RSS 1.0KAUST Solar Center (KSC)
Formerly the "Solar and Photovoltaic Engineering Research Center (SPERC)"
Recent Submissions
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Giant Nonlinear Optical Response via Coherent Stacking of In-Plane Ferroelectric Layers(Advanced Materials, Wiley, 2023-03-23) [Article]Thin ferroelectric materials hold great promise for compact nonvolatile memory, nonlinear optical and optoelectronic devices. Herein, we report an ultrathin in-plane ferroelectric material that exhibits a giant nonlinear optical effect: group-IV monochalcogenide SnSe. Nanometer-scale ferroelectric domains with ∼90°/270° twin boundaries or ∼180° domain walls are revealed in physical vapor deposited SnSe by lateral piezoresponse force microscopy. Atomic structure characterization reveals both parallel and antiparallel stacking of neighboring van der Waals (vdW) ferroelectric layers, leading to ferroelectric or antiferroelectric ordering. Ferroelectric domains exhibit giant nonlinear optical activity due to coherent enhancement of second harmonic fields, and the as-resulted second-harmonic generation was observed to be 100 times more intense than monolayer WS2. This work demonstrates in-plane ferroelectric ordering and giant nonlinear optical activity in SnSe, which paves the way for applications in on-chip nonlinear optical components and nonvolatile memory devices.
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Zero-dimensional Cu(i)-based organometallic halide with green cluster-centred emission for high resolution X-ray imaging screens(Chemical Communications, Royal Society of Chemistry (RSC), 2023-03-22) [Article]In this communication, we report a low-dimensional perovskite-related structure based on Cu(I) organometallic halide with strong green cluster-centred emission and near-unity photoluminescence quantum yield. The 0D [Rb(18-crown-6)]2Cu4I6 was sucessfully applied for X-ray imaging screens which exhibit high spatial resolution of 16.8 lp mm−1 and low detection limit of 458 nGy s−1.
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Artificial Leaf for Solar-Driven Ammonia Conversion at Milligram-Scale Using Triple Junction III-V Photoelectrode(Advanced Science, Wiley, 2023-03-22) [Article]Developing a green and energy-saving alternative to the traditional Haber-Bosch process for converting nitrogen into ammonia is urgently needed. Imitating from biological nitrogen fixation and photosynthesis processes, this work develops a monolithic artificial leaf based on triple junction (3J) InGaP/GaAs/Ge cell for solar-driven ammonia conversion under ambient conditions. A gold layer serves as the catalytic site for nitrogen fixation with photogenerated electrons. The Au/Ti/3J InGaP/GaAs/Ge photoelectrochemical (PEC) device achieves high ammonia production rates and Faradaic efficiencies in a two-electrode system without applying external potential. For example, at 0.2 sunlight intensity, the solar-to-ammonia (STA) conversion efficiency reaches 1.11% and the corresponding Faradaic efficiency is up to 28.9%. By integrating a Ni foil on the anode side for the oxygen evolution reaction (OER), the monolithic artificial leaf exhibits an ammonia production rate of 8.5 µg cm<sup>-2</sup> h at 1.5 sunlight intensity. Additionally, a 3 × 3 cm unassisted wireless PEC device is fabricated that produces 1.0039 mg of ammonia in the 36-h durability test. Thus, the new artificial leaf can successfully and directly convert solar energy into chemical energy and generate useful products in an environmentally friendly approach.
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Zero-dimensional Cu(i)-based organometallic halide with green cluster-centred emission for high resolution X-ray imaging screens(Chemical Communications, Royal Society of Chemistry (RSC), 2023-03-22) [Article]In this communication, we report a low-dimensional perovskite-related structure based on Cu(I) organometallic halide with strong green cluster-centred emission and near-unity photoluminescence quantum yield. The 0D [Rb(18-crown-6)]2Cu4I6 was sucessfully applied for X-ray imaging screens which exhibit high spatial resolution of 16.8 lp mm−1 and low detection limit of 458 nGy s−1.
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Observation of cnoidal wave localization in nonlinear topolectric circuits(Physical Review Research, American Physical Society (APS), 2023-03-21) [Article]We observe a localized cnoidal (LCn) state in an electric circuit network. Its formation derives from the interplay of nonlinearity and the topology inherent to a Su-Schrieffer-Heeger (SSH) chain of inductors. Varicap diodes act as voltage-dependent capacitors, and create a nonlinear on-site potential. For a sinusoidal voltage excitation around midgap frequency, we show that the voltage response in the nonlinear SSH circuit follows the Korteweg-de Vries equation. The topological SSH boundary state, which relates to a midgap impedance peak in the linearized limit is distorted into the LCn state in the nonlinear regime, where the cnoidal eccentricity decreases from edge to bulk.
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Visualization of Surface Charge Carrier Diffusion Lengths in Different Perovskite Crystal Orientations Using 4D Electron Imaging(Advanced Optical Materials, Wiley, 2023-03-20) [Article]Understanding charge carrier dynamics on the surface of materials at the nanometer and femtosecond scales is one of the key elements to optimizing the performance of light-conversion devices, including solar cells. Unfortunately, most of the pump-probe characterization techniques are surface-insensitive and obtain information from the bulk due to the large penetration depth of the pulses. However, ultrafast scanning electron microscopy (USEM) is superior in visualizing carrier dynamics at the surface with high spatial-temporal resolution. Here, the authors successfully used USEM to uncover the tremendous effect of surface orientations and termination on the charge carrier of MAPbI3 perovskite single crystals. Time-resolved secondary electrons snapshots and density functional theory calculations clearly demonstrate that charge carrier diffusion, surface trap density, surface work function, and carrier concentration are strongly facet-dependent. The results display a diffusion length of 22 micrometers within 6.0 nanoseconds along (001) orientation. While (100) facet forms defect states that prevent carrier diffusion and shows an increase in the surface work function leading to dark contrast and fast charge carrier recombination. These findings provide a new key component to optimizing the surface of perovskites, thus paving the way for even more efficient and stable solar-cell devices based on perovskite single crystals.
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Engineering grain boundaries in monolayer molybdenum disulfide for an efficient water/ion separation(Research Square Platform LLC, 2023-03-20) [Preprint]Atomically thin two-dimensional (2D) materials have long been considered as ideal platforms for developing separation membranes. However, it is difficult to generate uniform subnanometer pores over large areas on 2D materials. Herein, we report that the well-defined defect structure of monolayer MoS2, namely, eight-membered ring (8-MR) pores typically formed at the boundaries of two antiparallel grains, can serve as molecular sieves for efficient water/ion separation. The 8-MR pores (4.2 × 2.4 Å) in monolayer MoS2 allow rapid single-file water transport while rejecting various hydrated ions. Further, the density of grain boundaries and, consequently, the density of pores can be tuned by regulating the nucleation density and size of MoS2 grains during the chemical vapor deposition process. The optimized MoS2 membrane exhibited an ultrahigh water/NaCl selectivity of ~6.5 × 104 at a water permeance of 232 mol m−2 h−1 bar−1, outperforming the state-of-the-art desalination membranes. When used for direct hydrogen production from seawater by combining the forward osmosis and electrochemical water splitting processes, the membrane achieved ~40 times the energy conversion efficiency of commercial polymeric membranes. It also exhibited a rapid and selective proton transport behavior desirable for fuel cells and electrolysis. The bottom-up approach of creating precise pore structures on atomically thin films via grain boundary engineering presents a promising route for producing large-area membranes suitable for various applications.
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Ti3C2Tx MXene van der Waals gate contact for GaN high electron mobility transistors(Advanced Materials, Wiley, 2023-03-20) [Article]Gate controllability is a key factor that determines the performance of GaN high electron mobility transistors (HEMTs). However, at traditional metal-GaN interface, direct chemical interaction between metal and GaN can result in fixed charges and traps, which can significantly deteriorate the gate controllability. In this study, Ti3C2Tx MXene films were integrated into GaN HEMTs as the gate contact, wherein van der Waals heterojunctions were formed between MXene films and GaN without direct chemical bonding. The GaN HEMTs with enhanced gate controllability exhibited an extremely low off-state current (IOFF) of 10−7 mA/mm, a record high ION/IOFF current ratio of ∼1013 (which is six orders of magnitude higher than conventional Ni/Au contact), a high off-state drain breakdown voltage of 1085 V, and a near-ideal subthreshold swing of 61 mV/dec. This work shows the great potential of MXene films as gate electrodes in wide-bandgap semiconductor devices.
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Recombination in passivating contacts: Investigation into the impact of the contact work function on the obtained passivation(Solar RRL, Wiley, 2023-03-15) [Article]Improving the passivation of contacts in silicon (Si) solar cells is crucial for reaching high-efficiency devices. In this study, the impact of the contact work function on the obtained passivation is examined and quantified using a novel method—quasi-steady-state photoluminescence—which provides access to the surface saturation current density after metallisation (J0s,m). The obtained J0s,m indicates an improvement of the surface passivation when contacts with high work function are applied onto Si wafers passivated with aluminium oxide, regardless of the wafer doping type. This improvement is mainly due to the amplification of the imbalance between the electron and hole concentrations near the Si interface. The passivation quality is reduced when using contacts with low work function in which the recombination rate increases via the charge-assisted carrier population control. This study points to the vital importance of selecting suitable metals to minimise contact recombination in high-efficiency solar cells.
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Nucleation Stage for the Oriented Growth of Tantalum Sulfide Monolayers on Au(111)(The Journal of Physical Chemistry C, American Chemical Society (ACS), 2023-03-13) [Article]We study the nucleation stage in the epitaxial growth of monolayer TaS2 as a model system for monolayer transition-metal sulfides. The growth was done under ultrahigh-vacuum conditions with Au(111) as a substrate on which the metal atoms are evaporated, and the sulfur is provided from a background of H2S. Using scanning tunneling microscopy, we find atomic-scale protrusions with a well-defined triangular shape that act as nuclei for the further growth of extended tantalum sulfide monolayers. We identify these protrusions as TaS3 using density functional theory. We propose that their unique orientation is the cause of the well-defined orientation of a complete TaS2 layer found under favorable growth conditions.
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Copper Organometallic Iodide Arrays for Efficient X-ray Imaging Scintillators(ACS Central Science, American Chemical Society (ACS), 2023-03-10) [Article]Lead-free organic metal halide scintillators with low-dimensional electronic structures have demonstrated great potential in X-ray detection and imaging due to their excellent optoelectronic properties. Herein, the zero-dimensional organic copper halide (18-crown-6)2Na2(H2O)3Cu4I6 (CNCI) which exhibits negligible self-absorption and near-unity green-light emission was successfully deployed into X-ray imaging scintillators with outstanding X-ray sensitivity and imaging resolution. In particular, we fabricated a CNCI/polymer composite scintillator with an ultrahigh light yield of ∼109,000 photons/MeV, representing one of the highest values reported so far for scintillation materials. In addition, an ultralow detection limit of 59.4 nGy/s was achieved, which is approximately 92 times lower than the dosage for a standard medical examination. Moreover, the spatial imaging resolution of the CNCI scintillator was further improved by using a silicon template due to the wave-guiding of light through CNCI-filled pores. The pixelated CNCI-silicon array scintillation screen displays an impressive spatial resolution of 24.8 line pairs per millimeter (lp/mm) compared to the resolution of 16.3 lp/mm for CNCI-polymer film screens, representing the highest resolutions reported so far for organometallic-based X-ray imaging screens. This design represents a new approach to fabricating high-performance X-ray imaging scintillators based on organic metal halides for applications in medical radiography and security screening.
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Rationalizing the Influence of Tunable Energy Levels on Quantum Efficiency to Design Optimal Non-Fullerene Acceptor-Based Ternary Organic Solar Cells(Advanced Energy Materials, Wiley, 2023-03-09) [Article]Non-fullerene acceptor (NFA)-based ternary bulk heterojunction solar cells (TSC) are the most efficient organic solar cells (OSCs) today due to their broader absorption and quantum efficiencies (QE) often surpassing those of corresponding binary blends. The impact on QE of the energetics driving charge transfer at the electron donor:electron acceptor (D/A) interfaces is studied in blends of PBDB-T-2F donor with several pairs of lower bandgap NFAs. As in binary blends, the ionization energy offset between donor and acceptor (ΔIE) controls the QE and maximizes for ΔIE > 0.5 eV. However, ΔIE is not controlled by the individual NFAs IEs but by their average, weighted for their blending ratio. Using this property, the QE of a PBDB-T-2F:IEICO binary blend that has an insufficient ΔIE for charge generation is improved by adding a deep IE third component: IT-4F. Combining two NFAs enables to optimize the D/A energy alignment and cells’ QE without molecular engineering.
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Two-dimensional borocarbonitrides for photocatalysis and photovoltaics(Journal of Materials Chemistry C, Royal Society of Chemistry (RSC), 2023-03-08) [Article]We have designed two-dimensional borocarbonitrides (poly-butadiene-cyclooctatetraene framework BC2N) with hexagonal unit cells, which are stable according to the cohesive energy, phonon dispersion, ab initio molecular dynamics, and elastic modulus results. They are n-type semiconductors with strain-tunable direct band gaps (1.45–2.20 eV), an ultrahigh electron mobility (5.2 × 104 cm2 V−1 s−1 for β-BC2N), and strong absorption (an absorption coefficient of up to 105 cm−1). The intrinsic electric field due to the Janus geometry of α-BC2N reduces the recombination of photo-generated carriers. The band edge positions of α-BC2N and β-BC2N are suitable for photocatalytic hydrogen production, achieving high solar-to-hydrogen efficiencies of 17% and 12%, respectively, in excess of the typical target value of 10% for industrial application. Both γ-BC2N and δ-BC2N can be used as electron donors in type-II heterostructures with two-dimensional transition metal dichalcogenides, and the power conversion efficiency of a solar cell based on these heterostructures can be as high as 21%, approaching the performance of perovskite-based solar cells.
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Salts as Additives: A Route to Improve Performance and Stability of n-Type Organic Electrochemical Transistors(ACS Materials Au, American Chemical Society (ACS), 2023-03-06) [Article]Organic electrochemical transistors (OECTs) are becoming increasingly ubiquitous in various applications at the interface with biological systems. However, their widespread use is hampered by the scarcity of electron-conducting (n-type) backbones and the poor performance and stability of the existing n-OECTs. Here, we introduce organic salts as a solution additive to improve the transduction capability, shelf life, and operational stability of n-OECTs. We demonstrate that the salt-cast devices present a 10-fold increase in transconductance and achieve at least one year-long stability, while the pristine devices degrade within four months of storage. The salt-added films show improved backbone planarity and greater charge delocalization, leading to higher electronic charge carrier mobility. These films show a distinctly porous morphology where the interconnectivity is affected by the salt type, responsible for OECT speed. The salt-based films display limited changes in morphology and show lower water uptake upon electrochemical doping, a possible reason for the improved device cycling stability. Our work provides a new and easy route to improve n-type OECT performance and stability, which can be adapted for other electrochemical devices with n-type films operating at the aqueous electrolyte interface.
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Recent Progress in Colloidal Quantum Dot Thermoelectrics(Advanced Materials, Wiley, 2023-03-01) [Article]Semiconducting colloidal quantum dots (CQDs) represent an emerging class of thermoelectric materials for use in a wide range of future applications. CQDs combine solution processability at low temperatures with the potential for upscalable manufacturing via printing techniques. Moreover, due to their low dimensionality, CQDs exhibit quantum confinement and a high density of grain boundaries, which can be independently exploited to tune the Seebeck coefficient and thermal conductivity, respectively. This unique combination of attractive attributes makes CQDs very promising for application in emerging thermoelectric generator (TEG) technologies operating near room temperature. Herein, we review recent progress in CQDs for application in emerging thin-film thermoelectrics. We start by outlining the fundamental concepts of thermoelectricity in nanostructured materials, followed by an overview of the popular synthetic methods used to produce CQDs with controllable size and shape. Recent strides in CQD-based thermoelectrics are then discussed with particular emphasis on their application in thin-film TEGs. Finally, we highlight the current challenges and future perspectives in enhancing the performance of CQD-based thermoelectric materials for use in emerging applications.
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Preferential Pyrolysis Construction of Carbon Anodes with 8400 h Lifespan for High-Energy-Density K-ion Batteries.(Angewandte Chemie (International ed. in English), Wiley, 2023-03-01) [Article]Carbonaceous materials are promising anodes for practical potassium-ion batteries, but fail to meet the requirements for durability and high capacities at low potentials. Herein, we constructed a durable carbon anode for high-energy-density K-ion full cells by a preferential pyrolysis strategy. Utilizing S and N volatilization from a π-π stacked supermolecule, the preferential pyrolysis process introduces low-potential active sites of sp2 hybridized carbon and carbon vacancies, endowing a low-potential "vacancy-adsorption/intercalation" mechanism. The as-prepared carbon anode exhibits a high capacity of 384.2 mAh g-1 (90% capacity locates below 1 V vs. K/K+), which contributes to a high energy density of 163 Wh kg-1 of K-ion full battery. Moreover, abundant vacancies of carbon alleviate volume variation, boosting the cycling stability over 14,000 cycles (8,400 h). Our work provides a new synthesis approach for durable carbon anodes of K-ion full cells with high energy densities.
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A photo-responsive organic electrochemical transistor(JOURNAL OF MATERIALS CHEMISTRY C, Royal Society of Chemistry (RSC), 2023-02-28) [Article]The design of novel organic electrochemical transistor (OECT) channel materials that can be controlled by a whole range of external stimuli is key towards the emergence of unprecedented technologies in bioelectronics. Like the established multiresponsive field-effect transistors, multiresponsive OECTs can in principle be realised via blending, by combining multiple components with each one imparting a specific function to the device. Here we report the first example of an optically switchable OECT which is capable of undergoing a reversible modulation of its ON current by up to 30% upon irradiation with UV and visible light. By investigating the electrical characteristics of the channel material, in conjunction with the electronic characterisation performed by a macroscopic Kelvin probe technique and photoemission yield spectroscopy in air, we gained distinct insight into the electrochemical doping process occurring within the blend upon light irradiation. Such a proof-of-concept work opens perspectives towards the implementation of complex neuromorphic operations and algorithms in OECTs.
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(LaCrO3)m/SrCrO3 superlattices as transparent p-type semiconductors with finite magnetization(Nanoscale Advances, Royal Society of Chemistry (RSC), 2023-02-27) [Article]The electronic and magnetic properties of (LaCrO3)m/SrCrO3 superlattices are investigated using first principles calculations. We show that the magnetic moments in the two CrO2 layers sandwiching the SrO layer compensate each other for even m but give rise to a finite magnetization for odd m, which is explained by charge ordering with Cr3+ and Cr4+ ions arranged in a checkerboard pattern. The Cr4+ ions induce in-gap hole states at the interface, implying that the transparent superlattices are p-type semiconductors. The availability of transparent p-type semiconductors with finite magnetization enables the fabrication of transparent magnetic diodes and transistors, for example, with a multitude of potential technological applications.
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Closed-loop Error Correction Learning Accelerates Experimental Discovery of Thermoelectric Materials(arXiv, 2023-02-26) [Preprint]The exploration of thermoelectric materials is challenging considering the large materials space, combined with added exponential degrees of freedom coming from doping and the diversity of synthetic pathways. Here we seek to incorporate historical data and update and refine it using experimental feedback by employing error-correction learning (ECL). We thus learn from prior datasets and then adapt the model to differences in synthesis and characterization that are otherwise difficult to parameterize. We then apply this strategy to discovering thermoelectric materials where we prioritize synthesis at temperatures < 300{\deg}C. We document a previously unreported chemical family of thermoelectric materials, PbSe:SnSb, finding that the best candidate in this chemical family, 2 wt% SnSb doped PbSe, exhibits a power factor more than 2x that of PbSe. Our investigations show that our closed-loop experimentation strategy reduces the required number of experiments to find an optimized material by as much as 3x compared to high-throughput searches powered by state-of-the-art machine learning models. We also observe that this improvement is dependent on the accuracy of prior in a manner that exhibits diminishing returns, and after a certain accuracy is reached, it is factors associated with experimental pathways that dictate the trends.
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A comparison of para, meta, and ortho-carborane centred non-fullerene acceptors for organic solar cells(Journal of Materials Chemistry C, Royal Society of Chemistry (RSC), 2023-02-25) [Article]We report the first examples of carborane-containing non-fullerene acceptors (NFAs), and their use in organic photovoltaic (OPV) devices. NFAs employing an A–D–A′–D–A type design centred around a central electron withdrawing carborane unit (A′), using either para, meta, or ortho-carborane isomers are reported. We demonstrate that the nature of the isomer has a major impact on device performance, despite minor differences in optoelectronic and morphological properties, with the use of ortho-carborane resulting in the highest device efficiencies. We further show that end-group fluorination is an efficient strategy to modulate energy levels and improve device performance of such NFAs.
