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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: Research at the University of Washington regarding organic semiconductors is reviewed, covering four major topics: electro-optics, organic light emitting diodes, organic field-effect transistors, and organic solar cells.
Abstract: Research at the University of Washington regarding organic semiconductors is reviewed, covering four major topics: electro-optics, organic light emitting diodes, organic field-effect transistors, and organic solar cells. Underlying principles of materials design are demonstrated along with efforts toward unlocking the full potential of organic semiconductors. Finally, opinions on future research directions are presented, with a focus on commercial competency, environmental sustainability, and scalability of organic-semiconductor-based devices.

27 citations

Journal ArticleDOI
TL;DR: In this article, a Mach-Zehnder interferometer is used to decouple the electro-optic and piezoelectric tensor effects occurring in a poled polymer film.
Abstract: A Mach–Zehnder interferometer (MZI) is utilized to decouple the electro-optic and piezoelectric tensor effects occurring in a poled polymer film. This method has significant advantages over the commonly used Teng-Man reflection ellipsometry technique by allowing for the independent determination of the Pockel’s coefficients r13 and r33 and the piezoelectric coefficient d33. The r33 value of a guest host polymer that consists of AJLZ53 amorphous polycarbonate was found to be 122.69 pm/V and 123.03 pm/V using the MZI and reflection ellipsometry method, respectively. The r33 data fits well to the dispersion of the second order susceptibility tensor based on the two-level model approximation.

27 citations

Journal ArticleDOI
TL;DR: It is explained that the best material for the slab waveguide is an inorganic material because of the low RF permittivity combined with the high electro-optic coefficient.
Abstract: This paper provides a detailed analysis of electric field sensing using a slab-coupled optical fiber sensor (SCOS). This analysis explains that the best material for the slab waveguide is an inorganic material because of the low RF permittivity combined with the high electro-optic coefficient. The paper also describes the fabrication and testing of a SCOS using an AJL chromophore in amorphous polycarbonate. The high uniform polymer slab waveguide is fabricated using a hot embossing process to create a slab with a thickness of 50 μm. The fabricated polymer SCOS was characterized to have a resonance slope of ΔP/Δλ=6.83E5 W/m and a resonance shift of Δλ/E=1.47E−16 m2/V.

26 citations

Journal ArticleDOI
TL;DR: In this paper , a multifunctional interface manipulation strategy was developed by introducing a pyridine-functionalized fullerene derivative, which was placed at the interface between the tin perovskite and the electron transport layer (ETL) to improve the photovoltaic performance and stability.
Abstract: In tin perovskite solar cells (PSCs), fullerene (C60) and fullerene derivative [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) are commonly utilized electron transport materials. However, the energetic disorder, inadequate passivation, and energy level mismatch of C60 and PCBM limit the improvement of power conversion efficiency (PCE) and lifespan of tin PSCs. In this work, a multifunctional interface manipulation strategy is developed by introducing a pyridine‐functionalized fullerene derivative, fullerene‐n‐butyl‐pyridine (C60‐BPy), into the interface between the tin perovskite and the electron transport layer (ETL) to improve the photovoltaic performance and stability of tin PSCs. The C60‐BPy can strongly anchor on the perovskite surface via coordination interactions between the pyridine moiety and the Sn2+ ion, which not only reinforces the passivation of the trap‐state within the tin perovskite film, but also regulates the interface energy level alignment to reduce non‐radiative recombination. Moreover, the improved interface binding and carrier transport properties of C60‐BPy contribute to superior device stability. The resulting devices have achieved the highest PCE of 14.14% with negligible hysteresis, and are maintained over 95% of their initial PCE under continuous one‐sun illumination for 1000 h.

26 citations

Journal ArticleDOI
TL;DR: In this article, the authors identify the most significant controlling variables determining morphological evolution in a new class of hybrid perovskite alloy, namely CH3NH3Pb(Mn)yI3.
Abstract: Modification of CH3NH3PbI3 and related hybrid organic–inorganic semiconductors has become an increasingly important effort because of the need to control fundamental material properties. Herein, we closely study material growth to identify the most significant controlling variables determining morphological evolution in a new class of hybrid perovskite alloy. Specifically, drop-casting based perovskite analysis shows that CH3NH3Pb(Mn)yI3, CH3NH3Pb(Fe)yI3, CH3NH3Pb(Co)yI3, and CH3NH3Pb(Ni)yI3 constitute a unique class of hybrid organic–inorganic perovskite in which growth route most strongly determines morphology. Mn, Fe, Co, and Ni consistently modify CH3NH3PbI3 growth, enabling direct perovskite nucleation to compete with growth through solvent induced intermediate states. We show unambiguously that solvent-perovskite co-crystal formation is responsible for the rod-like thin-film morphology that a great deal of work optimizing perovskite growth in planar heterojunction solar cells endeavors to circumvent. In addition to providing insight into the role of growth route in morphological evolution, we also identity the impact of CH3NH3I stoichiometry and the impact of magnetic properties on growth as secondary variables that significantly affect optoelectronic properties. Leveraging this understanding to minimize the impact of morphological phenomena on performance, we closely analyze the compositional impact of these transition metals on optoelectronic quality using CH3NH3Pb(Fe)yI3 as a model system showing that transition metal inclusion of this type leads to trap-assisted recombination within the perovskite bulk that both sharply limits Jsc and causes significant hysteresis. By comparing device performance of Mn, Fe, Co, and Ni based systems, we show that Mn relieves this sharp limitation on Jsc and almost completely eliminates hysteresis. CH3NH3Pb(Mn)yI3 thus allows the implementation of direct perovskite nucleation while minimizing the deleterious impact of transition metal inclusion. PL analysis shows that this material is also more emissive than CH3NH3PbI3, making it ideal for light production as well. Methodology and insights developed herein outline a generalizable approach for navigating complexity of perovskite compositional modification.

26 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