Alex K.-Y. Jen

Bio: Alex K.-Y. Jen is an academic researcher from City University of Hong Kong. The author has contributed to research in topics: Perovskite (structure) & Polymer solar cell. The author has an hindex of 128, co-authored 921 publications receiving 61811 citations. Previous affiliations of Alex K.-Y. Jen include University of Nebraska–Lincoln & Zhejiang California International NanoSystems Institute.

Broadband electric field sensor with electro-optic polymer micro-ring resonator on side-polished optical fiber

Abstract: A novel broadband E-field sensor based on electro-optic polymer micro-ring resonator directly coupled to the core of optical fiber is proposed and demonstrated. A flat is made on the side of the optical fiber by polishing and an electro-optic polymer waveguide in the shape of a ring is placed on the polished flat. One side of the ring is directly above the core of the fiber and light is evanescently coupled between the fiber and the micro-ring. External electric fields change the index of refraction of the ring resonator and therefore its resonant wavelengths. The sensor is all dielectric without metal layers to distort the measured E-field. The resonance structure allows the sensor to potentially have much higher sensitivity than other electro-optic sensors based on interferometry or polarization modulation. Since electro-optic polymers have higher electro-optic coefficients, lower dielectric constants and faster electro-optic responses than inorganic crystals, higher sensitivity, lower invasiveness and higher bandwidth of E-field sensing can be expected. The sensor with EO polymer micro-ring directly coupled to side-polished fiber eliminates unreliable and possibly lossy fiber to waveguide butt coupling as well as the high propagation loss which comes from the long straight EO polymer waveguides. Unlike devices based on waveguide technology, a supporting substrate is not necessary in this device. This leads to sensors of small size and low disturbance to the measured electric field. In the proof-of-concept experiment, a sensitivity of 100 mV/m has been achieved at frequencies up to 550 MHz (limited by the measurement system) using AJLS103 electro-optic polymer.

Methoxy-substituted bis-tridentate iridium(III) phosphors and fabrication of blue organic light emitting diodes

Abstract: Both monoanionic dicarbene pincer chelate and dianionic azole-pyridine-carbazole cyclometalate were successfully employed in the preparation of respective bis-tridentate Ir(III) metal complexes (Cz6–9) in moderate yields. Tuning of emission to blue was achieved by the addition of dual methoxy substituents at the carbazole cyclometalate, as well as the introduction of either methoxy or dimethylamino group at the central pyridinyl fragment of the azole-pyridine-carbazole cyclometalate. Single-crystal X-ray diffraction study was conducted on complex Cz6 in an attempt to provide an unambiguous structural proof. Photophysical properties were next measured in degassed CH2Cl2 solution at RT, giving structureless emission with peak maximum spanning 460–508 nm and photoluminescence quantum yield of 22–87%. A TD-DFT calculation confirmed the dominance of azole-pyridine-carbazole cyclometalate in controlling the emission energy gap, while the carbene pincer was less influential on the detected emission peak wavelength, i.e. acted as an ancillary. Importantly, the experimentally detected radiative lifetime (e.g. 2.78–17.18 μs) showed a clear correlation to the calculated metal-to-ligand charge transfer percentage of the electronic transitions (10.8–7.5%). Organic light-emitting diode devices gave turn-on voltages of 4.3 and 4.3 V, maximum EQE = 16.3 and 12.2%, power efficiencies of 17.6 and 10.4 lm W−1 and with CIEx,y coordinates of (0.16, 0.26) and (0.16, 0.20) for Cz6 and 7 at 15 wt%, confirming their potential as blue dopants for phosphorescent organic light-emitting diodes.

Poling Dynamics and Effects of Trapped Charge in Poled Polymer Films for Nonlinear Optical Applications

Abstract: The behavior of surface and trapped charge that originates during contact electric-field poling of thin films of optically nonlinear polymers has been investigated. Thin films of poly(methyl methacrylate) doped with two different tricyanothiophene chromophores have been studied during and after the poling process by simultaneous measurement of the current through the poling circuit and the second harmonic generation signal (SHG) from the polymer system. The poling current present in a fresh sample was found to differ in both magnitude and temporal dependence to that observed in the sample during subsequent polings. The magnitude of the steady-state current eventually reached during the poling process was found to depend on temperature and was related to the mobility of charge. A sharp drop off in SHG signal intensity occurring when the poling electrodes were grounded is considered to be due to the removal of surface charge that orients chromophore dipoles near the surface of the film; the decay in the mac...

Highly sensitive thermal damage sensors for polymer composites: time temperature indicator based on thermochromic fluorescence turn-on response

Abstract: Carbon fiber epoxy composites have become prevalent in a variety of industries, especially in aerospace. The significant non-destructive evaluation (NDE) challenges of composites require new solutions, especially in detecting the onset of thermal damage. This work proposes the use of thermochromic fluorescent molecules dispersed in the composites as sensors for such detection. A molecule has been developed which transitions from a colorless, non-fluorescent state to a colorful, highly fluorescent state when exposed to temperature-time combinations that can cause damage in composites. This molecule dispersed in polymer composites of epoxy and PDMS matrices shows unique activation kinetics that can be used to quantitatively simulate the degradation kinetics of aerospace epoxies. The novel sensor materials based on the thermochromic activation of fluorescence can provide highly efficient and widely applicable NDE materials and techniques for carbon fiber epoxy composites.

Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber.

Abstract: Organic-inorganic perovskites have shown promise as high-performance absorbers in solar cells, first as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, we report transient absorption and photoluminescence-quenching measurements to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide (CH3NH3PbI(3-x)Cl(x)) and triiodide (CH3NH3PbI3) perovskite absorbers. We found that the diffusion lengths are greater than 1 micrometer in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. In contrast, the triiodide absorber has electron-hole diffusion lengths of ~100 nanometers. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a critical parameter to optimize for future perovskite absorber development.

Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber

Abstract: Organic-inorganic perovskites have shown promise as high-performance absorbers in solar cells, first as a coating on a mesoporous metal oxide scaffold and more recently as a solid layer in planar heterojunction architectures. Here, we report transient absorption and photoluminescence-quenching measurements to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide (CH3NH3PbI(3-x)Cl(x)) and triiodide (CH3NH3PbI3) perovskite absorbers. We found that the diffusion lengths are greater than 1 micrometer in the mixed halide perovskite, which is an order of magnitude greater than the absorption depth. In contrast, the triiodide absorber has electron-hole diffusion lengths of ~100 nanometers. These results justify the high efficiency of planar heterojunction perovskite solar cells and identify a critical parameter to optimize for future perovskite absorber development.

Interface engineering of highly efficient perovskite solar cells

Abstract: Advancing perovskite solar cell technologies toward their theoretical power conversion efficiency (PCE) requires delicate control over the carrier dynamics throughout the entire device. By controlling the formation of the perovskite layer and careful choices of other materials, we suppressed carrier recombination in the absorber, facilitated carrier injection into the carrier transport layers, and maintained good carrier extraction at the electrodes. When measured via reverse bias scan, cell PCE is typically boosted to 16.6% on average, with the highest efficiency of ~19.3% in a planar geometry without antireflective coating. The fabrication of our perovskite solar cells was conducted in air and from solution at low temperatures, which should simplify manufacturing of large-area perovskite devices that are inexpensive and perform at high levels.

Aggregation-Induced Emission: Together We Shine, United We Soar!

Abstract: Ju Mei,†,‡,∥ Nelson L. C. Leung,†,‡,∥ Ryan T. K. Kwok,†,‡ Jacky W. Y. Lam,†,‡ and Ben Zhong Tang*,†,‡,§ †HKUST-Shenzhen Research Institute, Hi-Tech Park, Nanshan, Shenzhen 518057, China ‡Department of Chemistry, HKUST Jockey Club Institute for Advanced Study, Institute of Molecular Functional Materials, Division of Biomedical Engineering, State Key Laboratory of Molecular Neuroscience, Division of Life Science, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China

High-performance photovoltaic perovskite layers fabricated through intramolecular exchange

Abstract: The band gap of formamidinium lead iodide (FAPbI3) perovskites allows broader absorption of the solar spectrum relative to conventional methylammonium lead iodide (MAPbI3). Because the optoelectronic properties of perovskite films are closely related to film quality, deposition of dense and uniform films is crucial for fabricating high-performance perovskite solar cells (PSCs). We report an approach for depositing high-quality FAPbI3 films, involving FAPbI3 crystallization by the direct intramolecular exchange of dimethylsulfoxide (DMSO) molecules intercalated in PbI2 with formamidinium iodide. This process produces FAPbI3 films with (111)-preferred crystallographic orientation, large-grained dense microstructures, and flat surfaces without residual PbI2. Using films prepared by this technique, we fabricated FAPbI3-based PSCs with maximum power conversion efficiency greater than 20%.