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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.


Papers
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Journal ArticleDOI
TL;DR: In this article, a tetraene structure with a strong dialkylaminophenyl donor and a CF3-TCF acceptor is used as the primary chromophore due to its large hyperpolarizability, and a short cyanoacetate dye with a smaller dipole moment and a very different molecular aspect ratio for dipole engineering.
Abstract: A new design of tethered binary chromophores has been studied for nonlinear optics. In this work, a push–pull tetraene structure with a strong dialkylaminophenyl donor and a CF3-TCF acceptor is used as the primary chromophore due to its large hyperpolarizability, and a short cyanoacetate dye with a smaller dipole moment and a very different molecular aspect ratio is used as a secondary dipolar structure for dipole engineering. We found that such binary chromophore systems (exemplified as chromophore C1) exhibited significantly improved poling efficiency and thermal stability in poled films of guest–host polymers and monolithic glass. A systematic study of materials' physical properties, including analyses of poling-induced order thermally stimulated depolarization and comparison with simple dipolar polyenes, correlates the improved EO performance of poled films containing C1 well with its tethered binary structure. It provides an effective electrostatic screening mechanism for excellent solution processibility of materials, and a cooperative enhancement for higher polar order of poled thin films under the force of the applied poling field. An ultrahigh r33 value of 273 pm V−1 and a high refractive index of 2.12 at the wavelength of 1300 nm have been achieved for monolithic glass of C1, which represents a record-high n3r33 figure-of-merit of 2601 pm V−1 with good temporal stability. This exceptional result is a great demonstration of the advantages offered by dynamically assisted dipolar polarization enhancement of tethered binary chromophores, for significantly improving the poling efficiency and thermal stability of organic EO materials for efficient optical modulation.

46 citations

Journal ArticleDOI
TL;DR: In this article, an electrooptic (EO) electric field (E-field) sensor based on side-polished fiber coupled with an EO polymer microring resonator is proposed and demonstrated.
Abstract: A novel electrooptic (EO) electric field (E-field) sensor based on side-polished fiber coupled with an EO polymer microring resonator is proposed and demonstrated. An EO polymer waveguide with a ring shape is fabricated on the polished flat of an optical fiber. Light in the fiber evanescently couples into the resonator and forms resonant modes for certain wavelengths and produces notches in the output intensity of the fiber. External electric fields change the index of refraction of the ring waveguide and therefore dither its resonant wavelengths. For light of wavelength on the slope of a resonance notch, a change in the output intensity can be detected. The sensor is all dielectric without metal layers to distort the measured E-field. The resonant structure allows the sensor to potentially have much higher sensitivity than other electrooptic sensors based on Mach-Zehnder or polarization modulation. Since electrooptic polymers have higher electrooptic coefficients, lower dielectric constants and faster electrooptic responses than inorganic crystals, higher sensitivity, lower invasiveness, and higher bandwidth of E-field sensing can be expected. This sensor eliminates unreliable fiber-to-waveguide butt coupling as well as the high propagation loss encountered in the long straight EO polymer waveguides of sensors based on Mach-Zehnder structures. By using the fiber itself as the supporting substrate of the ring waveguide, the sensor can have small size and low disturbance to the measured electric field. The concept is demonstrated using AJLS103 EO polymer. A sensitivity of 100 mV/m has been achieved at frequencies up to 550 MHz (limited by the measurement system)

46 citations

Journal ArticleDOI
TL;DR: In this paper, two donor-π linker-donor (D-π-D) HTMs, N01 and N02, were reported, which consist of facilely synthesized 4,8-di(n-hexyloxy)-benzo[1,2-b:4,5-b]-dithiophene as a π linker, with 10-bromohexyl-10H-phenoxazine and 10-hexylon-10 H-phenoxide as donors, respectively.
Abstract: Delicately designed dopant-free hole-transporting materials (HTMs) with ordered structure have become one of the major strategies to achieve high-performance perovskite solar cells (PSCs). In this work, we report two donor-π linker-donor (D-π-D) HTMs, N01 and N02, which consist of facilely synthesized 4,8-di(n-hexyloxy)-benzo[1,2-b:4,5-b']dithiophene as a π linker, with 10-bromohexyl-10H-phenoxazine and 10-hexyl-10H-phenoxazine as donors, respectively. The N01 molecules form a two-dimensional conjugated network governed by C-H⋅⋅⋅O and C-H⋅⋅⋅Br interaction between phenoxazine donors, and synchronously construct a three-dimension lamellar structure with the aid of interlaminar π-π interaction. Consequently, N01 as a dopant-free small-molecule HTM exhibits a higher intrinsic hole mobility and more favorable interfacial properties for hole transport, hole extraction and perovskite growth, enabling an inverted PSC to achieve a very impressive power conversion efficiency of 21.85 %.

46 citations

Journal ArticleDOI
TL;DR: In this paper, a selenium-substituted small-molecule acceptor (NFAs) was proposed to increase the short-circuit current density of organic photovoltaics.
Abstract: ConspectusOrganic photovoltaics (OPVs) with a photoactive layer containing a blend of organic donor and acceptor species are considered to be a promising technology for clean energy owing to their unique flexible form factor and good solution processability that can potentially address the scalability challenges. The delicate designs of both donors and acceptors have significantly enhanced the power conversion efficiency of OPVs to more than 18%. Nonfullerene small-molecule acceptors (NFAs) have played a critical role in enhancing the short-circuit current density (JSC) by efficiently harvesting near-infrared (NIR) sunlight. To take full advantage of the abundant NIR photons, the optical band gap of NFAs should be further reduced to improve the performance of OPVs. Incorporating highly polarizable selenium atoms onto the backbone of organic conjugated materials has been proven to be an effective way to decrease their optical band gap. For example, a selenium-substituted NFA recently developed by our group has attained a JSC of approximate 27.5 mA cm-2 in OPV devices, surpassing those of most emerging photovoltaic systems. Inspired by this advance, we concentrate on the topic of selenium-containing materials in this Account to incite readers' interest in further exploring this series of materials.In this Account, we first compare the differences among chalcogen heterocycles and discuss the influence of fundamental electronic behavior on the collective photoelectrical properties of the resulting materials. The superior features of selenium-substituted materials are summarized as follows: (1) The large covalent radius of selenium can diminish the π-orbital overlap, rendering enhanced quinoidal resonance character and a narrowed optical band gap of resulting materials. (2) The selenium atom is more polarizable than sulfur owing to its larger and looser outermost electron cloud, enabling enhanced intermolecular Se-Se interaction and increased charge carrier mobility of relevant materials in the solid state. We then focus on summarizing the design rules for various categories of selenium-containing materials including polymer donors, small-molecule acceptors, and polymer acceptors, especially those composed of ladder-type polycyclic units. The motivation for incorporating selenium atoms into these materials and the structure-property relationships were thoroughly elucidated. Specifically, we discuss the changes in the optical band gap, charge carrier mobility, and molecular packing induced by selenium substitution and correlate the effects of these changes with the exciton behaviors, energy loss, and nanoscale film morphology of corresponding OPV devices. Furthermore, we point out the intrinsic stability of selenium-containing materials under maximum-power-point tracking and long-term photo- or thermostress and indicate their potential use in semitransparent and tandem solar cells. At the end, the prospect of future research focuses and the possible applications of selenium-containing materials in the OPV field are discussed.

46 citations

Journal ArticleDOI
TL;DR: In this paper, it is shown that methylthio-terminated self-assembled monolayers (SAMs) can induce doping effects in graphene transistors and these values are directly correlated to changes in performance seen in graphene-transistor transistors.
Abstract: Recent reports have shown that self-assembled monolayers (SAMs) can induce doping effects in graphene transistors. However, a lack of understanding persists surrounding the quantitative relationship between SAM molecular design and its effects on graphene. In order to facilitate the fabrication of next-generation graphene-based devices it is important to reliably and predictably control the properties of graphene without negatively impacting its intrinsic high performance. In this study, SAMs with varying dipole magnitudes/directions are utilized and these values are directly correlated to changes in performance seen in graphene transistors. It is found that, by knowing the z-component of the SAM dipole, one can reliably predict the shift in graphene charge neutrality point after taking into account the influence of the metal electrodes (which also play a role in doping graphene). This relationship is verified through density functional theory and comprehensive device studies utilizing atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and electrical characterization of graphene transistors. It is shown that properties of graphene transistors can be predictably controlled with SAMs when considering the total doping environment. Additionally, it is found that methylthio-terminated SAMs strongly interact with graphene allowing for a cleaner graphene transfer and enhanced charge mobility.

46 citations


Cited by
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Journal ArticleDOI
18 Oct 2013-Science
TL;DR: In this article, transient absorption and photoluminescence-quenching measurements were performed to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide and triiodide perovskite absorbers.
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.

8,199 citations

Journal Article
TL;DR: In this paper, transient absorption and photoluminescence-quenching measurements were performed to determine the electron-hole diffusion lengths, diffusion constants, and lifetimes in mixed halide and triiodide perovskite absorbers.
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.

6,454 citations

Journal ArticleDOI
01 Aug 2014-Science
TL;DR: Perovskite films received a boost in photovoltaic efficiency through controlled formation of charge-generating films and improved current transfer to the electrodes and low-temperature processing steps allowed the use of materials that draw current out of the perovskites layer more efficiently.
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.

5,789 citations

Journal ArticleDOI
TL;DR: This paper presents a meta-analysis of the chiral stationary phase transition of Na6(CO3)(SO4)2, a major component of the response of the immune system to Na2CO3.
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

5,658 citations

Journal ArticleDOI
12 Jun 2015-Science
TL;DR: An approach for depositing high-quality FAPbI3 films, involving FAP bI3 crystallization by the direct intramolecular exchange of dimethylsulfoxide (DMSO) molecules intercalated in PbI2 with formamidinium iodide is reported.
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%.

5,458 citations