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.

Doping of Fullerenes via Anion‐Induced Electron Transfer and Its Implication for Surfactant Facilitated High Performance Polymer Solar Cells

Abstract: Simple and solution-processible tetrabutyl-ammonium salts (TBAX) can dope fullerene and its derivatives to achieve conductive thin films (σ as high as 0.56 S/m). The electron transfer between the anions of TBAXs and n-type semiconductors induces doping without encountering any harsh activation. These provide valid support for the surfactant interfacial doping of fullerene in polymer solar cells for enhanced device performance.

Nonlinear polymer-clad silicon slot waveguide modulator with a half wave voltage of 0.25 V

Abstract: We report on an electro-optic modulator fabricated from a silicon slot waveguide and clad in a nonlinear polymer. In this geometry, the electrodes form parts of the waveguide, and the modulator driving voltage drops across a 120nm slot. As a result, a half wave voltage of 0.25V is achieved near 1550nm. This is one of the lowest values for any modulator obtained to date. As the nonlinear polymers are extremely resistive, our device also has the advantage of drawing almost no current, suggesting this type of modulator could operate at exceedingly low power.

Highly Efficient and Thermally Stable Nonlinear Optical Dendrimer for Electrooptics

Abstract: For the fabrication of practical E -O devices, critical material requirements, such as large E -O coefficients, high stability (thermal, chemical, photochemical, and mechanical), and low optical loss, need to be simultaneously optimized. 1 In the past decade, a large number of highly active nonlinear optical (NLO) chromophores have been synthesized, and some of these exhibit very large macroscopic optical nonlinearities in high electric field poled guest/host polymers. 2 To maintain a stable dipole alignment, it is a common practice to utilize either high glass-transition temperature ( Tg) polymers with NLO chromophores as side chains or cross-linkable polymers with NLO chromophores that could be locked in the polymer network. 3 However, it is difficult to achieve both large macroscopic nonlinearities and good dipole alignment stability in the same system. This is due to strong intermolecular electrostatic interactions among high dipole moment chromophores and high-temperature aromatic-containing polymers, such as polyimides and polyquinolines that tend to form aggregates. The large void-containing dendritic structures 4-8 may provide an attractive solution to this critical issue because the dendrons can effectively decrease the interactions among chromophores due to the steric effect. Furthermore, these materials are monodisperse, well-defined, and easily purifiable compared to polymers that are made by the conventional synthetic approaches. In this paper, we report the synthesis and characterization of a cross-linkable NLO dendrimer 3 exhibiting very large optical nonlinearity and excellent thermal stability. This NLO dendrimer was constructed through a double-end functionalization of a 3-D shape phenyl-tetracyanobutadienyl (Ph-TCBD) thiophene -stilbene-based NLO chromophore 9 as the center core and the crosslinkable trifluorovinyl ether-containing dendrons 10,11as the exterior moieties (Scheme 1). Spatial isolation from the dendrimer shell decreases chromophore -chromophore electrostatic interactions, and thus enhances macroscopic optical nonlinearity because electrostatic interactions among chromophores play a critical role in defining the maximum macroscopic optical nonlinearity that can be achieved for a given chromophore. 12 In addition, the NLO dendrimer can be directly spin-coated without the usual prepolymerization process needed to build up viscosity, since it already possesses a fairly high molecular weight (4664 Da). The chromophore loading density of the dendrimer is 33 w/w %, which is confirmed by elemental analysis. There are also several other advantages derived from this approach, such as excellent alignment stability and mechanical properties, which are obtained through the sequential hardening/cross-linking reactions during the high-temperature electric-field poling process. Very large E -O coefficient (r33 ) 60 pm/V at 1.55μm), and long-term alignment stability (retaining>90% of its originalr33 value at 85°C for more than 1000 h) were achieved for the poled dendrimer. The dendrimer3 was synthesized by the Mitsunobu condensation between the carboxyl groups on the three branches of the desirable core molecule2 and the hydroxy-containing chromophore precursor 1 that has cross-linkable trifluorovinyl ether on the dendrons. Then, the intermediate was reacted with tetracyanoethylene (TCNE) to activate the Ph-TCBD electron acceptor (Scheme 1). The purity and structure of the dendrimer 3 were fully characterized by gel permeation chromatography † Department of Materials Science and Engineering. ‡ Department of Chemistry. (1) (a) Robinson, B. H.; Dalton, L. R.; Harper, A. W.; Ren, A.; Wang, F.; Zhang, C.; Todorova, G.; Lee, M.; Aniszfeld, R.; Garner, S.; Chen, A.; Steier, W. H.; Houbrecht, S.; Persoons, A.; Ledoux, I.; Zyss, J.; Jen, A. K -Y. Chem. Phys. 1999, 245, 35. (b) Marder, S. R.; Kippelen, B.; Jen, A. K -Y.; Peyghambarian, N. Nature1997, 388, 845. (c) Marks, T. J.; Ratner, M. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 155. (2) (a) Ahlheim, M.; Barzoukas, M.; Bedworth, P. V.; Hu, J. Y.; Marder, S. R.; Perry, J. W.; Stahelin, C. M.; Zysset, B. Science1996, 271, 335. (b) Cai, Y. M.; Jen, A. K-Y. Appl. Phys. Lett. 1995, 117, 7295. (c) Shi, Y. Q.; Zhang, C.; Zhang, H.; Bechtel, J. H.; Dalton, L. R.; Robinson, B. H.; Steier, W. H. Science2000, 288, 119. (3) (a) Verbiest, T.; Burland, D. M.; Jurich, M. C.; Lee, V. Y.; Miller, R. D.; Volksen, W.Science1995, 268, 1604. (b) Saadeh, H.; Wang, L. M.; Yu, L. P. J. Am. Chem. Soc. 2000, 122, 546. (c) Chen, T.-A.; Jen, A. K -Y.; Cai, Y. M. J. Am. Chem. Soc. 1995, 117, 7295. (d) Ma, H.; Wang, X. J.; Wu, X. M.; Liu, S.; Jen, A. K.-Y.Macromolecules1998, 31, 4049. (e) Ma, H.; Wu, J. Y.; Herguth, P.; Chen, B. Q.; Jen, A. K.-Y. Chem. Mater.2000, 12, 1187. (4) Newkome, G. R.; Moorefield, C. N.; Vo ̈gtle, F.Dendritic Molecules, Concepts, Syntheses, Perspecti Ves; VCH: Cambridge, 1996. (5) (a) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Re V. 1999, 99, 1665. (b) Fischer, M.; Vo ̈gtle, F.;Angew. Chem., Int. Ed. 1999, 38, 885. (6) (a) Miller, L. L.; Duan, R. G.; Tully, D. C.; Tomalia, D. A. J. Am. Chem. Soc.1997, 119, 1005. (b) Newkome, G. R.; Narayanna, V. V.; Echegoyan, L.; Perez-Cordero, E.; Luftmann, H. Macromolecules1997, 30, 5187. (7) (a) Wang, P. W.; Liu, Y. J.; Devadoss, C.; Bharathi, P.; Moore, J. S. AdV. Mater. 1996, 8, 237. (b) Adronov, A.; Gilat, S. L.; Fre ́chet, J. M. J.; Ohta, Kaoru, K.; Neuwahl, F. V. R.; Fleming, G. R. J. Am. Chem. Soc. 2000, 122, 1175. (8) (a) Zimmerman, S. C.; Wang, Y.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1998, 120, 2172. (b) Zeng, F. W.; Zimmerman, S. C. hem. Re V. 1997, 97, 1681. (9) Wu, X. M.; Wu, J. Y.; Liu, Y. Q.; Jen, A. K.-Y.J. Am. Chem. Soc. 1999, 121, 472. (10) Hawker, C. J.; Fre ́chet, J. M. J.J. Am. Chem. Soc. 1990, 112, 7638. (11) (a) Shah, H.; Hoeglund, A.; Radler, M.; Langhoff, C.; Smith, D. W., Jr. Polym. Prepr.1999, 40(2), 1293. (b) Smith, D. W., Jr.; Boone, H. W.; Traiphol, R.; Shah, H.; Perahia, D. Macromolecules2000, 33, 1126. (12) Harper, A. W.; Sun, S.; Dalton, L. R.; Garner, S. M.; Chen, A.; Kalluri, S.; Steier, W. H.; Robinson, B. H. J. Opt. Soc. Am. B. 1998, 15, 329. Figure 1. Temporal stability of the poled/cross-linked NLO dendrimer and guest/host polymer system at 85 °C in nitrogen. Normalizedr33 as a function of baking time.

Surface Doping of Conjugated Polymers by Graphene Oxide and Its Application for Organic Electronic Devices

Abstract: Conjugated polymers are a novel class of solution-processable semiconducting materials with intriguing optoelectronic properties. [ 1 ] They have received great attention as active components in organic electronic devices such as organic photovoltaic cells (OPVs), organic light-emitting diodes (OLEDs), and organic fi eld-effect transistors (OFETs) due to their light weight, facile tuning of electronic properties through molecular engineering, and ease of processing. The performance and lifetime of conjugated polymer-based electronic devices are critically dependent on the bulk properties of the active materials and the interfacial properties of electrode/polymer contacts. [ 2–4 ] In these devices, the electrode(s) either inject charge into or extract charges from the organic semiconductor layer(s). Mismatch of the work functions between metal or metal oxide electrodes and molecular orbital energy levels of organic semiconductors can lead to high contact resistance, which decreases the charge injection and extraction effi ciency. Therefore, it is essential to minimize contact resistance at the electrode/organic semiconductor interface. To improve charge injection/extraction across the electrode/ organic semiconductor interface, several strategies have been developed. One is to tune the interfacial dipole across the electrode/semiconductor interface to reduce the injection/collection energy barrier. This can be achieved by modifying the electrode surface with self-assembled dipolar molecules to tune the energy level alignment at the semiconductor/electrode interface. [ 5–7 ] Alternatively, the introduction of a thin layer of polymer surfactant that contains polar side chains between the conjugate polymer/electrode interface can also be used to improve the interfacial properties. The polar side chains can provide not

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%.