Xuedong Wang

Perovskite Microdisk Microlasers Self‐Assembled from Solution

Prof. Q. Liao, K. Hu, H. Zhang, Prof. H. Fu Beijing Key Laboratory for Optical Materials and Photonic Devices Department of Chemistry Capital Normal University Beijing 100048 , P. R. China E-mail: liaoqing@cnu.edu.cn; hongbing.fu@iccas.ac.cn Dr. X. Wang, Prof. J. Yao, Prof. H. Fu Beijing National Laboratory for Molecular Sciences (BNLMS) Institute of chemistry Chinese Academy of Sciences Beijing 100190 , P. R. China Prof. H. Fu Collaborative Innovation Center of Chemical Science and Engineering Tianjin 300072 , P. R. China

Near-Infrared Lasing from Small-Molecule Organic Hemispheres

Near-infrared (NIR) lasers are key components for applications, such as telecommunication, spectroscopy, display, and biomedical tissue imaging. Inorganic III-V semiconductor (GaAs) NIR lasers have achieved great successes but require expensive and sophisticated device fabrication techniques. Organic semiconductors exhibit chemically tunable optoelectronic properties together with self-assembling features that are well suitable for low-temperature solution processing. Major blocks in realizing NIR organic lasing include low stimulated emission of narrow-bandgap molecules due to fast nonradiative decay and exciton-exciton annihilation, which is considered as a main loss channel of population inversion for organic lasers under high carrier densities. Here we designed and synthesized the small organic molecule (E)-3-(4-(di-p-tolylamino)phenyl)-1-(1-hydroxynaphthalen-2-yl)prop-2-en-1-one (DPHP) with amphiphilic nature, which elaborately self-assembles into micrometer-sized hemispheres that simultaneously serves as the NIR emission medium with a photoluminescence quantum efficiency of ∼15.2%, and the high-Q (∼1.4 × 10(3)) whispering gallery mode microcavity. Moreover, the radiative rate of DPHP hemispheres is enhanced up to ∼1.98 × 10(9) s(-1) on account of the exciton-vibrational coupling in the solid state with the J-type molecular-coupling component, and meanwhile the exciton-exciton annihilation process is eliminated. As a result, NIR lasing with a low threshold of ∼610 nJ/cm(2) is achieved in the single DPHP hemisphere at room temperature. Our demonstration is a major step toward incorporating the organic coherent light sources into the compact optoelectronic devices at NIR wavelengths.

Whispering-Gallery-Mode Microlaser Based on Self-Assembled Organic Single-Crystalline Hexagonal Microdisks†

Whispering-gallery-mode (WGM) resonators of semiconductor microdisks have been applied for achieving low-threshold and narrow-linewidth microlasers, but require sophisticated top-down processing technology. Organic single-crystalline hexagonal microdisks (HMDs) of p-distyrylbenzene (DSB) self-assembled from solution can function as WGM microresonators with a cavity quality factor (Q) of 210. Both multiple- and single-mode lasing had been achieved using DSB HMDs with an edge length of 4.3 and 1.2 μm, respectively. These organic microdisks fabricated by bottom-up self-assembly approach may offer potential applications as low-threshold microlaser sources for photonic circuit integration.

Tunable Morphology of the Self-Assembled Organic Microcrystals for the Efficient Laser Optical Resonator by Molecular Modulation

Organic single-crystalline micro/nanostructures can effectively generate and carry photons due to their smooth morphologies, high photoluminescence quantum efficiency, and minimized defects density and therefore are potentially ideal building blocks for the optical circuits in the next generation of miniaturized optoelectronics. However, the tailor-made organic molecules can be generally obtained by organic synthesis, ensuring that the organic molecules aggregate in a specific form and generate micro/nanostructures with desirable morphology and therefore act as the efficient laser optical resonator remains a great challenge. Here, the molecular modulation of the morphology on the laser optical resonator properties has been investigated through the preparation of the elongated hexagonal microplates (PHMs) and the rectangular microplates (ORMs), respectively, from two model isomeric organic molecules of 1,4-bis(4-methylstyryl)benzene (p-MSB) and 1,4-bis(2-methylstyryl)benzene (o-MSB). Significantly, fluorescence resonance phenomenon was only observed in the individual ORM other than the PHM. It indicates that the rectangular resonators possess better light-confinement property over the elongated hexagonal resonators. More importantly, optically pumped lasing action was observed in the o-MSB rectangular morphology microplates resonator with a high Q ≈ 1500 above a threshold of ∼540 nJ/cm(2). The excellent optical properties of these microstructures are associated with the morphology, which can be precisely modulated by the organic molecular structure. These self-assembled organic microplates with different morphologies can contribute to the distinct functionality of photonics elements in the integrated optical circuits at micro/nanoscale.

Tuning the Solid State Emission of the Carbazole and Cyano‐Substituted Tetraphenylethylene by Co‐Crystallization with Solvents

Solid state emissive materials with high quantum yields and tunable emissions are desirable for various applications. A new TPE derivative (1) with two carbazole moieties and two cyano groups is reported, which shows typical aggregation induced emission behavior. Four crystals 1a, 1b, 1c, and 1d are obtained after crystallization from N,N-dimethylformamid (DMF), trichloromethane (CHCl3 ), tetrahydrofuran (THF), and dichloromethane (CH2 Cl2 ), respectively. Crystal structural analyses reveal that (i) molecules of 1 co-crystallize with DMF, CHCl3 , THF, and CH2 Cl2 in 1a, 1b, 1c, and 1d, respectively, and (ii) conformations of 1 are different within 1a, 1b, 1c, and 1d, and compound 1 within crystal 1a adopts the most twisting conformation. Crystalline solids 1a, 1b, 1c, and 1d exhibit high emission quantum yields up to 0.65, but their emission colors are varied from blue to green. In comparison, the amorphous solid of 1 is yellow-emissive with emission maximum at 542 nm. Moreover, the blue- or green-emissive crystalline solids and the yellow-emissive amorphous solid can be inter-converted by the grinding of crystalline solids and exposure of the amorphous solid to vapors of appropriate solvents. It is also demonstrated that microrods of 1a, 1b, and 1d show typical optical waveguiding behavior.

High performance quinacridone-based polymers in film transistors and photovoltaics: effects of vinylene linkage on crystallinity and morphology

A novel quinacridone-based polymer containing a vinylene linkage, PQTE, was synthesized and exhibited higher performance both in organic thin-film transistors and solar cells than that of the reference polymer PQ2T. On introducing the vinylene linkage, a strong interchain interaction is obtained with a short π–π stacking distance of 3.49 A in the polymer PQTE, which is responsible for the highest mobility of 0.67 cm2 V−1 s−1 reported to date for quinacridone-based semiconductors. More significantly, the incorporation of a thienylene–vinylene–thienylene unit contributes to good miscibility between PQTE and PC71BM. Because of the effective intercalation of PC71BM in the PQTE lamellar structure, a high power conversion efficiency of 3.9% was achieved without any additives or post-treatments. Our rational molecular design qualifies quinacridone as a promising building block simultaneously in high performance polymer thin-film transistors and solar cells.

White-Emissive Self-Assembled Organic Microcrystals

Organic semiconductor micro-/nanocrystals with regular shapes have been demonstrated for many applications, such as organic field-effect transistors, organic waveguide devices, organic solid-state lasers, and therefore are inherently ideal building blocks for the key circuits in the next generation of miniaturized optoelectronics. In the study, blue-emissive organic molecules of 1,4-bis(2-methylstyryl)benzene (o-MSB) can assemble into rectangular microcrystals at a large scale via the room-temperature solution-exchange method. Because of the Förster resonance energy transfer, the energy of the absorbed photons by the host matrix organic molecules of o-MSB can directly transfer to the dopant organic molecules of tetracene or 1,2:8,9-dibenzopentacene (DBP), which then emit visible photons in different colors from blue to green, and to yellow. More impressively, by modulating the doping molar ratios of DBP to o-MSB, bright white-emissive organic microcrystals with well-preserved rectangular morphology can be successfully achieved with a low doping ratio of 1.5%. These self-assembled organic semiconductor microcrystals with multicolor emissions can be the white-light sources for the integrated optical circuits at micro-/nanoscale.

High-efficiency quantum dot light-emitting diodes employing lithium salt doped poly(9-vinlycarbazole) as a hole-transporting layer

For the purpose of fabricating solution-processed quantum-dot light-emitting diodes (QLEDs) with high performance, the efficient hole–electron recombination at low current density is particularly pivotal. Herein, to enhance the charge balance of the QLED device, we employed lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI) as a p-type dopant into the hole-transporting material (HTM) of poly(9-vinlycarbazole) (PVK). In the experiment, the increased conductivity and the enhanced charge mobility of the Li-TFSI-doped PVK layer were confirmed by the J–V curves of the hole-only devices and conductive atomic force microscopy (c-AFM). Furthermore, on combining ultraviolet photoelectron spectroscopy (UPS) and the absorption spectra, it was found that the highest occupied molecular orbital (HOMO) of the Li-TFSI-doped PVK layers gradually shifted closer to the Fermi level upon increasing the doping ratios from 0 to 4.5 wt%. Therefore, the hole-injecting barrier decreases from 1.17 eV to 0.64 eV. As a result, the maximum current efficiency and the highest external quantum efficiency (EQE) of our fabricated QLED devices can reach as high as 15.5 cd A−1 and 11.46%, respectively. It was demonstrated that the p-type dopant Li-TFSI in the HTM can contribute to the fabrication of high-performance solution-processed light-emitting diodes.

Polymer as an Additive in the Emitting Layer for High-Performance Quantum Dot Light-Emitting Diodes

A facile but effective method is proposed to improve the performance of quantum dot light-emitting diodes (QLEDs) by incorporating a polymer, poly(9-vinlycarbazole) (PVK), as an additive into the CdSe/CdS/ZnS quantum dot (QD) emitting layer (EML). It is found that the charge balance of the device with the PVK-added EML was greatly improved. In addition, the film morphology of the hole-transporting layer (HTL) which is adjacent to the EML, is substantially improved. The surface roughness of the HTL is reduced from 5.87 to 1.38 nm, which promises a good contact between the HTL and the EML, resulting in low leakage current. With the improved charge balance and morphology, a maximum external quantum efficiency (EQE) of 16.8% corresponding to the current efficiency of 19.0 cd/A is achievable in the red QLEDs. The EQE is 1.6 times as high as that (10.5%) of the reference QLED, comprising a pure QD EML. This work demonstrates that incorporating some polymer molecules into the QD EML as additives could be a facile route toward high-performance QLEDs.

Controlled Substitution of Chlorine for Iodine in Single-Crystal Nanofibers of Mixed Perovskite MAPbI3–xClx

Longer carrier diffusion length and improved power conversion efficiency have been reported for thin-film solar cell of organolead mixed-halide perovskite MAPbI3- x Cl x in comparison with MAPbI3 . Instead of substituting I in the MAPbI3 lattice, Cl-incorporation has been shown to mainly improve the film morphology of perovskite absorber. Well-defined crystal structure, adjustable composition (x), and regular morphology, remains a formidable task. Herein, a facile solution-assembly method is reported for synthesizing single-crystalline nanofibers (NFs) of tetragonal-lattice MAPbI3- x Cl x with the Cl-content adjustable between 0 ≤ x ≤ 0.75, leading to a gradual blueshift of the absorption and photoluminescence maxima from x = 0 to 0.75. The photoresponsivity (R) of MAPbI3 NFs keeps almost unchanging at a value independent of the white-light illumination intensity (P). In contrast, R of MAPbI3- x Cl x NFs decreases rapidly with increasing both the x and P values, indicating Cl-substitution increases the recombination traps of photogenerated free electrons and holes. This study provides a model system to examine the role of extrinsic Cl ions in both perovskite crystallography and optoelectronic properties.