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.

Ultra-compact silicon nanophotonic modulator based on electro-optic polymer infiltrated slot photonic crystal waveguide

Abstract: We experimentally demonstrate a Mach-Zehnder modulator based on electro-optic (EO) polymer (AJ-CKL1/PMMA) infiltrated photonic crystal slot waveguide. The modulator design combines the advantage of excellent optical confinement in silicon slot waveguide, slow light enhancemen t in photonic crystal waveguide, and strong electro-optical response in EO polymer. This design allows us to achieve electro-optic modulation in active region only 352 m long. Matching the mode profile and group velocity between strip and photonic crystal slot waveguide enables coupling into slow light regime. The modulator shows a 22V switching voltage and an improved in-device EO coefficient of 51pm/V as compared to recen tly reported value. Keywords: silicon photonics, photonic crysta ls, slot waveguide, electro-optic pol ymer, and electro-optic modulator

Demonstration of Polymer-based Directional Coupler Modulator with High Linearity

Abstract: A linearized polymer-based directional coupler modulator is presented. The linearity was obtained by tailoring the coupling coefficient using photobleaching. A two-tone test of the device demonstrated an enhancement in intermodulation distortion compared to a Mach-Zehnder modulator.

Dual Sub‐Cells Modification Enables High‐Efficiency n–i–p Type Monolithic Perovskite/Organic Tandem Solar Cells

Abstract: Monolithic perovskite/organic tandem solar cells (POTSCs) have attracted increasing attention owing to ability to overcome the Shockley–Queisser limit. However, compromised sub‐cells performance limits the tandem device performance, and the power conversion efficiency (PCE) of POTSCs is still lower than their single‐junction counterparts. Therefore, optimized sub‐cells with minimal energy loss are desired for producing high‐efficiency POTSCs. In this study, an ionic liquid, methylammonium acetate (MAAc), is used to modify wide‐bandgap perovskite sub‐cells (WPSCs), and bathocuproine (BCP) is used to modify small‐bandgap organic solar cells. The Ac− group of MAAc can effectively heal the Pb defects in the all‐inorganic perovskite film, which enables a high PCE of 17.16% and an open‐circuit voltage (Voc) of 1.31 V for CsPbI2.2Br0.8‐based WPSCs. Meanwhile, the BCP film, inserted at the ZnO/organic bulk‐heterojunction (BHJ) interface, acts as a space layer to prevent direct contact between ZnO and the BHJ while passivating the surface defects of ZnO, thereby mitigating ZnO defect‐induced efficiency loss. As a result, PM6:CH1007‐based SOSCs exhibit a PCE of 15.46%. Integrating these modified sub‐cells enable the fabrication of monolithic n–i–p structured POTSCs with a maximum PCE of 22.43% (21.42% certified), which is one of the highest efficiencies in such type of POTSCs.

New developments in thermally stable second order nonlinear optical chromophores and electro-optic polymers

Abstract: A new class of thermally and chemically stable donor-acceptor substituted thiophenes is presented. It is shown that exceptionally large electro-optic activity (as high as 30 pm/V at 1.3 micrometers ) can be achieved by doping/covalently bonding highly efficient trycyanovinylthiophenes into high temperature polymers such as polymidies and polyquinolines. Monitoring the long-term stability of electro-optic activity at temperatures above 100 degrees C has revealed that the covalent bonding approach is quite superior to the guest-host approach in polymide as well as polyquinoline based materials.

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%.