Kou Yoshida

Afterglow Organic Light-Emitting Diode.

An afterglow organic light-emitting diode (OLED) that displays electroluminescence with long transient decay after it is turned off is demonstrated. This OLED exhibits blue and green dual emission originating from fluorescence and phosphorescence, respectively. A phosphorescence lifetime of 4.3 s is achieved.

Suppression of roll-off characteristics of organic light-emitting diodes by narrowing current injection/transport area to 50 nm

Using e-beam nanolithography, the current injection/transport area in organic light-emitting diodes (OLEDs) was confined into a narrow linear structure with a minimum width of 50 nm. This caused suppression of Joule heating and partial separation of polarons and excitons, so the charge density where the electroluminescent efficiency decays to the half of the initial value (J0) was significantly improved. A device with a narrow current injection width of 50 nm exhibited a J0 that was almost two orders of magnitude higher compared with that of the unpatterned OLED.

Toward continuous-wave operation of organic semiconductor lasers

Organic semiconductor laser operating in the quasi-CW regime at 80 MHz and under 30 ms long pulse photoexcitation is demonstrated. The demonstration of continuous-wave lasing from organic semiconductor films is highly desirable for practical applications in the areas of spectroscopy, data communication, and sensing, but it still remains a challenging objective. We report low-threshold surface-emitting organic distributed feedback lasers operating in the quasi–continuous-wave regime at 80 MHz as well as under long-pulse photoexcitation of 30 ms. This outstanding performance was achieved using an organic semiconductor thin film with high optical gain, high photoluminescence quantum yield, and no triplet absorption losses at the lasing wavelength combined with a mixed-order distributed feedback grating to achieve a low lasing threshold. A simple encapsulation technique greatly reduced the laser-induced thermal degradation and suppressed the ablation of the gain medium otherwise taking place under intense continuous-wave photoexcitation. Overall, this study provides evidence that the development of a continuous-wave organic semiconductor laser technology is possible via the engineering of the gain medium and the device architecture.

Suppression of external quantum efficiency roll-off of nanopatterned organic-light emitting diodes at high current densities

We developed organic light-emitting diodes (OLEDs) with nanopatterned current flow regions using electron-beam lithography with the aim of suppressing singlet–polaron annihilation (SPA). Nanopatterns composed of lines and circles were used in the current flow regions of nano-line and nano-dot OLEDs, respectively. Excitons partially escape from the current flow regions where SPA takes place. As such, current densities where external quantum efficiencies were half of their initial values (J0) increased as line width and circle diameter were decreased to close to the exciton diffusion length. Circles were more efficient at enhancing exciton escape and increasing J0 than lines. The J0 increase in the nano-dot OLEDs containing nanopatterned circles with a diameter of 50 nm was approximately 41-fold that of a conventional OLED with a current flow region of 4 mm2. The dependence of J0 on the size and shape of the nanopatterns was well explained by an SPA model that considered exciton diffusion. Nanopatterning of...

Frequency conversion in a long Josephson junction with a moving vortex array

The dispersion relation of small electromagnetic waves applied in the uniform flux-flow state of a long Josephson junction has been obtained. The results contain exponentially growing waves due to the parametric interaction between the applied fields and the moving vortices, which indicates the possibility of a Josephson-junction frequency converter of the traveling-wave type.

Low threshold amplified spontaneous emission and ambipolar charge transport in non-volatile liquid fluorene derivatives

Highly fluorescent non-volatile fluidic fluorene derivatives functionalized with siloxane chains were synthesized and used in monolithic solvent-free liquid organic semiconductor distributed feedback lasers. The photoluminescence quantum yield values, the amplified spontaneous emission thresholds and the ambipolar charge carrier mobilities demonstrate that this class of materials is extremely promising for organic fluidic light-emitting and lasing devices.

Joule heat-induced breakdown of organic thin-film devices under pulse operation

We investigated the influence of the substrates thermal conductivities (k) and the widths of the electrical pulses (τpulse) on the maximum current densities (Jmax) in organic thin-film devices. We also estimated the temperature rise (ΔT) inside devices under the pulse operation using numerical calculations to interpret the observed differences in Jmax. For a long τpulse of 5 μs, Jmax is higher for devices with high-k sapphire substrates (around 1.2 kA/cm2) than devices with low-k plastic substrates (around 0.4 kA/cm2). This is because high-k sapphire substrates can work as heat sinks to relax ΔT for such a long τpulse. Operation of devices with high-k sapphire substrates for a short τpulse of 70 ns resulted in further relaxation of ΔT, leading to an increase of Jmax to around 5 kA/cm2. Interestingly, for such a short τpulse, devices with high-k sapphire and low-k plastic substrates showed similar Jmax and ΔT values, the reason for which may be that it is difficult for the generated Joule heat to travel to the substrate across a low-k organic layer within this short time.We investigated the influence of the substrates thermal conductivities (k) and the widths of the electrical pulses (τpulse) on the maximum current densities (Jmax) in organic thin-film devices. We also estimated the temperature rise (ΔT) inside devices under the pulse operation using numerical calculations to interpret the observed differences in Jmax. For a long τpulse of 5 μs, Jmax is higher for devices with high-k sapphire substrates (around 1.2 kA/cm2) than devices with low-k plastic substrates (around 0.4 kA/cm2). This is because high-k sapphire substrates can work as heat sinks to relax ΔT for such a long τpulse. Operation of devices with high-k sapphire substrates for a short τpulse of 70 ns resulted in further relaxation of ΔT, leading to an increase of Jmax to around 5 kA/cm2. Interestingly, for such a short τpulse, devices with high-k sapphire and low-k plastic substrates showed similar Jmax and ΔT values, the reason for which may be that it is difficult for the generated Joule heat to travel t...

Quasi continuous-wave lasing in organic thin-film semiconductors(Conference Presentation)

Since the discovery of organic solid-state lasers, great efforts have been devoted to the development of continuous-wave (cw) lasing in organic materials. However, the operation of organic solid-state lasers under optical cw excitation or pulse excitation at a very high repetition rate (quasi-cw excitation) is extremely challenging. In this work, we have demonstrated quasi-continuous-wave (quasi-cw) surface-emitting lasing in a distributed feedback device which combines a second-order grating with an organic thin film of a host material 4,4’-bis(N-carbazolyl)-1,1’-biphenyl (CBP) blended with an organic laser dye 4,4’-bis[(N-carbazole)styryl]biphenyl (BSBCz). When pumping the device with optical picosecond pulse excitation, the quasi-cw laser operation maintained up to a repetition rate of 8 MHz. The lasing threshold was around 0.25 J cm−2 which was almost independent of the repetition rates. For our laser devices, the maximum repetition rate (8 MHz) is the highest ever reported, and the lasing threshold (0.25 J cm−2) is the lowest ever reported. These superior quasi-cw lasing characteristics in BSBCz are accomplished by the less generation of triplet excitons via intersystem crossing because a photoluminescence quantum yield of the blend film is nearly 100% and there is no significant spectral overlap between laser and triplet absorption.[1,2] Triplet quenchers, generally used for the fabrication of organic thin-film lasers, were not necessary in our devices because of negligible accumulation of triplet excitons and a small spectral overlap between emission and triplet absorption. Therefore, we believe that BSBCz is the most promising candidate for the first realization of electrically pumped organic laser diodes in terms of optical characteristics. However, electrical characteristics such as charge carrier mobility, charge carrier capture cross section, etc., are also extremely important and will need further investigation and enhancement for realization of electrically pumped organic lasers. 1. Aimono, T.; Kawamura, Y.; Goushi, K.; Yamamoto, H.; Sasabe, H.; Adachi, C. Appl. Phys. Lett. 2005, 86, 071110–071112. 2. Nakanotani, H.; Adachi, C.; Watanabe, S.; Katoh, R. Appl. Phys. Lett. 2007, 90, 231109.