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.

Optimization of organic NLO materials for integration with silicon photonic, plasmonic (metal optics), and metamaterial devices

Abstract: Specific spatially-anisotropic interactions are identified that enhance noncentrosymmetric order required for electro-optic activity. Enhancement of electric-field-poling-induced noncentrosymmetric order by these specific interactions is shown to result from a reduction of lattice dimensionality from three to two dimensions. New analytical techniques for measurement of centrosymmetric and noncentrosymmetric order and lattice dimensionality are introduced. Measurement of order parameters is correlated with viscoelastic data to gain further insight into the influence of specific interactions on poling efficiency and thus material electro-optic activity. The integration of organic electro-optic materials into silicon photonic, plasmonic, and metamaterial devices is also discussed. These device structures can affect the "effective" optical nonlinearity of organic materials but care must be exercised to control optical loss.

Electro-optic polymer prism beam deflector

Abstract: In this paper, the design and fabrication of a prism shape optical beam deflector utilizing a thermoplastic electro-optic polymer are presented. Experimental results of deflection angles versus various applied electric fields are investigated.

Third-order nonlinearity contribution to electro-optic activity in polymer materials in a constant bias field

Abstract: A method is described to separately determine the contributions from the third-order nonlinearity and from the linear electro-optic (EO) effect in EO polymer system under dc biased poling conditions. An experiment was carried out using a dc biased, controlled temperature technique in a waveguide Mach-Zehnder modulator. The drive voltages of the EO device were measured at various bias voltages and temperatures. Based on these data, it is found that the modulation of the device is predominantly due to the linear EO effect and the contribution from the third-order effect was not significant when the chromophores are effectively aligned in strong bias fields and at temperatures close to the glass transition temperature of EO polymer.

Second-Order Nonlinear Optical Properties Of Thiophene Containing Chromophores with Extended Conjugation

Abstract: The synthesis and first hyperpolarizabilities (β) of various donor-acceptor substituted thiophene containing compounds with extended conjugation are reported. Results indicate that replacing phenyl rings with less aromatic thiophene moieties enhances the second-order hyperpolarizability. Incorporating the acceptor group, N,N’ diethylthiobarbituric acid, that can gain aromaticity upon charge-separation has also led to an increase in the nonlinearity. Some of the molecules have been incorporated into poIy(methyl methacrylate) and the electro-optic coefficients of these host-guest polymers were measured.

Development of an Electro-Optic Scanner for Potential Endoscope Application

Abstract: Technological advancements in endoscopy design are in current development due to the increased demand for minimally invasive medical procedures. One such advancement is reducing the overall size of the endoscope system while maintaining the resolution and field-of-view (FOV). Reduction of size results in less tissue damage and trauma during operation as well as faster recovery times for patients. Additionally, areas that are inaccessible by today's endoscope designs will be possible to examine. Current endoscopes use either a bundle of optical fibers (optical waveguides) and/or one or more cameras having an array of detectors to capture an image. Thus, the diameter of these devices employed for remote imaging cannot be reduced to smaller than the image size. Even if one ignores additional optical fibers used for illumination of a region of interest, the scope diameter is therefore limited by the individual pixel size of a camera or by the diameter of optical fibers used to acquire the image. Therefore, it is apparent to achieve scopes with less than 3 mm overall diameter using current technologies, resolution and/or FOV must be sacrificed by having fewer pixel elements. All commercially available scopes suffer from this fundamental tradeoff between high image quality and small size. More recently, our research has been working on developing a 2-D electro-optic scanner potentially be implemented for clinical endoscopic imaging application. The proposed optical device has several unique advantages. Electro-optical scanning offers a sensitive, facile, accurate, and superb quality method to capture images of physical and biological tissues. In addition, the minute physical size of the imaging system has a much needed advantage over conventional imaging systems. The proposed design is based on the fact that the propagation direction of a light beam can be changed when the index of refraction of an electro-optic medium is altered by the application of an external electric field. The basic design of the system consists of a thin film electro-optic polymer waveguide with built-in cascaded prisms structure for horizontal beam deflection and an electro-optic grating structure for vertical beam deflection. The cascaded prisms are combined with the electro-optic polymer to create a voltage-controlled horizontal beam deflection. A grating coupler, a structure that is commonly used as light coupling device for dielectric waveguide, is combined with the EO polymer to create the vertical controlled beam deflection. A collimated light beam coupled into the waveguide by a mechanical coupler via an optical fiber cascaded down these two deflection stages. When the beam exits, the emitted light beam is displaced along two orthogonal directions in a raster pattern. A photodetector array integrated in the same substrate captured the reflected intensity. The scanned imaged is then analyzed and reconstruct based on the received signal.

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%.