity reduced owing to excessive dilution. Mott–Schottky analysis of the capacitance–voltage characteristics revealed opposite trends for dilution with PS and PVK; i.e., PS increased the accumulated charge density in the bulk of the blend film, whereas PVK reduced it. These results demonstrate that isotropy was induced in the PS blend films, whereas anisotropy was induced in the PVK blend films. Anisotropy is an important factor for understanding the hole transport characteristics in the diluted films.
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•Anisotropy of hole transport property was found in the diluted polymer films.•Anisotropy in the diluted films strongly depends on the inert host material.•Anisotropy is an important factor for understanding the hole transport properties.
Ambipolar carrier transport is demonstrated in an optically controllable organic field-effect transistor, where a benzothienothiophene-substituted diarylethene (BTT-DAE) thin film is employed ...directly as the transistor channel. A closed-ring isomer, which is produced by ultraviolet (UV) light irradiation, allows the carrier injection of both holes and electrons from source-drain electrodes into the BTT-DAE layer. Moreover, alternate UV or visible (VIS) light irradiation induces marked switching in the drain currents caused by reversible photoisomerization between closed-ring (semiconductor) and open-ring (insulator) isomers. The light-driven on/off ratio, which is defined by the ratio of the drain currents in the sample after UV or VIS light irradiation, reaches 240 for hole transport. The value is comparable to the gate-voltage-induced on/off ratio of 160. Our findings, therefore, have a potential to lead to the construction of new optoelectronic devices such as photoreconfigurable logic circuits and light emitting transistors.
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•Ambipolar operation is achieved in a diarylethene-channel transistor.•The gate voltage well controlled both hole and electron transport.•Ultraviolet or visible light irradiation realized a large modulation in both hole and electron currents.•The light-induced on/off ratio reached 240 for p-type operation and >25 for n-type operation.
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.
Translocation-related sarcomas (TRSs) harbor an oncogenic fusion gene generated by chromosome translocation and account for approximately one-third of all sarcomas; however, effective targeted ...therapies have yet to be established. We previously reported that a pan-phosphatidylinositol 3-kinase (PI3K) inhibitor, ZSTK474, was effective for the treatment of sarcomas in a phase I clinical trial. We also demonstrated the efficacy of ZSTK474 in a preclinical model, particularly in cell lines from synovial sarcoma (SS), Ewing's sarcoma (ES) and alveolar rhabdomyosarcoma (ARMS), all of which harbor chromosomal translocations. ZSTK474 selectively induced apoptosis in all these sarcoma cell lines, although the precise mechanism underlying the induction of apoptosis remained unclear. In the present study, we aimed to determine the antitumor effect of PI3K inhibitors, particularly with regards to the induction of apoptosis, against various TRS subtypes using cell lines and patient-derived cells (PDCs). All of the cell lines derived from SS (six), ES (two) and ARMS (one) underwent apoptosis accompanied by the cleavage of poly-(ADP-ribose) polymerase (PARP) and the loss of mitochondrial membrane potential. We also observed apoptotic progression in PDCs from SS, ES and clear cell sarcoma (CCS). Transcriptional analyses revealed that PI3K inhibitors triggered the induction of PUMA and BIM and the knockdown of these genes by RNA interference efficiently suppressed apoptosis, suggesting their functional involvement in the progression of apoptosis. In contrast, TRS-derived cell lines/PDCs from alveolar soft part sarcoma (ASPS), CIC-DUX4 sarcoma and dermatofibrosarcoma protuberans failed to undergo apoptosis nor induce PUMA and BIM expression, as well as cell lines derived from non-TRSs and carcinomas. Thus, we conclude that PI3K inhibitors induce apoptosis in selective TRSs such as ES and SS via the induction of PUMA and BIM and the subsequent loss of mitochondrial membrane potential. This represents proof of concept for PI3K-targeted therapy, particularly such TRS patients.
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