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Wallace C. H. Choy

Bio: Wallace C. H. Choy is an academic researcher from University of Hong Kong. The author has contributed to research in topics: Perovskite (structure) & Organic solar cell. The author has an hindex of 57, co-authored 326 publications receiving 13436 citations. Previous affiliations of Wallace C. H. Choy include Hong Kong Baptist University & University of Surrey.


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TL;DR: The use of two metallic nanostructures to achieve broad light absorption enhancement, increased shortcircuit current, and improved fi ll factor ( FF) simultaneously based on the new small-bandgap polymer donor poly{[4,8-bis(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b ′ ]dithiophene2,6-diyl]alt -[2-(2
Abstract: Polymer-fullerene-based bulk heterojunction (BHJ) solar cells have many advantages, including low-cost, low-temperature fabrication, semi-transparency, and mechanical fl exibility. [ 1 , 2 ] However, there is a mismatch between optical absorption length and charge transport scale. [ 3 , 4 ] These factors lead to recombination losses, higher series resistances, and lower fi ll factors. Attempts to optimize both the optical and electrical properties of the photoactive layer in organic solar cells (OSCs) inevitably result in a demand to develop a device architecture that can enable effi cient optical absorption in fi lms thinner than the optical absorption length. [ 5 , 6 ] Here, we report the use of two metallic nanostructures to achieve broad light absorption enhancement, increased shortcircuit current ( J sc ), and improved fi ll factor ( FF ) simultaneously based on the new small-bandgap polymer donor poly{[4,8-bis(2-ethyl-hexyl-thiophene-5-yl)-benzo[1,2-b:4,5-b ′ ]dithiophene2,6-diyl]alt -[2-(2 ′ -ethyl-hexanoyl)-thieno[3,4-b]thiophen-4,6-diyl]} (PBDTTT-C-T) in BHJ cells. [ 7 ] The dual metallic nanostructure consists of a metallic nanograting electrode as the back refl ector and metallic nanoparticles (NPs) embedded in the active layer. Consequently, we achieve the high power conversion effi ciency (PCE) of 8.79% for a single-junction BHJ OSC. Recently, plasmonic nanostructures have been introduced into solar cells for highly effi cient light harvesting. [ 5 , 8–17 ] Two types of plasmonic resonances, surface plasmonic resonances (SPRs) [ 18–22 ] and localized plasmonic resonances (LPRs), [ 11–14 ] can be used for enhancing light absorption. Metallic gratingbased light-trapping schemes have been investigated in traditional inorganic photovoltaic cells. [ 18–20 ] For metallic nanogratings, which can support SPRs, it is still challenging to experimentally demonstrate the enhancement of PCE in OSCs owing to the obvious issue of solution processing of

678 citations

Journal ArticleDOI
TL;DR: By introducing a thin film of perfluorinated ionomer (PFI) sandwiched between the hole transporting layer and perovskite emissive layer, the device hole injection efficiency has been significantly enhanced and the three-fold increase in peak brightness reaching 1377 cd m(-2).
Abstract: High photoluminescence quantum yield, easily tuned emission colors, and high color purity of perovskite nanocrystals make this class of material attractive for light source or display applications. Here, green light-emitting devices (LEDs) were fabricated using inorganic cesium lead halide perovskite nanocrystals as emitters. By introducing a thin film of perfluorinated ionomer (PFI) sandwiched between the hole transporting layer and perovskite emissive layer, the device hole injection efficiency has been significantly enhanced. At the same time, PFI layer suppressed charging of the perovskite nanocrystal emitters thus preserving their superior emissive properties, which led to the three-fold increase in peak brightness reaching 1377 cd m–2. The full width at half-maximum of the symmetric emission peak with color coordinates of (0.09, 0.76) was 18 nm, the narrowest value among perovskite based green LEDs.

650 citations

Journal ArticleDOI
TL;DR: In this article, the fabrication conditions significantly influence the electron paramagnetic resonance (EPR) and photoluminescence excitation (PLE) spectra obtained, and it is found that green PL emission can be suppressed by coating the nanostructures with a surfactant for all fabrication conditions.
Abstract: ZnO tetrapod nanostructures have been prepared by the evaporation of Zn in air (no flow), dry and humid argon flow, and dry and humid nitrogen flow. Their properties have been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies (at different temperatures), and electron paramagnetic resonance (EPR) spectroscopy at –160 °C and room temperature. It is found that the fabrication conditions significantly influence the EPR and PL spectra obtained. While a g = 1.96 EPR signal is present in some of the samples, green PL emission can be observed from all the samples. Therefore, the green emission in our samples does not originate from the commonly assumed transition between a singly charged oxygen vacancy and a photoexcited hole [K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant, J. A. Voigt, Appl. Phys. Lett. 1996, 68, 403]. However, the green emission can be suppressed by coating the nanostructures with a surfactant for all fabrication conditions, which indicates that this emission originates from surface defects.

575 citations

Journal ArticleDOI
26 Jan 2016-ACS Nano
TL;DR: The flawless and surface-nanostructured NiOx film can make the interfacial recombination and monomolecular Shockley-Read-Hall recombination of PVSC reduce and improve the hole extraction and thus PVSC performances, which contributes to the evolution of flexible PVSCs with simple fabrication process and high device performances.
Abstract: Recently, researchers have focused on the design of highly efficient flexible perovskite solar cells (PVSCs), which enables the implementation of portable and roll-to-roll fabrication in large scale. While NiOx is a promising material for hole transport layer (HTL) candidate for fabricating efficient PVSCs on a rigid substrate, the reported NiOx HTLs are formed using different multistep treatments (such as 300–500 °C annealing, O2-plasma, UVO, etc.), which hinders the development of flexible PVSCs based on NiOx. Meanwhile, the features of nanostructured morphology and flawless film quality are very important for the film to function as highly effective HTL of PVSCs. However, it is difficult to have the two features coexist natively, particularly in a solution process that flawless film will usually come with smooth morphology. Here, we demonstrate the flawless and surface-nanostructured NiOx film from a simple and controllable room-temperature solution process for achieving high performance flexible PVSCs...

446 citations

Journal ArticleDOI
TL;DR: In this article, the role of light trapping and angular restriction in improving the maximal output power of thin-film perovskite photovoltaics has been clarified and the influence of trap-assisted nonradiative recombination on the device efficiency is investigated.
Abstract: With the consideration of photon recycling effect, the efficiency limit of methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells is predicted by a detailed balance model. To obtain convincing predictions, both AM 1.5 spectrum of Sun and experimentally measured complex refractive index of perovskite material are employed in the detailed balance model. The roles of light trapping and angular restriction in improving the maximal output power of thin-film perovskite solar cells are also clarified. The efficiency limit of perovskite cells (without the angular restriction) is about 31%, which approaches to Shockley-Queisser limit (33%) achievable by gallium arsenide (GaAs) cells. Moreover, the Shockley-Queisser limit could be reached with a 200 nm-thick perovskite solar cell, through integrating a wavelength-dependent angular-restriction design with a textured light-trapping structure. Additionally, the influence of the trap-assisted nonradiative recombination on the device efficiency is investigated. The work is fundamentally important to high-performance perovskite photovoltaics.

444 citations


Cited by
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[...]

08 Dec 2001-BMJ
TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

33,785 citations

Journal ArticleDOI
TL;DR: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature.
Abstract: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

10,260 citations

Journal ArticleDOI
17 Apr 2019-Joule
TL;DR: In this paper, a ladder-type electron-deficient core-based central fused ring (Dithienothiophen[3.2-b]- pyrrolobenzothiadiazole) with a benzothiadiadiazoles (BT) core was proposed to fine-tune its absorption and electron affinity.

3,513 citations