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.

Organic Thin‐Film Transistors: Simultaneous Modification of Bottom‐Contact Electrode and Dielectric Surfaces for Organic Thin‐Film Transistors Through Single‐Component Spin‐Cast Monolayers (Adv. Funct. Mater. 8/2011)

Abstract: An efficient process is developed by spin-coating a single-component, self-assembled monolayer (SAM) to simultaneously modify the bottom-contact electrode and dielectric surfaces of organic thin-film transistors (OTFTs). This effi cient interface modifi cation is achieved using n-alkyl phosphonic acid based SAMs to prime silver bottom-contacts and hafnium oxide (HfO2) dielectrics in low-voltage OTFTs. Surface characterization using near edge X-ray absorption fi ne structure (NEXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, atomic force microscopy (AFM), and spectroscopic ellipsometry suggest this process yields structurally well-defi ned phosphonate SAMs on both metal and oxide surfaces. Rational selection of the alkyl length of the SAM leads to greatly enhanced performance for both n-channel (C60) and p-channel (pentacene) based OTFTs. Specifi cally, SAMs of n-octylphos-phonic acid (OPA) provide both low-contact resistance at the bottom-contact electrodes and excellent interfacial properties for compact semiconductor grain growth with high carrier mobilities. OTFTs based on OPA modifi ed silver electrode/HfO2 dielectric bottom-contact structures can be operated using < 3V with low contact resistance (down to 700 Ohm-cm), low subthreshold swing (as low as 75 mV dec−1), high on/off current ratios of 107, and charge carrier mobilities as high as 4.6 and 0.8 cm2 V−1 s−1, for C60 and pentacene, respectively. These results demonstrate that this is a simple and efficient process for improving the performance of bottom-contact OTFTs.

Interfacial-shear strength of the perfluorocyclobutane films on silicon

Abstract: The debonding behavior of perfluorocyclobutane (PFCB) films on silicon (Si) has been investigated using Vickers indentation as a function of cure temperature and film thickness. PFCB films on Si were processed by spin coating (1–4 μm) and solution casting (20–60 μm). The interfacial shear strength of solution-cast PFCB films was independent of film thickness. The interfacial shear strength increased with cure temperature. The PFCB/Si cured at 225 °C exhibited interfacial shear strength of 123 MPa, and the strength increased to 163 MPa when the cure temperature was raised to 275 °C. The increase in interfacial-shear strength with temperature has been attributed to the increase in the density of bonds between PFCB and Si due to an increase in the density of crosslinks. Spin-coated films exhibited cracking due to the penetration of the indenter into the substrate, and the extent of cracking increased with the load.

Hybrid electro optic modulators with subvolt drive voltages

Abstract: Summary form only given. Hybrid organic-solgel nonlinear materials are being investigated as high speed low voltage modulators. This paper provides a summary of our recent results in this area. Various design structures will be reviewed. We have also demonstrated near ~100% poling efficiency in several generations of hybrid electro-optic (EO) polymer/sol-gel waveguide modulators, using crosslinkable and guest-host EO polymers. In these devices, the EO polymer was poled using an electrically conductive sol-gel cladding in the hybrid modulator, such that the poling voltage was completely applied to the EO polymer layer. Stable long-term operation is preferred for industrial applications. The hybrid polymeric modulator has the additional benefits of stable and low loss optical coupling to standard optical fiber, and stable waveguiding, since the thermally and optically stable sol-gel core in the passive regions plays the primary role in optical coupling. Fine refractive index control of the EO polymer is not required in the hybrid modulator, which is another advantage for long-term stability. In the hybrid approach, whenever the EO polymer core has higher refractive index (e.g. 1.63-1.68) than the sol-gel core (1.50), coupled and waveguided light in the sol-gel core can be adiabatically transferred to the EO polymer core and back through vertical tapers in the sol-gel. We will review our recent results including demonstration of a Vpi of 0.65 V at 1550 nm.

Electro-optic properties of hybrid sol-gel doped with nonlinear chromophore

Abstract: This paper reports the electro-optic properties of hybrid silica sol-gel doped with a chromophore of high nonlinearity. High electro-optic coefficient and temporal stability have been demonstrated, showing the promise of the material for the development of high-speed electro-optic devices.

Gated lateral charge transport in self-assembled 1-pyrylphosphonic acid molecular multilayers

Abstract: We have synthesized a p-type organic semiconductor, 1-pyrylphosphonic acid, that can self-assemble onto silicon dioxide to form either a 0.9nm monolayer or a multilayer with well-defined 2.5nm step heights depending on the assembly conditions. We allowed polycrystalline multilayers to self-assemble between interdigitated gold electrodes on a SiO2∕Si substrate and measured the charge carrier transport as a function of applied voltages to the electrodes and the gate substrate, temperature, and electrode spacing. At room temperature for channel lengths larger than 400nm, the conduction was dominated by Poole-Frenkel emission at the metal/molecule interface. At low temperatures (∼240K) the space charge limited conduction dominated and an on-off ratio of 1000 was 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%.