Munetomo Inoue

Dual enhancement of electroluminescence efficiency and operational stability by rapid upconversion of triplet excitons in OLEDs.

Recently, triplet harvesting via a thermally activated delayed fluorescence (TADF) process has been established as a realistic route for obtaining ultimate internal electroluminescence (EL) quantum efficiency in organic light-emitting diodes (OLEDs). However, the possibility that the rather long transient lifetime of the triplet excited states would reduce operational stability due to an increased chance for unwarranted chemical reactions has been a concern. Herein, we demonstrate dual enhancement of EL efficiency and operational stability in OLEDs by employing a TADF molecule as an assistant dopant and a fluorescent molecule as an end emitter. The proper combination of assistant dopant and emitter molecules realized a “one-way” rapid Förster energy transfer of singlet excitons from TADF molecules to fluorescent emitters, reducing the number of cycles of intersystem crossing (ISC) and reverse ISC in the TADF molecules and resulting in a significant enhancement of operational stability compared to OLEDs with a TADF molecule as the end emitter. In addition, we found that the presence of this rapid energy transfer significantly suppresses singlet-triplet annihilation. Using this finely-tuned rapid triplet-exciton upconversion scheme, OLED performance and lifetime was greatly improved.

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.

Horizontal molecular orientation in solution-processed organic light-emitting diodes

Horizontal orientation of the emission transition dipole moments achieved in glassy vapor-deposited organic thin films leads to an enhancement of the light out-coupling efficiency in organic light-emitting diodes (OLEDs). Here, our combined study of variable angle spectroscopic ellipsometry and angle dependent photoluminescence demonstrates that such a horizontal orientation can be achieved in glassy spin-coated organic films based on a composite blend of a heptafluorene derivative as a dopant and a 4,4′-bis(N-carbazolyl)-1,1′-biphenyl as a host. Solution-processed fluorescent OLEDs with horizontally oriented heptafluorene emitters were then fabricated and emitted deep blue electroluminescence with an external quantum efficiency as high as 5.3%.

Tunable and flexible solvent-free liquid organic distributed feedback lasers

We report on optically pumped blue, green, and red liquid organic distributed feedback (DFB) lasers based on solvent-free fluidic organic semiconductors, and prepared on highly flexible corrugated polymeric patterns. By the appropriate selection of laser dyes doping a liquid 9-(2-ethylhexyl)carbazole host, the lasing wavelength is effectively tuned across the visible spectrum via a cascade energy transfer scheme. We also demonstrate a mechanical tunability of the flexible liquid DFB laser emission, which is due to the deformation of the high-aspect ratio DFB grating under bending. Overall, this work provides an important step in the development of flexible liquid organic optoelectronic devices.

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...

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.

Low amplified spontaneous emission threshold and suppression of electroluminescence efficiency roll-off in layers doped with ter(9,9′-spirobifluorene)

We demonstrate that ter(9,9′-spirobifluorene) (TSBF) doped in a host matrix layer of 4,4′-bis(carbazol-9-yl)biphenyl (CBP) shows a low amplified spontaneous emission (ASE) threshold (Eth = 1.0 μJ cm−2) and suppressed electroluminescence efficiency roll-off at high current densities (no roll-off up to 100 mA cm−2). One origin of the low ASE threshold is that the TSBF-doped CBP layer possesses a very large radiative decay constant (kr = 1.1 × 109 s−1). Singlet–triplet annihilation is almost suppressed in the TSBF-doped CBP layer, which can be ascribed to the small overlap between the emission and triplet absorption of TSBF. Also, the small energy level difference between TSBF and CBP minimizes carrier trapping in TSBF, leading to the suppression of singlet–polaron annihilation. TSBF showed one of the lowest Eth and the most suppressed efficiency roll-off among organic laser dyes investigated in this study and, therefore, is believed to be a promising candidate to realize electrically pumped organic semicond...