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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

The control of laser emission angles in laser devices is essential for many optoelectronic and photonic applications such as optical sensing and displays. In this context, we studied the light diffraction pattern of laser beams in a one-dimensional sampled distributed feedback resonator having organic gain media. The gratings consist of the repetition of supercells having a mixed-order sampled grating in which 1st-order gratings surround a 2nd-order grating. The experimental results showed that the diffraction angles of the laser beams are quite diverse depending on the supercell structures. We demonstrate that the interval of the diffraction angle (θ) of the laser beams is inversely proportional to the length of the supercell experimentally and theoretically. By tuning the length of supercells as well as the length of 1st-order and 2nd-order regions, the interval θ was tuned from 0.1° to 43° with the different arc emission patterns. With the reduction of θ, i.e., the longer 1st-order region, a significant decrease in the laser threshold was obtained, resulting in the lowest lasing threshold of 2.5 ± 0.1 μJ/cm2 with ∼3.5 ns of a long pulse width excitation source.

DOI： 10.1063/5.0123476

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

Abstract

In this study, electron and hole injection, and transport in semi-transparent organic light-emitting diodes, including dielectric/metal/dielectric (DMD) electrodes, were investigated. The DMD electrode was modified by incorporating a 5 nm-thick Cs:MoOx layer and a 10 nm-thick 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN) layer to improve the electron and hole injection in standard and inverse structures, respectively. The inverse structure showed a reasonably good and stable external quantum efficiency due to the well-balanced carrier densities at both low and high voltages which was confirmed by an electrical simulation study. Also, the peak position of the hole and electron recombination rate in the inverse structure was located on the ITO side, which is beneficial for reducing exciton quenching and photon absorption by metallic electrodes.

DOI： 10.35848/1347-4065/ac7e96

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Other Link： https://iopscience.iop.org/article/10.35848/1347-4065/ac7e96/pdf

Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.jpclett.1c03983

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Publishing type：Research paper (scientific journal)   Publisher：Frontiers Media SA  

A series of bis(triphenylamine)benzodifuran chromophores have been synthesized and fully characterised. Starting from suitably functionalized benzodifuran (BDF) precursors, two triphenylamine (TPA) moieties are symmetrically coupled to a central BDF unit either at 4,8-positions through double bonds (1) and single bonds (2) respectively, or at 2,6-positions through double bonds (3). Their electronic absorption and photoluminescence properties as well as redox behaviour have been investigated in detail, indicating that the π-extended conjugation via vinyl linkers in 1 and 3 leads to comparatively strong electronic interactions between the relevant redox moieties TPA and BDF. Due to intriguing electronic properties and structural planarity, 3a has been applied as a dopant emitter in organic light-emitting diodes. A yellowish-green OLED exhibits a high external quantum efficiency (EQE) of 6.2%, thus exceeding the theoretical upper limit most likely due to energy transfer from an interface exciplex to an emissive layer and/or favorable horizontal orientation.

DOI： 10.3389/fchem.2021.721272

Open Access

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Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Abstract

The development of near‐infrared (NIR) light sources has attracted much interest due to their attractive applications, such as biosensing and light detection and ranging (LiDAR). In particular, organic semiconductor laser diodes with NIR emission are emerging as a next generation technology. However, organic NIR emitters have generally suffered from a low quantum yield, which has resulted in only a few examples of organic solid‐state NIR lasers. In this study, the authors demonstrate a highly efficient NIR emitter based on a boron difluoride curcuminoid structure, which shows a high photoluminescence (PL) quantum yield (Φ_PL) at >700 nm and a high fluorescence radiative rate constant in a solid‐state film. Amplified spontaneous emission and lasing occurs at >800 nm with very low thresholds. The large redshift of the stimulated emission is attributed to the transition from the lowest excited state to the different vibrational levels of the ground state owing to the overlap between the emission and the singlet–singlet excited‐state absorption.

DOI： 10.1002/adom.202001947

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/adom.202001947

Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acsami.0c05449

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Publishing type：Research paper (scientific journal)   Publisher：Frontiers Media SA  

DOI： 10.3389/fchem.2020.00427

Open Access

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Abstract

The organic material 4,4′‐bis[(N‐carbazole)styryl]biphenyl (BSBCz) is an excellent gain medium for laser devices. However, BSBCz laser output quickly degrades during photoexcitation, which is an issue that must be overcome before it can be used for practical applications. In this study, the photodegradation mechanisms of BSBCz are investigated with the aim of enhancing its excited‐state stability. The photodegradation of BSBCz is attributed to instability of the triplet excited states that would occasionally decompose into other species. This decomposition reduces absorption and introduces exciton quenchers. Incorporating the triplet managing material 9,10‐di(naphtha‐2‐yl)anthracene (ADN) into BSBCz films greatly improves photoluminescence and amplified spontaneous emission stability because of the effective removal of the unstable triplets by ADN. This triplet managing method makes it possible to increase operational stability for BSBCz‐based organic light‐emitting diodes. Therefore, these results will contribute toward the fabrication of stable optically and electrically pumped organic laser diodes.

DOI： 10.1002/adfm.201807148

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/adfm.201807148

Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Abstract

The study reports the development of a solution‐processed phosphorescent tandem organic light‐emitting device (OLED) exhibiting extremely small efficiency roll‐off. The OLED comprises two light‐emitting units (LEUs) connected by an interconnecting unit and employs a thermally activated delayed fluorescence host material. One of the most difficult tasks in the fabrication of OLEDs is to form a multilayer structure without dissolving the underlayer during the coating of the upper layer. The developed host materials exhibit high tolerance to methanol. The upper‐layer adjacent to the light‐emitting layer consists of ZnO nanoparticles, which could be dispersed in methanol by improving the preparation method. This results in the successful fabrication of a solution‐processed phosphorescent tandem OLED comprising two LEUs. The maximum external quantum efficiency (EQE) of the tandem device is 22.8%, and the EQE is 21.9% even at a high luminance of 10 000 cd m⁻². The suppression of efficiency roll‐off is among the best of those previously reported. Moreover, the operational stability of the tandem device is much higher compared with single‐LEU devices.

DOI： 10.1002/adfm.201808022

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/adfm.201808022

Publishing type：Research paper (scientific journal)   Publisher：Wiley  

Abstract

Although imidazole derivatives are promising as blue fluorescent emitters in organic light‐emitting devices (OLEDs), the corresponding blue thermally activated delayed fluorescence (TADF) emitter is rarely explored. In this study, a series of imidazo[1,2‐f]phenanthridine‐based TADF materials in the blue to sky‐blue region is developed. The resultant benzimidazole (BIP)‐based TADF materials, with π‐expanded acceptor and electron‐donor phenoxazine (PXZ), namely PXZ‐BIP, shows a high maximum external quantum efficiency (η_ext,max) of 21% with CIE coordinates of (0.21, 0.37). These results have clearly demonstrated that PXZ‐BIP exhibits the highest η_ext,max among imidazole‐based fluorescent sky‐blue OLEDs. The corresponding imidazophenanthridine derivative, with a less π‐expanded acceptor, shows inferior OLED performances, with a η_ext,max of 5.9% and CIE of (0.19, 0.30). This study provides a critical molecular design to realize high‐efficiency TADF materials based on imidazole derivatives.

DOI： 10.1002/adom.201801282

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/adom.201801282

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：SPIE-Intl Soc Optical Eng  

DOI： 10.1117/1.jpe.8.032108

Open Access

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.chemmater.7b02403

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

By introducing an acceptor which contains double sulfonyl groups and a twisted biphenyl core, an efficient blue thermally activated delayed fluorescence emitter is successfully developed. Comparative study and analysis are carried out on it with an analogous molecule, which also imports two sulfonyl groups as acceptors and exactly same donor units but shows stretched shape and displays green emission. Results of quantum chemistry computation estimate that the introduction of such a twisted backbone is beneficial to increase energies of both singlet and triplet states. As a contrast to its counterpart, the newly developed blue emission compound with contorted structure enables a desirable emission with a peak wavelength of 460 nm and superior photoluminescence quantum yield up to 82%. Moreover, remarkable external quantum efficiency over 24% and Commission International de l'Eclairage coordinates with total (x + y) value <0.4 are achieved from the electroluminescent device based on the blue emitter, while this value is almost twice to that of the green emission device based on its analog.

DOI： 10.1002/adom.201700334

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/adom.201700334

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acsami.6b13482

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

DOI： 10.1002/adom.201600843

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

DOI： 10.1002/chem.201604303

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley  

DOI： 10.1002/adom.201600675

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Other Link： https://onlinelibrary.wiley.com/doi/full-xml/10.1002/adom.201600675

Authorship：Lead author   Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry (RSC)  

<p>A series of TADF emitters composed of pyrimidine/acridine moieties are developed. The optimized device realized its highest power efficiency of 62 lm W⁻¹, a high external quantum efficiency of 25%, and a low turn-on voltage of 2.8 V with light-blue emission.</p>

DOI： 10.1039/c5tc04057d

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.synthmet.2015.02.009

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