Organic materials, which are composed of weakly interacting molecules, make possible to fabricate devices that cannot be achieved with inorganic materials. In recent year, organic devices have been in the limelight again as organic EL displays have been used in smartphones and PCs and now on the market. In this special issue, we will introduce research results on organic devices that may put or will put to practical use next.
As a post-OLED (organic light emitting diode) device, expectations are rising for an organic semiconductor laser diode (OSLD), which is the ultimate current injection device. In order to realize OSLD, the achievement of high current density of several kA cm－2 and the development of laser molecules exhibiting ultra-low threshold has been required. This paper introduces the recent progress in molecular design for laser molecules aimed for OSLDs.
Many optoelectronic devices, such as displays, photovoltaics and sensor arrays, benefit greatly by fabricating them on flexible substrates. Although flexible organic devices have many advantages, such as versatility in shape and light weight, it is not easy to mass-produce them since reactive materials such as alkali metals, which are essential to inject/extract electrons around the cathode, degrade rapidly owing to the entry of oxygen and moisture. Thus, there is a strong desire to eliminate reactive materials from devices. Here, I show the feasibility of an air-stable organic light-emitting diode (OLED), which is realized by combining an inverted stacking structure and an inert electron injection layer. Then, the first ever flexible OLED display that can emit light over one year even in the presence of oxygen and moisture is introduced. Finally, I show a novel strategy for electron injection into organic semiconductors, which is realized by forming hydrogen bonds.
Electret-based vibrational energy generators (E-VEGs) have attracted much attention because they can generate electrical power from ambient vibration. The electret is a key material for the E-VEG because the output power of the device is proportional to the square of the surface charge density of the electret. A challenge is that charging process is indispensable for making the electret from a dielectric material, which involves a factor that limits device productivity. To solve this problem, we developed a novel E-VEG that does not require any charging process by utilizing spontaneous orientation of polar organic molecules which have been widely used for organic light-emitting diodes. In this paper, the property and stability of the device are introduced. We believe that the application of polar molecules opens up a new pathway for the development of electret-based devices such as VEGs, sensors, and microphones, etc.
In this paper, we introduce the development of a sheet-type magnetic sensor system using flexible organic thin-film transistors. Flexible transistors that make up the system circuit utilize a newly developed thin-film polymer insulator technology, which enables 2 V operation of transistors with high device yield. We also demonstrated two-dimensional magnetic field mapping using a developed magnetic sensor system. Besides, we discuss the importance of 1/f noise, which is an essential internal noise in analog circuits for sensor systems. And finally, we introduce the technical development of the world's lowest 1/f noise in flexible organic transistors.
For electronic devices with molecular semiconductors, surface of the molecular semiconductor thin-films plays a key role in the device performance. Despite its importance, the method to control the surface structure without changing bulk properties is limited. Here we introduce a concept of a surface-segregated monolayer that enables us to control the structures at the film surface with ease. We discuss its formation mechanism, characterization, and application to investigating the physics of organic solar cell devices.
Near-field Raman spectroscopy has attracted significant attention as a tool for performing chemical analysis with submolecular spatial resolution. The mechanism in atomic and molecular scale Raman spectroscopy, however, is not well understood due to the strong perturbation of the molecule during measurements in previous studies. In particular, the resonance effect that greatly enhances Raman scattering has not been clarified on a single molecule scale. Here we demonstrate single-molecule resonance Raman imaging with a scanning tunneling microscope (STM) at submolecular resolution of copper naphthalocyanine molecules. In our experiment, an ultra-thin insulating NaCl film is used as the substrate to keep the molecule in an undisturbed condition for performing a precise analysis. We have succeeded in resonance Raman spectroscopy by tuning the excitation wavelength to the intrinsic electronic transitions of the molecule. This makes it possible to acquire accurate Raman spectra with a short measurement time at single molecule sensitivities. The resonance Raman maps show three different spatial distribution patterns depending on the symmetry of the vibration modes. These results are explained by considering the interaction between the electromagnetic field of the plasmon in the tip-substrate gap and the intrinsic molecular symmetry, thereby a selection rule for the plasmon-enhanced resonant Raman effect is established.
Isolated single-walled carbon nanotubes (SWNTs) adsorbed on an atomically flat and clean Cu(111) surface were studied by means of ultra-high vacuum (UHV) scanning tunneling microscopy and spectroscopy (STM / STS) at 300 K. A dispersion of SWNTs without unzipping molecules was sprayed in vacuum. Subsequently upon annealing of 800 K atomically flat terraces were recovered. STM/STS measurements revealed that some of the SWNTs had similar morphology and electronic local density of states as graphene nanoribbons (GNRs), indicating that the unzipping process could occur even without unzipping molecules.