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.

Hybrid silicon-organic racetrack resonator designs for electro-optical modulation

Abstract: Racetrack resonators based on the silicon-on-insulator platform are proposed for electro-optical modulation. The resonators are functionalized by a cladding of a second order nonlinear optical polymer. Two different concepts for the racetrack design employing different waveguide geometries for quasi-TE and quasi-TM polarization operation are presented. In both resonator designs the electrical contact is established by fully etched segmented electrode sections to allow for an easy fabrication process. For quasi-TM polarization the width of the strip waveguide is optimized to 400 nm. The Q factor of 2000 is measured for a sample with segmented electrode. A loss of 0.4 dB per segmented waveguide is deducted. For the quasi-TE polarization the slot waveguide geometry is optimized to 470 nm total width including a vertical slot of 90 nm width. Only the straight parts of the racetrack are slotted, while the bends are built from strip waveguides. To convert the mode from strip to slot geometry stub like couplers of 100 nm length are employed. The measured Q factor is 550. The in device Pockels coefficient is measured to r 33 = 1 pm/V. This small value indicates a very low poling induced polar order which needs to be improved. This is a topic of current investigation.

Ultra-broadband Mach-Zehnder hybrid electro-optic polymer/sol-gel silica waveguide modulators

Abstract: We demonstrate ultra-broadband Mach-Zehnder (MZ) electro-optic polymer/sol-gel silica waveguide modulators. A bandwidth of an electrical transmission S 21 is measured at a modulation frequency of up to 50 GHz (upper limit of our electrical equipments), which extrapolates the 6 dB bandwidth of 130 GHz base on an experimentally obtained device parameters. The electrical transmission in dB scale is linearly dropped with respect to a modulating frequency with extremely small deviation dip of less than 0.5 dB. Large signal measurements at a transmission rate of 56 Gbit/s (upper limit of our electrical equipments) show clearly opened eye-diagram. The half-wave voltage and electrode length product is 4.15 Vcm (a single-driving) for an electrode distance of 13.3 μm, which corresponds to 2.08 Vcm for a dual-driving. An in-device electro-optic coefficient r 33 is 140 pm/V at a wavelength of 1.55 μm for the ultra-broadband MZ modulators.

Feasibility study of integration of electro-optic polymer waveguide device with MOSFET circuitry on silicon

Abstract: Systematic development of electro-optic (EO) polymers is leading to optical and material properties such that they present an increasingly viable alternative to crystalline-based technologies for integrated optics. EO polymers demonstrate an inherent velocity match between radio-frequency and optical waves, making them excellent candidates for applications in high-speed telecommunication switching and optical interconnects for VLSI circuitry. In addition, EO polymer devices are relatively simple to fabricate at conditions compatible with microelectronics industry processes, making same-substrate integration of optical and electronic circuitry possible. In this paper, we describe two vertical integration schemes whereby a polymer-based electro-optic modulator may be controlled by metal-oxide semiconductor field effect transistor (MOSFET) circuitry. One scheme described is an insitu integration on the same silicon (Si) substrate. The second scheme is the integration of a modulator built on a flexible substrate with a MOSFET circuit on a second Si substrate. Both schemes have potential applications for integrated electro-optics.

Efficient Two-Photon Absorbing Chromophores with Fine-Tuned л-Bridges

Abstract: A new series of symmetric two-photon absorbing chromophores with various л-bridges were synthesized. It has been shown that the spectra of two-photon absorption and two-photon induced fluorescence can be fined tuned by structural variations in the electronic nature and twist angle of л-bridges of D-л-D type chromophores. These chromophores exhibited large twophoton absorption cross-sections (up to 1800 GM), which were determined by two-photon induced fluorescence technique using femtosecond pulse excitation. The colors of two-photon pumped emission of these chromophores varied from blue to red (433 ∼ 604 nm) as the structure of л-bridges changed.

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%.