Showing papers by "Hoi Sing Kwok published in 2017"
TL;DR: It is shown that the requirements of low fill factor and short‐circuit current commonly observed in sputtered NiOx‐derived PSCs can be satisfied by using the magnetron sputtering, which is well established in the industry.
Abstract: Perovskite solar cells (PSCs) are one of the promising photovoltaic technologies for solar electricity generation. NiO x is an inorganic p-type semiconductor widely used to address the stability issue of PSCs. Although high efficiency is obtained for the devices employing NiO x as the hole transport layer, the fabrication methods have yet to be demonstrated for industrially relevant manufacturing of large-area and high-performance devices. Here, it is shown that these requirements can be satisfied by using the magnetron sputtering, which is well established in the industry. The limitations of low fill factor and short-circuit current commonly observed in sputtered NiO x -derived PSCs can be overcome through magnesium doping and low oxygen partial pressure deposition. The fabricated PSCs show a high power conversion efficiency of up to 18.5%, along with negligible hysteresis, improved ambient stability, and high reproducibility. In addition, good uniformity is also demonstrated over an area of 100 cm2. The simple and well-established approach constitutes a reliable and scale method paving the way for the commercialization of PSCs.
156 citations
TL;DR: The photoalignment technique facilitates the fabrication of mixed and multiple stacked NREF for LCDs, which improves the color gamut and polarization efficiency, and is thus expected to increase the optical efficiency of conventional LCDs by ≈60%.
Abstract: Semiconductor nanorods (NR) emit polarized light, which is expected to bring manifold benefits, in terms of brightness and color enhancement, for modern liquid-crystal displays (LCD). In this regard, photoaligned nanorod enhancement films (NREF) for color and polarization conversion for LCD backlights are introduced here. The photoinduced anchoring forces, by the photoalignment layer, stimulate well-ordered self-assembly of NR in the thin polymer films. Green and red emitting NR with a quantum yield of ≈80% are aligned unidirectionally and in-plane, showing a polarization ratio of >7:1 and a degree of polarization of >0.81. The photoalignment technique facilitates the fabrication of mixed and multiple stacked NREF for LCDs, which improves the color gamut and polarization efficiency, and is thus expected to increase the optical efficiency of conventional LCDs by ≈60%.
136 citations
TL;DR: The results are promising for the fabrication of complex nanostructures for photonic applications and the possibility of controlling the orientation of anisotropic nanocrystals with strongly polarized emission on microscopic scale.
Abstract: Photoalignment technology provides high alignment quality with an exceptional control over the local director of liquid crystals. Because of the reorientation ability of sulfonic azo dye molecules, they offer high azimuthal and polar anchoring energy with a low pretilt angle for the orientation of liquid crystals and liquid crystal composites. In this work, we make use of this approach to align thin film composites of light-emitting semiconductor nanorods dispersed in a liquid crystal polymer into both one-dimensional and two-dimensional microscale patterns. After unidirectional alignment, the patterns are fabricated by a second irradiation with different polarization azimuth and the employment of a photomask. Fluorescence micrographs reveal the nanorod pattern alignment in domain sizes down to 2 μm. Apart from demonstrating the possibility of controlling the orientation of anisotropic nanocrystals with strongly polarized emission on microscopic scale, our results are promising for the fabrication of comp...
62 citations
TL;DR: In this paper, a diffractive bifocal vortex lens is used to generate and sort light beams of different orbital angular momentum (OAM) by means of polarization control, and a vortex-beam laser with a chosen OAM can be realized.
Abstract: In an optical vortex beam, each photon carries orbital angular momentum (OAM), a useful resource for applications in optical communication and quantum information processing. The authors demonstrate a diffractive bifocal vortex lens that generates and sorts light beams of different OAM, by means of polarization control. Placing the lens inside a cavity, a vortex-beam laser with a chosen OAM can be realized. Moreover, the lens's OAM sorting can be used in a multifocal optical-trapping system that facilitates the manipulation of nanoparticles, molecules, and biological samples.
44 citations
TL;DR: In this paper, a bottom-up additive technique has been developed by manipulating wetting/dewetting of the substrate to achieve controllable and fine patterning of the EGaIn electrodes.
Abstract: Liquid metals based on eutectic gallium–indium (EGaIn) alloys are promising printable electrode materials for various printed devices such as light-emitting diodes or solar cells. To achieve a controllable and fine patterning of the EGaIn electrodes, herein, a facile bottom-up, additive technique has been developed by manipulating wetting/dewetting of the substrate. The substrate is first dewetted by coating a hydrophobic layer; then, to selectively dewet the substrate, the coated hydrophobic layer is patterned by laser ablation. EGaIn, which is painted onto the substrate, can only adhere to the wetting regions and thus can perfectly copy the patterns of the wetting regions. Via this method, high-resolution EGaIn patterns with a line width smaller than 100 μm can be obtained. Using the patterned EGaIn electrodes, all-printed quantum dot light-emitting diodes were successfully demonstrated. The proposed method offers a feasible route for fast patterning of EGaIn and thus allows for inexpensive, rapid fabrication of various devices without the need of costly vacuum processing.
42 citations
TL;DR: In this paper, a hybrid inverter constructed by p-channel low-temperature poly-silicon (LTPS) TFTs and n-channel amorphous-indium-gallium-zincoxide (a-IGZO) transistors was demonstrated.
Abstract: The applications of a-InGaZnO thin-film transistors (TFTs) to logic circuits have been limited owing to the intrinsic n-channel operation. In this paper, we demonstrated a hybrid inverter constructed by p-channel low-temperature poly-silicon (LTPS) TFTs and n-channel amorphous-indium–gallium–zinc–oxide (a-IGZO) TFTs. Hydrogenated LTPS TFTs and a-IGZO TFTs have been successfully fabricated on the same panel, followed by a rapid thermal annealing treatment to remove the hydrogens in the a-IGZO TFTs. The resulted hybrid inverter exhibits large noise margin closed to ${V}_{\textsf {DD}}/\textsf {2}$ and a high voltage gain as 68.3. Due to the complementary configurations in the static state, the inverter shows small current and thus consumes low power in hundreds of picowatts. As all the fabrication processes are compatible with conventional techniques, the reported results may open new opportunities in circuit design and applications for oxide TFTs.
38 citations
TL;DR: In this article, a switchable dual frequency liquid crystal (DFLC) Fresnel lens with a patterned hybrid photo-aligned nematic (HPAN) cell assembled by one substrate providing the homeotropic anchoring and the other one providing the in-plane patterned alignment, with mutually orthogonal easy axis in the neighboring alignment domains.
Abstract: In this article we disclose a method to fabricate a polarization independent dual frequency liquid crystal (DFLC) Fresnel lens (FL) with relatively high efficiency. The switchable FL is based on a patterned hybrid photo-aligned nematic (HPAN) DFLC cell assembled by one substrate providing the homeotropic anchoring and the other one providing the in-plane patterned alignment, with mutually orthogonal easy axis in the neighboring alignment domains, which has been prepared by means of two-step photo-alignment technique. Due to the electro-optical properties of dual frequency LC, the proposed HPAN DFLC FL, which manifests relatively high diffraction efficiency of 38.5% and low driving voltage, has the possibility of achieving a fast response by alternatively switching between high frequency and low frequency electric fields. Thus, with an excellent rewritable performance leading to the alternative focus/defocus switch, the HPAN DFLC FL would definitely be practical and promising in many modern devices.
33 citations
TL;DR: In this paper, the properties of hybrid-phase microstructural indium tin oxide-stabilized ZnO thin films and the relevant high-performance thin-film transistors (TFTs) were systematically investigated.
Abstract: In this paper, the properties of hybrid-phase microstructural indium tin oxide-stabilized ZnO thin films and the relevant high-performance thin-film transistors (TFTs) were systematically investigated. The optically extracted Urbach energy revealed that such thin films owned less band-tail state trapping in comparison with that of the corresponding amorphous thin films. This was determined by better atomic arrangement and might realize higher drift mobility theoretically. The influence of deposition parameters such as oxygen partial pressure ratio (PO2) and direct-current power ( $\text{P}_{\mathrm{ DC}})$ on thin films was discussed in detail. Then, the TFTs with optimal co-sputtering conditions were fabricated. Such devices exhibited a typical field-effect mobility of 26.1 cm2/ $\text{V}\cdot \text{s}$ , threshold voltage of 0.5 V, on–off ratio of over $10^{9}$ , and extremely low subthreshold swing of 89 mV/decade. Meanwhile, the spatial uniformity, air stability, and repeated switching behavior of devices were demonstrated to be excellent.
32 citations
TL;DR: The proposed 2D-3D switchable lens unit is characterized by low power consumption, ultrafast response, and 3D crosstalk <5%, and can therefore find application in TVs, cell phones, etc.
Abstract: We reveal a 2D–3D switchable lens unit that is based on a polarization-sensitive microlens array and a polarization selector unit made of an electrically suppressed helix ferroelectric liquid crystal (ESHFLC) cell. The ESHFLCs offer a high contrast ratio (∼10k∶1) between the crossed polarizers at a low applied electric field (∼1.7 V/μm) with a small switching time (<50 μs). A special driving scheme, to switch between a 2D and 3D mode, has been developed to avoid unwanted issues related to DC accumulation in the ferroelectric liquid crystal without affecting its optical quality. The proposed lens unit is characterized by low power consumption, ultrafast response, and 3D crosstalk <5%, and can therefore find application in TVs, cell phones, etc.
22 citations
TL;DR: In this article, the dependence of device reliability against a variety of stress conditions on the annealing atmosphere was studied using a single metal-oxide thin-film transistor with thermally induced source/drain regions.
Abstract: The dependence of device reliability against a variety of stress conditions on the annealing atmosphere was studied using a single metal-oxide thin-film transistor with thermally induced source/drain regions. A cyclical switch between an oxidizing and a non-oxidizing atmosphere induced a regular change in the stress-induced shift of the turn-on voltage, with the magnitude of the shift being consistently smaller after annealing in an oxidizing atmosphere. The observed behavior is discussed in terms of the dependence of the population of oxygen vacancies on the annealing atmosphere, and it is recommended the last of the sequence of thermal processes applied to a metal-oxide thin-film transistor be executed in an oxidizing atmosphere.
20 citations
06 Mar 2017
TL;DR: In this paper, a ferroelectric liquid crystal (FLC) dammann grating is demonstrated based on the patterned photoalignment technology, which can switch between a diffractive state with 7 × 7 optical spots array and a non-diffractive state, depending on the polarity of electric field.
Abstract: In this article, a ferroelectric liquid crystal (FLC) dammann grating (DG) is demonstrated based on the patterned photoalignment technology. By applying low electric field (10 V) on the FLC DG, the grating can switch between a diffractive state with 7 × 7 optical spots array and a non-diffractive state, depending on the polarity of electric field. The FLC DG shows very fast switching speed with switching on time and off time to be only 81 μs and 59 μs respectively. Comparing with other fast LC DGs such as the ones based on blue phase LC or dual-frequency LC, the switching speed of the proposed FLC DG is about one order faster, which provides great potential and perspective for the FLC DG to be applied in a broad range of optical applications such as optical communication and beam shaping.
TL;DR: Based on the distinct effects of oxidizing thermal annealing on the properties of zinc oxide and indium-gallium-zinc oxide (IGZO) under covers of different gas-permeabilities, the elevated-metal metaloxide (EMMO) thin-film transistor (TFT) architecture has been proposed and demonstrated using IGZO as the channel material as mentioned in this paper.
Abstract: Based on the distinct effects of oxidizing thermal annealing on the properties of zinc oxide and indium-gallium-zinc oxide (IGZO) under covers of different gas-permeabilities, the elevated-metal metal-oxide (EMMO) thin-film transistor (TFT) architecture has been proposed and demonstrated using IGZO as the channel material However, the speculation that the EMMO architecture is more generally applicable to semiconducting metal oxides other than IGZO has yet to be verified Presently reported is an EMMO TFT with a modified structure employing indium-tin-zinc oxide as the channel material The resulting TFT exhibited good performance metrics: a relatively higher field-effect mobility of 232 ± 08 cm2/Vs, an ON/OFF current ratio of at least $31\times 10^{10}$ , a pseudo subthreshold slope of 165 ± 15 mV/decade, a width-normalized off-state current of at most $81\times 10^{-19}\text{A}/\mu \text{m}$ , and robust stability against gate-bias stress
TL;DR: The transition from amorphous solid at low relative humidity to semicrystal at high relative humidity provides a new perspective of understanding the hydrophilic photoalignment materials.
Abstract: Azobenzene photoalignment materials are highly effective for liquid crystal alignment with high sensitivity and rewritability. A strong relationship between relative humidity and the alignment quality of a thin layer of azobenzenesulfonic dye has been investigated, where the photoinduced phase retardation, order parameter, and anchoring strength of the alignment layer are influenced dramatically by relative humidity. Our results provide fabrication guidance for the photoalignment process in both display and photonic applications. In addition, an exotic substantial ordering enhancement is observed by increasing the relative humidity without further light illumination, where the self-assembly of the photoaligned material incorporated with water molecules is the underlying reason for the enhanced high ordering (S > 0.8). Based on X-ray diffraction and depolarized optical microscopy observation, together with the photoalignment quality, a semicrystalline structure of the humidified azobenzenesulfonic material is proposed. The transition from amorphous solid at low relative humidity to semicrystal at high relative humidity provides a new perspective of understanding the hydrophilic photoalignment materials.
TL;DR: Three-order and eight-order binary-phase liquid crystal circular Dammann gratings with two mutually orthogonal photo-induced alignments in every two adjacent alignment domains, fabricated by a micro-patterned liquid crystal polymer phase mask, are proposed to generate annular uniform-intensity patterns in the far field.
Abstract: As one of the diffractive optical elements, circular Dammann grating has shown its excellent versatility in practical applications. The electrically switchable Dammann grating has been extensively investigated; however, the research on the optically tunable circular Dammann grating has received less attention and reports on this subject have been insufficient in the past decade. In this paper, three-order and eight-order binary-phase liquid crystal circular Dammann gratings with two mutually orthogonal photo-induced alignments in every two adjacent alignment domains, fabricated by a micro-patterned liquid crystal polymer phase mask, are proposed to generate annular uniform-intensity patterns in the far field. A simple maskless optical tuning of an eight-order liquid crystal circular Dammann grating is demonstrated by controlling the polarization of an ultraviolet light as well as the energy dose. The proposed liquid crystal circular Dammann gratings with high efficiencies and desirable uniformities exhibit outstanding optical as well as electrical tunabilities, enabling the widespread prospective applications in adaptive photonic chips stimulated flexibly by only light or by the combination of light and electric field.
TL;DR: Broadband reflective polarizers for liquid crystal displays (LCDs) are designed and show an ultra-thin profile, high transmission, large extinction ratio and wide acceptable angle, all of which are highly desirable for high-efficiency ultra- thin LCDs.
Abstract: Broadband reflective polarizers for liquid crystal displays (LCDs) are designed. The working principle is based on giant form birefringence generated by engineering the materials of the subwavelength gratings. The optical performances of the proposed polarizers are investigated by the finite difference time domain (FDTD) method. The proposed polarizers show an ultra-thin profile, high transmission, large extinction ratio and wide acceptable angle, all of which are highly desirable for high-efficiency ultra-thin LCDs. Finally, the fabrication tolerance of the proposed structure is discussed in detail.
01 May 2017
TL;DR: In this paper, a novel azo dye stabilization method was presented that only requires single-shot UV light exposure, which offers highly acceptable thermal and photostability, as well as low pre-tilt angle (0.2°) and low image sticking (1.01).
Abstract: A novel azo dye stabilization method was presented that only requires single-shot UV light exposure. The proposed photoalignment layer offers highly acceptable thermal and photostability, as well as low pre-tilt angle (0.2°) and low image sticking (1.01). Also, high VHR (99%) and low RDC (<10mV) comparable to that of PI rubbing have been achieved. Furthermore, the ability of multi-domain alignment has been demonstrated that shows a great potential in a wide range of electro-optical devices. Author
TL;DR: In this article, the degradation of polycrystalline silicon (poly-Si) thin-film transistors (TFTs) under "driving" stress is characterized and analyzed for the first time.
Abstract: In this letter, degradation of polycrystalline silicon (poly-Si) thin-film transistors (TFTs) under “driving” stress is characterized and analyzed for the first time. Dynamic hot carrier (HC) effect, related to pulse falling time, dominates device degradation. To suppress such “driving”-stress-induced dynamic HC degradation, a bridged-grain (BG) structure is applied to the active channel of poly-Si TFTs. Due to the lateral electric field reduction at source/drain junctions, “driving”-stress-induced dynamic HC degradation is significantly improved by the BG structure. Incorporated with transient simulations, the degradation mechanism is elucidated.
TL;DR: In this article, a hybrid-phase microstructural ITO-stabilized ZnO thin films, which were capped by tetraethyl-orthosilicate-sourced plasma-enhanced chemical vapor deposition (PECVD) SiO2 layers, were found to present remarkably different resistivity variation after O2 annealing treatment.
Abstract: In this letter, hybrid-phase microstructural ITO-stabilized ZnO thin films, which were capped by silane-sourced or tetraethyl-orthosilicate-sourced plasma-enhanced chemical vapor deposition (PECVD) SiO2 layers, were found to present remarkably different resistivity variation after O2 annealing treatment. Besides hydrogen-related factors, the silicon-doping phenomenon along with the silane-sourced PECVD process was also responsible for the durable low-resistivity state of the hybrid-phase microstructural ITO-stabilized ZnO thin films. Via a combination of differentiatedPECVD andO2 annealingprocesses, the self-aligned coplanar TFTs were fabricated, and they exhibited good electrical characteristics with an average field-effect mobility of 19.56 cm2/Vs, and a subthreshold swing of 105 mV/decade, as well as robust stability against gate-bias stress.
01 May 2017
TL;DR: A green top-emitting quantum-dot light emitting diode (TQLED) with conventional structure is demonstrated in this article with all its functional layers, including hole transportation layer, emitting layer, and electron transportation layer fabricated by spin-coating processes.
Abstract: A green top-emitting quantum-dot light-emitting diode (TQLED) with conventional structure is demonstrated in this study with all its functional layers, including hole transportation layer, emitting layer and electron transportation layer, fabricated by spin-coating processes This green QLED exhibits an external quantum efficiency (EQE) of 64 % and a maximum brightness of 200000 cd/m2 , which is the highest values reported for green TQLEDs in literature Besides, this is the first report of all-solution-processed TQLEDs
Patent•
10 May 2017TL;DR: A photoaligned quantum rod enhancement film (Qas discussed by the authors) as discussed by the authors consists of a substrate (802, 805), a photoalignment layer, a polymer layer, and a plurality of quantum rods, which emit one or more wavelengths of light in response to pumping light.
Abstract: A photoaligned quantum rod enhancement film (QREF) includes: a substrate ( 802, 805 ); a photoalignment layer deposited on the substrate ( 802, 805 ); and a polymer layer deposited on the photoalignment layer, the polymer layer comprises a plurality of quantum rods, the plurality of quantum rods are configured to emit one or more wavelengths of light in response to pumping light, and are aligned to an alignment axis based on the photoalignment layer
01 May 2017
01 Jan 2017
TL;DR: In this article, a photo-induced coatable polarizer with a dichroic ratio of more than 50, a contrast ratio more than 300, and a thickness less than 500 nm is presented.
Abstract: A coatable polarizer with high performance is a perpetual goal because of its numerous advantages. Here, we demonstrate a photo-induced coatable polarizer with a dichroic ratio of more than 50, a contrast ratio of more than 300, and a thickness less than 500 nm. This polarizer offers a simple and economical technique to realize advanced displays with low cost, ultra-thin profile, flexible style, and high performance.
01 Jan 2017
01 May 2017
01 May 2017
01 Jan 2017
01 Jan 2017
TL;DR: In this article, the authors proposed the ITO-stabilized ZnO thin film with less native defects, and introduced a novel hybrid-phase microstructure, where nanocrystals are embedded in an amorphous matrix.
Abstract: The advancement of displays towards large area and ultrahigh definition claim thin film transistors (TFTs) with higher electrical performance, uniformity and stability. The most prevalent a-IGZO is theoretically limited by its composition and randomness, and seems insufficient to provide demanded mobility (> 20 cm/Vs) [1]. Binary ZnO suffers from instability issues that related to native defects [2]. In this paper, we propose the ITO-stabilized ZnO thin film with less native defects, and introduce a novel hybrid-phase microstructure, where nanocrystals are embedded in an amorphous matrix. The proposed thin film can provide relatively high mobility and stability. Following a sufficient study on microstructural and electrical properties of hybrid-phase ITOstabilized ZnO thin films, both staggered bottom-gate and top-gate TFTs with this optimal ITO-stabilized ZnO channel layers are fabricated, and exhibit a fairly high electrical performance especially in field-effect mobility and subthreshold swing. Additionally, the electrical performance of devices is proved to be spatially uniform and stable.
Patent•
31 May 2017TL;DR: In this paper, a fabrication method of a polarization grating is described, which includes providing a polarization sensitive material; and causing two orthogonally polarized lights to scan the polarization-sensitive material and to meet on the polarizationsensitive material.
Abstract: A fabrication method of a polarization grating is provided. The method includes providing a polarization-sensitive material; and causing two orthogonally polarized lights to scan the polarization-sensitive material and to meet on the polarization-sensitive material.
TL;DR: In this article, trace nickel that diffuses from the MIC window into the a-Si matrix during the MIC heat-treatment is experimentally discovered, which is responsible for the crystallization of the amorphous silicon beyond the MIC front.
Abstract: Crystallization of amorphous silicon (a-Si) which starts from the middle of the a-Si region separating two adjacent metal-induced crystallization (MIC) polycrystalline silicon (poly-Si) regions is observed. The crystallization is found to be related to the distance between the neighboring nickel-introducing MIC windows. Trace nickel that diffuses from the MIC window into the a-Si matrix during the MIC heat-treatment is experimentally discovered, which is responsible for the crystallization of the a-Si beyond the MIC front. A minimum diffusion coefficient of cm2/s at 550 °C is estimated for the trace nickel diffusion in a-Si.