Organic light-emitting diode (OLED) technology is promising for applications in next-generation displays and lighting. However, it is difficult-especially in large-area mass production-to cover a ...large substrate uniformly with organic layers, and variations in thickness cause the formation of shunting paths between electrodes
, thereby lowering device production yield. To overcome this issue, thicker organic transport layers are desirable because they can cover particles and residue on substrates, but increasing their thickness increases the driving voltage because of the intrinsically low charge-carrier mobilities of organics. Chemical doping of organic layers increases their electrical conductivity and enables fabrication of thicker OLEDs
, but additional absorption bands originating from charge transfer appear
, reducing electroluminescence efficiency because of light absorption. Thick OLEDs made with organic single crystals have been demonstrated
, but are not practical for mass production. Therefore, an alternative method of fabricating thicker OLEDs is needed. Here we show that extraordinarily thick OLEDs can be fabricated by using the organic-inorganic perovskite methylammonium lead chloride, CH
NH
PbCl
(MAPbCl
), instead of organics as the transport layers. Because MAPbCl
films have high carrier mobilities and are transparent to visible light, we were able to increase the total thickness of MAPbCl
transport layers to 2,000 nanometres-more than ten times the thickness of standard OLEDs-without requiring high voltage or reducing either internal electroluminescence quantum efficiency or operational durability. These findings will contribute towards a higher production yield of high-quality OLEDs, which may be used for other organic devices, such as lasers, solar cells, memory devices and sensors.
Organic-inorganic lead halide quasi-two-dimensional (2D) perovskites are promising gain media for lasing applications because of their low cost, tunable colour, excellent stability and solution ...processability
. Optically pumped continuous-wave (CW) lasing is highly desired for practical applications in high-density integrated optoelectronics devices and constitutes a key step towards electrically pumped lasers
. However, CW lasing has not yet been realized at room temperature because of the 'lasing death' phenomenon (the abrupt termination of lasing under CW optical pumping), the cause of which remains unknown. Here we study lead halide-based quasi-2D perovskite films with different organic cations and observe that long-lived triplet excitons considerably impede population inversion during amplified spontaneous emission and optically pumped pulsed and CW lasing. Our results indicate that singlet-triplet exciton annihilation is a possible intrinsic mechanism causing lasing death. By using a distributed-feedback cavity with a high quality factor and applying triplet management strategies, we achieve stable green quasi-2D perovskite lasers under CW optical pumping in air at room temperature. We expect that our findings will pave the way to the realization of future current-injection perovskite lasers.
Near-infrared organic light-emitting diodes and semiconductor lasers could benefit a variety of applications including night-vision displays, sensors and information-secured displays. Organic dyes ...can generate electroluminescence efficiently at visible wavelengths, but organic light-emitting diodes are still underperforming in the near-infrared region. Here, we report thermally activated delayed fluorescent organic light-emitting diodes that operate at near-infrared wavelengths with a maximum external quantum efficiency of nearly 10% using a boron difluoride curcuminoid derivative. As well as an effective upconversion from triplet to singlet excited states due to the non-adiabatic coupling effect, this donor–acceptor–donor compound also exhibits efficient amplified spontaneous emission. By controlling the polarity of the active medium, the maximum emission wavelength of the electroluminescence spectrum can be tuned from 700 to 780 nm. This study represents an important advance in near-infrared organic light-emitting diodes and the design of alternative molecular architectures for photonic applications based on thermally activated delayed fluorescence.
A very high hole mobility of 15 cm2 V−1 s−1 along with negligible hysteresis are demonstrated in transistors with an organic–inorganic perovskite semiconductor. This high mobility results from the ...well‐developed perovskite crystallites, improved conversion to perovskite, reduced hole trap density, and improved hole injection by employing a top‐contact/top‐gate structure with surface treatment and MoOx hole‐injection layers.
Abstract
Large external quantum efficiency rolloff at high current densities in organic light-emitting diodes (OLEDs) is frequently caused by the quenching of radiative singlet excitons by long-lived ...triplet excitons singlet–triplet annihilation (STA). In this study, we adopted a triplet scavenging strategy to overcome the aforementioned STA issue. To construct a model system for the triplet scavenging, we selected 2,6-dicyano-1,1-diphenyl-λ
5
σ
4
-phosphinine (DCNP) as the emitter and 4,4′-bis(
N
-carbazole)styrylbiphenyl (BSBCz) as the host material by considering their singlet and triplet energy levels. In this system, the DCNP’s triplets are effectively scavenged by BSBCz while the DCNP’s singlets are intact, resulting in the suppressed STA under electrical excitation. Therefore, OLEDs with a 1 wt.%-DCNP-doped BSBCz emitting layer demonstrated the greatly suppressed efficiency rolloff even at higher current densities. This finding favourably provides the advanced light-emitting performance for OLEDs and organic semiconductor laser diodes from the aspect of the suppressed efficiency rolloff.
Near-infrared (NIR) organic light-emitting devices have aroused increasing interest because of their potential applications such as information-secured displays, photodynamic therapy, and optical ...telecommunication. While thermally activated delayed fluorescent (TADF) emitters have been used in a variety of high-performance organic light-emitting diodes (OLEDs) emitting in the visible spectral range, efficient NIR TADF materials have been rarely reported. Herein, we designed and synthesized a novel solution-processable NIR TADF dimeric borondifluoride curcuminoid derivative with remarkable photophysical, electroluminescence and amplified spontaneous emission properties. This dye was specifically developed to shift the emission of borondifluoride curcuminoid moiety toward longer wavelengths in the NIR region while keeping a high photoluminescence quantum yield. The most efficient OLED fabricated in this study exhibits a maximum external quantum efficiency of 5.1% for a maximum emission wavelength of 758 nm, which ranks among the highest performance for NIR electroluminescence. In addition, this NIR TADF emitter in doped thin films displays amplified spontaneous emission above 800 nm with a threshold as low as 7.5 μJ/cm2, providing evidence that this material is suitable for the realization of high-performance NIR organic semiconductor lasers.
Advanced organic laser dyes exhibiting high solubility and bipolar behavior are developed based on a structure combining bis‐stilbene with carbazole (BSBCz). The materials show high photoluminescence ...quantum yields and large radiative rate constants in solutions, crystals, and blend and neat films. The introduction of alkyl groups significantly improves the solubility of BSBCz, and solution‐processed films of the alkyl‐substituted derivatives exhibit amplified spontaneous emission thresholds as low as 0.59 µJ cm−2, which is comparable to those of vacuum‐deposited BSBCz films. On the other hand, cyano‐substitution on BSBCz (BSBCz‐CN) increases electron‐accepting properties, resulting in a bathochromic shift of the emission wavelength and improved bipolar behavior. In a BSBCz‐CN‐doped film, a low ASE threshold of 0.63 µJ cm−2 is achieved, which is one of the lowest values for organic laser dyes with green emission. In addition, organic light‐emitting diodes based on BSBCz‐CN neat films exhibit external quantum efficiencies of 1.8% and could withstand injection of high current densities of up to 500 A cm−2 under pulse operation. These properties along with low excited‐state absorption cross sections make these materials an outstanding addition to the existing library of organic laser dyes, especially for consideration in electrically pumped lasers.
Low amplified spontaneous emission thresholds below 1 µJ cm−2 are achieved in a series of bis‐stilbene with carbazole derivatives and are attributed to high photoluminescence quantum yields (≈100%), large radiative decay rates (10−8–10−9 s−1), and low excited state absorption cross sections. In addition, the new organic laser dyes have high solubility and/or bipolar properties.
Degradation of organic light-emitting diodes (OLEDs) operated continuously at a constant current density is investigated using photoluminescence techniques. The OLEDs contained the thermally ...activated delayed fluorescence emitting dopant (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN). OLED degradation proceeds mainly on the basis of excited-state instability of host molecules rather than processes related to 4CzIPN. Additionally, the electrochemical instability of radical cations and anions influences long-term OLED degradation. The formation of exciton quenchers and nonradiative carrier recombination centers acts to reduce OLED luminance. These findings highlight the need for new host material development to fabricate more stable TADF-OLEDs.
A continuous‐wave (CW) organic solid‐state laser is highly desirable for spectroscopy, sensing, and communications, but is a significant challenge in optoelectronics. The accumulation of long‐lived ...triplet excitons and relevant excited‐state absorptions, as well as singlet–triplet annihilation, are the main obstacles to CW lasing. Here, progress in singlet‐ and triplet‐state utilizations in organic gain media is reviewed to reveal the issues in working with triplets. Then, exciton behaviors that inhibit light oscillations during long excitation pulses are discussed. Further, recent advances in increasing organic lasing pulse widths from microseconds toward the indication of CW operation are summarized with respect to molecular designs, advanced resonator architectures, triplet scavenging, and potential triplet contribution strategies. Finally, future directions and perspectives are proposed for achieving stable CW organic lasers with significant triplet contribution.
An organic solid‐state laser under continuous‐wave (CW) excitation is one of the most challenging areas in organic optoelectronics. Recent advances in long‐pulsed organic lasers are comprehensively summarized with respect to molecular designs, optical‐resonator architectures, triplet scavenging, and potential triplet‐contribution strategies. Future directions and perspectives for CW operation are discussed.
In organic light‐emitting diodes (OLEDs) based on materials that show thermally activated delayed fluorescence (TADF), the internal quantum efficiency of 100 % can be obtained without using ...phosphorescence‐based organometallics that contain rare metals. Therefore, with TADF‐based emitters, it is possible to fabricate high‐performing OLEDs at a lower cost. However, compared with fluorescence‐ and phosphorescence‐based OLEDs, an understanding of degradation mechanisms in TADF‐based OLEDs is still insufficient for future commercialization. In particular, it is widely recognized that the development of electron transport materials is crucial for improving OLED characteristics, especially driving voltages and operational durability. In this study, it was demonstrated that the operational durability of TADF‐based OLEDs was greatly improved by introducing a triazine‐based material of 2,4,6‐tris(1,1′‐biphenyl‐4‐yl)‐1,3,5triazine (pT2T) as a hole‐blocking layer (HBL) compared with a conventional HBL material of 2,4,6‐tris(biphenyl‐3‐yl)‐1,3,5triazine (T2T). Several experiments were carried out to make the reasons of the improved durability clearer, and attributed the improved durability to the shift of a carrier recombination zone from the emitting layer/HBL interface and the suppressed formation of excited‐state quenchers in the pT2T HBL, because of the higher electron mobility of pT2T and the better stability of its radical anion state.
Thermally activated delayed fluorescence‐based organic light‐emitting diodes were fabricated using an electron transport material (ETM) of pT2T. These devices had excellent operational durability, along with low driving voltages and high external quantum efficiencies, when compared with devices with a conventional ETM of T2T. This is because pT2T has higher electron mobility and excellent electrochemical stability than T2T has.
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