The vibration-based electret generators (EGs) for energy harvesting have been extensively studied because they can obtain electrical energy from ambient vibrations. EGs exhibit a sandwich structure ...of electrodes surrounding an air gap and an electret, which is a dielectric material with a quasi-permanent electrical charge or dipole polarisation. Various charging processes have been developed because the surface charge density (σ) of the electret determines the output power of the device. However, such processes are considered to constitute a key productivity-limiting factor from the mass production viewpoint, making their simplification or elimination a highly desired objective. Herein, a model EG that does not require any charging process by utilising the spontaneous orientation polarisation of 1,3,5-tris(1-phenyl-1H-benzimidazole-2-yl)benzene (TPBi) is demonstrated. The surface potential (V
) of an evaporated TPBi film has reached 30.2 V at a film thickness of 500 nm without using a charging process. The estimated σ of 1.7 mC m
is comparable with that obtained using a conventional polymer-based electret after charging. Furthermore, V
is considerably stable in environmental conditions; thus, TPBi can be considered to be "self-assembled" electret (SAE). Application of SAE leads to developing an EG without requiring the charging process.
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.
Recently, a spontaneous orientation polarization (SOP) has attracted much attention as an important factor improving the performance of organic light‐emitting devices. However, so far, SOP is ...reported only for films fabricated by vacuum vapor deposition, and no direct attempt is made to examine the existence of SOP for wet‐processed film. To examine this phenomenon, it is necessary to completely exclude light illumination in the sample system, as photocarriers can reduce or eliminate the surface potential caused by the SOP formation. Herein, the SOP of wet‐processed tris(8‐hydroxyquinolinato)aluminum (Alq3) film on an indium‐tin‐oxide substrate is investigated using a novel rotary Kelvin probe method. Film formation and surface potential measurement are conducted under completely dark condition. A surface potential shift of −0.33 V is observed for a 1 μm thick film of Alq3, indicating that the SOP of wet‐processed Alq3 film is negligible, in contrast to vacuum‐evaporated film.
A spontaneous orientation polarization (SOP) of wet‐processed tris(8‐hydroxyquinolinato)aluminum (Alq3) film on an indium‐tin‐oxide substrate is investigated using a novel rotary Kelvin probe method under completely dark condition. A surface potential shift of −0.33 V is observed for a 1 μm thick film of Alq3, indicating that the SOP of wet‐processed Alq3 film is negligible, in contrast to vacuum‐evaporated film.
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.
Multi-heme cytochromes (MHCs) are fascinating proteins used by bacterial organisms to shuttle electrons within, between, and out of their cells. When placed in solid-state electronic junctions, MHCs ...support temperature-independent currents over several nanometers that are 3 orders of magnitude higher compared to other redox proteins of similar size. To gain molecular-level insight into their astonishingly high conductivities, we combine experimental photoemission spectroscopy with DFT+Σ current–voltage calculations on a representative Gold-MHC-Gold junction. We find that conduction across the dry, 3 nm long protein occurs via off-resonant coherent tunneling, mediated by a large number of protein valence-band orbitals that are strongly delocalized over heme and protein residues. This picture is profoundly different from the electron hopping mechanism induced electrochemically or photochemically under aqueous conditions. Our results imply that the current output in solid-state junctions can be even further increased in resonance, for example, by applying a gate voltage, thus allowing a quantum jump for next-generation bionanoelectronic devices.
The electronic structure and field emission characteristics of spray-coated single-walled carbon nanotubes (sprayed-SWCNTs) and vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) were ...evaluated to discuss the feasibility of applying carbon nanotubes (CNTs) to electrodes in organic electronics. For the electronic structure evaluation, high-sensitivity ultraviolet photoelectron spectroscopy and constant final state yield spectroscopy (CFS-YS) were used. For the field emission characterization, ultraviolet photoelectron spectroscopy (UPS) and field emission spectroscopy (FES) were used simultaneously (UPS/FES). As a result, the work functions of the sprayed-SWCNTs and VA-MWCNTs were determined as 4.75 and 4.64 eV, respectively, which are comparable to those of materials used as anodes in organic devices. CFS-YS measurement allowed a detailed characterization of the electronic structure near the Fermi level and showed that there were no defects that could affect the electrical conducting properties. Furthermore, the transmission probability of field emission was experimentally determined using UPS/FES. It was also confirmed that the sprayed-SWCNTs started emitting at approximately 1.1 V µm−1. This electric field is less than those for operating organic thin-film devices. This indicates that using CNT transparent electrodes as cathodes can improve the injection efficiency of organic devices such as inverted organic light-emitting diodes.
The function of electronically functional organic materials often originates at an interface, one example being organic electroluminescent devices (the Figure shows a typical energy diagram). ...Therefore, elucidation of the electronic structure at interfaces will lead to a better understanding of these devices, enabling their performance to be improved. Basic concepts are reexamined and recent progress in the area is reviewed.
The electronic structure of rubrene single crystals was studied by angle-resolved ultraviolet photoelectron spectroscopy. A clear energy dispersion of the highest occupied molecular orbital-derived ...band was observed, and the dispersion width was found to be 0.4 eV along the well-stacked direction. The effective mass of the holes was estimated to be 0.65(+/-0.1)m0. The present results suggest that the carrier conduction mechanism in rubrene single crystals can be described within the framework of band transport.
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