Effect of Low Temperature on Charge Transport in Operational Planar and Mesoporous Perovskite Solar Cells

Low-temperature optoelectrical studies of perovskite solar cells using MAPbI₃ and mixed-perovskite absorbers implemented into planar and mesoporous architectures reveal fundamental charge transporting properties in fully assembled devices operating under light bias. Both types of devices exhibit inverse correlation of charge carrier lifetime as a function of temperature, extending carrier lifetimes upon temperature reduction, especially after exposure to high optical biases. Contribution of bimolecular channels to the overall recombination process should not be overlooked because the density of generated charge surpasses trap-filling concentration requirements. Bimolecular charge recombination coefficient in both device types is smaller than Langevin theory prediction, and its mean value is independent of the applied illumination intensity. In planar devices, charge extraction declines upon MAPbI₃ transition from a tetragonal to an orthorhombic phase, indicating a connection between the trapping/detrapping mechanism and temperature. Studies on charge extraction by linearly increasing voltage further support this assertion, as charge carrier mobility dependence on temperature follows multiple-trapping predictions for both device structures. The monotonously increasing trend following the rise in temperature opposes the behavior observed in neat perovskite films and indicates the importance of transporting layers and the effect they have on charge transport in fully assembled solar cells. Low-temperature phase transition shows no pattern of influence on thermally activated electron/hole transport

Zn Single Atoms/Clusters/Nanoparticles Embedded in the Hybrid Carbon Aerogels for High-Performance ORR Electrocatalysis

Carbon-supported zinc single-atom catalysts have received considerable attention in the electrocatalytic oxygen reduction reaction (ORR) owing to the strong reduction capacity of zinc atoms and the abundant reserves of zinc elements. The common preparation method has been limited to the high-temperature pyrolysis of nitrogen-rich organic molecules and zinc ions, which makes it difficult to further improve the ORR performance. Herein, we first prepared ZnO/PNT/rGO aerogels as precursors via a simple hydrothermal method combined with freeze-drying, in which reduced graphene oxides (rGO) and polypyrrole nanotubes (PNT) together assembled into three-dimensional frames and numerous ZnO nanoparticles were anchored in the three-dimensional skeletons. Then, ZnO/PNT/rGO aerogels were calcined at 800 °C in the argon atmosphere, in which PNT/rGO were derived carbon aerogels, ZnO nanoparticles were reduced to Zn0 by carbon, and generating zinc single atoms were captured by the surrounding nitrogen atoms or aggregated into Zn clusters/nanoparticles in the carbon substrates. The obtained products were Zn single atoms/clusters/nanoparticles embedded into PNT/rGO-derived carbon aerogels, named Zn/NC catalysts. Zn/NC catalysts display a much higher half-wave potential and a larger limiting current density than pure rGO aerogels, NC, and Zn/C catalysts, indicating the synergy of excellent electronic transportation, high mass efficiency from outstanding porosity, and several active centers. Tailoring the quantity of zinc acetate can provide the optimal ORR performance with the Eonset of 0.96 V, the E1/2 of 0.845 V, and remarkable durability. This work exploits a novel strategy of carbon thermal reduction to construct high-performance Zn-based low-dimensional ORR catalysts

Robust Superamphiphobic Film from Electrospun TiO₂ Nanostructures

Rice-shaped TiO₂ nanostructures are fabricated by electrospinning for creating a robust superamphiphobic coating on glass substrates. The as-fabricated TiO₂ nanostructures (sintered at 500 °C) are superhydrophilic in nature which upon silanization turn into superamphiphobic surface with surface contact angle (SCA) values achieved using water (surface tension, γ = 72.1 mN/m) and hexadecane (surface tension, γ = 27.5 mN/m) being 166° and 138.5°, respectively. The contact angle hysteresis for the droplet of water and hexadecane are measured to be 2 and 12°, respectively. Thus, we have successfully fabricated superior self-cleaning coatings that possess exceptional superamphiphobic property by employing a simple, cost-effective, and scalable technique called electrospinning. Furthermore, the coating showed good mechanical and thermal stability with strong adherence to glass surface, thus revealing the potential for real applications

Polypyrrole Nanorod Networks/Carbon Nanoparticles Composite Counter Electrodes for High-Efficiency Dye-Sensitized Solar Cells

Polypyrrole­(PPy) nanorod networks with a high electrical conductivity of 40 S cm^–1 have been obtained in a high yield by employing an ion association of heparin–methylene blue as a new morphology-directing agent. The polypyrrole nanorod networks are mixed with different content of carbon nanoparticles to make PPy nanorod networks/carbon nanoparticles­(PPy/C) counter electrodes. It is found that the PPy/C composite with 10% carbon content shows a lower charge transfer resistance and better catalytic performance for the reduction of I₃^–, compared with the pristine PPy and carbon electrodes. The better catalytic performance is attributed to the interaction of the superior electrocatalytic activity of the unique polypyrrole nanorod networks and the carbon nanoparticles, which can accelerate triiodide reduction and electron transfer in the electrode. Under standard AM 1.5 sunlight illumination, the dye-sensitized solar cell based on the PPy/C composite with 10% carbon content as the counter electrode demonstrates a high efficiency of 7.2%, which is much higher than that of pristine PPy and carbon electrode-based DSCs and comparable to that of the thermal decomposed Pt-based DSC

Facile Fabrication of TiO₂–Graphene Composite with Enhanced Photovoltaic and Photocatalytic Properties by Electrospinning

We report the fabrication of one-dimensional TiO₂–graphene nanocomposite by a facile and one-step method of electrospinning. The unique nanostructured composite showed a significant enhancement in the photovoltaic and photocatalytic properties in comparison to TiO₂ as demonstrated in dye-sensitized solar cells and photodegradation of methyl orange

Charge Transport through Electrospun SnO₂ Nanoflowers and Nanofibers: Role of Surface Trap Density on Electron Transport Dynamics

A larger amount of tin precursor was dispersed in electrospun polyvinyl acetate fibers than that required for SnO₂ fiber formation upon annealing, thereby creating a constraint such that all nuclei formed during annealing could not be accommodated within the fiber, which leads to enhanced reaction kinetics and formation of highly crystalline–cum–higher surface area SnO₂ flowers. The flowers are shown to have a lower density of surface trap states than fibers by combining absorption spectra and open circuit voltage decay (OCVD) measurements. Charge transport through the SnO₂ flowers in the presence of the iodide/triiodide electrolyte was studied by OCVD, electrochemical impedance spectroscopy, and transient photodecay techniques. The study shows that the flowers are characterized by higher chemical capacitance, higher recombination resistance, and lower transport resistance compared with fibers. Photocurrent transients were used to extract the effective electron diffusion coefficient and mobility which were an order of magnitude higher for the flowers than that for the fibers. The flowers are also shown to have an enhanced Fermi energy, on account of which as well as higher electron mobility, dye-sensitized solar cells fabricated using the SnO₂ flowers gave <i>V</i>_OC ∼700 mV and one of the highest photoelectric conversion efficiencies achieved using pure SnO₂

Fabrication of Spinel One-Dimensional Architectures by Single-Spinneret Electrospinning for Energy Storage Applications

A facile and general method is developed to fabricate one-dimensional (1D) spinel composite oxides with complex architectures by using a facile single-spinneret electrospinning technique. It is found that precursor polymers and heating rates could control the structures of the products, such as 1D solid, nanotube and tube-in-tubes structures. Especially, the tube-in-tube structures have been successfully fabricated for various mixed metal oxide, including CoMn₂O₄, NiCo₂O₄, CoFe₂O₄, NiMn₂O₄ and ZnMn₂O₄. Benefiting from the unique structure features, the tube-in-tube hollow nanostructures possess superior electrochemical performances in asymmetric supercapacitors and Li–O₂ batteries

Enhanced Charge Carrier Transport and Device Performance Through Dual-Cesium Doping in Mixed-Cation Perovskite Solar Cells with Near Unity Free Carrier Ratios

PbI₂-enriched mixed perovskite film [FA_0.81MA_0.15Pb­(I_0.836Br_0.15)₃] has been widely studied due to its great potential in perovskite solar cell (PSC) applications. Herein, a FA_0.81MA_0.15Pb­(I_0.836Br_0.15)₃ film has been fabricated with the temperature-dependent optical absorption spectra utilized to determine its exciton binding energy. A ∼13 meV exciton binding energy is estimated, and a near-unity fraction of free carriers out of the total photoexcitons has been obtained in the solar cell operating regime at equilibrium state. PSCs are fabricated with this mixed perovskite film, but a significant electron transport barrier at the TiO₂–perovskite interface limited their performance. Cs₂CO₃ and CsI are then utilized as functional enhancers with which to substantially balance the electron and hole transport and increase the carriers (both electrons and holes) mobilities in PSCs, resulting in much-improved solar-cell performance. The modified PSCs exhibit reproducible power conversion efficiency (PCE) values with little hysteresis effect in the <i>J</i>–<i>V</i> curves, achieving PCEs up to 19.5% for the Cs₂CO₃-modified PSC and 20.6% when subsequently further doped with CsI

Exceptional Performance of TiNb₂O₇ Anode in All One-Dimensional Architecture by Electrospinning

We report the extraordinary performance of an Li-ion battery (full-cell) constructed from one-dimensional nanostructured materials, i.e. nanofibers as cathode, anode, and separator-cum-electrolyte, by scalable electrospinning. Before constructing such a one-dimensional Li-ion battery, electrospun materials are individually characterized to ensure its performance and balancing the mass loading as well. The insertion type anode TiNb₂O₇ exhibits the reversible capacity of ∼271 mAh g^–1 at current density of 150 mA g^–1 with capacity retention of ∼82% after 100 cycles. Under the same current density, electrospun LiMn₂O₄ cathode delivered the discharge capacity of ∼118 mAh g^–1. Gelled electrospun polyvinylidene fluoride-<i>co</i>-hexafluoropropylene (PVdF-HFP) nanofibers membrane is used as the separator-cum-electrolyte in both half-cell and full-cell assembly which exhibit the liquid like conductivity of ∼2.9 mS cm^–1 at ambient conditions. Full-cell, LiMn₂O₄|gelled PVdF-HFP|TiNb₂O₇ is constructed by optimized mass loading of cathode with respect to anode and tested between 1.95 and 2.75 V at room temperature. The full-cell delivered the reversible capacity of ∼116 mAh g^–1 at current density of 150 mA g^–1 with operating potential and energy density of ∼2.4 V and ∼278 Wh kg^–1, respectively. Further, excellent cyclability is noted for such configuration irrespective of the applied current densities

Engineering Poly(lactide)–Lignin Nanofibers with Antioxidant Activity for Biomedical Application

Biodegradable poly­(lactic acid) (PLA)–lignin composites are considered to be promising renewable plastic materials toward a sustainable world. The addition of lignin to PLA may assist to combat the oxidative stress induced by PLA as biomaterials. In this study, PLA–lignin copolymers with various contents of alkylated lignin (10–50%) were synthesized by ring-opening polymerization. The molecular weight of such copolymers ranged from 28 to 75 kDa, while the PLA chain length varied from 5 to 38. These PLA–lignin copolymers were further blended with poly­(l-lactide) (PLLA) and fabricated into nanofibrous composites by electrospinning. The PLLA/PLA–lignin nanofibers displayed uniform and bead-free nanostructures with fiber diameter of 350–500 nm, indicating the miscibility of PLLA and lignin copolymers in nanoscale. Unlike bulk materials, incorporation of PLA–lignin copolymers did not enhance the mechanical properties of the nanofibrous composites. Antioxidant assay showed that the lignin copolymers and PLLA/PLA–lignin nanofibers rendered excellent radical scavenging capacity for over 72 h. Moreover, three different types of cells (PC12, human dermal fibroblasts, and human mesenchymal stem cells) were cultured on the electrospun nanofibers to evaluate their biocompatibility. Lignin-containing nanofibers exhibited higher cell proliferation compared to neat PLLA nanofibers. PLLA/PLA-Lig20 nanofibers displayed the best biocompatibility as it achieved a balance between the antioxidant activities and the cytotoxicity. With excellent antioxidant activities and good biocompatibility, the PLLA/PLA–lignin electrospun nanofibers hold great potential to be used as biomedical materials for protecting cells from oxidative stress conditions