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.

Microring Resonators Made in Poled and Unpoled Chromophore-Containing Polymers for Optical Communication and Sensors

Abstract: Poled and unpoled chromophore-containing polymers offer some unique advantages in device functionality and fabrication. UV light and electron beam (e-beam) can bleach out the color of chromophores and reduce the index of refraction of the polymer. The photobleaching and e-beam bleaching methods form optical waveguides in a single step and do not involve solvents or wet chemicals, and can be applied to polymers that are not compatible with other waveguide fabrication techniques. A variety of microring resonator devices for fiber-optic telecommunication and sensors have been realized with chromophore-containing polymers. A novel broadband fiber-optic electric field sensor is presented as an example. The sensor uses a polymer with chromophores preferentially aligned after electric poling, and the microring resonator is directly coupled to the core of optical fiber. The feasibility of vertical integration of a poled electrooptic polymer waveguide device interfaced with silicon microelectronic circuits is also demonstrated.

Manipulation of optical field distribution in ITO-free micro-cavity polymer tandem solar cells via the out-of-cell capping layer for high photovoltaic performance

Abstract: Series-connected tandem organic photovoltaic devices (SCTOPVs) have been shown to provide higher power conversion efficiencies (PCEs) than the single junction devices due to the improved light harvesting. To achieve the optimal device performance of SCTOPVs, balancing the photocurrents generated from the sub-cells is critical according to the Kirchhoff law. In this work, we demonstrate that the out-of-cell capping layer of an ITO-free microcavity SCTOPV plays an important role in manipulating the optical field distribution in the constituent sub-cells for achieving balanced photocurrents and optimal photovoltaic performance. Two mirror-like electrodes, a semi-transparent ultrathin Ag capped with a dielectric TeO2 layer and a thick Ag electrode were used to construct an ITO-free top-illuminated microcavity configuration, in which certain frequencies of solar irradiance can resonate between the reflective surfaces. As a result, a top-illuminated ITO-free SCTOPV with a comparable performance (7.4%) to the ITO-based counterpart (7.5%) was demonstrated despite the inferior transmittance of the ultra-thin Ag relative to ITO.

Highly efficient and thermally stable organic/polymeric electro-optic materials by dendritic approach

Abstract: A series of dendron-modified nonlinear optical (NLO) chromophores and multiple chromophore-containing crosslinkable NLO dendrimers have been developed. The enhancement of poling efficiency (40%) in the dendritic NLO chromophore/polymer guest/host system was obtained due to the significant minimization of intermolecular electrostatic interactions among chromophores by the dendritic effect. Multiple NLO chromophore building blocks can be further placed into a dendrimer to construct precise molecular architecture with predetermined chemical composition. The site-isolation effect, through the encapsulation of NLO moieties by dendrons, can greatly enhance the performance of electro-optic (E-O) materials. A very large E-O coefficient (r33=60 pm/V at 1.55 micrometers ) and high temporal stability (85 degree(s)C for more than 1000 h) were achieved in a NLO dendrimer developed through the double-end functionalization of a 3D shape phenyl-tetracyanobutadienyl (Ph-TCBD)- containing NLO chromophore with thermally crosslinkable trifluorovinylether-containing dendrons.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Surface plasmon enhanced fluorescence of cationic conjugated polymer on periodic nanoarrays.

Abstract: The fluorescence from conjugated polymer assembled onto lithographically fabricated gold nanoarrays using genetically engineered peptides as molecular linkers is studied. A 16-fold increase in the photoluminescence of the conjugated polymer is observed when assembled on the optimized nanostructures due to surface plasmon enhanced fluorescence. This is achieved using a water-soluble cationic conjugated polymer, poly[(9,9-bis(6′-((N,N,N-trimethylammonium)hexyl)-2,7-fluorene)-co-4,7-di-2-thienyl-2,1,3-benzothiadiazole] dibromide (PFDBT-N+), systematically tuning the vertical distance of PFDBT-N+ from the gold nanopillar surface using solid-specific peptide linkers and horizontally optimizing the localized surface plasmon resonance by varying the geometric arrangements of the patterned metal nanoarrays. The diameter and tip-to-tip spacing of the nanopillars along with vertically tuning the distance of PFDBT-N+ from the nanopillar affected the observed fluorescence enhancements. The collective optical properti...

Solvent-vapor annealing-induced growth, alignment, and patterning of π-conjugated supramolecular nanowires

Abstract: The self-assembling properties of two rationally designed discotic π-conjugated hexaazatrinaphthylene (HATNA) molecules have been studied. In appropriate solvent systems, both ester-dodecyl-substituted and amide-dodecyl-substituted HATNAs self-assembled into nanowires and formed organogels. These nanowires could be easily transferred onto solid supports through spin casting for morphological study. In addition to the solution-based self-assembly method, solvent-vapor annealing (SVA) was explored as an alternative way to control the organization of supramolecular nanowires on surfaces. It was found that amorphous thin film of HATNA molecules transformed gradually into nanowire structures through a nucleation and growth mechanism during the SVA process. Several parameters including the preordering of molecules in the original thin film, choice of solvent vapors, annealing times, and surface properties were tuned to create different supramolecular organizations. Under particular conditions, aligned nanowires with preferential direction can be achieved.

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%.