Microscopic characterization of radical states in organic light‐emitting diodes (OLEDs) during device operation is useful for elucidating the degradation mechanism because the radical formation has been considered as non‐radiative recombination centers. Electron spin resonance (ESR) spectroscopy is suitable for such characterization because it can directly observe radicals in OLEDs. In this work, the detailed ESR investigation into the radical states in OLEDs during device operation is firstly reported using a typical light‐emitting Alq3‐based OLEDs. The simultaneous measurements of the ESR signal and the luminance of the same OLED are performed to study the direct correlation between the radical states and the performance degradation. These characteristics show that the luminance monotonically decreases and an ESR signal concomitantly increases as the duration of the device operation increases after operating the OLED. Using the analysis of density functional theory (DFT) calculation, the origin of the newly emerged ESR signal is ascribed to the cationic species due to decomposed Alq3 molecules. The elucidation of the radical species formed in OLEDs during device operation has been demonstrated at a molecular level for the first time. This ESR analysis would provide useful knowledge for understanding the degradation mechanism in the OLEDs at the molecular level.

Spin-states and charge-trappings in blue organic light-emitting diodes (OLEDs) are important issues for developing high-device-performance application such as full-color displays and white illumination. However, they have not yet been completely clarified because of the lack of a study from a microscopic viewpoint. Here, we report operando electron spin resonance (ESR) spectroscopy to investigate the spin-states and charge-trappings in organic semiconductor materials used for blue OLEDs such as a blue light-emitting material 1-bis(2-naphthyl)anthracene (ADN) using metal–insulator–semiconductor (MIS) diodes, hole or electron only devices, and blue OLEDs from the microscopic viewpoint. We have clarified spin-states of electrically accumulated holes and electrons and their charge-trappings in the MIS diodes at the molecular level by directly observing their electrically-induced ESR signals; the spin-states are well reproduced by density functional theory. In contrast to a green light-emitting material, the ADN radical anions largely accumulate in the film, which will cause the large degradation of the molecule and devices. The result will give deeper understanding of blue OLEDs and be useful for developing high-performance and durable devices.

Elucidating hole and electron states in organic semiconductor materials is one of the important issues for both their fundamental science and device applications. However, the detailed charge states, in particular, their spin states, have not yet been fully elucidated from a microscopic viewpoint. Here we show electrically controllable spin states of holes and electrons in typical organic semiconductor materials, a polymer regioregular poly­(3-hexyl­thiophene) (RR-P3HT) and a small molecule pentacene, using electron spin resonance (ESR) spectroscopy. By use of their ambipolar organic semiconductor devices, these states were revealed as a function of accumulated charge density. The spin states of the electrically accumulated electrons in RR-P3HT and pentacene are clarified for the first time. Moreover, the formation of spinless states of electrons in RR-P3HT and holes in pentacene are demonstrated under high charge density, showing a contrast to the spin states under low charge density. This result would be important for further understating hole and electron states in organic semiconductor materials and for improving the performance of organic semiconductor devices from a microscopic viewpoint.

Silicon nanowire (SiNW) biosensors have attracted a lot of attention due to their superior sensitivity. Recently, the dependence of biomolecule detection sensitivity on the nanowire (NW) width, number, and doping density has been partially investigated. However, the primary reason for achieving ultrahigh sensitivity has not been elucidated thus far. In this study, we designed and fabricated SiNW biosensors with different widths (10.8–155 nm) by integrating a complementary metal-oxide-semiconductor process and electron beam lithography. We aimed to investigate the detection limit of SiNW biosensors and reveal the critical effect of the 10-nm-scaled SiNW width on the detection sensitivity. The sensing performance was evaluated by detecting antiovalbumin immunoglobulin G (IgG) with various concentrations (from 6 aM to 600 nM). The initial thickness of the depletion region of the SiNW and the changes in the depletion region due to biomolecule binding were calculated. The basis of this calculation are the resistance change ratios as functions of IgG concentrations using SiNWs with different widths. The calculation results reveal that the proportion of the depletion region over the entire SiNW channel is the essential reason for high-sensitivity detection. Therefore, this study is crucial for an indepth understanding on how to maximize the sensitivity of SiNW biosensors.

Organic light-emitting diodes (OLEDs) using thermally activated delayed fluorescence (TADF) materials have advantages over OLEDs using conventional fluorescent materials or high-cost phosphorescent materials, including higher efficiency and lower cost. To attain further high device performance, clarifying internal charge states in OLEDs at a microscopic viewpoint is crucial; however, only a few such studies have been performed. Here, we report a microscopic investigation into internal charge states in OLEDs with a TADF material by electron spin resonance (ESR) at a molecular level. We observed operando ESR signals of the OLEDs and identified their origins due to a hole-transport material PEDOT:PSS, gap states at an electron-injection layer, and a host material CBP in the light-emitting layer by performing density functional theory calculation and studying thin films used in the OLEDs. The ESR intensity varied with increasing applied bias before and after the light emission. We find leakage electrons in the OLED at a molecular level, which is suppressed by a further electron-blocking layer MoO3 between the PEDOT:PSS and light-emitting layer, resulting in the enhancement of luminance with a low-voltage drive. Such microscopic information and applying our method to other OLEDs will further improve the OLED performance from the microscopic viewpoint.