Photoinduced Self-Gating of Perovskite Photovoltaic Cells in Ionic Liquid.

We demonstrate that the power conversion efficiency (PCE), photocurrent, and fill factor (FF) of perovskite solar cells (PSC) can be significantly improved by the photoinduced self-gating in ionic liquids (ILs) via n-doping of the carbon nanotube (CNT) top electrode on the fullerene electron transport layer (ETL). CNTs, graphene, and other carbon electrodes have been proven to be stable electrodes for PSC, but efficiency was not high. We have previously shown that the performance of PSCs with CNT electrodes can be improved by IL gating with gate voltage (Vg) applied from an external power source. Here we demonstrate that effective self-gating in ILs is possible by a photoinduced process, without an external source. The open circuit voltage (Voc) generated by the PSC itself can be applied to the CNT/C60 electrode as Vg leading to photogating. This self-gating with Voc is compared to photocharging of CNTs in ILs without any gating for two types of fullerene ETLs: C60 and C70, Two types of ILs, DEME-TFSI and BMIM-BF4, are tested for two types of nanotubes electrodes: single wall (SWCNT), and multiwall (MWCNT). The resulting improvements are analyzed using the effective diode-circuit (DC) and the drift-diffusion (DD) models. Self-gating allows the PCE improvement from 3-5% to 10-11% for PSCs with a thick ETL, while for optimal combination of a thin SWCNT/ETL with added layers for improved stability, the PCE reached 13.2% in DEME-TFSI IL.

More Powerful Twistron Carbon Nanotube Yarn Mechanical Energy Harvesters.

Stretching a coiled carbon nanotube (CNT) yarn can provide large, reversible electrochemical capacitance changes, which convert mechanical energy to electricity. Here, it is shown that the performance of these "twistron" harvesters can be increased by optimizing the alignment of precursor CNT forests, plastically stretching the precursor twisted yarn, applying much higher tensile loads during precoiling twist than for coiling, using electrothermal pulse annealing under tension, and incorporating reduced graphene oxide nanoplates. The peak output power for a 1 and a 30 Hz sinusoidal deformation are 0.73 and 3.19 kW kg-1 , respectively, which are 24- and 13-fold that of previous twistron harvesters at these respective frequencies. This performance at 30 Hz is over 12-fold that of other prior-art mechanical energy harvesters for frequencies between 0.1 and 600 Hz. The maximum energy conversion efficiency is 7.2-fold that for previous twistrons. Twistron anode and cathode yarn arrays are stretched 180° out-of-phase by locating them in the negative and positive compressibility directions of hinged wine-rack frames, thereby doubling the output voltage and reducing the input mechanical energy.

Ionically Gated Small-Molecule OPV: Interfacial Doping of Charge Collector and Transport Layer.

We demonstrate an improvement in the performance of organic photovoltaic (OPV) systems based on small molecules by ionic gating via controlled reversible n-doping of multi-wall carbon nanotubes (MWCNTs) coated on fullerene electron transport layers (ETLs): C60 and C70. Such electric double-layer charging (EDLC) doping, achieved by ionic liquid (IL) charging, allows tuning of the electronic concentration in MWCNTs and the fullerene planar acceptor layers, increasing it by orders of magnitude. This leads to the decrease of the series and increase of the shunt resistances of OPVs and allows use of thick (up to 200 nm) ETLs, increasing the durability of OPVs. Two stages of OPV enhancement are described upon the increase of gating bias Vg: at small (or even zero) Vg, the extended interface of ILs and porous transparent MWCNTs is charged by gating, and the fullerene charge collector is significantly improved, becoming an ohmic contact. This changes the S-shaped J-V curve via improving the electron collection by an n-doped MWCNT cathode with an ohmic interfacial contact. The J-V curves further improve at higher gating bias Vg due to the increase of the Fermi level and decrease of the MWCNT work function. At the next qualitative stage, the acceptor fullerene layer becomes n-doped by electron injection from MWCNTs while ions of ILs penetrate into the fullerene. At this step, the internal built-in field is created within OPV, which helps in exciton dissociation and charge separation/transport, increasing further the Jsc and the fill factor. The ionic gating concept demonstrated here for most simple classical planar small-molecule OPV cells can be potentially applied to more complex highly efficient hybrid devices, such as perovskite photovoltaic with an ETL or a hole transport layer, providing a new way to tune their properties via controllable and reversible interfacial doping of charge collectors and transport layers.

Surface Energy-Driven Preferential Grain Growth of Metal Halide Perovskites: Effects of Nanoimprint Lithography Beyond Direct Patterning.

Hybrid organic-inorganic lead halide perovskites have attracted much attention in the field of optoelectronic devices because of their desirable properties such as high crystallinity, smooth morphology, and well-oriented grains. Recently, it was shown that thermal nanoimprint lithography (NIL) is an effective method not only to directly pattern but also to improve the morphology, crystallinity, and crystallographic orientations of annealed perovskite films. However, the underlining mechanisms behind the positive effects of NIL on perovskite material properties have not been understood. In this work, we study the kinetics of perovskite grain growth with surface energy calculations by first-principles density functional theory (DFT) and reveal that the surface energy-driven preferential grain growth during NIL, which involves multiplex processes of restricted grain growth in the surface-normal direction, abnormal grain growth, crystallographic reorientation, and grain boundary migration, is the enabler of the material quality enhancement. Moreover, we develop an optimized NIL process and prove its effectiveness by employing it in a perovskite light-emitting electrochemical cell (PeLEC) architecture, in which we observe a fourfold enhancement of maximum current efficiency and twofold enhancement of luminance compared to a PeLEC without NIL, reaching a maximum current efficiency of 0.07598 cd/A at 3.5 V and luminance of 1084 cd/m2 at 4 V.

Enhanced Thermoelectric Properties of Poly(3-hexylthiophene) through the Incorporation of Aligned Carbon Nanotube Forest and Chemical Treatments.

Carbon nanotube/polymer composites have recently received considerable attention for thermoelectric (TE) applications. The TE power factor can be significantly improved by forming composites with carbon nanotubes. However, the formation of a uniform and well-ordered nanocomposite film is still challenging because of the creation of agglomerates and the uneven distribution of nanotubes. Here, we developed a facile, efficient, and easy-processable route to produce uniform and aligned nanocomposite films of P3HT and carbon nanotube forest (CNTF). The electrical conductivity of a pristine P3HT film was improved from ∼10-7 to 160 S/cm thanks to the presence of CNTF. Also, a further boost in TE performance was achieved using two additives, lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) and tert-butylpyridine. By adding the additives to P3HT, the degree of interchain order increased, which facilitated the charge transport through the composite. Under the optimal conditions, the incorporation of CNTF and additives led to values of the Seebeck coefficient, electrical conductivity, and power factor up to rising 92 µV/K, 130 S/cm, and 110 µW/m K2, respectively, at a temperature of 344.15 K. The excellent TE performance of the hybrid films originates from the dramatically increased electrical conductivity and the improved Seebeck coefficient by CNTF and additives, respectively.

Suppression of Electric Field-Induced Segregation in Sky-Blue Perovskite Light-Emitting Electrochemical Cells.

Inexpensive perovskite light-emitting devices fabricated by a simple wet chemical approach have recently demonstrated very prospective characteristics such as narrowband emission, low turn-on bias, high brightness, and high external quantum efficiency of electroluminescence, and have presented a good alternative to well-established technology of epitaxially grown III-V semiconducting alloys. Engineering of highly efficient perovskite light-emitting devices emitting green, red, and near-infrared light has been demonstrated in numerous reports and has faced no major fundamental limitations. On the contrary, the devices emitting blue light, in particular, based on 3D mixed-halide perovskites, suffer from electric field-induced phase separation (segregation). This crystal lattice defect-mediated phenomenon results in an undesirable color change of electroluminescence. Here we report a novel approach towards the suppression of the segregation in single-layer perovskite light-emitting electrochemical cells. Co-crystallization of direct band gap CsPb(Cl,Br)3 and indirect band gap Cs4Pb(Cl,Br)6 phases in the presence of poly(ethylene oxide) during a thin film deposition affords passivation of surface defect states and an increase in the density of photoexcited charge carriers in CsPb(Cl,Br)3 grains. Furthermore, the hexahalide phase prevents the dissociation of the emissive grains in the strong electric field during the device operation. Entirely resistant to 5.7 × 106 V·m-1 electric field-driven segregation light-emitting electrochemical cell exhibits stable emission at wavelength 479 nm with maximum external quantum efficiency 0.7%, maximum brightness 47 cd·m-2, and turn-on bias of 2.5 V.

Room-Temperature Lasing from Mie-Resonant Nonplasmonic Nanoparticles.

Subwavelength particles supporting Mie resonances underpin a strategy in nanophotonics for efficient control and manipulation of light by employing both an electric and a magnetic optically induced multipolar resonant response. Here, we demonstrate that monolithic dielectric nanoparticles made of CsPbBr3 halide perovskites can exhibit both efficient Mie-resonant lasing and structural coloring in the visible and near-IR frequency ranges. We employ a simple chemical synthesis with nearly epitaxial quality for fabricating subwavelength cubes with high optical gain and demonstrate single-mode lasing governed by the Mie resonances from nanocubes as small as 310 nm by the side length. These active nanoantennas represent the most compact room-temperature nonplasmonic nanolasers demonstrated until now.

Remote heteroepitaxy of GaN microrod heterostructures for deformable light-emitting diodes and wafer recycle.

There have been rapidly increasing demands for flexible lighting apparatus, and micrometer-scale light-emitting diodes (LEDs) are regarded as one of the promising lighting sources for deformable device applications. Herein, we demonstrate a method of creating a deformable LED, based on remote heteroepitaxy of GaN microrod (MR) p-n junction arrays on c-Al2O3 wafer across graphene. The use of graphene allows the transfer of MR LED arrays onto a copper plate, and spatially separate MR arrays offer ideal device geometry suitable for deformable LED in various shapes without serious device performance degradation. Moreover, remote heteroepitaxy also allows the wafer to be reused, allowing reproducible production of MR LEDs using a single substrate without noticeable device degradation. The remote heteroepitaxial relation is determined by high-resolution scanning transmission electron microscopy, and the density functional theory simulations clarify how the remote heteroepitaxy is made possible through graphene.

Electronic structure of CsPbBr3-xClx perovskites: synthesis, experimental characterization, and DFT simulations.

All-inorganic lead halide perovskites of various compositions have emerged as a prospective family of materials for light-emitting devices and photonic applications. However, a comprehensive study of their structural and electronic properties is still missing. Moreover, thin film fabrication of these perovskites comprising heterohalide anions by wet chemistry approaches also remains challenging. Here we fabricate high-quality CsPbBr3-xClx perovskite thin films using a wet chemical method accompanied by a chemical vapor anion exchange procedure, which allows rigorously studying their optical and structural properties at different compositions. Namely, we present both the numerical and experimental studies of the electronic properties of all-inorganic mixed-halide perovskites, showing their optical absorption, excitonic photoluminescence and exciton binding energy, phase, chemical composition, and band structure and the band gap evolution with a gradual change in x in CsPbBr3-xClx. The results reveal that as Cl ions substitute for Br ones in the perovskite crystal lattice the room-temperature phase does not change its orthorhombic symmetry, whereas the energy of the direct electronic transition from the valence to conduction band at the Γ-point increases nonlinearly. By using the experimentally derived nonlinear dependence it is easy to accurately predict the band gap for any CsPbBr3-xClx perovskite thin film consisting of grains with sizes beyond the quantum confinement regime.

Silver Nanowires on Carbon Nanotube Aerogel Sheets for Flexible, Transparent Electrodes.

Flexible, free-standing transparent conducting electrodes (TCEs) with simultaneously tunable transmittances up to 98% and sheet resistances down to 11 Ω/sq were prepared by a facile spray-coating method of silver nanowires (AgNWs) onto dry-spun multiwall carbon nanotube (MWNT) aerogels. Counterintuitively, the transmittance of the hybrid electrodes can be increased as the mass density of AgNWs within the MWNT aerogels increases; however, the final achievable transmittance depends on the initial transparency of the MWNT aerogels. Simultaneously, a strong decrease in sheet resistance is obtained when AgNWs form a percolated network along the MWNT aerogel. Additionally, anisotropic reduction in sheet resistance and polarized transmittance of AgNW/MWNT aerogels is achieved with this method. The final AgNW/MWNT hybrid TCEs transmittance and sheet resistance can be fine-tuned by spray-coating mechanisms or by choosing initial MWNT aerogel density. Thus, a wide range of AgNW/MWNT hybrid TCEs with optimized optoelectronic properties can be achieved depending of the requirements needed. Finally, the free-standing AgNW/MWNT hybrid TCEs can be laminated onto a wide range of substrates without the need of a bonding aid.

Single-Mode Lasing from Imprinted Halide-Perovskite Microdisks.

Halide-perovskite microlasers have demonstrated fascinating performance owing to their low-threshold lasing at room temperature and low-cost fabrication. However, being synthesized chemically, controllable fabrication of such microlasers remains challenging, and it requires template-assisted growth or complicated nanolithography. Here, we suggest and implement an approach for the fabrication of microlasers by direct laser ablation of a thin film on glass with donut-shaped femtosecond laser beams. The fabricated microlasers represent MAPbBr xI y microdisks with 760 nm thickness and diameters ranging from 2 to 9 µm that are controlled by a topological charge of the vortex beam. As a result, this method allows one to fabricate single-mode perovskite microlasers operating at room temperature in a broad spectral range (550-800 nm) with Q-factors up to 5500. High-speed fabrication and reproducibility of microdisk parameters, as well as a precise control of their location on a surface, make it possible to fabricate centimeter-sized arrays of such microlasers. Our finding is important for direct writing of fully integrated coherent light sources for advanced photonic and optoelectronic circuitry.

A Few-Minute Synthesis of CsPbBr3 Nanolasers with a High Quality Factor by Spraying at Ambient Conditions.

Inorganic cesium lead halide perovskite nanowires, generating laser emission in the broad spectral range at room temperature and low threshold, have become powerful tools for the cutting-edge applications in the optoelectronics and nanophotonics. However, to achieve high-quality nanowires with the outstanding optical properties, it was necessary to employ long-lasting and costly methods of their synthesis, as well as postsynthetic separation and transfer procedures that are not convenient for large-scale production. Here we report a novel approach to fabricate high-quality CsPbBr3 nanolasers obtained by rapid precipitation from dimethyl sulfoxide solution sprayed onto hydrophobic substrates at ambient conditions. The synthesis technique allows producing the well-separated nanowires with a broad size distribution of 2-50 µm in 5-7 min, being the fastest method to the best of our knowledge. The formation of nanowires occurs via ligand-assisted reprecipitation triggered by intermolecular proton transfer from (CH3)2CHOH to H2O in the presence of a minor amount of water. The XRD patterns confirm an orthorhombic crystal structure of the as-grown CsPbBr3 single nanowires. Scanning electron microscopy images reveal their regular shape and truncated pyramidal end facets, while high-resolution transmission electron microscopy ones demonstrate their single-crystal structure. The lifetime of excitonic emission of the nanowires is found to be 7 ns, when the samples are excited with energy below the lasing threshold, manifesting the low concentration of defect states. The measured nanolasers of different lengths exhibit pronounced stimulated emission above 13 µJ cm-2 excitation threshold with quality factor Q = 1017-6166. Their high performance is assumed to be related to their monocrystalline structure, low concentration of defect states, and improved end facet reflectivity.

Room-Temperature Continuous-Wave Operation of Organometal Halide Perovskite Lasers.

Solution-processed organic-inorganic lead halide perovskites have recently emerged as promising gain media for tunable semiconductor lasers. However, optically pumped continuous-wave lasing at room temperature, a prerequisite for a laser diode, has not been realized so far. Here, we report lasing action in a surface-emitting distributed feedback methylammonium lead iodide (MAPbI3) perovskite laser on a silicon substrate at room temperature under continuous-wave optical pumping. This outstanding performance is achieved because of the ultralow lasing threshold of 13 W/cm2, which is enabled by thermal nanoimprint lithography that directly patterns perovskite into a high- Q cavity with large mode confinement, while at the same time, it improves perovskite's emission characteristics. Our results represent a major step toward electrically pumped lasing in organic and thin-film materials as well as the insertion of perovskite lasers into photonic integrated circuits for applications in optical computing, sensing, and on-chip quantum information.

Tunable Hybrid Fano Resonances in Halide Perovskite Nanoparticles.

Halide perovskites are known to support excitons at room temperatures with high quantum yield of luminescence that make them attractive for all-dielectric resonant nanophotonics and meta-optics. Here we report the observation of broadly tunable Fano resonances in halide perovskite nanoparticles originating from the coupling of excitons to the Mie resonances excited in the nanoparticles. Signatures of the photon-exciton (" hybrid") Fano resonances are observed in dark-field spectra of isolated nanoparticles, and also in the extinction spectra of aperiodic lattices of such nanoparticles. In the latter case, chemical tunability of the exciton resonance allows reversible tuning of the Fano resonance across the 100 nm bandwidth in the visible frequency range, providing a novel approach to control optical properties of perovskite nanostructures. The proposed method of chemical tuning paves the way to an efficient control of emission properties of on-chip-integrated light-emitting nanoantennas.

Continuous-wave operation in directly patterned perovskite distributed feedback light source at room temperature.

We report a directly patterned perovskite distributed feedback (DFB) resonator and show narrow amplified spontaneous emission (ASE) at pump powers as low as 0.1 W/cm2 under continuous-wave (CW) optical pumping conditions at room temperature. Compared to the pristine thin film photoluminescence spectrum, a 16-fold reduction in emission linewidth in the MAPbI3 DFB cavity was observed. The direct nanostructuring of perovskites was achieved by thermal nanoimprint lithography. Our findings pave the way toward realizing CW pumped perovskite lasers at room temperature and energy-efficient perovskite light sources.

Biophysical interactions between pancreatic cancer cells and pristine carbon nanotube substrates: Potential application for pancreatic cancer tissue engineering.

Novel synthetic biomaterials able to support direct tissue growth and retain cellular phenotypical properties are promising building blocks for the development of tissue engineering platforms for accurate and fast therapy screening for cancer. The aim of this study is to validate an aligned, pristine multi-walled carbon nanotube (CNT) platform for in vitro studies of pancreatic cancer as a systematic understanding of interactions between cells and these CNT substrates is lacking. Our results demonstrate that our CNT scaffolds-which are easily tuneable to form sheets/fibers-support growth, proliferation, and spatial organization of pancreatic cancer cells, indicating their great potential in cancer tissue engineering. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1637-1644, 2018.

Nanoimprinted perovskite metasurface for enhanced photoluminescence.

Recently, solution-processed hybrid halide perovskite has emerged as promising materials for advanced optoelectronic devices such as photovoltaics, photodetectors, light emitting diodes and lasers. In the mean time, all-dielectric metasurfaces with high-index materials have attracted attention due to their low-loss and high-efficient optical resonances. Because of its tunable by composition band gap in the visible frequencies, organolead halide perovskite could serve as a powerful platform for realizing high-index, low-loss metasurfaces. However, direct patterning of perovskite by lithography-based technique is not feasible due to material instability under moisture. Here we report novel organolead halide perovskite metasurfaces created by the cost-effective thermal nanoimprint technology. The nanoimprinted perovskite metasurface showed improved surface morphology and enhanced optical absorption properties. Significantly enhanced optical emission with an eight-fold enhancement in photoluminescence (PL) intensity was observed under room temperature. Temperature-dependent PL of perovskite nanograting metasurface was also investigated. Based on our results, we believe that thermal nanoimprint is a simple and cost-effective technique to fabricate perovskite-based metasurfaces, which could have broad impact on optoelectronic and photonic applications.

Polar-Electrode-Bridged Electroluminescent Displays: 2D Sensors Remotely Communicating Optically.

A novel geometry for electroluminescent devices, which does not require transparent electrodes for electrical input, is demonstrated, theoretically analyzed, and experimentally characterized. Instead of emitting light through a conventional electrode, light emission occurs through a polar liquid or solid and input electrical electrodes are coplanar, rather than stacked in a sandwich configuration. This new device concept is scalable and easily deployed for a range of modular alternating-current-powered electroluminescent light sources and light-emitting sensing devices. The polar-electrode-bridged electroluminescent displays can be used as remotely readable, spatially responsive sensors that emit light in response to the accumulation and distribution of materials on the device surface. Using this device structure, various types of alternating current devices are demonstrated. These include an umbrella that automatically lights up when it rains, a display that emits light from regions touched by human fingers (or painted upon using a mixture of oil and water), and a sensor that lights up differently in different areas to indicate the presence of water and its freezing. This study extends the dual-stack, coplanar-electrode device geometry to provide displays that emit light from a figure drawn on an electroluminescent panel using a graphite pencil.

Pristine carbon nanotube scaffolds for the growth of chondrocytes.

The effective growth of chondrocytes and the formation of cartilage is demonstrated on scaffolds of aligned carbon nanotubes; as two dimensional sheets and on three dimensional textiles. Raman spectroscopy is used to confirm the presence of chondroitin sulfate, which is critical in light of the unreliability of traditional dye based assays for carbon nanomaterial substrates. The textile exhibits a very high affinity for chondrocyte growth and could present a route to implantable, flexible cartilage scaffolds with tuneable mechanical properties.

Nanoimprinted Perovskite Nanograting Photodetector with Improved Efficiency.

Recently, organolead halide-based perovskites have emerged as promising materials for optoelectronic applications, particularly for photovoltaics, photodetectors, and lasing, with low cost and high performance. Meanwhile, nanoscale photodetectors have attracted tremendous attention toward realizing miniaturized optoelectronic systems, as they offer high sensitivity, ultrafast response, and the capability to detect beyond the diffraction limit. Here we report high-performance nanoscale-patterned perovskite photodetectors implemented by nanoimprint lithography (NIL). The spin-coated lead methylammonium triiodide perovskite shows improved crystallinity and optical properties after NIL. The nanoimprinted metal-semiconductor-metal photodetectors demonstrate significantly improved performance compared to the nonimprinted conventional thin-film devices. The effects of NIL pattern geometries on the optoelectronic characteristics were studied, and the nanograting pattern based photodetectors demonstrated the best performance, showing approximately 35 times improvement on responsivity and 7 times improvement on on/off ratio compared with the nonimprinted devices. The high performance of NIL-nanograting photodetectors likely results from high crystallinity and favored nanostructure morphology, which contribute to higher mobility, longer diffusion length, and better photon absorption. Our results have demonstrated that the NIL is a cost-effective method to fabricate high-performance perovskite nanoscale optoelectronic devices, which may be suitable for manufacturing of high-density perovskite nanophotodetector arrays and to provide integration with state-of-the-art electronic circuits.