Steering lithium and potassium storage mechanism in covalent organic frameworks by incorporating transition metal single atoms.

Organic electrodes mainly consisting of C, O, H, and N are promising candidates for advanced batteries. However, the sluggish ionic and electronic conductivity limit the full play of their high theoretical capacities. Here, we integrate the idea of metal-support interaction in single-atom catalysts with π-d hybridization into the design of organic electrode materials for the applications of lithium (LIBs) and potassium-ion batteries (PIBs). Several types of transition metal single atoms (e.g., Co, Ni, Fe) with π-d hybridization are incorporated into the semiconducting covalent organic framework (COF) composite. Single atoms favorably modify the energy band structure and improve the electronic conductivity of COF. More importantly, the electronic interaction between single atoms and COF adjusts the binding affinity and modifies ion traffic between Li/K ions and the active organic units of COFs as evidenced by extensive in situ and ex situ characterizations and theoretical calculations. The corresponding LIB achieves a high reversible capacity of 1,023.0 mA h g-1 after 100 cycles at 100 mA g-1 and 501.1 mA h g-1 after 500 cycles at 1,000 mA g-1. The corresponding PIB delivers a high reversible capacity of 449.0 mA h g-1 at 100 mA g-1 after 150 cycles and stably cycled over 500 cycles at 1,000 mA g-1. This work provides a promising route to engineering organic electrodes.

Dopant-Free All-Back-Contact Si Nanohole Solar Cells Using MoOx and LiF Films.

We demonstrate novel all-back-contact Si nanohole solar cells via the simple direct deposition of molybdenum oxide (MoOx) and lithium fluoride (LiF) thin films as dopant-free and selective carrier contacts (SCCs). This approach is in contrast to conventionally used high-temperature thermal doping processes, which require multistep patterning processes to produce diffusion masks. Both MoOx and LiF thin films are inserted between the Si absorber and Al electrodes interdigitatedly at the rear cell surfaces, facilitating effective carrier collection at the MoOx/Si interface and suppressed recombination at the Si and LiF/Al electrode interface. With optimized MoOx and LiF film thickness as well as the all-back-contact design, our 1 cm(2) Si nanohole solar cells exhibit a power conversion efficiency of up to 15.4%, with an open-circuit voltage of 561 mV and a fill factor of 74.6%. In particular, because of the significant reduction in Auger/surface recombination as well as the excellent Si-nanohole light absorption, our solar cells exhibit an external quantum efficiency of 83.4% for short-wavelength light (∼400 nm), resulting in a dramatic improvement (54.6%) in the short-circuit current density (36.8 mA/cm(2)) compared to that of a planar cell (23.8 mA/cm(2)). Hence, our all-back-contact design using MoOx and LiF films formed by a simple deposition process presents a unique opportunity to develop highly efficient and low-cost nanostructured Si solar cells.

Unlocking the Potential of Bi2S3-Derived Bi Nanoplates: Enhanced Catalytic Activity and Selectivity in Electrochemical and Photoelectrochemical CO2 Reduction to Formate.

Various electrocatalysts are extensively examined for their ability to selectively produce desired products by electrochemical CO2 reduction reaction (CO2RR). However, an efficient CO2RR electrocatalyst doesn't ensure an effective co-catalyst on the semiconductor surface for photoelectrochemical CO2RR. Herein, Bi2S3 nanorods are synthesized and electrochemically reduced to Bi nanoplates that adhere to the substrates for application in the electrochemical and photoelectrochemical CO2RR. Compared with commercial-Bi, the Bi2S3-derived Bi (S-Bi) nanoplates on carbon paper exhibit superior electrocatalytic activity and selectivity for formate (HCOO-) in the electrochemical CO2RR, achieving a Faradaic efficiency exceeding 93%, with minimal H2 production over a wide potential range. This highly selective S-Bi catalyst is being employed on the Si photocathode to investigate the behavior of electrocatalysts during photoelectrochemical CO2RR. The strong adhesion of the S-Bi nanoplates to the Si nanowire substrate and their unique catalytic properties afford exceptional activity and selectivity for HCOO- under simulated solar irradiation. The selectivity observed in electrochemical CO2RR using the S-Bi catalyst correlates with that seen in the photoelectrochemical CO2RR system. Combined pulsed potential methods and theoretical analyses reveal stabilization of the OCHO* intermediate on the S-Bi catalyst under specific conditions, which is critical for developing efficient catalysts for CO2-to-HCOO- conversion.

Scaling-Up Insights for Zinc-Air Battery Technologies Realizing Reversible Zinc Anodes.

Zinc-air battery (ZAB) technology is considered one of the promising candidates to complement the existing lithium-ion batteries for future large-scale high-energy-storage demands. The scientific literature reveals many efforts for the ZAB chemistries, materials design, and limited accounts for cell design principles with apparently superior performances for liquid and solid-state electrolytes. However, along with the difficulty of forming robust solid-electrolyte interphases, the discrepancy in testing methods and assessment metrics severely challenges the realistic evaluation/comparison and commercialization of ZABs. Here, strategies to formulate reversible zinc anodes are proposed and specific cell-level energy metrics (100-500 Wh kg-1 ) and realistic long-cycling operations are realized. Stabilizing anode/electrolyte interfaces results in a cumulative capacity of 25 Ah cm-2 and Coulomb efficiency of >99.9% for 5000 plating/stripping cycles. Using 1-10 Ah scale (≈500 Wh kg-1 at cell level) solid-state zinc-air pouch cells, scale-up insights for Ah-level ZABs that can progress from lab-scale research to practical production are also offered.

Heterointerface promoted trifunctional electrocatalysts for all temperature high-performance rechargeable Zn-air batteries.

The rational design of wide-temperature operating Zn-air batteries is crucial for their practical applications. However, the fundamental challenges remain; the limitation of the sluggish oxygen redox kinetics, insufficient active sites, and poor efficiency/cycle lifespan. Here we present heterointerface-promoted sulfur-deficient cobalt-tin-sulfur (CoS1-δ/SnS2-δ) trifunctional electrocatalysts by a facile solvothermal solution-phase approach. The CoS1-δ/SnS2-δ displays superb trifunctional activities, precisely a record-level oxygen bifunctional activity of 0.57 V (E1/2 = 0.90 V and Ej=10 = 1.47 V) and a hydrogen evolution overpotential (41 mV), outperforming those of Pt/C and RuO2. Theoretical calculations reveal the modulation of the electronic structures and d-band centers that endorse fast electron/proton transport for the hetero-interface and avoid the strong adsorption of intermediate species. The alkaline Zn-air batteries with CoS1-δ/SnS2-δ manifest record-high power density of 249 mW cm-2 and long-cycle life for >1000 cycles under harsh operations of 20 mA cm-2, surpassing those of Pt/C + RuO2 and previous state-of-the-art catalysts. Furthermore, the solid-state flexible Zn-air battery also displays remarkable performance with an energy density of 1077 Wh kg-1, >690 cycles for 50 mA cm-2, and a wide operating temperature from +80 to -40 °C with 85% capacity retention, which provides insights for practical Zn-air batteries.

The tradeoff between plasmonic enhancement and optical loss in silicon nanowire solar cells integrated in a metal back reflector.

We perform a systematic numerical study to characterize the tradeoff between the plasmonic enhancement and optical loss in periodically aligned, silicon nanowire (SiNW) arrays integrated with a silver back reflector (Ag BR). Optimizing the embedded depth of the wire bottoms into a silver reflector achieved a highly efficient SiNW solar cell. Compared to the SiNW solar cell employing a flat back reflector, the embedded depth of ~20 nm resulted in the relative increase of ~5% in ultimate solar cell efficiency.

A stamped PEDOT:PSS-silicon nanowire hybrid solar cell.

A novel stamped hybrid solar cell was proposed using the stamping transfer technique by stamping an active PEDOT:PSS thin layer onto the top of silicon nanowires (SiNWs). Compared to a bulk-type counterpart that fully embeds SiNWs inside PEDOT:PSS, an increase in the photovoltaic efficiency was observed by a factor of ∼4.6, along with improvements in both electrical and optical responses for the stamped hybrid cell. Such improvements for hybrid cells was due to the formation of well-connected and linearly aligned active PEDOT:PSS channels at the top ends of the nanowires after the stamping process. These stamped channels facilitated not only to improve the charge transport, light absorption, but also to decrease the free carriers as well as exciton recombination losses for stamped hybrid solar cells.

Supramolecular Polymer Intertwined Free-Standing Bifunctional Membrane Catalysts for All-Temperature Flexible Zn-Air Batteries.

Rational construction of flexible free-standing electrocatalysts featuring long-lasting durability, high efficiency, and wide temperature tolerance under harsh practical operations are fundamentally significant for commercial zinc-air batteries. Here, 3D flexible free-standing bifunctional membrane electrocatalysts composed of covalently cross-linked supramolecular polymer networks with nitrogen-deficient carbon nitride nanotubes are fabricated (referred to as PEMAC@NDCN) by a facile self-templated approach. PEMAC@NDCN demonstrates the lowest reversible oxygen bifunctional activity of 0.61 V with exceptional long-lasting durability, which outperforms those of commercial Pt/C and RuO2. Theoretical calculations and control experiments reveal the boosted electron transfer, electrolyte mass/ion transports, and abundant active surface site preferences. Moreover, the constructed alkaline Zn-air battery with PEMAC@NDCN air-cathode reveals superb power density, capacity, and discharge-charge cycling stability (over 2160 cycles) compared to the reference Pt/C + RuO2. Solid-state Zn-air batteries enable a high power density of 211 mW cm-2, energy density of 1056 Wh kg-1, stable charge-discharge cycling of 2580 cycles for 50 mA cm-2, and wide temperature tolerance from - 40 to 70 °C with retention of 86% capacity compared to room-temperature counterparts, illustrating prospects over harsh operations.

Optical properties of Si microwires combined with nanoneedles for flexible thin film photovoltaics.

A combined wire structure, made up of longer periodic Si microwires and short nanoneedles, was prepared to enhance light absorption using one-step plasma etching via lithographical patterning. The combined wire array exhibited light absorption of up to ~97.6% from 300 to 1100 nm without an anti-reflection coating. These combined wire arrays on a Si substrate were embedded into a transparent polymer. A large-scale wire-embedded soft film was then obtained by peeling the polymer-embedded wire portion from the substrate. Optically attractive features were present in these soft films, making them suitable for use in flexible silicon solar cell applications.

Effective method to extract optical bandgaps in Si nanowire arrays.

A simple method to extract the optical bandgap of Si nanowire (SiNW) arrays that utilizes the reflection spectra of freestanding SiNW arrays is presented in this Letter. At a fixed nanowire diameter, three different wire lengths reproducibly formed a cross point in their reflectance curve plots. The cross point wavelength corresponded to the optical bandgap, as verified by the classical Tauc's model. The optical bandgap of the SiNW arrays (112 nm in average diameter) was measured to be ~1.19 eV, which is larger than the ~1.08 eV bandgap of bulk Si. Further decreasing the wire diameter to 68 nm caused an increase of the bandgap to ~1.24 eV, which is closer to the optimal bandgap (~1.40 eV) required to achieve the highest conversion efficiency in single-junction photovoltaic devices. Our method suggests that the multijunction tandem structure can be realized via control of the diameter of SiNW arrays.

Epitaxial insertion of gold silicide nanodisks during the growth of silicon nanowires.

Nanodisk-shaped, single-crystal gold silicide heterojunctions were inserted into silicon nanowires during vapor-liquid-solid growth using Au as a catalyst within a specific range of chlorine-to-hydrogen atomic ratio. The mechanism of nanodisk formation has been investigated by changing the source gas ratio of SiCl4 to H2. We report that an over-supply of silicon into the Au-Si liquid alloy leads to highly supersaturated solution and enhances the precipitation of Au in the silicon nanowires due to the formation of unstable phases within the liquid alloy. It is shown that the gold precipitates embedded in the silicon nanowires consisted of a metastable gold silicide. Interestingly, faceting of gold silicide was observed at the Au/Si interfaces, and silicon nanowires were epitaxially grown on the top of the nanodisk by vapor-liquid-solid growth. High resolution transmission electron microscopy confirmed that gold silicide nanodisks are epitaxially connected to the silicon nanowires in the direction of growth direction. These gold silicide nanodisks would be useful as nanosized electrical junctions for future applications in nanowire interconnections.

Silicon Microwire Arrays with Nanoscale Spacing for Radial Junction c-Si Solar Cells with an Efficiency of 20.5.

Structural optimization of microwire arrays is important for the successful demonstration of the practical feasibility of radial junction crystalline silicon (c-Si) solar cells. In this study, we investigated an optimized design of tapered microwire (TMW) arrays to maximize the light absorption of c-Si solar cells, while minimizing the surface recombination, for simultaneously improving the open-circuit voltage and short-circuit current density (Jsc). Finite-difference time-domain simulations confirmed that controlling the spacing between the TMWs at the nanometer scale is more effective for increasing the light absorption than increasing the TMW length. The photogenerated current of a c-Si TMW array with a 200 nm spacing was calculated to be 42.90 mA/cm2, which is close to the theoretical limit of 43.37 mA/cm2 in the 300-1100 nm wavelength range. To experimentally demonstrate the TMW arrays with a nanometer-scale spacing of 200 nm, which cannot be realized by conventional photolithography, we utilized a soft lithography method based on polystyrene beads for patterning a c-Si wafer. The solar cells based on optimized TMW arrays exhibited a Jsc of 42.5 mA/cm2 and power conversion efficiency of 20.5%, which exceed those of the previously reported microwire-based radial junction solar cells.

A strong antireflective solar cell prepared by tapering silicon nanowires.

Vertically aligned silicon nanowires (SiNWs) were cost-effectively formed on a four-inch silicon wafer using a simple room temperature approach, i.e., metal-assisted electroless etching. Tapering the NWs by post-KOH dipping achieved separation of each NW from the bunched NW, resulting in a strong enhancement of broadband optical absorption. As electroless etching time increases, the optical crossover feature was observed in the tradeoff between enhanced light trapping (by graded-refractive index during initial tapering) and deteriorated reflectance (by decreasing the areal density of NWs during later tapering). Compared to the bunched SiNWs, tapered NW solar cells demonstrated superior photovoltaic characteristics, such as a short circuit current of 17.67 mA/cm² and a cell conversion efficiency of ~6.56% under 1.5 AM illumination.

A waferscale Si wire solar cell using radial and bulk p-n junctions.

Silicon nanowires (NWs) and microwires (MWs) are cost-effectively integrated on a 4-inch wafer using metal-assisted electroless etching for solar cell applications. MWs are periodically positioned using low-level optical patterning in between a dense array of NWs. A spin-on-doping technique is found to be effective for the formation of heavily doped, thin n-type shells of MWs in which the radial doping profile is easily delineated by low voltage scanning electron microscopy. Controlled tapering of the NWs results in additional optical enhancement via optimization of the tradeoff between increased light trapping (by a graded-refractive-index) and increased reflectance (by decreasing areal density of NWs). Compared to single NW (or MW) arrayed cells, the co-integrated solar cells demonstrate improved photovoltaic characteristics, i.e. a short circuit current of 20.59 mA cm(-2) and a cell conversion efficiency of ∼ 7.19% at AM 1.5G illumination.

Electrostatically Doped Silicon Nanowire Arrays for Multispectral Photodetectors.

Nanowires have promising applications as photodetectors with superior ability to tune absorption with morphology. Despite their high optical absorption, the quantum efficiencies of these nanowire photodetectors remain low due to difficulties in fabricating a shallow junction using traditional doping methods. As an alternative, we report nonconventional radial heterojunction photodiodes obtained by conformal coating of an indium oxide layer on silicon nanowire arrays. The indium oxide layer has a high work function which induces a strong inversion in the silicon nanowire and creates a virtual p-n junction. The resulting nanowire photodetectors show efficient carrier separation and collection, leading to an improvement of quantum efficiency up to 0.2. In addition, by controlling the nanowire radii, the spectral responses of the In2O3/Si nanowire photodetectors are tuned over several visible light wavelengths, creating a multispectral detector. Our approach is promising for the development of highly efficient wavelength-selective photodetectors.

Phosphorescent Energy Downshifting for Diminishing Surface Recombination in Silicon Nanowire Solar Cells.

Molecularly engineered Ir(III) complexes can transfer energy from short-wavelength photons (λ < 450 nm) to photons of longer wavelength (λ > 500 nm), which can enhance the otherwise low internal quantum efficiency (IQE) of crystalline Si (c-Si) nanowire solar cells (NWSCs) in the short-wavelength region. Herein, we demonstrate a phosphorescent energy downshifting system using Ir(III) complexes at short wavelengths (300-450 nm) to diminish the severe surface recombination that occurs in c-Si NWSCs. The developed Ir(III) complexes can be considered promising energy converters because they exhibit superior intrinsic properties such as a high quantum yield, a large Stokes shift, a long exciton diffusion length in crystalline film, and a reproducible synthetic procedure. Using the developed Ir(III) complexes, highly crystalline energy downshifting layers were fabricated by ultrasonic spray deposition to enhance the photoluminescence efficiency by increasing the radiative decay. With the optimized energy downshifting layer, our 1 cm2 c-Si NWSCs with Ir(III) complexes exhibited a higher IQE value for short-wavelength light (300-450 nm) compared with that of bare Si NWSCs without Ir(III) complexes, resulting in a notable increase in the short-circuit current density (from 34.4 mA·cm-2 to 36.5 mA·cm-2).

Embedded Metal Electrode for Organic-Inorganic Hybrid Nanowire Solar Cells.

We demonstrate here an embedded metal electrode for highly efficient organic-inorganic hybrid nanowire solar cells. The electrode proposed here is an effective alternative to the conventional bus and finger electrode which leads to a localized short circuit at a direct Si/metal contact and has a poor collection efficiency due to a nonoptimized electrode design. In our design, a Ag/SiO2 electrode is embedded into a Si substrate while being positioned between Si nanowire arrays underneath poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), facilitating suppressed recombination at the Si/Ag interface and notable improvements in the fabrication reproducibility. With an optimized microgrid electrode, our 1 cm2 hybrid solar cells exhibit a power conversion efficiency of up to 16.1% with an open-circuit voltage of 607 mV and a short circuit current density of 34.0 mA/cm2. This power conversion efficiency is more than twice as high as that of solar cells using a conventional electrode (8.0%). The microgrid electrode significantly minimizes the optical and electrical losses. This reproducibly yields a superior quantum efficiency of 99% at the main solar spectrum wavelength of 600 nm. In particular, our solar cells exhibit a significant increase in the fill factor of 78.3% compared to that of a conventional electrode (61.4%); this is because of the drastic reduction in the metal/contact resistance of the 1 µm-thick Ag electrode. Hence, the use of our embedded microgrid electrode in the construction of an ideal carrier collection path presents an opportunity in the development of highly efficient organic-inorganic hybrid solar cells.

Cold Isostatic-Pressured Silver Nanowire Electrodes for Flexible Organic Solar Cells via Room-Temperature Processes.

Transparent conducting electrodes (TCEs) are considered to be an essential structural component of flexible organic solar cells (FOSCs). Silver nanowire (AgNW) electrodes are widely used as TCEs owing to their excellent electrical and optical properties. The fabrication of AgNW electrodes has faced challenges in terms of forming large uniform interconnected networks so that high conductivity and reproducibility can be achieved. In this study, a simple method for creating an intimate contact between AgNWs that uses cold isostatic pressing (CIP) is demonstrated. This method increases the conductivity of the AgNW electrodes, which enables the fabrication of high-efficiency inverted FOSCs that have a power conversion efficiency of 8.75% on flexible polyethylene terephthalate with no short circuiting occurring as the CIP process minimizes the surface roughness of the AgNW electrode. This allows to achieve 100% manufacturing yield of FOSCs. Furthermore, these highly efficient FOSCs are proven to only be 2.4% less efficient even for an extreme bending radius of R ≈ 1.5 mm, compared with initial efficiency.

18.4%-Efficient Heterojunction Si Solar Cells Using Optimized ITO/Top Electrode.

We optimize the thickness of a transparent conducting oxide (TCO) layer, and apply a microscale mesh-pattern metal electrode for high-efficiency a-Si/c-Si heterojunction solar cells. A solar cell equipped with the proposed microgrid metal electrode demonstrates a high short-circuit current density (JSC) of 40.1 mA/cm(2), and achieves a high efficiency of 18.4% with an open-circuit voltage (VOC) of 618 mV and a fill factor (FF) of 74.1% as result of the shortened carrier path length and the decreased electrode area of the microgrid metal electrode. Furthermore, by optimizing the process sequence for electrode formation, we are able to effectively restore the reduction in VOC that occurs during the microgrid metal electrode formation process. This work is expected to become a fundamental study that can effectively improve current loss in a-Si/c-Si heterojunction solar cells through the optimization of transparent and metal electrodes.

17.6%-Efficient radial junction solar cells using silicon nano/micro hybrid structures.

We developed a unique nano- and microwire hybrid structure by selectively modifying only the tops of microwires using metal-assisted chemical etching. The proposed nano/micro hybrid structure not only minimizes surface recombination but also absorbs 97% of incident light under AM 1.5G illumination, demonstrating outstanding light absorption compared to that of planar (59%) and microwire arrays (85%). The proposed hybrid solar cells with an area of 1 cm(2) exhibit power conversion efficiencies (Eff) of up to 17.6% under AM 1.5G illumination. In particular, the solar cells show a high short-circuit current density (Jsc) of 39.5 mA cm(-2) because of the high light-absorbing characteristics of the nanostructures. This corresponds to an approximately 61.5% and 16.5% increase in efficiency compared to that of a planar silicon solar cell (Eff = 10.9%) and a microwire solar cell (Eff = 15.1%), respectively. Therefore, we expect the proposed hybrid structure to become a foundational technology for the development of highly efficient radial junction solar cells.