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.

A novel approach to achieve higher order using pseudo-discotic chromophores in electro-optic materials and devices

Abstract: Organic materials have great potential for electro-optic devices. Achieving higher unidirectional ordering of chromophores is the main challenge in using organic materials in such devices. The prime reason for this decreased order is due to interchromophore interactions which lead to a decrease in the electro-optic coefficient, r33. A material consisting of a chromophore surrounded by dendrons and possessing an appropriate aspect ratio should facilitate the near Ferro-electric ordering as observed in the discotic liquid crystals. For that purpose, a prototype dendrimeric chromophore has been synthesized and will be used in Mach-Zehnder devices to achieve lower drive voltages.

Low-voltage organic electro-optic modulators using transparent conducting oxides as electrodes

Abstract: In this paper, we present a novel device structure for organic electro-optic modulators using transparent conducting oxides (TCOs) as electrodes to substantially reduce the switching voltage, and describe their fabrication. We report two different types of device geometry, a top conducting and a side conducting geometry, and discuss their strengths and weaknesses. We discuss how the voltage and speed performance of such modulators are dependant on the conductivity/optical loss ratio of the TCO electrodes. Our device simulation shows that by appropriately engineering the high TCO conductivity/optical loss ratio, 4-6x lower switching voltage can be achieved while still maintaining high modulation frequencies and low optical loss. We show that certain new TCO materials are capable of achieving the high conductivity/optical loss required for efficient modulation in the 1300-1550 nm wavelength range. We summarize the optical loss characteristics at 1300 nm of different types of thin-film TCO materials grown using different deposition techniques. TCO electrodes based on different types of materials, such as In 2 O 3 , ZnO, and ITO have been investigated for our device structures. Fabrication issues associated with the deposition of TCO electrodes directly on organic EO materials and our approach to addressing them are discussed. Initial results for organic EO modulators fabricated with TCOs as electrodes are presented, and the performance of these modulators are compared with theoretical modeling results. The new device structures presented here will enable next generation low-voltage organic EO modulators targeting RF photonics applications.

Cryogenic optical characterization of nonlinear polymers

Abstract: We report a second-order electro-optic coefficient r 33 of 155±5 pm/V at a telecommunication wavelength of 1550 nm from nonlinear poled polymer thin films at a cryogenic temperature of ∼4.2 Kelvin using a reflection ellipsometry technique.

Alignment-free Fabrication of a Hybrid Electro-Optic Polymer Modulator Platform

Abstract: A hybrid platform for electro-optic (EO) polymer modulators was realized on glass substrates with a simplified fabrication technique. The coplanar configuration device has 4.5 dB insertion loss with 7.5 μm electrode spacing.

Optimization of second-order nonlinear optical properties of push-pull conjugated chromophores using heteroaromatics

Abstract: In continuation of our studies on donor-acceptor substituted heteroaromatic nonlinear optical chromophores, we have developed a new series of thiophene derivatives containing dialkylamino electron-donor and tricyanovinyl electron-acceptor substituents. EFISH measurements indicate that the combination of tricyanovinyl acceptors and thiophene conjugating moieties yields dramatically enhanced molecular nonlinear properties [(beta) (mu) (at (lambda) equals 1.907 micrometers ) equals 6000 - 9100 X 10-48 esu].

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%.