Xuanhua Li

Dual Plasmonic Nanostructures for High Performance Inverted Organic Solar Cells

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

Carbon Nanotube–Multilayered Graphene Edge Plane Core–Shell Hybrid Foams for Ultrahigh‐Performance Electromagnetic‐Interference Shielding

Materials with an ultralow density and ultrahigh electromagnetic-interference (EMI)-shielding performance are highly desirable in fields of aerospace, portable electronics, and so on. Theoretical work predicts that 3D carbon nanotube (CNT)/graphene hybrids are one of the most promising lightweight EMI shielding materials, owing to their unique nanostructures and extraordinary electronic properties. Herein, for the first time, a lightweight, flexible, and conductive CNT-multilayered graphene edge plane (MLGEP) core-shell hybrid foam is fabricated using chemical vapor deposition. MLGEPs are seamlessly grown on the CNTs, and the hybrid foam exhibits excellent EMI shielding effectiveness which exceeds 38.4 or 47.5 dB in X-band at 1.6 mm, while the density is merely 0.0058 or 0.0089 g cm-3 , respectively, which far surpasses the best values of reported carbon-based composite materials. The grafted MLGEPs on CNTs can obviously enhance the penetration losses of microwaves in foams, leading to a greatly improved EMI shielding performance. In addition, the CNT-MLGEP hybrids also exhibit a great potential as nano-reinforcements for fabricating high-strength polymer-based composites. The results provide an alternative approach to fully explore the potentials of CNT and graphene, for developing advanced multifunctional materials.

Polarization-independent efficiency enhancement of organic solar cells by using 3-dimensional plasmonic electrode

Plasmonic back reflectors have recently become a promising strategy for realizing efficient organic solar cell (OSCs). Since plasmonic effects are strongly sensitive to light polarization, it is highly desirable to simultaneously achieve polarization-independent response and enhanced power conversion efficiency (PCE) by designing the nanostructured geometry of plasmonic reflector electrode. Here, through a strategic analysis of 2-dimensional grating (2D) and 3-dimensional patterns (3D), with similar periodicity as a plasmonic back reflector, we find that the OSCs with 3D pattern achieve the best PCE enhancement by 24.6%, while the OSCs with 2D pattern can offer 17.5% PCE enhancement compared to the optimized control OSCs. Importantly, compared with the 2D pattern, the 3D pattern shows a polarization independent plasmonic response, which will greatly extend its uses in photovoltaic applications. This work shows the significances of carefully selecting and designing geometry of plasmonic nanostructures in a...

Au NPs@MoS2 Sub‐Micrometer Sphere‐ZnO Nanorod Hybrid Structures for Efficient Photocatalytic Hydrogen Evolution with Excellent Stability

MoS2 shows promising applications in photocatalytic water splitting, owing to its uniquely optical and electric properties. However, the insufficient light absorption and lack of performance stability are two crucial issues for efficient application of MoS2 nanomaterials. Here, Au nanoparticles (NPs)@MoS2 sub-micrometer sphere-ZnO nanorod (Au NPs@MoS2 -ZnO) hybrid photocatalysts have been successfully synthesized by a facile process combining the hydrothermal method and seed-growth method. Such photocatalysts exhibit high efficiency and excellent stability for hydrogen production via multiple optical-electrical effects. The introduction of Au NPs to MoS2 sub-micrometer spheres forming a core-shell structure demonstrates strong plasmonic absorption enhancement and facilitates exciton separation. The incorporation of ZnO nanorods to the Au NPs@MoS2 hybrids further extends the light absorption to a broader wavelength region and enhances the exciton dissociation. In addition, mutual contacts between Au NPs (or ZnO nanorods) and the MoS2 spheres effectively protect the MoS2 nanosheets from peeling off from the spheres. More importantly, efficiently multiple exciton separations help to restrain the MoS2 nanomaterials from photocorrosion. As a result, the Au@MoS2 -ZnO hybrid structures exhibit an excellent hydrogen gas evolution (3737.4 μmol g-1 ) with improved stability (91.9% of activity remaining) after a long-time test (32 h), which is one of the highest photocatalytic activities to date among the MoS2 based photocatalysts.

Breaking the Space Charge Limit in Organic Solar Cells by a Novel Plasmonic-Electrical Concept

As a fundamental electrostatic limit, space charge limit (SCL) for photocurrent is a universal phenomenon and of paramount importance for organic semiconductors with unbalanced photocarriers mobility and high exciton generation. Here we proposed a new plasmonic-electrical concept to manipulate electrical properties of organic devices including photocarriers recombination, transport and collection. As a proof-of-concept, organic solar cells (OSCs) comprising metallic planar and grating electrodes are systematically investigated with normal and inverted device structures. Interestingly, although strong plasmonic resonances induce abnormally dense photocarriers around a grating anode, the grating-inverted OSC is exempt from space charge accumulation (limit) and degradation of electrical properties in contrast to the planar-inverted and planar-normal ones. The particular reason is that plasmonically induced photocarriers redistribution shortens the transport path of low-mobility holes, which are collected by the grating anode. The work demonstrated and explained the SCL breaking with the plasmonic-electrical effect. Most importantly, the plasmonic-electrical concept will open up a new way to manipulate both optical and electrical properties of semiconductor devices simultaneously.

The nanotipped hairs of gecko skin and biotemplated replicas impair and/or kill pathogenic bacteria with high efficiency

We show that gecko microspinules (hairs) and their equivalent replicas, bearing nanoscale tips, can kill or impair surface associating oral pathogenic bacteria with high efficiency even after 7 days of repeated attacks. Scanning Electron Microscopy suggests that there is more than one mechanism contributing to cell death which appears to be related to the scaling of the bacteria type with the hair arrays and accessibility to the underlying nano-topography of the hierarchical surfaces.

Multi-Physical Properties of Plasmonic Organic Solar Cells

(Invited Paper) Abstract|Organic solar cells (OSCs) have recently attracted considerable research interest. For typical OSCs, it is highly desirable to have optically thick and physically thin thickness for strong light absorption and e-cient carrier collection respectively. In the meantime, most organic semiconductors have short exciton difiusion length and low carrier mobility (1{3). As a consequence, the active layers of OSCs are generally thin with a thickness of a few hundred nanometers to ensure the e-cient extraction of carriers, hence limiting the total absorption of incident light. Optimizing both the optical and electrical (i.e., multi-physical) properties of OSCs is in demands for rationally designed device architectures. Plasmonic nanomaterials (e.g., metallic nanoparticles (4{6), nanorods (7,8), nanoprisms (9,10), etc.) have recently been introduced into difierent layers of multilayered solar cells to achieve highly e-cient light harvesting. The multilayered solar cells structures commonly have active layer, carrier (electron and hole) transport layer and electrode (anode and cathode). Through the localized plasmonic resonances (LPRs) (11{16) from metallic nanomaterials, very strong near-flelds will be generated, which can provide a large potential for enhancing optical absorption in the multilayered OSCs. Besides the optical efiects, it has been reported that metallic nanomaterials can modify the morphology, interface properties as well as the electrical properties of OSCs which will signiflcantly modify the performances of OSCs (17{23). In this article, the efiects of various optical resonance mechanisms and the theoretical studies of the multi- physical properties of OSCs will be reviewed. Meanwhile, the experimental optical and electrical efiects of metallic nanomaterials incorporated in difierent layers of OSCs will be studied. The morphology and interface efiects of metallic nanomaterials in the carrier transport layers on the performances of OSCs will also be described.

Facile preparation of a SiO2–Al2O3 aerogel using coal gangue as a raw material via an ambient pressure drying method and its application in organic solvent adsorption

Because of its outstanding properties such as a large specific area and Bronsted acidity, the SiO2–Al2O3 aerogel has been used for various applications such as adsorption, catalysis, and in synthetic chemistry. Regarding preparation of the SiO2–Al2O3 aerogel, however, expensive raw materials are always used. In addition, the traditional preparation method is supercritical drying, which is complex and has safety issues. Here, we successfully prepare a SiO2–Al2O3 aerogel utilizing coal gangue as a raw material, which is a zero-cost mining waste. More importantly, ambient pressure drying used in this work overcomes the drawbacks of supercritical drying. Characterizations show that the specific surface area of the SiO2–Al2O3 aerogel prepared from coal gangue can be comparable to that prepared from relative costly raw materials such as Al(NO3)3·9H2O or Al isopropoxide and tetraethoxysilane with supercritical drying conditions. In addition, adsorption tests show that the SiO2–Al2O3 aerogel demonstrates good adsorbability of an organic solvent. Our work provides an effective route to synthesize a SiO2–Al2O3 aerogel in mass quantities and opens a new direction for the comprehensive utilization of coal gangue.

Green synthesis of monodispersed LaCO3OH microgears with novel plum blossom-like structure via a glycerol-mediated solvothermal method

Monodispersed LaCO3OH microgears with a novel plum blossom-like structure are prepared by a simple, reliable, environmentally-friendly and glycerol-mediated solvothermal method for the first time. By studying the effect of the experimental parameters on the morphology of LaCO3OH structures, we find that urea and glycerol at high concentration play a significant role in the formation of the plum blossom-like LaCO3OH microgears. In addition, we observe that the morphology evolution of the LaCO3OH microgears is from wires, spheres composed of rods, to plum blossom-like products with the increase of reaction time. More importantly, PL results demonstrate that the LaCO3OH microgears show a stronger PL than that of other structures, such as wires and spheres.

Improved light outcoupling and mode analysis of top-emitting OLEDs on periodically corrugated substrates

Bragg scattering by one dimensional periodic structures is investigated in order to enhance the outcoupling effciency of optically optimized planar top-emitting OLEDs. Using a soft imprint process, we fabricate extremely homogeneous gratings with sub- m period. These gratings are integrated beneath the bottom contact of topemitting OLEDs, without affecting the electrical device performance. The reflective contacts of the top emission geometry introduce pronounced micro-cavity effects for directly outcoupled and internally trapped light modes. Bragg scattering of the trapped waveguided and surface plasmon modes into the air cone, i.e. the forward direction, leads to interference with the directly outcoupled mode. As a result, constructive and destructive interference of the modes is detected and analyzed. Overall, we find that the introduction of shallow one dimensional sub- m periodic grating structures underneath top-emitting OLEDs leads to an EQE and luminous efficacy enhancement by up to 42%.